Expression of SLAMF7 on malignant plasma cells and recognition of primary myeloma cells by SLAMF7-CAR T cells in vitro. (A) Expression of SLAMF7 on CD38⁺CD138⁺ primary myeloma cells obtained from patients with ND or R/R multiple myeloma. Data are presented as differences in MFI (ΔMFI) obtained by staining with anti-SLAMF7 mAb 162.1 and isotype control. Histograms show SLAMF7 expression on primary myeloma cells obtained from the patient with the highest (upper) and the lowest ΔMFI (lower histogram) in the R/R myeloma group, respectively. Staining with anti-SLAMF7 mAb (red) and isotype control (blue). (B) Primary myeloma cells were labeled with eFluor670 fluorescent dye and cocultured with autologous SLAMF7-CAR or CD19-CAR (control) CD8 T cells (10 000 myeloma target cells, E:T ratio = 10:1 to 1:1). After 4 hours of incubation, live (7-AAD^–) CD138⁺eFluor⁺ myeloma cells were quantified by flow cytometry using counting beads and specific lysis calculated using untreated myeloma cells as a comparator. (C) Exemplary dot plots obtained in the flow cytometry–based cytotoxicity assay (E:T ratio = 10:1). (D) Specific lysis of primary myeloma obtained from a patient with ND (left) and a patient with R/R multiple myeloma (right bar diagram) by autologous SLAMF7-CAR or CD19-CAR (control) T cells (E:T ratio = 10:1). Native SLAMF7^– K562 cells and K562/SLAMF7 cells are included as a negative and positive control, respectively. (E) Aggregate data on specific lysis of primary myeloma cells obtained from 3 ND and 4 R/R multiple myeloma patients (E:T ratio = 10:1). The x-axis shows SLAMF7 expression on primary myeloma cells before the assay as ΔMFI, and the y-axis shows the specific lysis obtained with SLAMF7-CAR T cells.