Progressive multifocal leukoencephalopathy (PML), also known as progressive multifocal leukoencephalitis, is a rare and usually fatal viral disease that is characterized by progressive damage (-pathy) or inflammation of the white matter (leuko-) of the brain (-encephalo-) at multiple locations (multifocal).
It occurs almost exclusively in people with severe immune deficiency, such as transplant patients on immunosuppressive medications, patients receiving certain kinds of chemotherapy, patients receiving natalizumab (Tysabri) for multiple sclerosis, psoriasis patients on long-term efalizumab (Raptiva) or AIDS patients.
It is caused by a virus, the JC virus, which is normally present and kept under control by the immune system. Immunosuppressive drugs prevent the immune system from controlling the virus.
The cause of PML is a type of polyomavirus called the JC virus (JCV), after the initials of the patient from whose tissue the virus was first successfully cultured. Recent publications indicate 39% to 58% of the general population are seropositive for antibodies to JCV, indicating current or previous infection with virus. The virus can cause persistent asymptomatic infection in approximately one third of the adult population, based on viral shedding into the urine from the site of asymptomatic infection in the kidney. The virus causes disease only when the immune system has been severely weakened.
Prior to the advent of effective antiretroviral therapy, as many as 5 percent of people with AIDS eventually developed PML. It is unclear why PML occurs more frequently in AIDS than in other immunosuppressive conditions; some research suggests that the effects of HIV on brain tissue, or on JCV itself, make JCV more likely to become active in the brain and increase its damaging inflammatory effects.
There are case reports of PML being caused by pharmacological agents, although there is some speculation this could be due in part to the existing impaired immune response or 'drug combination therapies' rather than individual drugs. These include efalizumab, rituximab, belatacept, infliximab, natalizumab, chemotherapy, corticosteroids, and various transplant drugs such as tacrolimus.
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