Science Editor

From the great blue whale (Balaenoptera physalus — 150 tons) to the Old World pygmy shrew (Suncus estruscus — two grams), there are something like 4,000 species of mammalia alive on earth today.

Homo sapiens, averaging 70 kilograms in weight, fits somewhere between these two extremes. So does Pan troglodytes, the common chimpanzee.

But the evolutionary stretch between chimps and people is very small, as measured by the differences between their genes.

"Actually," observed molecular pathologist David Weiner, at the University of Pennsylvania, "humans and chimpanzees are 99 percent identical, genomically. That one-percent difference is based on their genetic map. Statistically, any gene you might pick up could differ between the species by one percent."

But the identity is far more important immunologically than the disparity, Weiner told BioWorld Today: "Pretty much every human immunological reagent that we have -- antibodies, cytokines, T-cell markers -- works in chimps and cross-reacts, which, of course, is unique in the animal kingdom."

Weiner is senior author of a paper in the May issue of Nature Medicine, titled: "Protection of chimpanzees from high-dose heterologous HIV-1 challenge by DNA vaccination."

Among its 15 co-authors from seven research centers, three represent Apollon Inc., of Malvern, Pa., which specializes in naked-DNA vaccines. The company's president and CEO, Vincent Zurawski, told BioWorld Today: "Most of the immunology was done in Dr. Weiner's lab. We created the vaccine here at Apollon. The animal work itself took place in Alamogordo, New Mexico, at the Coulston Foundation's White Sands Research Center."

The primate technicians injected three out of four chimpanzees with the DNA vaccine late in 1995. Some months earlier, Zurawski recalled, "Back in the middle of '95 we injected the first patient, who was HIV-infected, with some of our DNA constructs." (See BioWorld Today, June 19, 1995, p. 1.) "And now we have 20 patients in that therapy trial."

In March of this year, he recounted, "we initiated a study at the National Institutes of Health in volunteers. By the time we finish that NIH trial," he added, "we expect to enroll 16 subjects." More recently, Apollon began another volunteer trial, also at the University of Pennsylvania.

"Then," Zurawski continued, "we intend to expand beyond that, hopefully later this year, with some additional volunteer trials, broader than those we're doing right now." The chimp vaccinations, he pointed out, "were set up as part of a safety and immunogenicity study that we put together for taking the program into humans."

Recipe For A Naked DNA Vaccine

The cocktail of human viral DNA injected into the chimpanzees consisted of two antigenic constructs. One of these vaccine components contained env genes encoding gp160, the viral envelope glycoprotein, together with rev, an accessory gene which the HIV uses during replication. The second plasmid package contained gag, for the p24 core protein, along with pol, the gene that expresses the viral polymerase.

All of these HIV sequences have been used elsewhere in experimental AIDS vaccines before, but in attenuated virus rather than naked DNA. What is unique to the Apollon/Penn recipe is an ingredient its authors call "the facilitator." This compound, a local anesthetic named bupivacaine, is a chemical cousin to the more familiar lidocaine.

"We noticed early on," Weiner explained, "that some of the cells our vaccine was transfecting were myoblasts, which exist within muscles. We reasoned that if we could increase their transfection efficiency by maybe making them divide, or by coating them with local anesthetics that would help transfect, we thought that would improve transfection efficiency. And in our first lab experiments, transfection did go up significantly." Their facilitator has since "substantially enhanced DNA uptake and gene expression," Zurawski observed.

Over 52 weeks, the team inoculated three chimpanzees, two male, one female, with their gene insert vector. The fourth animal received an identical control package lacking the HIV-1 DNA.

Then, at week 60, they inoculated that control animal and two of the three vaccinated chimps with enough high-dose live HIV-1 virus to infect 250 chimpanzees.

As expected, within two weeks the control had virus in its bloodstream, and remained infected for the 48-week follow-up period. After brief bouts with infection, at eight weeks, the other two challenged animals have remained free of viremia.

Who Said Chimps Don't Get AIDS?

Until recently, Weiner pointed out, use of the chimpanzee in modeling HIV disease was hampered by the supposition that Pan troglodytes, like other primates, never actually came down with AIDS, human or simian.

But this month's Journal of Virology, dated May 1997, negates that long-held folk wisdom with a paper titled: "Development of AIDS in a chimpanzee infected with human immunodeficiency virus type 1." Its authors are scientists at the Yerkes Regional Primate Research Center and the Centers for Disease Control, both in Atlanta.

The chimp in question, named Jerom, was one of 13 animals infected with three different isolates of human HIV-1 virus in the mid-1980s, as a platform for researching AIDS and possible vaccines.

On Jan. 30, 1997, Yerkes primatologist Frank Novembre told the Conference on Retroviruses and Opportunistic Infections, in Washington, D.C., of this first recorded case of an animal infected with human virus succumbing to AIDS.

In the late summer of 1995, Jerom began showing signs of overt AIDS, notably cryptosporidiosis, a parasite-borne intestinal infection marked by fulminating diarrhea, which is often fatal in AIDS patients.

Another of the 13 closely watched experimental chimpanzees, Nathan, received a transfusion of Jerom's blood, and promptly suffered a dramatic decline of his CD4 cells, an index of HIV infection.

On Feb. 13, 1997, Yerkes announced that Jerom had been euthanized.

Weiner commented: "Chimps take a very long time to get the disease, but the median time for humans is somewhere between seven and 13 years. Could it be that chimpanzees take a few years more than humans to start to develop AIDS? We'll start to know that in the next little while."

He concluded: "I think this puts to rest the minority view that HIV is not the cause of AIDS. And the chimp results of our vaccine suggest positive implications for human AIDS vaccine design." *