Antiterror Agenda Promotes Ebola Vaccine and Immunotherapy

Photo © Alfred Pasieka, SPL, Photo Researchers Inc. RNA RAIDERS: Computer artwork depicts Ebola viruses releasing their RNA inside an infected human cell. RNA strands are in the upper right. Tightly packed segments of released RNA float through the cell. Though smallpox and anthrax loom as the likeliest boogeymen in a bioterrorism nightmare, the rare Ebola virus still evokes particular dread. This untreatable virus rapidly kills 80% to 90% of the humans it infects, and no one knows where it lurks in the years between its small-scale outbreaks, up to now confined to central Africa. Scientists in the former Soviet Union reportedly weaponized the virus.1 The US Centers for Disease Control and Prevention (CDC) listed Ebola as a "select agent" in 1996, meaning that at least three antiterrorism statutes regulate it. Ebola is also one of the potential bioterrorism agents now targeted by the National Institute of Allergy and Infectious Diseases. NIAID is seeking $1.4 billion for biodefense in fiscal 2003, a $1.2 billion jump over 2002. The National Institutes of Health (NIH) "has had a longstanding commitment to work on emerging infectious diseases, and this is a huge opportunity to advance that agenda and at that same time deal with the problems of bioterrorism," notes Jack Killen, NIAID's assistant director for biodefense research. Soon to be showered with money, the Ebola research enterprise nevertheless will remain hamstrung by safety precautions that require space-suited scientists to toil in the few available biosafety level 4 facilities. An Ebola vaccine, though questionable for protecting large populations against an uncertain threat, might reduce investigators' infection risk and speed their progress. Two years ago, an NIH-CDC team reported that a combination DNA/adenovirus vaccine kept Ebola-infected monkeys alive.2 "We are in the process of working through the regulatory approvals to begin Phase I studies" in humans, says Gary J. Nabel, the team's leader and director of NIH's Vaccine Research Center. Questions linger, however, about the vaccine's efficacy. Scientists are also examining whether antibody injections, after an Ebola infection has begun, can dampen the rampant viremia that eventually leads to death. Passive immunization showed promise in a rodent study published last month,3 but researchers doubt whether such treatment alone can save primates. "I don't think there's going to be one magic bullet from a therapeutic perspective," remarks Thomas W. Geisbert, chief of the viral pathology and ultrastructure department at the US Army Medical Research Institute of Infectious Diseases (USAMRIID) in Fort Detrick, Md. CYTOKINE STORMS AND APOPTOSIS Discovered in the former Zaire in 1976, Ebola is a single-stranded RNA virus belonging to the Filoviridae family. Its nearly 19,000-nucleotide genome encodes eight proteins, including the surface glycoprotein (GP) typically incorporated into vaccines. The virus does not mutate much, suggesting a complex life cycle. (A simpler life cycle, involving passage among a few organisms, would tolerate more mutations.) Once in the body, Ebola infects monocytes, says Nabel. "Subsequently, infection spreads to multiple cell types, including endothelium and liver," he continues. "At that point you compromise the integrity of the vasculature. Probably with the liver damage, you end up with clotting abnormalities that contribute to bleeding." Nonhuman primates die within 5 to 12 days. Peter Jahrling, USAMRIID's principal scientific advisor, observes that Ebola-infected macrophages release a host of chemical mediators known as a "cytokine storm." Nearby lymphocytes, though not themselves infected, undergo apoptosis upon exposure to the cytokines. Ebola consequently has an immunosuppressive effect. As if that were not bad enough, the cytokine storm also triggers an intravascular coagulation cascade that generates many fibrin clots in organs such as the spleen, says Geisbert. Hemorrhaging in patients with Ebola infection probably occurs because clotting factors are used up, he adds. An alternative theory holds that GP, which Nabel's lab found can be cytotoxic, directly damages blood-vessel endothelial cells, leading to hemorrhaging. Geisbert and Jahrling discount this hypothesis as possibly applicable only to the last stage of infection. Image: Courtesy of T.W. Geisbert, Usamriid FATAL FILAMENTS: A transmission electron micrograph of Ebola virus particles isolated from culture fluid. SUCCESSFUL PRIMATE VACCINE Ebola does not naturally infect and kill rodents; it must be adapted to do so. As a practical matter, however, researchers use mice and guinea pigs to test Ebola treatments, and several vaccines are effective in rodents. Geisbert and Jahrling recently discovered that four such vaccines did not save infected primates.4 In the wake of that finding, Nabel's 2000 paper shines as the most promising for a successful Ebola vaccine in primates. Nabel's team injected monkeys three times, at four-week intervals, with DNA encoding GP. Three months after the last DNA injection, the animals received a booster shot containing a GP-expressing recombinant adenovirus. After another three months, each primate was injected with six "plaque-forming units" of Ebola (corresponding roughly to six viral particles). Four treated animals survived infection without symptoms, while four controls died within a week.2 Some investigators argue that the study's major flaw was its low Ebola dose. Geisbert and Jahrling routinely inject 1,000 infectious units. In response, Nabel notes that USAMRIID's