Inhibitory effect of activin A on IL-9 production by mouse NK cells through Smad3 signaling.

Interleukin-9 (IL-9) is a cytokine secreted by T-helper (Th)9 cells, and activin A can enhance Th9 cell differentiation. However, whether activin A affects IL-9 production by natural killer (NK) cells remains unclear. Herein, we found that not only Th cells, but also CD3-CD49b+NKp46+ NK cells of Balb/c mice produced IL-9. Although activin A promoted IL-9 expression in CD4+ Th cells, it inhibited IL-9 production by CD49b+NKp46+ NK cells in mice. Furthermore, the enzyme-linked immunosorbent assay (ELISA) results showed that mouse NK cells could secrete mature IL-9 protein, and activin A inhibited IL-9 release by NK cells. Additionally, activin A inhibited interferon (IFN)-γ production in splenic NK cells in mice, but promoted IL-2 production, and did not alter the production of IL-10. Western blotting results showed that levels of activin type IIA receptor (ActRIIA), Smad3 and phosphorylated-Smad3 (p-SMAD3) protein increased in activin A-treated splenic NK cells, compared with that in control NK cells. The inhibitory effects of activin A on IL-9 production by NK cells were attenuated in the presence of activin antagonist follistatin (FST) or Smad3 knockdown to NK cells. These data suggest that although activin A up-regulates IL-9 expression in Th cells, it inhibits IL-9 production in NK cells through Smad3 signaling.