the immune system Flashcards
What is are pathogens?
Organisms that cause disease
Give some examples of pathogens
Bacteria - (10^9 different species, approx. 30,000 named)
Viruses - (Small pox, rotavirus, Sars-Cov-2, etc)
Fungi
Parasites - Worms + protozoa (Malaria, elephantitis, etc)
For an effective immune response, the immune system must be able to do what?
- be able to recognise and respond to any invading organism.
- not over react to benign or self
- be able to direct different effector mechanisms against different pathogens
has to do this by linking innate and adaptive immune responses
what is specific/adaptive immunity?
- is induced by exposure to a particular infection.
- shows a high degree of specificity
- exhibits / generates ‘memory’
Name the features of specific immunity?
- meditated by lymphocytes (B/T cells, only seen in vertebrates and above)
- has clonally distributed receptors (each B/T cell has a diff. receptor making them very specific)
- large repertoire of lymphocytes - low frequency of cells specific for any antigen.
- specific immune response takes time to develop. Memory cells are produced so works better + faster the next time.
Describe the clonal selection theory
- Potentially self-reactive, immature lymphocytes are removed by clonal deletion.
- We are left with a pool of mature, naive lymphocytes. Some of these will be capable of recognising foreign antigens.
- Foreign antigen will cause activated specific lymphocyte to proliferate + differentiate to form a clone of effector cells.
- Effector cells eliminate antigen.
What is the function of antibodies?
- Importance shown in cases where absent - very sick patients
- Important for responses to bacteria
- Activation of complement cascade
- opsonisation
- classical pathway activation + MAC
- Activation of effector cells; cells that express FcR (receptor that binds ‘Fc’ region of antibody)
Draw and explain the structure of antibody
2 heavy chains, 2 light chains
N-terminus, C-terminus
- antibody is bivalent, can bind the same antigen twice
- constant regions form ‘FC’ region, interact with complement, and there are Fc receptors which are expressed on lots of cells of the innate immune system and these interact with this constant region
Name the antibody classes
lg: M, D, A, G and E (isotypes)
How many domains does each chain have?
- L chain = 2 domains
- H chain = 4 /5 domains
- each domain comprises 2 beta sheets linked by a disulphide bridge. domains are paired - folded units within the protein
What are hypervariable domains and how many are there?
- In the variable regions (binding Ags) are even more variable regions. These are called hypervariable regions
- There are 3 hypervariable regions in every light and heavy chain variable domain (so 12 on an antibody)
What are epitopes? How are antigens recognised?
- the part of an antigen recognised by antibodies
- they can be continuous or conformational
- CDRs in antibody V regions determine the specificity + the affinity of an antibody for an Ag
Describe a T cell receptor
- Heterodimer of ‘a’ and ‘B’ chain
- Each chain has a V and a C region
- Domains (x4) are lg-like
- V domains interact with Ag = peptide bound to MHC molecule
- Each chain contributes 3 CDRs to Ag binding (6)
What are the different types of MHC molecules? What are their structures and what are they recognised by? What are their associated HLAs, and what are each of the MHCs associated with?
- Class 1 - expressed by nearly all the cell types in the body. Structure: a1, a2, a3, B2-
- Class 2 - expression restricted to a specialised group of immune cells (APC). Structure: a1, a2, B1, B2
- Related but different structures + expression patterns
- Many alleles (polymorphic)
- MHC class 1 = HLA -A, HLA -B, HLA -C, CD8+ T cells
- MHC class 2 = HLA -DP, HLA -DQ, HLA -DR, CD4+ T cells
Any cell can be killed by a CD8+ T cell
How many antibodies can we make, from how many genes?
- Potential to make >10^9 antibodies (<3x10^4 genes in human genome)
- A range of mechanisms allows so many antibodies to be generated from so few genes
What gene segments form the different chains of BCR and TCR?
- Heavy chain of antibody/BCR and ‘B’ chain of TCR:
- 3 gene segments (V, D and J) which come together to encode the variable region of heavy chain / ‘b’ chain - Light chain and TCR ‘a’:
- V region encoded by 2 gene segments (V and J)
What do V D and J stand for? Which is the biggest?
V = Variable
D = Diversity
J = Joining
- V is the biggest
What happens during the rearrangement of antibody genes?
- In B cells (as they are developing in the bone marrow), the DNA containing the lg gene segments is deliberately broken and the gene segments are rearranged (joined together) to form functional lg genes. This is called non-homologous end joining (NHEJ) recombination
- Only B cells, T cells, sperm and egg cells do this - Each B cell performs this breakage and rearrangement randomly
Describe light chain recombination
- After DNA breaks, a single V and a single J gene segment are joined together to encode the variable region of the light chain
- There are 2 loci containing L chain genes ‘lambda’ and ‘kappa’
Describe heavy chain recombination
- Similarly, a single random V, D and J segment are joined together in a single B cell to encode the variable region of the heavy chain
- Breaks D and J first and brings them together, then brings that to a V segment
In what order do gene segments rearrange? What is the loci if the different chains?
