TFR cells regulate the specificity of anti-commensal IgA in the gut and repress global IgA responses via IL-10

Immunoglobulin A (IgA) most predominant antibody isotype at mucosal surfaces and is regulated by both T cell-dependent (TD) and independent (TI) mechanisms. In the TD pathway, T follicular helper (TFH) cells provide help to B cells to generate high-affinity antibodies. T follicular regulatory (TFR) cells fine-tune antibody responses, but precisely how these cellsregulate microbiota-directed IgA at mucosal sites has been unclear. Here, we used a TFR-deficient mouse model (Foxp3Cre Bcl6fl/fl; Bcl6FC) in which the gut microbiota develops in the absence of TFR cells, while the broader regulatory T cell compartment remains intact. While Bcl6FC mice showed similar levels of IgA-coated commensal bacteria to wild-type mice in the naïve state, following oral immunization, Bcl6FC mice exhibited a durable increase in IgA-coated commensal bacteria in feces and small intestine, without changes in overall community composition or epithelial barrier permeability. IgA-seq analysis of Bcl6FC mice revealed an increased diversity of IgA-coated taxa, both in naïve and orally immunized mice. IgA coating index analysis, a measure of IgA affinity, identified higher IgA binding to multiple taxa in Bcl6C mice compared to control mice. Consistent with these findings, sera from immunized Bcl6FC mice showed increased IgA binding to Alloprevotella and Klebsiella species but not to E. coli or Group A/B Streptococcus compared to control mice. B cell receptor repertoire sequencing demonstrated divergent patterns of somatic hypermutation (SMH) for IgA and IgG, where TFR cells repressed IgA SMH but enhanced IgG somatic hypermutation. Mechanistically, suppression of commensal-directed IgA required TFR-derived IL-10 but was independent of CTLA-4. IL-10 directly inhibited TGF-β-driven IgA class switching of B cells in vitro. Together, our data identify TFR cells as critical gatekeepers of mucosal IgA responses, constraining commensal-specific IgA responses through a novel IL-10-dependent pathway. Our findings haveimportant implications for regulation of the microbiome by TFR cells and IgA.