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. Nipah virus (NiV) disease is a highly lethal bat-borne virus with epidemic potential causing inflammation of the brain and a severe respiratory syndrome and is a high priority for vaccine development. We developed a novel single-dose vaccine that protects animals against disease and death caused by NiV and have started clinical trials. The vaccine is a live, recombinant vesicular stomatitis virus (VSV) vector identical to the recently approved Ebola vaccine (Ervebo) but also expressing the NiV G protein responsible for attachment of the virus to cell receptors. Vaccination results in antibodies to the G protein that block entry of the virus into cells. Since addition of the NiV receptor-binding G protein to a live virus could potentially target it to receptors on brain cells, extensive safety tests for neurovirulence were required involving direct inoculation of the vaccine virus into brains of different animal models. We showed that the vaccine candidate was significantly less neurovirulent in non-human primates than an unrelated approved live viral vaccine against yellow fever which has a long record of safe use and a known incidence of rare neurological adverse events. The use of an unrelated vaccine as a comparator is unprecedented in regulatory science and provides a novel approach to safety testing that may be applicable to other vaccines.