Vesicular stomatitis virus-based vaccines protect nonhuman primates against aerosol challenge with Ebola and Marburg viruses
Introduction
Members of the family Filoviridae, Ebola virus (EBOV) and Marburg virus (MARV), cause severe hemorrhagic fever (HF) in humans and nonhuman primates. The Marburgvirus genus contains a single species: Lake Victoria marburgvirus (LVMARV). The Ebolavirus genus is subdivided into four distinct species: Ivory Coast ebolavirus (ICEBOV) (also known as Cote d’Ivoire ebolavirus, CIEBOV), Reston ebolavirus (REBOV), Sudan ebolavirus (SEBOV), and Zaire ebolavirus (ZEBOV) [1], [2]. MARV, ZEBOV, and SEBOV are important human pathogens with case fatality rates frequently ranging between 70% and nearly 90% for ZEBOV, around 50% for SEBOV, and up to 90% for MARV outbreaks depending on the strain of MARV (reviewed in Ref. [2]). Currently, there are no vaccines or postexposure treatment modalities available for preventing or managing filoviral infections. However, remarkable progress has been made over the last few years in developing candidate preventive vaccines that can completely protect nonhuman primates against EBOV and MARV [3], [4], [5], [6], [7], [8], [9], [10], [11]. Among the most promising vaccines is a system based on recombinant vesicular stomatitis virus (VSV) which not only can protect nonhuman primates against EBOV and MARV when used as a single injection preventive vaccine but astonishingly showed 50–100% efficacy when employed as a postexposure treatment [12], [13], [14].
Little is known regarding how filoviruses are maintained in nature. All human outbreaks to date have been traced to Central Africa [2]. EBOV has decimated populations of wild apes in this same region [15]; however, apes and other nonhuman primates that have been associated with filoviral outbreaks are reservoir-incompetent species and like humans are accidental hosts [16]. Recent work has shown that bats may serve as a reservoir species for filoviruses [17], [18] but it remains unclear whether other species are involved or exactly how transmission to humans and/or apes occurs. Once an individual is exposed to EBOV or MARV person-to-person transmission occurs by direct contact with blood or bodily fluids (saliva, sweat, stool, urine, tears, and breast milk) of an infected patient during the acute phase of illness [2], [19]. Care-givers both at home and in hospitals are among populations at greatest risk. While studies have shown that EBOV and MARV can be spread through airborne particles/aerosols under controlled laboratory conditions [20], [21], [22], [23], this type of spread rarely occurs among humans in a hospital or household setting during outbreaks.
Despite the relatively low and localized global occurrence of cases and the fact that transmission in nature is primarily by contact exposure, the filoviruses may be exploitable as agents of bioterrorism since they are highly infectious by the aerosol route, produce high morbidity and mortality in primates, and can be readily propagated in vitro. Indeed, the filoviruses have been classified as Category A bioterrorism agents by the Centers for Disease Control and Prevention [24]. When planning defenses against biological warfare agents such as filoviruses, it is important to consider that the inhalation route is the most likely portal of entry for agents disseminated as aerosols [25]. Moreover, it is known that the former Soviet Union experimented with aerosolized EBOV and MARV [26], [27]. However, all previous vaccine candidates have been evaluated against peripheral or intraperitoneal filovirus injections while no vaccine has been tested against an aerosol challenge in nonhuman primates. Here, we used cynomolgus macaque models of filoviral hemorrhagic HF to test the ability of our recombinant VSV-based ZEBOV and MARV vaccines to protect against homologous ZEBOV and MARV aerosol exposures, respectively.
Section snippets
Vaccine vectors and viruses
The recombinant VSVs expressing either the glycoprotein (GP) of ZEBOV (VSVΔG/ZEBOVGP) or MARV (Musoke strain) (VSVΔG/MARVGP) were generated as described recently using the infectious clone for the VSV, Indiana serotype [28]. ZEBOV (strain Kikwit) was isolated from a patient of the ZEBOV outbreak in Kikwit in Ref. [29] while the Musoke strain of MARV was isolated from a human case in 1980 in Kenya [30].
Animal studies
Twelve healthy, filovirus-seronegative male cynomolgus macaques (Macaca fascicularis) (5–9 kg)
Clinical observations
We employed twelve cynomolgus macaques, of which five animals were immunized by i.m. injection with a single dose of VSVΔG/ZEBOVGP (Subjects #1–3 and Controls #4–5) and the remaining seven with a single dose of VSVΔG/MARVGP (Subjects # 4–7 and Controls #1–3). The animals were monitored closely for clinical symptoms and shedding of recombinant VSVs. Following vaccination none of the animals showed any signs of clinical symptoms indicating that the recombinant VSVs are apathogenic for these
Discussion
Few studies have evaluated the pathogenic potential of filoviruses in animals exposed by the aerosol route [20], [21], [22], [23]. Infection of nonhuman primates after aerosol exposure to ZEBOV has been reported and was uniformly lethal in both studies [21], [23]. With regard to MARV, aerosol exposure was also shown to be lethal [20], [22]. However, in one study in African green monkeys there appeared to be a reduced lethality associated with aerosol exposure to MARV [20]. Despite an overall
Acknowledgments
From USAMRIID, the authors thank John Crampton and Carlton Rice for animal care and Matthew Lackemeyer and Adam Hedge for assistance with aerosol exposures. From the National Microbiology Laboratory (NML) of the Public Health Agency of Canada (PHAC), the authors thank Jason Gren and Anders Leung for technical assistance in biocontainment. We are grateful to John Rose (Yale University) for kindly providing us with the vesicular stomatitis virus reverse genetics system. Work on filoviruses at
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Current address: Laboratory of Virology, Division of Intramural Research, National Institutes of Allergy and Infectious Diseases, Rocky Mountain Laboratories, 903 South 4th Street, Hamilton, MT 59840.