Clues to How Men Exposed to HIV Stay Virus-Free

According to researchers, men who have sex with HIV-positive women but remain virus-free carry relatively high levels of antibodies that specifically fight HIV infection in the tissue that first encounters the virus. These antibodies may help protect people exposed to the virus from becoming infected.

Study author Dr. Mario Clerici of the University of Milan in Italy explained that people become infected with HIV during intercourse when the virus binds to proteins in genital tissue, allowing HIV to penetrate mucosal cells, after which it eventually spreads in the body. But in a small number of people exposed to the virus, this process does not occur, Clerici said.

The study "Mucosal and Systemic HIV-1-Specific Immunity in HIV-1-Exposed but Uninfected Heterosexual Men," published in the journal AIDS (2003;17:531-539), compared 14 HIV-negative men whose female partner was HIV-positive with seven men infected with the virus and seven men without any known risk factor for HIV infection. All of the virus-free men had been having unprotected sex with an HIV-positive women for at least four years. On average, the couples said they had unprotected sex 14 times per year, the most recent time being within the four months before they enrolled in the study.

The authors found that 11 of these 14 men carried relatively high levels of the antibody known as IgA that specifically targets HIV in their seminal fluid. These "good" HIV-targeted antibodies were not present in the seminal fluid of the men who were at low risk of HIV exposure, according to Clerici. In addition, the concentration of these antibodies tended to be highest in men who had recently had unprotected sex with their infected female partner. Scientists have shown that IgA helps protect the body against HIV by changing its shape and barring its entry into target cells.

Previous studies have also detected the presence of IgA targeted to HIV in secretions from commercial sex workers in Africa and Thailand, who have a high risk of having been exposed to HIV. Clerici said studies underway in mice and monkeys are seeking to reproduce such natural mechanisms for an HIV vaccine.

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