Tocilizumab Promotes Regulatory T-cell Alleviation in STAT3 Gain-of-function−associated Multi-organ Autoimmune Syndrome

Purpose Signal transducer and activator of transcription 3 is a member of a family of proteins involved in the regulation of inflammation, differentiation, proliferation, and survival of cells. Here we describe a 38-year-old male who has experienced gastrointestinal, dermatologic, pulmonary, and malignant manifestations. Methods Whole-exome sequencing, validated by Sanger sequencing, was performed after extensive investigations. Findings Whole-exome sequencing revealed a heterozygous missense mutation in the signal transducer and activator of transcription 3 gene, c.1261G>A (p.G421R). Fluorescence-activated cell sorting analysis of peripheral T lymphocytes revealed low levels of CD4⁺CD25⁺FoxP3 and CD8⁺CD25⁺FoxP3 regulatory T cells. After treatment with 2 cycles of tocilizumab, an interleukin-6 receptor antibody, a significant increase in the level of regulatory T cells was observed, accompanied by clinical improvement. Implications This case sheds light on the emerging role of signal transducer and activator of transcription 3 gain-of-function mutation in the pathogenesis of autoimmune diseases, and further addresses the therapeutic role of interleukin-6 blocker treatment in this syndrome.