Selection of the MHC class II-associated peptide repertoire by HLA-DM.

During the past five years considerable progress has been made in the field of major histocompatibility complex (MHC) class II-restricted antigen presentation. Several observations made in mutant cell lines with a presentation defect led to the identification of a novel protein, the nonclassic MHC class II molecule human leukocyte antigen (HLA)-DM. Cell biological and biochemical characterization of HLA-DM provided deeper insight into the molecular mechanism underlying the loading process: HLA-DM accumulates in acidic compartments where it binds to classic class II molecules as long as no high-stability ligand occupies the peptide-binding groove. Thus, HLA-DM prevents empty alpha beta dimers from functional inactivation in a chaperone-like fashion. At the same time HLA-DM acts as an editor by removing low-stability ligands, thereby skewing the class II peptide repertoire presentable to T-helper cells.