Applied Immunology Flashcards
Coursework
Pick out the KEY aspect of the paper and summarise it / critically review
Limited slides for 5 min presentation > use Biorender etc.
HOW TO CRITICALLY ANALYSE A PAPER
RESOURCES
» Academic skills and development https://study.surrey.ac.uk/study-support/academic-skills-and-development
» ‘Quick guide to Critical Thinking’ and ‘Quick Guide to Critically Evaluating Literature’ (PDFs available on Surrey Learn)
» Panopto on Critiquing a Journal article- with an example (available on Surrey Learn) https://surrey.cloud.panopto.eu/Panopto/Pages/Viewer.aspx?id=f1994eba- 45a3-437d-8163-ac5900ae21f9
TIPS
- Identify the KEY parts first and summarise these bits and explain their purpose.
- Distinguish the relative importance of figures
- Title should give summary of the paper > you must find the title interesting!
- All the info for the summary for the paper should be in the abstract > use this wisely!
- Words like ‘may’, ‘at least’ and ‘suggest’ only claim a finding, NOT definite, look out for these words! Are they unclear or vague?
- Abstract and intro: the aim - Is this a good one? Is it clearer in the introduction?
e.g. the methods are sometimes mentioned very briefly, and mixed with results.
Are conclusions and impact clear? In line with the findings and discussion? - Introduction must have: What is known? What is NOT known? How do we address the question?
Essentially what’s the bigger picture and how do we answer this question? USE THIS FUNNEL FORMAT FOR RESEARCH PROJECT INTRO TOO, FOR ANY INTRO WRITING YOU DO! - Methods: are they explicit? Transparent? Try to identify the controls used (and judge them!), and stats (and judge them!). Do they have a negative / positive control? Do they use the correctly? T-test? Have they used the right test? Is the test group large enough?
What is the readout? (=what you should find in the figures) - When analysing a figure, ask:
» Why was the figure drawn? What research question does it attempt to answer? What do the results claim?e.g. Differential disease progression following VEEV and EEEV infection
I selected this figure as the key figure because…
e.g. Are infections with EEEV and VEEV clinically different?
»What does it show? Does it successfully address the research questions? Are further graphs/ data/ statistical test required to fully answer the question? Are you convinced, would you like to see other graphs or data?
e.g. At the same dose, in outbred mice, VEEV infection is more severe.
2 experiments with 3 animals each, unclear what stats data is shown (mean? SD? SEM? Geomean??
»Is anything unclear or misleading in the way the data is presented? How might you address these concerns? Could make the last slide as what you would do differently, pros and cons. Visually there could be a difference but if the control group is really small you wouldn’t actually be able to tell, not enough stats.
e.g. Differences are small, are they relevant? (Low n, unclear data analysis - show raw data!
» What additional scientific questions does the figure raise?
e.g. If the whole paper is based on this difference, is it relevant?
Making a summary you can present in 5 mins > 1 min per slide, 5 slides using the structure below:
Introduction slide(s): remember the funnel, try to identify the key questions / hypothesis (background and aim), are they relevant / hot topic /’ controversial in the field? Mention extra reading to get extra marks, have to mention as a reference!
Select the main findings that you want to present (the most important / relevant?), and try to understand the methods, explain them in a slide or 2
Show the results that you have selected. Take the time to explain the figures (as I have done today). Select one good figure to analyse, normally the last one is the best. Could use Biorender to create a better figure > AI? = extra marks!
Make a judgement on the quality: why is it good / clear / relevant / trustworthy? Or why is is falling short? JUSTIFY
Mention the main discussion points related the figures you presented. Does the field agree? What would you do different / next? What HAS BEEN done after?
WHERE TO START:
You will prepare and record a presentation with a critical analysis of the paper and proposal for next steps > LOOK AT OTHER PAPERS. This will be a presentation for a set amount of time (maximum 5 minutes) with slides that you will record and submit.
introduce the problem and the hypothesis /question addressed (background and aim)
how the experiments were conducted and controlled, and why (methods)
what the key findings were and whether you agree
How the findings impact in the field and propose possible next steps
DO:
Take enough time to read all parts of the paper
Perform a search on the topic of interest, to understand the importance and relevance of the problem discussed
Identify if the topic is the source of debate in the field
Identify and select the methods and results that you think are most relevant to the story (you may not have time to present all methods and results). This is the part where you want to critically evaluate the paper: are controls missing? Is the interpretation fair or is it missing the statistics ? What could be done better? (other methods described in similar papers? .. etc…
Discuss the findings, do you agree with the conclusions? What would you do next?
Think outside of the box – element not listed here that you think would be of interest.
Don’t:
Do not try to put too much information in the 3 to 5 minutes presentation. You are expected to extract the key information and to be able to summarise! The longer is not always the better… DON’T TALK TOO FAST, YOU WILL LOSE MARKS!
You don’t have to perform a comprehensive literature search on the topic, but you should find that you need to read a few more papers on the topic to be able to give an informed appraisal of the paper.
The presentation quality is really important to deliver your message, don’t keep that to the last minute, keep time to have fun with it.
Logistics:
Submit your 3 to 5 minutes video on SurreyLearn, the admin team will create the folder near the time.
Use slides (power point or equivalent)
Record with yourself visible on the slide (as the speaker or in person presenting as in a Panopto).
You are welcome to be creative and add own visuals, e.g. add the image of the figure in: you have to explain the results and findings from the paper, but you can create visuals to explain the methods (e.g on biorender > a schematic, I propose the paper should present its methods like this…) for example or the scale of the problem in your introduction.
Add the references used on each slide when relevant, as a small box in a bottom corner (the DOI) > embed references in the slides, just put first name, year and the link.
Grading:
the scientific aspect (the level of understanding of the problem in the field, its importance, the critical evaluation of the methods and controls used, and the level of interpretation). This will require to search the literature and read beyond the paper itself, and include proper referencing = 30% of the mark
The capacity to extract the key information, focus and summarise it= 30% > JUSTIFY YOUR REASONING. e.g. I chose this figure because… it’s the central one etc
The presentation style (whether you engage, smile, pace of the narrative…) = 20%
The presentation content: quality of the slides, clear and logical slide, use of visuals… = 20%
Submissions over 5 minutes will also incur a penalty, do rehearse and time yourself! Any videos over time limit will be deducted 1% per 10 seconds
Coursework Essay of Choice:
Immune response to respiratory disease in pigs
https://surreylearn.surrey.ac.uk/content/enforced/267450-BMS3108_2024-5_SEMR1_1/Respiratory%20disease%20in%20pigs.pdf
OR
https://www.sciencedirect.com/science/article/pii/S0165242724000734?via%3Dihub
To critically analyse this abstract, you might consider focusing on the following key points:
Effectiveness and Limitations of WIV Vaccines:
The abstract states that Whole-Inactivated Virus (WIV) vaccines are highly effective against homologous viruses but provide limited protection against antigenically divergent viruses. This limitation is crucial as it suggests that the vaccine’s effectiveness is restricted to specific strains, which could be problematic in the face of diverse viral mutations. This raises questions about the broader applicability and effectiveness of these vaccines in diverse viral environments.
Vaccine-Associated Enhanced Respiratory Disease (VAERD):
The mention of VAERD as a potential consequence of heterologous infection is a significant concern. This phenomenon, where vaccination might exacerbate disease severity, raises concerns about the safety and broader implications of WIV vaccines. The reproducibility of VAERD in laboratory settings but difficulty in clinical diagnosis highlights a gap between experimental and practical applications.
This phenomenon may present challenges for vaccine development and deployment.
Challenges in Clinical Diagnosis:
Diagnosing VAERD clinically is challenging due to the need for prior vaccine history and necropsy evidence. This point underscores the complexity of diagnosing VAERD in real-world scenarios, which could hinder timely and accurate identification and treatment.
Consider the practical implications of these diagnostic challenges in real-world settings.
Study Objective and Design:
The study aims to identify potential biomarkers for VAERD for antemortem clinical diagnosis. The design involves splitting naïve pigs into vaccinated and non-vaccinated groups, challenging them with a heterologous virus, and then assessing various biomarkers. This experimental setup appears robust, allowing for controlled comparison between groups.
Evaluate whether it adequately addresses the research questions.
Biomarkers Assessed:
The identification of potential biomarkers (elevated white blood cells, neutrophils, C-reactive protein, haptoglobin, and CitH3) for antemortem diagnosis is a key finding. Additionally, cytokine and CitH3 levels in bronchoalveolar lavage fluid (BALF) are measured. The comprehensive approach to biomarker assessment is a strength, potentially providing a detailed profile of VAERD.
Assess the validity and potential clinical utility of these biomarkers.
Results and Interpretation of Key Findings:
The findings indicate that vaccinated and challenged pigs with VAERD had elevated levels of white blood cells, neutrophils, C-reactive protein, haptoglobin, and CitH3 in blood, and elevated IL-8 and CitH3 in BALF. These results suggest a specific biomarker profile associated with VAERD, which could be valuable for clinical diagnosis.
Critically analyse the interpretation of these results and their implications for understanding VAERD and improving diagnostic methods.
Conclusion and Clinical Relevance:
The conclusion proposes that a profile of elevated white blood cells, neutrophils, C-reactive protein, haptoglobin, and CitH3 may be relevant for diagnosing VAERD antemortem. The conclusion suggests a profile of biomarkers for clinical diagnosis of VAERD.
This finding is significant as it offers a potential method for early diagnosis, which could improve management and outcomes for affected swine.
Consider the broader relevance of these findings for the swine industry and potential next steps for research and application.
Critical Evaluation:
Strengths: The study addresses a critical issue in swine health, uses a robust experimental design, and identifies specific biomarkers that could aid in the clinical diagnosis of VAERD.
Limitations: The abstract does not discuss the potential variability in biomarker levels among different swine populations or the feasibility of implementing these diagnostic methods in field conditions. Additionally, the reliance on necropsy for definitive diagnosis remains a practical challenge.
Overall, this abstract presents a well-structured study with significant findings, but further research and practical considerations are necessary to translate these findings into real-world applications
Tips to analyse figure 6:
- Understand the Context
Figure Description: The figure shows cytokine and citrullinated H3 histone levels in bronchoalveolar lavage fluid (BALF) at 5 days post-infection (dpi).
Assay Used: Multiplex bead-based assay for detecting IL-10, IL-6, IL-8, TNF-⍺, and IFN-⍺.
Groups: NV (non-vaccinated), NC (non-challenged), V (vaccinated), C (challenged). - Examine the Data Presentation
Box and Whiskers Plot:
Median Line: Indicates the central tendency.
Box: Represents the interquartile range (25th to 75th percentile).
Whiskers: Show the range of the data (min to max).
Sample Size: ( N = 8 ), which is relatively small but can still provide insights. - Identify Key Findings
Statistical Significance: Look for asterisks indicating significant differences between groups:
*p ≤ 0.05
**p ≤ 0.01
Comparisons: Note which groups are being compared and the significance of these comparisons. - Interpret the Results
Cytokine Levels: Assess how the levels of IL-10, IL-6, IL-8, TNF-⍺, and IFN-⍺ differ between the groups.
Citrullinated H3 Histone: Evaluate its presence and significance in the context of infection and vaccination. - Evaluate the Methodology
Assay Validity: Consider if the multiplex bead-based assay is appropriate for detecting these cytokines.
Sample Exclusion: Note that NV/NC samples from a previous study were not included in the statistical analysis, which might affect the interpretation. - Consider Limitations
Sample Size: A small sample size (N=8) may limit the generalizability of the findings.
