Inflammatory mediators in demyelinating disorders of the CNS and PNS.
Work in both experimental models and human disorders of the central and peripheral nervous system has delineated multiple effector mechanisms that operate to produce inflammatory demyelination. The role of various soluble inflammatory mediators generated and released by both blood-borne and resident cells in this process will be reviewed. Cytokines such as interleukin (IL)-1, interferon (IFN)-gamma, and tumor necrosis factor (TNF)-alpha are pivotal in orchestrating immune and inflammatory cell-cell interactions and represent potentially noxious molecules to the myelin sheath, Schwann cells, and/or oligodendrocytes. Arachidonic acid metabolites, synthesized by and liberated from astrocytes, microglial cells and macrophages, are intimately involved in the inflammatory process by enhancing vascular permeability, providing chemotactic signals and modulating inflammatory cell activities. Reactive oxygen species can damage myelin by lipid peroxidation and may be cytotoxic to myelin-producing cells. They are released from macrophages and microglial cells in response to inflammatory cytokines. Activation of complement yields a number of inflammatory mediators and results in the assembly of the membrane attack complex that inserts into the myelin sheath-creating pores. Activated complement may contribute both to functional disturbance of neural impulse propagation, and to full-blown demyelination. Proteases, abundantly present at inflammatory foci, can degrade myelin. Vasoactive amines may play an important role in breaching of the blood-brain/blood-nerve barrier. The importance of nitric oxide metabolites in inflammatory demyelination merits investigation. A better understanding of the multiple effector mechanisms operating in inflammatory demyelination may help to devise more efficacious antigen non-specific therapy.