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 chr omatin fragmentation assays. In infected cultures, virtually all of th e cells that had become detached (a hallmark feature of RU B-induced c ytopathology) were apoptotic; they were predominantly dead as shown by propidium iodide and trypan blue exclusion tests. In contrast, the ma jority of the cells in the infected monolayers that remained adherent were alive and contained intact chromatin. Thus simple counting of det ached 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 thro ugh 6 days postinfection) indicates that the induction of apoptosis is asynchronous since cells become uniformly virus antigen-positive by d ay 2 postinfection. To elucidate whether this asynchronicity and the a bility of RUB to persistently infect Vero cells were due to a suppress ion of apoptosis, we examined whether RUB can suppress chemically indu ced apoptosis. Staurosporine (ST) was found to be an efficient inducer of apoptosis in Vero cells. ST treatment of RUB-infected and RUB pers istently infected cells resulted in a much higher proportion of detach ed cells, higher even than in Vero cells treated with ST alone. This i ndicates that RUB does not suppress ST-induced apoptosis and, rather, that ST and RUB acted cumulatively in inducing apoptosis, possibly ind icating that they use different induction pathways. (C) 1998 Academic Press