Anti-PD-L1 antibody limits the survival of activated CAR T cells by disrupting the PD-L1/CD80 cis-heterodimer

Chimeric antigen receptor (CAR) T cells have revolutionized the treatment of hematologic malignancies but remain less efficacious in solid tumors. While PD-L1 blockade is widely used to counter tumor immune evasion, its direct effects on activated T cells, which also express PD-L1 and may engage in PD-L1/CD80 cis -interactions, remain largely unexplored. To investigate this, we blocked PD-L1 by engineering CAR T cells that secrete PD-L1-blocking single-chain variable fragment (scFv) antibodies derived from atezolizumab. Secretion of the scFv conferred no benefit: 4-1BB-based CAR T cells accumulated less upon repeated antigen stimulation, and CD28-based CAR T cells killed less efficiently when effector cells were limiting. Mechanistically, PD-L1 and CD80 form a cis -heterodimer on T cells that is disrupted by PD-L1 blockade, rendering CD80 susceptible to CTLA-4-mediated depletion and impairing CD28 co-stimulation. Consistent with this model, exogenous atezolizumab reduced CD80 expression and T cell viability, while CTLA-4 blockade with ipilimumab restored CD80 levels and rescued survival. These findings were corroborated by structural modeling demonstrating that atezolizumab disrupts the PD-L1/CD80 complex, and were reproduced in unmodified T cells and across both CD28- and 4-1BB-based CARs. Together, our findings uncover a T cell-intrinsic vulnerability to PD-L1 inhibitors that warrants consideration when PD-L1 blockade is combined with adoptive T cell therapy.