Semen May Sometimes Protect Against HIV As Well As Transmit It

Claims semen can have health benefits is usually restricted to the sleaziest sort of fake news. However, a paper in a respected peer-reviewed journal has provided evidence that exposure to semen triggers changes that boost resistance to HIV. The research could explain some puzzling aspects of HIV transmission, but definitely shouldn’t be used as an excuse to engage in unsafe sex.



Professor Luis Montaner of the Wistar Institute noted reports that some sex workers in Puerto Rico tested negative to HIV despite saying they did not consistently use condoms. Based on the incidence of HIV in the community, they had almost certainly been exposed to the virus many times.

In 2015, Montaner published evidence that frequent semen exposure triggers changes in the vagina that increase HIV immunity. The idea that the primary medium for the transmission of the virus could also provide partial protection brings to mind the saying “extraordinary claims require extraordinary proof”, and that paper was far from conclusive.

Now, Montaner has co-authored a paper in Nature Communications reporting what happened after female macaques were exposed to semen twice a week. Some of the samples included inactivated particles of the simian immunodeficiency virus (SIV), a relative of HIV used by researchers to model the behavior of the human virus. After 20 weeks, the macaques, along with a control group, were exposed to low vaginal SIV doses.

The semen-exposed monkeys were 42 percent less likely to be infected by the SIV than the controls. They also showed reduced expression of CCR5 receptors on the CD4+ T cells that are HIV and SIV’s prey. The viruses target these receptors to bind to and enter the cells, so fewer receptors could explain the reduced risk of infection. Moreover, semen exposure was associated with increased levels of the HIV-suppressing factor CCL5 in the cervix and vagina. Finally, exposure to semen induced increases in vaginal tissue known to be associated with viral and bacterial fighting capacity.

"While HIV infection has been with us for more than 30 years, this is the first study that describes how semen exposure over time could result in local tissue changes that limit HIV infection in humans," Montaner said in a statement.

Animals, even our fellow primates, are not always reliable guides to how the human body will behave. Nevertheless, the results are consistent with what Montaner previously reported about the immune systems of sex workers who had frequent condomless sex compared to a control group.

A 42 percent reduction is still far below the protection provided by condoms and even the macaques who resisted the low doses succumbed to higher SIV exposure. Consequently, as a method of avoiding HIV, lots of unprotected sex is a non-starter. However, as a pointer to possible pathways for developing drugs to prevent HIV transmission, the work could be invaluable.

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