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

ABSTRACT 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.