Two strides toward a workable AIDS vaccine.

In a development that they term "the most impressive... we have seen in any of our vaccine experiments:' five AIDS researchers report they have used a vaccine made of crippled, but live virus to completely protect a group of monkeys from the simian form of AIDS. And in a second, separate finding with implications for the AIDS vaccine search, a research team has determined that AIDS viruses pick up bits of the cells they infect, perhaps as a means of improving their ability to latch onto and invade new cells.

The first discovery, by Ronald C. Desrosiers and four of his colleagues at the New England Regional Primate Research Center in Southborough, Mass., paves the way for the first human tests of an AIDS vaccine made of live human immunodeficiency virus (HIV), the virus that causes AIDS. For safety reasons, the only AIDS vaccines that have so far entered clinical trials consist of killed HIV, pieces of HIV, or other viruses with HIV genes.

Desrosiers and his co-workers hobbled a strain of simian immunodeficiency virus (SIV) by removing a gene called nef, which is thought to regulate the virus' ability to reproduce. They found that a single immunization with the live, hobbled virus allowed each of four rhesus monkeys to fend off infection following repeated injections of enough virulent SIV to infect 10 animals -even though the vaccine was administered more than two years previously In contrast, Desrosier's group reports in the Dec. 18 SCIENCE, a control group of four monkeys that did not receive the immunization succumbed to the SIV infection and died.

The Massachusetts researchers say their results suggest that a similar strategy involving live HIV "may also be the most potent, effective vaccine for the prevention of AIDS" in humans. "If other vaccine approaches [now in clinical trials] indeed show little or no efficacy under field conditions, limited safety testing of live [disabled] HIV-I in high-risk human volunteers seems warranted," they conclude.

Larry O. Arthur, director of the AIDS vaccine program at the National Cancer Institute's Frederick Cancer Research and Development Center in Frederick, Md., says Desrosier's team's results "move AIDS vaccine work a major step forward." However, he cautions, "it's going to be very difficult to show that a vaccine like that is safe enough for human volunteers."

A team led by Arthur reports the second AIDS vaccine development, which also appears in the Dec. 18 SCIENCE. Arthur and his colleagues found on the surfaces of HIV particles clusters of molecules that human immune system cells normally use to communicate with one another.

They suggest their finding might explain a puzzling result reported last year: Monkeys inoculated with human cells that had never been infected with SlV nonetheless resisted infection by the virus (SN: 11/23/91, p. 328). At the time, some researchers thought the human cells had caused the monkeys' immune systems to make antibodies that could attack SIV as well as the foreign human cells. However, other researchers subsequently conducted experiments that ruled out such a cross-reaction scenario (SN: 2/1/92, p. 71).

Arthur and his colleagues now suggest that the human cells might have protected the unvaccinated monkeys from later infection with SIV because both the human cells and the SIV particles bore proteins called the major histocompatibility complex (MHC) on their surfaces. The researchers found that both SIV and HIV contain more MHC proteins than they could have been expected to pick up at random as they budded off infected cells. Instead, Arthur says, "it appears the viruses may selectively concentrate these cellular proteins:' perhaps to enable them to better stick to and infect other cells. To test the idea, he plans to administer human MHC proteins to monkeys to see if they protect the animals against SIV grown in human cells.