Abstract 2335: Targeting a positive feedback loop of MERTK and STAT3 during macrophage differentiation may provide anti-tumor immune function
Authors: Yan Dan , Huelse Justus M. , Bhasin Swati Sharma , Bhasin Manoj , DeRyckere Deborah , Graham Douglas K. 
Published: April 2023

Abstract
MERTK tyrosine kinase expression is upregulated upon monocyte to macrophage differentiation. This receptor tyrosine kinase enables macrophages to efficiently clear apoptotic cells to maintain tissue homeostasis. Activation of MERTK in macrophages during efferocytosis promotes an immunosuppressive phenotype, which is hijacked by tumors to inhibit anti-tumor immunity. Further, this immunosuppressive phenotype in MERTK expressing macrophages in the tumor microenvironment is reversed using MERTK selective inhibitors, suggesting that inhibiting MERTK expression or activity in the tumor microenvironment may be of therapeutic benefit in the treatment of cancer. In an attempt to further understand the mechanism of MERTK upregulation in macrophages, we treated the monocytic leukemia cell line THP1 with PMA to differentiate the cells from a monocytic morphology to an adherent macrophage-like morphology. Following differentiation of the THP-1 cells, MERTK upregulation was confirmed by western blot and flow cytometry. In a similar fashion, treatment with of murine bone marrow derived monocytic cells with PMA induced MERTK expression. Proteomics cytokine array analysis also revealed increased levels of multiple chemokines including MCP-1 and RANTES. Treatment of THP-1 cells with a pan STAT inhibitor in the presence of PMA abrogated the induction of MERTK expression, suggesting a critical role for STAT pathways in the regulation of MERTK expression during monocyte differentiation to macrophages. Specifically, the STAT3 pathway was found to be important in MERTK regulation, as treatment with a STAT3 selective inhibitor was sufficient to abrogate MERTK expression in the presence of PMA treatment. Single cell sequencing of the immune cells in the bone marrow demonstrated Stat3 expression in monocytic lineage cells. Furthermore, both CD11C+Ly6c+CD45+ population and MERTK+CD11C+Ly6c+CD45+ populations were lower in Stat3 -/- bone marrow cells treated with PMA relative to the untreated cells. Collectively, these data suggest that MERTK expression during macrophage maturation may be mediated by STAT3 activation. Previously published data have also demonstrated that STAT3 can be activated downstream of MERTK activation. Thus, we propose that MERTK and STAT3 form a positive feedback loop during macrophage maturation. Treatment with either a MERTK and/or STAT3 inhibitor may interfere with this feedback pathway, potentially reversing an immunosuppressive phenotype in the macrophages in the tumor microenvironment.
Citation Format: Dan Yan, Justus M. Huelse, Swati Sharma Bhasin, Manoj Bhasin, Deborah DeRyckere, Douglas K. Graham. Targeting a positive feedback loop of MERTK and STAT3 during macrophage differentiation may provide anti-tumor immune function [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2335.