CD4⁺ T‐cell subsets in transplantation

The identification of T-helper 9 (Th9), Th17, Th22 cells as distinct subsets of CD4(+) T cells has extended the Th1/Th2 paradigm in the adaptive immunity. In the past decade, many studies in animal models and clinical transplantation have demonstrated that interleukin-17 (IL-17) is involved in allograft rejection. It appears that Th17 cells together with Th1 and Th2 cells play an important role in mediating allograft rejection. Here, we summarize our current knowledge on the contribution of Th1, Th2, Th9, Th17, Th22, and follicular T-helper (Tfh) cells in allograft rejection. We also discuss the regulation of CD4(+) T-cell subsets by CD4(+) Foxp3(+) regulatory T cells (Tregs) in the context of transplantation tolerance.