Pathogenesis of visna. III. Immune responses to central nervous system antigens in experimental allergic encephalomyelitis and visna.

Experimental allergic encephalomyelitis (EAE) was induced in sheep in pursuit of the hypothesis that an immune response against central nervous system antigens might play a role in the pathogenesis of visna. Nine to 12 days after sensitization with whole sheep brain and complete Freund's adjuvant, approximately 50% of sheep developed a fulminating lethal form of EAE. Following a second sensitization, another 20% of animals developed EAE whereas a residual 30% failed to develop any signs or histologic evidence of disease. A histologic comparison of EAE and visna indicated considerable similarity in the nature of the pathologic process. However, the distribution of lesions was quite different, suggesting cellular responses to two different antigens, Cell-mediated immunity to myelin basic protein, as measured by lymphocyte blast transformation, was minimally elevated in sheep sensitized with whole brain suspension in complete Freund's adjuvant, whereas no response could be detected in visna-infected sheep. Complement-fixing antibody titers to basic protein and to a lipid antigen of brain, probably galactocerebroside, rose briskly after sensitization. In visna-infected sheep, on the other hand, there was no increase in either antibody. A large proportion of both Hampshire and Icelandic sheep had low levels of complement-fixing antibody to central nervous system antigens prior to induction of EAE or infection with visna virus. The origin of this antibody is undetermined, but it appeared to have no effect on the course of either disease. Immunosuppression of sheep with antilymphoid serum prevented induction of EAE. Acute EAE was, thus, successfully induced in sheep and used as a model to measure immune responses to central nervous system antigens and as an index of immunosuppression. However, these comparative studies did not provide any evidence for the role of an autoimmune response, to the two central nervous system antigens tested, in the pathogenesis of visna.