Kirchhoff and his team screened through many of the 900 proteins found in seminal fluid in their hunt for potential inhibitors and enhancers of HIV transmission. Among the enhancing factors uncovered were fragments of a protein called prostatic acid phosphatase that is secreted by the prostate gland. An analysis of the peptide's structure in semen indicated that it hooked up with similar fragments to create amyloid fibers (clusters of protein fragments that have also been implicated in diseases such as Alzheimer's). The scientists refer to the amyloid fibers as "semen-derived enhancer of virus infection" (SEVI). If they do not link to become fibers, the researchers report, the peptide segments remain inactive and do not enhance viral transmission.
When assembled, however, these fibers then act like ferries, trapping and shuttling HIV virus particles to target cells. The researchers found that HIV spiked into semen was more successful than the virus alone at infecting T cells and macrophages (immune system cells that are believed to be the infection's initial targets in the body). They also tested the threshold of virus needed to infect human tonsil cells, noting that in the presence of semen, far fewer HIV particles were needed for transmission.
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 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.
(Yes, the title of the article is amusing.)
This suggests new directions for research into HIV prevention that focus on men's physiology rather than on women's. I hope this will result in a paradigm shift that does more than tell women to "just say no" to sex and shares the burden of HIV transmission more equitably between men and women. Oddly enough, this is reminiscent of the potential paradigm shift that must have happened when it was discovered that men, and not women, were responsible for the sex of their offspring (rendering all those executions of wives who "failed" to bear sons completely erroneous). Or, even better, we will be able to leave the moralistic "blame game" behind completely and focus on what actually works: access to condoms and medically accurate sex education and actual legal and societal empowerment of women to enable them to be proactive actors rather than passive defenders regarding their health.