Work

902 Reprogrammed regulatory T cells as cytotoxic effectors drive anti-tumor immunity under bispecific T cell engager therapy

Year: 2026

Type: conference-abstract

Abstract: Background Bispecific T-cell engagers (BTCEs) have transformed the treatment of multiple myeloma (MM) and B-cell malignancies by redirecting cytotoxic T-cells toward malignant targets. While clinical responses can be deep and durable, the immunological determinants of BTCE efficacy, and mechanisms of eventual immune escape, remain poorly understood. Regulatory T cells (Tregs) are enriched early in MM and are classically immunosuppressive, but their modulation by BTCEs has not been systematically investigated.Methods We performed longitudinal single-cell RNA/TCR/ATAC-sequencing and immune profiling of 17 newly diagnosed multiple myeloma patients across induction therapy. 11 patients were treated with teclistamab plus daratumumab, lenalidomide, and dexamethasone (Tec-DRD, n = 11) in the ongoing MajesTEC-5 trial ( NCT05695508). As controls, patients receiving standard-of-care DRD (n = 6) were included. Functional validation used newly established NDMM plasma cell-T-cell co-cultures, real-time killing assays, cytokine ELISAs, intracellular flow cytometry and arrayed quantitative PCRs of key T-cell signaling pathways. Mechanistic studies employed CRISPR-mediated perturbations and combination treatments with IMiDs. Generalizability was tested using three BTCEs in vitro:teclistamab (BCMA × CD3), blinatumomab (CD19 × CD3), and glofitamab (CD20 × CD3) to redirect primary human Tregs towards MM, lymphoma, and B-ALL cell lines. Preclinical in vivo studies used an immunocompetent Bcma+ Vk*MYC tumor model treated with murinized BTCE surrogates and IMiDs.Results BTCE therapy induced robust transcriptional and epigenetic reprogramming of bone marrow Tregs into a FOXP3-low IL2RA-low cytotoxic phenotype (‘exTregs’) expressing NKG7, GZMA/B, TBX21, and CCL5. This conversion was absent in DRD-treated controls. Although comprising only ~2% of marrow-infiltrating T-cells, exTregs mediated direct plasma cell lysis in vitro, strictly dependent on BTCE-mediated antigen-specific synapse formation and not recapitulated by CD3 stimulation alone. IMiDs synergized with BTCEs by destabilizing the suppressive Treg phenotype via Helios (IKZF2 ) downregulation, augmenting granzyme and cytokine production. Single-cell ATAC-seq revealed increased accessibility at AP-1 motifs, and functional studies identified BATF/JUNB-driven AP-1 signaling as a central regulator of exTreg conversion. This cytotoxic reprogramming was conserved across BTCEs targeting BCMA, CD19, and CD20, and validated in immunocompetent mouse models, indicating a generalizable, BTCE-driven mechanism of Treg repurposing, rather than target- or engager-specific biology.Conclusions Our findings uncover a novel mechanism by which BTCEs convert suppressive Tregs into cytotoxic, pro-inflammatory effectors that contribute to tumor clearance. We identified the BATF/JUNB transcriptional axis and IMiD-mediated Helios destabilization as targetable regulators of this process. These results redefine Treg function under BTCE pressure, support rational combination regimens with IMiDs, and suggest exTreg signatures as biomarkers and therapeutic targets to enhance T cell-redirecting immunotherapies across hematological malignancies.Trial Registration MajesTEC-5 (HD10/DSMMXX) trial (ClinicalTrials.gov, NCT05695508)Ethics Approval All participants provided written informed consent for sample collection, tissue sequencing, and review of medical records. The study was conducted in accordance with the principles of the Declaration of Helsinki and the Belmont Report. Ethical approval for the collection, functional testing, and sequencing of human samples as well as for clinical data analysis was granted by the Ethics Committee of Heidelberg University Medical Faculty (reference numbers S-096/2017, S-777/2024).Consent Written informed consent was obtained from the patient for publication of this abstract and any accompanying images. A copy of the written consent is available for review by the Editor of this journal.

Source: Journal for ImmunoTherapy of Cancer

DOI: 10.1136/jitc-2026-sitc2026.0902