IL-12 and its Receptor

Patents:    US Patent No. 6, 168, 923, "Compositions and methods for use of IL-12 as an adjuvant

See also IL-12 production by DCs

IL-12 was identified as a factor secreted by EBV transformed B cell lines mediating several biological activities on human T and Natural Killer cells, including induction of IFN-γ production, enhancement of cell mediated cytotoxicity, and co-mitogenic effects on resting T cells. IL-12 is a heterodimeric proinflammatory cytokine. The IL-12 family of hterodimeric cytokines also cincludes IL-23, IL-27 and IL35.

What Cells produce IL-12?

IL-12 is produced by APCs, including dendritic cells, monocyte/macrophages, neutrophils and other phagocytic or APC-like cells. The primarily produces of IL-12, however,  are macrophages and dendritic cells.

What Does IL-12 Do?

• Th1 cell differentiation: IL-12 s a key regulator of Th1 cell differentiation by inducing IFNy production by activated T and NK cells. It binds to a unique, high affinity receptor on activated Th cells and NK cells. This has been confirmed in IL-12 knockout mice, that are deficient in generating a normal Th1 response and in producing IFNy. IL-12 primes T cells to differentiate into Th1 cells producing high levels of IL-2 and IFN-γ, resulting in improved innate defense mechanisms through enhanced activation of NK and antigen-presenting cells.
• Intracellular Bacteria Defense: Deficiencies in either IL-12 or IL-12 receptors in humans have been associated with impaired immunity against intracellular bacteria. Moreover, administration of IL-12 is a component of cancer vaccines enhances the activity of CD8+ cytotoxic T cells, resulting in protection and tumor regression in vivo.
• Inflammatory disease: Also involved in the pathogenesis of Th1-mediated chronic inflammatory diseases in mice and humans such as diabetes mellitus, multiple sclerosis, arthritis, and inflammatory bowel disease. Thus its downregulation may ameliorate these autoimmune diseases.
• cell immunity: IL-12 is a key cytokine for strong cell mediated immunity by promoting Th1 lymphocyte development (see above) and by inhibiting that of Th2 cells. Il-12 indirectly promotes Th1 and inhibits Th2 development by inducing the secretion of IFN-y by Th1 and NK cells. IL-12 has autocrine and paracrine effects. As the major autocrine growth factor for T cells, two important functions are the induction of proliferation and the prevention of apoptosis in polyclonal and monoclonal expansion of activated T-cells.
• Simulates Innate Immunity: IL-12 production by DCs stimulates natural killer cells as well as T cells, helping to activate both the innate immune system and acquired immunity. IL-12 promotes the production of IFNy by NK cells.
• Allograft rejection: IL-12 produced by accessory cells during early antigenic stimulation induces Th1 responses. Th1 cytokines promote both cytotoxic T lymphocytes and delayed-type hypersentsitivity responses, which are considered to be the principal effector mechanisms of allograft rejection. Thus inhibition of IL-12 production consequently blocks Th1 polarization and prolongs transplant survival. Thus IL-12 production is up-regulated in FL (dramatically augments the number of interstitial myeloid and lymphoid DC) liver grafts in associated with pronounced increases in DC.
• Tolerance: DCs are also involved in both central and peripheral tolerance.
• Apoptosis: IL-12 has been shown to inhibit activation-induced and Fas-mediated apoptosis of T cells, whereas anti-IL-12 enhances these events in vitro and in vivo. It enhances the expression of antiapoptotic factors (bcl₂ and bcl_xl).

Structure

IL-12 is a heterodimeric, disulfide-linked cytokine composed of 35- and 40-kDa subunits encoded by separate genes. Il-12 p70 is composed of p40 and p35 molecules in a 1:1 molar ratio. The genes encoding the two heterologous chains of IL-12, p40 and p35, are located on different human and mouse chromosomes. Expression of the p35 subunit is constitutive and ubiquitous. Thus the biological activity of IL-12 is mainly regulated by induction of p40. Both subunits are co-expressed in the same cell to generate the bioactive form.

Whereas IL-12p40 predominantly interacts with the Il12Rβ1 chain, IL-12p35 primarily binds to IL-12Rβ2.

There is currently no Il-12 p35 specific ELISA. IL-12p70 is often used as an indirect indicator of IL-12 p35 synthesis.

IL-12 Receptor:

The IL2 receptor is composed of 3 chains, alpha, beta and y. The alpha chain is expressed by activated but not resting T cells. This insures that only antigen activated CD4 and CD8 T cells will express the high affinity IL12 receptor and proliferate in response to IL-12. Like other members of the cytokine receptor family, the IL2 receptor lacks domains with tyrosine kinase activity. But when IL12 binds to the IL2 receptor, rapid tyrosin phosphorylation occurs. Signaling is mediated by interaction of the long cytoplasmic domain of the B chain with Src-family tyrosine kinases, leading to the phosphorylation of cellullar proteins.

How is IL-12 Activated?:

How Does IL-12 Signal?:

Signal transduction through the IL-12R induces tyrosine phosphorylation of the Janus family kinases JAK2 and TYK2, which in turn phosphorylate and activate signal transducer and activator of transcription 4 (STAT4). IL-12 and IFN-alpha are the only cytokines that induce the activation of STAT4. STAT4, through the production of IFN-y, mediates most but not all of the biological activites of IL-12 in host defense against microbial infection.

Regulation of IL-12