Rubella virus induces apoptosis in culture cells

The replication of rubella virus (RUB) in Vero cells, an adherent cell line, results in apoptotic death of infected cells as detected by chromatin fragmentation assays. In infected cultures, virtually all of the cells that had become detached (a hallmark feature of RUB-induced cytopathology) were apoptotic; they were predominantly dead as shown by propidium iodide and trypan blue exclusion tests. In contrast, the majority of the cells in the infected monolayers that remained adherent were alive and contained intact chromatin. Thus simple counting of detached cells in the medium is a convenient way of measuring the extent of RUB-induced apoptosis. RUB-induced cytopathology was inhibited by z-VAD-fmk, an inhibitor of caspases that are involved in the execution stages of apoptosis, confirming the induction of apoptosis by RUB. The lack of apoptotic adherent cells (maximally 1% at any time point through 6 days postinfection) indicates that the induction of apoptosis is asynchronous since cells become uniformly virus antigen-positive by day 2 postinfection. To elucidate whether this asynchronicity and the ability of RUB to persistently infect Vero cells were due to a suppression of apoptosis, we examined whether RUB can suppress chemically induced apoptosis. Staurosporine (ST) was found to be an efficient inducer of apoptosis in Vero cells. ST treatment of RUB-infected and RUB persistently infected cells resulted in a much higher proportion of detached cells, higher even than in Vero cells treated with ST alone. This indicates that RUB does not suppress ST-induced apoptosis and, rather, that ST and RUB acted cumulatively in inducing apoptosis, possibly indicating that they use different induction pathways.