Progressive multifocal leukoencephalopathy is a fatal demyelinating disease of the central nervous system frequently seen in patients with impaired immune systems, particularly acquired immunodeficiency syndrome. JC virus (JCV), a human neurotropic polyomavirus, is the etiologic infectious agent of this disease.
The significantly higher incidence of progressive multifocal leukoencephalopathy in patients with acquired immunodeficiency syndrome than in patients with other immunosuppressive conditions suggests that molecular interactions between human immunodeficiency virus 1 and JCV, via the Tat protein, are responsible for the activation of the JCV enhancer/promoter and the development of progressive multifocal leukoencephalopathy. An indirect mechanism through activation of cytokines, such as transforming growth factor beta1 and Smads 3 and 4, may also be responsible for the enhancement of JCV gene expression.
Immunohistochemical analysis in progressive multifocal leukoencephalopathy samples and chloramphenicol acetyl transferase assays on cell cultures were performed to corroborate this hypothesis.
The JCV capsid protein VP-1 was found in the nuclei of oligodendrocytes and in the nuclei and cytoplasm of bizarre astrocytes. Human immunodeficiency virus proteins, including p24 and Tat, were detected in the cytoplasm of astrocytes. Tat, but not p24, was detected in oligodendrocytes, suggesting that extracellular Tat accumulates in the nuclei of oligodendrocytes, where JCV gene transcription takes place. High levels of transforming growth factor beta1 and Smads 3 and 4 were detected in JCV-infected oligodendrocytes. Results from in vitro studies confirm activation of the JCV early and late promoters by Smads 3 and 4.
These observations support our model, suggesting that the induction of transforming growth factor beta1 by human immunodeficiency virus 1 Tat can stimulate its downstream factors, including Smads 3 and 4, which in turn augment transcription of the JCV promoter in glial cells.