HLA-DO and its effect on peptide binding to MHC Class II (106.29)

Abstract

Abstract True function of HLA-DO (DO), an MHC Class II accessory molecule remains a mystery. The leading model on it function proposes that DO inhibits the effects of DM. Most of those studies were have used in vivo mouse models, looking at the endpoint of peptide presentation, not taking into account the individual steps in the process. To directly study DO function, we designed a recombinant soluble DO that included leucine zipper for better dimerization of the DOα- and β- chains and expressed it in insect cells. Kinetic experiments involving DR1 and the peptide with or without DM and DO revealed unexpected results. We found that; a) DO diminishes the binding of two peptides that are DM-sensitive, b)DO has positive effects on binding of peptides that form DM-resistant complexes, c) DO only has an effect on peptide association and virtually no effect on peptide dissociation, and d) DO functionsdirectly on DR molecules.We propose that DO binds to the peptide-receptive MHC II molecules that DM generates either by dissociating the bound peptides, or opening of the empty groove. The role for DO then is to enforce another layer of control on peptide selection. In support of this hypothesis, using BIACore Surface Plasmon Resonance, we have measured direct binding of DO to an open conformation of HLA-DR1 in real time, while no binding to a closed DR1 conformation wasdetected.