Safety, Immunogenicity, and Efficacy of a Recombinant Vesicular Stomatitis Virus Vectored Vaccine Against Severe Fever with Thrombocytopenia Syndrome Virus and Heartland Bandavirus

Abstract

Background: Severe fever with thrombocytopenia syndrome virus (SFTSV) is a recently emerged tickborne virus in east Asia with over 18,000 confirmed cases. With a high case fatality ratio, SFTSV has been designated a high priority pathogen by the WHO and the NIAID. Despite this, there are currently no approved therapies or vaccines to treat or prevent SFTS. Vesicular stomatitis virus (VSV) represents an FDA-approved vaccine platform that has been considered for numerous viruses due to its low sero-prevalence in humans, ease in genetic manipulation, and promiscuity in incorporating foreign glycoproteins into its virions.

Methods: In this study, we developed a recombinant VSV (rVSV) expressing the SFTSV glycoproteins Gn/Gc (rVSV-SFTSV) and assessed its safety, immunogenicity, and efficacy in C57BL/6, Ifnar^-/-, and AG129 mice.

Results: We demonstrate that rVSV-SFTSV is safe when given to immunocompromised animals and is not neuropathogenic when injected intracranially into young immunocompetent mice. Immunization of wild type (C57BL/6) and Ifnar^-/- mice with rVSV-SFTSV resulted in high levels of neutralizing antibodies and protection in a lethal SFTSV challenge model. Additionally, passive transfer of sera from immunized Ifnar^-/- mice into naïve animals was protective when given pre- or post-exposure. Finally, we demonstrate that immunization with rVSV-SFTSV cross protects AG129 mice against challenge with the closely related Heartland bandavirus despite negligible neutralizing titers to the virus.

Conclusions: Taken together, these data suggest that rVSV-SFTSV is a promising vaccine candidate for SFTSV and Heartland bandavirus with a favorable safety profile.

Keywords: Dabie bandavirus; bandavirus; heartland bandavirus; heartland virus; immunity; neutralizing antibodies; rVSV; severe fever with thrombocytopenia syndrome virus; tick-borne virus; vaccine; vesicular stomatitis virus.

Figure 1

rVSV-SFTSV expresses SFTSV glycoproteins and is attenuated in vitro. (A) Schematic of mutations in SFTSV Gn/Gc that arose during passage. (B) Expression of SFTSV Gn and Gc by cells infected with rVSV-SFTSV. Gn band intensity: 94.86, Gc band intensity: 137.47. The uncropped, unedited blots are shown in Supplementary Materials. (C) Growth kinetics of rVSV-SFTSV and VSV in Vero E6 cells infected at a multiplicity of infection of 0.01. (Two-way ANOVA with Tukey’s multiple comparisons test; *, p < 0.0458; **, p < 0.0024). Images (D) and surface area (E) of plaques created by VSV and rVSV-SFTSV on Vero E6 cell monolayers 48 h post infection. (Unpaired t-test with unequal variance; ****, p < 0.0001).

Figure 2

rVSV-SFTSV has a favorable safety profile compared to rVSV-EBOV and parental VSV. (A) Weight change, (B) survival proportions, (C) and maximal neurologic disease severity score in C57BL/6 mice challenged intracranially (IC) with 10¹, 10², or 10³ PFU of parental VSV or rVSV-SFTSV into the right cerebral hemisphere (Mantel–Cox test and ordinary one-way ANOVA; *, p < 0.0332; **, p < 0.0021; ***, p < 0.0002; ****, p < 0.0001). (D) Survival proportions and (E) weight loss of Ifnar^−/− mice challenged intraperitoneally with PBS or 10¹, 10², 10³, or 10⁴ PFU of either rVSV-SFTSV or rVSV-EBOV. Weight changes were reported as percentages of body weight measured immediately pre-challenge. (Mantel–Cox test; *, p < 0.0332; **, p < 0.0021).

Figure 3

rVSV-SFTSV induces neutralizing antibodies across different mouse strains. (A) Ifnar^−/− mice were immunized with PBS, 10², 10³, or 10⁴ PFU rVSV-SFTSV. Serum neutralizing antibodies were quantified by measuring 50% decrease in pseudovirus foci, the reciprocal endpoint dilution is shown (Ordinary one-way ANOVA; *, p < 0.0332; **, p < 0.0021; ****, p < 0.0001). (B,C) AG129 mice were vaccinated with varying doses of rVSV-SFTSV and monitored for survival (B) and had serum collected 21 days post vaccination and FRNT₅₀ was assessed (C) (Mantel–Cox test and ordinary one-way ANOVA; *, p < 0.0332; **, p < 0.0021; ***, p < 0.0002). (D) Wild-type C57BL/6 mice were immunized with rVSV-SFTSV and had serum neutralization titers determined at 21 days post treatment (Ordinary one-way ANOVA; *, p < 0.0332; **, p < 0.0021). Horizontal dotted lines indicate the limit of detection (LOD) of the assay.

Figure 4

Vaccination with rVSV-SFTSV protects Ifnar^−/− mice from lethal SFTSV challenge. (A) Survival proportions and (B) percent weight change in Ifnar^−/− mice challenged subcutaneously with 10 PFU SFTSV (blue arrow) 23 days after IP vaccination with PBS, or 10², 10³, or 10⁴ PFU rVSV-SFTSV (red arrow). Weight change is reported as percentage change in body weight relative to starting weight prior to vaccination. One group of mice received favipiravir daily for eight days following SFTSV challenge to serve as a positive control for protection. (Mantel–Cox test; **, p < 0.0021; ****, p < 0.0001). (C) SFTSV titers in serum liver, spleen, and kidney five days post-challenge from mice subjected to the same vaccination schedule as those in (A,B). Horizontal dotted lines indicate the limit of detection of the assay (Ordinary one-way ANOVA; **, p < 0.0021; ***, p < 0.0002; ****, p < 0.0001).

Figure 5

Passive transfer of sera from immunized mice protects naïve mice against SFTSV challenge. Survival (A) and weight loss (B) curves are shown from naïve animals receiving immune sera either 2 days prior to or 2 days post challenge with 10 PFU of SFTSV. Mice immunized with 10³ PFU of the rVSV-SFTSV 7 days prior to challenge served as the positive control. Blue arrow, immunization with rVSV-SFTSV 7 days prior to challenge; Red arrow, passive transfer 2 days prior to challenge; Yellow arrow, SFTSV challenge; Teal arrow, passive transfer 2 days post SFTSV challenge. (Mantel–Cox test; **, p < 0.0021; ***, p < 0.0002; ****, p < 0.0001).

Figure 6

rVSV-SFTSV vaccination cross-protects animals against MA-HRTV challenge. AG129 mice were IP immunized with escalating doses of rVSV-SFTSV then challenged with MA-HRTV 21 days post immunization. (A) Survival and (B) weight loss curves are shown from immunization until completion of experiment. Black arrows indicate vaccination and challenge times at −21 and 0 days respectively (Mantel–Cox test; **, p < 0.0021; ****, p < 0.0001). (C) Four animals in each vaccination group were sacrificed 5 days post challenge to assess serum, liver, and spleen virus titers (Ordinary one-way ANOVA; **, p < 0.0021; ***, p < 0.0002) (D) Sera was collected from subsets of animals 21 days post immunization and prior to HRTV challenge. Sera was analyzed for neutralizing antibodies against HRTV using a pseudotyped virus with the HRTV Gn/Gc glycoprotein. Horizontal dotted lines indicate the limit of detection (LOD) of the assay (Ordinary one-way ANOVA; **, p < 0.0021; ***, p < 0.0002).