IL10 restrains autoreactive B cells in transgenic mice expressing inactive RAG1

IL10 plays a dual role in supporting humoral immunity and inhibiting inflammatory conditions. B cells producing IL10 are thought to play a key regulatory role in maintaining self-tolerance and suppressing excessive inflammation during autoimmune and infectious diseases, primarily by inhibiting associated T cell responses. The extent to which B cells, through the provision of IL10, might function to sustain or inhibit autoantibody production is less clear. We previously described transgenic mice expressing catalytically inactive RAG1 (dnRAG1 mice), which show expansion of an IL10-compentent CD5⁺ B cell subset that phenotypically resembles B10 B cells, hypogammaglobulinemia, and a restricted B cell receptor repertoire with features indicative of impaired B cell receptor editing. We show here that B10-like B cells in dnRAG1 mice bind the membrane-associated autoantigen phosphatidylcholine (PtC), and that in vitro lipopolysaccharide (LPS) stimulation of dnRAG1 splenocytes induces a robust IgM response enriched in reactivity toward lupus-associated autoantigens. This outcome was correlated with detection of sIgM^hi B cell populations that were distinct from, but in addition to, sIgM^int populations observed after similar treatment of wild-type splenocytes. Loss of IL10 expression in dnRAG1 mice had no significant effect on B10-like B cell expansion or the frequency of PtC⁺ B cells. Compared to IL10^+/+ dnRAG1 mice, levels of serum IgM, but not serum IgG, were highly elevated in some naïve IL10^-/- dnRAG1 mice, and was correlated with a significant increase in serum BAFF levels. Differentiation of sIgM^int B cells from LPS-stimulated dnRAG1 splenocytes was enhanced by loss of IL10 expression and IL10 blockade, but was suppressed by treatment with recombinant IL10. In vitro LPS-induced differentiation and antibody production was inhibited by treatment with JAK/STAT inhibitors or a synthetic corticosteroid, independent of IL10 expression and genotype. Taken together, these data suggest that IL10 expression in dnRAG1 mice maintains suppression of IgM levels in part by inhibiting BAFF production, and that regulatory B10-like B cells, through the provision of IL10, constrains B cell differentiation in response to mitogenic stimuli. Furthermore, autoantibody profiling raises a possible link between CD5⁺ B cell expansion, mitogenic stimulation, and autoantibodies associated with autoimmune complications observed in lupus and lupus-related disorders.