Synergistic effect of adoptive immunotherapy and docetaxel inhibits tumor growth in a mouse model.

Adoptive T cell transfer therapy (ACT) has emerged as a promising approach to cancer immunotherapy; however, the efficacy of ACT is limited by the T-cell suppressive activity of myeloid-derived suppressor cells (MDSCs), which accumulate in the tumor microenvironment after ACT. We sought to determine whether the efficacy of ACT could be enhanced by co-treatment with docetaxel, a taxane chemotherapy agent that has been shown previously to inhibit MDSC function. Using a mouse tumor model, we demonstrated that ACT and docetaxel synergistically inhibit the growth either of engrafted CT26 colon cancer or 4T1 mammary carcinoma cells. While ACT mediated an increase in the recruitment of MDSCs to the site of the tumor, docetaxel reversed this increase. Furthermore, ex vivo cultures of tumor-associated MDSCs suppressed the cytotoxic activity of tumor-specific T cells, and this suppressive activity was abolished by docetaxel treatment. These results suggest that docetaxel inhibits both the tumor recruitment and T cell suppressive activity of MDSCs. Inhibitors of iNOS and arginase partially inhibited ex vivo MDSC activity, and combined inhibition of iNOS and arginase had a similar effect as docetaxel, which supports the possibility that docetaxel may function by inhibiting ACT-associated activation of these pathways. Furthermore, docetaxel mediated inhibition of the T cell suppressive activity of MDSCs from human blood, which supports the potential clinical applicability of these findings. On the basis of these findings, docetaxel treatment may represent an effective therapeutic approach for reversing immunosuppression by MDSCs subsequent to ACT-based therapy.