Characterization of interstitial nephritis in pigs with naturally occurring Postweaning Multisystemic Wasting Syndrome

Kidney samples with interstitial nephritis from 26 pigs affected by postweaning multisystemic wasting syndrome (PMWS) were selected. A histological evaluation was carried out to describe the type of inflammation and its relationship with viral load, assessed by in situ hybridization (ISH). Out of 26 cases, 10 revealed a tubulo-interstitial lympho-plasmacytic nephritis, 11 an interstitial granulomatous nephritis and 5 both types of inflammation (mixed type). In 4 cases of granulomatous inflammation the pattern was not classically nodular and a population of macrophages and lymphocytes was present (interstitial lympho-histiocytic nephritis). ISH confirmed the presence of PCV2 nucleic acid in all cases. The epithelium of the renal tubules was the most constant ISH positive structure. In tubulo-interstitial nephritis, the higher the number of positive inflammatory cells, the more severe the inflammation. The ISH reaction was more heterogeneous and unpredictable in granulomatous nephritis, with some epithelioid and giant cells positive by ISH. To quantify macrophages distributed in the three patterns of nephritis, immunohistochemical methods using anti-MHC-II and anti-lysozyme antibodies were undertaken and semiquantitative evaluation was carried out. MHC-II was mainly expressed by lymphocytes in tubulo-interstitial nephritis, while not always stained macrophages in cases of granulomatous (including lympho-histiocytic) nephritis; the anti-lysozyme antibody revealed macrophages when present in tissues. The amount of PCV2 nucleic acid was not apparently associated with the pattern of inflammation (tubulo-interstitial or granulomatous). PCV2 load seems to reflect the severity of the lympho-plasmacytic inflammation but not that of granulomatous and lympho-histiocytic types.