Procaspases 3/7 activation in SJPL cells infected by PRRSV. MARC-145 and SJPL cells were infected at 0.5 MOI with PRRSV IAF-Klop strain or incubated with a combination of four apoptotic inducers (actinomycin D, vinblastine sulfate, cycloheximide and puromycin) as a positive control. At 24 hrs post-incubation with the apoptotic inducers, MARC-145 cells have developed high CPE level compared to SJPL cells which showed low to mild CPE. At 72 hrs, PRRSV infected cells were disrupted for the detection of caspase 3 using a specific fluorogenic substrate. The results were expressed as relative fluorescence released (relative fluorescence units or RFU) per second per μg of cell lysates. Values are presented as ± standard deviation (SD). When 2 sets of data within a cell type are labeled with superscripts of different letters or when only one set is labeled with a superscript, it indicates that these 2 sets of data are statistically different (P < 0.05). When 2 sets of data from different cell types are labeled with asterisk, it indicates that these 2 sets of data are statistically different (*** P < 0.001).