Abstract 502: In vitro characterization of antibodies targeting the SIRPα-CD47 phagocytosis checkpoint using tumor-associated macrophages from HuGEMM™ mice

Abstract Introduction: The signal-regulatory protein SIRPα-CD47 pathway is a phagocytosis checkpoint for macrophages and other innate immune cells. Tumor cells ubiquitously express CD47, and blocking SIRPα-CD47 interactions has been shown to promote cancer cell elimination by macrophage phagocytosis. The impact of SIRPα-CD47 blockade may be further improved by targeting prophagocytic receptors (Fc receptors) to enhance phagocytosis for anti-CD47 antibodies. Tumor-associated macrophages (TAMs) are the most abundant innate immune cells in late-stage tumors. Currently, drug development targeting CD47-SIRPα is rapidly growing, requiring robust in vitro macrophage/tumor cell co-culture assays to evaluate new agents targeting this pathway. Here, we report on the use of macrophages from an engineered mouse model expressing human SIRPα, which may serve as a good tool to test drug candidates. Methods: We have developed and characterized the human CD47 and SIRPα double knock-in mouse model (CD47/SIRPα HuGEMM) via CRISPR/Cas9 targeting. To characterize the effects of anti-human CD47 antibodies on phagocytosis and tumor cell killing, TAMs were isolated from tumor-bearing HuGEMM mice for in vitro macrophage/tumor cell co-culture. Results: Human CD47 expressing tumor cells, such as murine MC38-hCD47 and human Jurkat cells, were labeled with CFSE and used as target cells for antibody-dependent cellular phagocytosis (ADCP) and tumor cell death. The percentage of CFSE-positive among macrophages, and the percentage of dead cells among CFSE-positive tumor cell death, were measured by flow cytometry. We observed significant ADCP effects by TAM and killing of tumor cells after in vitro anti-CD47 antibody treatment. The contribution to ADCP by SIRPα-CD47 blockade and/or Fc effector function of antibodies was further investigated. We also evaluated the effect of SIRPα-CD47 blockade on the modulation of TAM phenotypes. Conclusion: Our data show that the CD47/SIRPα HuGEMM system provides an excellent tool for in vitro functional assays for drug candidates targeting the SIRPα-CD47 pathway, to evaluate the effect on ADCP by macrophages, activation of macrophages, and overall tumor cell death. Citation Format: Xuefei Yan, Hongjuan Zhang, Yi Liu, Yahong Zhang, Lei Zheng, Mingfa Zang, Annie Xiaoyu An, kevin Qijin Xu, Davy Ouyang, Henry Li, Yujun Huang. In vitro characterization of antibodies targeting the SIRPα-CD47 phagocytosis checkpoint using tumor-associated macrophages from HuGEMM™ mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 502.