Extended budded virus formation and induction of apoptosis by an AcMNPV FP-25K/p35 double mutant in Trichoplusia ni cells

An Autographa californica nucleopolyhedrovirus (AcMNPV) mutant (AcdefrT) isolated from virus-infected Trichoplusia ni (TN-368) cells produced plasma membrane blebbing and caspase-3-like activity late in infection. It also synthesized less polyhedra, but displayed enhanced budded virus formation in TN-368 cells. This phenotype resulted from dual mutations in p35 and FP-25. In this study we showed that enhanced budded virus production occurs because the hourly rate of release of virus from AcdefrT-infected cells is higher than that for AcMNPV and it continues for longer. This may be the trigger for the induction of apoptosis late in AcdefrT-infected TN-368 cells. However, laddering of host DNA was absent in TN-368 cells infected with AcdefrT, but was observed in Spodoptera frugiperda cells. Very late polyhedrin protein production and occlusion body formation was reduced in AcdefrT-infected TN-368 cells, but chitinase and capsid late gene expression remained unchanged. The AcdefrT was rescued with a copy of a baculovirus iap3, to replace the absent p35. This modification abolished most plasma membrane blebbing in AcdefrT-infected TN-368 cells, but did not affect enhanced budded virus production. These data suggest that inhibitors of apoptosis are required in T. ni cells, particularly when the production of budded virus is enhanced