Effects of glycine 64 substitutions in RNA-dependent RNA polymerase on ribavirin sensitivity and pathogenicity of coxsackievirus A6

Hand, foot, and mouth disease (HFMD) caused by a group of enteroviruses is a global public health problem. In recent years, coxsackievirus A6 (CVA6) has emerged as an important HFMD agent. Previous studies have shown that mutations of glycine 64 in RNA-dependent RNA polymerase (3D polymerase), which is central to viral replication, cause phenotypic changes such as ribavirin resistance, increased replication fidelity, and virulence attenuation in poliovirus and enterovirus A71. In this study, we constructed CVA6 mutants with G64R, G64S, and G64T substitutions by site-directed mutagenesis in full-length cDNA of an infectious CVA6 strain cloned in pcDNA3.1. Viral RNA was obtained by in vitro transcription, and the rescued virus strains were propagated in RD cells. Sequencing after six passages revealed that G64S and G64T mutations were stably inherited, whereas G64R was genetically unstable and reversed to the wild type. Comparison of the biological characteristics of the wild-type and mutant CVA6 strains in an in vivo model (one-day-old ICR mice) revealed that the pathogenicity of CVA6-G64S and CVA6-G64T was significantly reduced compared to wild-type CVA6. In vitro experiments indicated the mutant CVA6-G64S and CVA6-G64T strains had increased resistance to 0.8 mM ribavirin and a decreased replication rate in the presence of 0.8 mM guanidine hydrochloride. Our results show that mutation of residue 64 reduces CVA6 susceptibility to ribavirin and increases CVA6 susceptibility to guanidine hydrochloride, together with increased replication fidelity and attenuated viral pathogenicity, thus laying a foundation for the development of safe and effective live attenuated CVA6 vaccine.

African Swine Fever Virus MGF-110-9L-deficient Mutant Has Attenuated Virulence in Pigs

African swine fever virus (ASFV) is the etiological agent of African swine fever (ASF), an often lethal disease in domestic and wild pigs. ASF represents a major threat to the swine industry worldwide. Currently, no commercial vaccine is available because of the complexity of ASFV or biosecurity concerns. Live attenuated viruses that are naturally isolated or genetically manipulated have demonstrated reliable protection against homologous ASFV strain challenge. In the present study, a mutant ASFV strain with the deletion of ASFV MGF-110-9L (ASFV-Δ9L) was generated from a highly virulent ASFV CN/GS/2018 parental strain, a genotype II ASFV. Relative to the parental ASFV isolate, deletion of the MGF-110-9L gene significantly decreased the ability of ASFV-Δ9L to replicate in vitro in primary swine macrophage cell cultures. The majority of animals inoculated intramuscularly with a low dose of ASFV-Δ9L (10 HAD₅₀) remained clinically normal during the 21-day observational period. Three of five ASFV-Δ9L-infected animals displayed low viremia titers and low virus shedding and developed a strong virus-specific antibody response, indicating partial attenuation of the ASFV-Δ9L strain in pigs. The findings imply the potential usefulness of the ASFV-Δ9L strain for further development of ASF control measures.