Immune tolerance in multiple sclerosis.

Multiple sclerosis is believed to be mediated by T cells specific for myelin antigens that circulate harmlessly in the periphery of healthy individuals until they are erroneously activated by an environmental stimulus. Upon activation, the T cells enter the central nervous system and orchestrate an immune response against myelin. To understand the initial steps in the pathogenesis of multiple sclerosis, it is important to identify the mechanisms that maintain T-cell tolerance to myelin antigens and to understand how some myelin-specific T cells escape tolerance and what conditions lead to their activation. Central tolerance strongly shapes the peripheral repertoire of myelin-specific T cells, as most myelin-specific T cells are eliminated by clonal deletion in the thymus. Self-reactive T cells that escape central tolerance are generally capable only of low-avidity interactions with antigen-presenting cells. Despite the low avidity of these interactions, peripheral tolerance mechanisms are required to prevent spontaneous autoimmunity. Multiple peripheral tolerance mechanisms for myelin-specific T cells have been identified, the most important of which appears to be regulatory T cells. While most studies have focused on CD4(+) myelin-specific T cells, interesting differences in tolerance mechanisms and the conditions that abrogate these mechanisms have recently been described for CD8(+) myelin-specific T cells.