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

Abstract

Tyro3, Axl, and Mertk (collectively TAM receptors) are three homologous receptor tyrosine kinases 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 radioresistance to targeted therapeutics, but also have been implicated as inhibitory receptors on infiltrating myeloid-derived cells in the tumor microenvironment that can suppress host antitumor immunity. In the present study, we utilized TAM-IFNγR1 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 wild-type TAM receptors and show that although each receptor can promote PS-mediated efferocytosis, AKT-mediated chemoresistance, as well as upregulate 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 existence 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. Many tumor cells are known to upregulate the immune checkpoint inhibitor PD-L1. This study demonstrates a role for PS and TAM receptors in the regulation of PD-L1 on cancer cells. .

Efferocytosis of dying cells differentially modulate immunological outcomes in tumor microenvironment

“…Together, these studies suggest that antagonistic Mertk antibodies (via their role as PS receptors) may be attractive therapeutic targets in cancer (Figure 4).Moreover, analogous to the above-mentioned strategies to combine PS targeting antibodies anti-PD1 or anti-PDL1 antibodies, combinatorial strategies that combine TAM inhibitors with other checkpoint inhibitors may also be meritorious of further development. In support of this model, we have recently shown that PS (apoptotic cells) promotes TAM-mediated upregulation of PDL1 on several tumor cell lines 129. Further studies showing combined action of generalized TAM inhibitors with anti-PD1 in non-small cell lung cancer, or TAM inhibitors with anti-PD1 head and neck cancers supports this notion 130,131.…”

Regulation of efferocytosis as a novel cancer therapy

“…Hence, the safety of MerTK-targeted therapies should be explored further. Several preclinical studies have reported similar adaptive responses caused by single TKI therapy, and co-targeting of the receptor tyrosine kinase family could, therefore, be a novel strategy for overcoming drug resistance and increasing efficacy [61,[121][122][123][124][125]. Notably, cells of the innate immune system are involved in the initiation and regulation of adaptive immune response.…”

Annexin A5 as an immune checkpoint inhibitor and tumor-homing molecule for cancer treatment

“…2d). It has been previously reported that many tumor cells and immunosuppressive cells within the TME upregulate the expression of receptor tyrosine kinases Tyro3, Axl, and Mertk (collectively known as TAM receptors), which can interact with PS to stimulate the expression of PD-L1 46 . Thus, our results further support the notion that PS may act akin or upstream of PD-L1, CTLA-4, TGF-β, and TIM-3, representing an important immune checkpoint to be targeted for enhancing the efficacy of antitumor immunotherapy.…”