TH17 is a newly identified pathogenic memory CD4 T-cell lineage with potent agonist effects in some murine experimental autoimmunity models, however, its role in tumor immunology is still unclear. Clinical experience with interleukin (IL)-2 and ipilumumab [anti-cytotoxic T-lymphocyte antigen 4 (CTLA4) antibody], particularly in treating immunogenic malignancies such as melanoma and renal cell carcinoma (RCC), has suggested an association between a variety of autoimmune phenomena and tumor regression. To investigate this issue in patients with RCC, we isolated T-cell clones from peripheral blood that released IL-17 on stimulation with their autologous RCC tumor line. Clones were generated from 1 patient before any systemic treatment by in vitro stimulation with dendritic cells, autologous tumor, and IL-2 in the presence of anti-CTLA4 antibody. That patient subsequently received treatment with ipilimumab and showed both objective tumor regression and immune-mediated colitis. Limiting dilution cloning of his tumor-dendritic cell-stimulated T cells produced the 3G8D CD4+ clone, which secreted both IL-17 and interferon-gamma when cocultured with the autologous RCC line (transduced with class II transactivation molecule to induce major histocompatibility complex-class II expression). Broader analysis of its cytokine secretion profile showed large amounts of IL-8 when cocultured with RCC, but not when triggered with phorbol 12-myristate 13-acetate and ionomycin. This led to the discovery that IL-8 was being produced by the RCC cells in response to T-cell-derived IL-17. This effect of exogenous IL-17 on IL-8 release from tumor was seen in 5 of 8 RCCs, but not in other tumors tested. Preliminary data on the frequency of IL-17-secreting T cells in the lymphocytes infiltrating RCCs suggest that there may be a positive correlation between this frequency and IL-8 production by nonlymphoid cells as determined by quantitative reverse transcription-polymerase chain reaction. This report extends the known bidirectional interactions between immune cells and malignant cells in the tumor microenvironment that can shape and modulate the host immune response to cancer.