Classical swine fever virus induces tumor necrosis factor-α and lymphocyte apoptosis

The expression of tumor necrosis factor (TNF)-alpha and apoptosis was studied in lymph nodes from pigs infected with Classical swine fever virus (CSFV). Pigs were inoculated with CSFV and euthanized at 3, 5, 7, and 10 days postinoculation. An increase in TNF-alpha expression was detected in CSFV-infected lymph nodes using a reverse transcription-polymerase chain reaction, and TNF-alpha protein was detected in lymph nodes by immunohistochemistry. The majority of TNF-alpha-positive cells also expressed the SWC3a antigen, a specific marker for porcine leukocytes. By combined use of in situ hybridization and immunohistochemistry, CSFV infection was detected in lymph nodes macrophage. Lymphocytes death occurred by apoptosis that was characterized by condensed shrunken cells and the formation of apoptotic bodies, some of them contained pyknotic nuclear remnants. Apoptosis was detected in situ by TUNEL (terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling) reaction. A double-labeling experiment using immunohistochemistry and TUNEL reaction for the detection of CSFV and apoptosis demonstrated that the majority of labeled cells were positive for CSFV or apoptosis. This suggestes that CSFV can induce apoptosis directly and indirectly. Apoptotic cells induced by viral infection were more abundant than CSFV-infected cells in all lymph nodes tested. A double-labeling experiment using immunohistochemistry and TUNEL reaction for the detection of TNF-alpha and apoptosis demonstrated that labeled cells were positive for either TNF-alpha or apoptosis, and both. The present study addressed two important issues regarding CSFV-induced apoptosis: (i) viral infection and apoptosis colocalize at the cell level; and (ii) one or more factors (e.g., TNF-alpha) released from macrophages may induce apoptosis in uninfected bystander cells.