Serine Phosphorylation of the STAT1 Transactivation Domain Promotes Autoreactive B Cell and Systemic Autoimmunity Development.

Although STAT1 tyrosine-701 phosphorylation (designated STAT1-pY701) is indispensable for STAT1 function, the requirement for STAT1 serine-727 phosphorylation (designated STAT1-pS727) during systemic autoimmune and antipathogen responses remains unclear. Using autoimmune-prone B6.Sle1b mice expressing a STAT1-S727A mutant in which serine is replaced by alanine, we report in this study that STAT1-pS727 promotes autoimmune Ab-forming cell (AFC) and germinal center (GC) responses, driving autoantibody production and systemic lupus erythematosus (SLE) development. In contrast, STAT1-pS727 is not required for GC, T follicular helper cell (Tfh), and Ab responses to various foreign Ags, including pathogens. STAT1-pS727 is also not required for gut microbiota and dietary Ag-driven GC and Tfh responses in B6.Sle1b mice. By generating B cell-specific bone marrow chimeras, we demonstrate that STAT1-pS727 plays an important B cell-intrinsic role in promoting autoimmune AFC, GC, and Tfh responses, leading to SLE-associated autoantibody production. Our analysis of the TLR7-accelerated B6.Sle1b.Yaa SLE disease model expressing a STAT1-S727A mutant reveals STAT1-pS727-mediated regulation of autoimmune AFC and GC responses and lupus nephritis development. Together, we identify previously unrecognized differential regulation of systemic autoimmune and antipathogen responses by STAT1-pS727. Our data implicate STAT1-pS727 as a therapeutic target for SLE without overtly affecting STAT1-mediated protection against pathogenic infections.