Systemic lymphopenia is common among glioblastoma (GBM) patients after chemoradiotherapy and is observed up to six months to 1 year after irradiation. Systemic lymphopenia is associated with a worse survival outcome in GBM. To delineate the underlying mechanisms of lymphopenia in GBM, we prospectively evaluated peripheral blood of GBM patients treated with standard radiation therapy (RT) and concurrent temozolomide as well as conducted in vivo experiments using a syngeneic murine GBM model. Among 20 patients prospectively evaluated, 10 patients (50%) developed lymphopenia and the other 10 did not. The RNA sequencing analysis of the peripheral blood mononuclear cell (PBMC) showed an elevated level of myeloid-derived suppressor cells (MDSCs) regulatory genes including Arg-1 in lymphopenic patients compared to non-lymphopenic patients after chemoradiotherapy. The single-cell RNA sequencing (scRNA-Seq) analysis of two selected phenotypical patients also demonstrated that a severe lymphopenic patient had elevated levels of two types of monocytic MDSCs (M-MDSCs) after RT compared to a non-lymphopenic patient. Immunophenotyping analysis of the PBMCs using flow cytometry confirmed reductions of CD4 and CD8 T cells as well as increased polymorphonuclear MDSCs (PMN-MDSCs) and M-MDSCs in lymphopenic patients compared to non-lymphopenic patients. The in vitro T cell proliferation and functional assays showed that MDSCs from lymphopenic patients exhibited were more suppressive compared to MDSCs from non-lymphopenic patients. Using a syngeneic orthotopic mouse (C57BL/6) GBM (GL261) model, we demonstrated that fractionated RT (2 Gy/day x 5) reduced CD4/CD8 cells and increased MDSCs compared to sham irradiated mice. The MDSCs from the irradiated mice also had increased Arg-1 expression. The addition of Arg-1 inhibitor (Arg1i) or phosphodiesterase type 5 inhibitor (PDE5i) to RT blocked the radiation-induced increase of MDSCs and reduction of CD4/CD8 cells. The combination of either Arg1i or PDE5i also enhanced the survival of mice with orthotopic tumors when compared to mice treated with RT alone or drug alone. In conclusion, we found that RT can induce an increase in circulating MDSCs in both patients and mice and the resulting circulating MDSCs can suppress T-cell proliferation and function. Inhibition of MDSCs using either Arg1i or PDE5i during RT may prevent systemic lymphopenia and affect disease control, which should warrant further investigation in clinical trials.

Citation Format: Subhajit Ghosh, Matthew Inkman, Jin Zhang, Sukrutha Thotala, Ekaterina Tikhonova, Natalia Miheecheva, Felix Frenkel, Ravshan Ataullakhanov, Xiaowei Wang, Dennis Hallahan, Jiayi Huang, Dinesh Thotala. Association of circulating myeloid derived suppressor cells with systemic lymphopenia following chemoradiotherapy in glioblastoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 507.