- First H chain gene segments rearrange (D-J, then V-DJ)
- greater variability in H chain as V, D, J segments - Then light chain gene segments rearrange: K segments (V-J) first, and if this is unsuccessful, then ‘lambda’ gene segments (V-J) rearrange
- Consequently there are multiple V, D and J gene segments encoded at 2 different loci:
- H chain locus @ chromosome 14- Kappa chain locus @ chromosome 2
- Lambda chain locus @ chromosome 22
What are recombination signal sequences? What does recombinase do and what do RAG genes do?
- recombination signal sequences are bits of DNA recognised to guide Ig gene segment rearrangement
- recombinase recognises the sequence and cuts the DNA at the right places to bring these segments together
- RAG-1 and RAG-2 genes encode lymphoid specific components of the recombinase. mutations in RAG genes results in immunodeficiency.
What is a B cell receptor expressed by? What does it recognise?
- expressed by B lymphocytes
- it is the membrane bound form of an antibody or immunoglobulin
- recognises virtually anything that it interacts with (proteins, DNA, lipids, sugars)
What is the difference between a B cell receptor and an antibody?
- a B cell receptor/membrane Ig is anchored to the membrane, Ig/antibody is when the receptor has been released.
- when the B cell is activated, the BCR is secreted and known as an antibody
What is a T cell expressed by? What does it recognise and where is it attached?
- expressed by T cells
- membrane anchored, so it is never secreted, always on the surface of T cells
- only recognises short peptide fragments, and these have to be presented to the TCR by molecules encoded within the MHC
What are the structural differences between BCRs and TCRs?
- TCR smaller than BCR if comparing the domains of the receptor
- TCR has alpha and beta chains rather than heavy and light chains
What are isotypes and what are they determined by?
- isotypes are different antibody classes
- they are determined by the heavy chain/C region, and they differ in structure and function
What is meant by allelic exclusion? And light chain isotype exclusion? What does this ensure?
- in each B cell, only one rearranged H chain gene from one chromosome is expressed. similarly only one light chain is expressed by each B cell.
- light chain isotype exclusion - each B cell expresses either a rearranged kappa or lambda; never both.
- this ensures that each B cell produces just one BCR/antibody that is different from those made by every other B cell
What are the 5 mechanisms for generating antibody diversity?
- There are multiple gene segments for each chain
- Combinatorial diversity
- Combinations of heavy and light chains
- Junctional diversity
- Somatic hypermutations
Give examples of junctional diversity
- imprecise joining (small differences in sequences where V-D and D-J segments join)
- N regions (random additions of nucleotides at junctions of V-D and D-J by terminal transferase
Where does SHM occur? What is it performed by and what does this do?
- occurs in germinal centres (GC) as B cells recognise and proliferate/become activated
- performed by enzyme activation-induced deaminase (AID)
- AID acts on DNA to deaminate cytosine to uracil
- uracil is then recognised by error-prone DNA repair pathways leading to mutations
Which antibody is the first to be expressed by a developing B cell? Why?
IgM, because at the H chain locus, Cmu is physically closest to the V, D, and J gene segments
Which region is next to Cmu and can be co-expressed with IgM by differential processing of the RNA from the 2 C region genes?
Cdelta
What is class switching? What does it involve?
- When a B cell production of immunoglobulin from one type to another, by changing the heavy chain.
- Requires further DNA recombination guided by switch regions - this process involves AID.
- pathogen -> cytokine -> switch (after B cell stimulated by antigen)
When do the gene segments of TCR genes rearrange?
During T cell development in the thymus
How is diversity in TCR generated?
- Similar mechanisms to BCR: multiple VDJ segments, combinatorial diversity between VDJ, junctional diversity.
- no SHM occurs in TCR genes
Gene segments and chromosome of TCRa?
V, J, C
Chromosome 14
Gene segments and chromosome of TCRB?
V, D, J, C
Chromosome 7
Gene segments and chromosome of Ig (heavy)?
V, D, J, C
Chromosome 14
Gene segments and chromosome of Ig (light)?
V, J, C
Chromosome 2 (kappa) or chromosome 22 (lambda)
Where are all MHC molecules located in humans?
on chromosome 6
What are the advantages and disadvantages for MHC molecules being so polymorphic?
