Cutting Edge: Systemic Autoimmunity in Murine STAT3 Gain-of-Function Syndrome Is Characterized by Effector T Cell Expansion in the Absence of Overt Regulatory T Cell Dysfunction

Germline gain-of-function mutations in the transcriptional factor STAT3 promote early-onset multisystemic autoimmunity. To investigate how increased STAT3 promotes systemic inflammation, we generated a transgenic knock-in strain expressing a pathogenic human mutation STAT3K392R within the endogenous murine locus. As predicted, STAT3K392R mice develop progressive lymphoid hyperplasia and systemic inflammation, mirroring the human disease. However, whereas the prevailing model holds that increased STAT3 activity drives human autoimmunity by dysregulating the balance between regulatory T cells and Th17 cell differentiation, we observed increased Th17 cells in the absence of major defects in regulatory T cell differentiation or function. In addition, STAT3K392R animals exhibited a prominent accumulation of IFN-γ–producing CD4+ and CD8+ T cells. Together, these data provide new insights into this complex human genetic syndrome and highlight the diverse cellular mechanisms by which dysregulated STAT3 activity promotes breaks in immune tolerance. Key Points Human STAT3 gain-of-function mutation promotes autoimmunity in a murine model. Regulatory T cell development and function in STAT3 GOF mice are preserved. Enhanced STAT3 promotes in vivo expansion of Th1 and Th17 CD4+ T cells.