primary goal is to protect lab workers against an accidental needle-stick. "They've gone through the calculations, they know the concentration of virus they use, so they figure somebody could get stuck with a thousand [Ebola] units," he says. "Now I don't know that our vaccine won't protect against a thousand. But I also don't think that the average person in a natural outbreak would get exposed to a thousand units intramuscularly. I seriously doubt that they would." To settle the issue, Nabel's lab and USAMRIID are now jointly testing various vaccine doses and regimens in primates. Contemplating Phase I trials of the vaccine, NIH has engaged the services of two companies: San Diego-based Vical, to make the DNA component; and Crucell, headquartered in the Netherlands, to develop the recombinant adenovirus. Crucell has already engineered what Jaap Goudsmit, head of vaccine research, expects will be a more effective vector. It contains adenovirus type 35, which rarely infects humans, rather than the standard type-5 virus currently used in HIV vaccine trials. The rationale for the switch, Goudsmit explains, is that 50% to 90% of humans (depending on location) already have antibodies to adeno-5, and these preexisting antibodies might neutralize a GP-containing vector before it could induce immunity to Ebola. MORE SENSIBLE APPROACH? From a public-health perspective, treating existing Ebola infections with antibodies makes more sense than vaccinating large populations to prevent what still is a rare affliction. Passive immunization would also seem appropriate to bypass viral immunosuppression. But Nabel observes that antibody therapy generally has failed to abort Ebola infections, as it can polio. Jahrling flatly remarks, "I just don't think [such therapy] has very much likelihood of success with this virus, which has such an explosive potential." Nevertheless, a study conducted during a 1995 outbreak in the Democratic Republic of the Congo suggests that passive immunization might work in certain cases. Eight seriously ill patients received transfused blood containing Ebola antibody that was donated by convalescent patients, and only one of the eight died.5 In addition, Dennis R. Burton, an immunology professor at Scripps Research Institute in La Jolla, Calif., recently showed that guinea pigs could survive Ebola if they received a human monoclonal antibody one hour after infection.3 Geisbert, a coauthor on Burton's paper, calls the finding "promising" but reiterates that Ebola experiments in rodents do not predict efficacy in primates. For the past three years, USAMRIID has been collaborating with Abgenix, based in Fremont, Calif., to develop antibodies against filoviruses such as Ebola. According to chief scientific officer C. Geoffrey Davis, Abgenix so far has supplied the Army with its XenoMouse, which it has engineered to generate human instead of murine antibodies. Under an agreement signed in May, the company plans to bring the antibody production in-house, where it has high-throughput capability. Last year, a report by Yoshihiro Kawaoka, a professor of pathobiological sciences at the School of Veterinary Medicine, University of Wisconsin, Madison, indicated a possible drawback to antibody therapy. Applying anti-GP antibodies to a vesicular stomatitis virus (VSV) engineered to express GP, Kawaoka's team found that the antibodies somehow enhanced VSV's ability to infect a cell line.6 This result suggested that antibody therapy might strengthen, not dampen, an Ebola infection. Cell-culture experiments and monkey observations at USAMRIID failed to support the hypothesis of infectivity-enhancing GP antibodies.7 Geisbert and Jahrling contend that the conditions in Kawaoka's study were too artificial. Kawaoka responds that a more naturalistic, unpublished investigation confirms his finding. He also claims that "there are antibodies that neutralize the virus but do not enhance infectivity" and that certain epitopes induce infectivity-enhancing antibodies. Despite such unresolved controversies, Jahrling is optimistic about further progress against Ebola, citing the molecular tools now available. "Twenty years ago, about the only thing we knew how to do was kill [Ebola] with paraformaldehyde or pass it in cells to attenuate it," he recalls. "We now have so many other options to explore that the only crunch is going to be availability of monkeys and availability of space." Douglas Steinberg (dougste@attglobal.net) is a freelance writer in New York City. References 1. K. Alibek, Biohazard, New York: Random House, 1999. 2. N.J. Sullivan et al., "Development of a preventive vaccine for Ebola virus infection in primates," Nature, 408:605-9, 2000. 3. P.W. Parren et al., "Pre- and postexposure prophylaxis of Ebola virus infection in an animal model by passive transfer of a neutralizing human antibody," Journal of Virology, 76:6408-12, June 2002. 4. T.W. Geisbert et al., "Evaluation in nonhuman primates of vaccines against Ebola virus," Emerging Infectious Diseases, 8:503-7, May 2002. Available online at www.cdc.gov/ncidod/eid. 5. K. Mupapa et al., "Treatment of Ebola hemorrhagic fever with blood transfusions from convalescent patients," Journal of Infectious Diseases, 179 (suppl 1):S18-23, 1999. 6. A. Takada et al., "Infectivity-enhancing antibodies to Ebola virus glycoprotein," Journal of Virology, 75:2324-30, 2001. 7. T.W. Geisbert et al., "Evidence against an important role for infectivity-enhancing antibodies in Ebola virus infections," Virology, 293:15-19, Feb. 1, 2002.

Antiterror Agenda Promotes Ebola Vaccine and Immunotherapy

Interested in reading more?

Become a Member of