Gut microbiota influence B cell function in a TLR5-dependent manner

Toll-like receptor (TLR) 5-deficient mice display aberrantly low levels of flagellin-specific antibodies (Flic-IgA) secreted into the gut, combined with excess bacterial flagellin in the gut, and together these attributes define microbiome dysbiosis (T5-dysbiosis). How TLR5 signaling deficiency results in T5-dysbiosis is unclear. Here, we address the role of B cells in T-dysbiosis. We observed that B cells do not express TLR5, and that B cell transplantation from TLR5-/- mouse donors into B-cell deficient mice resulted in a slight reduction in Flic-IgA levels compared to B-cells from WT donors. Bone marrow transplants from WT and TLR5-/- donors into recipients of both genotypes confirmed that TLR5 signaling by non-hematopoietic cells is required for T5-dysbiosis. We observed TLR5 deficiency was associated with an expanded population of IgA+ B cells. TLR5-/- mice tended to have higher richness for the IgA gene hypervariable region (CDR3 gene) variants. Transplantation of microbiomes from TLR5-/- and WT microbiomes donors into germfree mice resulted in a higher proportion of IgA-secreting B cells, and higher overall fecal IgA and anti-Flic IgA for TLR5-/- microbiome recipients. This observation indicated that the TLR5-/- mouse microbiome elicits an anti-flagellin antibody response that requires TLR5 signaling. Together these results indicate that TLR5 signaling on epithelial cells influences B cell populations and antibody repertoire.