Repeated exposure to semen can alter vaginal tissue immune cells that are prime targets for HIV, reducing chances of infection, according to new research out of Philadelphia’s Wistar Institute and the University of Puerto Rico.
The sprawling list of co-authors on the ambitious study — which was supported by the National Institutes of Health and other top AIDS research agencies — also hail from the University of Nebraska, Duke University, Mount Holyoke College, Ulm University, the University of Pennsylvania and Tulane University.
The findings, published Tuesday in the journal Nature Communications, could explain why a small number of female sex workers worldwide continue to test negative despite continuous high-risk intercourse.
Previous research published in 2015 revealed that semen exposure resulted in changes in prostitutes’ cervicovaginal tissue, suggesting increased resistance to HIV infection.
The new study, however, directly investigated if semen is indeed a factor in resistance.
“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,” Wistar scientist Luis J. Montaner, lead author of the study, told Scienmag. “Apart from defining a new factor that may regulate HIV transmission, this unexpected finding could directly impact the design of future HIV vaccine studies that commonly recruit female sex workers.”
Montanter added, “Currently, condomless sex is assumed to only promote the likelihood of infection. Our observation, however, raises the hypothesis that frequent semen exposure may potentially reduce HIV transmission.”
Edmundo N. Kraiselburd, a professor at the University of Puerto Rico, co-directed the pre-clinical research project and supervised the use of macaque primates from the Caribbean Primate Research Center to test prophylactic anti-HIV interventions.
“This research clearly shows the valuable information the macaque model can provide when used to study what may determine HIV infections in humans,” said Kraiselburd.
The primates were exposed to semen twice a week over five months with or without inactivated particles of the simian immunodeficiency virus (SIV is an HIV-like virus that infects primates, causing a disease similar to AIDS). After this “conditioning period,” they received low doses of intravaginal SIV.
The semen-exposed monkeys showed a 42 percent decrease in the risk of infection.
The repeated semen exposure resulted in elevated cervicovaginal tissue levels of antivirals such as MX1, which also positively correlated with levels of what’s called IFN-epsilon. IFN-epsilon, which can be induced by semen and protects human cells from bacterial and viral pathogens, has “direct anti-HIV properties and was described to be induced in tissues from sex workers in association with condomless sex.”
But whether this is good news for the 17.4 million women living with HIV — 51 percent of the global total, according to the World Health Organization — is still very unclear.
Primates that remained uninfected after exposure to low viral amounts became infected when later challenged with high virus doses.
The scientists’ bottom line: Repeated semen exposure provides partial protection — but does not block HIV infection.
“Importantly, we show that semen exposure can promote host resistance but does not protect against infection,” said Montaner. “Therefore, our data do not change the fact that prevention methods, such as condom use and PrEP (pre-exposure prophylaxis) remain our best strategies to prevent infection.”