Phosphatidylserine Sensing by TAM Receptors Regulates AKT-dependent Chemoresistance and PD-L1 Expression

Tyro3, Axl and Mertk (collectively TAM receptors) are three homologous receptor tyrosine kinases (RTKs) that bind vitamin K-dependent endogenous ligands, Protein S (ProS) and Growth arrest specific factor 6 (Gas6), and act as bridging molecules to promote phosphatidylserine (PS)-mediated clearance of apoptotic cells (efferocytosis). TAM receptors are overexpressed in a vast array of tumor types, whereby the level of expression correlates with the tumor grade and the emergence of chemo- and radio-resistance to targeted therapeutics, but also have been implicated as inhibitory receptors on infiltrating myeloid-derived cells in the tumor microenvironment (TME) that can suppress host anti-tumor immunity. In the present study, we utilized TAM-IFNgammaR1 reporter lines and expressed TAM receptors in a variety of epithelial cell model systems to show that each TAM receptor has a unique pattern of activation by Gas6 or ProS, as well as unique dependency for PS on apoptotic cells and PS liposomes for activity. In addition, we leveraged this system to engineer epithelial cells that express WT TAM receptors, and show that while each receptor can promote PS-mediated efferocytosis, AKT-mediated chemo-resistance, as well as up-regulate the immune checkpoint molecule PD-L1 on tumor cells, Mertk is most dominant in the aforementioned pathways. Functionally, TAM receptor-mediated efferocytosis could be partially blocked by PS-targeting antibody 11.31 and Annexin V, demonstrating the existing of a PS/PS-Receptor (i.e. TAM-receptor) /PD-L1 axis that operates in epithelial cells to foster immune escape. These data provide a rationale that PS-targeting, anti-TAM receptor, and anti-PD-L1 based therapeutics will have merit as combinatorial checkpoint inhibitors. Implications: Many tumor cells are known to up-regulate the immune checkpoint inhibitor PD-L1. This study demonstrates a role for PS and TAM receptors in the regulation of PD-L1 on breast cancers cells.