- allows the binding of a vast range of peptides that can be presented to T cells - this provides a clear evolutionary advantage to the population as we can respond to an almost unlimited number of pathogens
- however increases the risk of immune-mediated disease (increases likelihood of presenting self antigens)
- reduces pool of available donor organs for transplantation (as MHC alleles should match for best outcome)
Which proteins are usually presented by class 1 MHC molecules?
peptides derived from protein antigens synthesised inside a cell (endogenous) (virus)
Which proteins are usually presented by class 2 MHC molecules?
peptides derived from protein antigens taken up from outside a cell (exogenous) (bacterium)
How are antigens presented by MHC class 1 molecules?
- Antigen processing to peptides in proteosome
- Peptide transport into ER by TAP transporter
- Peptide binding by MHC class 1
- MHC class 1 presents peptide at cell surface
What is TAP? What does it include?
a component of a multi-protein assembly, the peptide loading complex - includes tapasin and calreticulin
How are antigens presented by MHC class 2 molecules?
- Antigens (e.g. bacteria) are endocytosed into intracellular vesicles inside the cell
- Protein cleaved to peptides by acid proteases in vesicles
- Vesicles fuse with vesicles containing MHC-II molecules
- Peptides bind MHC-II molecules
- MHC-II/peptide complex then transported inside vesicles to cell surface.
-if MHC-II synthesised in the ER, MHC-II molecules bind to invariant chain (prevents peptides binding in the groove)
-in endocytic pathway, lysosomal enzymes degrade this, leaving CLIP peptide associated with the binding groove.
-peptides from antigen displace CLIP when they bind
-HLA-DM, a class-II like molecule, is required for loading of peptides into the groove
What are accessory molecules? Give an example for each MHC class.
involved in antigen processing
MHC-I: TAP and LMP
MHC-II: HLA-DM
What can be killed by cytotoxic CD8+ T cells?
as all nucleated cells express MHC-I molecules, any cell infected with a virus can present viral peptides on MHC-I molecules and be recognised and killed
What can activate CD4+ helper T cells?
relatively few, specialised cell types can express MHC-II molecules (often APC) which take up and present extracellular Ag to activate helped CD4+ T cells
What do B cells develop from?
Haematopoietic stem cells in bone marrow that express PAX5 transcription factor
What does B cell development involve?
rearrangement and expression of Ig. genes
expression of lymphocyte, then B cell-specific, markers, then CD19
removal of self-reactive cells (negative selection in attempt to avoid autoimmunity)
What are surrogate light chains?
Light chain-like proteins, product of VpreB and lambda 5 genes
What are pre-B cell receptors made up of?
Antibody heavy chains (mu chain) with signalling molecules Iga and IgB, and expressed with surrogate light chain
What are immature B cells?
B cells in bone marrow which have put together a heavy chain and then a light chain
How is IgM BCR produced?*
H chains rearrange, if it looks ‘ok’ then a signal is sent to the B cell.
The light chains then rearrange, and displace surrogate light chains -> IgM
What does a signal from pre-BCR do?
- turns off RAG-1, RAG-2 genes
- 5/6 rounds of cell division
- surrogate light chain expression stops
- RAG-1 and RAG-2 turned on again
- L chain rearrangement starts
(RAG genes needed for gene rearrangement)
What happens if a cell fails to productively rearrange both H and L genes?
It dies
Following successful H chain rearrangements, pre-B cells that initially fail to generate non-productive re-arrangements of light chain kappa genes can be ‘rescued’ by up to 10 further rearrangements at the same locus. What happed if it is still out of frame after all these attempts?
The lambda locus will begin to rearrange
Immature B cells can only express what?
IgM (membrane) only
What do immature B cells that bind multivalent self-antigens undergo?
Either clonal deletion (cell dies by apoptosis) or receptor editing (further light chain gene rearrangements of variable regions - get another chance)
What do immature B cells that bind soluble self-antigens do?
Cell becomes unresponsive (anergic)
How are T cells developed?
Initially, similar to that of B cells:
-originate from bone marrow stem cells, rearrange receptor genes once in thymus, express pre-T receptor, elimination of self-reactive T cells by negative selection
But, unlike B cells:
-undergo development/selection in the thymus
-rearrange alpha/beta TCR genes (CD4+ or CD8+), or gamma/delta TCR genes
-T cells expressing a/b TCR must bind with self MHC expressed in thymus
Describe a/B T cell development
Precursors produced in bone marrow, migrate to thymus. Once in the thymus, develop into thymocytes:
1. Rearrange TCR genes (B first) and express TCR
2. Acquire other markers e.g. CD3, CD4, CD8
3. Undergo positive and negative selection, while still in the thymus
Describe the structures of the thymus and the cells found within it
-bi-lobed organ in anterior mediastinum
-each lobe divided into many lobules
-each lobule has outer cortex and inner medulla
-cells: lymphoid cells, epithelial cells, macrophages and dendritic cells
How does T cell maturation occur in the thymus?
