Lymphoid depletion and lymphopenia in peripheral blood is a consistent feature in pigs that develop clinical PMWS. The reduction of lymphocytes might be due to reduced production in the bone marrow, reduced proliferation in secondary lymphoid tissues, or due to increased loss of lymphocytes in the bone marrow or peripheral blood or secondary lymphoid tissues via necrosis or apoptosis. The lymph node reaction can be graded as initial, intermediate, and end stage (Sarli et al., 2001). Studies on pigs with naturally-acquired PMWS revealed that the absence of follicles and depletion of lymphocyteswas associated with a reduction in number of interfollicular dendritic cells and interdigitating cells as well as a reduction or absence of B cells and CD4+ T lymphocytes (Sarli et al., 2001). By IHC characterization of PCV2-associated lesions in naturally-PMWS-affected pigs, Chianini et al. (2003) found increased numbers of macrophages and partial loss and redistribution of antigen presenting cells throughout lymphoid tissues when compared to healthy control pigs. In particular, a reduction or loss of CD79α and 3C3/9-positive cells and diminution in T-cell areas of CD3-positive lymphocytes was observed. Decrease of proliferation of both, lymphoid and medulla-like tissues in the initial stage of PMWS, but not in the intermediate or final stage has been reported. It was also demonstrated, that there is increase of subcapsular and peritrabecular lysozyme-positive macrophages, and partial loss and redistribution of BL2H5-positive antigen presenting cells throughout lymphoid tissues (Chianini et al., 2003).
Shibahara et al. (2000) found a loss of lymphocytes in B-cell areas and attributed this to increased PCV2-induced apoptosis since PCV2-antigen was found in nuclei of macrophages and apoptotic lymphocytes. In contrast, Mandrioli et al. (2004) investigated inguinal lymph nodes of 10 pigs with naturally acquired PMWS and found that decreased cell proliferation and not increased apoptosis seemed to be the most important variable leading to lymphoid depletion in PMWS. Krakowka et al. (2004) investigated tissues from experimentally PCV2 infected pigs for TUNEL staining and found positive signals only within the cytoplasm of virus-positive phagocytic mononuclear cells. It was concluded that apoptosis is not the primary mechanism of lymphoid depletion and hepatocyte loss in PMWS (Krakowka et al., 2004).
Chianini F, Majo N, Segales J, Dominguez J, Domingo M: Immunohistochemical characterisation of PCV2 associate lesions in lymphoid and non-lymphoid tissues of pigs with natural postweaning multisystemic wasting syndrome (PMWS). Vet Immunol Immunopathol. 94:63-75, 2003
Krakowka S, Ellis J, McNeilly F, Meehan B, Oglesbee M, Alldinger S, Allan G: Features of cell degeneration and death in hepatic failure and systemic lymphoid depletion characteristic of porcine circovirus-2-associated postweaning multisystemic wasting disease. Vet Pathol. 41:471-481, 2004
Mandrioli L, Sarli G, Panarese S, Baldoni S, Marcato PS: Apoptosis and proliferative activity in lymph node reaction in postweaning multisystemic wasting syndrome (PMWS). Vet Immunol Immunopathol. 97:25-37, 2004
Sarli G, Mandrioli L, Laurenti M, Sidoli L, Cerati C, Rolla G, Marcato PS: Immunohistochemical characterisation of the lymph node reaction in pig post-weaning multisystemic wasting syndrome (PMWS). Vet Immunol Immunopathol. 83:53-67, 2001
Shibahara T, Sato K, Ishikawa Y, Kadota K: Porcine circovirus induces B lymphocyte depletion in pigs with wasting disease syndrome. J Vet Med Sci. 62:1125-1131, 2000