African Swine Fever Virus Georgia Isolate Harboring Deletions of MGF360 and MGF505 Genes Is Attenuated in Swine and Confers Protection against Challenge with Virulent Parental Virus

Abstract

African swine fever virus (ASFV) is the etiological agent of a contagious and often lethal disease of domestic pigs that has significant economic consequences for the swine industry. The control of African swine fever (ASF) has been hampered by the unavailability of vaccines. Experimental vaccines have been developed using genetically modified live attenuated ASFVs where viral genes involved in virus virulence were removed from the genome. Multigene family 360 (MGF360) and MGF505 represent a group of genes sharing partial sequence and structural identities that have been connected with ASFV host range specificity, blocking of the host innate response, and virus virulence. Here we report the construction of a recombinant virus (ASFV-G-⌬MGF) derived from the highly virulent ASFV Georgia 2007 isolate (ASFV-G) by specifically deleting six genes belonging to MGF360 or MGF505: MGF505-1R, MGF360-12L, MGF360-13L, MGF360-14L, MGF505-2R, and MGF505-3R. ASFV-G-⌬MGF replicates as efficiently in primary swine macrophage cell cultures as the parental virus. In vivo, ASFV-G-⌬MGF is completely attenuated in swine, since pigs inoculated intramuscularly (i.m.) with either 10 2 or 10 4 50% hemadsorbing doses (HAD 50 ) remained healthy, without signs of the disease. Importantly, when these animals were subsequently exposed to highly virulent parental ASFV-G, no signs of the disease were observed, although a proportion of these animals harbored the challenge virus. This is the first report demonstrating the role of MGF genes acting as independent determinants of ASFV virulence. Additionally, ASFV-G-⌬MGF is the first experimental vaccine reported to induce protection in pigs challenged with highly virulent and epidemiologically relevant ASFV-G. A frican swine fever (ASF) is a contagious viral disease of swine caused by ASF virus (ASFV), a large enveloped virus containing a double-stranded DNA genome of approximately 180 to 190 kbp (1). ASF causes a spectrum of disease manifestations, from highly lethal to subclinical, depending on host characteristics and the virus strain (2). Virulent ASFV infections in domestic pigs are fatal and characterized by fever, hemorrhages, ataxia, and severe depression.Currently, ASF is endemic in several sub-Saharan African countries. In Europe, the disease is endemic in Sardinia (Italy), and outbreaks have been recorded in the Caucasus region since 2007, affecting Georgia, Armenia, Azerbaijan, and Russia, and, more recently, in Ukraine, Belarus, Lithuania, Latvia, and Poland, threatening to disseminate into neighboring western European countries (3).There is no vaccine available for ASF, and outbreaks are usually controlled by animal quarantine and elimination of the affected animals. Experimental vaccines based on the use of different inactivated virus preparations have failed to induce protective immunity (4-6). Protective immunity against reinfection with homologous viruses and (rarely) against reinfection with heterologous viruses does develop in pigs surviving viral infection (7,8). Pigs im...

Simultaneous Deletion of the 9GL and UK Genes from the African Swine Fever Virus Georgia 2007 Isolate Offers Increased Safety and Protection against Homologous Challenge

“…Until recently, this type of genetic manipulation in field isolates (i.e., isolates not adapted to replication in any cell line) had not been reported. Recently, we developed ASFV-G-Δ9GL/ΔMGF (14), a recombinant derivative of the ASFV Georgia 2007 isolate harboring all the gene deletions individually present in two other attenuated derivatives, ASFV-G-Δ9GL (13) and ASFV-G-ΔMGF (12). ASFV-G-Δ9GL/ΔMGF presented a significantly more attenuated phenotype than ASFV-G-Δ9GL, which showed remaining virulence when used at relatively high doses and the inconvenience of decreased replication during infection in swine, losing all immunogenicity and protective effect (14).…”

“…ASFV-G-Δ9GL/ΔUK is the first attenuated virus that has been shown to induce protection against virulent challenge at early times postimmunization and, along with the rationally designed experimental vaccine viruses ASFV-G-Δ9GL (13) and ASFV-G-ΔMGF (12), is the only strain reported to protect against the highly virulent epidemiologically significant ASFV-G isolate.…”

Association of the Host Immune Response with Protection Using a Live Attenuated African Swine Fever Virus Model

“…We already observed this in animals infected with attenuated Georgia 2007 variants. Presence and level of viremia induced by the challenge virus do not correlate with the severity of the disease [16,23]. …”

“…The deletion of 9GL (ORF B119L) in the highly virulent ASFV isolates Malawi Lil-20/1, Pretoriuskop/96/4 (Pret4), and more recently Georgia 2007 resulted in complete attenuation of these viruses in swine [11,15,16]. Therefore, targeting the highly conserved 9GL (ORF B119L) gene for genetic modifications appeared to be a reasonable approach for developing attenuated viruses for use as vaccine candidates.…”

Unraveling the Armor of a Killer: Evasion of Host Defenses by African Swine Fever Virus

“…Experiments with deletion mutant viruses lacking multiple members of the multigene family 360 (MGF360) and MGF530 show that these genes inhibit the induction of IFN and may also have an impact on the STAT signaling cascade and/or components of the antiviral state (11)(12)(13). Importantly, recombinant viruses with similar deletions of MGF360 and MGF530 that were derived from three different genotypes are attenuated in pigs (11,13,14). This indicates that modulation of the IFN response plays a crucial role in viral pathogenesis.…”