A Single Dose of Live-Attenuated Rift Valley Fever Virus Vector Expressing Peste Des Petits Ruminants Virus (PPRV) H or F Antigens Induces Immunity in Sheep

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:FPPRV 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:FPPRV 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.