A RABBITPOX VIRUS SERPIN GENE CONTROLS HOST-RANGE BY INHIBITING APOPTOSIS IN RESTRICTIVE CELLS

Poxviruses are unique among viruses in encoding members of the serine proteinase inhibitor (serpin) superfamily. Orthopoxviruses contain thr ee serpins, designated SPI-1, SPI-2, and SPI-3. SPI-1 encodes a 40-MDa protein that is required for the replication of rabbitpox virus (RPV) in PK-15 or A549 cells in culture (A. N. Ali, P. C. Turner, M. A. Bro oks, and R. W. Moyer, Virology 202:305-314 1994). Examination of nonpe rmissive human A549 cells infected with an RPV mutant disrupted in the SPI-1 gene (RPV Delta SPI-1) suggests there are no gross defects in p rotein or DNA synthesis. The proteolytic processing of late viral stru ctural proteins, a feature of orthopoxvirus infections associated with the maturation of virus particles, also appears relatively normal. Ho wever, very few mature virus particles of any kind are produced compar ed with the level found in infections with wild-type RPV. Morphologica l examination of RPV Delta SPI-1-infected A549 cells, together with an observed fragmentation of cellular DNA, suggests that the host range defect is associated with the onset of apoptosis. Apoptosis is seen on ly in RPV Delta SPI-1 infection of nonpermissive (A549 or PK-15) cells and is absent in all wild-type RPV infections and RPV Delta SPI-2 mut ant infections examined to date. Although the SPI-1 gene is expressed early, before DNA replication, the triggering apoptotic event occurs l ate in the infection, as RPV Delta SPI-1-infected A549 cells do not un dergo apoptosis when infections are carried out in the presence of cyt osine arabinoside. While the SPI-2 (crmA) gene, when transfected into cells, has been shown to inhibit apoptosis, our experiments provide th e first indication that a poxvirus serpin protein can inhibit apoptosi s during a poxvirus infection.