Submitted to: Research in Veterinary Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/14/2010
Publication Date: 2/1/2012
Citation: Sinha, A., Schalk, S., Lager, K.M., Wang, C., Opriessnig, T. 2012. Singular PCV2a or PCV2b infection results in apoptosis of hepatocytes in clinically affected gnotobiotic pigs. Research in Veterinary Science. 92(1):151-156. Interpretive Summary: Porcine circovirus type 2 (PCV2) is a virus that can cause a disease in pigs that is responsible for significant losses to the swine industry. Although there are now commercially available PCV2 vaccines, this virus is still a problem for pork producers. This report describes microscopic studies evaluating the potential mechanism of how this virus can affect pig cells, and thus how it could cause disease in a pig. Sometimes a cell can die after it is infected with a virus. One mechanism for cell death is called "apoptosis," a process in which the cell dies following a predictable pathway that can be identified by looking at tissues with special microscopic tests. This study compared PCV2-infected and non-infected pigs by looking at liver tissue for signs of apoptosis. Apoptosis was identified in the PCV2-infected pigs indicating this mechanism of cell death may play an important role in course of the disease. This knowledge can provide a better understanding of how this virus can affect pigs and may lead to improved vaccines.
Technical Abstract: Porcine circovirus type 2 (PCV2) which can be further subdivide into two main genotypes, PCV2a and PCV2b, is often clinically associated with respiratory signs, failure-to-thrive, and diarrhea. The precise pathogenesis of PCV2, and in particular its involvement in apoptosis, is controversial. The objectives of this study were to determine (1) whether PCV2 is associated with apoptosis by investigating hepatic tissues from experimentally PCV2-infected clinically-unaffected and affected gnotobiotic pigs, (2) if there are differences between PCV2a and PCV2b in inducing apoptosis, and (3) if differences between apoptosis detection systems exist. Forty-eight gnotobiotic pigs were separated into five groups based on inoculation status and development of clinical disease: sham-inoculated, clinically-unaffected (n=4); PCV2a, clinically-unaffected (n=10); PCV2a, clinically-affected (n=6); PCV2b, clinically-unaffected (n=13) and PCV2b, clinically-affected (n=15). Formalin-fixed, paraffin-embedded sections of liver from all pigs were analyzed for signs of apoptosis [presence of single strand DNA breaks in the nucleus by the terminal transferase dUTP nick end labeling (TUNEL) assay or presence of intra-nuclear cleaved caspase 3 (CCasp3) demonstrated by CCasp3 immunohistochemistry (IHC)]. The tissues were also tested for presence of cytoplasmic and intra-nuclear PCV2 antigen by an IHC assay. CCasp3 and TUNEL labeling were detected in the nucleus of hepatocytes in PCV2a and PCV2b infected pigs with significantly (P < 0.05) higher levels of apoptotic cells in clinically-affected pigs with severe microscopic lesions. Regardless of PCV2 genotype (PCV2a; PCV2b), there were higher levels of PCV2 antigen in clinically-affected pigs compared to clinically-unaffected pigs. There was no difference in detection rate of apoptotic cells between the TUNEL assay and CCasp3 IHC.