March 8, 2001 -- An experimental vaccine appears to keep immunized monkeys from developing AIDS even after they have become infected with a particularly aggressive strain of the virus that causes the disease. The discovery, reported in the March 8 issue of the journal Science, holds promise for a human version of the vaccine that is currently in development.
When 24 rhesus macaque monkeys were given the vaccine and then received a dose of the monkey version of the virus seven months later, all became infected with the virus but remained free of the disease. In contrast, three of four unvaccinated monkeys that received the virus died of AIDS. The vaccinated monkeys have now been followed for more than two years and remain healthy, senior researcher Harriet L. Robinson, PhD tells WebMD.
"This is not just a proof of principle: We think it's something that could be used [in humans]. The key thing now is that we have to make sure that the components work as well in humans as they do in monkeys," says Robinson, chief of microbiology and immunology at the Yerkes Primate Center and professor of microbiology and immunology at Emory University School of Medicine in Atlanta.
The vaccine, delivered in three doses spread over 24 weeks, combines two methods for training the immune system to recognize the virus as a dangerous foreigner and erect defensive barriers against it.
In the study, the monkeys received two doses each of a "primer" containing bits of the virus that put the immune system on alert, in the way that a police officer going out on patrol memorizes a photo of a wanted criminal last seen in the neighborhood. The third dose of the vaccine consisted of a booster made out of a modified virus that was previously used as part of smallpox vaccines. The booster vaccine served to heighten the response of the immune system or, in other words, to increase the number of cops looking for the wanted criminal.
In the study, 24 monkeys were given either a high or low dose of the primer and the booster. Seven months after the last injection, their immune systems were challenged with a dose of a virus similar to HIV, the virus that causes AIDS in humans. The virus was delivered to a mucous membrane in the rectum of the monkeys (mucous membranes are the most common route of infection with HIV in humans). Four additional monkeys that did not receive the vaccine served as a comparison group.
The vaccinated monkeys and the unvaccinated monkeys all became infected with the virus, but the vaccinated animals remained healthy. Their immune systems continued to produce a large number of disease-fighting T-cells, and the number of copies of the virus in their blood quickly began to dwindle -- both signs that their bodies were successfully fighting off the infection.
The unvaccinated monkeys, however, had severe depletion of their T-cells and had evidence in their blood that the virus was reproducing itself with abandon. The unvaccinated monkeys developed multiple life-threatening infections in a disease course mimicking that of advanced AIDS in humans.
"This is as exciting an animal protection result as we've seen, due to the fact that they're protecting against [T-cell] loss, and virus loads are going down to undetectable," says James Bradac, PhD, chief of the preclinical research and development branch, division of AIDS vaccine and research prevention program, at the National Institute of Allergy and Infectious Diseases in Bethesda, Md.
Bradac, who was not involved in the study, tells WebMD that "for the human experience, we've got to see what happens when you hold these animals [for more time], and a key is to see whether this type of situation will prevent transmission to others. You don't want to merely keep someone alive a few years longer; we've got to have higher goals to prevent the spread of the epidemic."
Peggy Johnston, PhD, assistant director for AIDS/HIV vaccines at the National Institute of Allergy and Infectious Diseases, tells WebMD that the vaccine "may even be better in humans, because these animals are exposed to a very high amount of virus to ensure that all the [unvaccinated monkeys] get infected, and that is probably much larger than an average human exposure [to HIV], so it could even be better -- we don't know."
Robinson and researchers from the National Institute of Allergy and Infectious Diseases are currently collaborating. They are working on developing and testing the vaccine in early human trials, pending approval from the institute, which funded the current study.