Exclusion of NV/NC Samples: Understand why these samples were excluded and how this might impact the results. - Draw Conclusions
Overall Findings: Summarize the main outcomes and their implications for the study.
Future Directions: Suggest further research or experiments that could build on these findings
How to improve the figure:
Title and Labels:
Ensure the title is descriptive and concise, e.g., “Cytokine and Citrullinated H3 Histone Levels in BALF at 5 dpi”.
Clearly label all axes, including units of measurement where applicable.
Use consistent and clear abbreviations, and provide a legend for any abbreviations used (e.g., NV, NC, V, C).
Color and Symbols:
Use distinct colors or patterns for different groups (NV, NC, V, C) to make them easily distinguishable.
Ensure that the colors are colorblind-friendly.
Use different symbols or markers for each group if color alone is not sufficient.
Data Presentation:
Consider adding individual data points to the box and whiskers plot to show the distribution within each group.
Ensure the box and whiskers plot is clearly defined, with the median line, interquartile range, and whiskers easily distinguishable.
Statistical Significance:
Clearly mark statistically significant differences with asterisks and brackets, as described.
Consider adding a brief note or legend explaining the significance markers (e.g., *p ≤ 0.05; **p ≤ 0.01).
Annotations and Legends:
Include a legend that explains all symbols, colors, and abbreviations used in the figure.
Add annotations or labels directly on the figure to highlight key findings or significant differences.
Figure Layout:
Ensure the figure is not cluttered; maintain a balance between data density and readability.
Use appropriate spacing between elements to avoid overlap and ensure clarity.
Supplementary Information:
If space allows, include a brief caption or footnote summarizing the key findings and any relevant context from the study.
Exam - essay questions
Questions will be based on lectures > pick out the key points of lectures!
Only have 1 hour per essay question > use lecture notes! Write in your own words!
Intro, two paragraphs with examples from notes and then conclusion!
e.g. maternal transfer in …
Immunity to Infection is…
A conflict between the microbe/parasite’s ability to infect
and evade the host’s immune response and the ability of
the immune system to detect and eliminate the pathogen
without causing damage
External defences
Extra-cellular pathogens/defences
Intra-cellular pathogens/defences
The Immune System
Innate immunity
Fast, less specific
Physical & chemical barriers
Complement
Phagocytes
Inflammation
Cytokine regulation
Specific/adaptive immunity
Slower, highly specific
Ab responses (B cells)
Cytotoxic T cells
All controlled by
T helper cells, T-reg cells &
Cytokines
Both together = best immune responses!
- External Defences
Physical barriers
Antimicrobial peptides/proteins (AMPs)
Secretory Antibody
Microbial antagonism
Physical barriers
Skin, mucous membranes, desquamation
Pathogens gain entry by skin damage from:
burns, cuts → staphylococcal, clostridial infection
infected needles → hepatitis (B & C), HIV & AIDS
arthropod bites → malaria, yellow fever, leishmaniasis
mammal bites → rabies
Mucus, cerumen
* Strep. pneumoniae and Influenza virus secrete neuraminidase: binds sialic
acid and then cleaves mucus.
* Some Strep. spp secrete hyaluronidase: breaks down intracellular matrix
allowing further spread
Muco-ciliary escalator (in bronchi)
* Strep. pneumoniae secrete pneumolysin exotoxin: aids attachment then
creates pores in epithelial cells and can block chemotaxis of macrophages.
* Pseudomonas produces an exoenzyme that degrades cell membranes in
cystic fibrosis
Fluid flow such as tears, urine
can be overcome by pathogen
adhesion to mucosal cells Neisseria
gonorrhoeae pilin
Skin secretions such as lactic
acid, high salt, fatty acids, lysozyme
and AMPs, defects → ringworm
(fungal infection)
Non-specific mucosal
secretions such as gastric acid,
AMPs, lysozyme, Helicobacter pylori
secrete urease to neutralise pH
Antimicrobial Proteins (AMPs)
-defensins - neutrophils, intestinal paneth cells
-defensins - epithelia (skin, trachea), neutrophils
cathelicidins - epithelia, keratinocytes, phagocytes
* rapid release in infection/tissue injury
* broad anti-microbial action, cationic (+ve charge)
* chemotactic/activating effects on APCs (-defensins →immature DCs)
* strong enhancing effects in vivo on innate & adaptive responses - induce
inflammatory mediators (Histamine, Prostaglandins, TNF, chemokines)
Microbial Evasion of Defensins
* Produce inhibitory binding proteins (Streptococcus pyogenes)
* Inhibit signalling pathways for gene expression of defensins (Bordetella
bronchiseptica at bronchial epithelium)
* Reduce affinity for cell wall (Staphylococcus aureus modify
teichoic/lipoteichoic acids, phospholipids)
Secretory Immunoglobulins
- Secretory (s)IgA - as dimer, IgA1 and
IgA2 in human, more IgA2 at mucosal
surfaces, IgA1 dominates serum &
upper respiratory tract - Some sIgM in tears & saliva when sIgA not present (IgG
serum diffused - respiratory, urogenital, IgE in parasitic and
allergic responses, IgD trace amounts respiratory tract, saliva, tears)
Evasion: production of IgA1 protease (N. gonorrhoeae, H. influenzae,
S. pneumoniae)
Microbial antagonism
* block attachment sites
* consume nutrients
* produce inhibitors
Evasion: Lactobacilli → lactic acid pH5 → limits mucosal (vaginal)
colonisation of other microorganisms
Secretory Immunoglobulins
- Extracellular Defences/pathogens
Complement activation - alternative pathway
Acute phase proteins (opsonins)
Phagocytosis
Antibodies & complement activation (classical)
Complement (C’) activation
alternative pathway
C3 undergoes continuous low level
cleavage (hydrolysis) in serum
C3a + C3b
Binds to activator surface:
LPS on Gram– bacteria, some Gram+,
zymosan on yeast, trypanosomes, some
viruses and virus infected cells
+ factors B, D, properdin
C3 convertase C3bBb
Microorganism surfaces differ from host in lacking sialic acid &
complement control proteins (factors H & I) so more C3 is cleaved
C3bBb3b = C5 convertase
C5 is cleaved and binds to
surface attracting C6-C9
C5b6789 = membrane attack
complex mainly lyses Gram –ve
Other by-products
C5a, C3a mediate acute inflammation,
vasodilation, permeability, oedema,
neutrophil chemotaxis
Microbial Evasion of Complement
Anti-complement surfaces
Cell wall thick outer surface of Gram +ve bacteria resist C’-mediated lysis
Distant surface Antigens
long chains activate C5b-9 at a
distance eg Salmonella enteritides
Presence of sialic acid
eg Neisseria
Capsules prevent C’ access by
phagocytes e.g. capsular
polysaccharide Type 1 (mucoid
type) of Staph aureus
Factor H-binding proteins
Factor H = complement regulator, inhibits complement activation on host
cells by binding to deposited C3b and C3bBb on self-cells accelerating
destruction of C3b and C3bBb limiting C5 activation and MAC formation
eg: Strep pyogenes: M-proteins and N. meningitides: GNA1870
which bind factor H molecules…which then bind any deposited
C3b etc inducing their destruction…
Viral homologues of complement regulatory proteins (H, I)
Limit complement-mediated neutralisation
eg: herpes, poxviruses, vaccinia, Herpes simplex virus
Secreted anti-complement proteins
* Complement degrading enzymes
eg: Pseudomonas & Strep. (group A)
* Inhibitory factors eg: Staphylococcus
* Decoy proteins eg: Staphylococcus & Pseudomonas
Acute Phase Proteins
- Secreted by hepatocytes during first 24-48 hours of inflammatory responses
(the ‘acute’ phase) - induced by IL-6, TNF, IL-1 (from activated macrophages)
- Act as opsonins + ?
C-Reactive Protein - binds phosphorylcholine in teichoic acids of Strep pneumoniae,
Leishmania donovani, Neisseria meningitides - → partial complement activation → C3b opsonisation for
extracellular pathogens – intracellular pathogens
use this as route to infect phagocytes esp. Ms
Mannose Binding Protein (lectin) - binds terminal mannose
→ direct opsonisation
→ Lectin complement pathway activation
Phagocytosis
Neutrophils in Innate Immune Responses
Migration to sites of infection or injury (chemotaxis: C5a,
IL-8, LTB4): tethering, rolling on endothelium, adhesion,
extravasation, diapedesis
* Phagocytosis: recognition, uptake & digestion of microbes
in phagolysosomes
* Exocytosis: of granule proteins by formation of NETs
(neutrophil extracellular traps): a fibril network of granule
proteins, DNA
* Produce proinflammatory cytokines to recruit/activate
DCs, macrophages, T cells
* Support wound healing (angiogenesis, fibroblasts) →
phagocytosis by macrophages to resolve inflammation
Killing in the Phagolysosome
Non-oxidative
pH ↑ then ↓ - limits some viral growth, enhances host Enzymes
defensins - circular, cationic, pore-forming
lysozyme - degrades peptidoglycan
acid hydrolases - e.g. lipases, cathepsins
Lactoferrin - competes with bacteria for Fe2+
NRAMP1 - integral membrane transporter, extrudes Fe, Zn and Mn ions
Oxidative also called the ‘respiratory burst’, free radical formation
ROS reactive oxygen species, activity of NADPH oxidase and myeloperoxidase
O2 → O2- superoxide
(NADPH → NADP+)
O2 - → H2O2 If add superoxide dismutase
H2O2 → OCl- If add myeloperoxidase
RNS reactive nitrogen species, activity of inducible nitric oxide synthases (iNOS)
O2 + arginine → NO
peroxynitrite ONOO- etc
Evasion of Phagocytosis
Exotoxins - inhibit chemotaxis, cytolytic
(Staph aureus, Strep pyogenes)
Capsule - hydrophilic polysaccharide
(Strep pneumoniae, H. influenzae)
Surface slime - hide beneath and form biofilms - communication
(Pseudomonas aeruginosa)
M-protein - fimbriae/attachment pili (S. pyogenes)
Fibrin layer and abscess development
- Staphylococcal coagulase
Flagellae - motility (Trichomonas vaginalis)
Induced entry into non-phagocytes
- Salmonella → intestine epithelial cells
- Shigella → M cells in intestine epithelia
Blocking of phagolysosomal killing
Inhibition of phagolysosome fusion/maturation
Inhibition of phagolysosomal killing
Escaping the phagosome
Antibodies
IgG, IgA, IgM, IgD, IgE
Variable H/L domains (Fab)
Bind Antigen
* neutralise exotoxins (IgG, sIgA)
* block microbial adhesins
* block nutrient transport
* immobilise bacterial flagellae
* agglutinate (IgM)
Constant H domains (Fc)
Effector roles for Ag disposal
* complement binding (classical)
* cell attachment via FcR – phagocytes, NK-cells (ADCC)
Antibody opsonisation – enhances phagocytosis
Ag + IgG FcR on neutrophil, macrophage
Antibody activation of complement by classical pathway
Ag + Ab (IgM, IgG1,3,2) + C1qrs
+ C4, C2 → C4b2a= C3 convertase …..more C3 cleaved
forming C5 convertase and C6-9 and lysis of target
Mediator release → acute inflammatory response
Pre-formed mediators: histamine, serotonin, tryptans, heparin
Lipids: prostaglandins, leukotrienes, PAF
Cytokines: IL-4, IL-5, IL-13, TNF, GM-CSF, IL-8 etc
Antibody-dependent cell-mediated cytotoxicity (ADCC)
Antigen on virally-infected cell, fungal hyphae, worm larvae bound to IgG1
or IgG3 then binds Fc receptor on phagocytes, NK cells
Antigen on worm larvae binds IgG or IgE these bind to FcR or FcR on
eosinophils which secrete major basic protein & eosinophil cationic protein
Avoiding Antibody
Production of ineffective Ab
Ab consumed by soluble microbial Ag
Adsorption of host molecules
Inaccessible
Production of decoy receptors
Antigenic variation
Antigenic drift – rapid continuous point mutations of surface proteins
accumulated slowly over time
e.g. HIV and Influenza
Antigenic shift – sporadic gene re-assortment due to mixing of strains
of viruses = rapid change
Human influenza H,N suddenly changes by many amino acids gene
exchange with e.g. bird H,N leading to no immunity in humans →
pandemics
Coronaviruses are NOT prone to this due to their large size
and low mutation and recombination rates, but spread from
zoonotic reservoirs and large number of cases has caused
pandemics
Gene switching → vary microbial surface Ag: Trypanosoma
brucei (African sleeping sickness) ~1000 copies Variant Surface
Glycoprotein (VSG) gene
