The ability of frog virus 3 (FV3), the type species of the family Iridoviridae, 
to induce apoptosis was examined by monitoring DNA cleavage, chromatin 
condensation, and cell-surface expression of phosphotidylserine (PS) in fathead 
minnow (FHM) and baby hamster kidney (BHK) cells. In productively infected FHM 
cells, DNA fragmentation was first noted at 6-7 h postinfection and was clearly 
seen by 17 h postinfection, while chromatin condensation was detected at 8.5 h 
postinfection. As with some other viruses, FV3-induced apoptosis did not require 
de novo viral gene expression as both heat-inactivated and UV-inactivated virus 
readily triggered DNA fragmentation in FHM cells. Moreover, FV3-induced 
apoptosis was blocked in FHM cells by the pan-caspase inhibitor Z-VAD-FMK, 
suggesting that virus infection triggers programmed cell death through 
activation of the caspase cascade. FV3 infection also triggered apoptosis in BHK 
cells as monitored by TUNEL and annexin V binding assays. To determine whether 
FV3, similar to other large DNA viruses, encoded proteins that block or delay 
apoptosis, mock- and FV3-infected FHM cells were osmotically shocked and assayed 
for DNA fragmentation 3 hours later. DNA fragmentation was clearly seen whether 
or not shocked cells were previously infected with FV3, indicating that 
infection with FV3 did not block apoptosis induced by osmotic shock in FHM 
cells. The above results demonstrate that iridoviruses triggered apoptosis and 
that the induction of programmed cell death did not require viral gene 
expression. However, it remains to be determined if virion attachment to target 
cells is sufficient to induce cell death, or if apoptosis is triggered directly 
or indirectly by one or more virion-associated proteins.