Joechner, A. H.; Mach, M.; Yeh, M.; Mehta, R.; Khoury, T.; Kurangil, E.; Yan, J.; Barton, D.; Li, Z.

CAR T cells are highly effective in haematological malignancies, but limited post-infusion control and tumour antigen heterogeneity remain major challenges. Modular CAR systems address these limitations using interchangeable adaptor molecules (AM) to control CAR T cell engagement and redirect targeting toward different antigens, yet the design principles underlying effective modular function remain undefined. We present our modular platform, BRiDGE CAR, and systematically examine how CAR and adaptor configuration influence activity. BRiDGE CAR activity was influenced by CAR hinge length and adaptor format and valency; a medium-length hinge combined with a full-length, bivalent adaptor supported potent activity across multiple tumour antigens and consistently controlled tumour growth across the tested xenograft models. During repeated antigen challenge, CD33-directed BRiDGE CAR T cells retained less-differentiated phenotypes than direct CD33-targeting CAR T cells. In vivo, BRiDGE CAR T cells persisted during adaptor withdrawal and regained antitumour activity upon adaptor re-administration, while IL-18 armouring enhanced tumour control at a reduced CAR T-cell dose. We identified key design parameters governing rules for adaptor-mediated CAR activity, with antigen targeting and CAR T cell function remaining independently modifiable, establishing BRiDGE CAR as a flexible and controllable modular platform.