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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.

Visna is a persistent retrovirus infection of sheep which produces a chronic progressive paralytic disease after an incubation period lasting from months to years. The cerebrospinal fluid (CSF) was repeatedly sampled in a group of Icelandic sheep which were infected intracerebrally and followed up to 42 months. Minimal levels of infectious virus were isolated from the cerebrospinal fluid (CSF) up to 4 months after infection after which CSF neutralizing antibodies appeared in many sheep. These antibodies varied in titer and in some animals exceeded serum antibody levels which were moderate to high. CSF antibody is apparently produced within the CNS by local proliferation of B cell clones, and is accompanied by the appearance of considerable numbers of plasma cells in the neural parenchyma. Some sheep raised serum antibody to a second serotype of visna virus and in a number of these animals heterotypic antibody was also found in the CSF. An increase in CSF leukocytes often occurs within 1 to 3 months following infection and may then persist or wane. A persistent high level of CSF cells is an indicator of progressive CNS disease and such animals are more likely to yield virus, have higher CSF antibody levels, more severe CNS lesions, and an enhanced risk of clinical illness (progressive paralysis). CSF cells are predominantly macrophages and lymphocytes, with a consistent minority of plasma cells.

It was hypothesized that the lesions of visna might represent an immunopathologic process. To test this hypothesis, a 1-month schedule of immunosuppressive treatment was devised, using horse anti-sheep thymocyte serum. In Hampshire sheep, this regime was shown to protect against experimental allergic encephalomyelitis, to inhibit development of tuberculin hypersensitivity, to retard rejection of skin homografts by 3 weeks, and to markedly reduce the number and mitogenic responsiveness of peripheral blood lymphocytes. Two groups of Icelandic sheep received intracerebral inoculations of visna virus, and one group was treated with horse antisheep thymocyte serum supplemented by a short terminal course of cyclophosphamide. Central nervous system lesions were seen in only one of eight suppressed animals at sacrifice 25 days after infection, whereas definite lesions were present in eight of eight infected control animals. The frequency of central nervous system virus isolation was similar in the two groups, indicating that treatment suppressed the cellular proliferative response without preventing the central nervous system phase of infection. Sheep receiving horse antisheep thymocyte serum had a reduced number of virus isolations from peripheral lymphoid tissue, presumably reflecting the lympholytic effect of treatment. These observations are consistent with the immunopathologic hypothesis and suggest several different ways in which suppression could modify the immune response to visna virus.

Iceland offers a favourable opportunity to examine the suggested relationship between canine distemper and multiple sclerosis. Distemper is not enzootic in Iceland and distemper immunisation is not practised. However, importations result in occasional epizootics, three of which have occurred since 1909. Careful enumeration of multiple sclerosis indicates that there were 129 cases during the period 1946--65. When these cases are subdivided into six regions, by place of birth, regional period-prevalence rates were highest in two regions partially involved by distemper only once in the past 70 years. Also, there was substantial prevalence in a third region, encompassing Reykjavik, where the dog population has been kept very low for over 50 years. The Icelandic experience indicates that multiple sclerosis can occur at high incidence in the virtual absence of canine distemper or in the presence of a very small dog population.

Serological responses of Icelandic sheep experimentally infected with visna virus (vv) were contrasted with responses in American Targhee sheep naturally infected with progressive pneumonia virus (PPV). Precipitating antibodies assayed by immunodiffusion were compared with the neutralising and complementing fixing antibody response. In experimental infections with vv, complement fixing and neutralising antibodies appeared early after infection and rose to high levels in all sheep, while precipitating antibodies were detected only at minimal titre. In natural infections with PPV, immune responses were less consistent and precipitating antibodies were detected more frequently than complement fixing or neutralising antibodies against PPV. These results may suggest important biological differences between the lytic fibroblast-tropic virus strains used for experimental infection of Icelandic sheep and the nonlytic macrophage-tropic strains of PPV circulating in nature. Lytic strains evoke a brisk response against the viral glycoprotein with high titre neutralising antibody while nonlytic strains induce a less consistent response to the glycoprotein.

Icelandic sheep were injected intracerebrally with visna virus, which produces a persistent infection of the CNS accompanied by encephalomyelitis and focal demyelinating lesions. Studies were conducted on two groups of sheep, with short-term infections (25 sheep sampled 1-3 months after infection) and long-term infections (14 sheep sampled 5-6 years after infection). Quantitative determination of CSF immunoglobulin levels 5 years after infection indicated that IgM concentration was usually elevated, IgG2 was occasionally elevated and IgG1 was rarely elevated. CSF oligoclonal bands were seen in about half the sheep examined 5 years after infection. There was a correlation between high titers of CSF antiviral antibody and both elevated CSF IgM concentration and CSF oligoclonal bands. Serum/CSF IgG1 ratios indicated that the blood-brain barrier was apparently intact in long-term visna infection, consistent with intrathecal synthesis of IgM and of antiviral antibody. The alterations in CSF immunoglobulins in visna resemble those found in other persistent CNS virus infections and in multiple sclerosis.