Generation of diversity in the T cell repertoire Flashcards
What is an antigen?
A combination of ‘antibody’ and ‘generate.’ Any molecule that can bind specifically to an antibody
What does an antigen induce?
○ Will induce an immune response in host
What do adaptive immune reactions occur to?
• Adaptive immune reactions occur to specific epitopes (portions of the antigen) as opposed to the entire antigen itself
What does an infection or vaccination induce?
Infection and vaccination usually induce polyclonal T- and B-cell responses
What can epitopes be?
• Epitope can be small peptide
What is an epitope the target for?
• Epitope is the target for the antibodies and MHC and TCRs
Where can multiple epitopes be recognised?
• Multiple epitopes can be recognised on a single antigen
What form of antigens to T cells not recognise?
T cells do not recognize native antigens
What do B cells typically recognise and what happens to this B cell?
- B cells will typically recognise unprocessed antigens/intact antigens
- This B cell will proliferate and make clones which will all produce antibodies just like the original B cell – CLONAL EXPANSION
What do T cells not respond to?
○ T cells do not respond to an unprocessed antigen
What needs to be seen in order for T cells to be activated?
• Need to see antigenic peptides for T cells to be activated
What do antigens become presented through?
• Antigens become presented through phagocytosis or controlled membrane uptake via receptors and antibodies
What are antigen-presenting cells?
Immune cells that express high levels of surface MHC Class II and can efficiently induce T-cell responses
Where are monocytes found?
○ Monocytes are found in blood
Where are macrophages found?
○ Macrophages are found in tissue – are not in circulation
What are macrophages?
○ Macrophages are terminally differentiated monocytes
What are dendritic cells better at processing?
○ DCs are better at processing antigens and macrophages are better at phagocytosis
Where are macrophages and dendritic cells enriched in?
Enriched in mucosal tissues
What do highly phagocytic cells induce and what is it important for protection against?
Induce strong T-cell responses and inflammation.
Important for protection against Mycobacterium tuberculosis
What are macrophages good at?
equipped to kill pathogens
What are dendritic cells better at?
DCs are better at migrating to lymph nodes (via CCR7) and presenting antigen to T- cells
What are dendritics of dendritic cells?
DCs have dendrites that are extensions of the cell membrane to increase cell surface and interaction with the environment
What is mucosal tissue?
tissue in direct contact with external environment
What are B cells highly abundant in?
○ Highly abundant in blood and mucosal tissues
What is the primary function of B cells?
○ Primary function to make antibody (plasma cell) – but still very good at antigen presentation
What are B cells possibly main inducers of?
○ Possibly main inducer of T-cell immune response to pathogens
Steps in endogenous antigen processing
- Uptake
○ Antigens/pathogens already present in the cell - Degradation
○ Antigens synthesised in the cytoplasm undergo limited proteolytic degradation in the cytoplasm
○ Needs to be cleaved to smaller peptides - Antigen-MHC complex formation
○ Loading of peptide antigens onto MHC class I molecules is different to the loading of MHC class II molecules - Presentation
○ Transport and expression of antigen-MHC complexes on the surface of cells for recognition by T cells
IS EXOGENOUS ANTIGEN PROCESSING SUFFICIENT?
Macrophages have well developed lysosomal systems which most other cell types don’t. This means non-lysosomal mechanism to process antigens for presentation to T cells is required.
What are lysosomal systems specialised for?
○ Specialised for motility, phagocytosis and the introduction of particles to the lysosomal system
Where are peptide antigens produced and what are they seperated from?
Peptide antigens produced in the cytoplasm are physically separated from newly formed MHC class I
How are antigens from inactive viruses processed?
• Antigens from inactive viruses are processed via exogenous pathways
What response does an inactive virus raise?
• Inactive virus raises a weak CTL response
What is the processing of antigens from inactive viruses sensitive to?
• The processing of antigens from inactive viruses is sensitive to lysosomotrophic drugs
What response do infectious viruses raise?
• Infectious virus raises a strong CTL response
What is the processing of antigens from infectious viruses not sensitive to?
• The processing of antigens from infectious viruses is NOT sensitive to lysosomotrophic drugs
What do most CTL recognise and via what pathway?
• Most CTL recognise antigens generated via a non-lysosomal pathway
What is required for non-lysosomal antigen processing?
• Protein synthesis is required for non-lysosomal antigen processing
What are antigens from infectious viruses processed via?
• Antigens from infectious viruses are processed via the endogenous pathway
Exogenous pathway of antigen processing and presentation?
○ Antigen is processed, cleaved and presented on MHC I
Endosomal pathway of antigen processing and presentation
○ Antigens are endocytosed ○ Sequestered to lysosomes/endosomes ○ Loaded onto MHC class II which are in the lysosome unlike MHC class I which are in the cytosol
Where are antigens loaded in MHC II?
In MHC II, its loaded in the phagolysosome
Where are antigens loaded in MHC I?
In MHC I, it’s in the cytosol
What does MHC II activate and need assistance from?
○ MHC II activate CD4 T cell and also needs assistance from CD8
How many type of MHC are their?
2 types
What MHC do all cells in the body express?
• All cells in body express MHC class I
What are MHC I expressed on?
Expressed on all nucleated cells
What are MHC II expressed on?
Expressed on APCsand activated T-cells
What does MHC I bind?
Binds short peptides (8-10amino acids)
What does MHC II bind?
