CELLULAR COMPOSITION, CORONAVIRUS ANTIGEN EXPRESSION AND PRODUCTION OF SPECIFIC ANTIBODIES IN LESIONS IN FELINE INFECTIOUS PERITONITIS

Twenty-three cats with spontaneous feline infectious peritonitis (FIP) were examined by light microscopy including immunohistology and histo chemistry in order to determine the cellular composition and the expre ssion of viral antigen in lesions in FIP. Furthermore, the presence of plasma-cells producing coronavirus-specific antibodies was evaluated in situ. Macrophages and neutrophils were demonstrated by an antibody against calprotectin (leukocyte protein L1, myeloid/histiocyte antigen ), neutrophils were recognized due to their chloroacetate esterase act ivity, and B- and T-lymphocytes were identified by antibodies against the CD3 antigen and the CD45R antigen, respectively. Expression of vir al antigen was immunohistologically demonstrated by a monoclonal antib ody (mAb) against coronavirus while coronavirus-specific antibodies in situ were identified by the application of feline coronavirus prior t o the coronavirus antibody. Lesions were classified as diffuse alterat ions at serosal surfaces, granulomas with areas of necrosis, granuloma s without extended necrosis, focal and perivascular lymphoplasmocytic infiltrates, and granulomatous-necrotizing vasculitis. Diffuse alterat ions on serosal surfaces were represented either by activated mesothel ial cells with single coronavirus antigen-bearing macrophages or by la yers of precipitated exudate containing single to numerous granulomas with areas of necrosis. In liver and spleen, the exudate was often und erlaid by a small band of subcapsular B-cells with an occasional plasm a-cell producing coronavirus-specific antibodies. In other locations, a variably broad and of B-cells and plasma-cells, often infiltrating b etween underlying muscle fibers, separated the exudate from the unalte red tissue. Some of these plasma-cells were positive for coronavirus-s pecific antibodies. In granulomas with areas of necrosis, the central necrosis was surrounded by macrophages usually expressing considerable amounts of viral antigen. Few B-cells and plasma-cells were found in the periphery. In granulomas without extended necrosis, the number of macrophages were lower. Only few macrophages expressing low amounts of viral antigen were present. B-cells and plasma-cells formed a broad r im. Few plasma-cells stained positive for coronavirus-specific antibod ies. In both types of granulomas, few neutrophils were found between m acrophages. Few T-cells were seen scattered throughout the lesions. Fo cal and perivascular lymphoplasmocytic infiltrates were mainly seen in omentum and leptomeninx. B-cells were the predominant cells; some pla sma-cells were positive for coronavirus-specific antibodies. Viral ant igen was not readily detected in these alterations. Granulomatous-necr otizing vasculitis was occasionally found in kidneys and leptomeninx. It was dominated by macrophages which often stained strongly positive for coronavirus antigen. Different types of alteration were often seen in the same animal and even the same tissue. There was no obvious cor relation between the cat's age, gross pathological changes, and the hi stological types of alteration. Single plasma-cells positive for coron avirus-specific antibodies were found around blood vessels distant fro m inflammatory alterations, within the lung parenchyma, as infiltratin g cells in the mucosa of the small intestine, and in spleen and mesent eric lymph node. Results show that alterations in FIP are heterogeneou s concerning cellular composition and expression of viral antigen. The dominance of B-cells in part of the lesions together with the presenc e of plasma-cells positive for coronavirus-specific antibodies indicat e that these cells may play a role in the maintenance of inflammatory processes in FIP. (C) 1998 Elsevier Science B.V. All rights reserved.