CategorySwine Health-Foreign Animal Disease
Date Full Report Received02/09/2018
Date Abstract Report Received02/09/2018
Funded ByPork Checkoff and CEEZAD
The African Swine Fever Virus (ASFV) causes hemorrhagic fever in pigs, but there is no vaccine or treatment available. The ASFV continues to spread globally and the USA swine industry is at risk. Therefore, it is important that appropriate solutions are developed to safeguard the industry. The overall goal of this project was to test whether immunization of commercial pigs with an experimental vaccine can confer protection against ASFV. The experimental vaccine was previously developed and tested for its safety and ability to induce immune responses in commercial pigs [NPB Project #13-176]. The experimental vaccine is a cocktail of attenuated adenoviruses expressing multiple ASFV antigens. In this study, we immunized commercial pigs with the experimental vaccine and then challenged them with wild type ASFV.
Results from previous studies show that protection against ASFV can in principal be induced with a vaccine since pigs which recover from infection with less virulent isolates are protected from challenge by related virulent isolates. However, due to safety concerns, a subunit vaccine, but not an attenuated ASFV vaccine, is likely to be approved for use in the USA. The main challenges facing development of a subunit vaccine are the identification of protective antigens and development of a suitable method for inducing protective immunity in pigs. Previous studies have shown that immunization of pigs with one or two ASFV vaccine candidate antigens failed to induce immunity strong enough to confer significant protection. Successful development of an effective subunit vaccine will more likely require identification and validation of multiple suitable antigens that will induce significant protection in the majority of vaccinated pigs.
In the previous NPB-funded study [NPB Project #13-176], we showed that an experimental adenovirus-vectored ASFV vaccine formulated in an adjuvant was safe and well tolerated by commercial pigs. Importantly, the vaccine induced strong immune responses that were amplified after boosting. The objective of the current project was to evaluate the ability of the experimental vaccine to induce protection against ASFV.
Materials and Methods
Generation of prototype vaccine: Briefly, the recombinant replication-incompetent adenoviruses [AdA151R, AdB119L, AdB602L, AdEP402RΔPRR, AdB438L, and AdK205R-A104R] developed in the previous study mentioned above were scaled up. Protein expression by these recombinant viruses was validated using ASFV-specific convalescent serum from a pig that survived exposure to ASF virus. In addition, purified recombinant proteins were generated for evaluating immune responses after immunization of pigs.
Pig study: Twenty four weaned piglets (~30lbs) were acquired, allowed to acclimatize and then randomly divided into two groups of 12 piglets each. Each piglet in group 1 (vaccinated group) was immunized with a cocktail containing AdA151R, AdB119L, AdB602L, AdEP402RΔPRR, AdB438L, and AdK205R-A104R adenoviruses, whereas each piglet in group 2 (control group) was inoculated with an equivalent amount of an adenovirus, designated AdLuc, expressing an irrelevant protein (Luciferase). Four weeks post-immunization, the piglets were boosted with the respective formulation used above. Antibody responses were evaluated weekly post-immunization. Four weeks post-boost, the pigs were challenged with wild type ASFV and clinical outcomes post-challenge were evaluated to assess the protective efficacy of the vaccine.
Study Outcomes and significance:
Immunization of commercial piglets with the cocktail of the recombinant adenoviruses expressing the ASFV A151R, B119L, B602L, EP402RΔPRR, B438L, K205R and A104R proteins induced strong antibody responses in all the vaccinated piglets. Notably, all the piglets responded to each antigen in the cocktail. Overall, the experimental vaccine was well tolerated and no serious negative effects were observed. However, when challenged with wild type ASFV, the vaccinated pigs had higher mean body temperatures and clinical scores than the negative controls. The vaccinated pigs also had a sharper decrease in white blood cells after challenge as compared to the negative controls. Taken together, the outcomes from this study suggests that the antibody responses induced in the vaccinated pigs proved to be detrimental and enhanced virus infection rather than providing the expected protection. These findings advocate for the development of alternate vaccine delivery strategies that would focus on inducing stronger T-cell immune responses while limiting antibody responses.
Dr. Waithaka Mwangi
Dept. of Diagnostic Medicine/Pathobiology
College of Veterinary Medicine & Biomedical Sciences
Kansas State University