Several recent studies have found that people who are infected with the recently discovered bug, called GB virus C, are substantially less likely than usual to die from AIDS. Experts assumed that GBV-C somehow interferes with HIV, but just how this protection works has been a mystery.
Now experts think they have the answer: It thwarts HIV's ability to infect cells by wiping away one of the chemical docking posts that HIV needs to make its entry.
"There is not a way for the virus to get into the cells. The doors are gone," said Carolyn Williams of the National Institute of Allergy and Infectious Diseases.
Whether this is more than a medical curiosity, however, is still unclear. Some researchers say they would be reluctant to intentionally give GBV-C to people with HIV, because they fear the approach could backfire, ultimately doing more harm than good.
However, others say experiments to give it a try are possible, perhaps on patients whose HIV resists all standard drugs. "I think it may well be tested in a controlled setting," said Dr. Jack Stapleton of the University of Iowa.
Stapleton presented the latest discovery Thursday in Boston at the 10th Conference on Retroviruses.
HIV is harmless unless it gets inside blood cells. Once it enters a cell, it commandeers the cells' own machinery to copy itself, killing the cell in the process.
The AIDS virus uses three different chemical spots as attachment points to enter cells. Stapleton's team found that if the body is already infected with GBV-C, it makes fewer copies of one of these chemicals, called CCR5. The result: HIV cannot get in as readily as usual.
GBV-C was discovered in 1995. It was at first mistakenly thought to be a hepatitis virus and called hepatitis G. Now, however, scientists say that, while the virus is closely related to hepatitis C, it causes no disease they can find.
Like HIV, GBV-C is transmitted mainly through blood contact or sex. Because the virus is harmless, blood donations are not routinely screened for it. However, studies show that 1 percent to 2 percent of blood donors have active infections, as do about 40 percent of all people with HIV.
Research suggests that the virus has been around as long as people have. "The virus was present in Lucy and evolved in humans as the races evolved," Stapleton said.
A study by Williams followed 271 men who caught HIV in 1985. Those who also had GBV-C fared considerably better than usual, while men who lost the virus after getting HIV did substantially worse.
After 11 years, 75 percent of those with GBV-C were still alive, compared with 39 percent who never had the virus and 16 percent of those who had the virus but then lost it.
Her study looked at GBV-C's effects before the advent new drug combinations that have greatly improved the outlook of people with AIDS. Nevertheless, a variety of other research suggests that people with GBV-C respond better to these drugs.
Among other research at the meeting:
Doctors worry that AIDS drugs may damage the heart by causing arteries to harden. Researchers from Hvidovre University Hospital in Denmark studied 23,490 patients and found that AIDS drug treatment appears to raise the risk of heart attacks by about 27 percent a year.
People with HIV are routinely denied liver transplants because of concern that immune-suppressing drugs would aggravate their infections. However, a study from the University of Pittsburgh found that HIV patients' survival after three years is just as good as uninfected people's.
The cost of AIDS drugs has fallen to about $300 a year in poor countries, but the testing needed to monitor the drugs can cost as much as $1,000. Researchers from Harvard Medical School described a battery-operated, handheld machine that can do blood counts in remote areas. The device will cost about $600 and do the tests for about $1 apiece, compared with the usual cost of $25.