A trispecific antibody engaging T cells with tumour and myeloid cells augments antitumour immunity.

Immunologically unresponsive tumours often resist immune checkpoint inhibitors due to the low abundance of tumour-specific T cells and an immunosuppressive microenvironment, despite pronounced infiltration of non-tumour-specific (bystander) T cells. Here we analysed single-cell RNA sequencing data from 300 patients across 17 tumour types, identifying abundant but functionally restrained bystander T cells in multiple malignancies, including ovarian and colorectal cancer. To enhance antitumour immunity in such contexts, we engineered B7H3xCD3xPDL1, a trispecific immunoglobulin-based T cell engager targeting B7H3, CD3 and PDL1, to redirect T cells while mitigating immunosuppression. Functional validation in co-culture systems, patient-derived tumour suspensions and fragments, and humanized mouse models showed T cell activation and tumour killing. Imaging cytometry and single-cell transcriptomics revealed IFNγ-dependent macrophage reprogramming and IL-15 secretion, establishing a feed-forward loop that augments T cell functionality. A machine learning model trained on ex vivo cytotoxicity and transcriptomic data predicted patient responsiveness, supporting data-driven clinical stratification for solid tumour immunotherapy.