CONSTITUTIVE AND VISNA VIRUS-INDUCED EXPRESSION OF CLASS-I AND CLASS-II MAJOR HISTOCOMPATIBILITY COMPLEX ANTIGENS IN THE CENTRAL-NERVOUS-SYSTEM OF SHEEP AND THEIR ROLE IN THE PATHOGENESIS OF VISNA LESIONS

Expression of major histocompatibility complex (MHC) antigens was stud ied in the brains of 10 healthy sheep 2 months to 5 years old and 13 s heep infected with visna virus by intracerebral inoculation and killed one and 6 months post infection (p.i.). In healthy sheep there was pr ominent expression of class I, mainly on endothelial cells but also de tected on ependyma, choroid plexus and in the leptomeninges. Class II expression was sparse. It was observed on perivascular cells, in choro id plexus, leptomeninges and on microglial cells in the white matter. No definite increase with age in the constitutive expression of class I and II was observed, confirming that we are dealing with a true cons titutive expression. In visna-infected sheep a considerable induction of MHC antigens on microglia was observed, which correlated with sever ity of lesions and was mainly found in or adjacent to inflammatory inf iltrates of the white matter. Increase in class II antigen expression was detected in all sheep but class I only in sheep with the most seve re lesions 6 months p.i., an indication of a higher threshold for indu ction of class I than class II antigens on microglia. Few cells expres sed viral antigens, indicating that direct immune-mediated destruction of infected cells plays a minor role in evolution of lesions. Since t he preferential induction of MHC antigens on microglia in the white ma tter correlated with the lesion pattern, activated microglia may play a considerable role in the pathogenesis of lesions.