Pro-thymocytes enter the cortex from the bone marrow, then:
-rearrange TCRB genes
-expressed along with a pre-T cell receptor
-cells proliferate and rearrange TCRa genes
Initially, T cells have CD4+ AND CD8+ on their surface as well as CD3+ when in the thymus, however once out of the thymus they express CD4+ OR CD8+ (one or the other)
What is the CD3+ complex? What is it made up if and what does it do?
Required for TCR expression
Subunits:
-CD3ε (CD3 epsilon), CD3γ (CD3 gamma), CD3δ (CD3 delta) - form a heterodimeric structure
-CD3ζ (CD3 zeta) - form a homodimer
It is a signalling machinery that allows the T cells to deliver a signal to turn on genes if it is recognising an antigen (on MHC molecule)
How does γ/δ TCR differ from a/B TCR?
-Similar structure but don’t express CD4 or CD8 markers
-Less diverse
-Expressed on a different T cell population - 1-5% in circulation, majority in epithelial tissues at mucosal surfaces
-Recognise different antigens
Lineage commitment to a/B or γ/δ depends upon which genes are first to rearrange successfully
What are the 3 things that T cells expressing randomly rearranged a/B TCRs may do? Which do we want to eliminate?
- Recognise self-MHC and peptide from foreign antigen (immunity)
- Recognise self-MHC and peptide from self antigen (autoimmunity)
- Not be able to recognise self-MHC (as all cells express self-MHC, these are useless)
Need to keep T cells with TCR1, and eliminate those with TCR2 and 3 through positive and negative selection
Describe the positive selection of T cells
Occurs when double positive (CD4+ CD8+) T cells recognise MHC on cortical epithelial cells in thymus
-Apoptosis if can’t recognise self-MHC
Only 1-2% capable of recognising self-MHC
Positively selected cells move into the medulla
Where does positive and negative selection of T cells occur?
Positive selection occurs first in the cortex, and then negative selection occurs in the medulla
Describe the negative selection of T cells
Macrophages and dendritic cells acting on the T cells present self-peptides on their MHC
We don’t want to keep cells that are really specific for their self-peptides
-TCR binding to MHC/self peptide with high affinity causes T cell to die by apoptosis (clonal deletion)
What is the ultimate goal of positive and negative selection of T cells?
-A population of T cells with a low affinity for self-peptide and self-MHC (those with the highest affinity TCR are negatively selected)
-These cells represent the safest/most useful to keep with the highest probability that they will have a high affinity for self-MHC when presenting peptides derived from pathogens
What happens to the T cells that survive thymic selection?
-Express TCRs that are capable of binding self-MHC
-Are depleted of self-reactive cells
-Exit the thymus as mature, single positive T cells
What happens during T cell mediated immunity, after the T cells have left the thymus?
-Naïve T cells (CD4+ or CD8+, that have survived positive and negative selection) recirculate via blood/lymphatics through secondary lymphoid tissue.
-If they encounter antigens in these areas, then clonal proliferation and differentiation occurs
-Naïve T cells will either become effector T cells or memory cells
What are the two types of effector T cells and what do each do?
Cytotoxic effector T cells (CD8+) - kill infected cells
Helper effector T cells (CD4+) - secrete cytokines
What happens in lymphoid tissue?
-T cells recognise Ag/MHC on APCs
-An array of APCs are found, some are specialised, and present antigens: lymph nodes, spleen
-Following activation, T cells effectors leave these areas and migrate to sites of infection
What must occur to naïve T cells in order for them to survive? What happens if this doesn’t happen?
-Naïve T cells must encounter antigen for survival
-They enter lymph node from blood via high endothelial venules (HEV)
-Move into the T cell area, which is rich in dendritic cells and macrophages (APCs)
-APCs present antigens and deliver other activation signals (e.g., cytokines)
-T cells that are not activated (i.e. don’t see peptide/MHC) leave the lymph node and re-enter circulation, recycled for another day
-They don’t have a very long lifespan
What are cell adhesion molecules (CAMs)? Give some examples of CAMs and some examples of some interactions.
Molecules that hold the cells on to the APCs, mediate cell-cell interaction, e.g., LFA-1/ICAM-1
-Naïve T cell + HEV
-T cell + APC
-Effector T cell + target cell
What happens once T cells are in contact with APCs?
- T cells contact APCs using CAMs
- TCR scans APC peptide/MHC complexes
-No recognition –> disengages
-Recognition –> signal from TCR complex –> T cell-mediated response
-T cells require multiple signals to become activated
What is co-stimulation? Talk through and describe the 3 signals required to activate T cells.
- Naïve T cells receive a signal from TCR contacting MHC/peptide on APC (signal 1) - involves CD3 zeta chain.
- For signal 2, APCs also express ‘co-stimulatory molecules’ (B7.1/2). These bind CD28 expressed by naïve T cells and deliver signal 2 to the T cell.
