How does the immune system control infection? Flashcards
What is signal 1 in terms of T cell activation ?
T cell receptors recognise Ag presented by MHC
What is signal 2 in terms of T cell activation ?
Antigen-presenting cell upregulates B7 to signal to the T cell via CD28 that Ag comes from a microbe
What is signal 3 in directing T cell effector function ?
Usually cytokines that tell the T cell what microbe it is so it can differentiate accordingly
What cell gives B cells permission to activate in the lymph nodes ?
T cells
What are plasma cells ?
activated antibody-producing B cells
How do B cells differentiate ?
- Changes isotype of antibody it produces
- Different isotypes interact with different innate molecules
- Deal with different infection types
Do complement or antibodies activate first ?
complement
What are the 3 main effector functions of complement proteins ?
- Opsonisation to enhance phagocytosis
- Stimulating inflammation by recruiting and activating immune cells
- Lysing microbes and cells
How is opsonisation initiated using complement effector proteins ?
C3b binds to microbe, recognition by phagocyte by the C3b receptor, phagocytosis – CR1 is the complement receptor 1
How is inflammation initiated through complement effector functions ?
mediated by complement: C3a, C4a and C5a released during complement activation
- Act locally similar to inflammatory cytokines: recruit cells to infection site
- Can activate cells
How is cytolysis mediated using complement effector functions?
Membrane attack complex (MAC) forms in the membrane of bacteria, water rushes in, ions rush out, microbe bursts, can also kill host/foreign cells
Match Macrophage,
DC, Neutrophils to talkers killers on call and jack of all trades
Macrophage: jack of all trades
Neutrophils: killers on call
DC – talkers
Phagosome killing 1
- Vacoular ATPases pump hydrogen ions into phagosome to acidify the environment
- Phagocyte oxidase and cofactor NADPH oxidase initiate the respiratory burst, converting oxygen into ROS such as hydrogen peroxide
o Activated in neutrophiles - Neutrophils expression myeloperoxidase (MPO) that make HOCL (hypochlorite = bleach)
- Proteolytic enzymes that destroy microbes
o Neutrophils produces elastase that degrades bacteria
Phagosome killing 2
- Nitric oxide is made via nitric oxide synthases (iNOS)
o Combines superoxide or NO to create highly reactive peroxynitrite radicals - Phagosomes withhold nutrients from pathogens
o Lactoferrin and other scavenger proteins binds and transport out iron - Defensins – directly microbicidal proteins
How do phagocytes bind to antibodies ?
Fc region
Netosis
Neutrophils extracellular traps (NETs)
- Neutrophil dies via a process called NETosis
- Nucleus swells and burst extruding DNA like a net
- The DNA has anti-microbial molecules attached (defensins, proteases)
- Traps and kills bacteria, fungi and viruses (shown as pus)
What do T cells make to enhance macrophage and neutrophil killing ?
cytokines like IFN-gamma and IL-17
Granulocytes role in host defense
- Granulocytes prestore effector molecules in granules in the cytoplasm
- Ready to fire out when activation
Include mast cells, Baophil, Eosinophil, Neutrophils
Antibody-dependent cell-mediated cytotoxicity (ADCC)
- Antibodies bind to target cell or microbe
- Fc portion of Ab binds to FcR on innate cell
- Cell is activated if multiple FcR-Ab interactions occur
- Cells then fires out its granules at the target
- Destructive process, can cause a lot of collateral damage
How do Eosinophils target helminths ?
- Extracellular: helminths, too large for phagocytosis
o Eosinophils bind to antibody, coating parasite via FcR
o Eosinophils degranulates releasing toxic compounds
MBP, EDN, EPO, ECP
o Toxic to parasite and host, can cause a lot of collateral damage
What is the role of mast cells in extracellular pathogen control ?
Mast cell degranulation:
- Weep response: causes liquid to flow from the tissue into the lumen to push helminths away from the gut wall
- Sweep response: histamine cause muscle contractions, push helminths out of the gut with faeces, causes wheeze in allergy
- Cytokine production: amplifies inflammatory response
What cytokines allow for enhanced immunity to helminths ?
IL-4, IL-5, IL-13
Why are som pathogens extracellular ?
