Human seminal fluid could enhanced mpox virus infection in primary human cells, with increased viral attachment and uptake potentially contributing to the effect.
Researchers tested seminal fluid from 6 healthy donors in primary human fibroblasts and keratinocytes. They preincubated the mpox virus with varying concentrations of seminal fluid prior to cell exposure and subsequently examined 2 semen-derived amyloid fibrils: semen-derived enhancer of viral infection (SEVI), formed from prostatic acid phosphatase fragment PAP, and SEM, formed from semenogelin-1 fragment SEM1. Experiments were extended to multiple mpox clades, epithelial cells, primary monocytes, and ex vivo human skin.
Seminal fluid from all donors increased the number of mpox-positive cells in both primary fibroblasts and keratinocytes. When averaged across donors, seminal fluid concentrations of 50% and 10% during virion treatment increased infection 2.2- and 2.6-fold in fibroblasts and 4.1- and 3.5-fold in keratinocytes, respectively. Seminal fluid also increased infection at a lower concentration in fibroblasts and at a low viral dose. All 4 tested clade IIb isolates collected during the 2022 outbreak showed increased infectivity following exposure to seminal fluid.
The researchers examined whether amyloid fibrils found in semen contributed to the effect. Both SEVI and SEM increased mpox infection in a dose-dependent manner. In contrast, nonagitated peptides lacking fibrillar structures did not increase infection, indicating that fibril formation was necessary for the enhancement. The tested fibril concentrations did not show toxicity in fibroblasts.
The effect was observed across mpox isolates. At 10 µg/mL of SEVI, infection increased with all tested clades, while clades IIb and Ib also showed enhancement at lower concentrations. At progressively lower viral inocula, SEVI increased mpox infection 2.4-, 4.0-, and 12.7-fold. SEVI and SEM also increased infection with cowpox and camelpox viruses in the experimental models.
SEVI also increased infection across epithelial and immune cell models. In corneal epithelial cells, which showed relatively low susceptibility to mpox without SEVI, infection increased from about 4% to 54%. Increased infection was also observed in primary keratinocytes, nasal epithelial cells, and monocytes from 2 healthy donors.
In an ex vivo human skin model, the infected area increased by 10 percentage points with SEVI following 72 hours of infection. Although 4 biological replicates were evaluated, all skin pieces were obtained from a single donor.
Mechanistic experiments suggested that adding SEVI postinfection did not increase infection, whereas exposure prior to infection did. In attachment experiments, the proportion of cells with attached viral particles increased from 54% without SEVI to 73% with SEVI. Following internalization, viral particles were detected in 88% of cells exposed to SEVI vs. 75% of untreated cells. The researchers proposed that SEVI facilitated interactions between viral particles and the cell surface, increasing attachment and subsequent uptake.
The fibril-targeting molecular tweezer CLR01 reduced SEVI-enhanced viral internalization and infection as well as inhibited mpox in the absence of SEVI. In separate tests, tecovirimat and brincidofovir retained antiviral activity under SEVI-enhanced conditions, although tecovirimat showed a minor reduction in efficacy as viral burden increased.
The study was limited to cell-based and ex vivo experimental models and did not establish the magnitude of any effect on person-to-person transmission. The researchers noted that in vivo studies are needed and that semen is a complex fluid containing other components that may influence mpox infection. The contribution of other bodily fluids to mpox transmission during sexual activity also remains unclear.
“Collectively, our results suggest that semen is not simply a passive carrier of [mpox] but semen-derived fibrils may both increase the efficiency of viral transmission and enhance infection in hardly susceptible cells,” wrote lead study author Emma Torbica, of the Institute of Medical Virology at the University Hospital at Goethe University Frankfurt, and colleagues.
The study authors reported no conflicts of interest.
Source: Emerging Microbes & Infections
