Researchers reported a platform that uses intraperitoneal mRNA lipid nanoparticles to program chimeric antigen receptor (CAR) macrophages directly in the body, boosting antitumor activity in peritoneal cancer models. The approach avoids ex vivo cell manufacturing by delivering transient CAR programs to resident macrophages, improving tumor clearance in preclinical studies and offering a potential pathway for rapid, on‑demand cell immunotherapies for peritoneal malignancies.