CD3-engaging bispecific antibodies convert human regulatory T cells into cytotoxic effectors

Bispecific T cell engagers (BTCEs) have transformed the treatment of B cell malignancies, but how they remodel non-conventional T cell circuits in patients, and how they should be ideally combined with existing treatment backbones, remains unclear. Here we show that CD3-engaging BTCEs convert human FOXP3 regulatory T cells (Tregs) into an AP-1-imprinted hybrid cytotoxic state that contributes to tumor clearance. Leveraging longitudinal bone marrow and peripheral blood sampling from patients with newly diagnosed multiple myeloma (NDMM) treated with BTCE-based induction in the phase 2 (GMMG-HD10/DSMM-XX/MajesTEC-5) trial, which achieves uniformly deep, minimal residual disease (MRD)-negative remissions, we integrate single-cell RNA, ATAC and TCR sequencing with functional and metabolic profiling to define the underlying mechanism. BTCE-containing induction drives expansion of cytotoxic CD8 clones and the emergence of a Treg population characterized by GZMA, GNLY and NKG7 expression, enriched cytotoxic gene signatures, and licensed by BATF-AP-1-centered chromatin remodeling. In vitro, BTCE-exposed Tregs acquire bona fide lytic function, killing tumor cells in a target- and dose-dependent manner and undergoing metabolic rewiring towards an effector-like state while retaining canonical Treg lineage markers. Immunomodulatory drugs (IMiDs) do not initiate but quantitatively amplify and qualitatively reshape this BTCE-driven cytotoxic program, providing a mechanistic rationale for BTCE-IMiD combination regimens. Cytotoxic Treg reprogramming is conserved across BTCEs targeting BCMA, GPRC5D, and CD19 and across multiple myeloma, as well as acute lymphoblastic leukemia. These findings reveal unexpected plasticity of human Tregs under synthetic T cell engagement and establish cytotoxic Tregs as active effectors, candidate biomarkers and tractable levers for optimizing T cell-redirecting immunotherapy.