DDO-adjuvanted influenza A virus nucleoprotein mRNA vaccine induces robust humoral and cellular type 1 immune responses and protects mice from challenge

A challenge in viral vaccine development is to produce vaccines that generate both neutralizing antibodies to prevent infection and cytotoxic CD8⁺ T-cells that target conserved viral proteins and can eliminate infected cells to control virus spread. mRNA technology offers an opportunity to design vaccines based on conserved CD8-targeting epitopes, but achieving robust antigen-specific CD8⁺ T-cells remains a challenge. Here, we tested the viral-derived oligonucleotide DDO268 as an adjuvant in the context of a model influenza A virus (IAV) nucleoprotein (NP) mRNA vaccine in C57BL/6 mice. DDO268 when co-packaged with mRNA in lipid nanoparticles is sensed by RIG I-like receptors and safely induces local type I interferon (IFN) production followed by dendritic cells type 1 activation and migration to the draining lymph nodes. This early response triggered by DDO268 improved the generation of IgG2c antibodies and antigen-specific Th1 CD4⁺ and CD8⁺ T-cells (IFNγ⁺TNFα⁺IL2⁺) that provided enhanced protection against lethal IAV challenge. In addition, the inclusion of DDO268 reduced the antigen dose required to achieve protection. These results highlight the potential of DDO268 as an effective mRNA vaccine adjuvant and show that an IAV NP mRNA/DDO268 vaccine is a promising approach for generating protective immunity against conserved internal IAV epitopes.