Abstract 2519: Polymorphonuclear myeloid-derived suppressor cells reflect the status of peritoneal dissemination in colon cancer mouse model
Authors: Saito Masafumi , Sugita Yutaka , Yamashita Kimihiro , Fujita Mitsugu , Yamada Kota , Agawa Kyosuke , Watanabe Akihiro , Fukuoka Eiji , Kanaji Shingo , Oshikiri Taro , Matsuda Takeru , Kakeji Yoshihiro 
Published: June 2022

Abstract
Background 8.3% of colorectal cancer (CRC) patients have peritoneal dissemination (PD) and their prognosis is poor. Myeloid-derived suppressor cells, MDSC, are functional myeloid cells with immune suppressive properties. When infiltrated into the tumor, these cells induce dysfunction of cytotoxic T cells via the secretion of immunosuppressive molecules like arginase-1 and promote tumor progression. MDSC is classified into two major subsets in a mouse by their surface antigens: monocytic MDSCs (M-MDSCs; CD11b+Ly6ChighLy6G−) and polymorphonuclear MDSCs (PMN-MDSCs; CD11b+Ly6ClowLy6G+). To date, MDSCs are difficult to identify because they express the same surface markers as neutrophils and monocytes, respectively. CD244, an immunoregulatory transmembrane receptor molecule, has been highlighted as a surface antigen to distinguish PMN-MDSCs from normal neutrophils. However, few studies have demonstrated whether and how MDSCs, especially CD244+ PMN-MDSC, affect the generation and progression of PD.
Objective To clarify CD244+ PMN-MDSC affects the progression of PD and CD8+ T cells.
Methods Female C57BL6J mice were injected 5.0×105 ~ 2.0×106 cells of MC38 colon cancer intraperitoneally to establish the PD model, and we had conducted the following three experiments: Exp. 1) Investigation of the trend of immune cells including MDSC and CD8+ T cell in the PD nodule, peritoneal cavity, blood, and spleen; Exp. 2) PMN-MDSC depletion test in PD model; and Exp. 3) Investigation of direct influence of CD244+ or CD244- Ly6G+ cells in T cell proliferation in vitro.
Results Exp. 1) PD nodules became enlarged and increased over time. All mice had died by day 40 after the cancer was inoculated in the PD model, which was significantly shorter than the MC38-based subcutaneously inoculation model. In the PD model, a significant increase of PMN-MDSCs was observed in the peritoneal cavity, blood, and spleen, and also observed these cells in the PD nodule at day 20. Of note, the CD244+ PMN-MDSC predominantly existed in the population of myeloid cells in the peritoneal cavity. Exp. 2) To deplete the PMN-MDSC, the mice were administered a Ly6G antibody. Although Ly6G treatment could not improve the survival rate of the PD model, the tumor progression was significantly decreased, and the number of both CD4+ T cells and CD8+ T cells was notably increased in the peritoneal cavity at day 19. Exp. 3) Finally, we sorted CD244+/CD244- Ly6G+ cells and co-cultured them with splenocytes. The Ly6G+CD244+ cells significantly inhibited antigen-specific CD8+ T cell proliferation in a dose-dependent manner. T cell suppression mediated by the Ly6G+CD244+ cells was also observed in the numerical evaluation of CD8+ T cells at a high (1:1) ratio.
Conclusion The targeted therapy for PMN-MDSCs would provide not only new therapeutic value but also a novel strategy to synergize with T cell-based immunotherapy for CRC-derived PD.
Citation Format: Masafumi Saito, Yutaka Sugita, Kimihiro Yamashita, Mitsugu Fujita, Kota Yamada, Kyosuke Agawa, Akihiro Watanabe, Eiji Fukuoka, Shingo Kanaji, Taro Oshikiri, Takeru Matsuda, Yoshihiro Kakeji. Polymorphonuclear myeloid-derived suppressor cells reflect the status of peritoneal dissemination in colon cancer mouse model [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2519.