TRAF6-TAK1 signaling drives Th9 differentiation

Abstract

Abstract Naive CD4+ T cells differentiate into different subsets depending upon milieu of cytokine exposure. Interleukin-9 (IL-9)-producing CD4+ helper T cells (Th9 cells) are induced from naive CD4+ T cells by transforming growth factor-β (TGF-β) together with interleukin-4 (IL-4) during stimulation of the T cell antigen receptor (TCR) in vitro. The master regulator of Th9 cells as well as the molecular mechanisms by which TGF-β signals and IL-4 signals co-operatively drive the differentiation of Th9 cells are not fully understood. Previously, our group has reported that inhibitor of DNA-binding 3 (Id3), a transcription factor which inhibits DNA-binding of basic helix-loop-helix protein, such as E2A, controls the TGF-β-mediated differentiation of Th9 cells. We found that this process required activation of the kinase transforming growth factor beta-activated kinase 1 (TAK1). In addition, RNA-seq revealed that tumor necrosis factor receptor-associated factor 6 (TRAF6) was highly up-regulated by TGF-β together with IL-4 in CD4+ T cells. Since TRAF6 is one of the TAK-1 activator, we hypothesized TRAF6 is also involved in the differentiation of Th9 cells. We found that the expression and ubiquitination of TRAF6 was upregulated under the Th9 differentiation condition and it resulted in activation of TAK1. These regulations were attenuated both in TGF-β and Smad3 deficient mice, suggesting TRAF6-TAK1 signaling is involved in TGF-β-mediated Th9 differentiation.