Identification of secreted proteins as novel antigenic vaccine candidates of Haemophilus parasuis serovar 5

Haemophilus parasuis (H. parasuis) is a swine pathogen responsible for the Glässer's disease, which has received more attention in the past decade due to the increasing economic losses in the pig industry worldwide. As traditional inactive vaccine of H. parasuis has obvious disadvantage, to identify efficient immunoprotective antigens would undoubtedly contribute to the development of novel subunit vaccines. The putative secreted proteins of H. parasuis are potentially essential components of more potent vaccines. In the present study, six secreted proteins (PflA, Gcp, Ndk, HsdS, RnfC and HAPS_0017) were selected from the annotated H. parasuis serovar 5 genome as immunogenic protein with bioinformatic and experimental approaches. These proteins were successfully expressed in Escherichia coli and their immunogenicity was assessed in a mouse challenge model. The results showed that subcutaneous injection with the recombinant proteins resulted in the production of antibodies with high levels. Antigen-specific lymphoproliferative responses were detected in the splenocytes of the immunized animals. CD4+ T-cell populations were higher in the vaccinated animals 3 weeks after the booster immunization than those of the control animals. A significant increase was observed in the cytokine levels of IL-2, IL-4 and IFN-γ in the culture supernatants of splenocytes. Furthermore, immunized mice conferred different levels of protection against challenge with a lethal dose of highly virulent serovar 5 strain (H46). Our results indicate that these six secreted proteins induced a good Th1 response and protection against H. parasuis infection, could be potential subunit vaccine candidates.