CD47 and cancer: To test the killing activity in vitro, we tested CD47 CAR-T cells by RTCA assay (Real-time cytotoxicity assay) with CD47-positive cells expressing a high level of CD47 antigen: ovarian cancer cells, A1847, and SKOV-3 and with cancer cells expressing a low level of CD47: A549 cells and Hep3B cells (Figure 2E).