Shin-ichiro Kashiwamura

Regulation of interferon-γ production by IL-12 and IL-18

Abstract IL-18 (interferon-inducing factor) and IL-12 exhibit a marked synergism in interferon-γ induction in T cells. Investigations into the mechanism of this synergism have revealed that IL-12 upregulates expression of the IL-18 receptor on cells producing interferon-γ. Although IL-18 does not induce the development of Th1 cells, it is essential for the effective induction and activation of Th1 cells by IL-12. As for natural killer cells, IL-18 seems to activate them independently of IL-12. Although IL-12 and IL-18 activate both innate and acquired immunity, their excessive production by activated macrophages may induce multiple organ disorders including disruption of the immune system.

Interleukin-18 : a novel cytokine that augments both innate and acquired immunity

Publisher Summary Interleukin-18 (IL-18) is a multifunctional cytokine, the roles of which extend over both acquired and innate immunity, and its major role has been thought to be the strong ability to induce IFN-γ production from T or NK cells. This chapter describes the IL-18/IL-l8R system and its pathophysiological roles in immune response. Although IL-18-producing cells were thought to be activated macrophages at first, recent investigations reveal that various types of cells, including keratinocytes, osteoclasts, and adrenal cortex cells, express IL-18, suggesting that IL-18 plays important physiological roles in fields other than the immune system. IL-18 shares many similarities to IL-1, such as in its amino acid sequence. Preliminary experiments indicate that IL-18 is involved in the defense mechanisms against infection and in tumor rejection, and it is also involved in the suppressions of allergy via suppression of IgE production. Thus, it can be considered that IL-18 is involved in the various diseases in diverse manners; however, more investigation is needed to answer several questions about the regulation of production and processing of IL-18 along with the role that it plays in defense against pathogens.

Caspase-1-Independent, Fas/Fas Ligand–Mediated IL-18 Secretion from Macrophages Causes Acute Liver Injury in Mice

IL-18, produced as a biologically inactive precursor, is processed by caspase-1 in LPS-activated macrophages. Here, we investigated caspase-1-independent processing of IL-18 in Fas ligand (FasL)-stimulated macrophages and its involvement in liver injury. Administration of Propionibacterium acnes (P. acnes) upregulated functional Fas expression on macrophages in an IFNgamma-dependent manner, and these macrophages became competent to secrete mature IL-18 upon stimulation with FasL. This was also the case for caspase-1-deficient mice. Administration of recombinant soluble FasL (rFasL) after P. acnes priming induced comparable elevation of serum IL-18 in parallel with elevated serum liver enzyme levels. However, liver injury was not induced in IL-18-deficient mice after rFasL administration. These results indicate a caspase-1-independent pathway of IL-18 secretion from FasL-stimulated macrophages and its critical involvement in FasL-induced liver injury.

Plasmodium berghei infection in mice induces liver injury by an IL-12- and Toll-like receptor/myeloid differentiation factor 88-dependent mechanism

Malaria, caused by infection with Plasmodium spp., is a life cycle-specific disease that includes liver injury at the erythrocyte stage of the parasite. In this study, we have investigated the mechanisms underlying Plasmodium berghei-induced liver injury, which is characterized by the presence of apoptotic and necrotic hepatocytes and dense infiltration of lymphocytes. Although both IL-12 and IL-18 serum levels were elevated after infection, IL-12-deficient, but not IL-18-deficient, mice were resistant to liver injury induced by P. berghei. Neither elevation of serum IL-12 levels nor liver injury was observed in mice deficient in myeloid differentiation factor 88 (MyD88), an adaptor molecule shared by Toll-like receptors (TLRs). These results demonstrated a requirement of the TLR-MyD88 pathway for induction of IL-12 production during P. berghei infection. Hepatic lymphocytes from P. berghei-infected wild-type mice lysed hepatocytes from both uninfected and infected mice. The hepatocytotoxic action of these cells was blocked by a perforin inhibitor but not by a neutralizing anti-Fas ligand Ab and was up-regulated by IL-12. Surprisingly, these cells killed hepatocytes in an MHC-unrestricted manner. However, CD1d-deficient mice that lack CD1d-restricted NK T cells, were susceptible to liver injury induced by P. berghei. Collectively, our results indicate that the liver injury induced by P. berghei infection of mice induces activation of the TLR-MyD88 signaling pathway which results in IL-12 production and activation of the perforin-dependent cytotoxic activities of MHC-unrestricted hepatic lymphocytes.