- APCs also release cytokines, which bind cytokine receptors that are now up-regulated on naïve T cells, which deliver signal 3.
Essentially: 1. TCR, 2. co-stim, 3. cytokine
-Once effector T cell has been generated, cell only needs 1 signal.
What is the importance of CTLA-4?
CTLA-4 mutations are often associated with several autoimmune diseases, like type 1 diabetes.
Treatment of cancer patients with anti-CTLA-4 can enhance the immune response to the tumour (but can induce autoimmune reactions)
What happens once a T cell has been activated by the 3 signals? What does it express and what do these do?
Activated T cells no proliferate and express ICOS and CTLA-4.
-ICOS - related to CD28, binds ICOSL on APC to induce cytokine secretion by T cells
-CTLA-4 - highly related to CD28, shows stronger binding to B7.1/2 than CD28
These basically stop the cells from becoming activated:
-Binding of CTLA-4 to B7.1/2 on APC delivers a negative signal to the activated T cell
-It is an antagonist to CD28 and limits the T cell response
What is the purpose of signal 3 (cytokines)?
Dictates the differentiation of activated CD4 cells into different subsets of effector cells
Describe the activations of APCs.
APCs express receptors for microbial molecules.
Binding these pathogen-associated molecules activates APC, this is known as the danger signal.
This leads to APC upregulation of MHC and co-stimulatory molecules.
Ensures that signal 2 to activate T cell-mediated response only occurs during infection.
Thus, the innate response activates APCs, that then induce activation of acquired response.
What do APCs express? What are the different types and what does each do?
Express MHC-II molecules.
1. Dendritic cells (only function is to present antigens) - crucial for activation of naïve T cells.
2. Macrophages and B cells - present antigen to receive help from effector T cells
Characteristics of dendritic cells.
-Bone marrow derived, immature form found in epithelia
-Don’t express co-stimulatory molecules until activated/matured
-Migrate to lymph node following ‘danger signal’ activation
-DC MHC-1 and II will then be loaded with peptides from pathogens they encountered in peripheral tissues.
-Their levels of co-stimulatory molecules will be very high, and they will express high levels of adhesion molecules, hence why they are very efficient activators of naïve T cells.
What is meant by cross presentation?
Some specialised dendritic cells (DC1) take up and process exogenous antigens and present them via MHC-I molecules.
This allows these DC to activate naïve CD8+ T cells.
This means these CD8 effector cells (which don’t need co-stimulating) can kill infected cells that are not APC.
Characteristics and functions of macrophages
-Function as scavengers/killers of pathogens but also important APC for extracellular pathogens
-Highly phagocytic
-Express MHC-II and B7 which increase following help from T cell.
-Once activated by T cells, secrete many inflammatory cytokines.
Characteristics and functions of B cells
-Very poor at phagocytosis
-Internalise soluble antigen for processing and presentation by B cell receptors
-Antigen binding to BCR upregulates B7 –> B cells are capable of providing signal 2 to activate T cells.
-Have recently been shown to use BCR to physically extract antigens from other cells
What is interleukin-2 (IL-2)? What is its function What happens when it binds to T cells?
-A key cytokine for T cell survival.
-It is a potent autocrine T cell growth factor, but naïve T cells express a low affinity form of the IL-2R..
-However, once T cells become activated, they express a high affinity IL-2R, and secrete IL-2.
-Thus, IL-2 binding to IL-2R on activated T cells –> lots of T cell proliferation.
-This: allows rapid division of T cells, expands the population of antigen specific activated T cells, is a target of immunosuppressive drugs
Characteristics of CD8+ cells
Acquire cytotoxic activity, kill cells expressing MHC-I complexes
Characteristics of CD4+ cells
Function by secreting cytokines, help recognise MHC-II complexes
What does signal 2 of B cells depend on? How do they differ
The type of antigen they bind:
-Thymus-independent (TI) - signal 2 is provided by the antigen itself or extensive cross-linking of BCR.
-Thymus-dependent (TD) - signal 2 is provided by CD4+ T cells.
Describe signal 1 in B cells
Binding of non-self antigen in secondary lymphoid tissues provides signal 1 to B cell.
BCR-associated polypeptides involved in signalling (Iga and IgB) cause the signal to go into the nucleus.
Kinases are activated –> TF and gene expression
Ways in which signal 1 in B cells can be increased of enhanced.
-If antigen has activated complement cascade
-Lots of C3b
-Complement receptor 2 (CR2) on B cell surface (CD21)
-CR2/CD19/CD81 form the BCR co-receptor complex
-Augments the signal
What does a thymus independent (TI) antigen do?
Leads to antibody production (only IgM) with no requirement for T cell involvement. Essentially induce antibodies in the absence of T cells.
What are the two types of thymus independent antigens? What is the difference?
TI-1 antigen:
-As well as binding to BCR, TI-1 antigens also bind to other receptors on all B cells that provide signal 2.
