Johns Hopkins researchers reported an mRNA-loaded nanoparticle that programs a patient’s T cells to express an anti-CD19 chimeric antigen receptor (CAR) inside the body, producing functional CAR T cells and depleting B cells in mice. The team published preclinical data in Science Advances and described a simplified, biodegradable polymer nanoparticle that carries therapeutic CAR mRNA to T cells, bypassing ex vivo manufacturing. Lead investigator Jordan Green and colleagues engineered targeted polymeric nanoparticles to deliver anti-CD19 CAR mRNA to circulating T cells; treated animals showed durable B cell depletion consistent with CAR T activity. The approach aims to remove costly steps—leukapheresis, ex vivo engineering and expansion—that limit broader access to CAR T therapy. CAR T therapy uses patient T cells engineered to express synthetic receptors that recognize tumor antigens; in vivo CAR generation encodes the receptor inside the body rather than in a manufacturing facility. The Science Advances data are preclinical; translation will require demonstration of safety, targeted delivery, and control over off-target immune activation in larger models and humans.