New immune
responses required
for each new Ag
- Intra-cellular defences/pathogens
CD8+ Cytotoxic T Lymphocytes (CTL)
Natural Killer cells (NK)
Type I Interferons (IFN )
Cytotoxic T-Lymphocytes (CTL)
Target infected cells to stop microbial replication in cytosol:
Viruses: EBV (B cell), HIV- (CD4+ Th), hepatitis B (hepatocytes) +++
Bacteria: In macrophages, escaping phagolysosome to cytosol:
mycobacteria, listeria ++
* Initially activated from CTL (CD8+) precursors in lymphoid tissue via
MHC I/peptide Ag complex on dendritic cells + co-stimulation and IL-2
from Th1 cells – clonally replicate and leave lymphoid tissues to find
infected targets (peaks at ~9 days from 1st infection)
* CD8+T cell TCR: MHC I/peptide Ag on target cells produces: cytokines IFN,
TNF, perforin and granulysin → cytoplasmic bacterial lysis (via necrosis
and apoptosis) + FasL:Fas activation of apoptosis
CTL killing mechanisms
CTLs produce and release toxic
proteins perforins and
granzymes initiating lysis and
death via necrosis and apoptosis
Also kill via FasL on CTL
interacting with Fas on target
cell to induce apoptosis
Evasion of Virus-Infected Cell Killing by CTLs
Block cytokines for CTL production
Block Ag processing and presentation on MHC I
Natural Killer Cells
Large granular lymphocytes, 5-10% of circulating lymphocytes,
rapidly induced, (group 1 of the innate lymphoid cells), kill targets
in several ways:
* Dual receptor recognition of targets: particularly virallyinfected or tumourous cells
inhibitory (killer inhibitory receptors KIRs) - ligand: MHC I
activatory (killer activatory receptors KARs) - ligand: MICA/B,
UBLPs (‘MHCI-like molecules’ on ‘stressed’ cells)
granzyme, perforin released causing necrosis and apoptosis
* ADCC as NK FcRs bind IgG Abs → surface Ag on target cells
triggering release of perforin and granzyme
* Fas:FasL FasL on NK → Fas (target cell) leading to apoptosis
Interferons (Type I)
induced by microbial products - viral dsRNA, DNA
* are released by many infected cell types
* induce ‘anti-viral state’ in infected cell via upregulation of ‘interferon
stimulated genes’ that aid degradation of viral mRNA, inhibition of mRNA and
translation and affect enzymes for viral entry, assembly, budding and release
and increase MHC I expression for presentation to CD8+CTLs
* Induce changes in adjacent uninfected cells:
Signals neighbouring
cells to destroy RNA and
reduce protein synthesis
Signals neighbouring
cells to undergo
apoptosis and to express
NK cell receptors (KARs)
Activates nearby
immune cells (such as
NKs)
Evasion of Type I Interferons
Inhibition of IFN-induced enzymes by production of
low mw RNA – e.g. EBV, adenovirus, HIV
Synthesis of viral proteins – e.g. vaccinia E3L/K3L
→ Protein Kinase regulated by RNA
Production of soluble IFN-receptor homologues
e.g. vaccinia B18R
Interference with IFN signalling pathways
e.g. adenovirus AdE1A protein
Adaptive Immunity REVISION
The innate immune system provides a rapid but non-specific response, while the adaptive immune system offers a slower but highly specific and long-lasting defence!
The adaptive immune system is comprised of two major lymphocyte populations B cells and T cells,
and T cells can be further divided into CD4 positive help ourselves and CD8 positive, cytotoxic or killer T cells.
The adaptive immune response comprises both humoral and cell mediated components.
Whilst the innate immune system is rapidly activated, T and B cell responses,
particularly when exposed to pathogen for the first time, are slow to develop.
Both the innate and adaptive immune systems rely on recognition molecules.
You will have heard from Falco how the cells of the innate immune system utilise a relatively small number of pattern recognition receptors to bind to cancer pathogen and damage associated molecular patterns.
Molecules such as LPs and double stranded RNA.
In contrast, each lymphocyte B at a T-cell or a B cell expresses a unique antigen receptor on their surface,
which is highly specific to an individual antigen molecule.
Adaptive immunity is often described in terms of humoral and cell mediated immunity.
When activated, B cells secrete antibodies, which is the soluble form of the antigen receptor.
Antibodies then bind to antigens on the pathogen leading to its neutralisation or destruction by other cells of the immune system.
Healthy T-cells produce cytokines which aid the induction or activation of other immune responses,
including both antibody mediated and cell mediated responses.
Which involve the T cell receptor mediated recognition and killing of infected or transformed cells.
There is a huge repertoire of lymphocytes, each with distinct receptors patrolling the body.
Lymphocytes circulate through secondary lymphoid organs, looking for the specific antigen.
Once found and presented in the correct manner, this antigen specific cell will become activated and clonal expand to enable it to mediate its function more effectively.
Once the pathogen has been eliminated from the body,
there is no longer requirement for all these effector cells and most will die by apoptosis and only those that have differentiated two memory cells will remain.
Dendritic cells or DCS play a key role in activation and differentiation of
CD4 helper T cells through both cell contact and cytokine mediated signals.
These helper cells then activate other cells, including CD8 T cells, B cells,
macrophages and natural killer cells that all function to tackle the infection. Antibodies secreted by activated B cells also interact with other components of the innate immune system, such as enhancing macrophage phagocytosis, a process known as optimisation.
Memory is the hallmark of adaptive immunity.
Memory means that a secondary response is quicker, stronger and often of a greater quality compared to the primary response.
This means that the immune system can often control and eliminate a second infection with the same pathogen before clinical signs or transmission can occur.
And this occurs because the secondary response is primarily mediated by memory and not naive lymphocytes.
Memory is NOT a major feature of the innate immune system!
And of course, immunological memory is the basis for immunity provided by vaccination.
ANTIGEN SPECIFIC RECEPTORS
The highly specific antigen receptor is expressed on B cells, and T cells share similarities in terms of how their diversity is generated and in their basic structural features,
but they differ fundamentally in the way they recognise antigen.
The B cells receptor that sits on its outer surface recognises antigen in its natural or native state.
The antigen may be protein, lipid carbohydrate or even nucleic acid.
Once activated, the B cell expresses and secretes the receptor in a soluble form, which is known as antibody.
In contrast, most T cells recognise a short peptide antigen only after the antigen has been processed and presented in combination with a major histocompatibility or MHC molecule on the surface of another cell.
Killer T-cells recognise peptide antigens presented to them by MHC class one molecules, while helper t cells only recognise peptide antigens presented by MHC class two molecules.
This complicated arrangement assures that T cells act only on precise targets and at close range!
The B cell receptor is the membrane bound form.
An antibody is the secreted form of a complex protein known as immunoglobulin.
IMMUNOGLOBULIN STRUCTURE
The two paired arms of the immunoglobulin molecule are known as the antigen binding fragments or fabs, and they possess two identical antigen binding sites.
The paired constant regions of the two heavy chains are known as the Constant or FC fragment.
This bestows the effector activity of the immunoglobulin molecule when it is in the secreted antibody form.
The very short cytoplasmic tail on the B-cell receptors means that it does not itself transmit signals to the B cell upon engagement with antigen.
Instead, it forms a complex with other transmembrane proteins.
Confusingly known as Immunoglobulin Alpha and beta.
And it is these proteins that trigger signalling cascades through immuno receptor tyrosine kinase activator motifs or items on their cytosolic tails.
T Cell receptors are not immunoglobulins but they do belong to the immunoglobulin protein superfamily and share key structural features.
The T cell receptor is a hetero dimer and each chain contains a constant and a variable domain, just like immunoglobulin.
Hyper variable cedars are found on the variable regions of each chain, and this thus bestows the unique ability to bind a peptide presented by an MHC molecule,
but where immunoglobulin has two identical antigen binding sites.
The T cell receptor has one!
A subset of T cells carry a receptor composed of gamma and delta chains instead.
These cells recognise antigens in a different manner than alpha and beta T cells, they recognise PAMPS and DAMPS and thus appear to act as innate lymphocytes.
Gamma delta t cells are abundant in birds, especially in younger animals.
In humans, gamma delta T cells are abundant during foetal development,
but after birth they are found primarily at epithelial surfaces where they contribute to front line defences.
GENERATING DIVERSITY OF ANITGEN SPECIFIC RECEPTORS
When looking in germline DNA. It was found that there are multiple gene segments encoding different parts of the immunoglobulin gene, and during development, the B cell uses recombination to combine these different parts to create a unique gene.
Recombination of different gene segments is a major mechanisms that generates diversity.
T AND B CELL DEVELOPMENT
Very early T-cell development occurs in the bone marrow and progenitor cells, then migrate to the thymus for further development.
Within the thymus, they are known as thymocytes , and they undergo a variety of different stages.
Double negative thymocytes do not express CD4 or create the undergo beta selection, which involves recombination of T cell receptor gene segments.
Cells that survive beta selection then express both CD4 and CD8 and a describe as double positive.
These cells undergo both positive and negative selection to become single, positive or SP cells.
Since they now only express CD4 or CD8, there is then a final negative selection screening to remove unreactive cells.
Non-Self reactive lineage committed.
Naive t cells are then released from the thymus into the bloodstream, where they will survey secondary lymphoid organs looking for their specific antigen.
Thymocytes can express either an alpha beta or a gamma delta T-cell receptor, but never both!
Double positive thymus sites now expressing an alpha beta tcr make up 80% of cells and the thymus,
and they undergo positive and negative selection in the cortex of the thymus.
Here they interact with cortical thymic epithelial cells bearing both MHC Class one and Class two molecules.
Positive selection selects thymocytes bearing receptors capable of binding MHC molecules, and this results in what we call MHC restriction. About 95% of cells fail positive selection and so fail to receive the needed survival signals, leading to death by neglect.
If the T cell receptor can bind to MHC Class two, it also binds with the CD4 molecule selecting the cell to become CD4 positive.
The opposite happens if the T cell receptor binds to an MHC class one molecule resulting in the cell becoming CD8 positive.
In addition to becoming committed to either the CD4 or the CD8 t cell lineage, double positive thymus sites may commit to becoming other types of T cells.
Natural killer t cells express a T cell receptor with an invariant alpha chain. These receptors bind to CD1 molecules presenting lipid antigens.
A further stage of negative selection also referred to is central tolerance of lineage.
Committed thymus sites occurs in the middle of the thymus.
Again, cells interact with epithelial cells, but also also with dendritic cells, and they are trigger to undergo apoptosis if they bind too strongly.
This is an important step in ensuring self tolerance.
How does screening against tissue antigens take place?
