Abstract Select humans and animals are able to control persistent viral infections via adaptive immune responses that include the development of neutralizing antibodies (Abs). However, the mechanisms underlying these exceptional protective responses remain largely unknown. Using a positional cloning approach, we identified a gene responsible for virus-neutralizing Ab responses in mice from the I/LnJ strain following infection with two distinct retroviruses, Mouse Mammary Tumor Virus (MMTV) and Murine Leukemia Virus (MuLV). This gene is H2-Ob (Ob), which encodes the b subunit (H2-Ob) of the ab obligate heterodimer H2-O. H2-O (DO in humans), is a non-classical Major Histocompatibility Class II (MHCII)-like molecule and a known negative regulator of the MHCII antigen presentation pathway. The recessive loss-of-function I/LnJ Ob allele allowed for the production of potent neutralizing Abs in infected mice. Subsequent bioinformatics and functional analyses of the human homologues (DOa and DOb) revealed both loss- and gain-of-function variants. Several of these variants were genetically linked to the differential outcomes of hepatitis B and C viral infections in humans which are controlled by neutralizing Ab responses. The process of loading of MHCII molecules with high-affinity, pathogen-derived peptides is mediated by the interaction of MHCII with another non-polymorphic MHCII-like molecule, H2-M (HLA-DM in humans). H2-M function is opposed by H2-O, which acts as an MHCII mimic, binding to H2-M and blocking its ability to catalyze MHCII peptide loading. Importantly, our studies showed that this accepted paradigm of direct competition between H2-O/DO and MHCII for interaction with H2-M/DM was incomplete, because at least 2 mouse Ob, as well as some human DOa and DOb variants result in H2-O/DO molecules, which bind to H2- M/DM but fail to inhibit its function. Clearly, there are gaps in our knowledge of the mechanism by which H2- O/DO negatively regulates MHCII presentation, a process that is central to controlling adaptive immune responses. Using positional cloning in the mouse and a biochemical approach, novel modulators of the MHCII pathway which have not been previously described were identified. We propose to study how these new modulators regulate Ag presentation by H2-O to direct potent virus-neutralizing antibody responses.