Apoptosis during ectromelia orthopoxvirus infection is DEVDase dependent: in vitro and in vivo studies

Ectromelia virus (EV), which causes mousepox, is a member of the orthopoxviruses that are defined as being able to suppress apoptosis. Caspase-3 is one of the key effector proteases which regulates the apoptotic cascade and which is responsible for DNA fragmentation observed during apoptosis. It is well known that viruses, especially poxviruses, can inhibit caspase activity. Here, we report that EV can regulate apoptosis in vitro, suppressing the activity of caspases recognizing the DEVD (Asp-Glu-Val-Asp) motif (caspase-3 and -7) before successful virus replication is completed. Caspase-3 activity measurement showed that an increase in caspase-3 activity preceded the peak of DNA fragmentation demonstrated by TUNEL staining of L929 and RK-13 cells. By using specific caspase inhibitors (Ac-DEVD-CHO, Ac-IETD-CHO and zVAD-fmk), we showed that caspase-3 and -7 (DEVDases) are major effector caspases during EV-induced apoptosis in permissive L929 and RK-13 cell cultures. Apoptosis in vivo seems to play an important role during viraemia as well as during the clearance of EV from genetically susceptible BALB/c (H-2(d)) mice. However, as shown by measurement of caspase-3 activity, caspase-3 protein detection and M30-antibody staining, both DEVDases seem to play an important role during EV clearance from draining lymph nodes and conjunctivae at 15 days p.i. up to 20 days p.i., whereas in the liver and spleen DNA fragmentation coexisted with viral multiplication and secondary viraemia. Apoptosis was DEVDase dependent only in the liver, while spleen DNA fragmentation observed between 5 and 10 days p.i. was caspase independent. Therefore, we conclude that DEVDase- (caspase-3- and caspase-7-) dependent apoptosis is an important mechanism regulating the resolution of EV infection.