MHC (major histocompatibility complex) Class II molecules are found only on a few specialized cell types, including macrophages, dendritic cells and B cells, all of which are professional antigen-presenting cells (APCs).
The peptides presented by class II molecules are derived from extracellular proteins (not cytosolic as in class I); hence, the MHC class II-dependent pathway of antigen presentation is called the endocytic or exogenous pathway.
Loading of class II molecules must still occur inside the cell; extracellular proteins are endocytosed, digested in lysosomes, and bound by the class II MHC molecule prior to the molecule's migration to the plasma membrane.
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Structure
Like MHC class I molecules, class II molecules are also heterodimers, but in this case consist of two homologous peptides, an α and β chain, both of which are encoded in the MHC. [1]
Because the antigen-binding groove of MHC class II molecules is open at both ends while the corresponding groove on class I molecules is closed at each end, the antigens presented by MHC class II molecules are longer, generally between 15 and 24 amino acid residues long.
Reaction to bacteria
Because class II MHC is loaded with extracellular proteins, it is mainly concerned with presentation of extracellular pathogens (for example, bacteria that might be infecting a wound or the blood). Class II molecules interact exclusively with CD4+ ("helper") T cells (THC). The helper T cells then help to trigger an appropriate immune response which may include localized inflammation and swelling due to recruitment of phagocytes or may lead to a full-force antibody immune response due to activation of B cells.
Synthesis
During synthesis, MHC class II is the result of dimerization of α and β chains, with the assistance of an invariant chain.[2] The invariant chain is a special polypeptide involved in the formation and deliverance of MHC class II protein.
The nascent MHC class II protein in the rough ER has its peptide-binding cleft blocked by the invariant chain (Ii; a trimer) to prevent it from binding cellular peptides or peptides from the endogenous pathway. The invariant chain also facilitates MHC class II's export from the ER in a vesicle. This fuses with a late endosome containing the endocytosed, degraded proteins. It is then broken down in stages, leaving only a small fragment called CLIP which still blocks the peptide binding cleft. An MHC class II-like structure, HLA-DM, removes CLIP and replaces it with a peptide from the endosome. The stable MHC class-II is then presented on the cell surface.
Genes
| Alpha | Beta | |
| HLA-DM | HLA-DMA | HLA-DMB |
| HLA-DO | HLA-DOA | HLA-DOB |
| HLA-DP | HLA-DPA1 | HLA-DPB1 |
| HLA-DQ | HLA-DQA1, HLA-DQA2 | HLA-DQB1, HLA-DQB2 |
| HLA-DR | HLA-DRA | HLA-DRB1, HLA-DRB3, HLA-DRB4, HLA-DRB5 |
References
- ^ "Histocompatibility". http://www.cehs.siu.edu/fix/medmicro/mhc.htm. Retrieved 2009-01-21.
- ^ School of Crystallography The Invariant chain
See also
External links
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