Cinnamaldehyde causes apoptosis of myeloid-derived suppressor cells through the activation of TLR4

Malignant tumors are among the most life-threatening diseases in the world. Although many different types of antitumor agents are available, severe side effects and toxicity limit their applications. Myeloid-derived suppressor cells (MDSCs) inhibit the antitumor immune response by suppressing the proliferation of T cells, the production of cytokines and the killing of tumor cells. As MDSCs have become novel targets in cancer therapy, this research focused on the anti-MDSC function of cinnamaldehyde (CA), which is extracted from cinnamon, a traditional Chinese spice. In the present study, MDSCs isolated from the spleens of mice with colon cancer were used as an in vitro model to assess the efficacy of CA. Treatment of MDSCs with CA significantly decreased cell proliferation and induced apoptotic cell death. Subsequent experiments demonstrated that CA treatment enhanced the expression of Bax and caspase-9 and inhibited the expression of Bcl-2, suggesting that CA induced apoptosis in the MDSCs via the intrinsic pathway. Taken together, the results demonstrated that CA exhibited significant anti-MDSC activity and attenuated the suppression of the antitumor immune response, indicating a potential use for CA in cancer therapy.