In vivo generation of CAR myeloid cells through erythrocyte-mediated mRNA delivery for cancer immunotherapy

myeloid cells in the spleen, with minimal uptake by hepatocytes. We also demonstrated that mRNA-LNP-Ery is internalized through phagocytosis and avoids lysosomal degradation, resulting in enhanced cytosolic mRNA translation. Delivery of mRNAs encoding CARs targeting human epidermal growth factor receptor 2 (HER2) or CD19 generated functional CAR myeloid cells in vivo that adopted a proinflammatory, antigen-presenting phenotype. These cells migrated to tumors, eliminated cancer cells, and remodeled the tumor microenvironment, leading to increased infiltration of effector T and natural killer (NK) cells. The antitumor effect was abolished in splenectomized mice and partially diminished in nude mice, indicating that therapeutic activity depends on both CAR myeloid cell formation within the spleen and their cross-talk with adaptive immunity. Furthermore, repeated administration of mRNA-LNP-Ery achieved superior antitumor efficacy to conventional mRNA-LNPs at one-tenth the mRNA dose, with minimal systemic toxicity, underscoring the high efficiency and safety of spleen-targeted delivery. Together, our findings established a clinically translatable erythrocyte-based mRNA platform that enables direct in vivo immune cell programming and advances CAR myeloid therapies for solid tumors.