Johns Hopkins teams reported biodegradable, targeted polymeric nanoparticles that deliver mRNA encoding anti‑CD19 CARs directly to T cells in vivo, producing functional CAR T cells and depleting B cells in mice. The Science Advances paper led by Jordan Green and colleagues described polymer nanoparticles that traffic to T cells, translate CAR mRNA, and trigger durable B‑cell clearance—bypassing costly ex vivo manufacturing steps. A second Johns Hopkins report described similar biodegradable formulations that reprogram immune cells in vivo to eliminate diseased immune populations and malignancies in preclinical models. Both sets of preclinical data show feasibility for generating therapeutic CAR T cells inside the patient and signal a possible shift from centralized manufacturing to in‑patient, on‑demand cellular engineering.