Elsevier

Vaccine

Volume 25, Issue 11, 1 March 2007, Pages 1923-1934
Vaccine

Review
Status and challenges of filovirus vaccines

https://doi.org/10.1016/j.vaccine.2006.11.037Get rights and content

Abstract

Vaccines that could protect humans against the highly lethal Marburg and Ebola viruses have eluded scientists for decades. Classical approaches have been generally unsuccessful for Marburg and Ebola viruses and pose enormous safety concerns as well. Modern approaches, in particular those using vector-based approaches have met with success in nonhuman primate models although success against Ebola has been more difficult to achieve than Marburg. Despite these successes, more work remains to be done. For the vector-based vaccines, safety in humans and potency in the face of pre-existing anti-vector immunity may be critical thresholds for licensure. The immunological mechanism(s) by which these vaccines protect has not yet been convincingly determined. Licensure of these vaccines for natural outbreaks may be possible through clinical trials although this will be very difficult; licensure may also be possible by pivotal efficacy studies in animal models with an appropriate challenge. Nevertheless, nonhuman primate studies have shown that protection against Marburg and Ebola is possible and there is hope that one day a vaccine will be licensed for human use.

Introduction

The viruses that comprise the family Filoviridae cause some of the most lethal viral hemorrhagic fevers known. In 1967, an outbreak in Marburg, Germany occurred among laboratory personnel that handled monkeys or tissues subsequently determined to be infected with a small, unidentified, and negative-strand RNA virus [1], [2]. With a case-fatality rate of 22% and an unknown route of transmission, there was considerable concern about Marburg virus (MARV) (now termed Lake Victoria marburgvirus); however, there were only a limited number of secondary cases. In 1976, a MARV-like virus emerged in two nearly simultaneous outbreaks in Africa along the Ebola River; the case-fatality rates, however, were substantially higher (50–80%) than in the MARV outbreak. Two distinct viruses were isolated from these outbreaks, Zaire ebolavirus (ZEBOV) and Sudan ebolavirus (SEBOV), their names based on the locations of the initial outbreaks [3], [4].

Since 1976, there have been sporadic cases and outbreaks in Africa of ZEBOV, SEBOV, and MARV. Two other strains of Ebola virus have been identified, Cote d’Ivoire (CIEBOV) and Reston (REBOV) [5], [6]. While CIEBOV and REBOV are highly pathogenic in nonhuman primates, only one human case of CIEBOV has been reported, and it is not clear whether REBOV is virulent in humans. Until recently, ZEBOV was thought to be the most virulent of all filoviruses, with case-fatality rates around 80%, while SEBOV was slightly less pathogenic with case-fatality rates around 50%. Recent outbreaks in the Congo and Angola have demonstrated that MARV strains can be as virulent as ZEBOV [7], [8].

The number of cases in these outbreaks has generally been small and implementation of general barrier-nursing procedures appears to bring a halt to these outbreaks. However, there is still considerable concern about these viruses and much that is not known. No licensed vaccines or therapeutics exist that can offer protection against these viruses, so they can only be handled in biosafety level-4 (BSL-4) laboratories. Recent data suggest bats may be a host [9] but even if bats are proven to be the sole host species for all filoviruses, control of outbreaks in African may be exceedingly difficult. Epidemics among chimpanzees and great apes have occurred, with potentially catastrophic effects on the populations of these endangered animals [10], [11], [12]. In the last decade, the number of outbreaks for both EBOV and MARV viruses in Africa has risen, leading to concerns that it is only a matter of time before cases are seen in a developed nation. Of paramount concern are the assertions that the former Soviet Union considered using filoviruses as offensive biological weapons and may have weaponized MARV for aerosol dissemination [13].

The high mortality rates seen with filovirus outbreaks and the knowledge that these viruses could be employed as biological weapons are the primary reasons these viruses are listed as Category A Priority Pathogens by the National Institutes of Health [14]. Licensed vaccines and therapeutics that can protect against aerosol exposure to either MARV or EBOV are needed to protect against this threat.

