Researchers in Germany have discovered that a protein found in semen makes HIV 100,000 times more virulent than it is alone - thus helping to explain why more than 80 percent of human-immunodeficiency-virus (HIV) infections are transmitted via sexual intercourse.

The team of German scientists had initially set out to determine whether semen contained factors that inhibit the HIV infection, according to the report published in the journal Cell.

But surprisingly, the HIV/AIDS researchers in Hanover and Ulm, Germany, found that fragments of prostatic acidic phosphates isolated from human semen form tiny fibers known as amyloid fibrils, which they call Semen-derived Enhancer of Virus Infection or SEVI. 

Those fibrils capture HIV particles and help them to penetrate target cells, thereby increasing the infection rate by up to several orders of magnitude.

"We were not expecting to find an enhancer, and we were even more surprised about the strength," says report co-author Frank Kirchhoff, a virologist at the University of Ulm Clinic in Germany.

"Most enhancers have maybe a two or three-fold effect, but here the effect was amazing. More than 50-fold and, under certain conditions, more than 100,000-fold," Kirchhoff says.

Wolf-Georg Forssmann of VIRO PharmaCeuticals GmbH & Co KG and Hanover Medical School says the fibrils act like a ferry, picking up the viruses and then bringing them to the cell.

Researchers injected both the naked virus and SEVI-treated HIV into the tails of rats that had been given human immune system cells.

The HIV with the semen component was five times more effective at transmitting the virus.

In situations where low levels of the virus are transferred - as during intercourse - Kirchhoff says, SEVI can make HIV up to 100,000 times more likely to spread when compared with the virus alone. 

In an editorial accompanying the article, postdoctoral fellow Nadia Roan, along with Warner Greene, a senior investigator at the University of California, San Francisco's Gladstone Institute of Virology and Immunology, writes:

"If SEVI truly increases the real-world heterosexual spread of HIV by several orders of magnitude, then negating the activity of this factor could conceivably diminish these frequencies to levels that might virtually eliminate semen-driven HIV transmission."

HIV, the virus that leads to AIDS, has infected 60 million people worldwide (causing 25 million deaths) since it was discovered in humans in 1981.

The transmission rate from intra-vaginal sexual intercourse is estimated at one in every 200 to 2,000 acts. In Africa, 60 percent of new infections are in women who have had sex with HIV-positive men.