Investigating the potential binding partners of H2-O to further elucidate its role in MHCII antigen presentation

Abstract

Abstract The production of neutralizing antibodies in response to viral pathogens requires CD4+ T cell help. For this to occur, pathogen-derived peptides must be loaded onto class II major histocompatibility complex (MHC II) molecules in endosomal/lysosomal compartments and trafficked to the surface of antigen presenting cells. The αβ heterodimeric nonclassical MHCII molecules, H2-M and H2-O, modulate peptide loading. While H2-M directly catalyzes MHCII peptide loading, H2-O is a negative regulator of MHCII antigen presentation. Current dogma supports that H2-O is an MHCII structural mimic that exerts control over peptide loading via direct competition with MHCII for H2-M-binding. Work done in our labs, however, suggests this current model is incomplete and suggests the involvement of other, yet undefined, factors. I/LnJ mice, which are resistant to retroviruses through the production of potent neutralizing antibodies, inherit an H2-Ob allele that encodes nonfunctional H2-O. I/LnJ H2-O binds to but does not inhibit H2-M, directly challenging current dogma. Compared with H2-Oβ from retrovirus-susceptible mice, I/LnJ H2-Oβ contains three unique amino acid (AA) substitutions in the conserved immunoglobulin domain which we have found to be critical for the H2-O inhibitory activity of H2-M. We hypothesize that altered H2-O function in I/LnJ mice is due to an inability to interact with essential binding partners. We recently generated B6J mice in which we have knocked-in these three AA substitutions. This novel tool will be exploited to discover potential binding partners of variant H2-Oβ and elucidate the mechanism by which H2-O activity is modulated.