Abstract 1622: The role of the CCL-2 on lymphopenia-induced myeloid derived suppressor cells

Abstract The induction of lymphopenia prompts the expansion of CD11b+Ly6ChiLy6G- monocytic MDSCs (M-MDSCs) and CD11b+Ly6C+Ly6G+ polymorphonuclear MDSCs (PMN-MDSCs). Patients receiving adoptive cell therapy (ACT) require nonmyeloablative chemotherapy to induce lymphopenia and support anti-tumor immunity. However, the role of MDSCs in the setting of ACT is not fully understood. As high levels of CCL-2 can be measured in lymphopenic mice, in this study we investigate the role of CCL-2 in the expansion and function of lymphopenia-induced MDSCs using CCR2KO mouse models. Lymphopenia was induced in melanoma-bearing C57BL/6 (WT) and CCR2KO mice by 600rad of total body irradiation or combination therapy with cyclophosphamide and fludarabine. We first evaluated the percentages of splenic MDSCs in WT and CCR2KO mice after the induction of lymphopenia. At day 14, the expansion of total MDSCs were similar in WT (279% of normal) and CCR2KO (241% of normal). However, the mean percentages of M-MDSCs (3.5%) and PMN-MDSCs (30.3%) in CCR2KO mice were altered compared to M-MDSCs (22.2%) and PMN-MDSCs (21.7%) in WT mice. In addition, M-MDSCs in B16 tumors grown in CCR2KO mice were decreased compared to intratumoral M-MDSCs in WT mice (p<0.001). In contrast, the percentages of M-MDSCs and PMN-MDSCs were unchanged in the bone marrow (BM). To investigate the suppressive capacity of lymphopenia-induced MDSCs, OVA antigen-specific CD8+ T cells were co-cultured with OVA peptide in the presence of MDSCs purified from the spleens of CCR2KO and WT mice. Both CCR2KO and WT MDSCs potently suppressed T cell proliferation as measured by 3H thymidine incorporation. CCR2KO and WT MDSCs had significant arginase activity and production of nitrites. To investigate the efficacy of ACT, B16 tumor-bearing CCR2KO and WT mice were lymphodepleted and gp100 antigen-specific Thy1.2+CD8+ T cells were adoptively transferred followed by 3 days of IL-2 treatment. We found that donor T cell tumor infiltration was similar and no difference in tumor growth was observed between CCR2KO and WT mice. Collectively, these results suggest that the role of CCL-2 is important for the egress of lymphopenia-induced M-MDSCs from the BM, but has no effect on the expansion or function of PMN-MDSCs in the setting of lymphopenia. These data suggest that factors besides CCL-2 play an important role in the expansion and function of MDSCs after the induction of lymphopenia. Citation Format: Pasquale P. Innamarato, Amy Weber, Shari Pilon-Thomas. The role of the CCL-2 on lymphopenia-induced myeloid derived suppressor cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1622. doi:10.1158/1538-7445.AM2017-1622