VSV Vectors as Vaccines for Emerging Viruses and as Probes for Entry Pathways

Abstract

Viral fusion proteins (VFPs) are membrane-anchored machines that coat the surfaces of enveloped viruses and induce the fusion of virus and cell membranes. While enveloped viruses are evolutionarily diverse, their fusion proteins are organized into three classes based on structural and functional properties. Our work describes the evolutionary and functional characterization of a class-I VFP, EnvPb1, which is derived from an endogenous retrovirus and conserved in primate genomes. Additionally, we investigated vesicular stomatitis virus (VSV) vectors as a means to develop vaccines targeting the class-II VFP of the newly emergent Zika virus (ZIKV). Since VFPs are surface exposed and mediate viral entry, antibodies that bind and neutralize VFPs are potent inhibitors of infection. Following the Zika virus outbreak in South America, we sought to generate vaccine candidates that would induce a neutralizing antibody response against the ZIKV envelope protein, E. We generated and characterized a panel of recombinant (r)VSV–based vaccines against ZIKV. We show incorporation of functional flavivirus envelope protein into VSV particles. While ZIKV-E incorporation into VSV did not yield independently propagating virus, we discovered that rVSV-ZIKV infected cells readily produce ZIKV virus like particles (VLPs) containing ZIKV-E. We immunized mice with the VLP-producing rVSV-ZIKV or purified VLPs alone. We determined that both the rVSV and the VLPs induce a neutralizing antibody response against ZIKV. Not only are VFPs targeted by host defenses, but also, in rare cases, they have been captured and repurposed to provide a beneficial function to that host. We developed assays and tools to assess the evolutionary conservation and the function of the endogenous retrovirus envelope protein EnvPb1. Our work revealed that EnvPb1 was captured in a common ancestor to all simians and that evolutionarily distant primate EnvPb1 sequences retain the ability to fuse cells. Additionally, we detect EnvPb1 protein in placenta tissue – indicating that it may serve a physiological role in this tissue. Moreover, we show that EnvPb1 is released from cells in culture, suggesting it can be packaged into extracellular vesicles. Our work expands upon what is known about EnvPb1 and provides insights and tools to serve future studies.