The unique features of Th9 cells and their products.

Although it was discovered more than two decades ago, new information concerning the biological activities of IL-9 has been provided in recent years, after the isolation of cells that selectively produce this cytokine, designated "Th9." Th9 cells are generated in vitro by polarization, mainly by TGF-β and IL-4, during activation with the specific antigen, or with anti-CD3/CD28 antibodies. This review deals mainly with Th9 generated by the former, "physiological" mode of activation. Of particular interest is the unique production kinetics of IL-9: the cytokine is produced very rapidly, but after reaching its peak (day 3 in our studies), it declines sharply to trace levels. In addition to IL-9, Th9 cells also produce similar amounts of another cytokine, IL-10, but the production kinetics of these two cytokines are strikingly different. Antigen-activated Th9 in our studies also developed pathogenic capacity, but only during the short time period of peak IL-9 production. Interestingly, no IL-9-producing cells were detected in sites of inflammation induced by Th9, in contrast to Thl and Thl7. The unique features of Th9 cells and their products are discussed with regards to the known and assumed functions of the cytokine.