Exosomes are cell-derived vesicles that circulate in bio-fluids and enclose cell-associated cargo, producing various consequences in intercellular communication and may contribute to microbial pathogenesis. Exosomes are similar in composition to enveloped viruses, making differentiation between exosomes and enveloped viruses difficult. Yet, exosomes and enveloped viruses may vary considerably in function. Exosomes produced from infected cells may incorporate viral material as they are simultaneously produced with viruses and depending upon the cargo, contribute to disease pathogenesis. Conversely, exosomes from uninfected cells may contribute to protection from infection. Exosomes from breast milk, vaginal fluid, and semen of healthy donors protect against HIV-1. The functional dichotomy of exosomes is unknown. Here, we focus on the function and physical qualities of exosomes found in semen (SE) and how these influence HIV-1. As semen is the major body fluid involved in HIV-1 transmission, exosomes from semen that regulate HIV-1 may contribute to the low incidence of HIV-1 sexual transmission in vivo.
Previous studies indicate healthy donor derived SE but not blood exosomes (BE) inhibit HIV-1 in a donor-independent manner. The composition and function of exosomes depends on the status of the exosome-producing cell. Thus, donor characteristics that alter the condition of exosome-producing cells may alter the antiviral phenotype of SE. Illicit drug use enhances HIV-1 replication, negatively affects male fertility, and alters exosome biogenesis pathways. Thus, illicit drugs may alter SE physical, composition, and functional properties. Indeed, SE from donors with a history of illicit drugs were altered in composition which correlated with a diminished ability to inhibit HIV-1. Similarly, because exosomes derived from HIV-1 infected cell cultures promote infection, donor HIV status may contribute to a proviral phenotype of exosomes. Infectivity studies by HIV-infected ART-naïve SE revealed that SE from healthy donors and HIV-infected donors are inhibitory, but BE are not inhibitory. Therefore, the inhibitory phenotype of SE is conserved regardless of donor HIV status. Significantly, BE and SE from HIV-infected ART-suppressed donors not only inhibited HIV-1, but contained inhibitory levels of antiretroviral (ARV) medications, indicating that body-fluid derived exosomes may act as carriers of ARV drugs.
Previous studies found that the antiviral mechanism of SE was targeted to multiple HIV-1 lifecycle steps; however, the mechanism of inhibition was unclear. Transcription specific analysis revealed that SE reduced HIV-transcription at multiple steps including association of transcription factors NF-kB and Pol II with the viral promoter, as well as transcription initiation and elongation. SE inhibited HIV-driven promoter activation and viral gene expression. Importantly, SE targeted inhibition of viral protein Tat transcriptional activities.
Overall, these findings from donor characteristics that may alter the condition of the cellular source of SE demonstrate that SE antiviral factor(s) is highly conserved. Illicit drugs alter SE-associated factors and may reduce SE inhibitory activities. However, HIV status does not affect the antiviral function of SE. Significantly, donor-ARV medications are associated with SE and BE indicating a potential role of exosomes in drug delivery in vivo. Mechanistically, SE suppress HIV-1 by targeting host and viral transcription factors that could be therapeutically exploited for novel anti-HIV strategies. These data emphasize the need for additional studies on the composition and function of SE to harness the antiviral potential of SE inhibitory factors.