Madecassic acid alleviates colitis-associated colorectal cancer by blocking the recruitment of myeloid-derived suppressor cells via the inhibition of IL-17 expression in γδT17 cells

Introduction: Madecassic acid (MA), a triterpene compound isolated from Centeno Asiatica herbs, has previously been shown to attenuate colitis induced by DSS in mice. In the present study, we address whether and how MA ameliorates colitis-associated colorectal cancer (CAC), which accounts for a considerable proportion of colorectal cancer. Methods: CAC was induced by AOM/DSS in mice, and MA was administered orally once a day. To identify the source cells of IL-17 and the target cells for MA reducing the expression of IL-17 in the colons of CAC mice, single-cell suspensions were prepared from the colons of CAC mice and analyzed by flow cytometry. An adoptive transfer experiment was performed to verify the importance of the decreasing gamma delta T17 cell population in the anti-CAC effect of MA. Results: Oral administration of MA reduced the burden and incidence of tumors in the CAC mice. MA decreased the number of MDSCs in the colon tissues of CAC mice and ameliorated anti-tumor immune responses. MA could prevent the migration of MDSCs by inhibiting the activation of gamma delta T17 cells and the expression of chemokines. The population of activated-gamma delta T17 cells in the tumor microenvironment of CAC mice positively correlated with the number of MDSCs and tumors as well as tumor load. Moreover, the anti-CAC effect of MA was significantly counteracted by the adoptive transfer of gamma delta T17 cells. Conclusions: MA alleviates CAC by blocking the recruitment of MDSCs to increase the population of anti-tumor immune cells in tumor microenvironment via inhibition of the activation of gamma delta T17 cells.