415 Background: The emergence of immune checkpoint inhibitors (ICI) has brought hope for cure and survival for those suffering from various cancers, including bladder cancer. However, the response rate of ICI monotherapy is modest, and recent reports indicate that myeloid-derived suppressor cells (MDSC) might play a role in the resistant mechanism of ICI. In this study, we assess the effect of chemokine signal on the proliferation of bladder cancer and investigate whether MDSC could be a new target for the treatment of cisplatin-resistant bladder cancer. Methods: We established a cisplatin resistant strain (MB49R) of mice bladder cancer cell line MB49, and examined the alteration of the expression levels of inflammatory chemokines by chemokine array. Next, we isolated MDSCs from spleen and tumor in tumor-bearing mice to examine gene expression levels of chemokine receptors (CXCR2 and CCR2) and immunosuppression genes (Arg-1 and iNOS). Furthermore, we assessed the efficacy of CDDP, α-PD-L1 and chemokine antagonists against the proliferation of tumors in MB49 and MB49R xenograft models. Results: Expression levels of CCL2 and CXCL1/2, which are involved in the migration of MDSC, were significantly increased in the culture supernatant of MB49R compared to those in MB49 cell lines. This result was confirmed by real-time RT-qPCR of tumor extract, and this increase was also observed in human bladder cancer cell lines (T24 and T24R). CXCR2 and CCR2 were highly expressed in PMN-MDSC and M-MDSC, respectively, which were isolated from spleen or tumors in tumor-bearing mice, and gene expression levels of Arg-1 and iNOS were dramatically increased in M-MDSCs from the tumor tissues compared to those from spleen. Also, analysis by flow cytometry revealed that PMN-MDSC dramatically decreased in MB49R compared to parental MB49 tumors, while the proportion of M-MDSC was not changed in MB49R, which indicates that M-MDSC could be a target for the treatment of CDDP resistant bladder cancer. Conclusions: The results in the present study might indicate that the combination treatment with ICI and MDSC-targeting therapy could be an option for the treatment of cisplatin-resistant bladder cancer.
651. Oncolytic Reovirus Induces Reduction in Immunosuppressive Activity of Myeloid-Derived Suppressor Cells in Tumor-Bearing Mice
