What causes progressive multifocal leukoencephalopathy?
Progressive multifocal leukoencephalopathy is believed to be due to the Jacob-Creutzfeldt (JC) papovavirus. The cause of PML is a type of polyomavirus called the JC virus, after the initials of the patient in whom it was first discovered. The virus is widespread, found in at least 70 percent of the general population by some estimates, but usually remains latent, causing disease only when the immune system has been severely weakened. The virus infects oligodendrocytes (support cells in the brain). Although at least 80% of the adults in the United States have been exposed to JC virus (as evidenced by the presence of antibodies to this virus), very few will develop PML. Little is certain about what causes JCV to produce active disease, but the virus persists in the kidneys of otherwise healthy people without making them ill. Recent evidence suggests that after prolonged compromise of the immune system, the virus changes into a form that can reach brain tissue and cause disease. In PML, the JCV infects and kills the cells (oligodendrocytes) that produce myelin, which is needed to form the sheath that surrounds and protects nerves.