Long peptides (typically 15-24amino acids)
What does MHC I present to?
Presents to CD8+T-cells
What does MHC II present to?
Presents to CD4+T-cells
What does TCR bind and what does it need to find?
○ Binds to peptide-MHC (pMHC) complexes – cannot recognise peptide alone
○ TCR needs to find peptide groove on APC and recognise peptide in the complex with the MHC
T cell similarities to B cell receptor/antibody
○ Belongs to Ig superfamily
○ Fab like fragment of antibody
§ T cell has peptide groove instead of antigen binding site but is structurally similar
○ Large diversity
○ Single specificity
§ One TCR will only bind one peptide
T cell differences to B cell receptor/antibody
○ Antibody has very high affinity to antigens: 107-109
§ TCR has lower affinity – kd = 105
○ TCR cannot be released
§ Antibodies are first produced on surface of cell and then eventually released to become circulating antibodies
§ TCR remains on cell surface all the time
○ TCR does not have Fc fragment, so no cellular functions
○ TCR has single bind site rather than two binding sites
○ B cell receptor/Ab: 5 classes
○ T cell receptor: 2 classes (ab and gd)
What are the mechanisms which generate B-cell receptor diversity before and after antigen stimulation?
○ Before antigen stimulation: Somatic recombination
○ After antigen stimulation: Somatic hypermutation
What are the mechanisms which generate T-cell receptor diversity before and after antigen stimulation?
○ Before antigen stimulation: Somatic recombination
○ After antigen stimulation: None
What does receptor gene rearrangement take place during?
Receptor gene rearrangement takes place during T-cell development in thymus
What are the three signal mode of T cell activation?
- Peptide MHC
- Co-stimulaiton
- Cytokines
How is the Peptide MHC signal mode of T cell activation delivered?
The main signal (signal 1) is delivered from the APC by a peptide-MHC complex to the TCR
How is the co-stimulation of T cell activation delivered?
The co-stimulatory signal (Signal 2) 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
What is the cytokine signal mode of T cell activation formed of?
Lastly, Signal Three is formed of cytokines secreted by the APC to determine the T-cell phenotype
What does cytokine IL-12 promote?
IL-12 promotes TH1 cells
What does cytokine IL-4 promote?
IL-4 promotes TH2 cells
What does cytokine IL-23 promote?
IL-23 promotes TH17 cells
What does peptide MHC and co-stimulation activate?
Signal 1 and 2 alone will activate naïve T cel
What is cytokine signal not needed for and what does it do?
Signal 3 not needed for initiation but amplifies the response
What happens along the immunological synapse?
• Interaction of TCR with peptide-MHC complex happens first
○ Not very high affinity
○ Interaction is therefore very short lived unless additional interactions occur
§ Costimulatory molecules such as CD80 and CD86 interact with their T cell partners to strengthen the interaction
§ Keep the cells in contact for long enough so that the information can be exchanged along the synapse
• Complex interaction of many molecules – but simplistically Signals 1 and 2 are central, and surrounding integrins and accessory molecules help to stabilise the interaction
What are most of the CD4 cells and what do they help with?
Most of the CD4 cells are helper T cells so help cells kill pathogens
What do CD4 cells produce?
Produce cytokines which activate other cells
What do TH2 produce and what may they activate?
○ TH2 produces other types of cytokines which may activate b cells that produce antibodies
What are cells that express CD8?
Cells that express CD8 are cytotoxic cells so directly kill infected cells
What do negative regulators of antigen presentation provide?
Negative regulators of antigen presentation provide an ‘immune checkpoint’ to limit T-cell activation
What does the immune response need to do once a virus has been dealt with?
Once a virus has been dealt with, the immune response needs to subside and only a memory T cell should remain in circulation and reactivate upon second exposure
What is Pd-1 expressed on?
Pd-1 is expressed on T cells
What are Pd-1 on T cells be activated by?
Can be activated by PD-L1
What happens once Pd-1 on T cells is activated?
When activated, blocks TCR activation by causing SHP-2 to dephosphorylate TCR signalling molecules
What does CTLA-4 act on and to do what?
CTLA-4 acts on costimulatory molecules to block them and therefore block TCR activation
What does CTLA-4 compete with for APC attention?
CTLA-4 competes with CD28 for APC attention
What do T cells arise from?
T-cells arise from the thymus
What are T cells exposed to and tested for?
T-cells are exposed to self-antigens and tested for reactivity
What happens to T cells that bind self-antigen MHC too strongly and what is it done through?
○ All T cells that bind self-antigen-MHC too strongly are deleted from the repertoire as these cells are harmful as they are too self-reactive
○ Done through negative selection and induction of apoptosis
What happens to T cells that cant bind and what is it done through?
- T-cells that can’t bind self antigen-MHC are also deleted
* Done through positive selection
What transcription factor is expressed when a proportion of T cells are strongly reactive to self-antigen and what is it the master controller of?
○ A proportion of T- cells that are strongly reactive to self-antigen will express the transcription factor FOXP3, which is the ‘master controller’ of Regulatory T-cells (TREG)
• These cells will regulate any potential self-reactive T cells by inducing apoptosis or deletion
What does HSV produce and prevent?
○ Produce protein which binds to and inhibits TAP
○ Prevents viral peptide transfer to ER
What does adenovirus produce and prevent?
○ Produce protein which binds MHC class I molecule ○ Prevents MHC class I molecule from leaving ER