Well, it appears that this is facilitated by the expression of the autoimmune regulator protein or AIRE, this is
expressed by thymic epithelial cells and induces the expression of a huge variety of different proteins.
Whilst highly efficient, central tolerance is not perfect and other mechanisms outside of the thymus exist that help maintain self tolerance.
Regulatory T cells (Treg) play a central role in negatively regulating immune responses, including alter reactive t cell responses through a variety of mechanisms > deplete the local area of stimulating cytokines, producing inhibiting cytokines, inhibit APC activity and directly kill T cells.
For B-cells, there is no requirement for MHC restriction.
Therefore there is no positive selection and only negative selection occurs.
Immature B cells expressing IgM and Igd on their surface are then released from the bone marrow and undergo final steps of maturation in the spleen.
TO SUMMARISE
Both B and T cells express randomly generated receptors highly specific to individual antigen molecules through somatic gene rearrangement.
An almost infinite array of specific receptors is generated from a finite number of genes, both B and T.
Cell development pathways share many characteristics, such as the rearrangement of gene segments and negative screening processes to avoid self reactivity.
B cell and T cell development also have differences such as the location of the development and the screening processes used, such as positive selection of T cells but not B cells.
Understanding these development pathways gives us insights into how things should normally proceed and how abnormal abnormalities can lead to disease states such as autoimmunity.
T CELL RESPONSE
Antigen recognition by a naive T-cell takes place in secondary lymphoid organs following presentation of a process peptide antigen by MHC Class one or Class two molecules on the surface of mature professional antigen presenting cells.
This leads to the activation of the antigen specific cell and its clonal expansion.
This is driven by the autocrine and paracrine actions of interleukin two or IL2, the major t cell growth factor which binds to the IL2 receptor on the surface of the activated T cell.
Then engagement of the cytokine derived from the APC or other cellular sources drives the differentiation of the activated t cell,
which influences the effector functions of that cell.
During proliferation, a proportion of cells differentiate to become memory rather than effector cells.
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Both memory and effector cells leave the lymphoid organ and migrate to the infected tissue where they
perform effector functions which could include activating of macrophages or killing of virus infected cells.
Once the antigen the pathogen has been cleared, the effector cells will die by apoptosis and the memory cells will remain and may preferentially recirculate back to this tissue to survey for re encounter with the pathogen.
ANERGY
Engagement of positive core stimulatory receptors is essential to facilitate activation of the naive T cell.
A lack, of course, stimulatory signal results in the induction of ‘Anergy’ which is a programmed state of non responsiveness.
This is one of the peripheral mechanisms by which the body protects itself against auto reactive t cells which have not been removed in the thymus.
APCs
Dendritic cells, macrophages and B cells are all considered antigen presenting cells.
In all cases, when activated, these cells increase expression of MHC Class two and CD8 and CD86, essential for activation of T cells.
T helper 1 Cells (Th1)
Th1 cell differentiation is induced by IL12, IL18 and IFN-y.
th1 cells are associated with driving cell mediated immune responses, particularly against intracellular pathogens, as they support the differentiation and activation of antiviral CD8 killer t cells and they activate macrophages.
They also drive inflammation by promoting the class switching of B cells to produce IgG classes that support phagocytosis and complement fixation.
T helper 2 cells (Th2)
Th2 differentiation is promoted by IL4.
This triggers the master regulator GATA3 to then induce expression of IL4, IL5 and IL13 by the Th2 cell.
These cytokines act cooperatively to promote immune responses against extracellular pathogens and helminth parasites.
IL4 acts to promote the activities of eosinophils and induces class switching to IgE, which helps other immune cell types to release anti parasite inflammatory mediators.
T CELL MEMORY
Memory T cells can be broadly divided into either central or effector memory.
Central memory T-cells reside in or travel between secondary lymphoid tissues.
They are long lived, are rapidly activated by re-exposure to antigen and can differentiate into different subsets depending on the cytokine environment.
In contrast, effector memory t cells travel to and reside in tertiary tissues and so contribute better to front line defences.
They rapidly reacquire their effector function upon second exposure to antigen.
Two different types of B-cell response can be elicited by distinct antigen types.
The majority of B-cell responses are described as T dependent responses and they require help from t cells.T dependent b cell responses are typically generated upon recognition of protein antigens.
T independent responses do NOT require T cell help and they are typically generated on exposure to multivalent or polymerised antigen.
Often these are carbohydrate structures such as the capsules on the surface of bacteria.
Activation of the B-cell in this case occurs through the crosslinking of large numbers of B cell receptors and engagement of pattern recognition receptors or complement receptors on the surface of the B cell.
T DEPENDANT B CELL RESPONSE
Antigen recognition by this B-cell receptor stimulates activation and proliferation of the B cell.
Once activated, B cells move into extra follicular spaces and form primary follicles where they differentiate into antibody secreting cells, which are known as Plasma cells. And cells also can undergo proliferation in the follicles to form germinal centres.
Their antigen is internalised, processed and presented on the B cell surface by MHC molecules.
They then interact with antigen specific helper T cells, which provide conditions to support the differentiation of B cells and the production of memory cells.
As part of the B-cell differentiation programme that ensues within the germinal centres, TWO KEY processes occur.
The first is Somatic Hyper Mutation.
This is where individual point mutations occur in both the heavy and light chain genes of the immune globulin.
Mutations increase over time and with repeated exposure to the antigen.
This is followed by affinity selection that results in increased affinity for the antigen over time.
B Cells that can bind process and present antigen better to T cells receive greater cytokine assistance and so higher affinity B cells may even steal antigen!
The second process is known as immunoglobulin Class Switch recombination.
If you recall from earlier, I mentioned that there are different heavy chain and constant gene segments and the functional class of the antibody depends on the C-H gene segment.
Usually, naive B cells Express, IgG and IgM on their surface and will typically first secrete antibody as IgM, but exposure of b cells in the germinal
centre to polarising cytokines typically derive from helper t cells drives a class switch recombination event, which is the utilisation of an alternative C-H gene segment.
For example, exposure of responding B cells to IL4 will drive class switching to IgG1 to IgE.
Germinal Centre B cells complete their maturation as class switched antibody secreting plasma cells or as memory cells.
Memory B cells provide a rapid and strong response to secondary infection.
T AND B CELL EFFECTOR RESPONSES - ANTIBODIES
B CELLS
This is mediated through the antibodies they secrete as plasma cells.
Antibodies mediate the clearance and destruction of pathogen in a variety of ways.
They can act directly by neutralising viruses and other intracellular pathogens and also neutralise toxins preventing them from binding to host receptors.
Or they can work cooperatively with other cells and components of the immune system.
They can bind pathogens and enhance phagocytosis by macrophages and neutrophils, a process known as optimisation.
They can activate the complement cascade via the classical pathway leading to the formation of the membrane attack complex,
which kills pathogens or in this example, destroys a transformed cell, and they can mediate antibody mediated cellular cytotoxicity or ADCC, which results in natural killer cell mediated destruction of infected or tumour cells, or in the cytotoxic granulation of mast cells, basophils and eosinophils, which causes damage to the surface of multicellular pathogens.
The ability of the antibody molecule to mediate one or more of these effector functions often defends depends on its class, also known as Isotope.
And in mammals, Antibodies can be one of five different major classes, each differentiated by heavy chain constant domain usage, and each class presents a degree of functional specialisation.
There are three monomeric forms of IgG, IgD, an IgE and two multimeric forms.
IgA exists as a dimer held together by the j chain and IgM is secreted as a covalently linked pentameter with the inclusion of the j chain.
Antibodies mediate many of their functions through engaging with FC receptors on the surface of immune cells.
FC Gamma receptors bind to IgG. hey are the most diverse group of FC receptors and are the main mediators of antibody functions in the body.
Most are activating receptors and will enhance optimisation by macrophages and mediates activity of natural killer cells.
FC Alpha receptor binds to IgA.
It has a much more restricted expression to cells of the myeloid lineage.
It can contribute to pathogen destruction again by triggering ADCC and optimisation.
FC Epsilon receptors bind to IgG.
The best study is the high affinity FC Epsilon receptor one expressed by mast cells, basophils and eosinophils and which mediates type one or immediate hypersensitivity reactions.
The polymeric immunoglobulin receptor is expressed by epithelial cells and is involved in
the transport of IgA and IgM into the lumen of the GI respiratory and reproductive tracts, as well as transporting antibodies into tears and milk.
And finally, the neonatal FC receptor expressed on many different cell types during development.
But later the expression is restricted to epithelial and endothelial cells.
The neonatal FC receptor is involved in transporting antibodies ingested in milk across the epithelial barrier in the gut and into the bloodstream.
The neonatal FC receptor helps recycle IgG from tissues back into the bloodstream.
CELL MEDIATED EFFECTOR RESPONSES - responsible for the killing of infected host cells
These are the three major types of cytotoxic lymphocyte.
Firstly, we have the cytotoxic T lymphocyte, which is a CD8 expressing MHC class one restricted T cell.
Next we have the natural killer T cell, which expresses a limited repertoire of unconventional T cell receptors that recognise lipid antigens presented by molecules of the CD1 family
And finally, natural killer cells. These are innate lymphocytes that recognise infected and transformed cells, either through engagement with antibody or through the recognition of MHC Class one and related stress induced molecules.
All of these cytotoxic lymphocytes express effector cytokines.
They also express cytotoxic granules that contain toxins called Perforins and Granzymes.
And they also express the death ligand, Fas-L on their surface.
Granule exocytosis and Fas ligand engagement are the two mechanisms by which these cells mediate their killing effects.
How CD8+ kills cells
Cross presentation allows dendritic cells to acquire antigens from the extracellular environment and to present them to CD8 T cells.
Once activated and a cd8 t cell recognises its MHC class one peptide complex on the surface of an infected cell.
The CD8 t cell will rearranges cytoplasm to allow the directional release of cytotoxic granule contents.
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The granule contacts act directly on the target’s cell membrane.
The perforin proteins form a pore in the target cell membrane that enable grandes aims to enter the target cell where they trigger apoptosis.
Alternatively, the expression of Fas ligand on the CD8 t cell surface can also induce apoptosis if it engages with the Fas death receptor on the target cell surface.
Adaptive Immunity - Lecture
Only birds and mammals have germinal centre formation.
Lymphoid tissues in humans are the bone marrow and thymus > sites of
B cells mature with different organs > birds vs rodents vs ruminants and pigs.
T cells mature in the thymus > ALL species!
Gut associated lymphoid tissues. (GALT)
In a chicken, can easily remove the thymus and Bursa of Fabricius and can see what the consequences are. Clear evidence that functions of thymus and Bursa in chickens. If you remove T cells, antibody response decreases as helper T cells help to produce B cells which produce antibody.
Porcine lymphocyte tracking.
ANTIGEN RECEPTORS
Each receptor specific for a particular epitope. When a T cell develops, you get a recombination of different T cell receptor segments = diversity.
TCRA = T cell receptor alpha.
TCRB = T cell receptor beta.
Birds only have 1 V region segment to choose from. With limited V gene diversity, species use gene conversion to create new ‘V’ genes.
Camelid Antibodies. Only have single chain antibodies. They are exploited as therapeutic molecules.
Avian antibodies > IgY. Has an extra domain / constant region and lacks a hinge region. Lacks the Fc region, unclear why they have evolved this as Fc region is important for complement to bind such as phagocytes or macrophages which can then engulf a foreign body / cell.
Gamma and delta T cells
Lack the CD4 or CD8 co receptors as they do not recognise peptide antigen.
Arise from a common progenitor cell in the thymus.
Provide help for activation of other leukocytes and immunoregulation.
