Bethesda, Maryland - Scientists believe they know why a small group of Aids patients show no ill effects from the disease - in stark contrast to most sufferers.
Those who are apparently unaffected - a rare group - are called long-term nonprogressors.
Most people who have Aids progress to full blown AIDS within a few years. Drugs can hold back the illness, but only for a certain time.
But some people are known to have carried the virus for as long as 20 years without treatment and with no apparent ill effects.
Scientists now find that the key lies within immune system white blood cells which fight HIV and which are known as CD8+ T cells.
Scientists had thought that people who could not control HIV had too few CD8+ T cells. However, this new study suggests the difference is not the number but the quality of these cells: both nonprogressors and others have about the same number of HIV-fighting CD8+ T cells, but the cells of nonprogressors function better.
The findings come from
"Understanding the mechanisms by which the immune systems of long-term nonprogressors control HIV is important to our development of effective vaccines," says NIAID director Anthony S Fauci.
"Studies like this one, which reveal basic knowledge about how the immune system interacts with HIV, form the foundation of our effort to fight this disease."
Instead of attacking HIV directly, CD8+ cells inhibit virus spread by killing off other immune system cells infected with HIV.
"For some time we have known that even patients who cannot control HIV maintain high numbers of HIV-specific CD8+ T cells," says the report's senior author, Mark Connors, MD, of NIAID's Laboratory of Immunoregulation.
But this study represents the first time scientists have observed a difference in the HIV-specific CD8+ T-cell response of nonprogressors, he says. This study also suggests a mechanism whereby the CD8+ T cells of nonprogressors control HIV and those of most individuals do not.
Connors and his colleagues closely examined the immune systems of 40 people infected with HIV, including about 15 nonprogressors - people who have controlled HIV for up to 20 years without antiretroviral therapy.
They found no significant difference in the number of HIV- fighting CD8+ cells between nonprogressors and the others. Instead, they found that the nonprogressors' cells were better able to divide and proliferate when called on to go into action. They also produced higher levels of a molecule called perforin, which helps them to kill off cells infected with HIV.
Connors and colleagues next plan to analyze an even broader array of differences between the CD8+ T cells of nonprogressors and others infected with HIV, seeking to understand what causes the poor function of most HIV-infected people's CD8+ T cells. - Sapa-DP