mice were injected s.c. cells to eliminate cancer cells lacking cognate antigen expression in a locally restricted manner. to IL-2R/C on neighboring cells. Importantly, IL-15 is commonly found in the inflamed tissues of patients with autoimmune disorders and celiac disease, where it may promote tissue damage (11, 12), either by serving as a costimulatory molecule for the T-cell receptor (TCR) (13C15) or by endowing T Nos1 cells through the licensing of natural killer group 2D receptor (NKG2D) to exert lymphokine-activated killer (LAK) activity (13, 15C17). LAK activity by cytotoxic T cells, previously dismissed as an Eicosadienoic acid in vitro artifact, has been correlated with IL-15 expression by intestinal cells in individuals with celiac disease (13, 15, 18, 19). However, previous studies in humans were correlative in nature and could not determine whether killing of epithelial cells in a noncognate manner involves low-affinity TCR recognition of self or microbial antigens. Antitumor activity of IL-15 in vivo has been reported in two types of regimens. Eicosadienoic acid In the first type, IL-15 was added to cultures during activation of tumor-specific T cells in vitro before adoptive transfer (20C22); in the second, IL-15 was Eicosadienoic acid given systemically (23C25). These reports examined the effects of IL-15 in cancer models, although treatments either were given before tumors had been established or produced only partial responses. Other studies examining the effects of IL-15 expression by cancer cells have suggested that IL-15 can prevent tumor outgrowth and/or metastasis (26), and our laboratories have recently shown the eradication of established IL-15Cexpressing tumors by densely granulated natural killer (NK) cells (27). Based on accumulating evidence that IL-15 requires cell contact to function (27C29) and that it promotes organ-specific autoimmunity when expressed by tissue cells (30), we postulated that if cancerous cells expressed IL-15, then they could endow cytotoxic T cells with the ability to reject large established tumors and even prevent relapse. To test this idea, we adoptively transferred CD8+ T cells into mice bearing well-established tumors expressing IL-15 and evaluated tumor regression and regrowth. Our results show that IL-15 elicits a powerful response against established solid tumors and may be a more powerful costimulatory molecule for the TCR than previously thought, in that it could even endow the TCR with the ability to mediate cytolysis of tumors lacking expression of cognate antigens. Results We previously reported that cancer cells expressing low antigen levels relapse after treatment with specific CD8+ T cells, whereas tumors expressing high levels of antigens are completely rejected (31). We wanted to determine whether IL-15 Eicosadienoic acid in the tumor microenvironment would endow antigen-specific cytotoxic T cells with the ability to prevent tumor escape despite low levels of antigen expression in the same tumor model. To this effect, Eicosadienoic acid we transduced the fibrosarcoma mesenchymal cell line MC57 to express low levels of a fusion protein of an SIYRYYGL (SIY) peptide trimer and EGFP with either IL-15 (32) in an enhanced cyan fluorescent protein (ECFP) vector (M-SIY-IL15) or the vacant vector (M-SIY) (Fig. 1and Table S1). M-SIY and M-SIY-IL15 have comparable EGFP and ECFP fluorescence (Fig. 1and Fig. S1). Open in a separate windows Fig. 1. Expression of IL-15 by cancer cells prevents relapse after treatment with tumor-specific T cells. (mice were injected s.c. with M-SIY or M-SIY-IL15 cells, followed 2 wk later by 2C splenocytes i.v. or no further treatment. Lines represent individual tumors compiled from three individual experiments. The incidence of relapse of M-SIY tumors compared with M-SIY-IL15 tumors was statistically significant (< 0.05). (mice and analyzed for EGFP (SIY antigen) and ECFP (control vector) fluorescence. We opted to use mice as hosts because they are incapable of responding to IL-15, thus permitting uninhibited establishment of tumors. Either M-SIY or M-SIY-IL15 cells were injected s.c. into mice. After 2 wk, when.