Combinational Deletions of MGF360-9L and MGF505-7R Attenuated Highly Virulent African Swine Fever Virus and Conferred Protection against Homologous Challenge

Multigene family (MGF) gene products are increasingly reported to be implicated in African swine fever virus (ASFV) virulence and attenuation of host defenses, among which the MGF360-9L and MGF505-7R gene products are characterized by convergent but distinct mechanisms of immune evasion. Herein, a recombinant ASFV mutant, ASFV-Δ9L/Δ7R, bearing combinational deletions of MGF360-9L and MGF505-7R, was constructed from the highly virulent ASFV strain CN/GS/2018 of genotype II that is currently circulating in China. Pigs inoculated intramuscularly with 104 50% hemadsorption doses (HAD50) of the mutant remained clinically healthy without any serious side effects. Importantly, in a virulence challenge, all four within-pen contact pigs demonstrated clinical signs and pathological findings consistent with ASF. In contrast, vaccinated pigs (5/6) were protected and clinical indicators tended to be normal, accompanied by extensive tissue repairs. Similar to most viral infections, innate immunity and both humoral and cellular immune responses appeared to be vital for protection. Notably, transcriptome sequencing (RNA-seq) and quantitative PCR (qPCR) analysis revealed a regulatory function of the mutant in dramatic and sustained expression of type I/III interferons and inflammatory and innate immune genes in vitro. Furthermore, infection with the mutant elicited an early and robust p30-specific IgG response, which coincided and was strongly correlated with the protective efficacy. Analysis of the cellular response revealed a strong ASFV-specific interferon gamma (IFN-γ) response and immunostaining of CD4+ T cells coupled with a high level of CD163+ macrophage infiltration in spleens of vaccinated pigs. Our study identifies a new mechanism of immunological regulation by ASFV MGFs that rationalizes the design of live attenuated vaccine for implementation of improved control strategies to eradicate ASFV. IMPORTANCE Currently, the deficiency in commercially available vaccines or therapeutic options against African swine fever constitutes a matter of major concern in the swine industry globally. Here, we report the design and construction of a recombinant ASFV mutant harboring combinational deletions of interferon inhibitors MGF360-9L and MGF505-7R based on a genotype II ASFV CN/GS/2018 strain currently circulating in China. The mutant was completely attenuated when inoculated at a high dose of 104 HAD50. In the virulence challenge with homologous virus, sterile immunity was achieved, demonstrating the mutant's potential as a promising vaccine candidate. This sufficiency of effectiveness supports the claim that this live attenuated virus may be a viable vaccine option with which to fight ASF.