Study : Semen Exposure Increases HIV Resistance In Women

HIV is found in the semen of infected individuals, so naturally you’d assume that the more exposure a person has through sex, the more likely they are to become infected. As always, it seems that things aren’t that simple, at least for women.

According to an intriguing new study, continued genital exposure to this bodily fluid leads to changes in the environment of the vagina that could actually increase resistance to HIV. But it’s imperative these findings are not wrongly interpreted as a free pass to forgo the use of condoms: These remain one of the most effective measures to reduce the risk of transmission, not just of HIV but of STDs in general. And this doesn’t mean that women who have lots of unprotected sex won’t get HIV; the risk may be reduced, but it’s not eliminated.

Regardless, the results are helping scientists understand some seemingly paradoxical observations in the field. Sex remains the main route of HIV transmission worldwide, for both men and women, thus sex workers are inherently at a high risk of contracting the virus. Yet in certain areas of the world with a high prevalence of HIV, some female sex workers continue to test negative for the virus, despite being in the industry for prolonged periods and rarely using condoms.

This led scientists to believe that these women might have developed some kind of mechanism of resistance to the virus, whether intrinsic or adaptive, yet what this could be, and what drives these changes, has eluded scientists. That said, studies have suggested that semen can trigger acute immune system responses, perhaps leading to changes in the vagina’s microenvironment that might actually be beneficial in terms of resisting infection.

To examine this possibility, researchers from the Wistar Institute focused on a group of sex workers in San Juan, Puerto Rico. Participants had all been in the industry for at least three years and all tested negative for HIV and other STDs; controls they were compared with were recruited from the same area, and self-reported low exposure to semen.

Reported in the journal Mucosal Immunity, the researchers made three key observations that they believe could be contributing to the apparent reduced risk of infection among sex workers who report low condom use. Perhaps most notably, the immune systems of the sex workers appeared to show lower levels of activation. You might think that an active immune response would be beneficial in terms of fighting off infection, but actually HIV needs to be inside activated cells in order to replicate, so this find is significant.

The team also discovered that cells lining the cervix showed alterations in gene expression that could confer resistance. For example, they noted a reduction in the expression of genes that have been implicated in key stages of HIV’s life cycle, like when it slips its genome into our own, a prerequisite for replication. Interestingly, the researchers also discovered a positive correlation between semen exposure and ramped up levels of an immune molecule called type 1 interferon, a crucial player in antiviral responses. Taken together, the team concludes these results suggest that exposure to semen can lead to changes in the vagina that may contribute to a reduced susceptibility, not an eliminated risk, to HIV infection.

"It is important to note that the study does not make a case for sexual intercourse without a condom, as doing so will increase the overall risk of HIV infection and other sexually transmitted diseases," lead author Luis J. Montaner said. "Instead, this study identifies unexpected effects that long-term semen exposure may have on the cervix and vagina that may lower but not remove the likelihood of infection."

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