Chimeric antigen receptor (CAR) T cell therapies have transformed treatment of B cell malignancies. However, their broader application is limited by complex manufacturing processes and the necessity for lymphodepleting chemotherapy, restricting patient accessibility. We present an in vivo engineering strategy using targeted lipid nanoparticles (tLNPs) for messenger RNA delivery to specific T cell subsets. These tLNPs reprogrammed CD8+ T cells in both healthy donor and autoimmune patient samples, and in vivo dosing resulted in tumor control in humanized mice and B cell depletion in cynomolgus monkeys. In cynomolgus monkeys, the reconstituted B cells after depletion were predominantly naïve, suggesting an immune system reset. By eliminating the requirements for complex ex vivo manufacturing, this tLNP platform holds the potential to make CAR T cell therapies more accessible and applicable across additional clinical indications.