The structure and T cell stimulatory effects of the recently discovered cytokine Interleukin-23 (IL-23) are similar to, but distinct from, those of Interleukin-12 (IL-12). IL-23, which composed of the p40 subunit of IL-12 and a newly identified p19 molecule, is normally secreted by activated macrophage and dendritic cells. IL-23 preferentially induces proliferation of, and Interferon-
