NAIROBI (Reuters) - Agnes Munyiva has been a prostitute for 31 years in Kenya's Majengo red light district, and like many in the trade has had her share of unprotected sex.
Despite long exposure to HIV-positive men she is HIV negative. So are some other women working the seedy Majengo haunts.
That fact draws a very different sort of visitor to Majengo than Agnes's lonely regulars: AIDS researchers.
Some of the scientists suspect that if they can discover why a group of prostitutes has remained HIV-negative for so long, they may find the key to the development of an AIDS vaccine.
For more than 15 years, researchers have worked with the Majengo prostitutes to do just that. Opinions vary about the significance of the cluster of HIV-negative Majengo prostitutes.
Joshua Kimani, director of Kenya's AIDS Control Programme, believes that solving the Majengo riddle could be worthwhile.
"There is something in them that is protecting them and that is the information we are using to create the vaccine," he says.
The human body normally produces antibodies once it acquires the HIV virus, but, Kimani argues, due to genetic differences and factors involving repeated exposure to the virus the Majengo prostitutes react differently, developing cells called Cytotoxic T-Lymphocytes (CTLs) that kill the virus.
"The more they get exposed the higher the number of CTLs they get. They have to be constantly exposed to HIV to produce CTLs," Kimani told Reuters.
Professor Andrew McMichael, director of Britain's Weatherall Institute of Molecular Medicine, disagrees.
He said only about five percent of the Majengo prostitutes are resistant to HIV, reflecting similar patterns in other high-risk groups such as the partners of HIV infected people.
"There's always a few who don't become infected. There's nothing unique about them (Majengo women). It's just a concentrated group, there's more of them in one place," he told Reuters by telephone from Oxford.
REMAINING HIV-NEGATIVE
As she gets older, Munyiva finds it increasingly difficult to find customers. But there are the occasional regulars -- about three of them -- who take care of expenses like rent or school fees for her two daughters.
"Things were good back then, I could get about 10 people daily. If I get five today I'll be very lucky," she lamented.
To supplement her income, Munyiva sells potato crisps to school children. As she walks by, one young man asks: "Are you off (duty) today?" She answers in the negative.
Doctors have told her and other women who have managed to remain HIV-negative that paradoxically they must continually have sex with HIV-positive men to keep their status.
"Munyiva and her company have three options if they want to remain negative -- either to stop their trade and stay away from sex, or have keep one partner who is AIDS free or keep working as a prostitute," Kimani said.
Over the years, a clinic at Majengo run by government researchers has recorded data on 2,200 prostitutes. At any one time, there have been some 60 women considered immune to HIV.
The clinic provides free treatment for patients with sexually transmitted diseases and offers anti-retroviral drugs to 16 AIDS-hit prostitutes.
Researchers believe there are more women in Majengo working as prostitutes but many are too shy to walk through its gates because of the stigma attached to HIV/AIDS.
Munyiva says she has watched many friends sicken and die of AIDS. God, she says, has kept her safe, but Kenyan researchers argue that the immunity is genetic and runs in some families.
"It has nothing to do with your tribe or the food you eat, it is actually genetic. It depends on the host. Genes give you an edge over the others," Kimani said.
McMichael disagrees: "It's not a single gene causing resistance. We hypothesise it's the immune response that's protecting them. To get evidence, we have to find a vaccine that stimulates that kind of response and see if it protects people."
Other possible explanations point to the HIV infection triggering an immune response, without the virus becoming established in the body, McMichael said.
TESTING
A possible vaccine developed by Majengo researchers has passed a "phase one" test where it was checked for safety, although it still has to pass tests for long-term side effects.
Doctors are currently carrying out phase two to find out whether HIV negative people on the proposed vaccine produce CTLs instead of antibodies when exposed to AIDS.
The researchers say despite all the information available about HIV/AIDS, people are still having unprotected sex and importing different subtypes from other areas in Africa.
"It is a time bomb," Kimani said. "The virus is always a step ahead of us and a step ahead of the body because it mutates."
A hyena cools itself in mucky waters on the drying shores of Lake Nakuru in Kenya's Rift Valley, 160km (99 miles) west of the capital Nairobi, December 18, 2009. World leaders ...
