|Major Histocompatibility Complex, Class II, DO alpha|
|Alt. symbols||HLA-DZA, HLA-DNA|
|Locus||Chr. 6 p21.3|
|Major Histocompatibility Complex, Class II, DO beta|
|Locus||Chr. 6 p21.3|
Human leukocyte histocompatibility complex DO (HLA-DO) is an intracellular, dimeric non-classical Major Histocompatibility Complex (MHC) class II protein composed of α- and β-subunits which interact with HLA-DM in order to fine tune immunodominant epitope selection. As a non-classical MHC class II molecule, HLA-DO is a non-polymorphic accessory protein that aids in antigenic peptide chaperoning and loading, as opposed to its classical counterparts, which are polymorphic and involved in antigen presentation. Though more remains to be elucidated about the function of HLA-DO, its unique distribution in the mammalian body—namely, the exclusive expression of HLA-DO in B cells, thymic medullary epithelial cells, and dendritic cells—indicate that it may be of physiological importance and has inspired further research. Moreover, HLA-DO is stable in complex with HLA-DM, and its exhibited instability in the absence of HLA-DM, as well as its evolutionary conservation, further denote its biological significance and potential to confer evolutionary benefits to its host.
Studies on HLA-DO transfected fibroblast cells lines and on the HLA-DO mouse homolog, H-2O, provide most of the current knowledge on the protein. In 1985, the α- and β-chains were separately discovered, and in 1990, both chains were found to be co-expressed in one protein in H-2O. In contrast to other molecules of MHC class II, interferon gamma does not induce HLA-DO expression.
The binding of HLA-DO at the MHC class II peptide-exchange catalysis site suggested that it acts as a competitive inhibitor, although biochemical studies have established its complementary function to HLA-DM in fine tuning epitope selection.
During infection, exogenous antigen is internalized by phagocytosis or receptor-mediated endocytosis, and processed in hydrolytic enzyme-containing compartments of increasing acidity. Once the degraded antigen is 13-18 residues, it is ready to bind to MHC class II molecules. To bind to the MHC-class II protein, HLA-DM catalyzes the exchange of CLIP, a protein occupying the binding groove of MHC class II, with the antigenic oligopeptide. HLA-DO is strongly associated with HLA-DM throughout the catalyzed exchange.
Before the three-dimensional structure of complexed HLA-DO was elucidated by X-ray crystallography, its crystal structure was modeled after homology studies to classical MHC class II proteins. Following crystallization of the protein, HLA-DO was found to be conformationally similar to classical MHC class II protein, with alterations in the N-terminus. The structure of the free HLA-DO protein, however, remains to be elucidated.
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