As described above, the low pathogenicity of VSVwt and the further attenuation of the virus through the deletion of the glycoprotein (GP) made this vaccine design attractive.
Lack of reassortment & cytosolic replication
In addition, VSV is considered a promising vaccine vector because its simple 12 kb RNA genome does not undergo reassortment and therefore lacks the potential for reassorting with wt viruses. It replicates in the cytosol and cannot integrate into host cell DNA. Moreover attenuated VSV-based vaccine vectors
expressing foreign proteins induce potent immune responses and protect against viral and bacterial disease in several animal models, including in nonhuman primates.46,47
High expression of EBOV-GP
Another strength is that cells infected with VSV-EBOV express particularly large quantities of the EBOV envelope gylcoprotein GP, as demonstrated by immunofluorescence staining. This is likely the basis of their potent immunogenicity. Indeed, antibodies to EBOV-GP are considered essential in protection against EBOV (see below).
Modification of soluble EBOV-GP
As described above, the release of high quantities of soluble EBOV-GP (sGP) during EBOV infection plays a crucial role in disease manifestations, which include inflammatory dysregulation, loss of vascular integrity and viral escape through immune suppression.4 The pathogenic functions attributed to sGP are debated but are likely to include cytotoxicity,40 immunosuppressive properties (interference with type I interferon responses and T-cell activation, T-cell apoptosis, reduced Th1 and increased IL-10 cytokine production, interference with neutrophil activation),40 loss of adherence properties and endothelial dysfunction.12 To mitigate this risk, VSV-EBOV was engineered to contain the 8A variant of the EBOV G gene (the authentic EBOV has 7As at this position). This results in the exclusive expression of full-length GP and no expression of the soluble GP. Accordingly:
• The preclinical evaluation of a panel of EBOV-GP-expressing constructs identified no sign of immunosuppression; rather, it elicited potent protective anti-GP immune responses.
• There was no difference in tolerability between VSV-EBOV-GP and VSV-Lassa-GP.
• The preclinical evaluation of a large variety of vectors expressing EBOV-GP identified no evidence of coagulopathy, cytopenia, alterations of liver function or other observable toxicities, including in NHP (see below).
• The post-exposure administration of VSV-EBOV was not associated with any adverse outcomes in NHP (see below).
Pre-clinical data: safety, immunogenicity and efficacy of rVSVΔG-ZEBOV-GP in NHP
The characteristics of the VSV-EBOV vaccine were reviewed before its testing and use in the current epidemic47 and are detailed below.
Protection against disease was first reported in NHP in 2005: a single intramuscular (IM) injection of ≥ 107 pfu of VSVΔG/ZEBOV-GP elicited completely protective immune responses against lethal EBOV challenge.46 Notably vaccine vector shedding was not detectable in the monkeys and none of the animals developed fever or other symptoms of illness associated with vaccination. The EBOV vaccine induced humoral and cellular immune responses in all vaccinated monkeys. No evidence of EBOV replication was detected in any of the protected animals after challenge. A 50% protection (4/8
macaques) was reported following post-exposure vaccination with VSV-EBOV as late as 20-30 min after lethal challenge with Zaire stain Ebola virus.4 In 2008, the protective capacity of VSV-EBOV was evaluated against aerosol challenge: all macaques were completely protected whereas all control animals
succumbed.40 In 2009, 11 cynomolgus monkeys were vaccinated with a vaccine consisting of equal parts of vaccine vectors encoding for the Sudan, Zaire or Côte d’Ivoire EBOV GP proteins. Again, none of the vaccinated macaques succumbed to filovirus challenge with these strains.12
Safety in immunocompromised hosts such as HIV-infected patients was evaluated in six rhesus macaques infected with simian-human immunodeficiency virus (SHIV). None of the six animals showed evidence of illness following VSV-EBOV vaccination, and four of six SHIV-infected macaques (those with the highest CD4+ counts) were protected from death following EBOV challenge.20 That immune competence is not required to control VSV-EBOV was confirmed by the lack of symptoms following the injection of severely immunocompromised mice with 2 x 105 pfu/ml, i.e., 10 times greater than the normal mouse
immunization dose.21 These observations indicated that viral replication was terminated by innate immunity through the induction of an antiviral state in VSVwt-infected cells, as a result of a strong induction of type I interferons, long before the onset of B and T cell responses.
Potential for protection against, newly emerging, phylogenetically related strains was assessed by immunizing macaques with VSV-EBOV prior to challenge with Bundibugyo (BEBOV), a newly discovered species in the Ebolavirus genus. A single vaccination provided cross-protection (75% survival), suggesting that monovalent VSV-based vaccines may be useful against a newly emerging species.48
The lack of neurovirulence of VSV-EBOV in NHP was demonstrated in 2012, as summarized above.49
The immune mechanisms of protection conferred by VSVΔG/ZEBOV-GP were defined in groups of cynomolgus macaques which were depleted of specific immune cells before and during vaccination with rVSV/ZEBOV-GP, prior to challenge. CD8+ T cell responses were elicited by immunization but CD8+ T cell-depleted animals survived. CD4+ T cell depletion abrogated both the induction of anti-GP antibodies and vaccine protection. However, CD4+ T cell depletion only at time of challenge resulted in the survival of immunized animals, indicating a minimal role for CD4+ T-cell immunity and a critical role for antibodies in rVSV-mediated protection against ZEBOV.23
Among all of these studies, which include roughly 80 NHP vaccinated with VSV-vectored vaccines targeting filoviruses, adverse clinical symptoms were not identified post-vaccination, even in
immunocompromised animals. Significant changes in hematology and blood chemistry were also not observed. The NHP vaccinated with VSV-EBOV consistently exhibited only low-level (101 - 104 pfu/ml) and transient (day 2) recombinant VSV viremia. Vaccine virus could be detected only by means of PCR;
attempts at virus isolation were unsuccessful. In all animals, infectious viral shedding was never observed.
Clinical data in humans before phase I testing
Prior to the launch of the Geneva phase I VSV-EBOV vaccine trial, clinical experience with VSV-EBOV was limited. At that time, only three people worldwide were on record as having received the vaccine. The first was a laboratory worker who experienced a needlestick injury during an animal experiment in the biosafety level (BSL) 4 laboratory of Hamburg, Germany, in March 2009. The syringe contained EBOV mixed with Freund's adjuvant. A single dose of VSV-EBOV (5 × 107 pfu) was injected 48 hours after the accident. The vaccinee developed fever 12 hours later and low-level VSV viremia was detectable by PCR for 2 days. Infection with EBOV was not confirmed, precluding any conclusion related to protective efficacy. However, blood chemistry, coagulation, and hematology parameters remained in the normal range and the person remained healthy.45
In addition, two healthcare workers were vaccinated post-needlestick injury in the current outbreak.
Both received a high post-exposure dose of 1 x 108 pfu. The first had no symptoms and transient VSV viremia (< 1 day) appearing 1 day after vaccination. The other had transient fever (40° C) with chills the day after vaccination. Neither contracted EVD, and no other adverse events were reported (personal communication).
4 The Geneva VSV-EBOV phase I trial: preparation and launch