Engineering Soluble Recombinant BCMA for Ide‐Cel Labeling

Chimeric antigen receptor (CAR)-T-based strategies target the B-cell maturation antigen (BCMA) expressed on the surface of malignant plasma cells in patients with multiple myeloma (MM). Idecabtagene vicleucel (ide-cel), an anti-BCMA CAR-T therapy, has shown promising results in the treatment of relapsed or refractory multiple myeloma (R/RMM). Notably, the extent of CAR-T cell expansion correlates with objective response rates and complete responses, indicating that monitoring CAR-T cells is critical for predicting clinical outcomes in these patients. Although several tools are available to monitor CAR-T cells in patient blood samples, they remain costly, prompting us to develop novel fluorescent and soluble BCMA reagents for CAR-T cell labeling. BCMA is a type III transmembrane protein, and we found that a recombinant construct consisting of the BCMA extracellular domain fused to green fluorescent protein (GFP) failed to traffic through the Golgi apparatus and was not secreted. Incorporation of the interferon-α2 signal peptide restored intracellular trafficking and enabled efficient secretion of soluble BCMA-GFP and BCMA-mCherry fusion proteins. These reagents specifically labeled anti-BCMA CAR-T cells. Overall, our results establish a cost-effective method to generate soluble BCMA probes and provide accessible tools for monitoring CAR-T cells in translational settings.