TLR7 and IFNγ signaling converge at STAT1 in B cells to control germinal center mediated autoimmunity

Abstract

Abstract Toll like receptor 7 (TLR7) is required for the development of spontaneous germinal center (Spt-GC) mediated autoimmunity. Accordingly, autoimmune-prone B6.Sle1b mice carrying an extra copy of TLR7 in the Yaa locus (named B6.Sle1bYaa mice) exhibit significantly elevated Spt-GC responses that strongly correlate with exacerbation of lupus disease. However, the underlying molecular mechanisms in TLR7 driven autoimmunity are not clearly defined. Here we report that overexpression of TLR7 or stimulation with its agonist, imiquimod increases IFNγR and STAT1 expression, and the strength of IFNγR-STAT1 signaling in B cells. B6.Sle1bYaa mice deficient in IFNγR (Sle1bYaa.IFNγR−/−) fail to develop Spt-GCs, resulting in markedly reduced serum autoantibody (autoAb) titers and renal pathology. Further, B cell-intrinsic IFNgR or STAT1 deficient mice fail to generate Spt-GC responses upon imiquimod treatment. Interestingly, TLR7 stimulation induces STAT1 phosphorylation at serine-727 in B cells. Consistently, lupus-prone B6.Sle1b mice with alanine substitution for serine-727 in STAT1 (Sle1b.STAT1-S727A) show significantly reduced Spt-GC responses, resulting in decreased number of ANA-producing antibody forming cells and serum autoAb titers. Finally, using mixed bone marrow chimeras, we find that B cell-intrinsic STAT1-S727 phosphorylation is required for increased autoAb titers in B6.Sle1b mice. Mice with S727A mutation in STAT1, however, respond normally to T-dependent protein immunization. Together these data highlight the importance of a crosstalk between IFNγ and TLR7 signaling in B cells at STAT1 in Spt-GC responses and lupus-like autoimmunity.