- Hide from immune effector mechanisms: antibody, complement mechanisms, immune effector cells like macrophages, immune system trained not to attack self, innate immunity often fails to contain intracellular infections on its own and usually needs help from the adaptive immune system.
- Ready supply of resources: nutrients, cellular machinery to hijack like viruses
- Hitch a ride around the host
Name 3 intracellular pathogens and their cell
HIV: targets Th cells, macrophages, DC
Ebola: DC, macrophages, (fibroblasts and epithelial cells)
Measles: T and B cells, DC, macrophages (epithelial cells, endothelial cells, neurons)
Why are immune cells good targets for intracellular pathogens ?
- Macrophages are trying to rapidly eat you anyway:
o If you can evade their killing mechanisms then they make infection easier - Immune cells migrate readily around the host
o Perfect transporters - Immune evasion:
o Easier to manipulate immune pathways to host if you are inside the cell
Intracellular PRRs detect …
endosomes through TLR, cytosol through NOD-like for bacteria and RIG like for viral RNA
avoiding killing mechanism of phagocytosis
Legionella pneumophila: prevents lysosome fusion with phagosome, and ends up in a rough ER-like vesicle in which it can proliferate until it lyses the vesicle and then the cell.
- Salmonella and tuberculosis do this too
Escaping the phagosome
- Listeria monocytogenes: can break out of phagosome and then lives and replicates in the cytosole
- Shigella also does this
What is required for macrophages to control intracellular infections ?
IFN- gamma from CD4 cells
How does mycobacterium tuberclosis evade phagocytosis >
Mycobacterium tuberculosis is very good at evading phagocytosis, results in chronic infections with continual production of IFN-gamma and macrophage activation
- Continue release of macrophage killing molecules (NO) causes lung pathology
- Granulomas form around the infected macrophages to wall off infection.
What is the role of IFN-alpha and beta in protection from viral infections?
- This induces an ‘anti-viral’ state in cells that protects them from being infected
Type 1 IFN
- Inhibition of viral gene expression:
o Blocks viral transcription and translation
o Viral RNA degradation
o Autophagy (cells eats own organelles) - Induces apoptosis:
o Misfolded viral proteins triggers unfolded protein response causing apoptosis
o Alters cells response to TNF-alpha from pro-inflammatory to apoptosis - Promote T cell and NK cell activation
o Sequester lymphocytes in LN
o Increase cytotoxicity of CTL and NK cells
o Promote Th1 differentiation
o Upregulate MHC class 1
How are NK cells fully activated ?
IFN-gamma from Th cells and CTL
what is the Important back up mechanism to deal with intracellular pathogens that evade CTL by inhibiting MHC class 1
NK cells
Necrosis
uncontrolled cell death, cell ruptures releasing contents, high inflammatory
Apoptosis
programmed cell death, fragmentation of DNA, membrane blebbing, apoptotic cells cleared by phagocytosis, non-inflammatory
What is the balance that is depends on if NK cells are activated or not?
- If inhibitory receptor isn’t engaged or activating receptor is the NK cell will be activated
How do Antibodies affect NK cells
- Ab can enhance NK killing via ADCC if a microbe Ag remains on the cells surfaces
How does adaptive immunity clear intracellular infections ?
- Directly: cytotoxic T lymphocytes, neutralising Ab
- Indirectly by enhancing the innate: Cytokines and ADCC
How do CD4 and CD8 cooperate to kill?
Th produce IFN-gamma to activate macrophages, NK cells and CTL that go forward to kill infected cells
How does the precision vary between NK cell and CTL
More precise from CTL
How do CTL kill ?
perforin creates pores, granzyme enters pores and induces apoptosis
How do NK cells kill ?
Target cells express Fas, CTL/NK express FAS ligand, FAS activation signals apoptosis
How are CTL activated ?
- All naïve T cells have to be activated by APC
- Cross-presentations allows specialised DCs to take up extracellular antigens and present them on MHC class 1 instead of MHC 2
How do viral evasions ?
- Disrupting MHC1 Ag presentation key evasion strategy
- NK cells detect MHC class down regulation
Which complement protein causes lysis of bacterial cells via the membrane attack complex ?
