Deletion of the L7L-L11L Genes Attenuates ASFV and Induces Protection against Homologous Challenge

African swine fever (ASF), caused by the African swine fever virus (ASFV), is a major epidemic disease endangering the swine industry. Although a number of vaccine candidates have been reported, none are commercially available yet. To explore the effect of unknown genes on the biological characteristics of ASFV and the possibility of a gene-deleted isolate as a vaccine candidate, the strain SY18 Delta L7-11, with deletions of L7L-L11L genes from ASFV SY18, was constructed, and its biological properties were analyzed. The results show that deletion of genes L7L-L11L did not affect replication of the virus in vitro. Virulence of SY18oL7-11 was significantly reduced, as 11 of the 12 pigs survived for 28 days after intramuscular inoculation with a low dose (10(3) TCID50) or a high dose (10(6) TCID50) of SY18 Delta L7-11. All 11 surviving pigs were completely protected against challenge with the parental ASFV SY18 on 28 days postinoculation (dpi). Transient fever and/or irregularly low levels of genomic DNA in the blood were monitored in some pigs after inoculation. No ASF clinical signs or viremia were monitored after challenge. Antibodies to ASFV were induced in all pigs from 14 to 21 days postinoculation. IFN-gamma was detected in most of the inoculated pigs, which is usually inhibited in ASFV-infected pigs. Overall, the results demonstrate that SY18 Delta L7-11 is a candidate for further constructing safer vaccine(s), with better joint deletions of other gene(s) related to virulence.

Automated summary

The deletion of genes L7L-L11L did not affect the replication of African swine fever virus in vitro. A vaccine candidate, SY18 Delta L7-11, showed significantly reduced virulence and provided protection against challenge with parental ASFV SY18 in all survivor pigs.