Baccatin III inhibits the accumulation and suppressive activity of myeloid‐derived suppressor cells in tumor‐bearing mic (LB584)

Abstract

Myeloid‐derived suppressor cells (MDSCs) mediate tumor‐associated immune suppression in both cancer patients and tumor‐bearing animals. Reduction or elimination of MDSCs reduces the rate of tumor progression and improves cancer therapies that employ mechanisms of immunity. Here we show that baccatin III, which is the precursor for the semisynthesis of paclitaxel, exerts antitumor immunomodulatory activity in very low doses (0.05 ‐ 0.5 mg/kg), although it is regarded as an inactive derivative of paclitaxel. Oral administration of baccatin III significantly reduced the growth of tumors induced by engrafting mice with either 4T1 mammary carcinoma or CT26 colon cancer cells. Antitumor activity was immune‐mediated because baccatin III was not directly cytotoxic. Baccatin III decreased the accumulation of MDSCs in the spleens of the tumor‐bearing mice. Furthermore, MDSCs isolated from baccatin III‐treated mice, compared with those isolated from vehicle‐treated mice, had a significantly reduced suppressive effect on T cells treated with the anti‐CD3 and anti‐CD28 monoclonal antibodies. Moreover, these cells produced significantly reduced amounts of reactive oxygen species and nitric oxide. These results demonstrate that baccatin III reduced tumor progression by inhibiting the accumulation and suppressive function of MDSCs.