Attenuation of pathogenic Rift Valley fever virus strain through the chimeric S-segment encoding sandfly fever phlebovirus NSs or a dominant-negative PKR

Abstract

Rift Valley fever is a mosquito-borne zoonotic disease affecting ruminants and humans. Rift Valley fever virus (RVFV: family Bunyaviridae, genus Phlebovirus) causes abortions and fetal malformations in ruminants, and hemorrhagic fever, encephalitis, or retinitis in humans. The live-attenuated MP-12 vaccine is conditionally licensed for veterinary use in the US. However, this vaccine lacks a marker for the differentiation of vaccinated from infected animals (DIVA). NSs gene is dispensable for RVFV replication, and thus, rMP-12 strains lacking NSs gene is applicable to monitor vaccinated animals. However, the immunogenicity of MP-12 lacking NSs was not as high as parental MP-12. Thus, chimeric MP-12 strains encoding NSs from either Toscana virus (TOSV), sandfly fever Sicilian virus (SFSV) or Punta Toro virus Adames strain (PTA) were characterized previously. Although chimeric MP-12 strains are highly immunogenic, the attenuation through the S-segment remains unknown. Using pathogenic ZH501 strain, we aimed to demonstrate the attenuation of ZH501 strain through chimeric S-segment encoding either the NSs of TOSV, SFSV, PTA, or Punta Toro virus Balliet strain (PTB). In addition, we characterized rZH501 encoding a human dominant-negative PKR (PKRDE7), which also enhances the immunogenicity of MP-12. Study done on mice revealed that attenuation of rZH501 occurred through the S-segment encoding either PKRDE7 or SFSV NSs. However, rZH501 encoding either TOSV, PTA, or PTB NSs in the S-segment uniformly caused lethal encephalitis. Our results indicated that the S-segments encoding PKRDE7 or SFSV NSs are attenuated and thus applicable toward next generation MP-12 vaccine candidates that encode a DIVA marker.

The two faces of Rift Valley fever virus virulence factor NSs: The development of a vaccine and the elucidation of pathogenesis

“…To examine whether NSs genes derived from other sandfly fever phlebovirus contribute to the attenuation of RVFV, Nishiyama et al 13 generated chimeric recombinant RVFV based on the pathogenic RVFV strain ZH501 (rZH501), and they evaluated attenuation by using a mouse model. Several of the chimeras, including rZH501 encoding TOSV NSs (rZH501-TOSNSs), PTA NSs (rZH501-PTANSs), PTV Balliet strain NSs (rZH501-PTBNSs), or SFSV NSs (rZH501-SFSNSs), were inoculated into mice, and the survival rate was observed.…”

“…These results indicate that the exogenous NSs contributes to the attenuation of RVFV, and they also suggest the applicability of SFSV NSs as a DIVA marker in the next generation of MP-12 vaccine. Another important aspect of the approach used by Nishiyama et al 13 is that it demonstrated that phlebovirus NSs proteins function as virulence factors in vivo. It was shown in the same study that 95% of mice infected with rZH501 lacking a functional NSs survived.…”

“…It was shown in the same study that 95% of mice infected with rZH501 lacking a functional NSs survived. 13 On the other hand, all mice infected with rZH501-TOSNSs, rZH501-PTANSs, or rZH501-PTBNSs, and 50% of mice infected with rZH501-SFSNSs succumbed to infection, as described earlier. These results suggest that, although the insertion of TOSV, PTV, or SFSV NSs contributes to the attenuation of RVFV, each NSs itself still contributes to virus virulence.…”

“…Thus, the establishment of an RVF vaccine with a DIVA marker is much needed. The study entitled "Attenuation of pathogenic Rift Valley fever virus strain through the chimeric S-segment encoding sandfly fever phlebovirus NSs or a dominantnegative PKR" published by Nishiyama et al 13 in Virulence focuses on this issue and proposes an interesting approach, which will not only be useful for vaccine development but also for in vivo functional analysis of sandfly fever phlebovirus virulence factors.…”

“…26 A likely explanation for this result is that complete disruption of NSs obviates the virus's ability to counteract the host IFN response and reduces viral replication in vivo. 13 The interesting idea that Ikegami's group next put forward was not to completely disrupt NSs, but "to replace the RVFV NSs with a functional NSs derived from other phleboviruses." 24,25 RVFV belongs to the sandfly fever phlebovirus group, which also includes the Naples serocomplex (e.g., Toscana virus [TOSV]), Punta Toro serocomplex (e.g., Punta Toro virus (PTV) Adames strain [PTA]), Sicilian serocomplex (e.g., Sandfly fever Sicilian virus [SFSV]), Icoaraci serocomplex (e.g., Icoaraci virus), and Frijoles serocomplex (e.g., Frijoles virus).…”