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Objective: Therapeutic vaccination with dendritic cells (DC) showed promising results in first clinical trials in cases of metastatic renal cell carcinoma (RCC). Human telomerase reverse transcriptase (hTERT) could be a potential target because it is detectable in more than 85% of human tumors including RCC. Design: 10 patients with progressive metastatic RCC were enrolled in a clinical phase I/II trial using DC pulsed with hTERT-peptide. Beside toxicity and feasibility aspects, a complex immune monitoring including in vitro data were evaluated. In addition to detection of tumor-specific effector cells we investigated their functionality like IFN-γ secretion and cytotoxic activity against tumor cells. Results: The vaccine was well tolerated. Two patients showed a mixed response (MR) and one patient a stable disease (SD). Interestingly, responders showed cytotoxic activity already before start of therapy and there was a significant increase in cytotoxic activity of effector cells from all responders (SD and MR patients) after the first vaccination. In contrast non-responders showed no cytotoxic activity before and during treatment. Therefore, cytotoxic activity might be used as a predictive marker in the future. Tetramer staining detected higher amounts of tumor-specific cytotoxic cells in responding patients compared to non-responders. Also, responders possessed increasing amounts of IFN-γ producing immunological effector cells. Conclusion: Telomerase-pulsed DC could enhance a tumor-specific immune response against RCC.