Chronic Theiler's murine encephalomyelitis virus infection of susceptible mice is an animal model for human demyelinating diseases. Previously we described an altered and diminished pattern of central nervous system disease in immunocompetent SJL/J mice infected with a variant virus. This variant virus H7A6-2 was selected with a neutralizing mAb recognizing the capsid protein VP-1 of Theiler's virus. Here we characterize the variant virus by ELISA and neutralization assays and by sequencing selected regions of the viral RNA genome and relate the alteration to disease. The variant virus contains one single point mutation within a neutralizing epitope of VP-1. This nucleotide change lead to an amino acid replacement at amino acid 101 of VP-1, a threonine (wild type) to an isoleucine (variant). Model building based on sequence alignments and the known structure of the related Mengo virus indicates that the altered amino acid is located in an exposed loop on the surface of the virus at the periphery of a site that has been proposed to be the receptor binding site. The results of ELISA, neutralization assay, and direct RNA sequencing provide for the first time an opportunity to precisely map an important structural determinant of neurovirulence.

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