Reasons for rarity of Th17 cells in inflammatory sites of human disorders.

T helper 17 (Th17) cells have been reported to be responsible for several chronic inflammatory diseases. However, a peculiar feature of human Th17 cells is that they are very rare in the inflammatory sites in comparison with Th1 cells. The first reason for this rarity is the existence of some self-regulatory mechanisms that limit their expansion. The limited expansion of human Th17 cells is related to the retinoic acid orphan (ROR)C-dependent up-regulation of the interleukin (IL)-4 induced gene 1 (IL4I1), which encodes for a l-phenylalanine oxidase, that has been shown to down-regulate CD3ζ expression in T cells. This results in abnormalities of the molecular pathway which is responsible for the impairment of IL-2 production and therefore for the lack of cell proliferation in response to T-cell receptor (TCR) signalling. IL4I1 up-regulation also associates with the increased expression of Tob1, a member of the Tob/BTG anti-proliferative protein family, which is involved in cell cycle arrest. A second reason for the rarity of human Th17 cells in the inflammatory sites is their rapid shifting into the Th1 phenotype, which is mainly related to the activity of IL-12 and TNF-α. We have named these Th17-derived Th1 cells as non-classic because they differ from classic Th1 cells for the expression of molecules specific for Th17 cells, such as RORC, CD161, CCR6, IL4I1, and IL-17 receptor E. This distinction may be important for defining the respective pathogenic role of Th17, non-classic Th1 and classic Th1 cells in many human inflammatory disorders.