Interleukin 27 (IL-27) is a heterodimeric cytokine that consists of a four helix bundle cytokine-like subunit, termed p28, and a soluble cytokine receptor-like subunit, EBI3 (Epstein-Barr virus-induced gene 3). The p28 subunit has homology with other single-chain cytokines, whereas EBI3 is related to the extracellular domain of the hematopoietic cytokine receptor family. As such, IL-27 is a member of a small family of heterodimeric cytokines that includes IL-12 and IL-23. Efficient expression and secretion of the IL-27 heterodimer requires coexpression of p28 and EBI3. Both subunits are produced by activated antigen-presenting cells (APCs) and the presence of both is required for the biologic activity of IL-27. Cellular targets include naive CD4+ T cells and NK cells (Pflanz et aL, 2002). The hematopoietic cytokine receptor WSX-1/TCCR, together with at least one additional - yet to be identified - receptor subunit(s) is believed to mediate the biologic activity of IL-27.