Functional immune profiling translates T cell dynamics into predictive biomarkers for myeloma immunotherapy.

Bispecific T cell engagers (TCE) targeting BCMA or GPRC5D have improved outcomes in relapsed/refractory multiple myeloma (MM), yet up to 40% of patients are primarily refractory, and predictive biomarkers are scarce. To address this unmet clinical need, we developed an image-based, high-content ex vivo assay using patient-derived bone marrow mononuclear cells exposed to teclistamab or talquetamab. Multiplex immunofluorescence enabled single-cell quantification of plasma cell lysis, T cell expansion, T cell morphology, and spatial T cell-plasma cell engagement. Across all ex vivo-treated patient samples, we identified three functional response phenotypes: non-responder, cytotoxic, and cytotoxic-expansive. Based on this classification, we delineated a morphologic T cell activation trajectory from resting to polarized and effector states. Progression along this trajectory reflected ex vivo plasma cell killing efficacy and distinguished productive from abortive immunological synapses. Importantly, ex vivo response phenotypes stratified clinical response and time on therapy in patients receiving TCE monotherapy, positioning image-based ex vivo profiling as a functional biomarker for immune competence and therapy stratification in MM.