mRNA-Based Designer APCs Elicit Robust CD8⁺ and CD4⁺ T Cell Responses

mRNA-based therapeutics have demonstrated notable success in SARS-CoV-2 vaccines and are emerging in cancer immunotherapy. However, conventional mRNA cancer vaccines are limited by the low immunogenicity of tumor-associated and neoantigens. We addressed this limitation by formulating a modular, liposome-based mRNA cocktail comprising three distinct mRNAs encoding tumor antigens, the co-stimulatory molecule CD80, and membrane-tethered IL-2. Administration of this mRNA mixture transforms somatic cells into designer antigen-presenting cells (APCs) in vivo, that simultaneously express the antigen, a co-stimulatory molecule, and a cytokine. These designer APCs activated tumor antigen-specific CD8⁺ T cells more effectively than mRNA encoding the antigen alone and elicited robust anti-tumor immune responses. In addition, substituting IL-2 in the mRNA mixtures with membrane-tethered IL-12 led to the expansion and differentiation of endogenous antigen-specific Th1 helper T cells in vivo. Importantly, this platform activated NY-ESO-1-specific CD8⁺ T cells in both human peripheral blood monocytes (PBMCs) in vitro and in HLA-A*02:01-transgenic mice, highlighting its translational potential. This modular mRNA strategy reprograms somatic cells in situ into designer APCs, thereby providing a flexible and translatable platform for precision immunotherapy.