By Agencies via Xinhua

　　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 asgroupsof 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," said Kimani.

　　Professor Andrew McMichael, director of Britain's Weatherall Institute of Molecular Medicine, disagrees.

　　He said only about 5 per cent 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 said from Oxford.

　　Remaining safe

　　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.

　　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 one partner who is AIDS free or keep working as a prostitute," Kimani said.

　　Other possible explanations point to the HIV infection triggering an immune response, without the virus becoming established in the body, McMichael said.

　　A possible vaccine developed by Majengo researchers has passed a "phase one" testswheresit 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.