A unique role for IL-23 in promoting cellular immunity

Recent discoveries of interleukin (IL)‐23, its receptor, and its signal‐transduction pathway add to our understanding of cellular immunity. IL‐23 is a heterodimer, comprising IL‐12 p40 and the recently cloned IL‐23‐specific p19 subunit. IL‐23 uses many of the same signal‐transduction components as IL‐12, including IL‐12Rβ1, Janus kinase 2, Tyk2, signal transducer and activator of transcription (Stat)1, Stat3, Stat4, and Stat5. This may explain the similar actions of IL‐12 and IL‐23 in promoting cellular immunity by inducing interferon‐γ production and proliferative responses in target cells. Additionally, both cytokines promote the T helper cell type 1 costimulatory function of antigen‐presenting cells. IL‐23 does differ from IL‐12 in the T cell subsets that it targets. Whereas IL‐12 acts on naïve CD4+ T cells, IL‐23 preferentially acts on memory CD4+ T cells. This review summarizes recent advances regarding IL‐23, providing a functional and mechanistic basis for the unique niche that IL‐23 occupies in cellular immunity.