C6-C9
Activation of complement through antibody complexes via the classical pathway
C1
what complement protein is released from anaphylatoxins ?
C3, C4, C5
Select the correct mechanisms used by phagocytes to kill pathogens.
oxygen radicals, nitric oxide, hypochlorite, witholding nutrients, acid
3 types of phagocytes
macrophages, neutrophils and DC
Which phagocyte makes hypochlorite/bleach ?
Neutrophils
What activity of antibodies binds to a virus to prevent it entering the cell?
neutralisation
NK cells kills cells that have downregulated …
MHC class 1
What cells kills infected cell with viral proteins on its surface, bound by specific antibodies ?
NK cells
What cells kills virus infected cell with downregulated mHC class 1, and expressing cell stress markers?
NK cells
Which transcription factor is essential for regulatory T cell development?
foxp3
What allows dendritic cells to present extracellular antigens on MHC class I?
Cross-presentation
Which of the following statements about naive B cells is true?
o A. They can secrete antibodies immediately
o B. They express either IgA or IgE
o C. They express membrane-bound IgM and IgD
o D. They undergo somatic hypermutation before antigen exposure
They express membrane-bound IgM and IgD
What happens during somatic hypermutation?
Point mutations are introduced into the V region
Which cytokine promotes class-switch recombination to IgE?
IL-4
Which cytokine promotes neutrophil recruitment and activation?
IL-8
What is the main function of IL-2 in T cell biology?
promotes clonal expansion
Which Th subset is primarily involved in defense against helminths?
Th2
Th17 cells primarily protect against which of the following?
extracellular bacteria and fungi
Which complement component acts as a potent anaphylatoxin?
C5a
Which process increases the diversity of TCRs?
V(D)J recombination
Which of the following is an inhibitory receptor on T cells?
CTLA-4
Which immunoglobulin is most effective at activating the classical complement pathway?
IgM
What is the role of CD28 ?
Recepts B7
What do antibodies do ?
- Neutralises the toxin
- Opsonization
- ADCC
- Lysis
- Complement activity:
o Phagocytosis
o Inflammation
What defines specificity and function on an antibody ?
Variable region defines Ag specificity, constant Fc region defines function
How do Ab change constant region to allow for different effector functions ?
o Membrane and secreted form
o ‘Isotype switching’ or ‘class switching’
What is cross-linking ?
Two Ag binding sites permits cross-linking, improves neutralisation and controls innate cell activation
What is affinity maturation ?
Once created the variable Ag binding site can undergo further mutations to increase affinity for Ag – called ‘Affinity maturation’
How do cells switch between transmembrane and secreted forms of antibodies ?
Different RNA processing leads to a cell being able to create the transmembrane and secreted form of the antibody.
5 different types of antibodies
IgM, IgD, IgG, IgE, IgA
IgM and IgD
IgM and IgD: first isotypes produced, only ones produced by B cells, IgD is secreted in small quantities and its function is unknown, IgM is the first response to Ab.
- DNA isn’t changed, alternate RNA splicing
What is isotype switching ?
Isotype switching – allows the B cell to make the other 3
Irreversible change in DNA, once switched you cannot go back
- Can only produce one isotype at a time
- AID (Activation-induced deaminase)
IgM
- Initial response, produced prior to affinity maturation so lower affinity than other isotypes.
- Forms pentamers via J chain:
o Compensates for lower affinity
o Very good at binding repeating epitopes
o Very efficient at activating complement
structure of IgM
pentameric structure
IgA
- J chain facilitates transport across intestinal epithelium
- Resistant to low pH and digestive enzymes
- Good at cross-linking pathogens
- Keeps pathogens away from epithelium by clumping them together for expulsion
- Weak ADCC function and complement fixation, less likely to cause collateral damage
What allows antibodies to be opsonins ?
