Bluetongue virus (BTV) and African horse sickness virus (AHSV) are vector-borne viruses belonging to the Orbivirus genus, which are transmitted between hosts primarily by biting midges of the genus Culicoides. With recent BTV and AHSV outbreaks causing epidemics and important economy losses, there is a pressing need for efficacious drugs to treat and control the spread of these infections. The polyanionic aromatic compound aurintricarboxylic acid (ATA) has been shown to have a broad-spectrum antiviral activity. Here, we evaluated ATA as a potential antiviral compound against Orbivirus infections in both mammalian and insect cells. Notably, ATA was able to prevent the replication of BTV and AHSV in both cell types in a time- and concentration-dependent manner. In addition, we evaluated the effect of ATA in vivo using a mouse model of infection. ATA did not protect mice against a lethal challenge with BTV or AHSV, most probably due to the in vivo effect of ATA on immune system regulation. Overall, these results demonstrate that ATA has inhibitory activity against Orbivirus replication in vitro, but further in vivo analysis will be required before considering it as a potential therapy for future clinical evaluation.

The introduction of three single nucleotide mutations into the genome of the virulent RVFV ZH548 strain allows for the rescue of a fully attenuated virus in mice (ZH548-rA2). These mutations are located in the viral genes encoding the RdRp and the non-structural protein NSs. This paper shows the results obtained after the subcutaneous inoculation of ZH548-rA2 in adult sheep and the subsequent challenge with the parental virus (ZH548-rC1). Inoculation with the ZH548-rA2 virus caused no detectable clinical or pathological effect in sheep, whereas inoculation of the parental rC1 virus caused lesions compatible with viral infection characterised by the presence of scattered hepatic necrosis. Viral infection was confirmed via immunohistochemistry, with hepatocytes within the necrotic foci appearing as the main cells immunolabelled against viral antigen. Furthermore, the inoculation of sheep with the rA2 virus prevented the liver damage expected after rC1 virus inoculation, suggesting a protective efficacy in sheep which correlated with the induction of both humoral and cell-mediated immune responses.

Rift Valley fever (RVF) is an important zoonotic viral disease affecting several species of domestic and wild ruminants, causing major economic losses and dozens of human deaths in various geographical areas of Africa, where it is endemic. Although it is not present in Europe, there is a risk of its introduction and spread linked to globalisation and climate change. At present, the only measure that could help to prevent the disease is vaccination of flocks in areas at risk of RVF. Available live attenuated vaccines are an effective means of controlling the disease, but their use is often questioned due to residual virulence, particularly in susceptible hosts such as pregnant sheep. On the other hand, no vaccine is currently licensed for use in humans. The development of safe and effective vaccines is therefore a major area of research. In previous studies, we selected under selective mutagenic pressure a highly attenuated RVFV 56/74 virus variant called 40Fp8. This virus showed an extremely attenuated phenotype in both wild-type and immunodeficient A129 (IFNARKO) mice, yet was still able to induce protective immunity after a single inoculation, thus supporting its use as a safe, live attenuated vaccine. To further investigate its safety, in this work we have analysed the attenuation level of 40Fp8 in immunosuppressed mice (A129) when administered by the intranasal route, and compared it with other attenuated RVF viruses that are the basis of vaccines in use or in development. Our results show that 40Fp8 has a much higher attenuated level than these other viruses and confirm its potential as a candidate for safe RVF vaccine development.

Rift Valley fever virus (RVFV) is a mosquito-borne zoonotic disease that causes severe disease in both domestic and wild ungulates and humans, making it a significant threat to livestock and public health. The RVFV genome consists of three single-stranded, negative-sense RNA segments differing in size: small (S), medium (M), and large (L). Segment S encodes the virus nucleoprotein N and the virulence-associated factor non-structural (NSs) protein in opposite orientations, separated by an intergenic region (IGR). To overcome the current need to use secondary techniques to detect the presence of RVFV in infected cells, we used T7-driven polymerase plasmid-based reverse genetics to generate replication-competent recombinant (r)RVFV expressing Nanoluciferase (Nluc) or Venus fluorescent proteins. These reporter genes were used as valid surrogates to track the presence of RVFV in mammalian and insect cells. Notably, we explored the genome plasticity of RVFV and compared four different strategies by modifying the viral segment S to introduce the reporter gene foreign sequences. The reporter-expressing rRVFV were stable and able to replicate in cultured mammalian and insect cells, although to a lesser extent than the recombinant wild-type (WT) counterpart. Moreover, rRVFV-expressing reporter genes were validated to identify neutralizing antibodies or compounds with antiviral activity. In vivo , all mice infected with the reporter-expressing rRVFV displayed an attenuated phenotype, although at different levels. These rRVFV-expressing reporter genes provide a novel approach to better understand the biology and pathogenesis of RVFV and represent an excellent biotechnological tool for developing new therapeutics against RVFV infections. IMPORTANCE: Rift Valley fever virus (RVFV) is a mosquito-borne virus and zoonotic agent threat that can be deadly to domestic or wild ungulates, and humans. In this work, we used reverse genetics approaches to explore the genome plasticity of RVFV by generating a set of recombinant (r)RVFV that express fluorescent or luminescent proteins to track viral infection. All the generated reporter-expressing rRVFVs were able to propagate in mammalian or insect cells and a mouse model of infection. Our studies may contribute to advances in research on RVFV and other bunyaviruses and pave the way for the development of novel vaccines and the identification of new antivirals for the prophylactic and therapeutic treatment, respectively, of RVFV infections.

