Virus-like particles exhibit potential as a pan-filovirus vaccine for both Ebola and Marburg viral infections

A DNA vaccine for Ebola virus is safe and immunogenic in a phase I clinical trial

Ebola viruses represent a class of filoviruses that causes severe hemorrhagic fever with high mortality. Recognized first in 1976 in the Democratic Republic of Congo, outbreaks continue to occur in equatorial Africa. A safe and effective Ebola virus vaccine is needed because of its continued emergence and its potential for use for biodefense. We report the safety and immunogenicity of an Ebola virus vaccine in its first phase I human study. A three-plasmid DNA vaccine encoding the envelope glycoproteins (GP) from the Zaire and Sudan/Gulu species as well as the nucleoprotein was evaluated in a randomized, placebo-controlled, double-blinded, dose escalation study. Healthy adults, ages 18 to 44 years, were randomized to receive three injections of vaccine at 2 mg (n = 5), 4 mg (n = 8), or 8 mg (n = 8) or placebo (n = 6). Immunogenicity was assessed by enzyme-linked immunosorbent assay (ELISA), immunoprecipitation-Western blotting, intracellular cytokine staining (ICS), and enzyme-linked immunospot assay. The vaccine was well-tolerated, with no significant adverse events or coagulation abnormalities. Specific antibody responses to at least one of the three antigens encoded by the vaccine as assessed by ELISA and CD4(+) T-cell GP-specific responses as assessed by ICS were detected in 20/20 vaccinees. CD8(+) T-cell GP-specific responses were detected by ICS assay in 6/20 vaccinees. This Ebola virus DNA vaccine was safe and immunogenic in humans. Further assessment of the DNA platform alone and in combination with replication-defective adenoviral vector vaccines, in concert with challenge and immune data from nonhuman primates, will facilitate evaluation and potential licensure of an Ebola virus vaccine under the Animal Rule.

[Ebola and Marburg viruses: the humans strike back]

Ebola and Marburg viruses are the causative agents of rapidly progressive hemorrhagic fevers with high mortality rates. Pre- or post-exposure treatments against the diseases are currently not available for human use. In the field, establishment of strict quarantine measures preventing further virus transmission are still the only way to fight the infections. However, our knowledge of Ebola and Marburg viruses has markedly increased as a result of two recent discoveries discussed in this review. Chandran et al. have elucidated the mechanism by which Ebola GP is converted to a fusion-active form. Infectivity of Ebola virus was shown to be dependent on the cleavage of GP by cellular endosomal proteases, cathepsin B and L, thus opening new therapeutic approaches options. As for Jones SM et al., they have successfully vaccinated monkeys with recombinant vesicular stomatitis virus expressing Ebola or Marburg virus surface glycoprotein GP, a promising vaccine approach.

Immunologic aspects of select agents

We are on the cusp of exciting new developments in vaccine biology. The use of DNA constructs allows virtually unlimited access to previously inaccessible organisms. Next-generation adjuvants will boost innate and acquired immunity, and will provide protection from infection with any potential biowarfare organism. We are limited only by our imaginations in the construction of a protective armamentarium.

Ebola virus-like particles produced in insect cells exhibit dendritic cell stimulating activity and induce neutralizing antibodies

Recombinant baculoviruses (rBV) expressing Ebola virus VP40 (rBV-VP40) or GP (rBV-GP) proteins were generated. Infection of Sf9 insect cells by rBV-VP40 led to assembly and budding of filamentous particles from the cell surface as shown by electron microscopy. Ebola virus-like particles (VLPs) were produced by coinfection of Sf9 cells with rBV-VP40 and rBV-GP, and incorporation of Ebola GP into VLPs was demonstrated by SDS-PAGE and Western blot analysis. Recombinant baculovirus infection of insect cells yielded high levels of VLPs, which were shown to stimulate cytokine secretion from human dendritic cells similar to VLPs produced in mammalian cells. The immunogenicity of Ebola VLPs produced in insect cells was evaluated by immunization of mice. Analysis of antibody responses showed that most of the GP-specific antibodies were of the IgG2a subtype, while no significant level of IgG1 subtype antibodies specific for GP was induced, indicating the induction of a Th1-biased immune response. Furthermore, sera from Ebola VLP immunized mice were able to block infection by Ebola GP pseudotyped HIV virus in a single round infection assay, indicating that a neutralizing antibody against the Ebola GP protein was induced. These results show that production of Ebola VLPs in insect cells using recombinant baculoviruses represents a promising approach for vaccine development against Ebola virus infection.

Development of a guinea pig immune response-related microarray and its use to define the host response following Mycobacterium bovis BCG vaccination

Immune responses in the guinea pig model are understudied because of a lack of commercial reagents. We have developed a custom-made guinea pig oligonucleotide microarray (81 spots) and have examined the gene expression profile of splenocytes restimulated in vitro from Mycobacterium bovis BCG-vaccinated and naive animals. Eleven genes were significantly (P < 0.05) up-regulated following vaccination, indicating a Th1-type response. These results show that microarrays can be used to more fully define immune profiles of guinea pigs.

Induction of Humoral and CD8+ T Cell Responses Are Required for Protection against Lethal Ebola Virus Infection

Ebola virus (EBOV)-like particles (eVLP), composed of the EBOV glycoprotein and matrix viral protein (VP)40 with a lipid membrane, are a highly efficacious method of immunization against EBOV infection. The exact requirements for immunity against EBOV infection are poorly defined at this time. The goal of this work was to determine the requirements for EBOV immunity following eVLP vaccination. Vaccination of BALB/c or C57BL/6 mice with eVLPs in conjunction with QS-21 adjuvant resulted in mixed IgG subclass responses, a Th1-like memory cytokine response, and protection from lethal EBOV challenge. Further, this vaccination schedule led to the generation of both CD4(+) and CD8(+) IFN-gamma(+) T cells recognizing specific peptides within glycoprotein and VP40. The transfer of both serum and splenocytes, but not serum or splenocytes alone, from eVLP-vaccinated mice conferred protection against lethal EBOV infection in these studies. B cells were required for eVLP-mediated immunity to EBOV because B cell-deficient mice vaccinated with eVLPs were not protected from lethal EBOV challenge. We also found that CD8(+), but not CD4(+), T cells are absolutely required for eVLP-mediated protection against EBOV infection. Further, eVLP-induced protective mechanisms were perforin-independent, but IFN-gamma-dependent. Taken together, both EBOV-specific humoral and cytotoxic CD8(+) T cell responses are critical to mediate protection against filoviruses following eVLP vaccination.