Overactive STAT3 Dysregulates BCR Signalling and Drives Accumulation and Autoreactivity of Disease-Associated CD21low B Cells

Introduction Signal transducer and activator of transcription 3 (STAT3) regulates gene expression downstream of cytokine/hormone receptors. Germline heterozygous STAT3 gain-of-function mutations cause early-onset multi-organ autoimmune disease, including type-1 diabetes, autoimmune thyroid disease, rheumatoid arthritis, enteropathies, autoimmune cytopenias, and/or B memory lymphopenia, many pointing to B cell tolerance defects. In populations, JAK-STAT3 polymorphisms and STAT3 signalling cytokines are associated with increased risk of B cell-mediated autoimmune diseases, and the cytokines’ levels correlate with disease severity. Finally, STAT3 can be mutated to gain-of-function and is frequently constitutively active in B lymphomas. Despite this, B cell-intrinsic effects of overactive STAT3 remain unknown. Method and Results We address this in patients, and mice engineered to carry the most common pathogenic germline gain-of-function mutation, STAT3T716M, or STAT3G421R and STAT3K658N, also found in lymphomas/leukemias. Gain-of-function STAT3 causes aberrant accumulation of polyclonal CD21low B cells resembling those that accumulate with age, chronic infections, immunodeficiency, and autoimmune diseases. It causes accumulation of self-reactive SWHEL B cells recognising a blood cell surface autoantigen and, in humans, of VH4-34+ B cells recognising I/i self-antigen on blood cells. Using mouse and human BCR deep sequencing, flow cytometry, single-cell RNA, and chromatin immunoprecipitation sequencing, we show overactive STAT3 induces a surprising and striking dysregulation of BCR signalling molecules, many of which are direct pSTAT3 targets, and dysregulates BCR-induced Tbet and atypical differentiation. We use in vivo inhibition to test whether STAT3-mutant atypical B cells are specifically depleted by BCR-PI3K inhibition. In a first-in-kind analysis, we study additive effects of JAK-STAT3 and BCR polymorphisms on atypical B cell abundance and mRNA phenotypes in single-cell expression quantitative trait loci and RNA sequencing data from >1,000 individuals. Conclusion Our findings reveal the landscape of genes and proteins dysregulated by overactive STAT3 in B cells and a novel mechanism explaining over-accumulation of autoantibody-enriched CD21low B cells in multiple diseases characterised by JAK-STAT polymorphisms and/or over-abundant IL-6/IL-21 cytokines.