Antigen Recognition by T Cells Flashcards
What is an antigen?
- a combo of ‘antibody’ and ‘generate’
- any molecule that can bind specifically to an antibody
- ie generate antibody responses
- proteins, carbohydrates, lipids capable of binding to B-cell receptors, T-cell receptors and/or innate immune receptors
- induce immune response
What is the epitope?
- adaptive immune rxns occur to specific epitopes (portions of antigen) as opposed to the entire antigen itself
- infection + vaccination usually induce polyclonal T- and B-cell responses
What is another term for MHC?
HLA
What are “professional” antigen-presenting cells?
- immune cells that can express high levels of MHC class II
- can efficiently induce T-cell responses
- macrophages, dendritic cells, B cells
- monocytes less so, but differentiate into macrophages
What are the roles of macrophages and dendritic cells and how do they differ (what are they each better at)?
- rare in peripheral blood - enriched in mucosal tissues
- highly phagocytic cells - induce strong T-cell responses + inflammation
- important for protection against TB
- macrophages better-equipped to kill pathogens (higher NO production)
- DCs better at migrating to lymph nodes (via CCR7) + presenting antigen to T-cells
- specialised but ultimately overlapping functions
What is the role of B-cells?
- highly abundant in blood + mucosal tissues
- receptor-mediated internalisation of antigens, as opposed to phagocytosis
- primary function to make antibody (plasma cell) - but still v good at presentation
- possibly main inducer of T-cell immune response to pathogens such as Neisseria meningitidis
Will T-cells recognise just any antigen?
No, the antigen must be processed in order to be recognised by T cells - cell surface peptides of Ag presented by cells that express MHC antigens
The following will not induce a T cell response:
- soluble native Ag
- cell surface native Ag
- soluble peptides of Ag
- cell surface peptides of Ag
What evidence suggests antigen-processing is a catabolic process?
- catabolism reduces antigens to peptides that can be recognised by T cells
- whereas native ovalbumin (antigen) cannot induce a T cell response to a fixed APC
- digested ovalbumin (antigen) CAN induce a T cell response
- showing importance of peptides binding to MHC on APC surface in order to induce T cell
What is the difference between exogenous and endogenous antigen processing?
- Exogenous - antigen uptaken from outside cell eg. extracellular bacteria (S. aureus, N. men.)
- Endogenous - antigen from inside cell eg. viruses (HIV, tumour antigens)
What processes + receptors are important for uptake of exogenous antigens?
- membrane Ig receptor mediated uptake
- phagocytosis
- pinocytosis
- complement receptor mediated phagocytosis
- Fc receptor mediated phagocytosis
Uptake mechanisms direct antigen into intracellular vesicles for exogenous antigen processing
Describe the mechanism/cellular pathway of uptake of exogenous antigens and how they are expressed on MHC class II
- exogenous protein internalised into early endosome
- exposed to proteases
- if it’s a bug - exposed to NO, low pH, reactive oxygen species to kill bug
- then early endosomes become late endosomes or “lysosomes”
- these are increasingly toxic (lower pH, stronger enzymes)
- this fuses with MHC Class II compartment (MIIC)
- meanwhile MHC class II exported from ER + joins MIIC
- invariant chain sits where antigen would be - helps to stabilise MHC
- enzymes also start to degrade invariant chain - leaves behind CLIP (class II associated invariant chain peptide)
- MHC recognises antigen -> high-affinity rxn displaces CLIP, CLIP degraded
- HLA-DM helps to shuffle loaded MHC to surface of cell
- MHC can then interact with TCR on CD4+ cell !! :)
What are cathepsins?
- proteases / enzymes that degrade proteins
- found in the endosomes / lysosomes
Describe the mechanism/cellular pathway for endogenous antigen processing and presentation of MHC class I
- peptide antigens prod in cytoplasm are physically separated from newly formed MHC class I
- peptides need access to the ER in order to be loaded onto MHC class I molecules
- cytosolic protein (viral protein) goes through proteosome
- chopped up into peptides
- TAP (transporter associated w antigen processing) shuttled peptides into ER
- peptides undergo further degradation in ER by amino-peptidases (ERAPs) + bind to MHC I
- MHC I transported to cell surface to interact w CD8 T cell
Antigens generated by endogenous + exogenous antigen processing activate different effector functions. How are exogenous pathogens eliminated?
by antibodies + phagocyte activation by T helper cells that use antigens generated by exogenous processing
How are endogenous pathogens eliminated?
killing of infected cells by CTL (cytotoxic T killer cells) that use antigens generated by endogenous processing
What about pathogens that don’t infect APCs? So if a virus only infects lung ciliated epithelial cells (eg. avian influenza), then how would you ever generate a good T-cell response against it?
