STAT6 Signaling Attenuates Interleukin-17-Producing γδ T Cells during Acute Klebsiella pneumoniae Infection

gamma delta T cells are prevalent at mucosal and epithelial surfaces and are a critical first line of defense against bacterial and fungal pathogens. gamma delta 17 cells are a subset of gamma delta T cells which, in the presence of IL-23 and IL-1 beta, produce large quantities of interleukin-17A (IL-17A), a cytokine crucial to these cells' antibacterial and antifungal function. STAT6, an important transcription factor in Th2 differentiation and inhibition of Th1 differentiation, is expressed at high levels in the T cells of people with parasitic infections and asthma. Our group and others have shown that STAT6 attenuates IL-17A protein expression by CD4(+) T cells. By extension, we hypothesized that STAT6 activation also inhibits innate gamma delta 17 cell cytokine secretion. We show here that gamma delta 17 cells expressed the type I IL-4 receptor (IL-4R), and IL-4 increased STAT6 phosphorylation in gamma delta T cells. IL-4 inhibited gamma delta 17 cell production of IL-17A. IL-4 also decreased gamma delta 17 cell expression of IL-23R as well as Sgk1. To determine whether STAT6 signaling regulates gamma delta 17 cell numbers in vivo, we used a model of Klebsiella pneumoniae in mice deficient in STAT6. We chose K. pneumoniae for our in vivo model, since K. pneumoniae increases IL-17A expression and gamma delta 17 numbers. K. pneumoniae infection of STAT6 knockout mice resulted in a statistically significant increase in the number of gamma delta 17 cells compared to that of wild-type mice. These studies are the first to demonstrate that gamma delta 17 cells express the type I IL-4R and that STAT6 signaling negatively regulates gamma delta 17 cells, a cell population that plays a front-line role in mucosal immunity.