As a member of the IL-12 family of heterodimeric cytokines that also includes IL-12, IL-23, and IL-35, IL-27 is formed by the association of an IL-27-p28 subunit (also known as IL-30) with the Epstein-Barr Virus (EBV)-induced Gene 3 (EBI3) subunit (also known as IL-27B). Expressed by antigen-presenting cells (APCs) in the early phases of antigen-mediated activation, IL-27 acts as a critical initiator of adaptive immune responses by promoting the rapid clonal expansion of naive CD4+ T cells, IFN-gamma production, and Th1 polarization. IL-27 elicits its effects through receptor complexes IL-27R (also known as TCCR/WSX-1) and gp130, a receptor shared by IL-6. Mainly expressed in monocytes, endothelial cells, and dendritic cells, IL-27 plays an important role alongside IL-6 in the regulation of inflammation and autoimmunity; directly antagonizing IL-6's stimulation of CD4+ T cell proliferation and Th17 differentiation. Recombinant Human IL-27 produced from HEK293 cells is a non-disulfide-linked, heterodimeric protein composed of a 209 amino acid length EBI3 subunit and a 215 amino acid length IL-27-p28 subunit, for a total sequence length of 424 amino acid residues. The calculated molecular weight of the associated IL-27 subunits is 47.8 kDa.