Targeting CD74 may mitigate immune-escape features and enhance BCMA CAR-T activity in preclinical models of relapsed/refractory multiple myeloma

Background: B-cell maturation antigen (BCMA)-directed chimeric antigen receptor (CAR) T-cell therapy has demonstrated substantial clinical benefit in relapsed/refractory multiple myeloma (R/R MM). Nonetheless, immune escape and therapeutic resistance remain major challenges, and the molecular features of residual disease have not been fully elucidated. Methods: Single-cell RNA sequencing (scRNA-seq) was applied to paired bone marrow samples collected before and after BCMA CAR-T treatment to characterize treatment-persistent myeloma cells. Functional validation involved in vitro co-culture assays, genetic perturbation, and in vivo xenograft modeling. CD74 inhibition was performed using the anti-CD74 monoclonal antibody milatuzumab. Results: Residual myeloma subpopulations were identified with upregulated CD74 and migration inhibitory factor (MIF) expression. Exposure to BCMA CAR-T cells induced CD74 expression and MIF secretion in MM cells. CD74 overexpression enhanced proliferation and apoptosis resistance, while CD74 knockdown impaired cell viability. In an in vivo xenograft model, CD74 blockade synergized with BCMA CAR-T cells to reduce tumor burden, prolong survival, and suppress CD74 expression. The MIF-CD74/CD44 axis was implicated in sustaining residual disease under immune pressure. Conclusions: CD74 contributes to immune escape and CAR-T resistance in R/R MM. Therapeutic targeting of CD74 enhances BCMA CAR-T efficacy and may offer a viable combinatorial strategy to overcome resistance in R/R myeloma. Trial registration number: ChiCTR2000040368.