IL-27 is a cytokine with pro- and anti-inflammatory properties, that can regulate T helper cell development, suppress T cell proliferation, stimulate cytotoxic T cell activity, induce isotype switching in B cells, and that has diverse effects on innate immune cells. Among its target cells are CD4 T helper cells which can differentiate in type 1 effector cells (TH1), type 2 effector cells (TH2) and IL-17 producing helper T cells (TH17). It drives rapid clonal expansion of naive but not memory CD4 T cells. It also strongly synergizes with IL-12 to trigger interferon-gamma/IFN-gamma production of naive CD4 T cells, binds to the cytokine receptor WSX-1/TCCR. Another important role of IL-27 is its antitumor activity as well as its antiangiogenic activity with activation of production of antiangiogenic chemokines.