H2-O deficiency in B cells or dendritic cells drives retrovirus-neutralizing antibody responses.

Major histocompatibility complex class II (MHC II)-restricted antigen presentation is central to CD4+ T-cell activation and the generation of high-affinity, class-switched antibody (Ab) responses. Peptide loading of MHC II in endosomal compartments is modulated by the functional interplay of two nonclassical MHC II molecules, H2-M and H2-O. H2-M binds to MHC II and facilitates loading of MHC II with high-affinity peptides. H2-O, a structural mimic of MHC II, modulates the MHC II pathway by binding to H2-M and inhibiting its peptide-loading catalytic activity. Although H2-O has a modest effect on the overall cell-surface MHC II peptide repertoire, these changes are sufficient to drive significant immunological consequences. H2-O plays a protective role against infection with ubiquitous γ-herpesviruses, while H2-O deficiency enables the development of protective, pathogen-specific Ab responses to mouse retroviruses and Staphylococcus aureus. Here, we further investigate the role of H2-O in regulating humoral immunity to a mouse retrovirus, mouse mammary tumor virus (MMTV). Using mosaic and conditional genetic approaches, we demonstrate that H2-O deficiency, and thus enhanced H2-M-mediated peptide loading, in either B cells or dendritic cells, is sufficient to drive a robust neutralizing Ab response against MMTV. Additionally, we show that the pronounced reduction of H2-O protein in germinal center B cells is governed by a post-transcriptional mechanism, revealing a previously unappreciated mechanism of H2-O regulation during the germinal center reaction. Collectively, these findings further establish H2-O as a tunable checkpoint in the MHC II antigen presentation pathway that shapes the quality of humoral immune responses.IMPORTANCEProtective antibody responses depend on the presentation of pathogen-derived peptides by major histocompatibility complex (MHC) class II molecules to activate CD4+ helper T cells. This process is tightly regulated. H2-O functions as a negative regulator of antigen presentation by limiting the repertoire of peptides displayed by MHC class II molecules, thereby shaping the antibody response. In this study, we show that the loss of H2-O in either of two key antigen-presenting cell types-B cells or dendritic cells-is sufficient to elicit a robust protective antibody response against a mouse retrovirus. We further demonstrate that germinal center B cells downregulate H2-O through protein degradation during the immune response. Together, these findings identify H2-O as a tunable checkpoint that regulates the magnitude and quality of antibody responses and provide additional evidence that B cells are key antigen-presenting cells.