Inactivated Wild-Type Yellow Fever Virus (17D)

Yellow fever (YF) has a wide range of severity, with clinical manifestations ranging from febrile, self-limited cases to fatal cases. Several studies have implicated oxidative stress induced by viral infection as a pathogenic factor. Here, researchers evaluated whether yellow fever virus (YFV) induces an imbalance in redox homeostasis upon infection of human hepatocytes, ultimately leading to oxidative stress. YFV infection resulted in a significant increase in reactive oxygen species (ROS) levels from 2 to 4 days post infection (dpi). When measuring oxidative parameters at 4 dpi, YFV infection caused oxidative damage to lipids, proteins, and DNA as evidenced by increases in lipid peroxidation/8-isoprostanes, carbonyl proteins, and 8-hydroxy-2′-deoxyguanosine, respectively. In addition, the activities of the antioxidant enzymes superoxide dismutase (SOD) and glutathione peroxidase (GPx) were significantly reduced, in addition to a decrease in the ratio of reduced to oxidized glutathione (GSH/GSSG), indicating a pro-oxidative environment. However, there were no changes in the enzymatic activity of catalase (CAT) or the gene expression of SOD isoforms (1/2/3), CAT, or GPx. Thus, these results suggest that YFV infection leads to an imbalance in redox homeostasis, excessive production of ROS, and depletion of antioxidant enzymes, resulting in oxidative damage to cellular components.

ROS quantification was performed from 1 to 5 dpi (Figure 1). At 1 dpi, the ROS level in YFV-infected cells was at baseline, with no difference observed in the control cells (Figure 1a). From 2 to 4 dpi, ROS production gradually increased in YFV-infected cells, with a significant difference (p < 0.05) when compared to the cell control, and no difference when compared to the positive control (H2O2) at 3 and 4 dpi (Figure 1b–d). At 5 dpi, the ROS levels in YFV-infected cells were very close to those of the control cells (Figure 1e). As the cell death process itself naturally increases ROS, and the result was normalized by the fluorescence level of the cell control, no statistical difference was observed between the control and YFV-infected cells at 5 dpi. At all dpi, infection of cells with UV-inactivated YFV did not stimulate ROS production, indicating that the exacerbated production of ROS is related to the viral activity and multiplication of YFV in hepatocytes, and not just the presence of viral particles themselves.

Figure 1. Yellow fever virus (YFV) infection of human hepatocytes increases levels of reactive oxygen species (ROS). HepG2 cells were infected with YFV (MOI 1) and intracellular ROS levels quantified from 1 to 5 day post-infection (dpi) by the marker fluorogenic carboxy-H2DCFDA. UV-inactivated YFV (YFV UV) was used as a control and H₂O₂ was used as an ROS inducer. (Ferraz A C, et al., 2024)