FcR region which stimulates ADCC
FcgammaR and FcalphaR
Antigen-Antibody cross-linking: antigen cross-linking is required to bind to, and activate, FcgammaR and FcalphaR. Distinguishes Ab that are bound to pathogen
- Free Ab without Ag:
o No infection so no innate activation
o IgG has low affinity for FcGammaR so doesn’t bind to monomeric form
o Free Ab does not activate innate cells
- Ag links multiple Ab
o Indicates infection so innate activate
o Multiple IgG have higher avidity for FcgammaR allowing them to bind
o IgG-Ag complex cross-link FcgammaR resulting in activation
Crosslinks mediate neutralisation: clumps toxins, clears toxins, hinders pathogen motility
IgE
IgE makes innate cells into memory cells:
- FcER has higher affinity for IgE
- Binds to monomeric IgE, doesn’t require cross-linking
- FcER becomes loaded
- Gives the mast cell an Ag-specific receptor
Supposed to kill parasitic helminths, cause allergies, anaphylaxis is so fast for this reason
How much larger is the mutation rate in B cells ?
100,000 fold
What happens to the B cell after somatic hypermutation
After somatic hypermutation B cell tested to see if it can still bind its Ag:
1) Loses ability to bind to Ag – B cell dies
2) Binds Ag with worse affinity – survives but outcompeted
3) Binds to Ag with higher affinity – B cell outcompetes others
When does affinity maturation occur ?
same time as isotype switching
Which antibodies have the highest affinity ?
- IgG, IgA and IgE tend to be higher affinity than IgM and IgD
Where does affinity maturation happen ?
Only occurs in lymph nodes and spleen, B cells require T cell permission to undergo affinity maturation. Afterwards T cell must check and if it still matches the B cell is microbe specific if not it needs to be deleted to prevent self-attack
- T cells do not undergo affinity maturation
How is the simultaneous production of IgM and IgD made ?
Differential RNA processing
How does class/isotype switching occur ?
permanent DNA rearrangment
Difference between class and isotype
Class refers to the type of antibody based on its heavy chain constant region (e.g. IgG, IgA), while isotype includes these classes as well as light chain types (κ or λ) — so class is a subset of isotype.
Ab class switching involves …
IgM is the first Ab to be produced in large quantities, An IgA secreting B cell can never secrete IgG again, Requires the enzyme AID (activation-induced deaminase)
Affinity maturation can lead to self-reactive antibodies ?
True
Affinity maturation involves …
Involves spontaneous mutations in the VDJ region
B cell survival during affinity maturation depends on … Ab.
membrane not soluble
Monomeric antibodies
Monomeric = a single antibody unit
→ Most antibodies (like IgG, IgE, IgD) circulate as monomers
→ Each monomer has 2 antigen-binding sites
Pentameric antibodies
Pentameric = five antibody units joined together
→ IgM is typically pentameric when secreted
→ Held together by a J chain
→ Has 10 antigen-binding sites total (5 × 2)
→ High avidity (overall binding strength) even though each site has low affinity
Dimeric antibodies
IgA Dimeric (in secretions)
Monomeric (in blood) In mucosa, forms dimers via a J chain + secretory component
What allows antibodies to perform different effector functions without altering their antigen specificity?
Isotype (class) switching of constant Fc regions
Which antibody is most effective at activating the classical complement pathway?
IgM
Why is IgM particularly effective during early immune responses despite its lower affinity?
Its pentameric structure increases avidity
What enables IgA to function effectively at mucosal surfaces?
Resistance to low pH and digestive enzymes
Why do free antibodies in the blood not activate innate cells?
They have low affinity for Fc receptors in monomeric form
Which antibody class is LEAST likely to activate the complement system?
IgE
Which property of the IgG-Ag complex enables it to activate Fcγ receptors effectively?
Cross-linking of multiple FcγRs to enhance avidity
FcgammaR, FcalphaR and FcER bind to what
FcgammaR -IgG
FcalphaR - IgA
FcER - IgE
Central tolerance
Central tolerance deletes self-reactive T cells, but can’t deleted T cells recognising non-dangerous antigens like Ag on food.
signal 1 with no signal 2
food, non-microbe
What is meant by soluble Ag in the blood?
imme to that Ag
What happens with food Ag ?
