Date Full Report Received09/30/2020
Date Abstract Report Received09/30/2020
InvestigationInstitution: University of Illinois at Urbana Champaign
Primary Investigator: Federico A. Zuckermann
Funded ByPork Checkoff
The deliverable generated by this project is the demonstration that the incorporation of PRRS virus envelope proteins into soluble nanoscale membrane assemblies called nanodiscs (NDs) constitute a safe and effective vaccine against PRRS virus. Notably, because these assemblies only incorporate transmembrane proteins, they would not incorporate the highly immunogenic nucleocapside protein. Consequently, this novel biologic can be used as a DIVA vaccine against PRRS. To generate this deliverable, we compared the level of protective immunity obtained in cohorts of weaner pigs that were vaccinated with a ND-based vaccine displaying PRRS virus envelope proteins (P-ND), administered either intranasally (IN) or intramuscularly (IM). As a negative control, a third cohort received NDs that were void of viral proteins, i.e. empty NDs (E-NDs), which were delivered by both routes; IN and IM. As a positive control, a cohort was vaccinated IM with an in-house inactivated whole virus (IWV) prepared with the same virus strain (G16X) that was used as the source of the viral envelope proteins used to prepare the NDs. These four groups received the same respective biologic twice at a 25-day interval. As requested by the NPB, an additional positive control consisted of a cohort that was immunized once with a commercial modified live virus (MLV) vaccine. A strict control cohort consisted of pigs that were neither vaccinated nor challenged. Forty days after the first immunization, all of the pigs in the trial were challenged IN with virulent virus strain 16244B which, based on the amino acid sequence of GP5, is <97% homologous to G16X. The level of protection stimulated by the vaccines was assessed using objective and subjective parameters indicative of protective immunity including: viremia, gross lung pathology, peripheral blood oxygen saturation (SpO2), and weight gain. As compared to the non-vaccinated and unchallenged pigs in the strict control cohort, animals that received the E-NDs and were challenged with the virulent 16244B virus as a group exhibited: a three-fold decrease in their rate of weight gain; a significant level of lung dysfunction reflected by hypoxemia (oxygen saturation level of <90%); a significant area of the lung with gross lung pathology (45±9); and a sustained level of viremia (>3 log10 TCID50/ml of serum) for 12 days. Pigs in the group that received P-NDs via the intranasal route were not protected from the virus challenge, as they exhibited the same extent of adverse events as those just described for the E-ND group. On the other hand, pigs in the cohort immunized intramuscularly with the P-ND vaccine exhibited a statistically significant (p<0.05) improvement in all of the protective immunity parameters measured, which were not significantly different from the level of protective immunity afforded to pigs immunized with either the IWV or the MLV vaccine, with only one exception. Specifically, all three vaccines that were administered intramuscularly, namely the P-ND, the IWV, and the MLV vaccines, significantly improved the rate of weight gain after the virus challenge, eliminated the presence of hypoxemia, and substantially reduced the extent of gross lung pathology. The one exception consisted in the ability of the MLV to elicit protective immunity capable of terminating the viremia within 10 days after the virus challenge in every pig in this cohort. Nonetheless, the viremia in the cohort of pigs that received the IM P-ND vaccine was extinguished within 12 days after the challenge in 3 out of 8 pigs, with the other 5 pigs exhibiting a clear trend towards an impending viral elimination. Notably, there was no statistical difference in the rate of the extinction of viremia between the pigs immunized IM with the P-ND or with the IWV vaccine. The observations made in this project demonstrate that a vaccine based solely on viral envelope proteins incorporated into NDs are capable of providing a significant level of protective immunity against a virulent virus challenge. The only parameter of protective immunity measured in which the P-ND vaccine did not appear to be as effective as the MLV vaccine was the rate of cessation of viremia. Nonetheless, it was only a matter of days before the cessation of viremia in every pig immunized IM with either the P-ND or the IWV vaccines. Clearly, the notable advantage of P-ND and IWV vaccines would be their absolute safety. Unlike MLV vaccines, they possess no risk of reversion to virulence. Further, the P-ND vaccine can function as a DIVA vaccine. The use of this type of biologic could be used to effectively vaccinate pigs against PRRS virus while avoiding the generation of revertant vaccine viruses that continue to plague the industry as it strives to control and eliminate this pathogen.