Abstract 3805: Enhancing anti-tumor immunity via <i>in vivo</i> LNP delivery of a universal chimeric adapter protein to activate innate immune cells through T cell engagers

Abstract T cell engagers (TCEs) have achieved remarkable successes both in liquid and solid tumors. However, relapses tend to occur in a significant fraction of patients, driven by antigen escape or T cell exhaustion. Currently, TCEs are unable to engage innate immune responses and thereby fail to leverage the innate immune system’s crucial support functions, such as inducing a pro-inflammatory environment and promoting antigen spread and T cell licensing against tumor neoantigens. Here we describe a chimeric adaptor protein that renders innate immune cells engageable by existing T cell engagers. This T cell engager receptor (TCE-R) is comprised of an extracellular domain that is recognized by all CD3-targeting TCEs, and a set of intracellular signaling domains that drive innate cell effector function upon TCE binding. Following initial administration of the TCE alone to induce tumor debulking, TCE-R is delivered in vivo using mRNA encapsulated in a lipid nanoparticle (LNP) allowing trafficking of pro-inflammatory innate immune cells to the tumor, inducing a broader TCE-driven anti-tumor response. In a monocyte cell line stably expressing TCE-R, we have demonstrated TCE-mediated phagocytosis of target cancer cells as well as TCE-driven induction of pro-inflammatory signaling pathways. Further, TCE-R delivered as an LNP-encapsulated mRNA to primary human macrophages and dendritic cells in vitro resulted in expression of TCE-R on the cell surface, demonstrating the ability to deliver the transgene to relevant lineages of the innate immune system. Lastly, in vivo delivery of TCE-R LNP to innate immune cells in immune competent Balb/c and immunocompromised NSG mice has been demonstrated through surface expression of the human TCE-R on multiple myeloid cell lineages. Taken together, our data lay the foundation to endow any anti-CD3 TCE with the ability to engage innate immune cells, recruit them to the tumor site, activate them in situ, and to leverage new effector functions to aid, broaden, and expand TCE-driven T cell responses ultimately leading to antigen spread, T cell licensing and durable remission. Citation Format: Kevin Carbajal, Warren Anderson, Matthias Schroff, Maximilian Richter. Enhancing anti-tumor immunity via in vivo LNP delivery of a universal chimeric adapter protein to activate innate immune cells through T cell engagers [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3805.