Regulatory T cells (Tregs) are a major obstacle to the establishment of effective cancer immunotherapy. As mediators of immune tolerance, they are a critical target for pre-conditioning for adoptive immunotherapy. Here, we show that pre-treatment with cyclophosphamide or agonistic anti-OX40 mAb augments the anti-tumor immune effect of adoptive CD8+ T cell therapy in a clinically relevant wild-type model, as opposed to a TCR-transgenic mouse model. Tumor antigen-stimulated CD8+ T cells (7x106), including a small number (2.17x105) of tumor antigen-specific effector CD8+ T cells, were transferred into tumor-bearing mice. A response was detected in the adoptively transferred antigen-specific CD8+ T cells, but was insufficient for the eradication of the established tumor. However, pre-treatment with cyclophosphamide to reduce Tregs was shown to enhance the anti-tumor immune effect of the adoptively transferred CD8+ T cells. Moreover, we demonstrated for the first time that pre-treatment with OX40 costimulation, with the aim of nullifying Treg-mediated suppression, maintained the tumor-specific immune response of adoptively transferred CD8+ T cells, resulting in the eradication of the established tumor. These findings suggest that pre-conditioning with the aim of depleting Tregs is a useful strategy for adoptive cancer immunotherapy.

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