Generation of Diversity in the T-Cell Repitoire Flashcards
T and B cells recognise what differently?
antigens
what does antigen processing generate?
antigenic peptides
where can antigen processing take place?
lysosomes, and there is also a non-lysosomal mechanism of antigen processing
when can the TCR bind to antigenic peptides?
only when the peptides are in complex with MHC molecules on APC’s
how many types of MHC are there?
2 types of MHC (I and II)
-they interact with CD8 or CD4 T cells
what is an antigen?
any molecule that can bind specifically to an antibody
antibodies are normally immunogenic - will initiate an immune response in the host
what is an epitope?
a small portion of an antigen (could be a small peptide), it is a target for TCR’s and antibodies
adaptive immune reactions occur to specific epitopes as opposed to the entire antigen itself
immune reactions occur to specific epitopes and infection can induce polyclonal T or B cell response
how do B cells and T cells differ in the way they recognise antigens?
T cells cannot recognise native antigens – they must be processed into peptides and presented on surface of APC’s by MHC molecules
B cells will recognise and process native antigens, and undergo clonal expansion
B cell clonal expansion?
B cells proliferate and make multiple clones of the same cell, which all produce the same antibody
if an APC is fixed what is the consequence?
it will not induce cell activation
what do uptake mechanisms do?
the direct the antigen into intracellular vesicles for exogenous antigen processing
examples are:
- phagocytosis
- pinocytosis
- membrane Ig medicated uptake
- complement receptor mediated uptake
- Fc receptor mediated phagocytosis
Which immune cells recognise and process antigen?
APC’s!!!!
monocytes
macrophages (terminally differentiated monocytes)
dendritic cells
B cells
macrophages vs dendritic cells?
both 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
how are dendritic cells specialised for their function?
these cells have has multiple dendrites - extensions of the cell membrane which increase the cell surface interaction with the environment
these cells are enriched in mucosal tissues because these are exposed to the environment
B-cells
Highly abundant in blood and mucosal tissues
Receptor-mediated internalisation of antigens, as opposed to phagocytosis
Primary function is making antibodies, but still very good at antigen presentation
Possibly main inducer of T-cell immune response to pathogens, such as Neisseria meningitidis
Endogenous antigen processing - the steps
LHS slide 17
UPTAKE
Antigens/pathogens already present in cell cytoplasm
DEGRADATION
Antigens undergo proteolytic degradation in cytoplasm
ANTIGEN-MHC COMPLEX FORMATION Peptide antigens transported to ER, enter via TAP and loaded onto MHC class I molecules
PRESENTATION
Transport and expression of antigen-MHC complexes on the surface of cells for recognition by CD8 T cells
eliminated by death by cytotoxic T cells
why do we need endogenous antigen processing - isn’t exogenous processing sufficient
Macrophages have well-developed lysosomal systems
They are 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
Viruses can infect most cell types
A non-lysosomal mechanism to process antigens for presentation to T cells is required
Non-lysosomal antigen processing requires what?
protein synthesis
antigens from inactive viruses and infectious viruses - how are they processed?
ANTIGENS FROM INACTIVE VIRUSES ARE PROCESSED VIA THE EXOGENOUS PATHWAY
ANTIGENS FROM INFECTIOUS VIRUSES ARE PROCESSED VIA THE ENDOGENOUS PATHWAY
Exogenous antigen processing? (RHS slide 17)
antigens are endocytosed into the cell, in the endosome they are broken down into peptides
loaded onto MHC class II molecules, recognised by CD4 T cells
eliminated by antibodies and phagocyte activation by T helper cells
comparison of MHC class I and II?
class I expressed on all nucleated cells, class II expressed on APCs and activated T-cells
class I binds short peptides, class II binds long peptides
class I presents to CD8 T cells, II presents to CD4 T cells
class I presents antigens from the cytosol, class II presents antigens from phagosomes and endosomes
The T-cell receptor:
- what does it bind to?
- how does it exist?
T cell receptor binds to peptide MHC complexes, cannot recognise peptide alone
Exists in a TCR complex with accessory molecules such as CD3
T cell receptor and B cell receptor - similarities and differences
Similarities to B cell receptor/antibody:
Belongs to Ig superfamily
Fab-like fragment (peptide binding site for antibody)
Large diversity
Single specificity
Differences to B cell receptor/antibody:
Lower affinity Cannot be released (remains on cell surface) No Fc fragment, so no cellular functions Single binding site rather than 2 B cell receptor/Ab: 5 classes T cell receptor: 2 classes
Mechanisms which generate B-cell and T cell receptor diversity?
before antigen stimulation: somatic recombination
after antigen stimulation (BCR only): somatic hypermutation
when does T cell receptor gene rearrangement occur?
takes place during T-cell development in thymus
Three signal model of T-cell activation?
- Peptide-MHC (pMHC)
- main signal delivered from the APC by a peptide-MHC complex to the TCR - Co-stimulation
- delivered from the APC by germline-encoded accessory receptors, eg. CD80 and CD86 - Cytokines
- 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
TH1 and TH2 produce cytokines that activate other cells
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
needs to be a direct physical attraction between APC and TCR
The immunological 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
do CD4+ and CD8+ cells secrete cytokines?
Most cd4+ cells secrete cytokines to help the immune process, but cells that express cd8 are cytotoxic T cells which kill pathogens directly
Negative regulators of antigen presentation
2 important regulatory molecules on T cells which dampen the immune response
- CTLA4 (Cytotoxic T-Lymphocyte-Associated Protein 4)
- blocks TCR activation via SHP-2, which dephosphorylates TCR signalling molecules - PD-L1 (Programmed Death-Ligand 1) are crucial for dampening the T-cell response
- competes with CD28 for APC ‘attention’
why is negative regulation needed?
because an overly-vigorous immune response is harmful to the host
what is positive and negative selection?
removal of defected T cells - T-cells arise from the thymus (‘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
how is excessive T cell activation down regulated?
T regulatory cells which express FOXP3
secrete anti- inflammatory cytokines:
- Interleukin-10
- TGF-β
2 types of T cells?
helper (Th) or cytotoxic (Tc)
Pathogens impeding antigen presentation - examples?
Mycobacterium tuberculosis
- Blocks MHC Class II expression
- Up-regulates PD-L1 on APCs to shut down T-cell activation
Neisseria meningitidis
-Blocks DC activation
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-L1 on T-cells, which antagonises TCR signalling
HSV
-Produce protein which inhibits TAP, prevents viral peptide transfer to ER
Adenovirus Produce protein which binds MHC class I molecule, prevents it from leaving ER
