Mouse HLA-DPA homologue H2-Pa: a pseudogene that maps between H2-Pb and H2-Oa

The major histocompatibility complex (MHC) class II subregion contains several subclasses of genes. The classical class II genes, HLA-DP, DQ, and DR homologues, present antigens directly to CD4 super(+) T cells. HLA-DM homologues facilitate the efficacy and transport of antigens to the cell surface by removing the CLIP peptides from the classical class II molecules. HLA-DNA/DOB homologues show unusual expression patterns and limited polymorphism, but their function is yet to be elucidated. Many mammalian species contain in their genome three classical class II genes, HLA-DP, HLA-DQ, and HLA-DR homologues, but their usage is variable among species. In humans, all three classical class II genes are functional although HLA-DP is less polymorphic than others, so that its role in antigen presentation may be minor. In the rat, RT1.D (HLA-DR homologue) and RT1.B (RT1.DQ homologue) are functional but RT1.H may not contribute to immune function, since, as reported previously, the RT1.H locus consists of a weakly transcribed alpha chain gene and a defective beta chain gene. In the mole-rat, two subclasses, Smh-P (HLA-DP homologue) and Smh-Q (HLA-DQ homologue), have been identified, but the HLA-DR homologue is deleted or has yet to be found. Smh-P is probably functional, since three loci have been found in the Smh-P subclass, and at least one of these may be polymorphic.