Abstract B45: Phosphatidylserine suppresses T cells through GPR174, and co-inhibition of adenosine receptors and GPR174 synergistically enhances T cell responses
Authors: Gavin Marc A. , Gragerov Alexander , Espling Erik , Rohde Alex , Sexton Tim , Doulami Christiana , Gaitanaris George 
Published: March 2020

Abstract
Extracellular phosphatidylserine (PS) is a potent modulator of immune responses. Various phospholipid scramblases respond to different cellular processes to expose PS either during apoptosis or during activation of multiple cell types, including platelets, lymphocytes, endothelial cells, and tumor cells. While it is well established that PS exposed during apoptosis suppresses inflammatory responses in phagocytic cells during efferocytosis, whether either form of exposed PS acts directly on T lymphocytes has not been extensively studied. Here we show that PS suppresses T cells through GPR174, a Gαs-coupled GPCR previously described as a receptor for lysophosphatidylserine (lysoPS), a soluble catabolite of PS. PS liposomes were found to be 5x more potent than lysoPS in promoting GPR174-dependent cAMP generation, and PS exposed on apoptotic cells, platelets, and activated T cells all induced GPR174 signaling in a reporter cell line. Consistent with the well described immunosuppressive nature of cAMP signaling, PS liposomes suppressed human T cell IL-2 production and mouse IL-2 production from WT but not GPR174-KO T cells. Leveraging a novel GPCR-modulating chemical library screen, we have identified several GPR174 inhibitors covering multiple chemical classes, and a GPR174 inhibitor reversed PS liposome-mediated suppression of human and WT mouse T cells while having no effect on GPR174-KO mouse T cells. In a syngeneic mouse tumor model, GPR174-deficiency significantly increased control of tumor growth in the presence of sub-optimal anti-GITR co-therapy. In many respects, GPR174 is similar to the A2A/A2B adenosine receptors in that both suppress T cells through cAMP signaling in response to products of cell stress and death abundant in the tumor microenvironment, and we have found that both pathways work synergistically to restrain T cell responses. In mouse splenocyte cultures containing endogenous levels of adenosine and PS, an A2A inhibitor enhanced T cell responses only in GPR174-knockout cells but not WT cells. In similar cultures of human PBMC, GPR174 and A2A/A2B inhibitors, or GPR174 inhibitors and adenosine deaminase, synergistically enhanced IL-2 production. Individually, GPR174 inhibitors were typically more effective than adenosine pathway inhibitors at increasing cytokine release. Our findings suggest that inhibition of both GPR174 and the adenosine pathway will be important for effectively overcoming cAMP-mediated immunosuppression in the tumor microenvironment.
Citation Format: Marc A. Gavin, Alexander Gragerov, Erik Espling, Alex Rohde, Tim Sexton, Christiana Doulami, George Gaitanaris. Phosphatidylserine suppresses T cells through GPR174, and co-inhibition of adenosine receptors and GPR174 synergistically enhances T cell responses [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B45.