Differential effect of deleting members of African swine fever virus multigene families 360 and 505 from the genotype II Georgia 2007/1 isolate on virus replication, virulence and induction of protection

African swine fever virus multigene family (MGF) 360 and 505 genes have roles in suppressing the type I interferon response and in virulence in pigs, The role of the individual genes is poorly understood. Different combinations of these genes were deleted from the virulent genotype II Georgia 2007/1 isolate. Deletion of five copies of MGF 360 genes, MGF360-10L, -11L, -12L, -13L, -14L and three copies of MGF505-1R, -2R and -3R reduced virus replication in macrophages and attenuated virus in pigs. However only 25% of the immunised pigs were protected against challenge. Deletion of MGF360-12L, -13L, -14L and MGF505-1R in combination with a negative serology marker, K145R, (GeorgiaΔK145RΔMGF(A)) reduced virus replication in macrophages and virulence in pigs since no clinical signs or virus genome in blood were observed following immunisation. Four out of six pigs were protected after challenge. In contrast, deletion of MGF360-13L, -14L, MGF505-2R, -3R and K145R (GeorgiaΔK145RΔMGF(B)), did not reduce virus replication in macrophages. Following immunisation of pigs, clinical signs were delayed but all pigs reached the humane endpoint. Deletion of genes MGF360-12L, MGF505-1R and K145R reduced replication in macrophages and attenuated virulence in pigs since no clinical signs or virus genome in blood were observed following immunisation. Thus, the deletion of MGF360-12L and MGF505-1R, in combination with K145R, was sufficient to dramatically attenuate virus infection in pigs. However only two of six pigs were protected, suggesting that deletion of additional MGF genes is required to induce a protective immune response. Deletion of MGF360-12L, but not MGF505-1R, from the GeorgiaΔK145R virus reduced virus replication in macrophages indicating that MGF360-12L was most critical for maintaining high levels of virus replication in macrophages.

Importance African swine fever, has a high socio-economic impact and no vaccines to aid control. The virus (ASFV) has many genes that inhibit the host's interferon response. These include related genes that are grouped into multigene families including MGF360 and 505. Here, we investigated which MGF360 and 505 genes were most important for viral attenuation and protection against genotype II strains circulating in Europe and Asia. We compared viruses with deletions of MGF genes. Deletion of just two MGF genes in combination with a third gene, K145R, a possible marker for vaccination, is sufficient for virus attenuation in pigs. Deletion of additional MGF360 genes was required to induce higher levels of protection. Furthermore, we showed that the deletion of MGF360-12L, combined with K145R, impairs virus replication in macrophages in culture. Our results have important implications for understanding the roles of the ASFV MGF genes and for vaccine development.