Autoimmune and virus-induced demyelinating diseases. A review.

Patterns of demyelination are described in several autoimmune and virus-induced demyelinating diseases of the peripheral and central nervous system. Myelin can be destroyed by injuries that affect either the myelin-supporting cells and/or the myelin lamellae. After destruction of the supporting cells, the related disintegrating sheaths are stripped off axons by invading phagocytes. Virus-induced cytolysis can occur with or without participation of immune responses, as demonstrated in subacute sclerosing panencephalitis and progressive mutlifocal leukoencephalopathy, respectively. Autoimmune demyelination is characterized by disintegration of myelin sheaths in periventular, mononuclear cell infiltrates. Myelin lamellae rather than the myelin-supporting cells are the target of the allergic reaction. The lamellae are lysed in focal areas when in contact with presumably sensitized mononuclear cells. The damaged sheaths are then removed in a nonspecific manner by invading macrophages that strip the myelin remnant off the axons. This sequence of changes is best revealed in experimental and human autoimmune demyelination of peripheral nerves, ie, allergic neuritis and idiopathic polyneutris (the Guillain-Barré syndrome). Autoimmune demyelination triggered by virus infection is described in Marek's disease and postinfectious Theiler's virus myelitis. Changes in canine distemper are discussed with reference to both autoimmune and virus-induced demyelination. The observations are compared with lesions in multiple sclerosis, the most common human demyelinating disease of unknown etiology.