Project Summary mRNA holds great therapeutic potential as an agent to induce expression of proteins in cells. Precise delivery of mRNA to diseased cell types could have numerous applications, from correcting pathogenic mutations by gene editing to inducing killing in diseased cell types to cell state reprogramming. Despite this potential, it remains challenging to deliver mRNA to specific subsets of cells. A general method to safely deliver mRNA cargo to target cell types within a patient’s body would unlock the full therapeutic potential of mRNA. Living cells are ideal delivery vehicles because they can be programmed to use sensing and logic to specifically deliver mRNA to target cell populations. Towards this goal, our lab has recently developed delivery cells capable of exporting mRNA in synthetic export vehicles (synEVs). These particles can transfer mRNA cargo to receiver cells, where it is expressed. In this proposal, we aim to create, optimize, and integrate elements to generate an ideal cell-based mRNA delivery platform. In Aim 1, we will engineer primary human T cells, a clinically relevant cell type, as delivery cells. In Aim 2, we will develop strategies to control targeting specificity by engineering conditional sending and pseudotyping. In Aim 3, we will demonstrate cell-cell delivery and functionality of mRNA in vivo by delivering a circuit that induces cell death to tumor cells, which should bypass the immunosuppression that limits traditional immune cell-based therapies. Together, this work will expand our capabilities in cell-cell mRNA delivery and enable a novel, cell-based tumor killing strategy that circumvents immunosuppression. Further, while we focus on applications related to cancer in this proposal, this work will establish a platform with broad utility across biomedicine, including for in vivo cell reprogramming and genome editing.