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
usually refers to proteins, carbohydrates and lipids capable of binding to B-cell receptors, T-cell receptors and/or innate immune receptors
What is an epitope?
The part of an antigen molecule to which an antibody attaches itself
One antigen can have multiple epitopes
How do B cells produce monoclonal antibodies?
B cells recognise intact antigens (proteins / microbes) that havent been processed and go on to proliferate and produce → monoclonal antibodies ; produce antibody clones of the mother B cell
Why do T cells not produce a response to unprocessed antigens?
T cells don’t recognise unprocessed antigens ∴ there’s no proliferation or cytokine release
(same response with incorrect signalling)
What requirement must be met for T cell antigen recognition?
Antigens must be processed in order to be recognised by T cells
T cell requires APC interaction to produce a response
How are antigens processed for TCR?
An antigen will generate multiple peptides which are presented on APCs with MHC molecules via catabolism
How are antigens prepared for exogenous antigen processing?
Uptake mechanisms direct antigens into intracellular vesicles for exogenous antigen processing
What are the exogenous antigen processing mechanisms?
Phagocytosis
- Fc Receptor mediated
- Complement receptor mediated
- Pinocytosis
Membrane Ig receptor mediated uptake
What are professional APCs?
“Professional” Antigen-Presenting Cells (APCs): Immune cells that express high levels of surface MHC Class II and can efficiently induce T-cell responses
What are the 2 types of APCs?
Myeloid cells: monocytes & macrophages
Dendritic cells (most advanced APCs)
Where are monocytes located?
Monocytes are blood circulating cells
Where do macrophages reside?
Macrophages are found in mucosal tissues and are ∴ terminally differentiated monocytes
Rare in peripheral blood - enriched in mucosal tissues
Compare and contrast the functions of Dendritic cells and macrophages
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
Where are B cells found?
Highly abundant in blood and mucosal tissues
How do B cells process antigens?
Receptor-mediated internalisation of antigens, as opposed to phagocytosis
What is the primary function of B cells?
Primary function to make antibody (plasma cell) – but still very good at antigen presentation
How do B cells process antigens?
Contains membrane antibody on surface that can capture antigens and present it to other B cells or T cells
Outline how endogenous antigen processing occurs
Uptake: Antigens/pathogens already present in cell
Degradation: Antigens synthesised in the cytoplasm undergo limited proteolytic degradation in the cytoplasm
Antigen-MHC Complex Formation: Loading of peptide antigens on 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 cell
Why are macrophages so sufficient at exogenous antigen processing?
Macrophages have well developed lysosomal systems
Specialised for motility, phagocytosis and the introduction of particles to the lysosomal system
Most cell types do not have lysosomal systems developed as well as macrophages
Why is exogenous antigen processing not enough to combat viruses?
Viruses can infect cytosol of most cell types
A non-lysosomal mechanism to process antigens for presentation to T cells is required
What is the role of the proteasome in antigen processing?
Peptide antigens produced in the cytoplasm are physically separated from newly formed MHC class I by the proteasome
What is the role of ER in antigen processing?
Peptides need access to the ER in order to be loaded onto MHC Class I molecules
Outline the exogenous non-lysosomal antigen processing mechanism of CD8 cells
Inactive virus raises a weak CTL response
The processing of antigens from inactive viruses is sensitive to lysosomotropic drugs
Which antigens are processed via the exogenous non-lysosomal pathway?
Antigens from inactive viruses are processed via the exogenous pathway
Describe the non-lysosomal antigen processing endogenous pathway
- Infectious virus raises a strong CTL response
- Processing of antigens from infectious viruses is NOT sensitive to lysosomotropic drugs
Most CTL recognise antigens generated via a non-lysosomal pathway - Protein synthesis is required for non-lysosomal antigen processing
Which viruses are processed via non-lysosomal endogenous pathway?
