Abstract 2720: The CD47-blocking innate immune checkpoint inhibitor, TTI-621, triggers CD47-mediated tumor cell apoptosis

CD47 is a cell surface glycoprotein of the immunoglobulin superfamily that is utilized by tumor cells to avoid immune destruction. Overexpression of the CD47 ligand on tumor cells provides a “do-not-eat” signal when bound to its receptor, signal regulatory protein α (SIRPα), on macrophages, resulting in inhibition of phagocytosis and tumor survival. TTI-621 is a soluble SIRPα recombinant fusion protein with an IgG1 Fc tail that triggers macrophage phagocytosis of tumor cells in vitro, and potently inhibits tumor growth in vivo. It is currently being evaluated in two clinical studies in patients with hematologic and solid cancers (NCT02663518 and NCT02890368). TTI-6219s pro-phagocytic effect has been shown to be a result of blockade of the CD47-SIRPα pathway as well as activation of Fc gamma receptors (FcγR) on macrophages. However, engagement of CD47 has also been shown to directly induce apoptosis in tumor cells. Therefore, the objective of this study was to further examine the pro-apoptotic potential of TTI-621. The induction of apoptosis in tumor cells by TTI-621 was evaluated using flow cytometry and protein expression assays. Ligating CD47 with immobilized TTI-621 efficiently induced apoptosis in malignant T-ALL and DLBCL cells but had no effect on apoptosis in normal peripheral blood mononuclear cells. Immobilized TTI-621 or anti-CD47 mAb promoted caspase-independent but PLCγ-1-dependent apoptosis. Under conditions of cellular stress, both soluble and immobilized TTI-621 induced apoptosis in T-ALL cells. In contrast to immobilized TTI-621, treatment of stressed tumor cells with soluble TTI-621 triggered caspase-dependent but PLCγ-1-independent apoptosis. In addition, anchoring the IgG1 Fc tail of TTI-621 to FcγR-expressing cells enhanced tumor cell apoptosis in a dose-dependent and CD47-dependent manner. These results suggest that in addition to blocking the anti-phagocytic “do-not-eat” signal on tumor cells and activating FcγR on macrophages, binding of TTI-621 to FcγRs on tumor-infiltrating immune cells may provide a cross-linking scaffold to enhance TTI-621-mediated apoptosis via CD47 on cancer cells. Additionally, under conditions of cellular stress, TTI-621 may induce tumor cell death directly via binding to CD47 on malignant cells, in a cell-autonomous, cytotoxic mechanism. Thus, TTI-621 may employ multiple mechanisms to elicit its anti-tumor effects. Citation Format: Julia Bershadsky Izrailit, Natasja Nielsen Viller, Xinli Pang, Penka S. Petrova, Robert A. Uger, Jeff Winston, Emma Linderoth. The CD47-blocking innate immune checkpoint inhibitor, TTI-621, triggers CD47-mediated tumor cell apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2720.