Antigen Recognition by T-lymphocytes/ Generation of Diversity in the T cell repertoire Flashcards
What is the definition of an antigen?
Proteins, carbohydrates and lipids that are capable of binding to B-cell receptors, T-cell receptors and innate immune receptors (PRRs e.g lectin receptors, scavenger receptors, toll-like receptors)
What is an epitope?
An epitope is a small portion (e.g small peptide) of the antigen that the TCR and BCR (antibody) can bind to
Can the immune system recognise the whole antigen?
No normally it is only the epitope that will result in an adaptive immune response
Infection and vaccination usually induce polyclonal T cell and B cell responses as multiple epitopes can be recognised on an antigen
What is a key difference between B cells and T cells in terms of recognising antigens?
B cells will recognsie unprocessed antigens which will then cause proliferation of B-cell clones and produce Abs just like original B cell
T cells do not recognise unprocessed (native antigens) and therefore they will not proliferate or produce cytokines
How are antigens taken up by APCs?
- Phagocytosis/ Pinocytosis
- Membrane Ig receptor mediated
- Complement receptor mediated
- Fc receptor mediated phagocytosis
What are the main immune cells which present antigens?
APC’s which express high levels of MHC Class II can efficiently induce T cell responses
- Monocytes
- Macrophages
- Dendritic Cells
- B-cells
Where do monocytes reside and what do they become?
Monocytes are blood circulating immune cells which will differentiate into either macrophages or dendritic cells
What is the difference between dendritic cells and macrophages? Which cell is better at being an APC and which cell is better at being a phagocyte?
- They are both highly phagocytic cells that will induce a strong T-cell response and inflammation. (important protection against mycobacterium tuberculosis)
- Macrophages better at phagocytosis and killing pathogens (higher NO production)
- Dendritic cells better at migrating to lymph nodes (via CCR7) and presenting antigens to T cells
Where do macrophages and dendritic cells mainly reside?
- Rare in peripheral blood, mainly enriched in mucosal tissues
How do B-cells internalise antigens?
Ig receptor mediated internalisation of antigens (opposed to phagocytosis)
(However primary function of B-cells is to make antibodies via plasma cells, however still good at antigen presentation to other cells)
What are the two types of antigen processing?
Exogenous and Endogenous Antigen processing
Describe the endogenous/ intracellular pathway
- Infected (virally or malignant) nucleated cell (can include APC) will produce viral/malignant proteins in the cytosol
- These will be ubiquitinated and targeted to the proteasome which will recognise proteins and degrade them into peptides
- Peptides will be imported into the ER lumen via a transporter called TAP protein where there is MHC I production
- Tapasin will link MHC I molecules to the TAP protein allowing antigens to be easily captured by MHC I molecules as they enter
- MHC I with succesfully captured peptides will leave the ER and enter the golgi where they form exocytotic vesicles
- Exocytotic vesicles will fuse with the plasma membrane which can be recognised by CD8+ cytotoxic T cells
Describe the exogenous/ extracellular pathway?
- Dendritic cells/ phagocytes will ingest extracellular microbes e.g bacteria by phagocytosis/ receptor mediated endocytosis to form an endosome/phagsome
- Endosome will fuse with lysosome to form a phagolysosome which will break antigens down into its peptides
- MHC II proteins are produced ER and held by an invariant chain to stabalise the MHC II
- The invariant chain contains a sequence called CLIP which binds to the peptide binding groove
- MHC II will leave the ER and enter the golgi where it forms an exocytotic vesicle
- Exocytotic vesicle will fuse with the late-endolysosome where the antigenic peptides are
- Late endolysosome has a low pH, invariant chain will be broken down leaving CLIP in the MHC II groove
- HLA-DM has a high affinity for CLIP removing it from the MHC II molecule
- Antigenic peptides will bind to MHC II complex leaving the late endo-lysosome and fuses with the plasma membrane
- Infected APC can now be recognised by TCRs on CD4+ T cells
What do MHC I and MHC II activate respectively?
MHC I will activate CD8+ Cytotoxic Cells
MHC II will activate CD4+ T helper cells
Where is MHC Class I + II expressed?
- MHC I = Expressed on all nucleated cells (including APCs) (so only cell that doesnt is RBCs)
- Binds short peptides
- Antigens from cytosol (+ cross presentation)
- MHC II = Expressed on APCs and activated T cells
- Binds long peptides
- Antigens from phagosomes and endosomes
What is the tissue specific expression of MHC like?
Epithelial cells of the thymus will express MHC II at high levels (only exception)
What is the structure of MHC Class I?
- Large structural part containing alpha 3 domain and beta 2 microglobulin supporting the peptide binding cleft
- Peptide binding cleft containing alpha 1 and alpha 2 domain
What is the structure of MHC Class II?
- Large structural part with alpha 2 and beta 2 domains supporting peptide binding cleft
- Peptide binding cleft with alpha 1 and beta 2 domains
What are the MHC isotypes?
