T-helper 17 cells in psoriatic plaques and additional genetic links between IL-23 and psoriasis.

T-helper 17 (Th17) cells are a newly appreciated T-cell subset, distinct from both Th1 and Th2 cells, that have been implicated in the pathogenesis of psoriasis and other autoimmune inflammatory diseases (Figure 1) (Fitch et al., 2007; Kastelein et al., 2007). IL-23 stimulates survival and proliferation of Th17 cells and thus serves as a key master cytokine regulator for these diseases.