Class II Peptide-Major Histocompatibility Complex Lipid Nanoparticles Enable in vivo mRNA Delivery to Antigen-Specific CD4+ T Cells.

CD4+ T cells are critical central regulators of adaptive immunity and have emerged as promising candidates for engineered cellular immunotherapies. While current nonviral approaches enable pan-T cell targeting, strategies for selective genetic modulation of antigen-specific CD4+ T cells remain undeveloped. Here, we present a proof-of-concept platform employing lipid nanoparticles (LNPs) decorated with cognate peptide-MHC class II (pMHC-II) ligands (MHC-II LNPs) for antigen-specific mRNA delivery. Ex vivo, MHC-II LNPs achieved highly precise transfection of naïve antigen-specific CD4+ T cells without overt activation or toxicity, demonstrating comparable efficiency as pMHC-I based targeting, despite the lower affinity of pMHC-II:TCR interactions. In vivo administration in TCR-transgenic mice resulted in selective mRNA delivery to cognate CD4+ T cells in the spleen, liver, and blood. Furthermore, the delivery of CD19-directed CAR mRNA produced functional antigen-specific CD4+ CAR-T cells that mediated peripheral B cell depletion. These findings establish that pMHC-II ligands can be exploited for selective, nonviral genetic programming of CD4+ T cells. This strategy provides a foundational approach for precision engineering of antigen-specific CD4+ T subsets with potential applications in cancer, autoimmunity, and infectious disease.