Introduction/Background: Rift Valley fever virus (RVFV) and peste des petits ruminants virus (PPRV) are significant pathogens affecting small ruminants, causing substantial economic losses in the affected regions. The development of effective vaccines against both viruses is crucial for disease control. Recombinant viruses expressing heterologous antigens have shown promise as multivalent vaccine candidates. Unlike conventional PPRV vaccines, our recombinant RVFV-vectored vaccines offer a novel dual-protection strategy against RVF and PPR, combining safety, immunogenicity, and a DIVA strategy. Methods: Recombinant RVFVs (ZH548 strain) were generated to express either the hemagglutinin (H) or fusion (F) proteins from the PPRV strain Nigeria 75/1. The stability of these recombinant viruses was assessed through consecutive passages in cell culture. Immunogenicity studies were carried out in both mice and sheep to assess the induction of cellular and humoral immune responses capable of providing protection against RVFV and PPRV. These studies included intracellular cytokine staining (ICS), IFN-γ ELISAs, standard ELISAs for antibody detection, and virus neutralization assays. Results: The recombinant RVFVs expressing PPRV H or F proteins demonstrated stability in cell culture, maintaining high viral titers and consistent transgene expression over four passages. Immunization of mice resulted in the production of serum antibodies capable of neutralizing both RVFV and PPRV in vitro as well as cell-mediated immune responses specific to PPRV and RVFV antigens. In mice vaccinated with a high dose (10⁵ pfu), RVFV neutralizing titers reached ≥1:160 and PPRV neutralizing titers ranged from 1:40 to 1:80 by day 30 post-immunization. In sheep, neutralizing antibody titers against RVFV exceeded 1:160 as early as 2 days post-inoculation, while PPRV-specific neutralization titers reached up to 1:80 by day 21 in responsive individuals. In mice, administration of rZH548ΔNSs:F_PPRV elicited a detectable CD8+ IFNγ+ T-cell response against PPRV, with levels ranging from 1.29% to 1.56% for the low and high doses, respectively. In sheep, rZH548ΔNSs:F_PPRV also induced a robust IFNγ production against PPRV at 14 and 21 days post-infection (dpi). Conclusions: The successful generation and characterization of recombinant RVFVs expressing PPRV antigens demonstrate the potential of using rationally attenuated RVFV as a vector for multivalent vaccine development. Notably, the strategy proved more effective for the recombinant virus expressing the F protein, as it consistently induced more robust cellular and humoral immune responses. These results suggest that this approach could be a viable strategy for simultaneous immunization against Rift Valley fever and other prevalent ruminant diseases, such as peste des petits ruminants. Even though challenge studies were not performed in target species, the strong immune response observed supports including them in future studies.

Introduction/Background : Rift Valley fever virus (RVFV) and peste des petits ruminants virus (PPRV) are significant pathogens affecting small ruminants, causing substantial economic losses in the affected regions. The development of effective vaccines against both viruses is crucial for disease control. Recombinant viruses expressing heterologous antigens have shown promise as multivalent vaccine candidates. Methods : Recombinant RVFVs were generated to express either the Hemagglutinin (H) or Fusion (F) proteins from the PPRV strain Nigeria 75/1. The stability of these recombinant viruses was assessed through consecutive passages in cell culture. Immunogenicity studies were conducted in both mice and sheep to evaluate the induction cellular and humoral immune responses capable of protecting against both RVFV and PPRV. Results : The recombinant RVFVs expressing PPRV H or F proteins demonstrated stability in cell culture, maintaining high viral titers and consistent transgene expression over five passages. Immunization of mice resulted in the production of serum antibodies capable of neutralizing both RVFV and PPRV in vitro. In sheep, the recombinant viruses elicited both neutralizing antibodies and cell-mediated immune responses specific to PPRV and RVFV antigens. Conclusions : The successful generation and characterization of recombinant RVFVs expressing PPRV antigens demonstrate the potential of using rationally attenuated RVFV as a vector for multivalent vaccine development. These results suggest that this approach could be a viable strategy for simultaneous immunization against Rift Valley fever and other prevalent ruminant diseases, such as peste des petits ruminants.