Abstract 2546: Targeting MerTK-mediated efferocytosis in the prostate cancer TME

Abstract The prostate cancer tumor microenvironment (TME) is comprised of many different components and cell types that influence tumor progression and patient outcome. Macrophages are highly abundant immune cells in the prostate cancer TME. Macrophage phenotypes can be modeled on a continuous spectrum of M1-like (anti-tumor macrophages) to M2-like (pro-tumor macrophages) and most macrophages in the prostate cancer TME are M2-like. Efferocytosis, the phagocytosis of apoptotic cells, is a pro-tumor function of M2-like macrophages. We have developed a flow cytometry assay to quantify efferocytosis of the prostate cancer cell line LNCaP. With both cell line-based THP-1 macrophages and human monocyte-derived macrophages (HMDMs), we observe that M2 macrophages efferocytose LNCaP cells more than M1 macrophages. Based on literature from other models and contexts, efferocytosis further supports the M2-like phenotype. Efferocytosis also prevents the apoptotic cell from progressing to secondary necrosis, which would attract an M1-like macrophage phenotype. Due these aspects, we hypothesize efferocytosis in the prostate cancer TME is a tumor-promoting function of macrophages. Following efferocytosis of LNCaP cells by M2 HMDMs, we detect a decrease in the M1-like, anti-tumor marker CD80 and an increase in M2-like, pro-tumor markers CD206 and PDL1. Due to this role in modulating macrophage phenotype, we hypothesize targeting efferocytosis will slow prostate cancer growth and promote an anti-tumor immune infiltrate, including M1-like macrophages. MerTK is a receptor tyrosine kinase that mediates efferocytosis by binding phosphatidylserine on apoptotic cells. At both the protein and mRNA level, we detect higher MerTK expression in M2 than M1 THP-1 macrophages and HMDMs. Upon addition of apoptotic LNCaP cells, we observe an increase in phosphorylated MerTK (active form), suggesting the role of MerTK in prostate cancer cell efferocytosis. Currently, we are targeting MerTK to block prostate cancer cell efferocytosis in vitro and in vivo. We have generated a Mertk-/-, Hi-Myc mouse model on the FVB/N background. This prostate cancer GEMM will be used to assess the role of MerTK across different stages of prostate cancer progression. We will be comparing tumor size and immune infiltration between Mertk+/+ and Mertk-/- Hi-Myc mice aged to 2 months, 6 months and 12 months. We predict that the Mertk-/- mice will have smaller tumors and an overall anti-tumor immune infiltrate compared to Mertk+/+ mice in this model due to lack of efferocytosis. Citation Format: Kayla V. Myers, Amber E. de Groot, Anna L. Gonye, Luke V. Loftus, Sarah R. Amend, Kenneth J. Pienta. Targeting MerTK-mediated efferocytosis in the prostate cancer TME [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2546.