Vaccination with viral protein-mimicking peptides postpones mortality in domestic pigs infected by African swine fever virus

Periodic outbreaks of African swine fever virus (ASFV) infection around the world threaten local populations of domestic pigs with lethal disease and provide grounds for pandemic spread. Effective vaccination may bring this threat under control. We investigated the effectiveness of select peptides mimicking viral proteins in establishing a protective immune response. Forty-six synthetic peptides based on the analysis of the complete nucleotide sequence of ASFV were tested for immunogenicity in mice. The 17 best immune response-inducing peptide candidates were selected for further investigation. Twenty-four domestic pigs, 3-4 months old and weighing 20-25 kg, were divided into six groups (n=4) and immunized by subcutaneous injection using a standard three-round injection protocol with one of four peptide combinations prepared from the 17 peptides (Groups 1-4) or with carrier only (Group 5). Group 6, the control, was not vaccinated. Animal body temperature and behavior were monitored during and post immunization for health assessment. Two weeks after the last round of immunizations, the pigs were infected with live ASFV (Espania 70) at 6.0 Ig GAE50/cm3, and the survival rate was monitored. Blood samples were collected for analysis the day before infection and on days 3, 7 and 10 post-infection, or from deceased animals. The serum titers of specific immunoglobulins against synthetic peptides and whole inactivated ASFV were determined by enzyme immunoassay before and after infection. The presence of viral DNA in blood serum samples was determined by polymerase chain reaction. Viral infection activity in blood sera was determined by heme absorption in cultured porcine bone marrow and porcine leukocyte cells. Repeating the injection of synthetic peptides in both the mice and pigs produced an immune response specific to individual peptides, which differed widely in the intensity scale. Specific anti-whole virus immunoglobulin binding activity in the swine serum samples from all groups was below the detection limit. Viral DNA was positively identified in all the samples infected with viral preparations. Viral infection activity was present in all the infected animals and steadily increased with time. On day 3 after infection, the viral titer was significantly lower in Groups 1 and 3 than in the unvaccinated controls. In deceased animals, the viral titer was significantly lower in Groups 1 and 3 than in the controls. All infected animals died within 17 days of infection. The average survival rate was significantly higher in Groups 1 and 3 (12.0 and 14.3 days, respectively) than in the controls (9.8 days). Vaccination with specific synthetic peptides significantly delayed mortality in domestic pigs infected with ASFV. These results justify further investigation aimed at developing an effective vaccine against ASFV infection.