The role of immune responses in tumor development is a central issue for tumor biology and immunology. IL-17 is an important cytokine for inflammatory and autoimmune diseases. Although IL-17 producing cells are detected in cancer patients and tumor bearing mice, the role of IL-17 in tumor development is controversial and mechanisms remain to be fully elucidated. In the current study, we found that the development of tumors was inhibited in IL-17 receptor A (IL-17R) deficient mice. A defect in IFN-γ receptor increased tumor growth whereas tumor growth was inhibited in mice that were deficient in both IL-17R and IFN-γR compared to wild type animals. Further experiments showed that neutralization of IL-17 by antibodies inhibited tumor growth in wild type mice whereas systemic administration of IL-17 promoted tumor growth. The IL-17R deficiency increased CD8 T cell infiltration whereas it reduced the infiltration of myeloid derived suppressor cells (MDSC) in tumors. In contrast, administration of IL-17 inhibited CD8 T cell infiltration and increased MDSC in tumors. Further analysis indicated that IL-17 was required for the development and tumor promoting activity of MDSC in tumor bearing mice. These data demonstrate that IL-17 mediated responses promote tumor development through the induction of tumor promoting microenvironments at tumor sites. IL-17 mediated regulation of MDSC is a primary mechanism for its tumor promoting effects. The study provides novel insights into the role of IL-17 in tumor development and has major implications for targeting IL-17 in treatment of tumors.

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Toll-like receptors (TLR) activate multiple steps in inflammatory reactions in innate immune responses. They also activate signals that are critically involved in the initiation of adaptive immune responses. Many tumorigenic chemicals have been associated with endotoxin hypersensitivity mediated through TLR4. To determine the role of TLR4 in cutaneous skin carcinogenesis, we treated TLR4-deficient C3H/HeJ mice and the TLR4-normal C3H/HeN mice with the carcinogenic polyaromatic hydrocarbon 7,12-dimethylbenz(a)anthracene (DMBA). TLR4-deficient C3H/HeJ mice developed more tumors relative to the TLR4-normal C3H/HeN mice. Both C3H/HeN and C3H/HeJ mice developed a T-cell-mediated immune response to topically applied DMBA. Interestingly, the cell-mediated immune response was mediated by IFN-; in C3H/HeN mice and by interleukin (IL)-17 in C3H/HeJ mice. Moreover, C3H/HeN mice had elevated circulating levels of IFN-; following topical application of DMBA, whereas IL-17 was elevated in C3H/HeJ mice. The results of this study indicate that TLR4 plays an important role in the prevention of DMBA skin tumorigenesis and that this is associated with differences in the T-cell subtype activated. Efforts to divert the cell-mediated immune response from one that is IL-17 mediated to one that is IFN-; mediated may prove to be beneficial in the prevention of DMBA-induced cutaneous tumors. [Cancer Res 2008;68(2):615-22]