IL-18 Contributes to Host Resistance Against Infection with Cryptococcus neoformans in Mice with Defective IL-12 Synthesis Through Induction of IFN- Production by NK Cells 1 Kazuyoshi Kawakami, 2 * Yoshinobu Koguchi,* Mahboob Hossain Qureshi, 3 * Akiko Miyazato,* Satomi Yara,* Yuki Kinjo,* Yoichiro Iwakura, ² Kiyoshi Takeda, ‡ Shizuo Akira, ‡ Masashi Kurimoto, § and Atsushi Saito* The aim of this study was to examine the contribution of IL-18 in host defense against infection caused by Cryptococcus neoformans in mice with defective IL-12 production. Experiments were conducted in mice with a targeted disruption of the gene for IL-12p40 subunit (IL-12p40 / mice). In these mice, host resistance was impaired, as shown by increased number of organisms in both lungs and brains, compared with control mice. Serum IFN- was still detected in these mice at a considerable level (20 –30% of that in control mice). The host resistance was moderately impaired in IL-12p40 / mice compared with IFN- / mice. Neutralizing anti-IFN- mAb further increased the lung burdens of organisms. In addition, treatment with neutralizing anti-IL-18 Ab almost completely abrogated the production of IFN- and also impaired the host resistance. Host resistance in IL-12p40 / IL-18 / mice was more profoundly impaired than in IL-12p40 / mice. Administration of IL-12 as well as IL-18 increased the serum levels of IFN- and significantly restored the reduced host resistance. Spleen cells obtained from infected IL-12p40 / mice did not produce any IFN- upon restimulation with the same organisms, while those from infected and IL-12-treated mice produced IFN-. In contrast, IL-18 did not show such effect. Finally, depletion of NK cells by anti-asialo GM1 Ab mostly abrogated the residual production of IFN- in IL-12p40 / mice. Our results indicate that IL-18 contributes to host resistance to cryptococcal infection through the induction of IFN- production by NK cells, but not through the development of Th1 cells, under the condition in which IL-12 synthesis is deficient. The Journal of Immunology, 2000, 165: 941–947. C ryptococcus neoformans, a ubiquitous fungal pathogen, causes a life-threatening infection of the CNS in patients with impaired cell-mediated immunity, such as AIDS (1). In these patients, meningoencephalitis caused by this fungal patho- gen is often resistant to antifungal treatment and requires long- term chemotherapy even when patients are rescued from death (2). Therefore, for the development of effective immunotherapy, it is important to understand the precise mechanism of host resistance to this infection. Cell-mediated immunity plays a major role in host defense (3– 7), and Th1 cytokines, such as IFN-, which acts by inducing NO-dependent fungicidal activity of macrophages (8, 9), are es- sential for this response (10). Many investigators have reported the essential role of IL-12, an important cytokine for differentiation of Th1 cells (11), in host resistance to a variety of infectious patho- gens (12–17). Using mice with a targeted disruption of the gene for IL-12p40 or p35 subunit (IL-12p40 -/- or p35 -/- mice), Decken et al. (17) recently indicated that this cytokine is a prerequisite for protecting hosts against infection with C. neoformans. In a series of studies, we have previously demonstrated that administration of IL-12 promoted the clearance of fungal organisms from the lung and prevented dissemination to the brain (18 –20). In contrast, IL- 18, a novel cytokine identified as an IFN--inducing factor (21), is known to potentiate the differentiation of Th1 cells, although this cytokine by itself fails to induce this response (22). Several studies showed that IL-18 plays important roles in host defense against infection with Yersinia enterocolitica, Salmonella typhimurium, HSV1 and Leishmania major (23–26). We have recently shown that this cytokine plays an important role in the host resistance against cryptococcal infection, and its administration protects mice against this infection (27). Furthermore, IL-18 potentiates the pro- tective effects of IL-12 against this infection both in in vitro and in vivo studies (28, 29). However, the contribution of the former cytokine to host defense against infectious pathogens is not fully understood because it has been difficult to discriminate the activity of IL-18 from that of the counterpart cytokine. In the present study, we elucidated the role of IL-18 in host resistance to pulmonary and disseminated infections with C. neo- formans using IL-12p40 -/- mice. For this purpose, we examined the effect of neutralizing anti-IL-18 Ab on the clinical course of this infection and cytokine responses by measuring the serum lev- els of IFN-. Furthermore, we compared the host resistance of *First Department of Internal Medicine, Faculty of Medicine, University of the Ryukyus, Okinawa, Japan; ² Laboratory Animal Research Center, Institute of Medical Science, University of Tokyo, Tokyo, Japan; ‡ Department of Host Defense, Research Institute for Microbial Diseases, Osaka University, Osaka, Japan; and § Fujisaki In- stitute, Hayashibara Biochemical Laboratories, Okayama, Japan Received for publication September 28, 1999. Accepted for publication May 4, 2000. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 1 This work was supported in part by a grant-in-aid (C) (09670292) from the Ministry of Education, Science, and Culture, by grants from the Ministry of Health and Wel- fare, Japan, and by the Japan Health Science Foundation. 2 Address correspondence and reprint requests to Dr. Kazuyoshi Kawakami, First Department of Internal Medicine, Faculty of Medicine, University of the Ryukyus, 207 Uehara, Nishihara, Okinawa 903-0215, Japan. E-mail address: kawakami@ med.u-ryukyu.ac.jp 3 Current address: Division of Infectious Diseases, Department of Internal Medicine, University of Kentucky, Lexington, KY 40536. Copyright © 2000 by The American Association of Immunologists 0022-1767/00/$02.00