For over a decade, IL-12 has been recognized as the canonical cytokine that links innate and adaptive immunity, and with the discovery of IL-23 and IL-27 as cytokines related to IL-12, there has been a concerted effort to understand the relationship between these cytokines. Recent studies have provided new insights into the developmental pathways that promote the differentiation and function of CD4+ T helper cells and offer a dramatically altered perspective on the cause and prevention of autoimmune disease. Besides cell-cell contacts that provide activation signals via peptide-MHC/TCR and classical costimulatory interactions (B7/CD28), antigen-presenting cells communicate with T cells via cytokine production. As a consequence of interacting with various microbial products, antigen-presenting DCs and monocytes/macrophages, as well as other cell types, produce a variety of these soluble factors that are responsible for the expansion and differentiation of naive T cells to generate mature phenotypes such as Thl and Th2 cells. IL-12, discovered in 1989, is well known as a central player in promoting the differentiation of naive CD4+ T cells into mature Thl effector cells and is a potent stimulus for NK cells and CD8+ T cells to produce IFNγ.
All cytokines of the IL-12 family are produced by activated DCs and macrophages, which function as antigen-presenting cells. expression of IL-12p35, IL-23pl9, and IL- 27p28 has been shown in many different cell types. However, IL-12p40 transcription appears to be restricted to antigen-presenting cells. Stimuli for the expression of IL-12 family cytokines include pathogen-associated molecular patterns (PAMPs), which are ligands for toll-like receptors (TLRs) on monocytes/macrophages and DCs. The production of cytokines of the IL-12 family by monocytes/macrophages and DCs is triggered when TLR are activated by pathogen-associated molecular patterns present on bacterial, fungal, and viral microbes. In addition, optimal production of IL-12 (and probably also of IL-23 and IL-27) requires cytokines such as IFNγ, IL-4, and IL-13 depending on the cell types. Target cells for all IL-12 family cytokines are NK cells and T cells. In this study I have used IFNγ (10 ng/ml) to prime the monocytes to induce IL-23 and IL-27 expression following LPS stimulation. To date, the exact role of IFNγ-induced IL-23 and IL-27 expression in monocytes/macrophages is not known. Monocytes/macrophages and DCs appear to express functional receptors for IL-12 and IL-23. For IL-27, a receptor has also been described on monocytes/macrophages, Langerhans' cells, activated DCs, and endothelial cells. The individual members of the IL-12 family cytokines have overlapping, but also distinct, activities. This may partially be based on different receptor components expressed on different target cells or due to different developmental stages of the target cells (naive vs. memory T helper cells). Naive T helper (Th) cells express receptors for IL-27 and IL-12, but not for IL-23. On the other hand, memory Th cells express receptors for IL-23, but have only low or no expression of receptors for IL-27 or IL-12.
Recently, IL-23 has been demonstrated to play a key role in the development of several autoimmune diseases such as multiple sclerosis (MS) and experimental autoimmune encephalitis (EAE). Oppmann et al. demonstrated that the IL-23pl9 subunit, a four-a helix cytokine with an overall sequence identity of approximately 40% to IL-12p35, has no biological activity by itself, but binds to IL-12p40 to form IL-23, with biological activities similar as well as distinct from IL- 12. IL-23 induces proliferation of mouse memory T cells, but IL-12 does not. IL-23 has biological functions that are similar to IL-12, such as IFNγ production from CD4 T cells, antitumor and antimetastatic activity. Similar to IL-12, IL-23 induces IFNγ production in phytohemagglutinin (PHA)-stimulated T cells. Some earlier reports also suggest that IL-23 induces a more robust and sustained cytotoxic T lymphocyte and Thl immune response than IL-12. Furthermore, IL-23N220L, an N-glycosylation mutant of this protein that shows reduced expression of excess IL-12p40 but has no inhibitory effects upon IL-23 levels, has been proposed as an effective adjuvant for DNA vaccine to induce antigen-specific T cell immunity. Daily injections of IL-23 in wild-type mice led to a psoriasis-like phenotype with visually apparent erythema and induration and are associated with prominent dermal papillary blood vessel formation and possibly vasodilatation as soon as 2 days after starting treatment. Histological and immunohistochemical examination of IL-23-treated skin show epidermal hyperplasia and a mixed dermal infiltrate consisting of neutrophils, macrophages, CD11c+ DCs, and CD4+ T cells as early as 1 day after IL-23 treatment. Importantly, these activities are unique to IL-23, as IL-12 does not induce changes in epidermal thickness when injected into skin. The impact of IL-23 on autoimmunity and chronic inflammation is well established. Increased level of IL-23pl9 subunit expression in microglia cells from multiple sclerosis lesions has been reported. However, its role in protection against infectious diseases is poorly understood and our current understanding of the biology of IL-23 is still considerably less than IL-12, which to-date has been studied more extensively. IL-12 is critical for the generation of CMIR to infectious agents including the HIV-1 virus, as well as having potent anti tumour activity, and playing a significant role in autoimmunity.
In 2002, a new heterodimeric cytokine termed IL-27 that consists of Epstein-Barr virus-induced gene 3 (EBI3) and p28 was discovered. IL-27EBI3, a 33-34 kDa glycosylated protein is related to the IL-12p40 subunit of IL-12 and IL-27p28 is related to the IL-12p35 polypeptide. IL-27 is involved in early Thl initiation and possesses anti-inflammatory properties. IL-27 may also play a role in the induction of T-bet expression, a master transcriptional regulator for Thl differentiation, and regulation of immunoglobulin class switching in B cells. Its action is mediated through IL-27 receptor consisting of the orphan cytokine receptor WSX-1/TCCR and gpl30. The IL-27/WSX1 signaling system induces STAT 1-mediated T-bet expression to initiate Thl differentiation.
By comparison to IL-12 and IL-23, which share a similar structural makeup, IL- 27 differs in that its subunits are not linked by a disulfide bond. The absence of the disulfide bond theoretically allows for production of the two subunits by distinct cells followed by extracellular association. IL-27 has attracted considerable interest as an anticancer agent due to its similarities with IL-12. IL-12 is thought to be effective against tumors because it promotes Thl polarization and, thereby, promotes cellular immune responses and proliferation of cytotoxic T lymphocytes (CTL). IL-27 enhancement of CTL activity strengthens the case that IL-27 could be a potent antitumor agent. Over expression of IL-27 in mouse colon carcinoma C26 cells, TBJ neoblastoma cells, or B16F10 melanoma cells led to protection against tumors growth. It has been shown that IL-27 has anti-angiogenic properties, suppressing neovascularization of tumors as well as chick embryonic angiogenesis. On the other hand, it has also been shown that IL-27 suppresses the secretion of proinflammatory cytokines, including IL-23- induced IL-17 from activated T cells. Specifically, suppression of IL-2 and IL-17 expression and inhibition of IL-6 function are likely to contribute significantly to the effects of IL-27.