Key words: IL-18; caspase-1; prostate cancer; prognosis; IFN-␥; IFN-␣IL-18 was first identified as an IGIF based on its ability to induce high levels of IFN-␥ secretion by both NK cells and Th1 clones. 1,2 IL-18 belongs to the IL-1 family, 3 lacks a signal sequence 2 and is processed into an 18 kDa mature form by caspase-1. 4 It is mainly produced by macrophages and dendritic cells. However, we and others have shown that IL-18 is also synthesized by nonimmune cells. 5 IL-18 potentiates IL-12-induced Th1 development 6,7 and plays an important role in T-cell proliferation. 2 In addition, it enhances FasL-mediated cytotoxicity of NK cells and Th1 cells 8,9 and has proinflammatory properties by inducing chemotactic molecules for macrophages, polymorphonuclear neutrophils and inflammatory cytokines such as TNF-␣ or IL-1. 10 -12 Murine models have demonstrated the antitumor activity of IL-18, either by systemic administration 13,14 or in tumors genetically modified to constitutively express IL-18. 15 Antitumor effects are mediated by IL-18-induced IFN-␥, NK cells and T CD4 ϩ Fas-dependent cytotoxicity. [13][14][15][16] In vitro, IL-18 added to cultures of tumor cells, NK cells, dendritic cells and T cells induces tumor cell death by enhancing NK cell cytotoxicity, which activates dendritic cells to promote a specific CTL response. 17 This could position IL-18 as an important bridge between innate and adaptive antitumor immune responses. Some antitumor effects of IL-18 may also be mediated via nonimmunologic mechanisms as IL-18 has antiangiogenic properties in vitro and immunohistochemistry studies have revealed hypovascularization of IL-18-treated tumors. 15,18 Little is known about the in vivo presence of IL-18 in human tumors. In a previous study, we observed that IL-18 protein was expressed at various levels in colon cancers and that loss of transcripts at the tumor site of the 2 downstream targets of IL-18, i.e., IFN-␥ and FasL, was associated with the concomitant presence of distant metastases. 19 These results suggested a role of IL-18 in the control of tumor spreading in humans, in line with the antitumor properties of IL-18 observed in murine models.Research in prostate cancer has focused on the role of IFN-␥ in tumor progression. In vitro, IFN-␥ causes cycle arrest of prostate tumor cell lines, induces the cyclin-dependent kinase inhibitor p21 WAF1 , downregulates neu/HER-2 20 and decreases the metastatic potential of some prostate cancer cell lines. 21 IFN-␥ is produced by stromal and epithelial cells in normal prostate and prostate cancers, 22 and analysis of the transcriptional profile of prostate cancers by Affymetrix (Santa Clara, CA) Genechip technology has shown that about 30% of malignant tumors present downmodulation of IFN-␥-inducible molecules. 23 This may indicate modulation of IFN-␥ in tumors, but no information is available about the existence of the IFN-␥-inducing cytokine IL-18 in normal prostate and prostate cancers. We therefore investigated the IL-18 status of prostate cancers and its as...