The rapid progression of severe disease after Ebola virus infection allows little opportunity to develop protective immunity, and there is currently no effective antiviral therapy. Therefore, vaccination offers a promising intervention to prevent infection or severe disease and limit spread to contacts and would be an important public health benefit for health care workers involved in the care of patients and containment of outbreaks. Another compelling reason for accelerated development of an Ebola virus vaccine relates to its contribution to biodefense (17
). The Centers for Disease Control and Prevention (6
) Category A agents are highly contagious and largely lack effective vaccines or treatments (18
) and include the filoviruses (Ebola and Marburg viruses).
Gene-based vaccine technology provides a safe avenue for producing candidate vaccines for select agents without the need for extreme biocontainment. Gene-based vectors for filoviruses are particularly attractive vaccine approaches because of their capacity to induce both humoral and cell-mediated immune responses, both of which may be important for protection. The concept of using bacterium-derived plasmid DNA to deliver vaccine antigens has many attractive features, including (i) ease and flexibility of construction, (ii) scalable manufacturing capacity, (iii) stability, (iv) intracellular production of vaccine antigen, (v) transient expression, (vi) no induction of antivector immunity, (vii) induction of both CD4+
T-cell responses as well as antibody, and (viii) lack of local or systemic reactogenicity. However, DNA vaccines have not performed well enough to be considered as a vaccine platform in humans until recently. A hepatitis B virus DNA vaccine administered by a needle-free particle-mediated delivery was shown to be safe and immunogenic in a phase I clinical trial (20
). Additionally, DNA vaccination against malaria was shown to be safe and immunogenic, especially as a priming vaccination in a prime-boost regimen (16
). Recently, a multiclade HIV DNA vaccine based on a similar design to the Ebola virus DNA vaccine described here was shown to be safe and immunogenic in healthy adults (11
The broad immunogenicity of this Ebola virus DNA vaccine suggests that immunization by plasmid DNA delivery is a viable platform and merits further development. The consistent immunogenicity of the Ebola virus DNA vaccine described here likely reflects a combination of factors, including optimization of vector design, manufacturing methods, delivery, sample processing, and immunological assays. Additional work is needed to further improve the efficiency and consistency of DNA vaccination.
Nonhuman primate studies have shown that an rAd5 vaccine effectively prevents disease, and DNA vaccination prior to boosting with rAd5 also confers protection and markedly increases the magnitude of the immune response (22
). Further vector and construct optimization may further increase protective immunity of this DNA vaccine. Recently, the importance of the GP transmembrane region in the design of the immunogen has been described. Reduced protection with an Ebola virus rAd immunogen containing a GP transmembrane region deletion compared to a point mutation in this region or wild-type GP constructs has been found (22
). Additionally, it was found that the NP gene is dispensable for immune protection, and the addition of NP in a candidate vaccine may diminish the immune response to Ebola virus GP (23
). Therefore, future formulations of this DNA product will include multiple GP constructs encoding GP in either its wild-type form or a modified form to optimize vaccine potency. Because Ebola virus from Ivory Coast has been observed in only one limited outbreak and is closely related to Ebola virus Zaire, it is not included in vaccine formulations.
This is the first report of an evaluation of a candidate Ebola virus vaccine in humans. This three-plasmid DNA candidate Ebola virus vaccine was safe and well-tolerated in 21 healthy adults. Importantly, DNA immunization induced both Ebola virus-specific antibody and T-cell responses to the GP and NP antigens. While Ebola virus-specific neutralizing antibody could not be detected in vaccinees, the range of antibody titers measured by ELISA was similar to those seen in nonhuman primates following vaccination with similar vaccine constructs (24
). Recently, in a series of nonhuman primate studies demonstrating protection from Ebola virus with vaccine constructs expressing similar antigens as used in this clinic trial, IgG as measured by ELISA correlated with survival. In functional assays, serum antibodies were neither neutralizing nor enhancing, suggesting that IgG levels may reflect the overall level of immune stimulation. Although antibody-dependent enhancement of Ebola virus replication has been observed in tissue culture, there is no evidence of antibody-dependent enhancement in humans or in animal studies, and only protection, rather than enhancement, has been observed in animal studies evaluating DNA or rAd-based vaccine strategies (23
). In the clinical trial described here, the vaccine-induced antibody and T-cell-mediated immune responses were greatest to the GP immunogens, with a less frequent response to the NP immunogen, and Ebola virus-specific CD4+
T-cell responses were more frequent than CD8+
T-cell responses. While the presence of Ebola virus GP-specific IgG seems to predict survival in nonhuman primates, the definite correlate(s) of protection from Ebola virus infection is not known, and it is possible that T-cell responses also contribute to protection. Therefore, we believe it is important that a candidate Ebola virus vaccine be capable of eliciting both Ebola virus-specific antibody and T-cell responses.
Further studies are needed to determine the optimal preventive gene-based Ebola virus vaccine strategy. Our development plan includes evaluation of DNA vaccination alone, rAd5 vaccination alone, and a heterologous prime-boost strategy of DNA priming followed by rAd boosting. Even if the optimal strategy were determined to be heterologous prime-boost, the potential vaccines would need to be independently demonstrated as safe and immunogenic. Since the prophylactic efficacy of an Ebola virus vaccine cannot feasibly or ethically be demonstrated in a human trial, the combination of safety and immunogenicity data from phase I, II, and III human trials and efficacy data from nonhuman primate studies will ultimately need to be utilized to obtain licensure of an Ebola virus vaccine under the Animal Rule.
The successful evaluation of a DNA vaccine to multiple Ebola virus subtypes reported here provides the opportunity for further clinical evaluation of candidate Ebola virus DNA vaccines alone or in combination with Ebola virus rAd vaccines as a heterologous prime-boost strategy. Evaluation of gene-based candidate vaccines in humans will continue in parallel with efforts to define immunological correlates of vaccine-induced protection in nonhuman primate models of Ebola virus infection. Together, these studies will provide the scientific basis for identifying a vaccine strategy for the prevention of Ebola virus and other filovirus infections in humans.