Johns Hopkins Medicine researchers reported biodegradable targeted polymeric mRNA nanoparticles that program patients’ T cells in vivo to express a CD19 chimeric antigen receptor (CAR). In mouse studies the nanoparticles produced functional CAR T cells that depleted B cells, demonstrating an alternative to expensive ex vivo CAR T manufacturing. The nanoparticle approach packages mRNA encoding an anti-CD19 CAR with targeting ligands that preferentially deliver to T cells. Investigators highlight potential advantages: reduced cost, elimination of ex vivo cell processing, and simplified supply chains. The work was published in Science Advances and presented mechanistic and safety data from murine models. While translation to humans requires careful control of biodistribution, dosing and immune responses, the study sets a blueprint for in vivo cell engineering that could broaden access to cellular therapies for autoimmune disease and hematologic malignancies.