Abstract
Myeloid Derived Suppressor Cells (MDSCs) are present in most solid tumors, including glioblastoma (GBM), where they can block the activation of tumor-reactive T-cells and support tumor growth. Our lab and others have demonstrated the important roles of cyclooxygenase (COX)-2 as well as its enzymatic product, prostaglandin E2 (PGE2), in MDSC development, accumulation, and function. PGE2 mediates cellular responses through its receptors, EP1-EP4, of which EP4 is known to play an important role in anti-tumor immune responses. We therefore evaluated the impact of ONO-AE3-208, an EP4 antagonist, on MDSC function, tumor growth, and T-cell activation in gliomas. Our in vitro studies demonstrated that while PGE2 and ONO-AE3-208 treatment did not alter the growth of SB28 mouse glioma cells, ONO-AE3-208 reversed the MDSC phenotype of bone marrow cells induced by PGE2 or SB28 glioma cell line-derived conditioned media. Similarly, ONO-AE3-208 restored interferon-γ production by CD8 T-cells following PGE2 treatment. In vivo, ONO-AE3-208 treatment significantly prolonged survival of mice bearing established intracranial C57BL/6-syngeneic SB28 gliomas in wild-type but not RAG1-/- immunodeficient mice. Immunostaining and flow cytometry revealed that ONO-AE3-208-treated gliomas were infiltrated by fewer MDSCs compared with control mice. RT-PCR analysis of glioma-infiltrating MDSCs demonstrated that ONO-AE3-208 treatment suppressed Arg1 and Cox2 expression levels in both Ly6G+ and Ly6C+ MDSC populations as well as Tgfb1 and Il4ra levels in Ly6G+ MDSCs. Consistently, glioma-infiltrating lymphocytes in ONO-AE3-208 treated tumor-bearing mice revealed enhanced Ifng expression compared with control mice, suggestive of enhanced T-cell activity. Finally, combination therapy using anti-PD1 monoclonal antibody with ONO-AE3-208 in the anti-PD1-sensitive GL261 glioma model resulted in further prolonged survival compared with anti-PD1 therapy alone. Our data demonstrate that ONO-AE3-208 may be useful in the treatment of glioma patients to suppress MDSCs and promote anti-tumor immunity.