VETS2006 Lecture Notes - Lecture 9: Antigen-Presenting Cell, Mhc Class I, T Helper Cell
Helper T cells and MHC II
- CD4+ helper T cells recognize processed antigen presented to them in association with MHC
class II molecules
- MHC II oleules are epressed o the surfae of professioal atige presetig ells
(macrophages, dendritic cells and activated B cells)
- Professioal APCs preset eogeous atiges i MHC II to helper T ells. Eogeous
antigens are from outside the cell (e.g. Bacterial antigens) that have been phagocytosed +
processed.
Professional Antigen Presenting Cells
1. Macrophages
- Distributed throughout body, versatile.
- Not efficient presenters of antigen until activated.
- When activated (e.g. By gamma interferon) > increase expression of MHC II molecules and
production of co-stimulatory molecules.
2. Dendritic Cells
- Most effective antigen-presenters, distributed throughout body, including skin, spleen and
lymph nodes.
- A mixed population of cells, derived from three different sources, but all have characteristic
starfish shape.
- In resting state, express only small amounts of MHC II, but have large internal reserves of
MHC molecules so can rapidly increase expression following encounter with antigen.
3. Activated B Cells
- B cells are effective and important antigen-processing cells
- B cells bind specific antigen (i.e. Recognised by their BCRs) > ingest antigen > process antigen
> present in association with MHC II
- Advantage of B cells over other APCs = antigen concentration, i.e. If antigen in small
amounts, B cell acts like magnet to collect and concentrate it.
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Class II MHC molecules
- MHC II molecules consist of two linked protein chains (a and B)
- Peptide-binding groove formed between the two chains.
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-
- The peptide binding groove is open at either end, so ends of peptide can extend past the
groove.
- Peptides from 12-24 amino acids in length are presented.
- The critical amino acids that anchor the peptide are spaced along the groove (instead of
clustering at the ends as ith MHC class I)
- As with MHC class I, there is some promiscuity i.e. One type of MHC II might be able to bind
slightly different peptides, and different peptides might be able to fit into more than one
type of MHC class II molecule.
Processing of Exogenous Antigens
- Foreign antigens are phagocytosed by antigen presenting cells, and protein is broken down
into peptides.
- The endosomes containing antigenic peptides fuse with endosomes containing newly
synthesized MHC class II molecules.
- If they fit, the antigenic peptides are loaded into the peptide binding groove.
- MHC II plus peptide complex is transported to cell surface.
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Document Summary
Cd4+ helper t cells recognize processed antigen presented to them in association with mhc class ii molecules. Mhc ii (cid:373)ole(cid:272)ules are e(cid:454)pressed o(cid:374) the surfa(cid:272)e of (cid:862)professio(cid:374)al a(cid:374)tige(cid:374) prese(cid:374)ti(cid:374)g (cid:272)ells(cid:863) (macrophages, dendritic cells and activated b cells) Professio(cid:374)al apcs prese(cid:374)t (cid:862)e(cid:454)oge(cid:374)ous(cid:863) a(cid:374)tige(cid:374)s (cid:894)i(cid:374) mhc ii(cid:895) to helper t (cid:272)ells. E(cid:454)oge(cid:374)ous antigens are from outside the cell (e. g. bacterial antigens) that have been phagocytosed + processed. Not efficient presenters of antigen until activated. When activated (e. g. by gamma interferon) > increase expression of mhc ii molecules and production of co-stimulatory molecules: dendritic cells. Most effective antigen-presenters, distributed throughout body, including skin, spleen and lymph nodes. A mixed population of cells, derived from three different sources, but all have characteristic starfish shape. In resting state, express only small amounts of mhc ii, but have large internal reserves of. Mhc molecules so can rapidly increase expression following encounter with antigen: activated b cells. B cells are effective and important antigen-processing cells.
