Johns Hopkins researchers reported that targeted biodegradable polymer nanoparticles delivering mRNA encoding an anti‑CD19 CAR can reprogram T cells inside mice to produce functional CAR T cells and deplete B cells. The team published the preclinical results in Science Advances and framed the approach as a way to bypass labor‑intensive ex vivo CAR‑T manufacturing. Authors led by Jordan Green describe polymer NPs engineered to home to T cells, trigger CAR expression and produce systemic B‑cell depletion in animal models. The study underlines delivery and safety hurdles remain, but positions in vivo CAR‑generation as a pathway to cheaper, more scalable cell therapies.