Overview of IL-9 Signaling and its Primary Biological Effects of IL-9 Signaling in Different Immune Cell Types
Interleukin-9 (IL-9) is a pleiotropic cytokine that is produced by activated T lymphocytes. It signals through a receptor complex consisting of IL-9 R and the common gamma-chain/IL-2 R gamma subunit. IL-9 was initially identified as a mouse T cell and mast cell growth factor. It has subsequently been shown to regulate immunoglobulin production by B cells, enhance mast cell protease expression, and promote goblet cell hyperplasia and mucus production, suggesting a link between IL-9 and the development of allergic inflammation. Although IL-9 was originally thought to be produced primarily by Th2 cells, naïve CD4+ T cells differentiate into a distinct IL-9-secreting T cell subset known as Th9 cells in the presence of IL-4 and TGF-beta. Th9 cells secrete IL-9 and IL-10 (in mice), but do not produce cytokines characteristic of other T helper subsets. Since the precise role of Th9 cells in the pathogenesis of allergic inflammation and other human diseases is not currently well understood, growing interest in this area will help to better define the effects of IL-9 signaling.