In vitro cytotoxicity and demyelination induced by Theiler viruses in cultures of spinal cord slices.

The cytopathic effects caused by Theiler viruses to myelinated organotypic spinal cord cultures was studied by light and electron microscopy. Heavily myelinated cultures, 2-3 weeks in vitro were infected with WW and GD VII viruses. Mock infection served as control. On light microscopy cytopathic effects and demyelination became evident about 16-17 hr after infection. Demyelination observed in WW virus-infected cultures was much more pronounced than in cultures infected with GD VII viruses. The myelin in mock-infected cultures remained undamaged. Electron microscopy revealed that in control cultures cells were intact, exhibiting numerous synapses and a network of axons enwrapped by multilayered myelin sheaths. Virus-infected spinal cord slices showed that a more severe cytotoxicity was caused by GD VII virus than by WW virus. The cytopathology included accumulation of cytoplasmatic vacuoles, margination of chromatin, synapse and cell disintegration, and various degrees of demyelination. Several GD VII virions were observed, arranged in crystalline arrays, mainly in electron-opaque cells, but not within axons. WW virions on the other hand were only occasionally encountered.