Doxorubicin and T Helper Lymphocytes: Unexpected Allies Against Cancer

Despite considerable progress in conventional cancer therapies, major challenges persist in the treatment of patients with advanced stage malignancies. Cancer immunotherapy (harnessing the immune system against tumors) has demonstrated limited success to date, partially due to the immunosuppressive environment generated by tumors. The mechanisms of cancer-induced immune suppression are multiple and include the promotion of immunosuppressive cells such as regulatory T cells (Treg) and myeloid-derived suppressor cells (MDSC). MDSC expand in tumor-bearing hosts and play a central role in cancer immune evasion by inhibiting adaptive and innate immunity. Different approaches have been explored to negatively impact MDSC, each associated with specific pitfalls. In this study, we demonstrated that the anthracycline doxorubicin selectively eliminates MDSC in the spleen, blood and tumor beds. Furthermore, five days after doxorubicin treatment residual MDSC exhibited impaired suppressive function, which correlated with reduced reactive oxygen species (ROS) production, and down-regulation of arginase-1 and indoleamine 2,3-dioxygenase (IDO) expression. Of therapeutic relevance, the frequency of effector lymphocytes (CD4⁺ and CD8⁺ T cells) or natural killer cells (NK) to suppressive MDSC ratios was significantly increased following doxorubicin treatment of tumor-bearing mice. Importantly, the proportion of natural killer (NK) and cytotoxic T cells (CTL) expressing perforin and granzyme B and of CTL producing IFNγ was augmented following doxorubicin administration. The mechanism of doxorubicin-mediated elimination of MDSC was partly mediated by the increase of ROS production in MDSC at earlier time points after doxorubicin treatment. Consistently, MDSC isolated from gp91-/- mice were less sensitive to doxorubicin in vitro, and doxorubicin effects on MDSC in gp91-/- tumor-bearing mice were reduced. Of clinical significance, this drug efficiently combined with Th1 or Th17 lymphocytes to suppress tumor development and metastatic disease, resulting in better overall survival. MDSC isolated from patients with different types of cancer were also sensitive to doxorubicin-mediated cytotoxicity in vitro. Our results therefore indicate that doxorubicin may be used not only as a direct cytotoxic drug against tumor cells, but also as a potent immunomodulatory agent that selectively impairs MDSC-induced immunosuppression, thereby fostering the efficacy of T cell-based immunotherapy