- Main MHC Class I are HLA-A, HLA-B and HLA-C
- MHC Class II are split into HLA-DM, DO, DP, DQ and DR
How does the TCR receptor exist?
Exists in a complex with accessory molecules such as CD3
What are similarities between the B-cell receptor and T-cell receptor?
- Belongs to an Ig superfamily
- TCR also has Fab like fragment of BCR
- Large diversity
- Single specificity
What are the differences in the B-cell receptor and the T-cell receptor?
- T cell receptor has a lower affinity to their antigens compared to B-cell receptors
- T-cell receptor cannot be released and remains on T-cell surface, BCRs can become circulating antibodies
- TCR has no Fc fragment
- So not cellular function
-
TCR only has a single binding site
- Unlike the BCR that has multiple binding sites
- TCR only has two classes (αβ and γδ)
- BCR has 5 classes (IgM, IgD, IgG, IgE, IgA)
What differs in the mechanisms of creating receptor diversity between B-cells and T-cells?
B cells can generate receptor diversity by:
- Before antigen stimulation = somatic recombination
- After antigen stimualtion = somatic hypermutation
T-cells can generate receptor diversity by:
- Before antigen stimulation = somatic recombination
- After antigen stimulation = NONE!!!!
Can T-cell receptor diversity be stimulated after antigen stimulation?
No T-cell receptor diversity can only be stimulated after antigen stimulation as they lack somatic hypermutation (explains the lower affinity of the TCR compared to BCR)
What are the signals needed for naive T-cell activation?
- Peptide-MHC
- Co-stimulation (CD28 on T-cell and B7 family (CD80/CD86) on APC)
- Cytokine secretion
What is the level of importance of the three signals?
- Signal 1 + 2 → activate naïve T-cells
- Signal 3 is not required for naïve T-cell activation but it important for amplification of the response and determining the T-cell phenotype
Expand on the three signals that activate naive T-cells
- The main signal (Signal One) is delivered from the APC by a peptide-MHC complex to the TCR
- The co-stimulatory signal (Signal Two) is delivered from the APC by germline-encoded accessory receptors such as the B7 family (CD8- and CD86) = although many of these receptors are not fully characterised or understood
- Lastly (Signal Three) is formed of cytokines secreted by the APC to determine the T-cell phenotype
What do IL-2, IL-4 and IL-23 secreted by the APC determine?
The APC will secrete these cytokines to determine the T-cell phenotype
- IL-2 promotes TH1 cells
- IL-4 promotes TH2 cells
- IL-23 promotes TH17 cells
Describe the immunological synapse
- Region of contact and communication between T cell and APC required for T cell activation
- Surrounding integrins and accessory molecules (CD80 and CD86 from APC)
What co-receptor is expressed depending on the type of T-cell?
Either CD4 (T-helper cell) or CD8 (cytotoxic T-cell)
What is the difference between Th1 and Th2 cells?
Th1 cells will produce cytokines that activate macrophages which kill intracellular pathogens more efficiently than those without cytokines
Th2 cells will produce cytokines that activate B-cells which will produce plasma cells that produce antibodies which neutralise circulating viruses or prevent bacterial colonisation
Why are there negative regulators of antigen presentation?
An overly vigourous immune response is harmful to the host and so negative regulators provide an immune checkpoint to limit T-cell activation
What are examples of negative regulators of antigen presentation?
CTLA4 (Cytotoxic T-Lymphocyte-Associated Protein 4)
PD-L1 (Programmed Death Ligand 1)
How do PD-L1 and CTLA-4 inhibit T-cell function?
PD-L1 will bind to and activate PD-1 which is expressed on T-cells leading to production of SHP-2 which dephosphorylates TCR signalling molecules = inhibiting T-cell activation
CTLA-4 competes with CD28 on the TCR to bind co-stimulatory molecules on the APC (CD80/86)
Describe how T-cells are programmed and self antigens are removed
- T-cells arise from the thymus, which is a ‘school’ for T-cells
- T-cells are exposed to self-antigens and tested for reactivityT-cells that can’t bind to self-antigen are deleted = POSOTIVE SELECTION
- These T-cells are useless because they won’t protect against pathogens
- T cells that bind self-antigen-MHC too strongly are also deleted à NEGATIVE SELECTION
- These T-cells are dangerous because they are too self-reactive
- T-cells are exposed to self-antigens and tested for reactivityT-cells that can’t bind to self-antigen are deleted = POSOTIVE SELECTION
What are T-regualtory cells?
Subset of CD4+ T cells which carry an additional CD25 molecule and express the transcription factor FOXP3
Regulating any potential self-reacting T-cells by inducing their apoptosis or deletion
How can pathogens impede antigen presentation?
-
Herpes Simplex Virus
- Produces a protein which binds to and inhibits TAP
- Prevents viral peptide transfer to ER
-
Adenovirus
- Produce protein which binds MHC class I molecule
- Prevents MHC Class I molecule from leaving ER
