Th17 and Treg cells, two new lymphocyte subpopulations with a key role in the immune response against infection.

In addition to the T helper 1 (Th1) and Th2 lymphocyte subsets, two new subpopulations Th17 and regulatory T (Treg) cells have recently been described. Th17 cells, which produce high levels of interleukin (IL)-17, are dependent on the transcription factor orphan nuclear receptor RORC2/RORgammat and have been implicated in exacerbating the immune response to infections. Conversely, Treg cells, either thymus-derived or generated upon TCR activation of naîve T cells, express the transcription factor forkhead box P3 (Foxp3) and have regulatory functions mediated through either direct cell-cell contact or immuno-suppressive cytokines, being able to suppress the activation of T, B and NK cells. Based on the current knowledge of Th17 and Treg cell functions, new therapeutic strategies start to emerge, involving anti-cytokine treatments targeting Th17 functions or cell-based treatments in which Treg cells are generated from T cells either through Foxp3 gene transfer onto T cells with known specificities or transferring specific TCR genes onto Treg cells.