Recombinant vesicular stomatitis vaccine against Nipah virus has a favorable safety profile: Model for assessment of live vaccines with neurotropic potential

Abstract

Nipah virus (NiV) disease is a bat-borne zoonosis responsible for outbreaks with high lethality and is a priority for vaccine development. With funding from the Coalition of Epidemic Preparedness Innovations (CEPI), we are developing a chimeric vaccine (PHV02) composed of recombinant vesicular stomatitis virus (VSV) expressing the envelope glycoproteins of both Ebola virus (EBOV) and NiV. The EBOV glycoprotein (GP) mediates fusion and viral entry and the NiV attachment glycoprotein (G) is a ligand for cell receptors, and stimulates neutralizing antibody, the putative mediator of protection against NiV. PHV02 is identical in construction to the registered Ebola vaccine (Ervebo) with the addition of the NiV G gene. NiV ephrin B2 and B3 receptors are expressed on neural cells and the wild-type NiV is neurotropic and causes encephalitis in affected patients. It was therefore important to assess whether the NiV G alters tropism of the rVSV vector and serves as a virulence factor. PHV02 was fully attenuated in adult hamsters inoculated by the intramuscular (IM) route, whereas parental wild-type VSV was 100% lethal. Two rodent models (mice, hamsters) were infected by the intracerebral (IC) route with graded doses of PHV02. Comparator active controls in various experiments included rVSV-EBOV (representative of Ebola vaccine) and yellow fever (YF) 17DD commercial vaccine. These studies showed PHV02 to be more neurovirulent than both rVSV-EBOV and YF 17DD in infant animals. PHV02 was lethal for adult hamsters inoculated IC but not for adult mice. In contrast YF 17DD retained virulence for adult mice inoculated IC but was not virulent for adult hamsters. Because of the inconsistency of neurovirulence patterns in the rodent models, a monkey neurovirulence test (MNVT) was performed, using YF 17DD as the active comparator because it has a well-established profile of quantifiable microscopic changes in brain centers and a known reporting rate of neurotropic adverse events in humans. In the MNVT PHV02 was significantly less neurovirulent than the YF 17DD vaccine reference control, indicating that the vaccine will have an acceptable safety profile for humans. The findings are important because they illustrate the complexities of phenotypic assessment of novel viral vectors with tissue tropisms determined by transgenic proteins, and because it is unprecedented to use a heterologous comparator virus (YF vaccine) in a regulatory-enabling study. This approach may have value in future studies of other novel viral vectors.

Fig 1

Schematic of genome organization of wild-type VSV (top), rVSV-EBOV (Ebola) vector (center) showing the substitution of the EBOV GP gene for the glycoprotein (G) gene and principal virulence factor of wild-type VSV, and rVSV-Nipah (bottom) showing the addition of NiV G encoding the NiV attachment glycoprotein downstream of the EBOV GP.

Fig 2. Survival distribution, infant mice and hamsters inoculated by the IC route.

A. 8-day-old ICR mice. Twenty μL of PHV02 at graded doses or YF 1DD vaccine were inoculated into 2 litters (10 suckling pups/litter). The doses injected in 20 μL were 100,000, 1000, or 10 pfu for PHV02 groups and 640 pfu for YF 17DD. Animals were observed for 21 consecutive days and clinical signs and deaths recorded. All mice died in the intermediate and high dose groups, 85% died in the low dose (10 pfu) group and 70% died in the reference group receiving 640 pfu of YF17DD. B. 8-day-old Golden hamsters. Twenty μL of PHV02 at graded doses or YF 17DD were inoculated into 2 litters (8-10 suckling pups/litter). The doses injected in 20 μL were 1000 or 10 pfu for PHV02 groups and 640 pfu for YF 17DD. All infant hamsters died, but the YF 17DD animals had a statistically longer survival time (see text). Since all animals in the test article groups died by Day 7, the study was terminated at that point.

Fig 3. Survival distribution, 6- to 8-week-old ICR mice and Golden hamsters inoculated by the IC route.

6- to 8-week-old ICR mice Twenty μL of PHV02 at graded doses or YF 17DD were inoculated IC into groups of 5 mice. The doses injected in 20 μL were 100,000, 1000, or 10 pfu for PHV02 groups and 640 pfu for YF 17DD. 6- to 8-week-old Golden hamsters. Twenty μL of PHV02 or YF 17DD were inoculated IC into groups of 5 hamsters. The doses injected in 20 μL were 100,000, 1000, or 10 pfu for PHV02 groups and 640 pfu for YF 17DD. Animals were observed for 21 consecutive days and clinical signs and deaths recorded. Hamsters inoculated with 100,000 pfu of PHV02 virus had a 60% mortality ratio for an IC LD50 of 46,800 pfu. In contrast to adult mice which are susceptible to lethal IC infection with YF 17DD Fig 3A), adult hamsters showed no illness or deaths.

Fig 4. Survival distribution of infant and adult Swiss-Webster mice inoculated IC with rVSV-Nipah or rVSV-EBOV.

A. 8-day-old mice (10 animals/group) inoculated IC with 0.1, 1, 10 or 100 pfu of PHV02 (VSV-Nipah virus), rVSV-EBOV, or 0.9% saline. rVSV-EBOV was avirulent at all dose levels. The IC LD50 for PHV02 is 5.3 pfu, and the LD50 of rVSV-EBOV is >100 pfu. B. 6- to 8-week-old Swiss-Webster mice (10 animals/group) inoculated IC with 100,000 pfu of VSV-Nipah virus, rVSV-EBOV, or 0.9% saline.