Effect of viral infection on experimental allergic encephalomyelitis in mice.

BALB/c mice were irradiated with 350 R and injected with mouse spinal cord homogenate (MSCH) in complete Freund's adjuvant. Only 15-30% of these animals developed signs of experimental allergic encephalomyelitis (EAE) at 21-28 days after inoculation. Intraperitoneal infection with the non-lethal A7 strain of Semliki forest virus (SFV) 7 days after sensitization reduced the mean appearance time of the EAE symptoms to 14 days and the number of animals with clinical EAE increased up to 70%. In contrast, virus inoculation 10 days before induction of EAE decreased significantly the incidence of clinical EAE in both BALB/c and SJL mice. Demyelination with increased cellularity, presence of macrophages, stripping of myelin from the axons and sparing of oligodendrocytes was observed in spinal cords of animals at days 13-16 after induction of EAE and subsequent virus infection. No demyelination was seen in specimens taken at the same time from mice inoculated with MSCH or SFV alone. Combined MSCH and virus inoculations induced changes in the general immune response which may be one of the major reasons for the increase or decrease in demyelination in this model.