Cytokines are critical coordinators of the immune response necessary for resolving bacterial and viral assaults on the immune system. In particular, the IL-12 family of cytokines are key players in the regulation of T cell responses. These responses are orchestrated by monocytes, macrophages, and dendritic cells which produce the members of the IL- 12 family of cytokines in response to infection. IL-27 and IL-23 are two cytokines that are related to IL-12; these cytokines share homology at the subunit, receptor, and signalling levels. IL-12 is composed of p35 and p40 subunits, which, when combined together form the bioactive IL-12p70. IL-23 is composed of the IL-12p40 subunit as well as the IL-23p19 subunit, which shares homology with IL-12p35. IL-27 is composed of EBI3 and p28. These three cytokines activate similar members of the JAK/STAT signalling pathways as a result of homology in their receptor components. Production of these cytokines by activated monocytes, macrophages, and dendritic cells results in the activation and differentiation of T cells. In spite of their similarity, each of these cytokines has specific roles in the regulation of immune responses. IL-12 is required for the induction of IFN-γ production, critical for the induction of Th1 cells. IL-27 has been shown to play a role in the induction of Th1 cells from naive T cells, whereas IL-23 has been demonstrated to play a key role in the induction of the newly described Th17 cells. Recently, a novel heterodimeric and anti-inflammatory cytokine composed of the IL- 12p35 and EBI3 subunits has been identified as IL-35. The biological properties of the IL-12 family of cytokines, the signalling pathways mediated by these cytokines and their role in infection, inflammation, and autoimmune diseases will be the focus of this review.

Interleukin-10 (IL-10), a pleiotropic cytokine that inhibits inflammatory and cell-mediated immune responses, is produced by a wide variety of cell types including T and B cells and monocytes/macrophages. Regulation of pro- and anti-inflammatory cytokines has been suggested to involve distinct signaling pathways. In this study, we investigated the regulation of the human IL-10 (hIL-10) promoter in the human monocytic cell line THP-1 following activation with lipopolysaccharide (LPS). Analysis of hIL-10 promoter sequences revealed that DNA sequences located between base pairs −652 and −571 are necessary for IL-10 transcription. A computer analysis of the promoter sequence between base pairs −652 and −571 revealed the existence of consensus sequences for Sp1, PEA1, YY1, and Epstein-Barr virus-specific nuclear antigen-2 (EBNA-2)-like transcription factors. THP-1 cells transfected with a plasmid containing mutant Sp1 abrogated the promoter activity, whereas plasmids containing the sequences for PEA1, YY1, and EBNA-2-like transcription factors did not influence hIL-10 promoter activity. To understand the events upstream of Sp1 activation, we investigated the role of p38 and extracellular signal-regulated kinase mitogen-activated protein kinases by using their specific inhibitors. SB202190 and SB203580, the p38-specific inhibitors, inhibited LPS-induced IL-10 production. In contrast, PD98059, a specific inhibitor of extracellular signal-regulated kinase kinases, failed to modulate IL-10 production. Furthermore, SB203580 inhibited LPS-induced activation of Sp1, as well as the promoter activity in cells transfected with a plasmid containing the Sp1 consensus sequence. These results suggest that p38 mitogen-activated protein kinase regulates LPS-induced activation of Sp1, which in turn regulates transcription of the hIL-10 gene.

Toll-like receptor (TLR)-7 is an endosomal innate immune sensor capable of detecting single-stranded ribonucleic acid (ssRNA). TLR7-mediated induction of type I interferon (IFN) and other inflammatory cytokine production is important in anti-viral immune responses. Furthermore, altered TLR7 expression levels are implicated in various autoimmune disorders, indicating a key role for this receptor in modulating inflammation. This review is focused on the regulation of TLR7 expression and localization compared to that of the other endosomal TLRs: TLR3, 8, and 9. Endosomal TLR localization is a tightly controlled and intricate process with some shared components among various TLRs. However, TLR-specific mechanisms must also be in place in order to regulate the induction of pathogen- and cell-specific responses. It is known that TLR7 is shuttled from the endoplasmic reticulum (ER) to the endosome via vesicles from the Golgi. Several chaperone proteins are required for this process, most notably UNC93B1, recently identified to also be involved in the localization of the other endosomal TLRs. Acidification of the endosome and proteolytic cleavage of TLR7 are essential for TLR7 signaling in response to ligand binding. Cleavage of TLR7 has been demonstrated to be accomplished by furin peptidases in addition to cathepsins and asparagine endopeptidases (AEP). Moreover, triggering receptor expressed on myeloid cells like 4 (TREML4), a protein associated with antigen presentation and apoptosis in immune cells, has been implicated in the amplification of TLR7 signaling. Understanding these and other molecular mechanisms controlling TLR7 expression and trafficking will give insight into the specific control of TLR7 activity compared to the other endosomal TLRs.

IL-27 is a heterodimeric cytokine bridging innate and adaptive immunity by playing a role in the activation of naive T cells and in development of Th1 cells. Additionally, recent evidence supports a role for IL-27 in the activation of monocytic cells. Both pro-inflammatory and anti-inflammatory activities have been attributed to IL-27; however, the role played by IL-27 in the activation of human monocytic cells in terms of cytokine production has not been well described. Our results show that IL-27 is a strong inducer of proinflammatory cytokine and chemokine expression, including enhancement of IL-6, IP-10, MIP-1alpha, MIP-1beta, and TNF-alpha expression in human primary monocytes. Furthermore, we observed that IL-27-induced cytokine and chemokine production was mediated by STAT1, STAT3, and NF-kappaB activation. Understanding how IL-27 exerts its effects on monocytic cells will identify important molecular mechanisms in the regulation of immune responses, particularly in the modulation of monocyte activation.

The signaling pathways of mammalian Toll-like receptors (TLRs) are well characterized, but the precise mechanism(s) by which TLRs are activated upon ligand binding remains poorly defined. Recently, we reported a novel membrane sialidase-controlling mechanism that depends on ligand binding to its TLR to induce mammalian neuraminidase-1 (Neu1) activity, to influence receptor desialylation, and subsequently to induce TLR receptor activation and the production of nitric oxide and proinflammatory cytokines in dendritic and macrophage cells. The α-2,3-sialyl residue of TLR was identified as the specific target for hydrolysis by Neu1. Here, we report a membrane signaling paradigm initiated by endotoxin lipopolysaccharide (LPS) binding to TLR4 to potentiate G protein-coupled receptor (GPCR) signaling via membrane Gα_i subunit proteins and matrix metalloproteinase-9 (MMP9) activation to induce Neu1. Central to this process is that a Neu1-MMP9 complex is bound to TLR4 on the cell surface of naive macrophage cells. Specific inhibition of MMP9 and GPCR Gα_i-signaling proteins blocks LPS-induced Neu1 activity and NFκB activation. Silencing MMP9 mRNA using lentivirus MMP9 shRNA transduction or siRNA transfection of macrophage cells and MMP9 knock-out primary macrophage cells significantly reduced Neu1 activity and NFκB activation associated with LPS-treated cells. These findings uncover a molecular organizational signaling platform of a novel Neu1 and MMP9 cross-talk in alliance with TLR4 on the cell surface that is essential for ligand activation of TLRs and subsequent cellular signaling.