The role of sBCMA in anti-BCMA TCE cytotoxicity. (A) Surface BCMA and CD138 expression on OPM2 and OPM2_BCMA^high cells as measured by flow cytometry. (B) ABC of anti-BCMA PE antibody, estimated using the Quantibrite PE (BD). The data represent 3 technical replicates. ∗∗∗∗P ≤ .0001. (C) The figures represent 3 different anti-BCMA TCEs, namely teclistamab, elranatamab, and alnuctamab, and their respective reported BCMA K_D off rates. Figure created with BioRender.com. (D) TCE binding assay. Histograms depict binding of teclistamab, elranatamab, or alnuctamab (68 nM) on OPM2 or OPM2_BCMA^high cells in the presence of escalating doses of recombinant sBCMA. The numbers represent median fluorescent intensities (MFIs). (E) The TCE cytotoxicity assay dose-response curve. OPM2 or OPM2_BCMA^high target cell viability 48 hours after coculture with healthy donor PBMCs at an effector to target ratio of 10:1 with sBCMA at the indicated concentrations. The dotted black line represents the starting cell viability without the addition of TCE. The data are presented as mean values ± standard deviations. The raw values for triplicate experiments of cell viability measurement and the calculated P values are indicated in supplemental Table 4. ABC, antibody binding per cell.