Persistent Coronavirus Infection of Progenitor Oligodendrocytes

Abstract

Mouse hepatitis virus (MHV) is a prototype of murine coronavirus. It can infect rodents and causes enteritis, hepatitis, and central nervous system (CNS) diseases. Infection of mouse CNS with neurovirulent MHV strains usually results in acute encephalitis followed by demyelination.1,2 If the majority of the virus can be cleared from the CNS, encephalitis then resolves; if mice survive the acute phase, demyelination develops. Although acute demyelination can be detected histologically as early as 6 days postinfection (p.i.), extensive demyelination is often not seen until 4 weeks p.i.3 However, infectious virus can no longer be isolated from the CNS at this time, although viral RNAs continue to persist in the CNS for more than one year, during which time period demyelination is concomitantly detectable.4,5 The correlation between viral RNA persistence and demyelination in the CNS suggests that viral persistence may be a prerequisite for the development of CNS demyelination. However, virtually nothing is known as to how viral persistence contributes to demyelination. Previous studies attempted to establish an in vitro system of glial or fibroblast cell culture for viral persistence.6,7 Unfortunately, the persistent infection established in these cells is productive, i.e., generation of infectious viruses with significant virus liters. This type of persistence does not reflect on the infection of animal CNS. Recently we established a persistent MHV infection in a progenitor rat oligodendrocyte. We showed that MHV RNAs were continuously detected in infected cells of more than 20 passages. However, no infectious virus could be isolated from these cells. This phenomenon resembles the persistent, nonproductive infection in animal CNS. To understand the molecular basis of viral persistence in host cells, we analyzed the gene expression profiles of the persistently infected cells by using DNA microarray technology and RT-PCR. We found that the expression of a substantial number of cellular genes was altered by viral persistence. Interestingly, although persistently infected progenitor cells could be induced to differentiate into mature oligodendrocytes, the number of dendrites and level of myelin basic protein were markedly reduced in persistent cells. This finding indicates that MHV persistence has an inhibitory effect on oligodendrocyte differentiation and dendrite outgrowth and provides the first direct evidence linking viral persistence to demyelination.