Deletion Mutants of the Attenuated Recombinant ASF Virus, BA71ΔCD2, Show Decreased Vaccine Efficacy

Abstract

African swine fever (ASF) has become the major threat to the global swine industry. Lack of available commercial vaccines complicates the implementation of global control strategies. So far, only live attenuated ASF viruses (ASFV) have demonstrated solid protection efficacy at the experimental level. The implementation of molecular techniques has allowed the generation of a collection of deletion mutants lacking ASFV-specific virulence factors, some of them with promising potential as vaccine candidates against the pandemic genotype II ASFV strain currently circulating in Africa, Europe, Asia and Oceania. Despite promising results, there is room for improvement, mainly from the biosafety point of view. Aiming to improve the safety of BA71∆CD2, a cross-protective recombinant live attenuated virus (LAV) lacking the ASFV CD2v gene (encoding β-glucuronidase as a reporter gene) available in our laboratory, three new recombinants were generated using BA71∆CD2 as a template: the single mutant BA71∆CD2f, this time containing the fluorescent mCherry reporter gene instead of CD2v, and two double recombinants lacking CD2v and either the lectin gene (EP153R) or the uridine kinase (UK) gene (DP96R). Comparative in vivo experiments using BA71∆CD2f, BA71∆CD2DP96R and BA71∆CD2EP153R recombinant viruses as immunogens, demonstrated that deletion of either DP96R or EP153R from BA71∆CD2f decreases vaccine efficacy and does not improve safety. Our results additionally confirm ASFV challenge as the only available method today to evaluate the protective efficacy of any experimental vaccine. We believe that understanding the fine equilibrium between attenuation and inducing protection in vivo deserves further study and might contribute to more rational vaccine designs in the future.

The Development of a Multiplex Real-Time Quantitative PCR Assay for the Differential Detection of the Wild-Type Strain and the MGF505-2R, EP402R and I177L Gene-Deleted Strain of the African Swine Fever Virus

“…In June 2022, the attenuated vaccine using the ASFV strain with 121 bp deletion in the I177L gene was officially approved for marketing in Vietnam to prevent ASF, which was the first commercial vaccine approved for clinical application in the world (http://link.gov.vn/vtUM759t, accessed on 8 June 2022). In addition, the MGF360-505R and EP402R gene-deleted vaccine, which has been developed in recent years, has become one of the ideal vaccine candidates [23][24][25][26]. These vaccine candidates are the most promising for commercial applications in the field in the future.…”

New Insights in the Interplay Between African Swine Fever Virus and Innate Immunity and Its Impact on Viral Pathogenicity

“…Moreover, these effects can be strain-dependent: for instance, deletion of DP96R lead to attenuation in the virulent strain E70 but not in the virulent Georgia strain that emerged in Europe in 2007 ( Ramirez-Medina et al, 2019 ). In some cases, deletion of viral genes known to modulate the immune response had deleterious effects on the efficacy of live attenuated vaccines candidates ( Reis et al, 2020 ; Lopez et al, 2021 ; Rathakrishnan et al, 2022 ). These studies highlight the complexity of the mechanisms underlying ASFV virulence and suggest that multiple viral proteins act in concert to shape the innate immune response to ASFV.…”

Current efforts towards safe and effective live attenuated vaccines against African swine fever: challenges and prospects

“…I177L , I226R , and A137R ) have been identified, the underlying mechanisms by which these genes affect virulence remain unclear [ 21 , 25 , 26 ]. The combined deletion of VAGs may over-attenuate ASFV and lead to a decrease or loss of protection [ 27 , 28 ]. In the next 3 to 5 years, strengthening the functional research of VAGs and identifying new VAGs will accelerate the rational engineering of ASF LAVs with improved safety profiles.…”