Both melanoma and glioma cells are of neuroectodermal origin and share common tumor associated antigens. In this article, we report that the melanocyte differentiation antigen TRP2 (tyrosinase-related protein 2) is not predominately involved in the tumor rejection of a syngeneic murine glioma. Although GL261 glioma cells endogenously expressed TRP2 and were lysed by TRP2 specific cytotoxic T cells (CTLs) in vitro, vaccinations with TRP2 peptide-pulsed dendritic cells (DCs) could only induce minor antiglioma responses in a prophylactic setting and failed to work in a stringent setting where vaccine and tumor were administered on the same day. Further analysis revealed that TRP2 is not recognized by bulk CTLs after depletion of regulatory T cells which results in tumor rejections in vivo. In contrast to TRP2 peptidepulsed DC, tumor lysate-pulsed DCs were more potent as a vaccine and completely protected mice from tumor outgrowth in a prophylactic setting. However, the vaccine efficacy of tumor lysate-pulsed DC was not sufficient to prevent the tumor outgrowth when tumors were inoculated the same day. In this case, Treg depletion before vaccination was essential to boost antiglioma immune responses leading to the rejection of 80% of the mice and long-term immunity. Therefore, we conclude that counteracting the immunosuppressive glioma tumor environment via depletion of regulatory T cells is a prerequisite for successful eradication of gliomas after targeting multiple tumor antigens by using tumor lysate-pulsed DCs as a vaccine in a more stringent setting. ' 2007 Wiley-Liss, Inc.Key words: glioma; TRP2; tumor lysate; dendritic cell; regulatory T cell; CD25-depletion Dendritic cell (DC)-based immunotherapy is regarded as an attractive strategy for the therapy of malignant gliomas. Several animal studies have demonstrated that DCs can efficiently mediate the induction of antitumor responses within the central nervous system. 1-9 A number of phase I/II studies have been performed and others are ongoing to evaluate and improve DC vaccines for patients with malignant gliomas.
10-15Although DC-based immunotherapies for various cancers including malignant gliomas led to the induction of antitumor cytotoxic T cells (CTLs) and lymphocyte infiltration into tumors in situ, objective tumor regressions have been limited to a few cases. 16 In addition, most mouse glioma models primarily assessed the ability of DC-based immunotherapy to prevent the outgrowth of tumor in a prophylactic setting in which tumor cells are injected after vaccination. Attempts to treat tumors at more stringent conditions, like administration of tumor cells and vaccine on the same day (simultaneous setting) or a therapeutic setting were either not evaluated or shown to be unsuccessful, despite the presence of large numbers of antiglioma immune cells.A major obstacle for an active immunization against solid tumors is the presence of immunosuppressive factors at the local tumor environment. There is accumulating evidence that active suppressor mechanisms ar...