-In high concentrations, these antigens will act as polyclonal activators (mitogens) for B cells (will activate as many B cells, irrespective of their different BCR)
TI-2 antigen:
-Contain repeated epitopes and so will cross-link many BCR molecules on same B cell surface.
-Take longer (more antigen required) to induce B cell activation (antibody responses in TI-2 antigens typically don’t develop until 25 years in humans)
What are thymus dependent (TD) antigens?
Antibodies to TD antigens require the presence of CD4+ T cells (i.e. are absent in the absence of a thymus)
Antibody responses seen to TD antigens are much better than those to TI antigens (they are classic acquired response)
-T cells activated by MHC, BCR binds antigen, signal 1
-B cell internalises antigen, processes and presents antigen to CD4+ T cells, receives signal 2 via CD40/CD40-L interaction
-Cytokines secreted by T cell (help B cell to class switch) so all antibody classes can be produced to TD antigen.
-Thus, B cells act as APC for TD antigen.
-Epitopes recognised by antibody and T cell must be physically linked, either from different parts of the same molecule, or from different molecules of a complex.
What is a conjugate vaccine? Give some examples.
-Coupling a capsular polysaccharide (TI Ag) to a protein (TD Ag) to produce a protective response.
-Examples include Haemophilus influenzae type B, Men C, pneumococcal conjugate vaccine
Describe the activation of B cells with TD antigen
-Conjugates of B lymphoblasts and T cells move to primary follicles (B cell areas)
-Form germinal centres (GC) within a B cell follicle in secondary lymphoid tissues
-B cells divide rapidly to become centroblasts and undergo: SHM of Ig genes, isotype switching
-Differentiate into non-dividing centrocytes (smaller)
What is a germinal centre?
lots of proliferating B cells
What can happen to a B cell once in a germinal centre?
- Differentiate into plasma cells
-secrete various isotypes
-high affinity antibody, somatically mutated - Form long-lived memory cells
-recirculate - Die within lymphoid tissue
-happens if BCR no longer binds antigen (as a result of unsuccessful V region created or somatic mutation)
What happens during somatic hypermutation (SHM)?
-Randomly mutates the DNA at the V,D, J regions
-Mediated by AID enzyme, which is activated by CD40, as well as DNA repair genes
What are follicular dendritic cells?
-Also present in germinal centres
-Not the bone-marrow derived dendritic APC as we know
-They are cells in the primary follicle that capture intact antigens for centrocytes to bind via BCR. They capture antigen via FcR and CR, which can bind antigens and display antigens to the B cells
Follicular dendritic cells oversee B cell affinity maturation. What is meant by this?
-Centrocytes that have undergone SHM present mutated BCR on their surface.
-So centrocytes compete with each other for antigen on FDC, and for signals from TFH cell.
-If the mutated BCR binds to the antigen on the FDC better than the unmutated BCR, it will present more efficiently and receive CD40 signal from TFH cell. Failure = apoptosis or recycle to dark zone.
-Those centrocytes that have generated higher affinity BCR survive to differentiate into plasma cells
What are follicular T helper cells (TFH)?
-CD4+ TH subset found predominantly in the B cell follicles of the lymph node
-Specialised to provide help to B cells
-Secrete either TH1 or TH2 type cytokines
What is the role of CD40 expressed by B cells?
-CD40 signal via CD40-L expressed on TFH
-Protects centrocytes from apoptosis. Induces isotype switching (different cytokines induce different isotypes to be produced)
Which antigens produce which isotypes?
IgM (TI) –> polysaccharides
IgG (TD) –> proteins
IgA –> antigen at mucosal surfaces
IgE –> some antigens
Why is tolerance of the immune response required?
-Needed as due to the random generation of repertoire of BCR and TCR, many self-reactive specificities will be produced
-If no tolerance, autoreactivity would lead to serious pathology
What is AIRE (autoimmune regulator protein)? What does it do? What would people with a deficiency experience?
Transcription factor
Plays a key role in tolerance induction
Allows the expression of many tissue-specific antigens in the thymus, hence the negative selection of T cells that recognise these antigens
Patients with AIRE deficiency have major autoimmune syndromes
What is meant by immunological ignorance?
Antigens are not presented at sufficient levels to activate T cells
What is meant by tolerance via anergy?
-Lymphocytes that recognise self antigen can also become unresponsive i.e. anergic
-Immature B cells - when receptors encounter antigen that is not multivalent and leave the bone marrow as anergic
-T cells can also be made anergic
What is meant by the term ‘privileged sites’?
Antigen is sequestered from the immune system. E.g., eye, testis, CNS (barriers in place)
What is a reason why B cell tolerance is much more forgiving than T cell tolerance?
Many B cell responses are T cell dependent. If antigen specific T cells are absent, no help is available and so there is no antibody response.
