Antiviral Effects of Tecovirimat and Cellular Ultrastructural Changes in Human Bronchial Epithelial Cell Line Following Monkeypox Virus Infection

The mpox virus (MPXV) Clade IIb outbreak in 2022 marked the largest occurrence of this disease outside of Africa, presenting with a range of clinical symptoms and varying degrees of severity. Currently, there is no specific treatment approved for mpox. However, Tecovirimat, an antiviral drug, has shown efficacy against known orthopoxviruses in various animal models and Vero cell cultures. Given that serious complications, including lung involvement, have been reported, particularly in individuals with compromised immune systems, our study aimed to investigate the effects of MPXV infection on an in vitro model of the human lung airway epithelium, using the Calu-3 cell line.

We examined the kinetics of MPXV replication and associated ultrastructural changes within these cells. Additionally, we conducted dose-response studies to evaluate the antiviral activity of Tecovirimat. Our findings revealed active replication of MPXV in Calu-3 cells, which was associated with structural modifications in mitochondria, including their relocation around the nucleus, and the formation of cytoplasmic vacuoles within the infected cells.

Treatment with Tecovirimat consistently reduced viral replication, both in the cell culture supernatants (by 81%) and inside the cells themselves (by 77%). Ultimately, the drug effectively halted viral infectivity, resulting in a 92% reduction of the cytopathic effect (damage to the cells) after 48 hours of infection.

The administration of Tecovirimat inhibited the final wrapping stage of mature viral particles, leading to extensive cytoplasmic vacuolation in the infected cells. Our results demonstrate the in vitro effectiveness of Tecovirimat against MPXV at nanomolar concentrations in Calu-3 cells. This suggests a potential rationale for considering the use of this drug in patients with severe mpox disease, especially those experiencing lung involvement.