Evaluation of the Deletion of MGF110-5L-6L on Swine Virulence from the Pandemic Strain of African Swine Fever Virus and Use as a DIVA Marker in Vaccine Candidate ASFV-G-ΔI177L.

African swine fever virus (ASFV) is responsible for an ongoing pandemic that is affecting central Europe, Asia, and recently the Dominican Republic, the first report of the disease in the Western Hemisphere in over 40 years. ASFV is a large, complex virus with a double-stranded DNA (dsDNA) genome that carries more than 150 genes, most of which have not been studied. Here, we assessed the role of the MGF110-5L-6L gene during virus replication in cell cultures and experimental infection in swine. A recombinant virus with MGF110-5L-6L deleted (ASFV-G-ΔMGF110-5L-6L) was developed using the highly virulent ASFV Georgia (ASFV-G) isolate as a template. ASFV-G-ΔMGF110-5L-6L replicates in swine macrophage cultures as efficiently as the parental virus ASFV-G, indicating that the MGF110-5L-6L gene is nonessential for virus replication. Similarly, domestic pigs inoculated with ASFV-G-ΔMGF110-5L-6L presented with a clinical disease undistinguishable from that caused by the parental ASFV-G, confirming that the MGF110-5L-6L gene is not involved in producing disease in swine. Sera from animals inoculated with an efficacious vaccine candidate, ASFV-G-ΔMGF, strongly recognized the protein encoded by the MGF110-5L-6L gene as a potential target for the development of an antigenic marker differentiation of infected from vaccinated animals (DIVA) vaccine. To test this hypothesis, the MGF110-5L-6L gene was deleted from the highly efficacious ASFV vaccine candidate ASFV-G-ΔI177L, generating the recombinant ASFV-G-ΔI177L/ΔMGF110-5L-6L. Animals inoculated with ASFV-G-ΔI177L/ΔMGF110-5L-6L developed an ASFV-specific antibody response detected by enzyme-linked immunosorbent assay (ELISA). The sera strongly recognized ASFV p30 expressed in eukaryotic cells but did not recognize ASFV MGF110-5L-6L protein, demonstrating that deletion of the MGF110-5L-6L gene can enable DIVA capabilities in preexisting vaccine candidates. IMPORTANCE Currently, there are no African swine fever (ASF) commercial vaccines that can be used to prevent or control the spread of ASF. The only effective experimental vaccines against ASF are live-attenuated vaccines. However, these experimental vaccines, which rely on a deletion of a specific gene of the current circulating strain of ASF, make it hard to tell the difference between a vaccinated and an infected animal. In our search for a serological marker, we identified that the virus protein encoded by the MGF110-5L-6L gene induced an immune response, making a virus lacking this gene a vaccine candidate that allows the differentiation of infected from vaccinated animals (DIVA). Here, we show that deletion of MGF110-5L-6L does not affect virulence or virus replication. However, when the deletion of MGF110-5L-6L was added to vaccine candidate ASFV-G-ΔI177L, a reduction in the effectiveness of the vaccine occurred.