Section snippets

The Filoviridae

The genomes of all filoviruses are composed of a non-segmented, negative sense, single-strand RNA approximately 19-kb long, encoding genes for NP (major nucleoprotein), VP35 (P-like protein), VP40 (matrix protein), GP (glycoprotein), VP30 (minor nucleoprotein), and VP24 and L (RNA-dependent RNA polymerase). The known transcribed open reading frames of the viral genes, gene order, and presumptive protein functions are shown in Fig. 1. Expression of VP40 in combination with GP is sufficient to

Human disease

In humans, MARV and EBOV incubation periods range from 2 to 14 days. Typical presentation is an acute, unremarkable febrile illness with symptoms including chills, headache, and myalgia [30]. Mental confusion or changes in personality have been reported, particularly with MARV. Nausea, vomiting, abdominal pain, diarrhea, sore throat, and a maculopapular rash have been reported in some but not all cases. Within 6–8 days of fever, hemorrhagic complications can develop, and patients develop

Animal models of the human disease

Since the first known outbreak of MARV, animal models have been critical to the study of filoviruses. Both rodent and nonhuman primate (NHP) models exist for MARV and EBOV [39]. Because the number of human cases is low and the availability of human tissues from fatal cases is limited, animal models have been used to study the underlying pathology of the diseases caused by MARV and EBOV. Most of these studies have focused on infection of the viruses by i.p., s.c., or i.m. injections although a

‘Classical’ approaches

The earliest attempts to generate filovirus vaccines were based on the classical approach; i.e., inactivated virus. Classical attenuation by passage through cell culture or another species is not considered a viable option; guinea pig-adapted ZEBOV and MARV retain their virulence for NHP [40], [61], [62], [72] and the reversion rate of other attenuated virus vaccines make clinical trials and licensure of such a vaccine for filoviruses extremely improbable. Recombinant genetic engineering to

DNA vaccines

DNA vaccines expressing the GP of MARV and ZEBOV have been evaluated as potential vaccines in rodents. In guinea pigs, a DNA vaccine expressing MARV GP was weakly immunogenic compared to other strategies and offered incomplete protection when given alone but worked well when boosted with baculovirus-expressed GPΔTM [60]. In NHP, the DNA MARV GP vaccine protected four of six cynomolgus macaques from lethal MARV infection [80]. In mice, a DNA vaccine expressing GP from ZEBOV was able to fully

Virus-like particles (VLP)

VLP are an attractive alternative to ‘traditional’ subunit vaccines in that they are an aggregate of viral proteins in native conformation without the safety concerns that attend attenuated or replication-deficient viruses. Co-expression of ZEBOV GP and VP40 in 293T cells resulted in the production of ZEBOV VLPs (eVLPs) that were indistinguishable by electron microscopy from live ZEBOV particles [15], [101]. When eVLPs were cultured with mouse bone-marrow-derived dendritic cells, they induced

Immunological correlates and mechanisms

To license vaccines using the FDA's Animal Rule requires demonstrating an understanding of the immunological mechanisms responsible for that protection. The response in humans to the vaccine must be sufficiently similar to the response in protected animals so that one can infer that protection in the animal will predict protection in the human.

For filoviruses, this challenge is even more daunting than it may first appear. Much of what we know about the immune system is based on studies in mice

Conclusions and the path forward

A decade ago, there seemed little hope for a vaccine that would protect against filoviruses. Equally troubling were the revelations that the former Soviet Union had ‘weaponized’ MARV and possibly ZEBOV [13]. Since that time, several vaccine candidates have been generated by using modern technology and have shown immense promise by protecting animals, particularly NHP, against challenge with MARV and ZEBOV. If nothing else, these animal studies have demonstrated that a vaccine is certainly now

Acknowledgements

We would like to thank Dr. Alan Schmaljohn for providing materials used in the figures published in this review as well as critical comments and review of the manuscript prior to submission.

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    The views, opinions, and/or findings contained herein are those of the authors and should not be construed as an official Department of Army or John Hopkins University, policy, or decision unless so designated by other documentation.

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