This proof-of-principal study suggests that pretargeting CD138^-/CD20⁺ cells in patients with multiple myeloma (MM) with infusions of bispecific antibody-armed T cells (BATs) before autologous stem cell transplantation (SCT) is safe and does not impair engraftment or immune recovery.

BATs infusions reduced the proportions of clonogenic myeloma precursor cells (CMPCs) in bone marrow and induced cellular and humoral anti-MM immunity.

Both humoral (anti-SOX2 antibody) and cellular (anti-MM IFN-γ ELISPOT) responses could be detected after SCT and boosted by a single BATs infusion.

Anti-SOX2 antibody levels and IFN-γ ELISPOT responses were higher in patients who were in remission compared with those who relapsed.

The presence of anti-SOX2 antibodies may provide an anti-MM effect after SCT.