A pneumo-virulent United States isolate of porcine reproductive and respiratory syndrome virus induces apoptosis in bystander cells both in vitro andin vivo

Evidence of apoptosis was detected for the United States porcine reproducti ve and respiratory syndrome virus (PRRSV) in ATCC CRL11171 cells inoculated with strain ATCC VR2385 and in the tissues of pigs infected with the same strain. Apoptosis was detected by agarose gel electrophoresis, transmission electron microscopy and terminal deoxytransferase dUTP nick end labelling (TUNEL) techniques. By electron microscopy and double-labelling techniques, apoptosis was detected primarily in uninfected bystander cells in the cont inuous cell line rather than the PRRSV-infected cells. In the lungs, the ap optotic cells were predominantly alveolar and pulmonary intravascular macro phages, and mononuclear cells in the alveolar septa. In the lymph nodes, th e apoptotic cells were predominantly tingible body macrophages and mononucl ear cells. The induction of apoptosis in a large number of mononuclear cell s in the lungs and lymph nodes appears to be a mechanism of PRRSV pathogene sis and might be an explanation for a dramatic reduction in the number of a lveolar macrophages and circulating lymphocytes and monocytes in PRRSV-infe cted pigs.