Background Human genetic variations resulting in immunodeficiency and/or the propensity for autoimmunity represent “experiments of nature” that can advance our understanding of polygenic human diseases. Heterozygous gain- of-function (GOF) mutations in STAT1 induce immune dysregulation characterized by autosomal dominant chronic mucocutaneous candidiasis (CMC) and the propensity for humoral autoimmunity. However, given widespread expression of STAT1 in immune and non-immune lineages and engagement by multiple cytokine receptors, the immune mechanisms driving breaks in immune tolerance in STAT1 GOF syndrome remain poorly understood. In addition, STAT1 GOF variants are thought to enhance cytokine signaling by increasing total STAT1 protein levels, but the cause of this phenotype has not been identified.

Methods To gain insights into the complex roles for STAT1 in human immunity, we performed mass cytometry using PBMCs from STAT1 GOF patients. Affected subjects were studied prior to treatment with JAK inhibitors, allowing unique insight into the immune landscape of STAT1-driven immune dysregulation. In parallel, we generated a novel murine knock-in strain allowing cell-intrinsic expression of a pathogenic human STAT1 GOF mutation

Results We performed multiparameter immunophenotyping of pediatric STAT1 GOF patients and age-matched controls to identify immune characteristics of STAT1-driven inflammation. Affected patients exhibited expansion of CXCR3-expressing CD4+ T and B cell populations exhibiting surface markers indicative of B cell helper function and extrafollicular activation, respectively. Moreover, relative expansion of these adaptive immune populations correlated with serum autoantibody titers. To study underlying mechanisms, we generated a new Stat1 GOF transgenic model and confirmed the development of spontaneous humoral autoimmunity recapitulating the human phenotype. Despite clinical resemblance to human regulatory T cell (Treg) deficiency (IPEX syndrome), Stat1^GOF mice and humans exhibited normal Treg development and function. Rather, STAT1 GOF autoimmunity was driven by dysregulated STAT1-dependent signals downstream of the type 1 and type 2 interferon (IFN) receptors. Surprisingly, autoimmunity in Stat1^GOF mice lacking the type 1 IFN receptor (IFNAR) was only partially ameliorated, whereas loss of type 2 IFN (IFN-γ) signals prevented disease. Strikingly, IFN-γR deletion abolished the known increase in total STAT1 expression resulting in normalization of STAT1-dependent systemic inflammation.

Conclusions Since STAT1 regulates its own transcription, these findings highlight IFN-γ as the critical driver of a feedforward inflammatory cascade in STAT1 GOF syndrome. More broadly, our data provide new insights into the STAT1- dependent cellular mechanisms underlying both rare monogenic and more common polygenic autoimmune diseases, such as systemic lupus erythematosus.