Contrary to the long-held view that semen can only act as a way to transmit HIV-1 from men to women, scientists at The Wistar Institute and the University of Puerto Rico found that frequent and sustained semen exposure can change the characteristics of the circulating and vaginal tissue immune cells that are targets for infection, reducing the susceptibility to a future infection.
This finding, published in the journal Nature Communications, also provides a potential explanation as to why a small number of female sex workers worldwide continue to test negative for infection despite continuous high-risk sexual activity.
Research previously reported by the laboratory of Luis J. Montaner, D.V.M., D.Phil., the Herbert Kean, M.D., Family Professor and director of the HIV-1 Immunopathogenesis Laboratory at Wistar’s Vaccine & Immunotherapy Center, together with investigators at the University of Puerto Rico, showed in a 2015 paper how continued semen exposure in female sex workers resulted in changes in the cervicovaginal tissue that predicted an increased resistance to HIV infection.
The current study directly addressed if semen could be 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,” said Montaner, who is the lead author of the new study.
“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.
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, Ph.D., professor at the University of Puerto Rico (UPR), co-directed this research project and supervised the use of non-human primates (NHP) from the Caribbean Primate Research Center. NHPs are a principal pre-clinical research model used 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.
In the study, animals were exposed to semen twice a week over 20 weeks with or without inactivated particles of the simian immunodeficiency virus (SIV is an HIV-like virus that infects primates and causes a disease similar to AIDS); after this conditioning period, they received low-dose intravaginal SIV challenges.
Semen-exposed animals showed a 42% decrease in the risk of infection.
Scientists analyzed specific markers of immune activation in the cervicovaginal microenvironment and in the bloodstream. On circulating CD4+ cells, semen conditioning was associated with lower expression of the CCR5 receptor, which acts as a binding site for HIV to enter its host cells, supporting the observation of a lower susceptibility to SIV vaginal challenge.
Furthermore, semen-conditioned animals had higher levels of the CCL5 cytokine, a natural HIV-suppressive factor, in the cervicovaginal compartment in response to SIV challenge.
Additionally, repeated semen exposure resulted in elevated cervicovaginal tissue levels of antiviral factors such as MX1, which also positively correlated with levels of 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.
Of note, semen-treated animals that remained uninfected after exposure to low viral amount became infected when subsequently challenged with high doses of virus, confirming that they were still susceptible to infection and that repeated semen exposure provides only partial protection and 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.”
Women comprise more than 50% of HIV-1 infected individuals, with heterosexual transmission representing the leading route for infection. Understanding the factors that contribute to HIV-1 transmission in the cervicovaginal compartment is essential for the development and improvement of HIV vaccines and/or pre-exposure prophylaxis approaches.
Several factors have been associated with higher rates of HIV infection in women. These include pre-existing inflammation, as well as repeated exposure via commercial sex work.
However, studies in high HIV prevalence areas have also identified female sex workers (FSW) that remain seronegative despite long-term sexual work and low condom use (highly-exposed seronegative, HESN).
These observations led to the hypothesis that these women may have developed intrinsic or adaptive mechanism(s) of resistance .
Cell-mediated adaptive responses in blood 4 and mucosal anti-HIV IgA5 titers are frequently absent or weak in HESN FSW, and in one study did not exclude subsequent seroconversion6.
Thus, adaptive anti-HIV-1-specific responses are unlikely to be solely responsible for maintaining an HESN status.
Recent studies, largely supported by data in peripheral blood or cervicovaginal secretions, suggest that HESN FSW may sustain a state of immune quiescence or reduced immune activation that can counter infectivity in spite of repeated acute HIV-1/semen exposures.
Importantly, it has remained unknown to what extent semen versus other factors (e.g., repeated HIV-1 exposure) may contribute to these observed changes of the cervical tissue microenvironment and in systemic immune modulation.
Additional mechanisms proposed to maintain HESN status include genetic polymorphism , increased peripheral T-regulatory cell frequency , and an increase in anti-proteases within the cervico-vaginal mucosa.
Low condom use consistently exposes HESN FSWs to semen and HIV-1. Human semen, more than an inert vehicle for HIV-1 virions within the female reproductive tract, has been shown to be immunologically pleiotropic.
Studies have shown that semen can acutely upregulate inflammatory cytokines and chemokines (e.g.: IL-6, IL-8, CCL20 and CXCL3) in cervico-vaginal tissue-derived epithelial cell lines and induce infiltration of immune effectors into cervico-vaginal tissue shortly after coitus .
However, semen also includes factors able to mediate a tolerogenic Th2 profile (e.g., T-Regulatory Cell induction) driven by high-levels of IL-10, TGF-β, and Prostaglandin E2 in semen, which has been hypothesized to have an important role during fertilization .
Aside from effects of repeated signaling by IL-10 or PGE2 on myeloid and T-cell in cervix, the presence of Th2-polarized CD4s alone would not exclude infection as both Th1 and Th2 CD4 cells are susceptible to HIV-1 .
However, although seminal fluid-derived amyloid fibrils have been shown to have variable results in affecting HIV-1 infectivity in vitro no marked in vivo effects were noted when tested in non-human primates (NHP) for acute effects on SIV infection.
Independently of semen, work in non-human primate models have shown that acute exposure to high-titer SIV can also induce recruitment of CD4 T-cells, plasmacytoid dendritic cells and macrophages into the cervical and vaginal epithelium which if followed by infection can result in a greater depletion of CD4 T-cells.
In spite of the potential acute effects of semen and/or viral particle exposure, analysis of ectocervical tissue from HESNs has shown a steady-state of reduced rather than increased inflammation , suggesting that the tissue microenvironment alterations resulting from chronic semen or viral particle exposure (e.g.: from long-term sex work) may differ from those associated with acute semen/viral- exposure.
In order to assess the contribution of sex work in groups with low HIV exposure incidence, we hypothesized that in women with high levels of genital exposure to semen both localized tissue (i.e.: cervicovaginal mucosa) and/or systemic immune activation changes will contribute to a lower susceptibility for HIV-1 infection.
Furthermore, as no report outside Africa has yet described blood-based and cervical tissue changes in long-term sex worker cohorts, it also remains unknown if reported HESN or long-term sex work protective mechanisms, such as immune quiescence, are attributable to differential genetics or regional sex worker practices.
To address our stated hypothesis and reproducibility of mechanisms associated with long-term sex work, we analyzed a cohort of seronegative long-term FSW (and a comparator group of women with low semen exposure, as determined by self-reported frequency of unprotected intercourse) recruited from a geographical distinct region, San Juan, Puerto Rico. HIV-1 prevalence amongst FSW in San Juan has been previously reported at 4.8%, which is 5–15 fold lower than in sub-Saharan Africa.
Although Puerto Rican FSW cannot be directly compared to HESN-FSW African cohorts, if semen (and not HIV exposure itself) is responsible for the mucosal changes observed in HESN-FSW in Africa, we would expect that FSW with no HIV-specific immune responses but high levels of condomless sex (semen exposure) will have comparable peripheral and mucosal changes as described in HESN FSW.
More information: Repeated semen exposure decreases cervicovaginal SIVmac251 infection in rhesus macaques, Nature Communications(2019).
Journal information: Nature Communications
Provided by The Wistar Institute
