The mechanism underlying the differentiation of CD4+ T cells into functionally distinct subsets (Th1 and Th2) is incompletely understood, and hitherto unidentified cytokines may be required for the functional maturation of these cells. Here we report the cloning of a recently identified IFN-gamma-inducing factor (IGIF) that augments natural killer (NK) activity in spleen cells. The gene encodes a precursor protein of 192 amino acids and a mature protein of 157 amino acids, which have no obvious similarities to any peptide in the databases. Messenger RNAs for IGIF and interleukin-12 (IL-12) are readily detected in Kupffer cells and activated macrophages. Recombinant IGIF induces IFN-gamma more potently than does IL-12, apparently through a separate pathway. Administration of anti-IGIF antibodies prevents liver damage in mice inoculated with Propionibacterium acnes and challenged with lipopolysaccharide, which induces toxic shock. IGIF may be involved in the development of Th1 cells and also in mechanisms of tissue injury in inflammatory reactions.

Although interleukin-18 is structurally homologous to IL-1 and its receptor belongs to the IL-1R/Toll-like receptor (TLR) superfamily, its function is quite different from that of IL-1. IL-18 is produced not only by types of immune cells but also by non-immune cells. In collaboration with IL-12, IL-18 stimulates Th1-mediated immune responses, which play a critical role in the host defense against infection with intracellular microbes through the induction of IFN-gamma. However, the overproduction of IL-12 and IL-18 induces severe inflammatory disorders, suggesting that IL-18 is a potent proinflammatory cytokine that has pathophysiological roles in several inflammatory conditions. IL-18 mRNA is expressed in a wide range of cells including Kupffer cells, macrophages, T cells, B cells, dendritic cells, osteoblasts, keratinocytes, astrocytes, and microglia. Thus, the pathophysiological role of IL-18 has been extensively tested in the organs that contain these cells. Somewhat surprisingly, IL-18 alone can stimulate Th2 cytokine production as well as allergic inflammation. Therefore, the functions of IL-18 in vivo are very heterogeneous and complicated. In principle, IL-18 enhances the IL-12-driven Th1 immune responses, but it can also stimulate Th2 immune responses in the absence of IL-12.

The interleukin-1beta (IL-1beta) converting enzyme (ICE) processes the inactive IL-1beta precursor to the proinflammatory cytokine. ICE was also shown to cleave the precursor of interferon-gamma inducing factor (IGIF) at the authentic processing site with high efficiency, thereby activating IGIF and facilitating its export. Lipopolysaccharide-activated ICE-deficient (ICE-/-) Kupffer cells synthesized the IGIF precursor but failed to process it into the active form. Interferon-gamma and IGIF were diminished in the sera of ICE-/- mice exposed to Propionibacterium acnes and lipopolysaccharide. The lack of multiple proinflammatory cytokines in ICE-/- mice may account for their protection from septic shock.

IL-18 is a cytokine that is secreted from activated macrophages and induces IFNgamma production. To investigate the in vivo role of IL-18, we generated IL-18-deficient mice. In Propionibacterium acnes (P. acnes)-primed IL-18-deficient mice, LPS-induced IFNgamma production was markedly reduced, despite normal IL-12 induction. Natural killer cell activity was significantly impaired. Th1 cell response after injection of P. acnes or Mycobacterium bovis (bacillus Calmette-Guerin [BCG]) was significantly reduced. Similar results were observed in IL-12-deficient mice. Interestingly, Th1 response was induced after BCG infection in IL-12-deficient mice. We therefore generated mice lacking both IL-18 and IL-12. In these mice, NK activity and Th1 response were further impaired. This demonstrates the important role of both IL-18 and IL-12 in NK activity, as well as in in vivo Th1 response.

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IL-18 is a proinflammatory cytokine that plays an important role in natural killer cell activation and T helper 1 (Th1) cell responses. Mast cells and basophils are major inducers and effectors of allergic inflammation. Here we show that basophils and mast cells derived by culture of bone marrow cells with IL-3 for 10 days express IL-18Rα chain and that basophils produce large amounts of IL-4 and IL-13 in response to stimulation with IL-3 and IL-18. Injection of IL-12 and IL-18 inhibits IgE production in helminth-infected wild-type mice and abolishes the capacity of their basophils to produce IL-4 and IL-13 in response to stimulation either with IL-3 and IL-18 or with FcɛR cross-linkage. By contrast, this combination of cytokines actually increases IgE levels in helminth-infected IFN-γ −/− mice and enhances IL-4 and IL-13 production by their basophils. Furthermore, injection of IL-18 alone enhances basophil production of IL-4 and histamine both in wild-type and IFN-γ −/− mice. Thus, IL-18 has the potential to stimulate basophils but, when given with IL-12, exhibits an antiallergic action in vivo .

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