CD200 is expressed on aPCs in MM at levels that inhibit CAR T-cell activity. (A) Gating strategy used to identify MM aPCs in patient bone marrow biopsies for the purposes of measuring CD200 expression. (B) Expression of CD200 on normal PCs vs aPCs from 3 separate patients with MM in relation to matched fluorescence-minus-1 (FMO) control. (C) Differential expression of BCMA and CD200 in MM cell lines calculated based on published RNA sequencing data.⁴⁰ (D) Comparison of the expression level of CD200 on aPCs from MM specimens to levels achieved on the CD200⁻ MM cell line RPMI-8226 that were either electroporated with different doses of CD200 mRNA, or stably transduced with CD200 lentiviruses. Key (inset) indicates disease stage of patients, whether treated with idecabtagene vicleucel (ide-cel), and 2 matched samples from 1 patient before and after ide-cel treatment. (E) The expression of CD200R was examined on different T-cell subsets, including T_N (naïve T cells), T_CM (central memory T cells), T_EM (effector memory T cells), and T_EMRA (effector memory CD45RA T cells), derived from both healthy donors (n = 2) and patients with MM (n = 2). The figures presented display representative histogram plots from 1 healthy donor and 1 patient with MM. (F) BCMA CAR T or TnMUC1 CAR T cells were cocultured with RPMI-8226 cells that had been electroporated with different doses of CD200 or PD-L1 mRNA. For the CD200 mRNA experiments, the cocultures were established at an E:T ratio of 0.3. Similarly, an E:T ratio of 0.3 was used for the RMPI-8226 CD200 stable cell line experiments. To assess the cytotoxicity, a luciferase assay was conducted. The error bars displayed in the figures represent the standard deviation derived from 3 independent experiments.