- epithelial cells are not v good at activating the immune system
- APCs are best at inducing T-cell responses
- if the virus never infects an APC, would you never get good T-cell responses?
- the problem is overcome by antigen cross-presentation
What cells are primarily responsible for antigen cross-presentation?
Myeloid CD11c+CD8a+ dendritic cells
(CD8+ dendritic cells -> help induce CD8+ T cell response)
What is retrotranslocation, in terms of antigen cross-presentation?
- so you have both the CD8(endo) and CD4(exo) pathways
- each with their cytosolic proteins and then endosomal proteins
- retrotranslocation -> diversion of endosomal antigens to cytosol
- so endosomal proteins become cytosolic proteins, then can be presented to CD8+ cytotoxic killer cells
- even a dendritic cell this way that can’t be infected by a virus can still produce a really good immune response against that virus by cross-presentation
Both MHC classes I and II are highly polymorphic molecules. What is the structure of MHC Class I?
- single polypeptide chain
- 3 domains
- a1, a2, a3 & b2 microglobulin
- antigen binding site between a2 and a1
What is the structure of MHC Class II?
- 2 polypeptide chains
- a1-a2
- b1-b2
- peptide binding cleft between a1 and b1
What are key characteristics of MHC Class I?
- expressed on all nucleated cells
- binds short peptides (8-10 aa)
- presents to CD8+ T-cells
- present antigens from cytosol (+ cross-presentation)
What are key characteristics of MHC Class II?
- expressed on APCs, thymic epithelia + activated T-cells
- binds long peptides (typically 15-24 aas)
- presents to CD4+ T-Cells
- presents antigens from phagosomes + endosomes
When an antigen presenting cell intracts with a T-cell receptor, there are 3 main signals being relayed. What are these 3 signals?
- antigen presented on MHC complexes interacts w/ TCR - Zap70
- co-stimulatory (+inhibitory) molecules to activate/inhibit naïve T-cells delivered from APC by co-receptors such as ‘B7 family’ (CD80 + CD86) although many of these molecules are not fully understood - PI(3)K
- cytokines secreted by APC to determine T-cell phenotype
What are the 4 types of CD4 T cell and what do they each fight?
- Th1 - intracellular pathogens
- Th2 - helminths + parasites
- Th17 - extracellular pathogens
- Treg - autoimmunity
What does the T-cell receptor do?
- binds to peptide-MHC complexes - cannot recognise peptide alone
- huge diversity - potentially up to 1x1013 diff TCRs
- exists in a TCR complex with accessory molecules such as CD3
What are similarities between T and B cell receptors?
- belong to antibody superfamily
- similar Fab (binding site) fragment of antibody
- large diversity
- high specificity
How is the T cell receptor different to the B cell receptor/antibody?
- lower affinity
- cannot be released
- no Fc fragment
- single rather than two binding sites
- B cell receptor/ab : 5 classes (IgA, D, M etc)
- T cell receptor : 2 classes (alpha-beta + gamma-delta)
What do B cells do before and after antigen stimulaiton in order to generate B-cell receptor diversity?
- before antigen stimulation: somatic recombination
- after antigen stimulation: somatic hypermutation
both of these processes before and after allow for great diversity!
What do T cells do before and after antigen stimulation in order to generate T-cell receptor diversity?
- before antigen stimulation: somatic recombination
- after antigen stimulation: none
takes place in thymus (during T-cell development)
Describe the process of gene rearrangement that takes place during T-cell development in the thymus
- both alpha and beta chain germline undergo the following:
- germline DNA
- recombination -> rearranged DNA
- transcription, splicing, translation -> protein (TCR)
- V(D)J recombination uses similar machinery to the non-homologous end joining (NHEJ) DNA repair process
- C region stays same throughout
What is the immunological synapse?
