mRNA-Encoded TLR5 Agonist as an Immune Adjuvant

Activation of innate immunity enhances adaptive immune responses and underlies the mechanism of action of vaccine adjuvants. First-generation mRNA vaccines lack a dedicated adjuvant, relying instead on the self-adjuvanting properties of lipid nanoparticles (LNPs), and are thus expected to benefit from additional engagement of innate immune pathways not activated by LNPs. Among innate immune modulators, TLR5 agonists are particularly promising adjuvants due to their ability to induce a balanced Th1/Th2 immune response and their relatively favorable safety profile. Here we tested whether supplementing mRNA vaccines with mRNA encoding a TLR5 agonist could enhance immunization efficacy by induction of TLR5 signaling coordinated with vaccine antigen expression. We designed FL711, an mRNA encoding a derivative of Salmonella flagellin optimized for mammalian expression, functionally active in TLR5 signaling, deimmunized for pre-existing human T and B cell epitopes, and engineered for secretion to stimulate the TLR5 pathway in the local tissue microenvironment. We characterized FL711 in vitro and in vivo for functional and pharmacological parameters and assessed its adjuvant effect as a component of experimental anti-influenza and anti-SARS-CoV-2 mRNA vaccines. Supplementation with small amounts of FL711 mRNA (up to 30-fold less than antigen-encoding mRNA) significantly enhanced vaccine immunogenicity and protective efficacy, stimulating local NF-{kappa}B induction, boosting antibody production and T cell activation, and prolonging the durability of the response -- while enabling a marked reduction in mRNA dose per vaccine. These findings support the potential of FL711 as a broadly applicable mRNA-encoded adjuvant to improve the potency, durability, and dose efficiency of next-generation mRNA vaccines.