By David N. Leff

"EBOLA COULD SOON BE THE WEST'S PROBLEM, TOO"

That's the banner headline across a full-page article in this week's Business Week magazine, dated Dec. 4, 2000. Its lead paragraph warns: "There is a crisis brewing in the world that we ignore at our peril. The Ebola virus is back, and it's spreading." The story goes on to report the latest outbreak, in Uganda this month and last, with 329 people infected, of whom 107 (33 percent) have died.

This brings to 1,258 the number of cases recorded since Ebola virus emerged in 1976, decimating villages along the Ebola river in the African country of Zaire - since re-named the Democratic Republic of the Congo. In that grim box score, fatalities range from 33 percent in the current Uganda outbreak, up to 93 percent. That list includes zero percent at two U.S. primate centers - in Reston, Virginia, 1989, and Alice, Texas, 1996. (See BioWorld Today, April 19, 1996.)

Today's issue of Nature, dated Nov. 30, 2000, reports news of a contrapuntal kind, headlined: "Development of a preventive vaccine for Ebola virus infection in primates." Its senior author is molecular biologist Gary Nabel, director of the NIH Vaccine Research Center (VRC), in Bethesda, Md.

"Doctors have essentially been helpless against Ebola virus," Nabel told BioWorld Today. "We have not known if immunity to the virus exists or what parts of the immune response are important. Our studies show that animals can launch an effective immune response against Ebola virus, and we can use knowledge of this response to design a vaccine that protects humans from infection."

Evidently, that super-smart Ebola virus knows enough about the human immune system to get in its lethal licks before those defenses can pull themselves together and strike back. "Normally," Nabel noted, "people die anywhere between seven and 21 days. The pathology that we see in our primates," he added, "is similar to the pathology you see in humans. Put a virus from a person into a monkey; the monkey will die - and vice versa."

Environmental Controls? Out Of The Question

Unlike Yellow Fever and Dengue viruses, which are transmitted by mosquitoes, or Hantavirus, by small rodents, the natural reservoir of Ebola virus remains a mystery. One far-out speculation indicts bats inhabiting the rain forests of equatorial Africa and the Phillipines.

"Currently available antiviral drugs have no proven effect on Ebola virus," said Anthony Fauci, who heads NIAID - the National Institute of Allergy and Infectious Diseases, which funds VRC, "and not knowing its natural reservoir makes environmental control impossible. A vaccine is therefore the best hope for protecting humans from infection. This study makes some key advances toward realizing that goal."

As reported in Nature, Nabel's vaccine has already protected four cynomolgus macaque monkeys (Macaque fascicularis) from swift and certain death. He noted that "Four control primates which received dummy vaccination did die - in less than six days."

The business end of a conventional vaccine consists of antigenic glycoproteins extracted from the pathogen, which the vaccinee's immune system will recognize as foreign and target for destruction. However, the virulent Ebola virus exists in three main strains named for the countries where they emerged - Zaire, Sudan, Ivory Coast. Each has its own types of immunogenic target proteins. The current Uganda outbreak involves the Sudan strain.

Ebola-Zaire is responsible for the most deaths, but the co-authors covered all bases by combining genes encoding surface proteins from all three strains. DNA vaccines, they suggest, enter a cell and use that cell's machinery to manufacture new viral proteins - a strategy designed to trick the recipient's immune system into thinking a real viral infection has occurred.

Nabel compared this new vaccine with an earlier, rodent-specific version he had successfully tested in guinea pigs. The formulation the co-authors have now injected into the four primates produced an immune response of equal power. (See BioWorld Today, Dec. 31, 1997.)

"But the guinea pig immune system is different from the human," Nabel pointed out, "and the virus may well behave a little bit differently in that rodent model. So we wanted to get some indication that we were on the right track, before we would consider further testing." He said the guinea pig results showed that they had "a 99 percent chance of being protected against a lethal Ebola challenge. And that's been borne out in our monkeys."

But before going from rodent to primate, the co-authors added a booster stage to their original DNA prime vaccine. They attached the Zaire virus glycoprotein gene to a weakened adenovirus (AV) vector, and tested this new booster vaccine in mice. It produced a more vigorous immune response than that observed with the triple-strain DNA stage, and increased the amount of antibodies and T cells aimed against the Ebola virus target proteins.

The co-authors then tested their novel prime-boost immunization strategy on eight monkeys. Four received the three-strain Ebola DNA vaccine, followed by the AV booster. All four survived subsequent challenge by lethal doses of Ebola-Zaire virus. Six months later, they remained symptom-free.

DNA Cocktail Plus Adenoviral Booster

"The DNA is a cocktail," Nabel said. "We did that so we could provide broad coverage against all three strains of Ebola virus that occur in nature. It's the surface glycoprotein in those three strains, plus the nuclear protein of Zaire, which is a little more highly conserved of the three."

He said human trials lie not far ahead. "It's the typical Phase I - II - III drill," he noted. "We would obviously want to start with Phase I dose escalation and safety studies, doing immunogenicity assays at the same time. And once we've done that, extend the dosing, then move on to efficacy."

For the DNA part, he added, "that should be pretty straightforward to move ahead into Phase I. It's a two-part vaccine - a DNA prime and an AV boost. We obviously want to be very careful to use technologies that give very good quality AV. There we're going to do a number of preclinical safety studies, in light of some of the problems that have surfaced with AVs in the past."

Nabel said, "We are now in discussion with some biotechnology companies for the vaccine's DNA component and a Dutch firm, Crusell AV, of Leiden, the Netherlands, is involved in our AV production."