The induction and suppression of the apoptotic response of HSV-1 in human corneal epithelial cells

PURPOSE: HSV-1 has been shown to block apoptosis in some cell lines when the cells are exposed to exogenous agents (e.g., sorbitol). The purpose of this study was to determine whether HSV-1 infection of human corneal epithelial (HCE) cells alone induces an early proapoptotic response and whether this response is subsequently downregulated during the infection. METHODS: HCE cells were infected with HSV-1 or subjected to osmotic shock (sorbitol). Fluorescent staining for annexin V binding, mitochondrial membrane potential, and DNA condensation and assays for caspase 8, 9, and 3 activity and cytokeratin 18 cleavage were performed, to assess the apoptotic pathway. RESULTS: HSV-1 infection of HCE cells induced a rapid proapoptotic response, characterized by translocation of phosphatidylserine to the external membrane, activation of caspases 8 and 3 within 2 hours, and cleavage of cytokeratin 18. However, the induced response was downregulated during the infection, and later stages of the apoptotic responses (e.g., DNA condensation) were not produced. Sorbitol treatment led to terminal apoptosis by 12 hours, as indicated by DNA condensation of treated cells and reduction in the number of viable cells. CONCLUSIONS: HSV-1 can induce and subsequently suppress the apoptotic pathway in HCE. Suppression of apoptosis occurred only during HSV-1 infection and not after treatment with sorbitol, suggesting that the suppression of apoptosis may be a mechanism of viral survival.