108P Phosphatidylserine suppresses T cells through GPR174, and co-inhibition of adenosine receptors and GPR174 synergistically enhances Th1 cytokine production

Abstract Background Extracellular phosphatidylserine (PS) is a potent modulator of immune responses. In addition to exposure during apoptosis, PS is observed on activated platelets, leukocytes, endothelial cells, tumor cells, and exosomes. While PS exposed during apoptosis is known to suppresses inflammatory responses in phagocytic cells, whether either form of exposed PS acts directly on T lymphocytes has not been extensively studied. Methods HEK293 cells expressing GPR174 and GloSensor were used to detect GPR174 agonism. Immune cells were stimulated with anti-CD3/CD28 with GPCR inhibitors, NECA, or PS liposomes; and cytokines in media were measured. WT or GPR174-KO mice inoculated with syngeneic tumor cells and treated with anti-GITR (DTA-1) were evaluated for tumor growth. Results Here we show that PS suppresses T cells through GPR174, a Gαs-coupled GPCR. PS liposomes were more potent than lyso-PS in stimulating GPR174, and PS exposed on various cell types agonized GPR174. Several GPR174 inhibitors of different chemical classes were identified. PS liposomes attenuated Th1 cytokine production from human T cells and WT but not GPR174-KO mouse T cells, and GPR174 inhibitors reversed this suppression. Th1 cytokines were increased by GPR174 inhibition in the presence of tumor exosomes. GPR174 inhibition or genetic deletion also reduced CTLA-4 expression, an immune checkpoint known to be induced by cAMP. Compared to WT mice, GPR174-KO mice significantly controlled tumor growth when Treg were transiently depleted with anti-GITR. GPR174 is similar to A2A/B adenosine receptors in that both suppress Th1 immunity through cAMP in response to products of cell stress and death abundant in the tumor microenvironment. Inhibition of GPR174 and A2A/B synergistically increased cytokine production, GPR174 and A2A/B agonists suppressed T cells to the same extent as both combined, and A2A/B inhibition was more effective on GPR174-KO T cells vs. WT T cells. Conclusion Our findings suggest that for T cells to effectively overcome cAMP-mediated immunosuppression in the tumor microenvironment, both GPR174 and the adenosine pathway must be inhibited. Legal entity responsible for the study Omeros Corporation. Funding Omeros Corporation. Disclosure M.A. Gavin: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation. A. Gragerov: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation. E. Espling: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation. A. Rohde: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation. T. Sexton: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation. C. Doulami: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation. G. Gaitanaris: Shareholder / Stockholder / Stock options, Full / Part-time employment: Omeros Corporation.