Potentiality of Interleukin-18 as a Useful Reagent for Treatment and Prevention of Leishmania major Infection

ABSTRACT Interleukin-18 (IL-18) is a proinflammatory cytokine that plays an important role in natural killer cell activation and the T helper 1 (Th1) cell response, particularly in collaboration with IL-12. Since Th1 cells play a pivotal role in the host defense against infection with intracellular microbes, such as Leishmania major, we investigated whether IL-18 is critically involved in protection againstL. major infection by activation of Th1 cells. We administered IL-12 and/or IL-18 daily to L. major-susceptible BALB/c mice. Neither IL-12 (10 ng/mouse) nor IL-18 (1,000 ng/mouse) induced wound healing, while daily injection of IL-12 and IL-18 during the first week after infection strongly protected the mice from footpad swelling by induction and activation of Th1 cells. Furthermore, these mice acquired protective immunity. We also investigated a protective role of endogenous IL-18 by using anti-IL-18 antibody-treated C3H/HeN mice (an L. major-resistant strain) or IL-18 deficient (IL-18−/−) mice with a resistant background (C57BL/6). We found that in the absence of endogenous IL-18, these mice showed prolonged footpad swelling as well as diminished nitric oxide production. However, daily injection of IL-18 into IL-18−/− mice corrected their deficiencies, suggesting that these mice have Th1 cells that produce gamma interferon (IFN-γ) in response to IL-18. Indeed, these mice had normal levels of Th1 cells. Thus, IL-18 is not responsible for inducing Th1 cells but participates in host resistance by its action in stimulating Th1 cells to produce IFN-γ. Our results also indicate the high potentiality of IL-18 as a useful reagent for treatment as well as prevention against reinfection.

Roles of interleukin-18 in tissue destruction and compensatory reactions.

Interleukin (IL)-18 is a member of the IL-1 cytokine family. Pro-IL-18 is cleaved by caspase-1 (IL-1&bgr;-converting enzyme) to yield biologically active 18-kDa IL-18. Interleukin-18 is recognized by a heterodimeric receptor, consisting of a ligand-binding &agr;-chain (IL-18R&agr;/IL-1Rrp) and an associating &bgr;-chain (IL-18R&bgr;/AcPL), which transmits signals through MyD88/IRAK/TRAF-6 molecules. Interleukin-18 is expressed in various types of cells, including macrophages, keratinocytes, intestinal epitherial cells, osteoblastic cells, chondrocytes, and adrenal cortex cells. Interleukin-18 promotes IFN-&ggr; production and Th1 helper T-cell development, synergistically with IL-12. However, IL-18 itself shows capabilities to induce IL-4, IL-5, IL-10, and IL-13 from T and natural killer cells. It also induces PGE2 production from activated macrophages. Moreover, many diseases are characterized by the production of IL-18 in the lesion. Taking these data together, our working hypothesis on how IL-18 is involved in “destructive” and “compensatory” pathways is proposed in this issue.

Plasma interleukin-18 levels in patients with psychiatric disorders.

There are an increasing number of reports on an association between the alteration of circulating cytokine levels and pathophysiology of psychiatric disorders. Plasma concentrations of interleukin (IL)-18 were measured in 13 nonmedicated patients with psychiatric disorders. There was a significant elevation of IL-18 levels in patients with major depression (n = 8) and panic disorder (n = 5), compared with normal controls. The mean IL-18 value of our psychiatric patients was comparable with that of various somatic disorders reported. We suggest that the elevation of plasma IL-18 levels reflects the increased production and release of IL-18 in the central nervous system under stressful settings. We propose that the measurement of IL-18 plasma levels may provide a useful index for the involvement of immune system in psychiatric disorders.

The Role of IL-18 in Blood-Stage Immunity Against Murine Malaria Plasmodium yoelii 265 and Plasmodium berghei ANKA