Research in pigs suggests that they might be important in observed protection.
Cattle and pigs.
Functions in cows > anti microbial effects and immune activation. A major T cell regulatory population.
CD4 = MHC class 2 = helper T cells
CD8 = MHC class 1 = cytotoxic T cells > use perforin or Fas-L death receptor pathway to kill infected cells
Innate immune system > danger > activates PAMPs (pathogen associated and DAMPs such as toll like receptors.
Vaccines
Smallpox vaccine first ever vaccine > led to mass vaccination programs that led to control and eradication of Smallpox > 1st disease ever to be eradicated
The need for new / improved vaccines:
Global deaths are caused by vaccine-preventable diseases and diseases where effective vaccines are not yet available such as Malaria, HIV/AIDS, TB.
There are many more licensed veterinary vaccines but important gaps also remain: african swine fever and mastitis.
Licensed vaccines > typical pathogen features and vaccine approach:
Polio
MMR
Tetanus
Diptheria
Influenza
Pathogen features
Antigens do not change a lot
Immunity requires antibodies
Vaccine approach
Conventional vaccinology
Killed
Live attenuated
Subunit
Recombinant
No effective vaccines available, diseases > typical pathogen features and vaccine approach:
HIV
TB
Malaria
Staphylococcus
Chlamydia
Gonorrhoea
Parasite diseases
Pathogen features
High antigenic variability
And/or T cell-dependent
immunity more significant
Vaccine approach
Evolving vaccinology
Reverse vaccinology
Novel viral vectors
Reverse engineering/
structural vaccinology
Inactivated/killed vaccines
Inactivating or killing a pathogen renders it safe
Heat > not commonly used as it could damage the antigens, will denature the proteins and change their conformation, could generate an Ab response that does not recognise the antigen.
Chemical - commonly fixative agents that cross-link proteins e.g. formaldehyde
Radiation – effective for viruses, less so for larger/more complex pathogens (attenuation)
Inactivation must preserve the integrity of the antigen
Particularly important for conformational antigens recognised by antibodies
Heat may denature proteins
Relevant antigens must be expressed/present in the inactivated formulation
Parasite stage specific antigens may be absent
Presence of non-structural viral proteins may be absent/limited > non structural proteins can sometimes be the targets for T cell responses
May not mimic the infection process by the live pathogen
Stimulation of the innate immune response may be limited > important when you vaccinate as the innate system is required to activate the adaptive immune system for production of B cells and antibodies
Less immunogenic/efficacious than live vaccines – poorer stimulation of CD8 T cell responses
Requirement for adjuvants, multiple immunisations
Compared to live preparations, inactivated vaccines are inherently more stable
May be freeze dried, removing/reducing need for a cold chain
Examples of inactivated veterinary vaccines
Viruses
Influenza - pigs and poultry
Bovine viral diarrhoea [BVD] virus – cattle
Bovine Herpes virus-1 – cattle
Foot-and-mouth disease virus – ruminants and pigs
Bacteria
Actinobacillus [Haemophilus] pleuropneumoniae - pigs
Mycoplasma agalactiae – sheep and goats
Pasteurella multocida - cattle
Parasites
Giardia duodenalis - dogs
Sarcocystis neurona – horses
Neospora caninum - cattle
Examples of inactivated human vaccines
Viruses
Polio
Influenza
Hepatitis A
Japanese encephalitis virus
Bacteria
Cholera (Vibrio cholerae)
Parasites
- None
Live attenuated vaccines
Attenuation
Radiation – effective for complex pathogens e.g. parasites
Repeated passage of pathogen in vitro in cell culture or in a different host species > more common nowadays, ethically better and less expensive
Genetic engineering; removal of genes that encode virulence factors, more straightforward for simple viruses than for more complex pathogens
Live attenuated vaccines most resemble infection with the pathogen
- Most immunogenic/efficacious vaccines
Stimulates the relevant innate immune responses (PRRs)
Stimulate both antibody and T cell mediated immunity
Often there is no requirement for an adjuvant as they stimulate PRRs on their own
Often a single shot can be effective and immunity may be life long
For some live attenuated vaccines there are safety concerns
Attenuated pathogens may still be pathogenic in immunocompromised animals e.g. BVDV vaccines are not safe to use in pregnant animals
There may be reversion to virulence either through the attenuated pathogen mutating back to a virulent form or through genetic recombination with wild-type pathogens > particularly in RNA vaccines
Live attenuated vaccines are less stable and may require a cold-chain to remain potent
Examples of live attenuated veterinary vaccines
Viruses
Classical swine fever virus - pigs
Bovine viral diarrhoea [BVD] virus – cattle (not licensed in UK)
Porcine reproductive and respiratory syndrome [PRRS] virus - pigs
Bacteria
Lawsonia intracellularis - pigs
Streptococcus equi – horses (virulence factor gene deleted vaccine)
Brucella abortus - cattle
Parasites
Eimeria spp. - poultry
Theileria annulata – cattle
Toxoplasma gondii – sheep
Dictyocaulus viviparous - cattle
Examples of live attenuated human vaccines
Viruses
Polio
Influenza
Measles-Mumps-Rubella (MMR)
Smallpox
Rotavirus
Bacteria
Cholera (Vibrio cholerae)
Tuberculosis (BCG)
Parasites
- None
Subunit vaccines
Pathogens may not grow in the lab and therefore difficult to produce inactivated or live attenuated vaccines
Subunit vaccines are composed of the protective antigens; which may be determined empirically or more rationally through the understanding of the protective immune mechanism and identification of the antigens that they target e.g. antibody versus T cell responses or both
Early subunit vaccines were composed of pathogen components from the organism (native antigens) but currently the advent of recombinant DNA technology allowed antigens to be produced in the lab without the requirement for pathogen propagation
- Protective antigen must be defined
- Depending on the antigen the expression of the recombinant form may or may not be readily up-scaled
- Depending on the expression system used the antigen may not be expressed in a state resembling the native antigen > e.g. enveloped viruses are glycosylated and antigen must also resemble this
- Recombinant proteins are poorly immunogenic (absence of PAMPs) therefore there is a requirement for administration with an adjuvant
No safety concerns but cost and stability (cold chain etc.) may be antigen dependant
Examples of protein subunit veterinary vaccines
Viruses
Classical swine fever virus E2 protein (baculovirus/insect cell) - pigs
Newcastle disease virus HN protein (plant cell) – poultry
Porcine circovirus-2 ORF2 protein (baculovirus/insect cell) -pigs
Bacteria
Actinobacillus pleuropneumoniae recombinant or extracted ApxI, ApxII, ApxIII, and outer membrane proteins- pigs
Parasites
Leishmania donovani native fuccose-mannose-ligand antigen complex – dogs
Babesia canis in vitro cultured supernatant antigens – dogs
Eimeria maxima gamete antigens – poultry
Boophilus microplus recombinant tick gut membrane protein Bm86 - cattle
Examples of subunit human vaccines
Viruses
Hepatitis B (HBsAg expressed in yeast)
Bacteria
Anthrax (protective antigen collected from culture filtrates)
Meningococci (native and recombinant antigens)
Pertussis (whooping cough) (purified native antigens)
Parasites
- Malaria (RTS,S) (recombinant protein)
Adjuvants
An adjuvant (from Latin, adiuvare: to aid) is a pharmacological or immunological agent that enhances the immune response to an antigen, while keeping the injected foreign material to a minimum.
Exert their effects through different mechanisms. Some adjuvants, such as alum and emulsions (e.g. MF59®), generate depots that allow slow release of antigen in order to continue the stimulation of the immune system.
They also increase recruitment and activation of antigen presenting cells.
Particulate adjuvants (e.g. alum) have the capability to bind antigens to form multi-molecular aggregates which will encourage antigen presenting cell, APC uptake
Pathogen-associated molecular patterns > can be used as molecular adjuvants
More recently, adjuvants have been developed that incorporate defined PAMPs that may help alter the balance or direction of the immune response e.g. direct it towards a Th1 biased cell-mediated rather than a Th2 biased antibody response.
Examples of adjuvants used in veterinary vaccines
Water/Oil emulsions
Seppic adjuvants based on emulsion, nano-emulsion or polymer technology
Organic
Plant derived saponins or purified QuilA
Squalene from plant oils combined with surfactants
Inorganic
Aluminium salts (Alum)
Pathogen recognition receptor ligands (PAMPs)
CpG deoxynucleotides (TLR9 agonist)
Many more adjuvants are licensed for veterinary use compared to only 5 licensed for human vaccines > side effects and safety concerns, examples below:
- Feline vaccine-associated sarcoma
A vaccine-associated sarcoma (VAS) is a type of malignant tumour found in cats (and rarely, dogs) which has been linked to vaccines (rabies and feline leukaemia virus)
Associated with aluminium salt based adjuvants
VAS appears as a rapidly growing firm mass in and under the skin.
Treatment through surgery, radio- and chemotherapies
VAS limit type and frequency of vaccinations given to cats
Reduced frequency
Removal/replacement of adjuvants
Targeted to high-risk groups
Delivery to sites which could be readily excised e.g. injection to upper legs - amputation. - Adjuvant associated-autoimmune disease
In 2009, increase in the incidence of a bleeding disorder in young calves due to bone marrow damage in Germany which was subsequently reported in countries across Europe
Bovine neonatal pancytopenia (BNP) an alloimmune syndrome associated with vaccine-induced alloreactive antibodies
Associated with a ‘new’ adjuvant used in a commercial BVDV inactivated vaccine
Experimental data suggests that this potent adjuvant drives antibody responses to MHC I molecules present within the cell culture contaminants
If vaccinated dams are served by bulls that express these MHC molecules then their calves are at risk of developing BNP following ingestion of colostrum from its vaccinated mother > leads to destruction of cells in calf
Led to the withdrawal of this vaccine/adjuvant
Recombinant vectored vaccines
Use attenuated pathogens (vectors) to express vaccine antigens
Vectors may be pathogenic in non-target species e.g. canary pox virus or an attenuated strain e.g. modified vaccinia virus Ankara strain (MVA)
Vectors provide similar PAMPs to live attenuated vaccines thus adjuvanting the response to the antigen > we have a good activated immune system
Induction of immunity against the vector itself reduces the effectiveness of booster immunisations or to use the same vector to deliver other vaccines (anti-vector immunity). Concerns over effectiveness.
Examples of recombinant vectored veterinary vaccines
Licensed/commercialised vaccines*
Equine influenza virus (canary pox virus vectored ) - horses
Newcastle disease virus (fowl pox vectored) – poultry
Canine distemper virus (canary pox virus vectored) – dogs/fur animals
Rabies virus (canary pox virus vectored) – cats
Rabies virus (vaccinia virus vectored) -used via oral baits to control rabies in wild carnivores (foxes) in Europe & N America
Porcine circovirus 2 – a chimeric virus based on the avirulent PCV-1 backbone engineered to express the capsid protein of PCV-2
*Note:
Most are for companion animals rather than for animals bred for human consumption.
Pox viruses have the advantage of being able to carry large amount of exogenous genetic
Experimental vectors
Viruses
Adenoviruses
porcine pseudorabies virus
Bovine viral diarrhoea virus
Bacteria
Listeria monocytogenes
Salmonella spp.
Mycobaterium bovis-BCG
Parasites
Trypanosoma spp.
DNA vaccines
Mammalian expression plasmids carrying the gene encoding the vaccine antigen > vaccine made by the cells of the recipient.
Can be delivered by i.m. injection but best results if delivered into the skin using a ‘biolistic’ particle delivery system – enhanced entry into cells or cell damage (DAMPs) acting as an adjuvant?
