TAM receptor tyrosine kinase MERTK as a therapeutic target in the tumor immune microenvironment in a murine Kras mutant lung cancer model

TAM receptor tyrosine kinase MERTK is involved in apoptotic cell clearance to maintain tissue homeostasis. During efferocytosis, MERTK activation also promotes an immunosuppressive phenotype, which can be hijacked by tumor cells to escape immune attacking. Here, we determined the impact of MERTK inhibition on immune cell subsets in the tumor microenvironment in mice with subcutaneous murine CMT167 (Kras-G12V) lung cancer grafts. Although MERTK expression was not detected on tumor associated dendritic cells (DCs), there was a significant increase in the incidence of DCs in tumors from Mertk knockout (KO) mice compared to wild type mice by day 5 after inoculation of tumor cells. Characterization of tumor-associated macrophages by flow cytometry revealed three distinct populations expressing different levels of MHCII (CD11b +MHCII high, CD11b +MHCII +and CD11b highMHCII neg). Tumors from Mertk KO mice had significantly greater numbers of MHCII highmacrophages by day 10 after tumor cell inoculation and reduced numbers of MHCII negmacrophages by day 19. Together, the increased incidence of dendritic cells and the shift toward a more antigen-presenting macrophage phenotype are expected to promote anti-tumor immunity in Mertk KO mice. Indeed, CMT167 tumor growth was significantly reduced in Mertk KO mice and this effect was abrogated in Mertk KO scid mice, which lack functional B and T cells. Treatment with MRX-2843, a MERTK-selective inhibitor currently in Phase 1 clinical trials, also reduced tumor growth in mice. Together, these data demonstrate a critical role for MERTK as a mediator of tumor progression in the tumor immune microenvironment and implicate MRX-2843 as an effective strategy to exploit this finding in the clinic. The research was supported by the National Cancer Institute of the National Institutes of Health (Emory Lung Cancer Spore, Award Number P50CA217691). The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.