IFN-γ Suppresses STAT6 Phosphorylation by Inhibiting Its Recruitment to the IL-4 Receptor

Abstract

Polarized Th1 cells show a stable phenotype: they become insensitive to IL-4 stimulation and lose the potential to produce IL-4. Previously, we reported that IFN-γ played a critical role in stabilizing Th1 phenotype. However, the mechanism by which IFN-γ stabilizes Th1 phenotype is not clear. In this study, we compared STAT6 phosphorylation in wild-type (WT) and IFN-γ receptor knockout (IFNGR−/−) Th1 cells. We found a striking diminution of STAT6 phosphorylation in differentiated WT Th1 cells, but not in differentiated IFNGR−/− Th1 cells. The impairment of STAT6 phosphorylation in differentiated WT Th1 cells was not due to a lack of IL-4R expression or phosphorylation. Jak1 and Jak3 expression and phosphorylation were comparable in both cell types. No differential expression of suppressor of cytokine signaling 1 (SOCS1), SOCS3, or SOCS5 was observed in the two cell types. In addition, Src homology 2-containing phosphatase mutation did not affect IL-4-induced STAT6 phosphorylation in differentiated Th1 cells derived from viable motheaten (mev/mev) mice. These results led us to focus on a novel mechanism. By using a pulldown assay, we observed that STAT6 in WT Th1 cells bound less effectively to the phosphorylated IL-4R/GST fusion protein than that in IFNGR−/− Th1 cells. Our results suggest that IFN-γ may suppress phosphorylation of STAT6 by inhibiting its recruitment to the IL-4R.