T helper-9 cells and Interleukin-9 in transplantation: The open question

The role of main TCD4+ lymphocyte subsets including T helper 1 (Th1), Th2, Th17, and T regulatory cells in transplantation has already been described; however, the implication of newly defined lineages such as Th22, Th9, and T follicular helper cells in alloimmune responses remain to be elucidated. In addition to the low number of studies, most evidence about the role of these cells in transplantation has been obtained from experimental studies, which might be insufficient or irrelevant for clinical interpretations. In the present article, we have reviewed the studies that have investigated the role of Th9 and its principal cytokine interleukin-9 (IL-9) in allograft rejection and tolerance induction. However, the findings tend to be controversial since some investigations demonstrate positive effects of Th9 on transplantation outcomes whereas others are suggestive of its detrimental influences. A similar challenge is presented by IL-9 as both advantages and disadvantages of IL-9 expression in allografts have been reported. Moreover, different organs appear to be affected in different ways by Th9 cells and IL-9. Therefore, more research particularly in human patients is required to provide sufficient data for drawing a concrete conclusion about the implication of Th9 and IL-9 in transplantation.