Generation of Diversity in The T Cell Repertoire Flashcards
What is an antigen?
Antigen – A combination of ‘antibody’ and ‘generate’.
It is any molecule that can bind specifically to an antibody. They are normally immunogenic, so they will induce an immune response from the host.
However in everyday science, ‘antigen’ 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?
An epitope is a small portion of the antigen.
It is the target of antibodies, MHC and T cell receptors.
Adaptive immune reactions occur to specific epitopes (portions of the antigen) as opposed to the entire antigen itself.
Infection and vaccination usually induce polyclonal T- and B-cell responses – this means that multiple epitopes can be recognised on a single antigen.
What is the biggest difference in what B cells and T cells recognise?
The biggest difference between B cells and T cells is that T cells do not recognise native antigens.
How do T cells recognise antigens?
Instead, T cells process antigens in order to recognise them.
An antigen will generate multiple peptides, which can then be presented on the surface of an antigen- presenting cell.
Only when the peptide is presented on the surface will we get a T cell response.
How are the antigens taken up by APCs?
It can occur by phagocytosis, or via membrane Ig receptor-mediated uptake. In both instances the antigen will get presented, but they will produce slightly different immune responses.
Describe the types of APCs.
There are two main types: myeloid cells (which include monocytes and macrophages) and dendritic cells (the most advanced type of APC.
Dendritic cells are also known as ‘professional’ APCs, as there are other cells that do the same thing, but with a lower efficiency.
B cells can also present certain types of antigens, but they are less efficient in doing so.
Monocytes are blood-circulating cells, while macrophages are found in tissue. Macrophages are essentially terminally differentiated monocytes.
Describe macrophages and dendritic cells.
They’re rare in peripheral blood - but enriched in mucosal tissues.
They are highly phagocytic cells – induce strong T-cell responses and inflammation. This is important for protection against Mycobacterium tuberculosis.
Macrophages are better-equipped to kill pathogens (higher NO production); DCs are better at migrating to lymph nodes (via CCR7) and presenting antigen to T-cells.
They are both specialised, but ultimately have overlapping functions.
Describe B cells as a type of APC.
They are highly abundant in the blood and mucosal tissues.
They execute receptor-mediated internalisation of antigens, as opposed to phagocytosis.
Their primary function is to make antibody (plasma cell) – but they’re still very good at antigen presentation.
It is possibly the main inducer of the T-cell immune response to pathogens such as
Neisseria meningitidis.
Describe the process of endogenous antigen processing.
UPTAKE:
The antigens/ pathogens are already present in the cell.
DEGRADATION:
The antigens synthesised in the cytoplasm undergo limited proteolytic degradation in the cytoplasm.
ANTIGEN-MHC COMPLEX FORMATION: The loading of peptide antigens onto MHC class I molecules is different to the loading of MHC class II molecules.
PRESENTATION:
There is 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. 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, but viruses can infect most cell types.
A non-lysosomal mechanism to process antigens for presentation to T cells is required.
Describe cytosolic protein presentation.
The antigen/viral protein enters the proteasome, which cleaves it into multiple peptides. These are then loaded onto MHC Class I molecules and presented on the cell surface.
Describe non-lysosomal antigen processing.
Inactive virus raises a weak cytotoxic T lymphocyte (CTL) response.
The processing of antigens from inactive viruses is sensitive to
lysosomotrophic drugs.
Thus, antigens from inactive viruses are processed via the exogenous pathway.
Infectious virus raise a strong CTL response.
The processing of antigens from infectious viruses is NOT sensitive to lysosomotrophic drugs.
Most CTLs recognise antigens generated via a non-lysosomal pathway. Protein synthesis is required for non-lysosomal antigen processing.
Thus, antigens from infectious viruses are processed via the endogenous pathway.
How are antigens processed and presented via the two pathways?
In the endosomal pathway, the antigen is processed in the proteosome, cleaved into multiple peptides and presented on MHC Class I molecules that reside in the cytosol.
In the exogenous pathway, the antigen is endocytosed, then sequestered to lysosomes or endosomes and processed there. The resulting peptides are presented on MHC Class II molecules that reside in the lysosome/endosome. They are then loaded to MHC Class II on the surface of the cell.
Based on the way in which the antigen is acquired, the different pathways will activate different T cells: the exogenous pathway will activate CD8 T cells, and the endogenous pathway will activate CD4 T cells.
Different MHC will activate different T cells: MHC II will activate CD4 cells, and MHC I will activate CD8 cells.
How are exogenous and endogenous pathogens eliminated?
EXOGENOUS PATHOGENS:
They are eliminated by antibodies and phagocyte activation by T helper cells that use antigens generated by exogenous processing.
ENDOGENOUS PATHOGENS:
They are eliminated by the killing of infected cells by CTL that use antigens generated by endogenous processing.
Describe MHCs.
MHC stands for major histocompatibility complex. There are two types, I and II.
They are quite similar in structure as they both have alpha and beta subunits. The common thing between both of them is that both of them have the peptide binding groove, where the peptide binds.
The groove interacts with many different domains. If you modify that structure, the capacity of the MHC to bind to peptides will be lost.