- when TCR:peptide:MHC meet
- complex interaction of many molecules
- but signals 1 + 2 are central
- surrounding integrins + accessory molecules help stabilise the interaction
In the APC-T-Cell interactions, what happens in signal 1 (peptide-MHC) when the peptide-MHC complex binds to the TCR?
- induces phosphorylation of several kinases
- Lck
- Zap70
- PLC-gamma
- releases calcium flux in cell
- NFAT (transcription factor) activated
- responsible for T cell prolif + survival
The signal one phosphorylated molecules have some crosstalk with the signal two molecules. Similarly, what molecules does the co-stimulatory signal 2 pathway induce?
- CD80 + CD86 bind to CD28
- induces PI3K, Akt, MAPK
- cause NF-kappaB activation
- v important transcription factor in immune system - ‘master controller’
- induce pro-inflammatory survival signals
Briefly describe the structure of the T-cell co receptors: CD4 and CD8
- CD4 single polypeptide chain (4 domains)
- CD8 - 2 polypeptides bound by disulfide bridge, help stabilise rxn
An overly vigorous immune response is harmful to the host. Negative regulators of antigen presentation provide an ‘immune checkpoint’ to limit T-cell activation (homeostasis). What are two important molecules here?
- CTLA-4 (cytotoxic T-lymphocyte-associated protein 4)
- PD-L1 (programmed death-ligand 1)
- both crucial for dampening T-cell response
- TCR also down-regulates its own expression once antigen recognised
How does CTLA-4 inhibit T-cell function?
- CTLA-4 competes with CD28 for APC ‘attention’
- acts as an inhibitor of the co-stimulatory pathway
- interferes with signal 2
- CTLA-4 can also rip CD80/CD86 from surface of APC
How can PD-L1 inhibit T-cell function?
- PD-L1 binds to ligand on T cell (PD-1)
- activates SHP-2
- SHP-2 dephosphorylates TCR signalling molecules
- inhibits T cell activation
- interferes with signal 1
Mature T-cells come from the thymus, which is a ‘school’ for T-cells. T-cells are exposed to self-antigens and tested for reactivity. T-cells that can’t bind self antigen-MHC are deleted. What is this called?
- positive selection*
- these T-cells are useless because they won’t protect against pathogens
*although technically speaking, this is not positive selection - it’s death by neglect, positive selection is when there is a weak antigen binding so those T-cells are kept
Therefore, what is meant by ‘negative selection’, in regards to T-cell maturation?
- T-cells that bind self antigen-MHC too strongly are also deleted -> negative selection
- these T cells are dangerous bc they are too self-reactive
What does the Stochastic Model suggest?
- proportion of CD4+ T cells that are strongly reactive to self-ag will xpress transcription factor FOXP3
- FOXP3 = master controller of Treg cells
- secrete potent amounts of IL-10, TGF-b - anti-inflammatory
- xpress high levels of CTLA-4 so can rip CD80/CD86 from APCs
- xpress high levels of CD25 - IL-2 receptor, can act as a sink for IL-2 and deprive other T-cell subsets of a valuble resource
What are ‘obligate parasites’?
- many organisms depend exclusively on human host for survival
- need to co-exist with the host immune system - immune evasion
How does mycobacterium tuberculosis evade the immune system?
- up-regulates PD-L1 on APCs to shut down T-cell activation
- blocks MHC class II expression via multiple mechanisms
How does Neisseria meningitidis evade the immune system?
- blocks DC activation - low CD40, CD86 + MHC Class I + II expression
- antigens (capsule) with homology to self-antigen, therefore anergic T cells
How does Neisseria gonorrhoeae evade the immune system?
- expresses Opa protein, which binds to T cells
- this induces tyrosine phosphatases that ‘switch off’ key molecules involved in TCR signalling
How does HIV evade the immune system?
- up-regulates PD-1 on T-cells, which antagonises TCR signalling
- binds to DC-SIGN to suppress DC activation via Rho-GTPases
How does herpes simplex virus (HSV) evade the immune system?
- produces protein which binds to + inhibits TAP
- prevents viral peptide transfer to ER
How does adenovirus evade the immune system?
- produce protein which binds MHC class I molecule
- prevents MHC class I molecule from leaving ER