What are regulatory T cells (Treg)? Where do they arise? What does a deficiency cause?
-A CD4+ T cell subset that suppress immune responses by turning off other T cells once activated.
-Arise in thymus from T cells with high affinity receptors for self antigen.
-(n(atural)Treg, iTreg (induced in preiphery))
-Deficiency –> severe autoimmune syndrome (IPEX)
What is Breg/10? What does it prevent?
B cells that secrete IL-10, crucial in preventing autoimmunity
3 reasons why regulation is necessary?
- Ensures responses continue only for as long as they are needed.
- Minimises collateral (tissue) damage
- Ensures responses are qualitatively appropriate (right for the specific pathogen)
Name 5 types of CD4+ T cells
TH1, TH2, TH17, Treg, TFH
What is the function of Cd4+ TH1 cells?
Secretes IL-10
Activate macrophages, make them better either by the secretion of cytokines or by expressing CD40L which binds to CD40 on the macrophage, allowing the macrophage to upregulate CD40.
CD4+ TH1 can kill chronically-infected macrophages
What is the function and characteristics of CD4+ TH17 cells?
Secretes IL-17
Recruits neutrophils early in (fungal) infection
Also important in autoimmune disease
Probably the oldest of T cells
What is the function and characteristics of CD4+ Treg cells?
-Not a single population, it is a mixed bag of cells: CD4+, CD25+, some CD8+ cells
-Arise in the thymus (nTreg) of from circulating T cells in the peripheral tissues (iTreg)
-Act to suppress other T cell responses
-Secrete suppressive cytokines (TGF-B, IL-10)
What does IL-10 do?
Inhibits APC function
What determines whether a TH1 or TH2 response occurs?
The cytokines that are present when T cells are activated (signal 3)
-IL-12 and IFN-gamma = TH1
IL-4 = TH2
Why is polarisation of CD4+ T cell responses important?
-Ensures correct responses for different types of pathogens
-Can go wrong, may lead to allergy (excess of TH2)
-Control of pregnancy/autoreactivity
Different infections require difference effector mechanisms. What is it dependent on?
-Type of pathogen
-Localisation i.e. site
-Challenge
-Stage of infection
Give examples of some gram positive bacteria
staphylococcus aureus, streptococcus spp
Give examples of some gram negative bacteria
campylobacter, salmonella, shigella, haemophilus, neissera
How can components of cell walls in bacteria induce innate responses?
Bind to toll-like receptors on macrophages, receptors recognise distinct molecular patterns on microbes
NOD-like receptors in cytoplasm
What can the binding of pathogen-associated molecular proteins (PAMPs) to TLR do?
-Promote inflammation
-Promote dendritic cell maturation
-Influence differentiation of T cells
-Activate B cells (TI-1 Ags)
What is the role of antibodies in bacterial infection?
- Opsonisation
-Bind Fc receptors on phagocytes - Complement activation
-Promote inflammation via C3a, C5a
-Opsonise by binding C3b receptors on phagocytes
-Lysis of gram negative organisms - Bind to and neutralise toxins
- Bind to surface structures to prevent mucosal adherence
How can gram negative bacteria be killed by complement lysis?
Defects in terminal complement components can lead to susceptibility to certain bacteria.
MAC isn’t most important component, other defects have more widespread effects
Bacterial cell division –> vulnerable –> death
What is the outcome of a TH1 response of tuberculoid leprosy?
Strong TH1 response, few live bacteria, slow progression, granuloma formation
What is the outcome of a TH2 response of lepromatous leprosy?
Strong TH2 and antibody response, large number of bacteria in macrophages, disseminated infection, fatal
Are antibodies important against extracellular or intracellular mechanisms? Which are T cell effector mechanisms important for?
Antibodies important in protecting against extracellular pathogens, T cell effector mechanisms protect against intracellular organisms
What are viruses?
Obligate intracellular parasite, but may be extracellular at some stage in infection
What are the 2 innate defences against viruses?
Interferons and natural killer cells
What are the therapeutic uses of interferons?
rIFNa can be used to treat hep B and C
some cancers
side effects can be very severe
Function and characteristics of type 1 interferons
Synthesis of IFNa and IFNB induced in virus infected cells. Early response to infection.
Function and characteristics of type 2 interferons
IFN gamma secreted by activated T cells and NK cells. Inhibits TH2 response (antibodies) and promotes TH1 (cell killing).
Recruits macrophages
What are natural killer cells? Name some of their functions and characteristics
-Type of innate lymphoid cells
-Large granular lymphocytes
-Recognise structures on viral infected cells
-Can recognise stressed cells in absence of Igs and MHC
-Kill by extracellular mechanism - perforin and granzyme
-Fast
What must NK cells do?
Need to distinguish between infected and uninfected host cells otherwise it would be a disaster.
