IL-27 is a heterodimeric cytokine that consists of EBI3, an IL-12p40-related protein, and p28, a IL-12p35-related polypeptide. IL-27 triggers expansion of antigen-specific naive CD4-positive T cells and promotes polarization towards a Th1 phenotype with expression of gamma-interferon. IL-27 contributes to the development of an adaptive immune response through its action on CD4-positive T cells, and also directly acts on cells of the innate immune system. IL-27 protein levels increase upon activation of antigen-presenting cells. IL-27 protein induces orphan cytokine receptor IL-27R (WSX-1)-dependent clonal expansion of naive but not memory CD4(+) T cells. IL-27 signaling through IL-27R and gp130 also induces phosphorylation of Stat1-5.