This study investigates mechanisms of resistance to CAR T therapy across diverse cancer models and explores innovative strategies to overcome these challenges. A key focus is directed towards the membrane protein CD5, known to be a negative regulator of T cell activation in normal T cells. While T cell dysfunction poses a formidable challenge for CAR T cells against all cancers, fratricide emerges as a specific hurdle in CAR T cell therapies targeting T cell malignancies. This underscores the pivotal role of CD5 as a dual-action regulator: by knocking it out, we can prevent CART-T cell fratricide and enhance T cell activation, as CD5 serves as a negative regulator of T cell activation in normal T cells. Experimental findings across multiple models, encompassing immunodeficient and immunocompetent settings, reveal that CD5 knockout enhances the efficacy of adoptive T cell therapies, including CART5, CART19, CARTmeso, CART-HER2, and TCR-GP100. Mechanistically, we identified notable upregulation of cytotoxicity-related genes in CD5 KO T cells compared to CD5+ T cells in vivo that were collected and subjected to single-cell RNA sequencing. As a result of these promising discoveries, our platform technology has introduced its initial product: a CD5 KO CART5 formulation that employs a unique dual-population approach. This innovative design allows for the integration of a CD5-negative, CAR-negative population, offering the possibility of replenishing normal T cells in patients. These advancements pave the way for clinical translation, with the evaluation of a CD5 KO dual population product in a phase I clinical trial targeting CD5+ nodal T cell lymphomas, promising to revolutionize CAR T therapy outcomes.