A new study has found that a gene found only in people of African ancestry which evolved to prevent malaria infection may actually increase their odds of contracting AIDS by up to 40 percent.
The gene does, however, seem to protect against the progression of the disease, allowing those carrying it to live about two years longer.
Around 90 percent of people in Africa carry this genetic variant and it may be responsible for 11 percent of the infections there, the study published Wednesday in Cell Host & Microbe found.
"Now, with another global pandemic on the scene, this same variant renders people more susceptible to HIV."
About 68 percent of people infected with HIV live in Sub-Saharan Africa, according to the United Nations.
The US and British researchers who authored the study said sexual behavior and other social factors do not fully explain large discrepancies in HIV prevalence.
"It's well-known that individuals vary in their susceptibility to HIV and that after infection occurs, the disease progresses at variable rates," said co-author Sunil Ahuja of the South Texas Veterans Health Care System.
"The mystery of variable infection and progression was originally thought to be mainly the result of viral characteristics, but in recent years it has become evident that there is a strong host genetic component."
The gene in question, the Duffy Antigen Receptor for Chemokines (DARC), encodes a protein found mainly at the surface of red blood cells.
Earlier studies have shown that HIV can bind to red blood cells through this receptor. The receptor has also been found to bind a wide array of inflammatory molecules, including one which is highly effective in suppressing replication of HIV.
The researchers studied nearly 3,500 people in the US Air Force, including more than 1,200 who are HIV positive, who have been followed for nearly 22 years.
This allowed them to rule out difference in economic status, access to health care and other factors which would generally confound a genetic effect. They found that the prevalence of the variant in African Americans was greater amongst those with HIV than in those without.
Researchers in Canada, meanwhile, have isolated two genes which may prevent people from contracting HIV or at least slow the rate at which they develop AIDS, a second study published Wednesday.
The genes were isolated by comparing the genetic profiles of people in their first year of HIV infection with those who managed to resist infection despite repeated exposure to the virus.
The "good" versions of the two genes were present in 12.2 percent of those who resisted infection compared with only 2.7 of patients in primary HIV infection.
Researchers are not yet sure how this protection works.
One of the gene codes for a receptor on the surface of the immune system's natural killer cells which destroy infected cells in the body.
The other codes for a protein which binds the first gene and dampens the natural killer cell activity.
The most likely explanation is that HIV prevents the protein that dampens the killer cell activity from being expressed, allowing the killer cells to destroy cells infected with HIV.
Since this can happen very soon after the initial infection, people carrying those genes may be able to more efficiently destroy infected cells and lower their chances of developing AIDS.
"More research is needed to determine the exact mechanism behind the protection we have observed, but these findings have revealed a promising avenue," said co-author Nicole Bernard of the Research Institute of the McGill University Health Centre.
"In the future, our findings could be used to somehow 'boost' the innate immune system and thus fight the virus as soon as it enters the body."
The study was published Wednesday in the journal AIDS.