By David N. Leff

Two dozen rhesus monkeys (Macaca mulatta) spent the last two years testing the latest entry in the AIDS vaccine sweepstakes. Their three-shot immunization routine started in 1999 with two injections eight weeks apart of a DNA-based payload.

Twenty-four weeks later, the brownish animals received a third, final booster shot of the vaccine, packaged with a defused recombinant vaccinia viral vector. Then their scientific handlers at the Yerkes Regional Primate Research Center of Emory University in Atlanta let seven months go by. If their vaccine worked, the recipient monkeys would be able to withstand a lethal challenge combining pathogenic human and simian immunodeficiency viruses - HIV and SIV.

Knowing that HIV infection in humans is most often transmitted mucosally by rectal or vaginal intercourse, the Yerkes experimenters then introduced their deadly viral combo rectally, in the manner of a suppository.

Soon after this infection, four control monkeys that had not been vaccinated rapidly developed high levels of virus - 100 times the concentration typically found in HIV-positive humans - and progressed to full-blown AIDS. By the 28th week, all four were near death with AIDS-type opportunistic infections, and were euthanized.

By contrast, two weeks after the challenge, the 24 vaccined monkeys had only one-tenth as much viral burden in their blood as did those nonvaccinated controls. At 12 weeks post-infection, the lymph nodes (immune system powerhouses) of the vaccinated animals were intact and responding to the challenge, while those of the infected control animals had been functionally destroyed. Now, all 24 remain alive and healthy.

Today's issue of Science, dated March 9, 2001, reports this simian success story in an online article titled "Control of a mucosal challenge and prevention of AIDS in rhesus macaques by a multiprotein DNA/MVA vaccine." Its senior author is retrovirologist Harriet Robinson, chief of microbiology and immunology at the Yerkes Primate Center, and professor of both disciplines at Emory School of Medicine.

"This vaccine," she told BioWorld Today, "is the first to express three viral proteins from a single-component DNA sequence, or a single-component poxvirus vector. It comprises," she added, "the genes expressing the three principal HIV and SIV viral proteins - Gag, Pol and Env, which are very important to the structure and function of the virus."

Vaccinia Vector: Smart But Safe

"The vaccine's twofold vector," Robinson continued, "consists of a purified DNA plus MVA - the modified vaccinia Ankara booster, a live, recombinant virus. MVA seems to be particularly effective at raising immune responses. It doesn't undergo productive replication, so it can infect primate cells but doesn't produce progeny, which makes it a very safe virus. We gave just three inoculations, and that's one way our vaccine differs from others. It's really among the simplest regimens that have been used in any HIV or SIV vaccine trial."

Robinson added, "With this double vector, we get a very high cellular as well as humoral [antibody] immunity, much higher than you can get with the DNA or poxvirus vector alone. Thus, our DNA/MVA vaccine, by expressing the three major proteins of HIV, raised a memory immune response that could control a very virulent challenge." Robinson pictures the body's cellular immune defenses as fielding a two-force army: T cells flying the CD4 flag are the main ones killed by the virus, while CD8-marked T cells pick themselves up off the mat and fight back, enhanced by the DNA/MVA viral vectors.

"I think one reason our vaccine is so successful," Robinson observed, "is that it makes very broad CD4 and CD8 immune responses, and that's what allowed the T cells to fight this infection."

She cites another up-and-coming vaccine that's pursuing a similar strategy - but with a tactical difference. "It's out of Harvard," she noted. "Their protocol required six inoculations rather than our three, and used up to 100 times more DNA than we did. But they also achieved the kind of control that we achieved, and they attributed it mainly to cell-mediated immunity. The fact that we now have two studies showing this kind of control being mediated by T cells means that this kind of strategy is going to work.

"That doesn't mean," Robinson pointed out, "that it's going to succeed as smoothly in humans the first time out, but at least we have something that's working. And our vaccinated monkeys controlled the virus so that now we are at the level that only a very, very small fraction of HIV-positive humans achieve - a residual burden of 1,000 virus copies per milliliter of blood. Those are patients who are in HAART [highly active anti-retroviral therapy]. And so are people who are long-term disease nonprogressors and are nontransmitters. Our vaccinated monkeys have fewer than 1,000 copies of virus per milliliter.

"It's important to point out," Robinson noted, "that the vaccine did not prevent infection in the vaccinated monkeys. Instead, it controlled infection by keeping the virus from replicating in large numbers. Maintaining a chronic, low-level infection prevents progression to AIDS, and reduces the risk of viral transmission, since higher viral loads are associated with greater infectiousness.

Human Trial In 2002; Africa On Drawing Board

Could that 1000-copy, low viral load stage a comeback - a breakout?

"It could," Robinson said, "especially if the person underwent immunosuppression for some reason. And it could if the virus managed to mutate around the immune response that we have achieved. Now our immune response is very broad; it's like multidrug therapy.

"In the present status of our work," she recounted, "we're very actively monitoring our monkeys to watch for any viral comeback. And we're working hard to get our first Phase I initial vaccine safety trials on humans. They're scheduled for early next year. The Phase I studies will go through the HIV vaccine trials network, an academic operation sponsored by the U.S. government.

"Also, here in the lab, we are building a vaccine jointly with the Centers for Disease Control against one of the three HIV viral variants most prevalent in Africa. The cold chain for tropical conditions now has been largely set up. Both DNA and poxvirus are very stable molecules, so this vaccine is going to be an easy one to take into the field." n