A possible protective role of IL-18 in host defense against blood-stage murine malarial infection was studied in BALB/c mice using a nonlethal strain, Plasmodium yoelii 265, and a lethal strain, Plasmodium berghei ANKA. Infection induced an increase in mRNA expression of IL-18, IL-12p40, IFN-γ, and TNF-α in the case of P. yoelii 265 and an increase of IL-18, IL-12p40, and IFN-γ in the case of P. berghei ANKA. The timing of mRNA expression of IL-18 in both cases was consistent with a role in the induction of IFN-γ protein expression. Histological examination of spleen and liver tissues from infected controls treated with PBS showed poor cellular inflammatory reaction, massive necrosis, a large number of infected parasitized RBCs, and severe deposition of hemozoin pigment. In contrast, IL-18-treated infected mice showed massive infiltration of inflammatory cells consisting of mononuclear cells and Kupffer cells, decreased necrosis, and decreased deposition of the pigment hemozoin. Treatment with rIL-18 increased serum IFN-γ levels in mice infected with both parasites, delayed onset of parasitemia, conferred a protective effect, and thus increased survival rate of infected mice. Administration of neutralizing anti-IL-18 Ab exacerbated infection, impaired host resistance and shortened the mean survival of mice infected with P. berghei ANKA. Furthermore, IL-18 knockout mice were more susceptible to P. berghei ANKA than were wild-type C57BL/6 mice. These data suggest that IL-18 plays a protective role in host defense by enhancing IFN-γ production during blood-stage infection by murine malaria.

Relation between interleukin-18 and PGE2 in synovial fluid of osteoarthritis: a potential therapeutic target of cartilage degradation.

Osteoarthritis (OA) is characterized by articular cartilage degradation and hypertrophic joint changes. Interleukin (IL)-18 is a potent inducer of prostaglandin (PG) E2 in vitro. We determined the relation between IL-18 and PGE2 in synovial fluid (SF) of human OA, and discussed the role of IL-18 in the pathogenesis of OA and also its therapeutic consequences. SF was collected from 30 patients with knee OA. The concentrations of IL-18 and other cytokines including IL-1&bgr;, tumor necrosis factor (TNF)-&agr;, IL-6, and IL-8 were measured by enzyme-linked immunosorbent assay (ELISA). The concentration of PGE2 was also assessed by inhibitory ELISA. The average value of IL-18 was 248 ± 310 pg/mL. The average value of PGE2 was 93 ± 103 pg/mL. There was a relatively strong correlation between IL-18 and PGE2 (r = 0.78, p = 0.0001). In contrast, IL-1&bgr; was undetectable (cutoff point of 20 pg/mL), except for one case. TNF-&agr; was also undetectable (cutoff point of 20 pg/mL), except for two cases. The average value of IL-6 was 1,310 ± 2,623 pg/mL (n = 17), whereas IL-8 was 5,208 ± 6,031 pg/mL (n = 5). Furthermore, IL-6 and IL-8 correlated with IL-18 (r = 0.69, p = 0.0024 and r = 0.87, p = 0.0527, respectively). Our results suggest that IL-18 could play a major role in vivo in inducing the production of PGE2, which in turn can cause cartilage degradation in OA pathogenesis. Thus, targeting this cytokine appears to be an important therapeutic approach in OA.

Cytokine-induced injury of the lacrimal and salivary glands.

Damages to the lacrimal and salivary glands that accompany various autoimmune diseases are categorized as secondary Sjögren syndrome. Cytokines and free radicals are thought to be responsible for the pathologic changes, but the precise mechanisms are not clear. We evaluated whether cytokines alone can cause the damages in these exocrine tissues, and whether gaseous molecules such as nitric oxide (NO) play a role in these injuries. Various knockout (KO) mice as well as wild-type mice were injected intraperitoneally (i.p.) with the proinflammatory cytokines, IL-12 and IL-18, singly or in combination. Concurrent administration of IL-12 and IL-18 to mice caused serious atrophy in the lacrimal and salivary glands, which was spared when each cytokine was singly administered. Microscopically, there were apparently no infiltrating cells; nonetheless, numerous apoptotic cells were observed in the epithelium, which was confirmed by DNA ladder formation on gel electrophoresis. Serum levels of IFN-&ggr; and NO2/NO3 were markedly elevated. Combined injections of IL-12 and IL-18 caused the same changes in Fas-deficient and Fas-ligand deficient mice, as well as in perforin-KO mice, but the same changes were not detected in inducible NO synthase-KO mice or IFN-&ggr; KO mice. Thus, the synergistic effect of IL-12 and IL-18 was dependent on production of IFN-&ggr; and NO, but independent of Fas/Fas ligand system and perforin-dependent cytotoxic T cells. IL-18 together with IL-12 caused destructive changes in the glandular tissues without apparent lymphocyte infiltration. It is suggested that these cytokines can mediate apoptosis in glandular epithelial cells and that the elevated NO production is responsible for the change.