Apoptosis of lymphoid tissues after classical swine fever virus (CSFV) inoculation in CSFV-vaccinated piglets

Abstract

A study on the immune response and apoptosis of lymphoid tissues in CSFV-vaccinated pig was performed. The pigs from CSF-free herd 2 years before study were classified into 3 groups, 6 pigs of each; group A and B had average matermal antibody (Ab) ≥ 64 in group A and ≤ 8 in group B and they were vaccinated at 3 week old; group C which had matermal Ab range from ≤8 -≥ 64 was a control group. All pigs were challenged with high virulent CSFV (Bangkok 1950 strain) at 5 weeks of age. To study the vaccine efficacy. the relationship between apoptosis and CSFV-infected cells in lymphoid tissues, the blood samples were collected at 0, 3, 7,10, 14, and 21 days post infection (dpi) to evaluate white blood cell (WBC) count, virus isolation, and antibody level. All dead pigs, either during or at the end of experiment (21 dpi), were necropsied to collect lymphoid tissues for histopathology. Viral antigen detection was performed under immunohistochemistry using monoclonal Ab against gp55, while TdT mediated dUTP nick end labeling (TUNEL) was used to detect apoptosis. At the end of observation, no pigs from either group A or B died whereas 5 out of 6 in group C gradually died. Pigs from all groups developed distinctive Ab response after challenged, group B significantly yielded higher Ab response than group A and group C respectively (p<0.05). Though, the decreased of WBC in all pigs and viremia were detected at 3 dpi, the appearances were soon recovered at 7 dpi in those group A and B while they were continuously demonstrated in group C. Histopathology showed severe lymphoid depletion and viral infected cells in lymphoid tissues of pigs in group C, the mean±SD of number of CSFV-infected cells was 22.9±9.1 celss/0.1mm2, while none was found from either group A or B Mean±SD of number of apoptotic cells in lymphoid tissues of pigs from group C was 3.0±1.7 cells/0.1 mm² , which was significantly the lowest among group A (14.8±6.8 cells/0.1mm²) and group B (23.4±7.8 cells/0.1 mm²) respectively. In addition, the correlation between CSFV-infected cell and apoptotic cells in lymphoid tissues were weakly negative (r² =0.21).