Background: Carbohydrate sulfotransferase 15 (CHST15) is a proteoglycan biosynthetic enzyme and is responsible for tumor matrix remodeling. Intratumoral injection of CHST15 siRNA was shown to increase tumor-infiltrating T cells in patients with pancreatic cancer in the phase I/IIa study. However, the mechanism underlying the enhanced T cell accumulation is not fully explored. Here we investigated the effect of locally injected CHST15 siRNA on modulating tumor immune microenvironment in mice. Methods: For in vivo pharmacokinetics after a single local injection of CHST15 siRNA to normal C57BL/6J mice, RNA oligonucleotides were isolated from tissues and detected by HPLC. Mouse pancreatic cancer KPC cells were implanted subcutaneously into the left hind footpad of syngeneic C57BL/6J mice and the tumor growth was monitored. From week 1, intratumoral injections with 1 mg/mL of CHST15 siRNA or vehicle were performed twice a week for 2 weeks and mice were sacrificed at week 3. In separate experiments, human BxPC-3 cells were implanted subcutaneously in T cell-deficient nude mice, and the effect of intratumoral injections of CHST15 siRNA or vehicle for 2 weeks was also investigated. Immunohistochemical staining was performed for the tumor and tumor-draining lymph node (TDLN). Results: CHST15 siRNA was detectable in not only the local sites but also the draining LNs of normal mice 1 and 4 hours after locoregional injection. In the KPC-implanted syngeneic mouse model, CHST15 siRNA showed significant tumor growth inhibition compared to the vehicle. In tumor, CHST15 was expressed by tumor cells, and to a lesser extent, Ly6C/G⁺ MDSCs. CHST15 siRNA altered tissue remodeling by repressing dense fibrotic architecture and NETosis. CHST15 siRNA significantly diminished Ly6C/G⁺ MDSCs while increasing CD4⁺ and CD8⁺ T cells. In TDLN, CHST15 was expressed by Ly6C/G⁺ MDSCs. Locally injected CHST15 siRNA markedly diminished Ly6C/G⁺ MDSCs, while increasing Ki-67⁺ proliferating CD4⁺ and CD8⁺ T cells. In the BxPC-3-implanted nude mouse model, locally injected CHST15 siRNA repressed excessive fibrosis and NETosis in tumor, diminishing both tumoral and TDLN MDSCs, but tumor growth inhibition was marginal. These results suggest that locally injected CHST15 siRNA could enter into TDLN, directly diminishing CHST15⁺ MDSCs, and expanded T cells. Tumor growth inhibition was largely T-cell medicated. Conclusions: The present study demonstrated that CHST15 siRNA locally and remotely diminished MDSCs, leading to the enhancement of anti-tumor T cell activity. A novel role of locally-delivered RNA oligonucleotide is suggested.