Demyelination induced in mice by avirulent Semliki Forest virus. II. An ultrastructural study of focal demyelination in the brain.

Four-week-old mice were infected intraperitoneally with an avirulent strain A7(74) of Semliki Forest virus (SFV) and killed at 4, 10, 15, 21 and 29 days after inoculation. Focal demyelinating lesions were present by 10 days. These were usually accompanied by a mononuclear infiltrate which included lymphocytes possessing characteristic cytoplasmic projections. These latter extended deep into the cytoplasm of adjacent cells, which were usually astrocytes and macrophages. Other features of the focal lesions were expansion of the extracellular space and demyelination which appeared to be fragmentation or lysis rather than stripping of myelin by macrophages. Although healing occurred in some mice after 4 weeks, acute lesions were still found in others of the same age. It was concluded that the demyelination probably had an immunological basis, and interaction between elements of the immune system and glial cells was a factor which inhibited orderly remyelination of the relatively mild lesions resulting from this infection.