Common Protein in Human Semen Increases Infectious Potential of HIV, Study Says

A common protein in human semen increases the infectious potential of HIV 100,000-fold, according to a study published Friday in the journal Cell, the San Francisco Chronicle reports (Russell, San Francisco Chronicle, 12/14).

Researchers at the University of Ulm in Germany began looking for ingredients in human semen that could block infection with HIV-1, the most common strain of the virus. The researchers instead found that the protein prostatic acidic phosphatase, which is produced in the prostate and forms tiny fibers called amyloid fibrils, enhanced HIV transmission (Steenhuysen, Reuters, 12/13).

According to the Chronicle, the discovery could help explain why HIV "appears weakly infectious in laboratory dishes" but "can spread explosively through sexual contact." It takes between 1,000 and 100,000 HIV particles to successfully infect human cells in most lab experiments. However, when PAP was added, as few as three HIV particles can infect human cells, the study found. Although it is unclear why the protein increases HIV transmission, it is possible that the amyloid fibrils attach to cell surfaces, making it easier for HIV to enter cells.

According to the Chronicle, little research has been conducted to determine what role semen plays in the transmission of HIV. Finding a way to eliminate the protein could make HIV transmission more difficult, according to the Chronicle. Warner Greene, director of the Gladstone Institute of Virology and Immunology, has begun a project to determine how to block PAP, but he said that the project will not be easy. In addition, a German pharmaceutical company has begun early studies to determine whether a version of PAP could be tested as a potential HIV drug (San Francisco Chronicle, 12/14). The researchers said that additional studies are needed on the "role of amyloids in the transmission and pathogenesis of enveloped viruses" (Reuters, 12/13).

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Frank Kirchhoff, who led the study, said he was "so surprised" by the finding that at first he "did not believe the numbers." He added that the researchers "did the experiment multiple times, and the results were always the same." Greene called the finding "one of the most interesting new perspectives on HIV transmission to emerge in years."

Anthony Fauci, director of NIH's National Institute for Allergy and Infectious Diseases, said that the study is impressive but that the findings are a long way from producing a practical solution to reduce the risk of HIV transmission. "It is a surprising finding, but I would be cautious about how important this is going to be," he said. Fauci added that sexual contact is only one mode of HIV transmission and that it is unlikely similar proteins exist in breastmilk, which is a source of mother-to-child HIV transmission.

Ian McGowan, a researcher at the University of Pittsburgh and principal investigator with the Microbicide Trials Network, said he is skeptical that the findings would lead to advances in HIV prevention. He added that a new generation of microbicides -- which is made from antiretroviral drugs and has successfully blocked the virus in test tube studies -- soon will enter clinical trials and could be successful in blocking PAP and similar proteins. Jay Levy, a virologist of the University of California-San Francisco, said the finding likely will to lead to further research about the role PAP plays in HIV transmission, adding that studies could be conducted to determine PAP prevalence among populations at high risk of HIV (San Francisco Chronicle, 12/14).

The study is available online.