Main features of human T helper 17 cells

In addition to T helper type 1 (Th1) and Th2 cells, Th17 cells are a third arm of effector CD4+ T cells. Human Th17 cells express RORC and CD161 and originate from RORC-expressing CD161+ precursors, which migrate to lymphoid tissue and differentiate into mature Th17 cells in response to interleukin (IL)-1 and IL-23. Human Th17 cells are rare in inflammed tissues for two reasons: (1) Th17 cells do not produce IL-2 and, therefore, do not proliferate in response to TCR signaling, mainly because of RORC-dependent IL-4I1–mediated mechanisms that interfere with IL-2 gene activation; and (2) Th17 cell shift to a Th1 phenotype in the presence of IL-12; such Th17-derived Th1 cells are considered to be nonclassical Th1 cells and can be distinguished from classical Th1 cells. The possible role of Th17 cells in human tumors is still unclear and even controversial.