BCMA CARs recognize patient CD138⁺ myeloma cells in vitro. (A) Staining of BCMA on 8226 and 3 representative primary MM samples. Open histograms depict staining with isotype control. All plots are gated on CD38⁺ (thawed) or CD138⁺ (fresh) CD45⁻propidium iodide (PI)⁻ singlet cells after Ficoll separation and CD138⁺ enrichment of freshly collected BMAs. Summarized data for all evaluable primary MM samples (n = 22) are shown as pie charts. (B) IFN-γ concentrations in supernatants of CD8⁺ BCMA CAR T cells after stimulation with primary CD138⁺ myeloma cells for 24 hours at a ratio of 2:1 as measured by ELISA. BCMA CAR T cells were generated from different allogeneic donors. Untransduced T cells or T cells transduced with an irrelevant CAR from the same donor were included as a control for alloreactivity as well as the CD138-depleted fraction from patient samples as a control for BCMA specificity. Supernatants from control cultures were <275 pg/mL in all assays. Correlation between BCMA and IFN-γ is depicted with P values by linear regression.