African Swine Fever Virus Bearing an I226R Gene Deletion Elicits Robust Immunity in Pigs to African Swine Fever

Abstract

African swine fever (ASF) is a severe hemorrhagic infectious disease in pigs caused by the African swine fever virus (ASFV), leading to devastating economic losses in the epidemic regions. Its control currently depends on thorough culling and clearance of the diseased and the surrounding suspected pigs. ASF vaccine has been extensively explored for years worldwide, especially in hog-intensive areas where it is highly desired, but it is still unavailable due to numerous reasons. Herein, we reported another ASF vaccine candidate named SY18ΔI226R bearing a deletion of the I226R gene in replacement of an eGFP expression cassette at the right end of the viral genome. This deletion results in complete loss of virulence of SY18 as the gene-deleted strain does not cause any clinical symptoms in all pigs inoculated with either a dosage of 10 4.0 TCID 50 or 10 7.0 TCID 50 . An apparent viremia with the gradual decline was monitored, while the virus shedding was only occasionally detected in oral- or anal swabs. ASFV specific antibody appeared at 9 days post-inoculation. After intramuscular challenge with its parental strain ASFV SY18 on 21 days post inoculation, all the challenged pigs survived without obvious febrile or abnormal clinical signs. No viral DNA could be detected on the dissection of any tissue when viremia disappeared. These indicated that SY18ΔI226R is safe in swine and elicits a robust immunity to the virulent ASFV infection. IMPORTANCE: Outbreaks of African swine fever have resulted in devastating losses to the swine industry worldwide, but there is currently no commercial vaccine available. Although several vaccine candidates have been reported, none has been approved for use due to several reasons, especially the ones concerning bio-safety. Here, we identified a new undescribed functional gene, I226R. When deleted from the ASFV genome, the virus completely loses its virulence in the swine. Importantly, pigs infected with this gene-deleted virus were resistant to infection by an intramuscular challenge of 10 2.5 or 10 4.0 TCID 50 of its virulent parental virus. Furthermore, rarely the nucleic acid of the gene-deleted virus and its virulent parental virus was detected from oral- or anal swabs. Viruses could not be detected in any tissues after necropsy when viremia became negative, indicating that robust immunity was achieved. Therefore, SY18ΔI226R is a novel, ideal and efficacious vaccine candidate for genotype II ASF.

I226R Protein of African Swine Fever Virus Is a Suppressor of Innate Antiviral Responses

“…Thereafter, another report showed that the I226R protein could be expressed in the early and late stages of viral infection [ 32 ]. Notably, recent studies demonstrated that the expression of I226R was observed at 2 hpi and could be detected throughout the remaining infection cycle, although expression of I226R peaked at 8 hpi, implying that I226R is transcribed at the late stage of infection [ 33 ]. These data suggest that I226R could express during the early and late phases of viral infection, which may be associated with the host, virus strain, and challenge dose.…”

“…These data suggest that I226R could express during the early and late phases of viral infection, which may be associated with the host, virus strain, and challenge dose. In addition, it has been reported that I226R knockout attenuated ASFV in pigs [ 33 ]. However, the biological function of the I226R protein remains largely unknown.…”

“…Recently, it was reported that I226R knockout attenuated ASFV in pigs. Moreover, pigs infected with I226R-deleted virus were resistant to infection by its virulent parental virus, indicating that I226R is a promising candidate to design recombinant vaccines for genotype II ASF [ 33 ]. However, the biological function of the I226R protein is largely unknown.…”

Immune Escape Mechanism and Vaccine Research Progress of African Swine Fever Virus

“…ASFV-G-∆I177L could achieve 100% protection through oral and injection routes, and field trials have been completed in Vietnam [ 19 , 20 , 21 ]. Similarly, ASFV-G-ΔA137R and SY18ΔI226R were proven to confer 100% protection [ 22 , 23 ].…”

“…Researchers are actively conducting ASF vaccine-related research, including the evaluation of live attenuated vaccines (LAVs) vaccines by knocking out virulence genes, and the screening of antigens with protective effects to develop subunit vaccines, vector vaccines, etc. [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 ]. ASFV has evolved the ability to manipulate host immune responses by encoding many immune escape genes, including regulation of IFN expression, inhibition of autophagy, and apoptosis.…”