IL-12 Signaling and Production in Macrophages

Interleukin 12 (IL-12p70 or IL-12) is a heterodimeric pro-inflammatory cytokine produced by macrophages that are activated by pathogens or T lymphocytes. IL-12p70 is comprised of the monomeric subunits p40 and p35. The IL-12p40 monomer can homodimerize, or, heterodimerize with another monomer p19 giving rise to the IL-12 related cytokines: IL-12p80 and IL-23. In addition to producing IL-12, macrophages also respond to it; thus IL-12 demonstrates an autocrine effect in macrophages and other cell types like dendritic cells. The signaling by IL-12 is effected through heterodimeric IL-12 receptors (IL-12R). The IL-12R protein composed of β1 and β2 chains is found on resting macrophages; however expression of these chains is upregulated during cell activation.One of the major effects of IL-12 signaling on macrophages is the induction of interferon gamma (IFN γ) which in turn favors the differentiation of T helper 1 cells (TH1) cells, affecting the development of adaptive immunity. IL-12-induced production of interferon gamma (IFN γ) by macrophages is dependent on signal transducers and activators of transcription 4 (STAT4). The cytokine IL-18 secreted by several cell types including macrophages, synergizes with IL-12 in the production of IFN γ by activating nuclear translocation of STAT4. In addition, this synergy also increases the production of nitric oxide, which in turn promotes intracellular bacterial death. Complex reciprocal influences take place between IL-12 and IFN γ , both of which are involved in the initiation of cell-mediated immunity. IFN γ can enhance the expression of IL-12Rβ1 thereby increasing the number of IL-12 binding sites expressed on macrophages. Autocrine production of IL-12 seems to be another major effect of IL-12 on macrophages, which is reinforced in the presence of IFN γ. In addition to IFN γ, IL-12p40 monomer or IL-12p80 induce the tumor necrosis factor (TNF). This induction is triggered by the mitogen activated protein kinase (MAPK) pathway leading to the activation of factors like CCAAT/enhancer binding protein (C/EBP) and NF kappa B (NF-κB), which increase the gene expression of TNF. Thus IL-12 serves as a bridge between innate and adaptive immunity: Macrophages produce IL-12 when they encounter a pathogen. The signaling by IL-12 in macrophages, in turn, produces cytokines like IFN γ and TNF, which develop the TH1 response, thereby enhancing adaptive immunity.The subunits of IL-12, IL-12p40 and IL-12p35 are expressed from separate genes on different chromosomes. Both genes need to be expressed coordinately for the production of IL-12. The expression of IL-12p35 is constitutive and ubiquitous. On the other hand, IL-12p40 is expressed in cells like macrophages only upon activation. The transcription of IL-12p40 gene is thus regulated by a number of factors. Signaling via the toll like receptor (TLR) that is activated by several bacterial structural components, is one major mechanism that upregulates the transcription of the IL-12p40 gene. Stimulation of TLRs triggers the MAPK pathway in several ways. This stimulation could activate transcription factors like C/EBP, activator protein 1 (AP-1) and NF-κB, which in turn enhance transcription. The NF-κB mediated upregulation of IL-12p40 gene expression is enhanced by nuclear factors like steroid receptor coactivator-1 (SRC-1), p300 and PU.1 and inhibited by nuclear receptors like retinoid X receptor (RXR) and peroxisome proliferator activated receptor gamma (PPARγ). Signaling by IFN γ also enhances the transcription of the IL-12p40 gene via a STAT1/ interferon stimulated factor (IRF8) mechanism in conjunction with IRF-1.This pathway highlights the important components of IL-12 signaling and production in macrophages.