CHICAGO, Dec 13 (Reuters) - An ingredient in human semen may actually help the HIV virus infect cells, German researchers said on Thursday.

They said naturally occurring prostatic acidic phosphatase or PAP, an enzyme produced by the prostate, can form tiny fibers called amyloid fibrils that can capture bits of the human immunodeficiency virus and usher it into cells.

Researchers at the University Clinic of Ulm were looking for factors in semen that might block infection with HIV-1, the most common strain of the virus that causes AIDS.

Instead, they found one that enhanced transmission, in some experiments as much as 50-fold.

"In this study we show that fragments of PAP, a highly abundant semen marker, form amyloid fibrils that drastically enhance the infectiousness of HIV-1," they wrote in the journal Cell.

They said they are now looking for compounds that might block this process to slow transmission of the disease.

Amyloid fibrils formed from different proteins are associated with many diseases, including Alzheimer's and prion disease.

"Further studies on the role of amyloids in the transmission and pathogenesis of enveloped viruses are highly warranted," they wrote.

HIV has infected some 60 million people, killing more than 20 million, the researchers said. More than 90 percent of those infections are acquired through sexual intercourse. (Reporting by Julie Steenhuysen; editing by Todd Eastham)

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