B7-H3 Promotes Prostate Cancer Progression in Mice by Antagonizing Myeloid-Derived Suppressor Cell Apoptosis

Background: B7-H3 is an important immunomodulatory molecule, and clinical studies have confirmed that its expression level is closely correlated with prostate cancer prognosis. However, the mechanism of its biological action is unclear. Methods: An engineered cell line overexpressing B7-H3 was constructed. Cell apoptosis, growth and proliferation assays in vitro and an animal model in vivo were performed to analyze the role and possible mechanism of B7-H3 in promoting prostate cancer progression. Results: Compared with the control cell line (Mock-RM-1), the B7-H3-overexpressing prostate cancer cell line (B7-H3-RM-1) showed no significant growth differences in vitro, whereas the in vivo tumorigenesis rate of B7-H3-RM-1 was significantly higher than that of Mock-RM-1. These results suggest that B7-H3indirectly, rather than directly, promotes prostate cancer progression. Further analysis revealed that significantly higher levels of myeloid-derived suppressor cells (MDSCs) accumulated in vivo in B7-H3-RM-1 tumor-bearing mice than in Mock-RM-1 mice. In vitro and in vivo experiments showed that B7-H3-RM-1 cells significantly antagonized MDSC apoptosis. To further confirm the role of MDSCs in B7-H3-mediated prostate cancer progression, model mice were pretreated with cyclophosphamide before inoculation to clear immune cells and achieve myelo suppression. The results showed no significant differences in tumor growth between the B7-H3-RM-1 group and the Mock-RM-1 group. Conclusions: We found, for the first time, that B7-H3 can antagonize MDSC apoptosis, leading to MDSC accumulation in the tumor microenvironment and thereby promoting prostate cancer progression.