New retroviral restriction factor
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 positional cloning approach, we have 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 potently 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 which were genetically linked to the differential outcomes of hepatitis B and C viral infections in humans. The process of loading of MHCII molecules with peptides is mediated by the interaction of MHCII with another non-polymorphic MHCII-like molecule, H2-M (HLA-DM in humans), which loads MHCII molecules with high-affinity, pathogen-derived peptides. 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 a direct competition between H2-O/DO and MHCII for binding to H2-M/DM was incomplete, because I/LnJ Ob, as well as some human DOa and DOb variants result in H2-O/DO molecules, which fail to inhibit peptide loading of MHCII and yet retain the ability to interact with H2-M/DM. Clearly, there are severe gaps in our knowledge of how H2-O/DO negatively regulates MHCII presentation, a central process directing effective immune responses. The studies in this renewal application seek to address these gaps. Specifically, Aim1 will elucidate the mechanism underpinning the loss of inhibition of H2-M function by the mutant H2-O found in I/LnJ mice as a means to understand how H2-O normally functions. Aim 2 seeks to discover novel genes involved in the regulation of the H2-O-dependent control of MHCII peptide loading and Aim 3 will determine how two different viruses (a retrovirus and a gherpesvirus) exploit H2-O regulation to suppress the anti-viral immune response and/or to promote viral pathogenesis.