1. Activating receptors - recognise carb ligands, triggers killing.
2. Inhibitory receptors - recognise MHC-I molecules (no binding, only TCRs can do this)
Viruses that reduce MHC expression make cells more susceptible to NK killing.
How do CD8+ cells kill?
2 mechanisms, both induce apoptosis.
1. Secretion of cytotoxic granules
-perforin, polymerises in membrane
-granzymes (proteases enter cell)
2. Fas ligand on T cell interacts with Fas on target = death.
What are the functions of IFNγ?
-Inhibits viral replication
-Upregulates MHC-I and II expression and antigen presentation
-Increases macrophage phagocytosis of dead cells
-Promotes NK cell killing activity
What are the functions of antibodies?
-Neutralise free virus (prevent entry into host cell)
-Opsonise to increase phagocytosis
-Activate complement leading to lysis (enveloped viruses)
How does HIV work?
-Attacks specific immune system
-Targets CD4+ cell, macrophages and dendritic cells
-Progressive development of AIDS leads to opportunistic infections
Why don’t we have a vaccine for HIV? How else may we control infection?
-Antibodies to HIV don’t seem to protect
-Infection may be controlled by CD8+ T cell responses: patients with higher levels of CD8+ activity show slower disease progression, virus mutations that escape CD8+ recognition may lead to AIDS progression
What response may some helminths induce?
A strong IgE antibody response:
-Mast cell mediated inflammation
-Eosinophil ADCC
When is cell-mediated immunity important?
-Important for protozoa that can survive in macrophages hidden from Igs
-Cytokines important in inducing macrophage activation
How can passive immunity be provided to individuals with hypogammaglobulinemia?
Through IV IgG therapy every 2-4 weeks to maintain protective levels
How does active immunity differ from passive immunity?
-Exploits ‘immunological memory’
-Faster to develop
-Greater in magnitude
-May be qualitatively better (e.g., higher affinity antibody)
What is meant by herd immunity? Give an example
Disease can decline if the majority of the population are immune.
-MMR vaccine, 83-94% need to be immune to prevent outbreaks
At what point is a disease considered to be ‘eliminated’?
When it has stopped freely circulating for at least 3 years
Name some requirements of an effective vaccine.
-Safe
-High level of protection, and long-lasting
-Low cost
-Stable
-Minimal side effects
-Easy to administer
-Right type of response (local or systemic, antibody or cell-mediated immunity)
What is the main purpose of a vaccine?
Induce a protective response to a pathogen without causing disease
What are the 5 main types of of vaccine (prior to Nov. 2020)
-Inactivated (dead) organisms
-Attenuated (live but virulence disabled)
-Subunit vaccines - protein fragments
-Toxoid - bacterial toxin
-Conjugate - something with low antigenic property covalently bound to something with high antigenic property
Example and characteristics of dead/inactivated organisms vaccine
-e.g., Salk polio vaccine (parenteral injection)
-Important antigens must survive killing
-Mmay have side effects
-Being replaced by new vaccines
Example, pros, and cons of live/attenuated organisms vaccine
e.g., smallpox, MMR, Sabin pollo
Pros:
-Single dose effective
-May be given by natural route
-May induce local and systemic immunity
-May induce right type of response
Cons:
-Reversion to virulence
-Possibility of contamination
-Susceptible to contamination
-Causes disease in immunocompromised host
Example and characteristics of subunit vaccine
-Hep B - surface antigen of the virus
-Use isolated antigens from pathogen
-May get non-responders - genetic, general health, etc
Examples and characteristics of conjugate vaccine
-HiB, MenC
-Converts TI-2 polysaccharide antigen to a TD form
-Young children can respond
What is an adjuvant?
A substance administered with an antigen to promote the immune response.
Pure antigens often only elicit weak immune responses, adjuvants enhance immune responses by providing a ‘depot’ or by immunostimulatory properties
What are the different ways in which adjuvants can act?
-Activate dendritic cells via TLRs or NLRs
-Cause release of endogenous danger signals
-Promote antigen uptake by dendritic cells
-Stimulate release of chemokines/cytokines
-Promote ‘cross-presentation’ of exogenous antigens by class 1
What is most commonly used in human adjuvants?
Aluminium hydroxide or aluminium phosphate
What is the difference between serum and antiserum?
Serum is plasma once blood clots have been removed, serum from an immunised person is antiserum
What are the limitations of using antiserum?
-Would be better if each individual antibody could be separated
-Once the antiserum has been used, another individual will need immunising and the antibodies generated will never be exactly the same
What are the two types of epitopes?
Linear (continuous) and non-linear (discontinuous)
Which enzymes cleave the DNA at recombination signal sequences?
RAG-1 and RAG-2 complexes
Where are NOD-like receptors found?
Cytoplasm
