IL-12 Family Cytokines

IL-12 Family Cytokines, Receptors, and Intracellular Signaling Pathways

IL-12, IL-23, IL-27, IL-35, and IL-39 are heterodimeric cytokines belonging to the IL-12 family. Each cytokine in this family consists of an alpha subunit (p19, p28, or p35) that is linked through a disulfide bond to one of two beta subunits (p40 or EBI3). The alpha subunits are structurally related to IL-6 and have a four alpha-helix bundle structure oriented in an up-up-down-down topology, while the beta subunits are structurally similar to the soluble class I cytokine receptors (i.e. sIL-6 R alpha). As a result, IL-12 family cytokines have a structure that is equivalent to a cytokine subunit linked to a soluble receptor. Significantly, the alpha and beta subunits that make up these heterodimeric cytokines are shared among different members of the IL-12 family, and chain pairing promiscuity partially contributes to the broad range of effects that IL-12 family cytokines have on host immunity. While IL-12 consists of the p35 alpha subunit paired with the p40 beta subunit, IL-23 consists of the p19 alpha subunit, which is homologous to IL-12 p35, paired with the p40 beta subunit. Similarly, IL-27, IL-35, and IL-39 consist of the p28, p35, or p19 alpha subunits, respectively, paired with the Epstein-Barr virus-induced gene 3 (EBI3) beta subunit. The p28 alpha subunit of IL-27 is a polypeptide related to the IL-12/IL-35 p35 alpha subunit, while EBI3 is an IL-12/IL-23 p40-related protein. IL-12, IL-23, and IL-27 are primarily produced by activated antigen-presenting cells, including B cells, monocytes, macrophages, and dendritic cells. In contrast, IL-35 is primarily produced by regulatory T cells and activated B cells and IL-39 is produced by activated B cells and keratinocytes.

IL-12 Family Receptors

IL-12 family cytokines signal through heterodimeric receptors belonging to the class I cytokine receptor family. Each receptor subunit is a type I transmembrane protein with an extracellular domain that contains a cytokine receptor homology domain consisting of two fibronectin type III domains. The more N-terminal fibronectin type III domain contains four positionally conserved cysteine residues that form interstrand disulfide bonds, while the second fibronectin type III domain has a conserved Trp-Ser-X-Trp-Ser (WSXWS) sequence motif and is important for the tertiary structure. With the exception of IL-23 R, the cytokine receptor homology domain in the IL-12 family receptor subunits is followed by three additional fibronectin type III domains, and most of these receptor chains also contain an N-terminal immunoglobulin-like domain, with the exception of IL-27 R alpha/WSX-1. Similar to the chain promiscuity displayed by the IL-12 family cytokines, the heterodimeric receptors for these cytokines are also composed of shared subunits. IL-12 binds to a receptor complex composed of IL-12 R beta 1 and IL-12 R beta 2, while IL-23 binds to a receptor complex composed of IL-23 R and IL-12 R beta 1. Similarly, IL-27, IL-35 and IL-39 all bind to receptor complexes that contain the gp130 receptor subunit, which is common to the IL-6 cytokine family receptors. The gp130 subunit is paired with either IL-27 R alpha/WSX-1 to form the IL-27 receptor complex, IL-12 R beta 2 to form the IL-35 receptor complex on T cells, or IL-23 R to form the IL-39 receptor complex. In contrast to the other IL-12 family cytokines, IL-35 is unique in that it can bind to more than one receptor complex. Besides the IL-12 R beta 2-gp130 complex, IL-35 has also been shown to bind to gp130 or IL-12 R beta 2 homodimeric receptor complexes, as well as a receptor complex consisting of IL-27 R alpha/WSX-1 and IL-12 R beta 2.

IL-12 Family Signaling

IL-12 family receptors associate with members of the Jak family of kinases (Jak1, Jak2, and Tyk2), which upon activation, promote the recruitment and phosphorylation of STAT proteins. Phosphorylated STATs form homodimers or heterodimers that translocate to the nucleus where they regulate gene expression.  Although IL-12 family cytokines promote similar signaling pathways overall, they can activate multiple STAT proteins and their distinct biological effects have been attributed to the primary activation of one or two different STAT proteins. For example, while IL-12 primarily activates STAT4 and to a lesser extent STAT1, STAT3, and STAT5, IL-23 primarily activates STAT3 and to a lesser extent STAT4, STAT1, and STAT5.

Many of the distinct effects of the IL-12 family cytokines involve their abilities to differentially regulate the activities of specific T cell subsets. Although IL-12, IL-23 and IL-27 all promote pro-inflammatory immune responses, IL-12 primarily affects Th1 cells, IL-23 primarily affects Th17 cells, and IL-27 affects both Th1 and Th17 cells. IL-12 and IL-27 signaling promotes the differentiation of naïve CD4⁺ T cells into Th1 cells, while IL-23 signaling promotes the stabilization and maintenance of Th17 cells. Unlike IL-12 and IL-23, IL-27 can also suppress inflammation by inhibiting Th17 differentiation and inducing the conversion of effector T cells into IL-10-producing T regulatory type I-like (Tr1-like) cells. In contrast to the pro-inflammatory effects induced by IL-12, IL-23, and IL-27, IL-35 has anti-inflammatory effects. IL-35 enhances the activity of regulatory T cells, inhibits conventional T cell proliferation, and promotes the conversion of B cells into IL-10-, IL-35-secreting regulatory B cells. Although little is currently known about the effects of IL-39 on different immune cell types, it is also presumed to have pro-inflammatory effects as it was shown to promote neutrophil development and differentiation, which has been suggested to contribute to the pathogenesis of disease in a lupus mouse model. In addition, other recent studies have shown that IL-39 is secreted by keratinocytes and is increased in the serum of patients with acute coronary syndrome, but the significance of these observations is not currently well understood.