Ruxolitinib reverses dysregulated T helper cell responses and controls autoimmunity caused by a novel signal transducer and activator of transcription 1 (STAT1) gain-of-function mutation

Background: Gain-of-function (GOF) mutations in the human signal transducer and activator of transcription 1 (STAT1) manifest in immunodeficiency and autoimmunity with impaired T(H)17 cell differentiation and exaggerated responsiveness to type I and II interferons. Allogeneic bone marrow transplantation has been attempted in severely affected patients, but outcomes have been poor. Objective: We sought to define the effect of increased STAT1 activity on T helper cell polarization and to investigate the therapeutic potential of ruxolitinib in treating autoimmunity secondary to STAT1 GOF mutations. Methods: We used in vitro polarization assays, as well as phenotypic and functional analysis of STAT1-mutated patient cells. Results: We report a child with a novel mutation in the linker domain of STAT1 who had life-threatening autoimmune cytopenias and chronic mucocutaneous candidiasis. Naive lymphocytes from the affected patient displayed increased T(H)1 and follicular T helper cell and suppressed T(H)17 cell responses. The mutation augmented cytokine-induced STAT1 phosphorylation without affecting dephosphorylation kinetics. Treatment with the Janus kinase 1/2 inhibitor ruxolitinib reduced hyperresponsiveness to type I and II interferons, normalized T(H)1 and follicular T helper cell responses, improved T(H)17 differentiation, cured mucocutaneous candidiasis, and maintained remission of immune-mediated cytopenias. Conclusions: Autoimmunity and infection caused by STAT1 GOF mutations are the result of dysregulated T helper cell responses. Janus kinase inhibitor therapy could represent an effective targeted treatment for long-term disease control in severely affected patients for whom hematopoietic stem cell transplantation is not available.