- Food Ag result in deletion to the T cell, anergy (T cells becomes unresponsive but lives), inhibitory cytokines (inhibits or turns off T cells or innate cells), regulatory T cells (inhibit the function of the immune system)
Celiac disease
Th1 cells producing IFN-gamma, macrophages and B cells are upregulated
Food allergies
h2 cells producing IL-4, mast cells and B cells produce IgE
What is the predominant intestinal antibody ?
IgA
Structure of IgA
dimeric
Why does IgA have such high affinity ?
somatic hypermutation
Is IgA good at stimulating ADCC?
IgA is poor at stimulating ADCC so does not activate strong inflammatory responses that could impair the barrier
What are some mucosal immunity components ?
- Tight junctions - prevent penetration
- Mucus layer – prevents microbes getting to epithelium
- Anti-microbial peptides – damage microbes that get too close
- IgA – specialised for clumping, blocking and neutralising pathogens
Intestinal bacteria allow
detoxification, biosynthesis, immune maturation, metabolism and protection
- Commensals fill up intestinal niches
- Antibiotics kill commensal bacteria leading to pathogenic clostridium difficile infection
Appendicitis
appendix gets blocked, microbes build up, inflammation drops barrier integrity, commensals invade, increased inflammatory response, appendix burst leading to peritonitis, life threatening
Inflammatory bowel disease (IBS)
heterogenous group of disorders linked to over-enthusiastic immune responses against commensals.
- IFN-gamma, IFN-alpha and IgG (ADCC and complement) released
Regulatory cells inhibit Ag by
interacts with APC via MHC class II
- IL-10 inhibit T and b cells, macrophages, cytokines and DC
Hygiene hypothesis
reduced exposure to pathogens correlates with an increase in autoimmune and allergic diseases
which parameters increase during immune responses to commensal bacteria ?
IgG, IFN-gamma, IFN-alpha
What molecules down-regulate immune responses ?
Treg cells, IL-10
What factors need to be missing for T cell anergy ?
signal 2, B7
T cell anergy
T cell anergy is a state of functional unresponsiveness in T cells, typically induced when they encounter an antigen without the necessary co-stimulatory signals, preventing them from activating or responding to the antigen.
Paneth cells
anti-microbial peptides
M-cells
Ag transport across the epithelium
Which cytokine is primarily produced by Th1 cells in Celiac disease?
IFN-gamma
In the context of food allergies, which cytokine is primarily produced by Th2 cells?
IL-4
What is the main pathophysiological mechanism behind appendicitis?
The appendix is blocked, leading to microbial buildup, inflammation, and a life-threatening burst
What characteristics Herpesviruses ?
dsDNA genome, enveloped virus, 400-800nm
Categories of herpesvirus
alpha - neurons
beta - hematopoietic progenitors/ myeloid cells (monocytes)
gamma - lymphocytes
HSV1
2.3 global population infected, 80 ORFs, asymptomatic, cold sores
How is HSV1 transmitted ?
oral-to-oral, oral to genital, genital to newborn
How does HSV type alpha get reactivated in the neurone ?
UV, fever, fatigue
What is the balance in HSV infection?
between periodic reactivation and asymptomatic latency
How do HSV modulate the innate immune system ?
‘Dampening’ of Type 1 Interferon (IFNα/β) responses leading to the lack of the antiviral state
How do HSV modulate the adaptive immune system ?
Inhibition of MHC1 Ag presentation
What is the plasmodium parasite doing in humans ?
sporozoites -> invade liver -> erythrocytic cycle
What is the plasmodium parasite doing in mosquitos ?
gametocytes ingested and mature, fertilisation -> ookinete -> oocyst gut wall -> release of sporozoites through the salivary glands
What do infected hepatocytes release ?
type 1 IFN, signals to neighbours
What is FAS/FASL ?
death receptors that activate caspases (enzyme for apoptosis)
Hepatomegaly
enlargement of the liver through inflammation
Splenomegaly
Inflammation of the spleen
Role of spleen in managing malaria
filtration of parasitised red blood cells, immune cell axctivation, haematopoiesis (RBC production outside the bone marrow)
SMA
severe malarial anaemia, can lead to hypoxia and acidosis
How does malaria lead to less RBC?
lysis of infected RBC, removal of pRBC by spleen, suppressed erythropoiesis, removal uninfected RBC (1 pRBC for every 12 healthy RBC)
Haemozoin
brown crystalline, digestion of haemoglobin leads to free heme which is toxic so haemozoin is made, this is taken up by phagocytes of the spleen
Immunostimulatory macrophages in malaria
immunostimulatory macrophages with Hz produce large
amounts of cytokines (IL-10/IL-12,TNFa) & chemokines.
