New Investigator Symposium Presentation 49th Annual Scientific Meeting of the Australian and New Zealand Society for Immunology 2021

Signal transducer and activator of transcription 3 (STAT3) is a latent transcription factor with pleiotropic roles in hematopoietic and non-hematopoietic cells, that regulates gene expression downstream of surface cytokine and hormone receptors. STAT3 constitutive activation and somatic gain-of-function (GOF) SH2 domain mutations recur in B cell malignancies. In addition, germline heterozygous STAT3 GOF mutations were recently shown to 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 and autoimmune cytopenias, and hypogammaglobulinemia and B cell memory lymphopenia also observed in patients with STAT3 GOF syndrome, point to defects in B cell tolerance checkpoints.

However, little is known of the B cell-intrinsic effects of overactive STAT3. Here, we address this question using mice engineered to carry the most common mutation causing STAT3 GOF syndrome, STAT3^T716M, or a mutation at the SH2 domain dimerization interface, STAT3^K658N, found in both malignancy and STAT3 GOF syndrome.

We demonstrate that GOF STAT3 causes aberrant accumulation of polyclonal CD21^low B cells similar to those previously shown to accumulate in mice and humans with age, chronic infections, immunodeficiency and autoimmune disease. We show that STAT3 GOF allows aberrant accumulation of self-reactive SW_HEL B cells recognising a blood cell-surface autoantigen. We use ex vivo cultures, along with B cell receptor deep-sequencing, flow cytometric analyses and high-throughput single-cell RNA sequencing paired with chromatin immunoprecipitation sequencing (ChIP-Seq), to reveal the cell-intrinsic effects of overactive STAT3 in CD21^low B cells. Finally, we propose a novel mechanism by which STAT3 drives aberrant differentiation and accumulation of CD21^low B cells. Our findings help explain the over-accumulation of autoantibody-enriched CD21^low B cells in autoimmune diseases associated with IL-6 and IL-21 over-abundance. They reveal the landscape of genes and proteins dysregulated by overactive STAT3 in B cells.