Developing Therapies to Overcome Immunosuppressive Myeloid Cells in the Tumor Microenvironment

Abstract

Myeloid cells in the tumor microenvironment represent significant barriers to the development of successful cancer immunotherapies. A multi-kinase inhibitor, Regorafenib (Reg), and a DNA-PK inhibitor, NU7441 (NU) were shown in a previous study to reduce expression of immunoinhibitory proteins in adaptive immune cells while increasing stimulatory MHC-I on cancer cells. In this study, we explored whether these drugs could reverse the suppressive activity of myeloid-derived suppressor cells (MDSCs) and alternatively activated macrophages. To test this idea, we used splenocytes from tumor-bearing mice and a human monocytic cell line differentiated into suppressive macrophages and assessed Arginase activity, their ability to suppress effector T cells, and mRNA expression of immunosuppressive and activating markers. We showed that Reg/NU decrease arginase activity and increase immunoactivating markers. These data demonstrate that treatment of suppressive myeloid cells with Reg/NU confers a less suppressive phenotype and leads to the generation of a more activating phenotype.