53. Generation of Diversity in the T Cell Repertoire

Question 1

What is an antigen?

Answer 1

Antigen – A combination of ‘antibody’ and ‘generate’. Any molecule that can bind specifically to an antibody

‘antigen’ usually refers to proteins, carbohydrates and lipids capable of binding to B-cell receptors, T-cell receptors and/or innate immune receptors

Question 2

Update of exogenous antigens

Answer 2

• Membrane Ig receptor mediated uptake
• Phagocytosis
• Uptake mechanisms direct antigen into intracellular vesicles for exogenous antigen processing

Question 3

Which immune cells recognise and process antigen?

Answer 3

• Monocytes
• Macrophages
• Dendritic cells
• B-cells

Question 4

Expand on macrophages and dendritic cells.

Answer 4

• Rare in peripheral blood - enriched in mucosal tissues
• Highly phagocytic cells – induce strong T-cell responses and inflammation. Important for protection against Mycobacterium tuberculosis
• Macrophages better-equipped to kill pathogens (higher NO production); DCs better at migrating to lymph nodes (via CCR7) and presenting antigen to T- cells
• Specialised but ultimately overlapping functions

Question 5

Expand on B cells

Answer 5

• Highly abundant in blood and mucosal tissues
• Receptor-mediated internalisation of antigens, as opposed to phagocytosis
• Primary function to make antibody (plasma cell) – but still very good at antigen presentation

•Possibly main inducer of T-cell immune response to pathogens such as
Neisseria meningitidis

Question 6

Endogenous antigen processing

Answer 6

1) UPTAKE
Antigens/pathogens already present in cell

2) DEGRADATION
Antigens synthesised in the cytoplasm undergo limited proteolytic degradation in the cytoplasm

3) ANTIGEN-MHC COMPLEX FORMATION
Loading of peptide antigens onto MHC class I molecules  is different to the loading of MHC class II molecules

4) PRESENTATION
Transport and expression of antigen-MHC complexes on the surface of cells for recognition by T cells

Question 7

How is antigen processed and presented?

Answer 7

• Endogenous and exogenous processing pathways
• Endogenous includes the cytosolic proteins being processed by proteasome and fragments bind to MHC I and this migrates to the surface. ~ Interacts with CD8
• Exogenous is when the antigens are taken up by phagocytosis and this will bind to MHC II and moves to the surface. ~ Interacts with CD4.

Question 8

Exogenous and endogenous pathogen fate

Answer 8

•EXOGENOUS PATHOGENS
Eliminated by:
- Antibodies and phagocyte activation by T helper cells that use antigens generated by EXOGENOUS PROCESSING

•ENDOGENOUS PATHOGENS
Eliminated by:
- Killing of infected cells by CTL that use antigens generated by ENDOGENOUS PROCESSING

Question 9

Main points for MHC Class I

Answer 9

• Expressed on all nucleated cells
• Binds short peptides (8-10 amino acids)
• Presents to CD8+ T-cells
• Antigens from the cytosol (+ cross- presentation)

Question 10

Main points for MHC Class II

Answer 10

• Expressed on APCs and activated T-cells
• Long peptides (typically 15-24 amino acids)
• Presents to CD4+ T-cells
• Antigens from phagosomes and ensodomes

Question 11

Expand on TCR

Answer 11

• Binds to peptide-MHC (pMHC) complexes – cannot recognise peptide alone
• Huge diversity – potentially up to 1 x 1013 different TCRs
• Exists in a TCR complex with accessory molecules such as CD3

Question 12

Compare TCR and BCR

Answer 12

Similarities to B cell receptor/antibody:

• Belongs to Ig superfamily
• Like Fab fragment of antibody
• Large diversity
• Single specificity

Differences to B cell receptor/antibody:

• Lower affinity
• Cannot be released
• No Fc fragment, so no cellular functions
• Single rather than two binding sites
• B cell receptor/Ab: 5 classes
• T cell receptor: 2 classes (αβ and γδ)

Question 13

Mechanisms which generate B-cell receptor diversity

Answer 13

• Before antigen stimulation: Somatic recombination

* After antigen stimulation: Somatic hypermutation

Question 14

Mechanisms which generate T-cell receptor diversity

Answer 14

• Before antigen stimulation: Somatic recombination
• After antigen stimulation: None
• Receptor gene rearrangement takes place during T-cell development in thymus

Question 15

Three signal model of T-cell activation

Answer 15

1. Peptide-MHC (pMHC)
2. Co-stimulation
3. Cytokines

•Signals 1 + 2 alone will activate a naïve T-cell, but Signal 3 is also important for a strong response and also determining T-cell phenotype

• 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’ (CD80 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.
  IL-12 promotes TH1 cells
  IL-4 promotes TH2 cells
  IL-23 promotes TH17 cells

Question 16

CD8 T cell, CD4 TH1 cell and CD4 TH2 cell functions

Answer 16

• CD8 T cell - cytotoxic T cell that will kill the virus infected cell
• CD4 TH1 cell + macrophage → release cytokines and activate the macrophages
• CD4 TH2 cell + B cell → Release cytokines and stimulate differentiation into plasma cells which release antibodies

Question 17

Events of T cells in the thymus

Answer 17

•T-cells arise 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 → POSITIVE 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 → NEGATIVE SELECTION
•These T-cells are dangerous because they are too self- reactive

Question 18

How can Mycobacterium tuberculo, Neisseria meningitidis, Neisseria gonorrhoeae and
HIV impede antigen presentation?

Answer 18

•Mycobacterium tuberculo

• Up-regulates PD-L1 on APCs to shut down T-cell activation
• Blocks MHC Class II expression via multiple mechanisms

•Neisseria meningitidis
- Blocks DC activation –
low CD40, CD86 and MHC Class I & II expression
- Antigens (capsule) with homology to self- antigen, therefore anergic T-cells

•Neisseria gonorrhoeae
- Expresses Opa protein, which binds to T-cells and induces tyrosine phosphatases that ‘switch off’ key molecules involved in TCR signalling

•HIV

• Up-regulates PD-1 on T-cells, which antagonises TCR signalling
• Binds to DC-SIGN to suppress DC activation via Rho-GTPases

Question 19

Herpes Simplex Virus (HSV) and adenovirus impede antigen presentation?

Answer 19

•Herpes Simplex Virus (HSV)

• Produce 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