Chimeric antigen receptor (CAR) T cells are T cells genetically engineered to express an antigen binding domain, such as a single-chain variable fragment (scFv), fused to intracellular signaling domains, so to induce activation upon binding to the target epitope. Many clinically-used CAR designs are limited by up to 1,000-fold reduced sensitivity for antigen relative to T-cell receptors (TCRs), in part due to poor recruitment of downstream signaling molecules such as LCK to facilitate T-cell signaling. This inefficient signaling combined with observed downregulation of targeted antigens contribute to many patients relapsing even after exhibiting a complete response. Hence, more sensitive CAR designs are needed in the clinic to reduce instances of relapse. In this body of work, I first provide a brief history of CAR design, clinical use, and inefficiencies compared to the TCR (Chapter 1). Afterwards, I describe the iterative design process in which we engineered and evaluated a novel chimeric coreceptor (CoCAR), capable of improving antitumor functionality and increasing the sensitivity of a second-generation anti-CD22 CAR anywhere from five- to seven-fold (Chapter 3). In order to gain a better understanding of the underlying mechanisms behind the CoCAR we then modified or mutated selected features of the system (Chapter 4). Lastly, we successfully display a degree of modularity by re-engineering the CoCAR system to target BCMA (Chapter 5). I conclude with a brief review of strategies undertaken by others to combat CAR-T relapse, putting this study in perspective to the greater field (Chapter 6). Collectively, these findings describe a novel approach to increase the antitumor functionality of CAR-T cells with relatively minor modifications to existing CAR constructs. I hope that the further evaluation and eventual adoption of the CoCAR to clinically-approved constructs translate to more durable responses to CAR-T therapy.