Demyelination precedes oligodendrocyte loss in canine distemper virus-induced encephalitis

Canine distemper virus (CDV), a negative-stranded RNA morbillivirus, causes a persistent infection within the central nervous system resulting in a progressive, multifocal demyelinating disease. Demyelination is thought to be caused by a selective alteration of the myelin-producing oligodendrocytes. Metabolic impairment and morphological changes of the oligodendrocytes after CDV infection have previously been observed in vitro as well as in vivo. Until now it has been suggested that the oligodendrocytes completely disappear from CDV-induced demyelinating lesions. However, ultrastructural analysis in brain tissue sections and immunohistochemical examination of oligodendrocytes in dog brain cell cultures contradicted these observations. In this study oligodendrocytes from different categories of CDV-induced lesions were examined by in situ hybridization for proteolipid protein mRNA and – as a new tool employed on canine brain tissue sections – by immunohistochemistry using a monoclonal antibody against 2',3'-cyclic nucleotide 3'-phosphodiesterase, a myelin-specific enzyme. A down-regulation in the myelin gene transcription was detected already before demyelination occurred. However, a decrease in the number of oligodendrocytes was not observed until demyelination became evident. Although there was further depletion of oligodendrocytes in plaques with progressive demyelination, we demonstrated for the first time that these cells were still present in a significant amount even in chronic, completely demyelinated distemper lesions.