Oligodendrocyte infection and demyelination produced in mice by the M9 mutant of Semliki Forest virus

Intraperitoneal inoculation with the M9 mutant of Semliki Forest virus caused focal demyelinating encephalomyelitis in weanling BALB/c and C57BL/6 mice. Demyelination was more severe in BALB/c than in C57BL/6 mice. Virus particles were seen in oligodendrocytes in areas of myelin vacuolation 5 and 7 days post inoculation (DPI). Oligodendrocytes containing virus in BALB/c mice showed hypertrophy and vacuolar degeneration. There was a mononuclear cell infiltrate and lymphocytes and necrotic cells were present in vacuoles in myelin sheaths. Demyelinating plaques containing macrophages laden with myelin debris were most prominent 14 DPI when virus was cleared from the brain. Remyelination of the central type occurred 28 DPI in BALB/c mice. These findings indicate that direct virus-induced injury to oligodendrocytes has a major role in the initiation of inflammation and demyelination in this model system.