Detection of Porcine Circovirus Type 2 and Viral Replication by In Situ Hybridization in Primary Lymphoid Organs From Naturally and Experimentally Infected Pigs

Abstract

Porcine circovirus type 2 (PCV2) infection is the cause of postweaning multisystemic wasting syndrome (PMWS). It has been speculated whether cell types permissive of replication are found in the primary lymphoid organs and whether infection of these tissues has an important role in the pathogenesis of PMWS. The aim of this study was to determine if primary lymphoid organ cells support viral replication during PCV2 infection. This was done by histopathological examination of thymus and bone marrow from pigs experimentally inoculated with PCV2 (n ¼ 24), mock-infected pigs (n ¼ 12), pigs naturally affected by PMWS (n ¼ 33), and age-matched healthy control animals (n ¼ 29). In situ hybridization (ISH) techniques were used to detect PCV2 nucleic acid irrespective of replicative status (complementary probe, CP) or to detect only the replicative form of the virus (replicative form probe, RFP). PCV2 was not detected in the experimentally PCV2-inoculated pigs or the control animals. Among the PMWSaffected pigs, 19 of 20 (95%) thymuses were positive for PCV2 by CP ISH, and 7 of 19 (37%) of these also supported viral replication. By CP ISH, PCV2 was detected in 16 of 33 (48%) bone marrow samples, and 5 of 16 (31%) of these also supported replication. The 2 ISH probes labeled the same cell types, which were histiocytes in both organs and lymphocytes in thymus. The RFP labeled fewer cells than the CP. Thus, PCV2 nucleic acids and replication were found in bone marrow and thymus of PMWSaffected pigs, but there was no evidence that primary lymphoid organ cells are major supporters of PCV2 replication.

Global Gene Expression Profiling of Myeloid Immune Cell Subsets in Response to In Vitro Challenge with Porcine Circovirus 2b

“…Particularly alveolar macrophages (AMØs) and other cell-types of the monocyte/macrophage lineage seem to act as reservoirs and Trojan horse for PCV2, subsequently infecting other immune cells and specifically bone-marrow cells [14], [16]. Lymphocyte depletion, secondary infections with opportunistic pathogens, induction of apoptosis and other changes in immune cell subpopulations and PBMCs are all common characteristics of PMWS in severely affected pigs, strongly suggesting an immunosuppressive status [13], [17], [18].…”

“…Viruses have evolved complex immune evasion strategies for protection against host immune responses [15] , and PCV2-infected pigs seem to harbour the virus in different immune cell subsets [13] , [14] without infected cells showing signs of functional differences. Particularly alveolar macrophages (AMØs) and other cell-types of the monocyte/macrophage lineage seem to act as reservoirs and Trojan horse for PCV2, subsequently infecting other immune cells and specifically bone-marrow cells [14] , [16] . Lymphocyte depletion, secondary infections with opportunistic pathogens, induction of apoptosis and other changes in immune cell subpopulations and PBMCs are all common characteristics of PMWS in severely affected pigs, strongly suggesting an immunosuppressive status [13] , [17] , [18] .…”

Myeloid to Erythroid (M: E) ratio in the evaluation of bone marrow cytology of Porcine Circovirus type 2 affected pigs

“…The present study employed the PCR technique as a diagnostic tool to detect PCV-2 virus by using the viral nucleocapsid gene (ORF-2) specific PCR. Bone marrow suppression is a general manifestation in viral infection, especially in immunosuppressive infections like PCV-2 (Hansen et al, 2013;Pascutti et al, 2016). Even though immunosuppression is a feature of porcine circovirus associated diseases (Segalés, 2012), the mechanism by which PCV-2 produces immunosuppression is not clear.…”

Biology and Diseases of Swine

“…Macrophages in bone marrow, thymus, and thymic lymphocytes label positive for PCV2 in infected animals (Hansen et al, 2013;Nauwynck et al, 2012). Viral replication is present in both thymus and bone marrow (Hansen et al, 2013).…”