Expression of tumour necrosis factor-alpha is associated with apoptosis in lungs of pigs experimentally infected with porcine reproductive and respiratory syndrome virus.

Expression of tumour necrosis factor (TNF)-alpha and its association with apoptosis was examined in lungs from pig experimentally inoculated with porcine reproductive and respiratory syndrome virus (PRRSV). The TNF-alpha nucleic acid was detected in PRRSV-infected alveolar macrophages by reverse transcription-polymerase chain reaction (RT-PCR). The TNF-alpha antigen was detected in alveolar macrophages by immunohistochemisty. DNA fragmentation in apoptotic cells was detected by in situ terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labelling (TUNEL). A double-labelling procedure using immunohistochemisty for the detection of PRRSV and the in situ TUNEL assay for the detection of apoptosis demonstrated that a majority of labelled cells were infected with PRRSV or apoptotic, but not both. Apoptotic cells were more abundant than PRRSV-infected cells in all lungs examined. A double-labelling procedure using immunohistochemisty for the detection of TNF-alpha and the in situ TUNEL assay for the detection of apoptosis demonstrated that a majority of labelled cells were apoptotic or for TNF-alpha, but not both. These data suggested that TNF-alpha released from macrophages after infection by PRRSV may induce apoptosis in uninfected bystander cells.