Additional adjuvant by co-administering cytokine plasmids or incorporation of CpG motifs in the plasmid backbone
Overcomes the safety concerns of live vaccines and no issue of anti-vector immunity
Suffers from poor immunogenicity & efficacy
Three licensed veterinary DNA vaccines for infectious haematopoietic necrosis virus in salmon, influenza in poultry and West Nile virus in horses
None licensed for humans - linked to poor performance & negative public perception!
RNA vaccines
Messenger (m)RNA vaccines:
mRNA nucleosides modified to decrease innate immune activation and increase the mRNA ½ life in the host cell
mRNA encapsulated in lipid nanoparticles (LNP) for efficient delivery of mRNA into host cells
LNP also provide an adjuvant effect
Stimulates both Ab and T cell-mediated immunity
Requires a cold chain
Licensed human COVID-19 vaccines
No licensed veterinary vaccines – cost issue? Too expensive for animal use.
This may change with Self-amplifying (sa)RNA vaccines:
Utilises an alphavirus genome in which the genes encoding the structural proteins are removed and replaced with RNA encoding vaccine antigen > non infectious RNA vector
Requires LNP for delivery
Amplification limits the utility of modified nucleosides, more protein produced but may allow lower doses to be effective
No saRNA vaccine licensed.
Other vaccine approaches
Peptide vaccines
Can be chemically synthesised in the lab and modified
Linear B cell epitopes or T cell epitopes
Require a potent adjuvant
Toxoid vaccines
Inactivated bacterial toxins that may protect against disease rather than the pathogen
e.g. tetanus toxoid – humans, Clostridium Perfringens Type A toxoid – cattle
Conjugate vaccines
While the majority of vaccines are protein based, protection against some bacteria is dependant on raising an antibody response to their polysaccharide coatings. Carbohydrates on their own are weakly immunogenic so conjugates with immunogenic proteins (conjugate) aid the induction of immune response to the sugars
e.g. Haemophilus influenzae type B (Hib) – humans, Actinobacillus pleuropneumoniae in pigs
Allergy vaccines
Companion animals – cats, dogs and horses
“allergen-specific immunotherapy” (ASIT) consists of administering gradually increasing amounts of the allergen extract, either aqueous or precipitated with alum, over a period of several months, followed by yearly boosters.
Protects against atopic dermatitis
Cancer vaccines:
Companion animals – cats, dogs and horses
Canine malignant melanoma vaccines based on tumour cells expressing GM-CSF or a DNA vaccine expressing a tumour antigen
Fertility and production control vaccines:
Induce auto-immune reaction against components of the reproductive hormone cascade
Anti-luteinizing hormone-releasing hormone (LHRH) vaccines to control sexual behaviour
‘Immunoneutering’ - vaccines against gamete antigens being developed > to destroy gametes in possums and render them sterile e.g. to control possum populations in New Zealand
Vaccines targeting the steroid androstenedione increases the frequency of multiple ovulations in sheep.
Reverse vaccinology
Serogroup B Neisseria meningitides (MenB) vaccine development
Exploitation of genome sequence data to predict open-reading frames encoding surface-expressed or secreted proteins
350 proteins expressed & screened in mice for induction of bactericidal antibodies
4 proteins selected for evaluation as a multi-component vaccine
Immunogenic in children inducing antibody titres predicted to be protective in 90% of vaccines
Led to the first licensed MenB vaccine
Reverse vaccinology also applied to vaccines for A Streptococcus, B Streptococcus, Staphylococcus auereus, and Streptococcus pneumoniae
Structural vaccinology - a 3D view for vaccine development
Structural vaccinology combines a genome-based approach with structural biology and immunology - knowledge of the structure and variability of protective epitopes to selectively engineer vaccine antigens.
Create immunogens composed of multiple variants of a given antigen
Engineer antigens to display conserved epitopes not readily exposed/immunogenic e.g. haemagglutinin stem of influenza viruses, conserve the influenza stalk but target with other types of HA heads
The DIVA concept
Differentiating Infected from Vaccinated Animals
or more correctly
Differentiating Infection in Vaccinated Animals
A DIVA or marker vaccine is a vaccine which allows for the differentiation between infected and vaccinated animals or determines whether a vaccinated animal is/has been infected with the pathogen.
Most vaccines on the market are inactivated or attenuated forms of the targeted pathogen, therefore it is often impossible to differentiate by laboratory tests which animals are infected or only vaccinated. Need a way to discriminate.
The DIVA concept cont.
For many animal diseases, the primary goal of vaccination is to prevent or reduce clinical disease associated with the infectious agent, but it can also be used as a means of managing or eradicating a disease from a particular region
Vaccination can reduce clinical symptoms but may not completely protect the animal from infection and shedding of the pathogen (lack of sterilising immunity). In disease outbreak/control situations, it is therefore important to be able to detect active infection in vaccinated animals in order to avoid spread of the disease.
Particularly important for transboundary diseases that impact on trade in animals and animal products that we do not want to spread around the world:
Foot-and-mouth disease
Classical swine fever
Avian influenza
The success of a DIVA strategy enables a move away from solely relying traditional ‘stamping out’ (slaughter) eradication policies
Marker vaccines are available for FMD, Aujeszky’s disease, classical swine fever, avian influenza, infectious bovine rhinotracheitis, bovine herpesvirus-1
Research still focused on improving marker vaccines for the above, but also for other diseases such as: Newcastle disease, peste des petits ruminants, bluetongue, bovine viral diarrhoea, bovine TB.
DIVA Negative marker strategy:
Most common marker vaccines are a form of the virus without a particular gene coding for a non-protective immunogenic antigen (the DIVA antigen) so, do not produce a key antibody response, while still protecting against the disease.
Laboratory tests can test for the presence/absence of antibody specific for the DIVA antigen, confirming whether the animal is infected.
Alternatively, a subunit strategy may be used that only contains the protective antigen and lacks the DIVA antigen
Preferred approach?
DIVA Positive marker strategy:
Introduction of an artificial antibody epitope or ‘tag’ into the vaccine so that vaccinated animals can be identified.
Infected but unvaccinated animals will lack an anti-tag antibody response.
Disadvantage is that this strategy would not detect infection in vaccinated animals.
Development of a DIVA vaccine for classical swine fever > Case Study
Classical swine fever
Small enveloped, ss + sense RNA pestivirus ~12.5kb genome, single polypeptide precursor processed to produce 11 viral proteins
Non-cytopathic virus infects leukocytes and endothelial cells – causes a wide range of clinical symptoms (fever, leukopenia, haemorrhagic skin lesions, death)
A devastating disease that poses one of the greatest risks to the swine industry worldwide
Live attenuated classical swine fever vaccines
Live attenuated Chinese vaccine strain (C-strain) was initially developed by passage through rabbits and later by cell culture
Safe (including pregnant animals) and highly efficacious vaccines
Widely used in CSF endemic countries in Asia to help control the disease
In Europe they have been used successfully for bait vaccination of wild boar which act as virus reservoirs
Provide a remarkable rapid onset of protection (full protection after 5 days and partial protection after 1-3 days), which makes them an attractive tool to control CSF outbreaks
However, the lack of DIVA hampers the serological surveillance performed in pig-producing and -exporting countries that allows them to declare or redeclare freedom from infection > cannot discriminate between infection or not in a vaccinated pig! So wanted to develop of DIVA vaccine to get around this.
Subunit classical swine fever vaccines
Studies on immunity to CSFV revealed that the envelope glycoprotein E2 was the major target of the virus-neutralising antibody response (and T cell response)
Non-neutralising antibody responses are induced against the Erns glycoprotein and the non-structural protein NS3
Two commercial E2 subunit vaccines comprising baculovirus/insect cell expressed (to preserve conformational epitopes) were developed and sold in combination with DIVA companion diagnostic tests, which detected the presence of Erns specific antibodies, which would be absent in E2 vaccinated animals
However, there is a slow onset of immunity (>14 days) and incomplete protection, nowhere near as effective. So these vaccines are only suitable for prophylactic use, ideally with multiple boosters and are not appropriate for emergency use.
Chimeric pestivirus DIVA vaccine > vector approach next tested
CP7_E2alf is a BVDV virus carrying the E2 gene from CSFV
E2 specific antibodies are induced that neutralise CSFV
Erns specific antibodies are BVDV specific and are not detected by the CSFV Erns ELISA thus allow detection of DIVA
Efficacious vaccine – provides rapid and lasting protection and can be delivered orally (to vaccinate wild boar)
Genetically engineered vaccine!
Led to the approval of DIVA vaccine:
Suvaxyn® CSF Marker is the first live CSF marker vaccine that received EU product license in Europe by the centralized authorization procedure.
For the first time, emergency vaccination is feasible that allows deviations from the trade restrictions for vaccinated animals.
Adaptive Immune System - Lecture
Evolution of Immunoglobulins
So we’ve evolved to diversify our antibodies.
And these antibodies each have unique sets of functional properties.
And we’ve been able to create this diversity of antibodies through a process known as a class switching.
So it changes the class of antibody from the default setting which is IGM,
and which you can see is conserved across all these animals to become IgG or IGE or IgA.
Only birds and mammals have what we call germinal centres.
And these are specialised sites that exist within lymphoid tissues where B-cell responses mature.
So this is where class switching occurs. And what’s known as somatic hyper mutation occurs.
So that’s where we select for even antibodies with even higher affinity.
Lymphoid Tissues
There are two different types of lymphoid tissues.
There are primary lymphoid tissues. And in humans the primary lymphoid tissues are the bone marrow but also the thymus. So primary lymphoid tissues are the sites where immune cells are produced.
And they are also sites where B cells and T cells mature.
So in humans the maturation of B cells from the progenitors occurs in the bone marrow and T cells.
Progenitors move from the bone marrow and they mature in the thymus.
Secondary lymphoid organs.
And so these are the sites, the centres where B cells and T cells come to survey for antigens.
So they’re the sites of initiation of adaptive immune responses.
And they’re strategically located around the body to survey for infection in different parts of the body:
Tonsils > B cells circulate looking for infection.
Spleen > surveys for infection in the blood.
Small intestine > Peyer’s Patches > survey for infection in GI tract.
Species differences in lymphoid organs
B cells mature within different organs
Birds – Bursa of Fabricius, within the gut of bird (hence the name B cell)
Rodents & primates – Bone marrow
Ruminants & pigs – Peyer’s patches (intestinal lymphoid tissues)
T cells mature in the thymus
All species mammals & birds (and reptiles, amphibians, fish)
Gut-associated lymphoid tissues
Humans, ruminants, pigs, horses and dogs – Peyer’s patches (PP) 80-90% ileum
In sheep PP are the largest lymphoid tissues ~1% of body weight in lambs but largely disappear in adult sheep
Rabbits & rodents- PP span both jejunum & ileum
Birds GALT = caecal tonsils (caecum - pouch connected to junction of small & large intestines) and bursa of Fabricius
Chickens and Birds
if you removed the thymus from a bird, essentially you had a severe reduction in functions that are associated with T cell responses.
Why do you think animals that lack T cells may have a lower antibody response?
Because T cells help B cells.
So you have specialised helper T cells that actually help the B cell response and promote that B cell response.
If you remove T cells you remove that help. Therefore you have a poorer antibody response.
And in the the animals where they had removed the Bursa, you can see that essentially there was no antibodies in those animals, but they had an almost fully functional T cell.
So very clean, clear evidence that it’s the thymus that is the place for T cell development.
And in the chicken it’s the Bursa Fabricius where B cell development is taking place.