Antigens from infectious viruses are processed via the endogenous pathway
Outline how antigens are processed and presented exogenously by MHC I molecules
- Antigen is processed
- Cleaved into multiple peptides
- Presented on to MHC I on cell surface
Outline how MHC II molecules process and present antigens endogenously
- Antigens endocytosed by macrophages / dendritic cells
- Sequestered into lysosomes / endosomes and processed
- Loaded onto MHC II molecules residing in lysosomes onto cell surface
Outline the difference between MHC I and MHC II molecules
MHC I
- loaded in cytosol → CD8 T cells
MHC II
- loaded in phagolysosome → CD4 T cells
How do antigen effector functions differ?
Antigens generated by endogenous and exogenous antigen processing activate different effector functions
What are the functions of the exogenous antigen processing?
Eliminated by antibodies and Phagocyte activation by Th cells
What are the antigen effector functions of endogenous pathogens?
Eliminated by CD8 cells
Describe features of MHC I molecules
- expressed on all nucleated cells
- bind short peptides (8-10 a.a.)
- present to CD8+
- antigens from cytosol
Describe features of MHC II molecules
- expressed on APCs and activated T cells
- Long peptides (15-24 a.a.)
- present to CD4+
- Antigens from phagosomes and endosomes
Where are MHC I molecules highly expressed?
Epithelial cells express low density levels of MHC I whereas macrophages, T cells and B cells express high levels
Describe the expression of MHC II molecules
MHC II are expressed on a much smaller subset of cells (APCs) and epithelial cells
What are the different types of MHC I/II molecules?
Several types of MHC I molecules - main classes are HLA-A,B and C
MHC II are divided into HLA-DM, DO, DP etc..
These determine compatibility especially in organ transplants
Outline how TCR and BCR are similar
- Belongs to Ig superfamily
- Like Fab fragment of antibody
- Large diversity
- Single specificity
Describe how TCR differs from BCR
- 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 (ab and gd)
What mechanisms generate BCR diversity?
Before antigen stimulation: Somatic recombination
After antigen stimulation: Somatic hypermutation
|Which mechanisms generate TCR diversity?
Before antigen stimulation: Somatic recombination
After antigen stimulation: None
When does TCR recombination occur?
Receptor gene rearrangement takes place during T-cell development in thymus
Outline the 3 step model of T cell activation
Three signal model of T-cell activation:
- Peptide-MHC (pMHC)
- Co-stimulation
- Cytokines
Signals 1 + 2 alone will activate a naïve T-cell, but Signal 3 important for a strong response and determining T-cell phenotype
What is the rolw
e of cytokine release by APCs?
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
These have specialised functions in dealing with pathogens
Describe the immunological synapse structrue
Signals 1 and 2 are central, and surrounding integrins and accessory molecules help to stabilise the interaction
What causes T cell signalling molecules activation?
Signalling molecules (kinases) are activated following ligation of the TCR with co-stimulatory molecules
What is the role of CD4+?
CD4+ cells are helper T cells - recruit other cells to kill pathogens
What is the role of CD8+?
CD8+ are cytotoxic T cells - directly kill infected cells
What is the effect of CD4 activation?
CD4 Th1 and Th2 both produce cytokines which activate other cells
What is the role of Th1 cells?
Th1 release cytokines that activate macrophages which go on to kill cellular pathogens (occurs more efficiently than without cytokines)
What is the effect of Th2 cells?
Th2 cells release cytokines that may activate B cells that produce antibodies. The antibodies will neutralise circulating viruses or prevent bacterial colonisation
Which molecules are responsible for inactivating T cells?
CTLA4 (Cytotoxic T-Lymphocyte-Associated Protein 4)
PD-L1 (Programmed Death-Ligand 1) are crucial for dampening the T-cell response
How does PD-L1 inactivate T cells?
PD-1 expressed on T cells is activated by PD-L1 (programmed death ligand 1) which blocks TCR activation through multiple signalling pathways
How does CTLA4 mediate T cell activation?
CTLA-4 molecule competitively acts on the costimulatory CD28 molecule binding site
Where do self antigens arise form?
T-cells arise from the thymus, which is a ‘school’ for T-cells
T-cells are exposed to self-antigens and tested for reactivity
Describe the positive selection for self antigens
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
What is negative selection of self antigens?
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
