Protection against genotype VII Newcastle disease virus challenge by a minicircle DNA vaccine coexpressing F protein and chicken IL-18 adjuvant

Genotype VII Newcastle disease virus (NDV) is still one of the most important virus threats severely affecting poultry production worldwide. Although inactivated vaccines are commercially available, there is still an urgent need to develop novel vaccine candidates for convenient and affordable vaccine application. Oral immunization using live attenuated bacteria such as Salmonella has recently attracted increasing interest, and in a previous study, we used a regulated delayed lysis Salmonella vector to deliver a DNA vaccine encoding the F protein and chicken IL-18 adjuvant together, named pYL23. To further improve its efficiency, we employed a novel in vivo minicircle DNA (mcDNA) platform to construct pYL58, which could maintain the complete plasmid during in vitro culture conditions and then transform into mcDNA in vivo whenever the plasmid was delivered by Salmonella into host cells. Compared with immunization with the parental strain harboring plasmid pYL23, immunization with Salmonella with pYL58 induced increased levels of serum IgY and mucosal sIgA in chickens, especially the intestinal and tracheal sIgA levels. Production of cytokines, including IL-4, IFN-γ, IL-18 and IFN-α, was also determined in serum and spleen cell culture supernatants after the 3rd immunization, and the results showed that the production of IFN-γ in the pYL58 group was significantly increased compared with that in the negative control group. Interestingly, compared with pYL23, significantly increased production of IFN-α in the cell supernatants from the pYL58 group was also observed. In addition, the CCK-8 assay results showed that the minicircle pYL58 significantly increased spleen cell proliferation. After virulent VII NDV challenge, pYL58 immunization could provide 70% protection compared with 50% protection in the pYL23 group, together with decreased virus titers in chicken lung samples at Day 5 and virus shedding at Days 3 and 5 post-challenge. This study demonstrated that the application of mcDNA technology dramatically increased the DNA vaccine efficiency, providing additional support for the use of our mcDNA platform in the veterinary field.