MGF505-7R and I267L Deletions Attenuate African Swine Fever Virus and Confer Protection Against the Virulent Pandemic Virus in Pigs.

African swine fever virus (ASFV) poses a serious risk to wild boars and domestic pigs worldwide. Developing live-attenuated vaccines (LAVs) containing specific gene deletions is an effective approach to prevent and control African swine fever (ASF) epidemics. We created a novel ASFV mutant, ASFV-GS-Δ7R/ΔI267L, with deletions of genes MGF505-7R and I267L, using the highly pathogenic parental strain ASFV-GS/2018 (ASFV-GS). In vitro, the viral titer of ASFV-GS-Δ7R/ΔI267L was approximately 10 times lower than that of its parental strain. Pigs inoculated with 104 50% hemadsorption doses (HAD50) of this mutant did not exhibit the severe fever and sudden death that are characteristic of ASF. The viral load in the blood of vaccinated pigs remained low, and their body temperatures tended to revert to normal at the end of the immunization experiment. After being challenged with 102 HAD50 of virulent ASFV-GS, five of seven pigs in the immunization group remained disease-free, whereas seven pigs in the control group developed severe fever and died within 9 days after the challenge. Histological examination revealed that the protected pigs showed no visible pathological damage and had lower viral loads than the unprotected pigs. Furthermore, transcriptome analysis demonstrated that the host genes IL1B1 and CSF3 were significantly upregulated in ASFV mutant-infected porcine alveolar macrophages, but not in parental virus-infected macrophages. These findings provide important information for the development of ASF LAVs.