Antigen Recognition by T Cells Flashcards
1
Q
What is an antigen?
A
- 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
2
Q
What is the epitope?
A
- 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
3
Q
What is another term for MHC?
A
HLA
4
Q
What are “professional” antigen-presenting cells?
A
- 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
5
Q
What are the roles of macrophages and dendritic cells and how do they differ (what are they each better at)?
A
- 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
6
Q
What is the role of B-cells?
A
- 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
7
Q
Will T-cells recognise just any antigen?
A
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
8
Q
What evidence suggests antigen-processing is a catabolic process?
A
- 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
9
Q
What is the difference between exogenous and endogenous antigen processing?
A
- Exogenous - antigen uptaken from outside cell eg. extracellular bacteria (S. aureus, N. men.)
- Endogenous - antigen from inside cell eg. viruses (HIV, tumour antigens)
10
Q
What processes + receptors are important for uptake of exogenous antigens?
A
- 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
11
Q
Describe the mechanism/cellular pathway of uptake of exogenous antigens and how they are expressed on MHC class II
A
- 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 !! :)
12
Q
What are cathepsins?
A
- proteases / enzymes that degrade proteins
- found in the endosomes / lysosomes
13
Q
Describe the mechanism/cellular pathway for endogenous antigen processing and presentation of MHC class I
A
- 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
14
Q
Antigens generated by endogenous + exogenous antigen processing activate different effector functions. How are exogenous pathogens eliminated?
A
by antibodies + phagocyte activation by T helper cells that use antigens generated by exogenous processing
15
Q
How are endogenous pathogens eliminated?
A
killing of infected cells by CTL (cytotoxic T killer cells) that use antigens generated by endogenous processing
16
Q
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?
A
- 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
17
Q
What cells are primarily responsible for antigen cross-presentation?
A
Myeloid CD11c+CD8a+ dendritic cells
(CD8+ dendritic cells -> help induce CD8+ T cell response)
18
Q
What is retrotranslocation, in terms of antigen cross-presentation?
A
- 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
19
Q
Both MHC classes I and II are highly polymorphic molecules. What is the structure of MHC Class I?
A
- single polypeptide chain
- 3 domains
- a1, a2, a3 & b2 microglobulin
- antigen binding site between a2 and a1
20
Q
What is the structure of MHC Class II?
A
- 2 polypeptide chains
- a1-a2
- b1-b2
- peptide binding cleft between a1 and b1
21
Q
What are key characteristics of MHC Class I?
A
- expressed on all nucleated cells
- binds short peptides (8-10 aa)
- presents to CD8+ T-cells
- present antigens from cytosol (+ cross-presentation)
22
Q
What are key characteristics of MHC Class II?
A
- 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
23
Q
When an antigen presenting cell intracts with a T-cell receptor, there are 3 main signals being relayed. What are these 3 signals?
A
- 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
24
Q
What are the 4 types of CD4 T cell and what do they each fight?
A
- Th1 - intracellular pathogens
- Th2 - helminths + parasites
- Th17 - extracellular pathogens
- Treg - autoimmunity
25
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
26
What are similarities between T and B cell receptors?
* belong to antibody superfamily
* similar Fab (binding site) fragment of antibody
* large diversity
* high specificity
27
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)
28
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!
29
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)
30
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 **n**on-**h**omologous **e**nd **j**oining (**NHEJ**) DNA repair process
* C region stays same throughout
31
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
32
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
33
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
34
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
35
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
36
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
37
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
38
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
## Footnote
\*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
39
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
40
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
41
What are 'obligate parasites'?
* many organisms depend exclusively on human host for survival
* need to co-exist with the host immune system - immune evasion
42
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
43
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
44
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
45
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
46
How does herpes simplex virus (HSV) evade the immune system?
* produces protein which binds to + inhibits TAP
* prevents viral peptide transfer to ER
47
How does adenovirus evade the immune system?
* produce protein which binds MHC class I molecule
* prevents MHC class I molecule from leaving ER
