A hyper-attenuated variant of Rift Valley fever virus (RVFV) generated by a mutagenic drug (favipiravir) unveils potential virulence markers

Abstract

SUMMARY/ABSTRACTIn a previous work, we showed that favipiravir, a promising drug with antiviral activity against a number of RNA viruses, led to the extinction of RVFV from infected cell cultures. Nevertheless, certain drug concentrations allowed the recovery of a virus variant showing increased resistance to favipiravir. In this work, we characterized this novel resistant variant both at genomic and phenotypic level in vitro and in vivo. Interestingly, the resistant virus displayed reduced growth rates in insect cells and was highly attenuated but still immunogenic in vivo. Some amino acid substitutions were identified in the viral RNA-dependent RNA-polymerase (RdRp) gene and in the encoded IFN antagonist NSs gene, in catalytic core motifs and nuclear localization associated positions respectively. These data may help to characterize novel potential virulence markers, offering additional strategies for further safety improvements of RVF live attenuated vaccine candidates. SIGNIFICANCE STATEMENTLive attenuated virus vaccines usually provide long lasting immune responses upon administration. These vaccines are not recommended for use in immune compromised hosts, due to the presence of uncontrolled residual virulence. Cell culture virus propagation in the presence of mutagenic drugs often results in weakened virus lacking virulence as well as limited spreading capabilities. Here, we have characterized a mutagen-induced RVFV variant (40F-p8) that is not virulent in a highly sensitive mouse strain lacking antiviral response. The observed lack of virulence correlates with the presence of specific mutations along key residues in the viral genome, unveiling potential virulence determinants. Thus, 40F-p8 constitutes the basis for a novel RVFV vaccine strain with additional safety features.