–> Hz acts through TLR9, presenting parasite DNA
removal of phagocytes lead to …
lower level of anaemia and higher parasitaemia
Plasmodium leads to … not being expressed therefore … cant be used
MHC class 1, CTL
How is malaria killed ?
oxidative bursts using superoxide, acidified phagosomes
What happens at the germinal centre ?
affinity maturation and isotype switching
What do antibodies target with the plasmodium parasite ?
(1)Sporozoites; blocking trafficking to the liver.
(2) Erythrocytic Stages; targeting merozoites & infected RBCs.
(3) Gametocytes; blocking transmission to the mosquito
Naturally acquired immunity to malaria ?
Effective in adults after uninterrupted lifelong heavy exposure.
2. Lost upon cessation of exposure.
3. Species specific.
4. Somewhat stage specific.
5. Acquired at a rate which was dependent on exposure
Erythrocytic cycle
merozoites infect leading to an intracellular trophozoite then forms merozoites and the RBC bursts
PfEMP1
a erythrocyte membrane protein 1, different variants, binds to EPCR, ICAM1 and CD36
Hypothesis of malaria immunity
mmunity is due to acquisition of a repertoire of responses to many different isolates
(and/or)
2. The development of cross-protective responses to shared antigens.
An understanding of both immune mechanisms & antigen
targets is needed to understand vaccine-mediated immunity.
Th1 cell produce … which … immune cell function so there is a …. memory response
IL-10, inhibits, lower quality
which cytokine induces fever and promotes parasite killing ?
TNF
Which cytokine regulates the response to malaria ?
IL-10
What is the primary function of the innate immune response in Herpes Simplex Virus (HSV) infection?
Induce an antiviral state in surrounding cells via IFN-alpha/beta
What immune response is primarily responsible for killing infected hepatocytes during malaria infection?
Cytotoxic T lymphocytes (CTLs) recognizing antigen on MHC I
What immune response is primarily involved in the removal of infected red blood cells during malaria infection?
Antibodies targeting merozoites and infected RBCs
Which of the following immune mechanisms is thought to play a role in malaria immunity?
Cross-protective responses to shared antigens from multiple isolates
What is the role of antibodies in malaria immunity?
They block parasite replication in the liver and target merozoites in the blood
Which immune cells are primarily involved in the activation of a Th2 response during malaria infection?
Dendritic cells
What mechanism allows Plasmodium sporozoites to evade the immune response after entering the liver?
They remain in hepatocytes, preventing immune detection
Do T cells undergo affinity maturation ?
No
What is the PRR for flagella/adhesins ?
TLR5
What enzyme is responsible for class switch recombination (CSR) and somatic hypermutation?
AID (Activation-Induced Cytidine Deaminase)
Which subset of CD4+ T cells enhances B cell class switching and affinity maturation in germinal centers?
Tfh
How does celiac disease occur ?
Th1 cells increase in IFN-gamma that increases plasma cell and macrophage activity
How does allergies occur ?
Th2 cells increase production of IL-4 that increases mast cells and IgE production
How does appendicitis form ?
appendix gets blocked due to a build up of mucus. Inflammation occurs so commensals invade lamina propria via the broken epithelium. This increases the inflammation then leads to bursting.
How does IBD form ?
chronic remitting inflammation, enthusiastic immune response to commensals
Which cytokines contribute to IBD ?
IFN-gamma and TNF-alpha
what family is herpes in ?
Herpesviridae
Is HSV enveloped ?
yes
Genome of HSV
dsDNA
Where does HSV primarily infect ?
mucosa
HSV replication occurs in …
the nucleus
The reactivation of herpes can cause …
HSK, progessive corneal scarring and opacity
How does HSV modulate the immune system ?
dampens type 1 interferon, inhibits of MHC 1
IL-12
T helper cell 1 differentiation
