Retinoic-Acid-Derived Ionizable Lipids Enable Spleen-Selective Antigen Expression for mRNA Cancer Vaccination

Effective mRNA cancer vaccination requires productive antigen expression in lymphoid antigen-presenting cells rather than high total antigen output. Here, we report a retinoic-acid-inspired ionizable lipid platform for spleen-selective functional mRNA expression. Systematic variation of the ionizable scaffold, linker architecture, and retinoid-derived hydrophobic domain generated a modular 40-member library of retinoic-acid-derived ionizable lipids (RAILs). Paired in vitro and in vivo screening identified AT4-13CRA LNPs as the lead formulation, with the highest spleen-to-liver reporter-expression ratio and approximately 15.1-fold higher splenic expression than the tested Dlin-MC3/18PA selective organ targeting (SORT) formulation. Enhanced reporter expression was not explained by total cellular uptake alone and was accompanied by reduced lysosomal mRNA retention and attenuated TNF-α/NF-κB-associated activation. Serum-corona proteomics revealed an immunoglobulin heavy-chain- and J-chain-associated profile distinct from the apolipoprotein-associated profile of 18PA-containing SORT LNPs. In Ai9 reporter mice and an OVA mRNA vaccination model, AT4-13CRA LNPs promoted functional expression in splenic macrophages and dendritic cells, enhanced antigen-specific CD8⁺ T-cell and antibody responses, and improved prophylactic protection against B16F10-OVA tumor challenge. These findings establish retinoid-derived hydrophobic-domain engineering as a chemically defined route to spleen-selective, inflammation-restrained, antigen-presenting-cell-centered mRNA cancer vaccination.