Evidence that Fas and FasL contribute to the pathogenesis of experimental autoimmune encephalomyelitis
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The well established and characterized animal model for the human demyelinating autoimmune disease mulitple sclerosis (MS) is known as experimental autoimmune encephalomyelitis (EAE). EAE is clinically characterized by focal areas of inflammation and demyelination and an infiltrate composed of large numers of lymphocytes and macrophages, often found in a perivascular localization but also throughout the central nervous system (CNS). Active immunization of mice with several different protein components of myelin, including myelin basic protein (MBP), proteolipid protein (PLP) and myelin oligodendrocyte glycoprotein (MOG), are capable of eliciting an immune response resulting in the quintessential symptoms of EAE: ascending paralysis involving the tail and then the limbs. Depending on this mouse strain and myelin antigen utilized, the disease course can be acute or chronic relapsing, characterized by a rapid onset of hind limb weakness that commonly progresses to paralysis, followed by spontaneous remission starting 7-10 days after the initial appearance of symptoms. EAE can also be induced passively by the adoptive transfer of in vitro activated CD4+ T cell clones or lines, typically of the Th1 phenotype, into irradiated susceptible recipients. The mechanisms involved in the cellular pathogenesis leading to paralysis and demyelination have been extensively studied and are primarily mediated by CD4+ T cells of the Th1 phenotype, with specificity for myelin antigens. Following activation, Th1 CD4 T cells produce in abundance the inflammatory cytokine TNF-, IFN- and lymphotoxin-alpha (LT-alpha, also known as TNF-beta). IFN-gamma production is highly correlated with encephalitogenicity and may contribute to disease by up-regulation of adhesion molecules on endothelial cells, facilitating migration of lymphocytes into the CNS; by induction of MHC class I and MHC class II molecules on astrocytes, microglial cells and brain endothelium, facilitating Ag presentation in the CNS; and by activation of macrophages, leading to production of nitric oxide, a potent cytotoxic molecule. TNF-alpha and LT-alpha are both members of the TNF family of molecules and cause cell death by apoptosis following interaction with their counter-receptors, the TNFR1 and TNF2, leading to a cascade of proteolytic events culminating in the blebbing of the cytoplasmic membrane, nuclear condensation and DNA fragmentation. Consequently, the production of TNF-alpha and LT-alpha by Th1 clones has been correlated with encephalitogenic potential and Abs to both prevents EAE upon transfer of encephalitogenic clones. Even though substantial evidence exists for the role of inflammatory cytokines in the pathogenesis of EAE, other mechanisms of myelin destruction are thought to exist. To date, many reports have implicated a role for the cell death-inducing ligand pair Fas and Fas ligand (FasL).
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B.N. Dittel, Section of Immunobiology, Yale School of Medicine , New Haven, CT 06520-8011, USA