The Lymphatic System / Lymphocyte Recirculation:
We have this constant recirculation of lymphocytes because they’re constantly surveying the body looking for antigen.
So naive lymphocytes B cells and T cells come in through the blood.
They leave the blood within the lymph node. They look for their antigen and then they’ll leave the lymph node either still as a naive cell or as an activated cell if they found their antigen via the efferent lymph vessel. And then those lymph vessels eventually rejoin the bloodstream through the thoracic duct.
And then the lymphocytes are back in the blood system.
If there is infection here and the B cell and T cells become activated, what happens is that they change what are called homing receptors.
And so it means that those memory or effector cells change the pathway through which they recirculate.
And so rather than going back through the lymph node they’ll preferentially home to tissues where that infection was first found.
So it’s quite a smart system to try and encourage those cells to go back to where the infection was.
So that’s the system in most mammals.
The ‘inverted’ porcine lymph node structure: (pigs)
Lymphocytes do not randomly migrate but accumulate preferentially in specific compartments
B cells – cortex/follicles
T cells – paracortex/medulla
Three stage process
1. Entry – regulated by adhesion molecules on endothelial cells and lymphocytes
2. Transit – through the parenchyma of the lymph node
3. Exit – Directly into blood vessels (or via efferent lymph)
However very few cells are found in efferent lymph compared to other species
It is thought that the inverted structure means that cells do not migrate to the main efferent duct but rather re-enter the blood system via high endothelial venules (HEV). It is not clear whether distinct HEV are used for entry and exit
ANTIGEN RECEPTORS - key part of adaptive immune system
o T cells and B cells receptors are related but distinct. And the system has evolved that we have a huge diversity of different receptors.
And each receptor specific for a particular epitope which is a part of an antigen.
So how do we create such a diversity of receptors?
Well, the principal mechanism through which we create a diversity is through the recombination of different gene segments.
This is showing the T cell receptor alpha chain.
And this is the T cell receptor beta chain. And so the T cell receptor is a hetero dimer.
It’s got an alpha chain and a beta chain. And what happens is when a T cell develops you get recombination of what are called different gene segments.
o the V stands for variable, the J for joining the C for constant
And the beta has got an additional D for diversity segment.
So you get recombination of one v segment with one j segment with the C segment to give you a full chain.
So because you have different segments to choose from.
You create diversity.
So the alpha beta T cell receptor is often referred to as the conventional T cell receptor system.
But we also have another family of T cells, and those are T cells that express what’s known as the gamma delta T cell receptor.
So instead of the alpha beta chains, it uses a gamma chain and a delta chain to make that T cell receptor.
What you see is if you compare humans and mice and I guess horses,
you can see that cattle and to some extent sheep and pigs have many more V gene segments to choose from to create a receptor.
So what that implies is that those species can make a much bigger diversity of receptors, because there’s many different segments to choose from.
And that greater diversity is also in line with gamma delta T cells being much more abundant in these species.
So therefore probably playing a more prominent role in the immune systems of those species.
Generation of immunoglobulin diversity in birds by gene conversion:
Single functional V region at the heavy and light chain immunoglobulin loci is altered (i.e. converted) by the donation of sequences from an upstream array of pseudogenes.
The continued iterations of gene conversion produces a virtually unlimited antibody repertoire of starting antibodies
Limited V gene diversity, means other species also use this mechanism to create ‘new’ V genes
Camelid antibodies:
Camels and llamas have three IgG subclasses
IgG1 has a conventional structure
IgG2 and IgG3 (75% of total Ig) have no light chains and just utilise a variable heavy chain (VHH) to bind antigen
VHH can have convex Ag-binding site and therefore can bind concave active-sites of enzymes, which ‘conventional’ antibodies could not bind
– Exploited as therapeutic ‘nanobodies’
Avian Antibodies:
Principle antibody circulating in avian blood is IgY
Similar in structure mammalian IgG but with significant molecular differences
Extra domain on heavy chain
Lack hinge region – reduced flexibility
Some birds have a form of IgY that lacks the Fc region (∆Fc)
Function of the ∆Fc IgY is unclear; may be limited as it cannot fix complement or bind Fc receptors on immune cells:
And that’s a bit of a puzzle why animals would have evolved an antibody molecule that lacks that FC tail.
Why do you think that is?
It is because that Fc tale is the portion of the antibody that really interacts with either other cells that are the components of the immune system.
So the arms are the antigen binding site. So that will bind to the antigen.
So imagine binding the surface of the bacteria and the tail that sticking out.
That is then available for a phagocytes such as a macrophage to then bind and then engulf that bacteria.
So it’s often really considered the sort of functional tail of the antibody molecule.
So if the antibody doesn’t have that functional tail then it’s going to have limited or more limited functionality compared to the standardised Ab.
γδ T cells:
Unconventional.
γδ T cells are a major population in artiodactyls - cloven-hooved ungulates
Highest proportions of γδ T cells found in young animals
γδ T cells also a significant population in birds
*WC1 – major co-receptor for γδ-T cells not present in man/mouse
Human/murine γδ T cells:
Whilst γδ T cells make up only 1-5% of T cells in blood and peripheral lymphoid organs, they are enriched ~50% of the T cell population in epithelial rich tissues e.g. skin, reproductive, respiratory and intestinal tracts where pathogens are likely to enter
Recognise antigen in a distinct manner from conventional αβ TcR expressing T cells
Since they do not recognise peptide antigen presented in the context of MHC most lack the CD4 or CD8 co-receptors >
Because what CD4 does is it stabilises the binding of the receptor to meet C class two, and CD8 stabilises the binding the receptor to MHC class one.
So if the gamma T cell is not recognising MHC presented peptide, there’s no reason for it to have CD4 or CD8!
Can recognise soluble antigen in a manner akin to B cells
Also share recognition features of NK cells recognising MHC-like molecules e.g. MICA ad MICB that are expressed on the surface of stressed/infected cells
γδ T cell development:
γδ T cells and αβ T cells arise from a common progenitor cell in the thymus
Unclear what dictates thymocyte (progenitor of a T cell in the thymus) to rearrange and express γδ vs. αβ TCRs
Many tissue-specific γδ T cells express different specific subsets of TCRs with little or no diversity
The different subsets of γδ T cells arise in the thymus at different stages of ontogeny:
Skin & respiratory tract γδ T cells are only generated in the early foetal thymus
Gut and circulating γδ T cells generated later and are continued to be produced in the thymus until adulthood.
γδ TCR recognise distinct ligands:
So cells of the innate immune system have what are called pattern recognition receptors or PRR.
So these receptors, such as the toll like receptors and what these PRRs do because you don’t have a mass diversity of these is they recognise conserved molecules.
So molecules that are conserved on pathogens or molecules that are commonly elicited when there is stress or there is damage to to tissues.
And so we refer to these as pathogen associated molecular patterns or damage associated molecular patterns.
And so that’s how the innate system recognises danger is pathogen is the damage taking place.
And gamma delta T cells seem to be doing a very similar thing.
So they bind to microbial pumps and they also bind to damage associated molecular patterns.
Bind to microbial PAMPs (pattern associated molecular patterns), including - pathogen-derived lipids, organic phosphoesters, nucleotide conjugates and other non-peptide ligands with no requirement for antigen processing /presentation by MHC molecules
e.g. the major γδ T cell population in humans (Vγ9/Vδ2 T cells) recognise the small metabolite (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate (HMB-PP) produced by protozoa e.g. Plasmodium spp. and a broad range of bacterial pathogens including Mycobacteria
Also bind to DAMPs, host proteins associated with stress - damaged, transformed or infected cells, including: non-classical MHC molecules and heat-shock proteins
e.g. Human Vγ4Vδ5 T cells recognise the endothelial protein C receptor (EPCR), which is expressed on endothelial cells infected with cytomegalovirus and epithelial tumours.
Characteristics of αβ and γδ T cells
Characteristic
Antigen receptor
Form of antigen recognised
CD4/CD8 expression
Frequency in blood
MHC restricted
Function
αβ T cells
αβ TcR + CD3
MHC + peptide
Yes
60-75%
Yes
“Help for activation of other leukocytes
Immunoregulation
Cytotoxic killing”
γδ T cells
γδ TcR + CD3
MHC-like molecules plus non-protein ligands
Mainly no (some express CD8)
1-5%*
Mostly no
“Help
Immunoregulation
Cytotoxicity”
Porcine γδ T cells (PIG):
Originally defined a null T cells, but the development of γδ TCR mAbs enabled their specific detection & characterisation
Comprise ~15-35% of circulating lymphocytes, with a minority expressing CD8.
Limited systematic analysis of these cells in tissues, however appear to be primarily blood borne being recruited to skin and mucosal sites by inflammatory processes (differs from other species)
Antigens recognised by porcine γδ T cells are not well defined
Do not recognise the HMB-PP phosphoantigen
Mycobacterium bovis (antigen or PAMP?)
Porcine γδ T cell function:
Porcine γδ T cells do not express the cytotoxic pore-forming protein, perforin but may mediate cytotoxicity through the Fas-L death receptor pathway
Can produce both inflammatory e.g., IL-1, TNF & IFN-γ and anti-inflammatory cytokines e.g., TGF-β suggestive that these cells can exert both effector and regulatory functions
Vaccines for foot-and-mouth disease virus (FMDV) protect pigs after 4 days which precedes the appearance of antibodies
γδ T cells are the major responding cell population when stimulated with FMDV vaccine in vitro
Not clear whether γδ T cells are responding to
a viral antigen or to ‘danger signals’
Suggests γδ T cells might be important
for the observed protection.
A proportion of porcine γδ T cells express surface molecules usually associated with professional antigen presenting cells such as MHC class II, CD80/86, and CD40
These γδ T cells appear to have the capacity to take up antigen and present it with MHC-II to CD4 T cells
Demonstrated in vitro with the model antigen ovalbumin and with African swine fever virus
Subsequently also described for human γδ T cells but not in mice
It now appears that γδ T cells express a range of genes normally associated with antigen presenting cells (macrophages, dendritic cells and B cells
Functional significance remains to be fully elucidated.
Bovine γδ T cells (CATTLE):
Represent up to 60% of circulating T cells (greatest in calves)
Cattle express a greatly expanded number of TCR δ variable gene segments
Majority express the phenotype WC1+ CD4- CD8-
Workshop Cluster 1 (WC1) is a transmembrane glycoprotein belonging to the scavenger receptor cysteine rich (SRCR) family
Bovine γδ T cells were originally defined by mAbs to this co-receptor
Three subsets of WC1+ γδ T cells (WC1.1, WC1.2 & WC1.3) and a WC1- subset
Unclear whether WC1 expression defines functional subsets
Bovine γδ T cell activating ligands:
Viral, bacterial and protozoan PAMPs
Phosphate antigens from Mycobacterium bovis
Peptide antigen from the bacterium Anaplasma marginale
Stress-inducible molecules
Cytokines
Bovine γδ T cell function:
Anti-microbial effectors
Secrete IFN-γ
Cytotoxicity, express both granulysin and perforin; however this killer phenotype is predominantly observed in young animals & diminishes with time
Evidence that they display ‘memory’ with enhanced proliferation an
d IFN-γ secretion (dependent upon antigen-specific CD4 T cells)
Immune activation
Evidence for reciprocal activating signals between γδ T cells and DCs
Capable of acting as antigen-presenting cells and stimulation results in the expression of MHC-II and CD80 on a small proportion of cells
Immune regulation
In most species the expression of the transcription factor forkhead box P3 (FOXP3) by predominantly CD4 T cells confers a regulatory phenotype
FOXP3+ CD4+ T cells exist in cattle but these cells do not exhibit anergic or suppressive properties in vitro, but FOXP3-WC1+ γδ T cells do
γδ T cells spontaneously secrete IL-10 and proliferate in response to IL-10, TGF-β, and contact with APCs
IL-10–expressing γδ T cells inhibit Ag-specific and nonspecific responses of CD4+ and CD8+ T cells in vitro
These cells can inhibit can suppress the responses of our conventional T cells, stop them responding.
