TCR γδ cell-specific STAT5 gain of function induces a druggable chronic human immune dysregulation

Within a prospective cohort of patients with immune dysregulation, we identified several individuals with chronically increased proportions of TCR γδ cells but normal peripheral lymphocyte counts. Among those, we identified one individual with a TCR γδ cell-specific heterozygous p.Y665F STAT5B gain-of-function mutation. Recurrent oral aphthous lesions, susceptibility to infection, arthralgia, and fatigue, were linked to relatively elevated numbers of γδ T cells expressing a Vγ9Vδ2 TCR, displaying hyperphosphorylation of STAT5 upon in vitro IL-2 stimulation. The TCR Vγ9Vδ2 cells exhibited enhanced proliferative response to (E)-4-hydroxy-3-methyl-but-2-enyl pyrophosphate and dysregulated cytokine production. The TCR γδ cell transcriptome revealed the suppression of the default Th17 program, along with inhibition of RORC and MAF expression. The JAK inhibitor baricitinib improved clinical features of the observed immune dysregulation and reduced the frequency of peripheral TCR Vγ9Vδ2 cells. Thus, functionally altered TCR γδ cells may underlie chronic immune dysregulation of unknown molecular cause, demonstrated here to be amenable to tailored immune modulation.