Parallel 4-1BB Signaling in Dual DLL3/CD56-Targeting CAR-T Cells Triggers Adaptive Proliferation and Functional Persistence to Eradicate Small Cell Lung Cancer

Limited intratumoral persistence and insufficient proliferative capacity severely restrict the efficacy of chimeric antigen receptor (CAR)-T cell therapies in solid tumors. Here, we demonstrated that DLL3-targeting CAR-T cells co-expressing a CD56 chimeric switch receptor (CSR) and incorporating parallel 4-1BB costimulatory signaling (DBBζ.CBB) effectively address these limitations. In preclinical small cell lung cancer (SCLC) models, DBBζ.CBB exhibited sustained tumor infiltration, prolonged persistence, and superior antitumor activity. Mechanistically, parallel 4-1BB signaling dynamically programed CAR-T cell fate by promoting early expansion and memory maintenance, driving a highly proliferative effector state at the intermediate stage, and delaying terminal exhaustion at the later stage, thereby sustaining in vivo persistence and enabling durable antitumor responses. Building upon the intratumoral T-cell pool established by DBBζ.CBB, subsequent DLL3 trispecific T-cell engager (TriTCE) administration synergistically enhanced tumor eradication by further boosting CD8+ T cell infiltration and overall activation while mitigating exhaustion and terminal differentiation. Collectively, these findings establish a clinically translatable combinatorial framework to enhance the efficacy and durability of CAR-T therapy in solid tumors.