Abstract Text: Signal transducer and activator of transcription 3 (STAT3) plays pleiotropic roles in hematopoietic and non-hematopoietic cells, by regulating gene expression downstream of cytokine and hormone receptors. STAT3 constitutive activation and somatic gain-of-function (GOF) mutations recur in B-lymphomas and germline heterozygous STAT3 GOF mutations cause early-onset multi-organ autoimmune disease. Affected individuals variably present with type-1 diabetes, autoimmune thyroid disease (AITD), rheumatoid arthritis, gut enteropathies and autoimmune cytopenias. The AITD, cytopenias, hypogammaglobulinemia and B-memory lymphopenia observed in STAT3 GOF syndrome point to B cell tolerance defects. However, little is known of the B cell-intrinsic effects of overactive STAT3. Here, we address this question in human patients and mice engineered to carry the most common mutation causing STAT3 GOF syndrome, STAT3T716M, or STAT3K658N found in malignancy and STAT3 GOF syndrome. We demonstrate that GOF STAT3 causes aberrant accumulation of polyclonal CD21low B cells resembling those that accumulate in humans and mice with age, chronic infections, immunodeficiency and autoimmune disease. STAT3 GOF allowed aberrant accumulation of self-reactive SWHEL B cells recognising a blood cell-surface autoantigen, and in humans of VH4-34+ B cells recognising the I/i self-antigen. We use ex vivo cultures, BCR deep-sequencing, flow cytometry, single-cell RNA sequencing and chromatin immunoprecipitation sequencing to reveal cell-intrinsic effects of overactive STAT3 in CD21low B cells. Our findings reveal the landscape of genes and proteins dysregulated by overactive STAT3 in B cells. We propose a novel mechanism to help explain the over-accumulation of autoantibody-enriched CD21low B cells in autoimmune diseases associated with IL-6 and IL-21 over-abundance.
