Vesicular Stomatitis Virus Pseudotyped with Ebola Virus Glycoprotein Serves as a Protective, Noninfectious Vaccine against Ebola Virus Challenge in Mice

Abstract

The recent Ebola virus (EBOV) epidemic in West Africa demonstrates the potential for a significant public health burden caused by filoviral infections. No vaccine or antiviral is currently FDA approved. To expand the vaccine options potentially available, we assessed protection conferred by an EBOV vaccine composed of vesicular stomatitis virus pseudovirions that lack native G glycoprotein (VSVΔG) and bear EBOV glycoprotein (GP). These pseudovirions mediate a single round of infection. Both single-dose and prime/boost vaccination regimens protected mice against lethal challenge with mouse-adapted Ebola virus (ma-EBOV) in a dose-dependent manner. The prime/boost regimen provided significantly better protection than a single dose. As N-linked glycans are thought to shield conserved regions of the EBOV GP receptor-binding domain (RBD), thereby blocking epitopes within the RBD, we also tested whether VSVΔG bearing EBOV GPs that lack GP1 N-linked glycans provided effective immunity against challenge with ma-EBOV or a more distantly related virus, Sudan virus. Using a prime/boost strategy, high doses of GP/VSVΔG partially or fully denuded of N-linked glycans on GP1 protected mice against ma-EBOV challenge, but these mutants were no more effective than wild-type (WT) GP/VSVΔG and did not provide cross protection against Sudan virus. As reported for other EBOV vaccine platforms, the protection conferred correlated with the quantity of EBOV GP-specific Ig produced but not with the production of neutralizing antibodies. Our results show that EBOV GP/VSVΔG pseudovirions serve as a successful vaccination platform in a rodent model of Ebola virus disease and that GP1 N-glycan loss does not influence immunogenicity or vaccination success.IMPORTANCE The West African Ebola virus epidemic was the largest to date, with more than 28,000 people infected. No FDA-approved vaccines are yet available, but in a trial vaccination strategy in West Africa, recombinant, infectious VSV encoding the Ebola virus glycoprotein effectively prevented virus-associated disease. VSVΔG pseudovirion vaccines may prove as efficacious and have better safety, but they have not been tested to date. Thus, we tested the efficacy of VSVΔG pseudovirions bearing Ebola virus glycoprotein as a vaccine platform. We found that wild-type Ebola virus glycoprotein, in the context of this platform, provides robust protection of EBOV-challenged mice. Further, we found that removal of the heavy glycan shield surrounding conserved regions of the glycoprotein does not enhance vaccine efficacy.

Keywords: Ebola virus; filovirus; glycoproteins; glycosylation; pseudovirion; vaccine.

FIG 1

A prime/boost regimen provides better protection against lethal ma-EBOV challenge than a single dose of EBOV GP/VSVΔG. Groups (n = 10) of C57BL/6 mice received the indicated amounts of EBOV GP/VSVΔG SRIPs as either a single dose s.c. or a prime-plus-boost dose i.m., followed by lethal challenge with ma-EBOV. Data are presented as the percentage of survival at the indicated SRIP dose.

FIG 2

EBOV GP NGS mutant models and vaccination efficacy. (A) Left, top-down view of models of EBOV GP trimer (top) and a GP mutant deficient in GP1 N-linked glycans (7Gm8G) (bottom). Right, linear models of GP constructs utilized in this study. Positions of N-linked glycans are marked with “Y,” and the GP1-GP2 disulfide bond is shown. The RBD is shown in red, the glycan cap is shown in teal, and GP2 is shown in tan, N-linked glycans are shown in orange, and the MLD (not included in the crystal structure) is shown in gray and modeled as a sphere in the structure. The two conserved N-linked glycans found on GP2 were not mutated in our glycosylation mutants investigated in these studies and are shown around the base of GP2. (B) Dose-dependent protection conferred by WT or mutant EBOV GP/VSVΔG pseudovirions that are partially (7G) or fully (7Gm8G) deficient in N-linked glycans. Groups (n = 10) of C57BL/6 mice received a prime/boost of VSVΔG SRIPs bearing the indicated GP followed by lethal challenge with ma-EBOV. Data are presented as the percentage of survival at the indicated SRIP dose, normalized for EBOV GP immunogen expression as determined by quantitative Western blotting. AU, arbitrary units of GP.

FIG 3

Humoral responses of mice vaccinated by the prime/boost regimen. Sera were collected from vaccinated mice 3 weeks following the boost. (A) Correlation between the quantity of EBOV GP immunogen administered and Ig production. AU, arbitrary units of GP. (B) Correlation between group survival and anti-EBOV GP ΔMLD Ig in sera. The trend line for pooled results from all three groups is shown. (C) Lack of neutralizing activity present in sera from mice vaccinated with 2 × 10⁷ SRIPs that were normalized for VSV matrix. Serum was serially diluted and incubated with EBOV GP/VSVΔG for 30 min at 37°C prior to addition to Vero cells. Data are shown as infection (GFP-positive cells) as a percentage of the value for a PBS control. (D) Passive transfer of immunity. Groups (n = 10) of C57BL/6 mice received the indicated serum 1 day following lethal challenge with ma-EBOV. Significance was determined by a Cox proportional hazards model (***, P < 0.0001).

FIG 4

VSVΔG SRIPs bearing EBOV GP deficient in N-linked glycans do not provide protection against SUDV challenge. Groups (n = 10) of C57BL/6 IFNAR^−/− mice were primed and boosted with 2 × 10⁷ SRIPs bearing the indicated viral glycoprotein, followed by challenge with WT EBOV (A) or WT SUDV (B). The percentages of weight loss at the indicated times following infection are shown.