Targeting MerTK decreases efferocytosis and increases anti-tumor immune infiltrate in prostate cancer.

The prostate cancer tumor microenvironment (TME) is comprised of many cell types that can contribute to and influence tumor progression. Some of the most abundant prostate cancer TME cells are macrophages, which can be modeled on a continuous spectrum of M1-like (anti-tumor macrophages) to M2-like (pro-tumor macrophages). A function of M2-like macrophages is efferocytosis, the phagocytosis of apoptotic cells. Based on literature from other models and contexts, efferocytosis further supports the M2-like macrophage phenotype. MerTK is a receptor tyrosine kinase that mediates efferocytosis by binding phosphatidylserine on apoptotic cells. We hypothesize efferocytosis in the prostate cancer TME is a tumor-promoting function of macrophages and that targeting MerTK-mediated efferocytosis will slow prostate cancer growth and promote an anti-tumor immune infiltrate. The aims of this study are to measure efferocytosis of prostate cancer cells by in vitro human M1/M2 macrophage models and assess changes in the M2-like, pro-tumor macrophage phenotype following prostate cancer efferocytosis. Additionally, this study aims to demonstrate that targeting MerTK decreases prostate cancer efferocytosis and promotes an anti-tumor immune infiltrate. We have developed methodology using flow cytometry to quantify efferocytosis of human prostate cancer cells using the LNCaP cell line. We observed that M2 macrophages efferocytose the LNCaP cell line more than M1 macrophages. Following efferocytosis of LNCaP cells by M2 human monocyte-derived macrophages (HMDMs), we observed an increase in the M2-like, pro-tumor phenotype by flow cytometry cell surface marker analysis. By qRT-PCR, flow cytometry, and Western blot, we detected greater MerTK expression in M2 than M1 macrophages. Targeting MerTK with antibody Mer590 decreased LNCaP efferocytosis by M2 HMDMs, establishing the role of MerTK in prostate cancer efferocytosis. In the prostate cancer mouse model hi-myc, Mertk KO increased anti-tumor immune infiltrate including CD8 T cells. These findings support targeting MerTK-mediated efferocytosis as a novel therapy for prostate cancer.