A neutralization-resistant Theiler's virus variant produces an altered disease pattern in the mouse central nervous system.

Theiler's murine encephalomyelitis virus infection of mice is an animal model for human demyelinating diseases. To further define the role of this virus in the disease process, we selected a virus variant resistant to neutralization by a monoclonal antibody to VP-1. This virus variant was then injected into SJL/J mice. Central nervous system tissue was compared between variant virus- and wild-type virus-infected mice. Within the brain, no large differences were observed between the two groups as to the distribution of inflammatory infiltrates around the injection site and the number of viral antigen-positive cells during the first weeks of the observation period. In contrast, in the spinal cord major differences were found between variant virus- and wild-type virus-infected mice regarding the number of inflammatory lesions, infected cells, and the size of the areas involved with time. By immunohistochemistry, equivalent numbers of infected cells could be found in the spinal cord 1 week postinfection (p.i.): however, after that time, the number of infected cells in the wild-type virus-infected mice continued to increase, whereas the virus-positive cells from the variant virus-infected mice gradually decreased. Thus, the number of viral antigen-containing cells peaked by 1 week p.i. in the variant virus-infected animals. Conversely, the number of infected cells in the spinal cords from mice inoculated with wild-type virus steadily increased until 8 weeks p.i. At this time (8 weeks p.i.), no more variant virus antigen-positive cells could be observed within the spinal cord. Plaque assay of central nervous system tissue confirmed these differences between the two groups observed by immunohistochemistry. No infectious variant virus could be isolated after 2 weeks p.i. from the brain and 4 weeks p.i. from the spinal cord, whereas infectious wild-type virus could be detected up to the end of the observation period (12 weeks p.i.). Virus which was isolated from variant virus-infected mice still retained the neutralization-resistant phenotype. These studies emphasize the important biological in vivo activity of Theiler's virus VP-1 in determining neurovirulence.