Adoptive immunotherapy using T lymphocytes genetically modified to express a chimeric antigen receptor (CAR-T) holds considerable promise for the treatment of cancer. However, CAR-based therapies may involve on-target toxicity against normal tissues expressing low amounts of the targeted tumor-associated antigen (TAA). To specify T cells for robust effector function that is selective for tumor but not normal tissue, we developed a trans-signaling CAR strategy, whereby T-cell activation signal 1 (CD3ζ) is physically dissociated from costimulatory signal 2 (CD28) in two CARs of differing antigen specificity: mesothelin and a-folate receptor (FRa). Human T cells were genetically modified to coexpress signal 1 (anti-Meso scFv-CD3ζ) and signal 2 (anti-FRa scFv-CD28) CARs in trans. Trans-signaling CAR-T cells showed weak cytokine secretion against target cells expressing only one TAA in vitro, similar to first-generation CAR-T cells bearing CD3ζ only, but showed enhanced cytokine secretion upon encountering natural or engineered tumor cells coexpressing both antigens, equivalent to that of second-generation CAR-T cells with dual signaling in cis. CAR-T cells with dual specificity also showed potent anticancer activity and persistence in vivo, which was superior to first-generation CAR-T cells and equivalent to second-generation CARs. Importantly, second-generation CAR-T cells exhibited potent activity against cells expressing mesothelin alone, recapitulating normal tissue, whereas trans-signaling CAR-T cells did not. Thus, a dual specificity, trans-signaling CAR approach can potentiate the therapeutic efficacy of CAR-T cells against cancer while minimizing parallel reactivity against normal tissues bearing single antigen. Cancer Immunol Res; 1(1); 43–53. ©2013 AACR.