Characterization of IM virus, which is frequently isolated from cerebrospinal fluid of patients with multiple sclerosis and other chronic diseases of the central nervous system.

A transmissible agent, the IM virus, antigenically related to the Japanese subacute myelo-optico-neuropathy virus, has been isolated from several human cerebrospinal fluids obtained from American patients with multiple sclerosis and other chronic diseases of the central nervous system. The isolates were propagated in human diploid fibroblast (MRC5) cells, and virus was released into the culture medium in the absence of overt cytolysis. Infection of MRC5 cells resulted in a subtle alteration in the normal growth pattern of the cells. In unstained cultures, the cell changes were so mild that it was necessary to carry out all virus assays under code to eliminate bias. Cells in late passages were more susceptible than vigorously growing cells in early passages. Analysis of the kinetics of replication revealed that newly synthesized progeny virus was first detected about 12 h postinfection, that maximal virus release occurred by 48 h postinfection, and that virus production was persistent throughout an 8-day period. Several inhibitors of DNA synthesis were effective in blocking viral replication, including cytosine arabinoside, iododeoxyuridine, and phosphonoacetic acid. A substantial decrease in infectivity was observed upon treatment of IM virus with ether, suggesting that a lipid-containing structure is essential for infectivity. Ultrafiltration studies approximated the size (diameter) of IM virus to be between 100 and 200 nm.