And so from these from this work, it’s been proposed that rather than these fox P three CD4 cells,
gamma delta T cells may be a if not the major regulatory T cell population in in cattle.
Therefore proposed that γδ T cells are a major regulatory T cell population in cattle.
Protection of offspring from and by the maternal adaptive immune system:
Developing offspring need to be protected from the maternal immune system because the foetus, the developing foetus is comprised, of course, from the genetics of the mother and the father.
So it is expressing some proteins that are foreign to the maternal immune system.
So there is the possibility that the maternal immune system will see this as antigen and potentially respond and attack the, um developing foetus.
Extreme examples:
Marsupials versus placental mammals:
Marsupials are characterized by giving birth to relatively undeveloped young. They lack a complex placenta to protect the embryo from its mother’s immune system.
At birth the tissues of marsupial immune system are underdeveloped & not immunocompetent.
The survival of the neonatal marsupial in a microbially rich environment is dependent on maternal strategies, including immunoglobulin transfer via milk and, in some species, prenatally via the yolk sac placenta. It is also likely that pouch skin secretions e.g. lysozyme also play a role.
Example. Opossum (Monodelphis domestica)
Born after only 15 days gestation
Neonates have neither lymphoid tissues or organs
Neonates can produce their own antibodies by 7 days post-partum
During first 7 days rely on passive immunity from mother’s milk & suckle permanently until 16 days.
Weaned at 60 days when antibody adsorption across the intestinal epithelium ceases.
The role of mammalian placenta in protecting the foetus:
The placenta functions as an immunological barrier between the mother and the foetus, creating an immunologically privileged site
Secretes imunosuppressive enzymes e.g., IDO and Neurokinin B
Contains high proportion of suppressor cells e.g., Tregs
Placental trophoblasts lack classical MHC class I molecules and express atypical molecules to prevent attack by CTL & NK cells
Forms a syncytium without any extracellular spaces between cells in order to limit the exchange of migratory immune cells between the developing embryo and the body of the mother
The increased immune tolerance in pregnancy can cause an increased susceptibility to and severity of some infectious diseases
In some species, the placenta does not block maternal IgG antibodies, which pass through the placenta, providing immune protection to the foetus against infectious diseases.
Immune responses of neonates:
Foetal development occurs in the sterile environment of the uterus & then neonates are born into a microbe-rich environment
Neonates are ‘immunocompetent’ & capable of mounting both innate and adaptive immune responses
Adaptive responses are primary response with a prolonged lag period and low concentrations of antibodies
Responses skewed toward Th2 rather than Th1 response (as a result of hormonal influences in utero to prevent IFN-γ mediated placental damage)
As such, neonates are susceptible to infection with intracellular pathogens e.g., Rhodococcus equi in foals
Because > Th2 are helper cells that have evolved to really promote responses against extracellular pathogens,
whereas TH1 responses evolved to help responses against viruses and other intracellular pathogens.
The final development of the newborn immune system depends in large part to exposure to intestinal microflora
‘germ-free’ animal raised in a sterile environment may fail to develop normal GALT.
Antibodies in colostrum vs. milk:
Particularly important in ungulates due to the lack of IgG transfer in utero
Colostrum is rich in IgG and IgA, but also contains some IgM and IgE
As
lactation progresses and colostrum changes to milk differences among the species emerge
With primates, IgA predominates in both
With pigs and horses IgG predominates in colostrum and IgA in milk
With ruminants IgG predominates in both.
Colostrum and antibody absorption:
Colostrum is also rich in cytokines which may promote the development of the neonate’s immune system
Colostrum is also full of lymphocytes (milk – v. few)
Pigs 105-106 cells/ml
Cattle up to 106 cells/ml
Maternal antibody is absorbed across the intestine which is highly permeable immediately following birth, but species differ in selectivity & duration
Horse/pigs – selective absorption of IgM & IgG; IgA remains in the intestine
Ruminants – absorption is unselective, all classes are taken up although IgA is then gradually excreted back
Duration of absorption declines after 6 hours in ruminants/horses but can be retained for up to 4 days in pigs if milk products are withheld.
Maternal antibodies & vaccination:
Maternal antibodies can inhibit neonatal antibody responses & thus impair the induction of primary immune responses to vaccination
Maternal antibodies may ‘mask’ or ‘neutralise’ vaccines or block B cell activation by binding inhibitory FcRs
When to vaccinate young animals depends on the amount of maternal IgG transferred and the half-life of IgG involved:
Dogs – 10-12 wks
Horses/cattle – 12-16 wks.
Exotic Animal Immunology
Why study “other species”
immunology?
- To protect animal species
- To protect endangered wildlife e.g.
Tasmanian devil and cancer (contagious cancer?):
Tumour DFT1 downregulates MHC I expression
Tumour DFT2 mimics MHC I
Adenovirus-expressed IFN-γ upregulates MHC I
Species could go extinct without the assistance of immunologists! - To protect our animal food source:
e.g. Berry extracts could possibly boost poultry immunity:
Flavonoids found in berries could modulate the immune system by decreasing production
of pro-inflammatory cytokines, T-cell activation, and proliferation.
Could help animal fight the bacteria. Boosting their immunity in natural ways is beneficial for both animal and immunologists. Better welfare of animal = better immune system = better food products and less antibiotics in the environment. - To protect human health:
Bats > host defence of bats is a unique viral reservoir. They have a low cancer rate and low death rate from viruses. If we understand how they are able to balance defence and tolerance, humans could learn from this to improve immune resistance and kill viruses.
Bats enhanced host defence = higher levels of IFNS, ISGs, HSPs, ABCB1 and autophagy.
Their tolerance is also increased, they have suppressed immune response pathways such as Il-1B and NLRP3.
5.Because it is fascinating:
e.g. Discovery of unique antibodies in camels > a revelation in medicine, you can make them easily, they can get into cells, more soluble and durable etc. Nanobody treatment has been developed for a rare clotting disease by utilising camel antibodies > a novel scientific discovery that can be used to treat diseases.
LYMPHOID ORGANS
Primary organs:
* Bone marrow
* Thymus
* Bursa of Fabricius (B)
* Yolk sac (B)
- Secondary organs:
- Spleen
- MALT:
- NALT
- BALT
- GALT
Thymus
- 2 pairs of thymic lobes
in snakes - Elongated lobulated
gland with 7 lobes on
each side of the neck
in birds and
crocodilians - Seasonal
reactivation in the
summer, marked in
reptiles
Why are
lymphocytes B
called B ?
Due to the Bursa of Fabricius:
- Sac-like evagination of the dorsal wall of
the cloaca - Humoral immune system > If you remove the Bursa in chicken and do a skin graft, they will reject it > so only in humoral immune system.
- First IgM produced in 12-day egg
- 90% of bursa cells are lymphocytes B at
hatching - Not present in reptiles
- Reptile B lymphocytes appear between 2
and 6 months post-hatching in lizards.
Bone marrow plays no role in bone development > the bursa size correlates with bone health.
NO LYMPH NODES
Exception > geese and ducks.
1 pair near thyroid
1 pair near kidneys
no external capsule
Other organs:
Spleen * Very important for humoral
immunity in reptiles as they do not have a Bursa!
- Liver and kidneys
– Important
secondary lymphoid organs - Harder’s gland in birds
Cells of the immune system
* Very similar to mammals except:
*Heterophils
* Platelets can also phagocyte bacteria.
Heterophils
* Equivalent of mammalian
neutrophils
* Lack myeloperoxidase
* Rely mainly on
nonoxidative killing
mechanism
* Clinical application: Solid
pus versus liquid pus! Have to remove tumours as an abscess.
Eosinophils
*Not reliable indicators
of parasitism in birds
and reptiles
*Delayed hypersensitivity
in birds
INNATE IMMUNITY
Birds have unique TLR
(Toll-like receptors). > activation of these receptors enables recognition of PAMPs?, determines self vs non-self. Use in cancers…
Reptiles: no fever.
Ectothermic > thermal regulation to activate immune system. Will seek heat to bring body to appropriate temperature. Reason reptiles get ill in captivity, unable to regulate body temp.
Behavioural
thermoregulation.
Functional elements:
- Physical barriers
- Similar to mammals in many ways
- Low pH
- Colonization of the gut
- Lysozymes
- Defensins and cathelicidins
- Antimicrobial peptides, lysozymes and
complement much broader range of
activity in reptiles than in mammals.
ADAPTIVE IMMUNITY
Seasonal changes
Very slow humoral response
in reptiles (peak at 6-8 wks) > not very efficient because they do not have lymph nodes / germinal centres
Natural antibodies (Nabs)
Temperature dependant in
reptiles
Immunoglobulins
- Birds: IgM, IgY, IgA * Tendency to have lower molecular weight
immunoglobulins - Will not trigger hypersensitivity reactions
- Reptiles: Igm, IgY
- Allergies/anaphylactic reactions are very
rare in birds and reptiles. No IgE. Still a lot to learn on that, not well understood.
Major Histocompatibility Complex
- I, II and III in birds
- I, II in reptiles, maybe III?
- MHC much more complex in migrating birds > makes sense as they are exposed to a wide range of pathogens in many environments.
- Mammals: difficult to show a genetic resistance to a disease.
- Birds, in contrast, have multiple polymorphic antigen-processing
molecules. They are either resistant or sensitive to a specific disease.
Easy to see if genetically susceptible to disease. - More primitive pattern also seen in reptiles (and fish and amphibians).
Chicken have at least 12 blood group systems with multiple alleles.
* Blood transfusion can be done between different species!
* Rarely any reaction on first transfusion
* Cross-match done before transfusion >
* Not much known for reptiles
What affects the immune system?
* STRESS > inflammation. e.g. cystitis in cats, infection of the bladder.
* Breeding season (testosterone)
*Inbreeding (ex. colour mutation)
* Migration
*Age
Disorders of the
immune system
* Auto-immune diseases rare in birds
and reptiles
* Hypersensitivity rare too – No IgE
* Mast cells can be activated by
psychological stress!
* Feather picking behaviour?
* Virus-induced immune response:
* Avian bornavirus > inflammation in central nervous system and bird will die.
Immunodeficiencies
* Infections targetting the bursa:
* Chlamydia
* Salmonella
* Virus-induced:
* Circovirus
* Ex. Psittacine Beak and
Feather Disease
Toxin-induced:
* Oil
* Lead
* Benzimidazoles
* Corticosteroids
Perspectives:
New therapies: immune system modulation
* Vaccines
* Cytokines
e.g. APOQUEL, dogs surviving advanced melanoma after vaccination with xenogeniec human tyrosinase.
INTERESTING:
Elephants don’t get cancer! They Have extra copies of two cancer fighting genes: P53 (which hunts for cells with miscopied DNA) and LIF6 (which obliterates the mutated cells before they can form a tumour).
Conclusions
- Clinically-relevant specificities
- Studying the common factors and differences of immunity
across species allows us to: - better understand zoonotic diseases (ex. COVID)
- design preventive treatments (ex. new RNA vaccines)
- develop new innovative therapeutic protocols for infectious
diseases but also for cancers and pain (ex. melanoma, atopic
dermatitis, arthritis) - prepare for the future (ex. antibioresistance)
- preserve endangered species (ex. Tasmanian devil’s)
