Vaccine Delivery Platforms Affect Heterologous Protection by Hemagglutinin-Based Influenza Vaccines in Pigs

Vaccine-associated enhanced disease (VAED) poses a significant challenge in vaccine development for certain diseases. Vaccine-associated enhanced respiratory disease (VAERD), a form of VAED, has been observed in pigs vaccinated with whole-inactivated vaccine against influenza A virus (IAV) vaccines, followed by an exposure to antigenically mismatched virus strains. Similar outcomes have been reported in pigs vaccinated with a protein-based vaccine containing hemagglutinin (HA), a key viral surface antigen of IAV. We recently developed a lipid nanoparticle encapsulated DNA vaccine encoding the HA gene of IAV, which induced robust immune responses after a single immunization and protected pigs against homologous IAV challenges. This study compared the immunogenicity and efficacy of HA-protein-based and HA-DNA-based vaccines against an antigenically mismatched IAV strain in pigs. While the HA protein-based vaccine exacerbated lung consolidation compared to non-vaccinated controls, the HA-based DNA vaccine prevented gross lung lesion development. Transcriptome analysis revealed distinct gene expression profiles, highlighting differences in host immune responses. These findings provide direct evidence that the same vaccine antigen, when delivered through different platforms, can result in different outcomes: protection versus disease enhancement. This underscores the importance of selecting appropriate delivery platforms for vaccine development. Furthermore, the IAV vaccine in pigs could be a valuable model for studying the immunological mechanisms underlying vaccine-enhanced diseases. Overall design: A total of 18 pigs, 3 weeks old and seronegative for swine influenza A virus, were assigned to three treatment groups. After a 1-week acclimation period, Group 1 received a single intramuscular dose of the pdm09-LNP vaccine, which consisted of 500 µg of a DNA plasmid encoding the HA protein of the H1N1 pdm09 influenza virus, encapsulated in lipid nanoparticles. Group 2 received the pdm09-protein vaccine, administered as two intramuscular doses 21 days apart. This vaccine contained 10 µg of purified recombinant HA protein expressed using a baculovirus expression system and emulsified in an oil-in-water adjuvant. The doses were given at 4 and 7 weeks of age. Group 3 did not receive any treatment and served as a negative control. Peripheral blood mononuclear cells (PBMCs) were collected at 10 weeks of age and sent to Azenta Life Sciences for RNA sequencing. Total RNA was extracted, and poly(A) RNA was purified. RNA sequencing was performed on an Illumina platform with 2 × 150 bp reads, generating approximately 20 million paired-end reads per sample.