IL-12 Family Ligand Review

The IL-12 family of cytokines consist of IL-12 (IL-12p35/IL-12p40), IL-23 (IL-23p19/IL-12p40), IL-27 (IL-27p28/Ebi3) and IL-35 (IL-12p35/Ebi3) and has emerged as important regulators of host immunity.

Below will briefly introduce each IL-12 family ligand and their functions, including interleukin-12, interleukin-23, interleukin-27 and interleukin-35.

Interleukin-12

Interleukin 12 (IL-12) is the founding member of the IL-12 family. It is comprised of the IL-12p35 and IL-12p40 subunits and co-expression of both subunits in the same cell is required to secrete the disulfide-linked bioactive IL12p70 cytokine. Although it is secreted by a variety of hematopoietic cell types, the major physiological producers are antigen-presenting cells (APCs), such as dendritic cells (DCs) and macrophages.

IL-12 induces naïve CD4+ T cells to differentiate into Th1 cells, a T-helper subset that is implicated in the etiology of a number of human autoimmune diseases. High levels of IL-12 and Th1 cells are detected in the aqueous humor and vitreous of patients with autoimmune uveitis, suggesting a role for IL-12-induced expansion of Th1 cells in this group of sight-threatening intraocular inflammatory diseases.

Multiple sclerosis (MS) is another chronic CNS autoimmune disease which IL-12-induced expansion of Th1 cells is thought to play an important role. Similar to uveitis and MS, the levels of IL-12 and Th1 cells are elevated in the serum and synovial fluid of patients with rheumatoid arthritis (RA) and are correlated with disease activity.

Interleukin-23

A decade after the discovery of IL-12, homology search of the human DNA sequence database with a probe specific to the highly conserved ''D'' helical structure of IL-6-related cytokines led to identification of additional members of the IL-12 family. Interleukin-23 (IL-23) was discovered in 2003 and shares the IL-12p40 subunit with IL-12 but differs from IL-12 because of its unique IL-23p19 subunit. Similar to IL-12, co-expression of IL-12p40 and IL-23p19 subunits in the same cell is required to secrete the disulfide-linked bioactive IL-23 cytokine.

Connected with IL-6 and TGF-β1, IL-23 can drive naive CD4+ T cells to differentiate into Th17 cells, and has been shown to be crucial to the pathogenesis of T cell-mediated inflammatory diseases.

IL-23 has displayed both anti-tumor and pro-tumor effects. Recombinant IL-23 reduces B-ALL cell growth in vitro and in preclinical models by inhibiting proliferation and inducing apoptosis of tumor cells. Over-expression of IL-23 in mouse colon carcinoma or B16 melanoma significantly inhibits tumor growth and metastasis. Administration of high-dose IL-23 resulted in slower tumor growth and longer survival time of mice bearing fibrosarcoma. In contrast to exogenous IL-23, endogenous IL-23 has been shown to enhance tumor incidence and growth. The deficiency of IL-23 or IL-23R not only decreases tumor incidence but also inhibits tumor growth. IL-23 promotes inflammatory responses, increases angiogenesis, and the infiltration of macrophages, but reduces cytotoxic CD8+ T cell infiltration. Furthermore, IL-23p19 is over-expressed in most human cancers. The differential effects of exogenous and endgenous IL-23 in tumor growth suggest that the dual roles of the immune system in suppressing and promoting cancer formation.

Interleukin-27

Interleukin 27 (IL-27), was first identified in 1996 from a subtractive hybridization screen of genes expressed in Epstein-Barr virus (EBV) transformed B cell lines. It is comprised of IL-27p28 and EBV-induced gene 3 (Ebi3). Similar to IL-12 and IL-23, EBI3 and p28 co-expression in a single cell is required for IL-27 to exert its biological functions.

The nature of the association between IL-27p28 and EBi3 is uncertain and these subunits can be secreted independently and are differentially expressed in various cell types. EBi3 can partner with IL-12p35 to form IL-35, which has been linked to the activities of regulatory T cells.

The expression of human IL-27 has been found in monocytes, monocyte-derived DCs, endothelial cells, and trophoblast cells, while the expression of murine IL-27 has been verified in activated microglia cells and macrophages.

Signaling through toll-like receptors (TLR) are crucial inducers of IL-27. The expression of EBI3 in DCs was substantially decreased in the lack of TLR2, TLR4, TLR9, or MyD88, suggesting that TLR stimulation is required for IL-27 expression. In addition, the expression of EBI3 in DCs was induced through the activation of the transcription factors NF-κB and PU-1. Human IL-27p28 mRNA was specially induced by Toll/IL-1R-containing adaptor inducing IFN-β-coupled TLR ligands, and IRF3 activation is a chief switch for the synthesis of IL-27. Besides TLR ligands, many host-derived factors, such as CD40L, IL-1β, IFN-α, IFN-β, and IFN-γ, can up-regulate IL-27 expression as well.

IL-27 is a IL-12 family cytokine with strikingly diverse influences on the immune response. Although it was initially linked with the development of Th1 responses, it is now recognized as a potent antagonist of different classes of inflammation through its ability to directly modify CD4+ and CD8+ T cell effector functions, to induce IL-10, and to promote specialized T regulatory cell responses.

Interleukin-35

Interleukin 35 (IL-35) is the most recently identified member of the IL-12 family of cytokines and offers the potential to be a target for new therapies for autoimmune, inflammatory, and infectious diseases.

Similar to other members of the IL-12 family including IL-12, IL-23, and IL-27, IL-35 is composed of a heterodimer of α and β chains, which in the case of IL-35 are the p35 and Epstein-Barr virus-induced gene 3 (EBI3) proteins. IL-35 shares p35 subunit with IL-12, and shares EBI3 with IL-27.

Unlike other members of the IL-12 family which are secreted by a variety of myeloid cell types, IL-35 is restricted to be secreted by Foxp3+ regulatory T cells and a regulatory T cell population induced by IL-35. Naive T cells with IL-35 treatment induces a regulatory population, called 'iTR35 cells', that mediates T cell suppression via IL-35. Human Foxp3+ Tregs do not constitutively express IL-35 but can be induced to produce IL-35 by anergic dendritic cells characterized by cell surface expression of B7-H1 (CD274) and sialoadhesin (CD169).

IL-35 signaling in Tregs is mediated through unconventional receptors comprising IL-12Rβ2/gp130, IL-12Rβ2/IL-12Rβ2 or gp130/gp130. Although it is not clear which of these is the high affinity IL-35 receptor, binding of IL-35 to the receptor preferentially activates JAK1, JAK2, STAT1 and STAT4.

IL-35 has also been shown to induce regulatory B cells that produce IL-10 (Bregs) and/or IL-35 (i35-Bregs), suggesting potential use of autologous regulatory B cells in regulating immune responses in health and disease.

References

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