Immune-Depletion Related to Bovine Viral Diarrhoea Virus in a Heifer with Naturally Occurring Mucosal Disease

Introduction: Bovine viral diarrhoea virus (BVDV) induces muco- sal disease (MD) in young persistently infected cattle. MD affects the gastrointestinal (GI) tract and lymphoid tissues; however, the pathogenesis of lymphoid lesions is still poorly understood. Materials and Methods: Samples of different parts of the gastroin- testinal tract, lymph nodes and spleen from nine naturally MD-in- fected heifers were collected during necropsy, processed for histology and immunolabelled with antibodies specific for BVDV, CD3, CD79a, MAC387, MHCII and HSP72 (heat shock protein 72). Results: All animals had gross and microscopical lesions (mucosal ulcers, necrosis, vasculitis and intestinal crypt herniation) character- istic for MD. Lymphoid organs had follicular germinal centre necrosis and apoptosis with macrophage infiltration. Immunohistochemically, BVDV antigen was present only in GI epithelial cells. HSP72 was expressed by both mucosal epithelial cells and macrophages; macro- phages were also MAC387+ and MHCII+. Rare residual CD79a+ B lymphocytes, surrounded by a few CD3+ T lymphocytes, were detected within germinal centres. Conclusions: The presence of BVDV antigen with restriction to the epithelium, and the lack of reaction of lymphoid cells to BVDV anti- gen, suggested that immune depletion was mediated by up-regulation of pro-inflammatory cytokines, as previously reported in pigs infected with classical swine fever.