Antibodies to myelin components are routinely detected in multiple sclerosis patients. However, their presence in some control subjects has made it difficult to determine their contribution to disease pathogenesis. Immunization of C57BL͞6 mice with either rat or human myelin oligodendrocyte glycoprotein (MOG) leads to experimental autoimmune encephalomyelitis (EAE) and comparable titers of anti-MOG antibodies as detected by ELISA. However, only immunization with human (but not rat) MOG results in a B cell-dependent EAE. In this study, we demonstrate that these pathogenic and nonpathogenic anti-MOG antibodies have a consistent array of differences in their recognition of antigenic determinants and biological effects. Specifically, substituting proline at position 42 with serine in human MOG (as in rat MOG) eliminates the B cell requirement for EAE. All MOG proteins analyzed induced high titers of anti-MOG (tested by ELISA), but only antisera from mice immunized with unmodified human MOG were encephalitogenic in primed B cell-deficient mice. Nonpathogenic IgGs bound recombinant mouse MOG and deglycosylated MOG in myelin (tested by Western blot), but only pathogenic IgGs bound glycosylated MOG. Only purified IgG to human MOG bound to live rodent oligodendrocytes in culture and, after cross-linking, induced repartitioning of MOG into lipid rafts, followed by dramatic changes in cell morphology. The data provide a strong link between in vivo and in vitro observations regarding demyelinating disease, further indicate a biochemical mechanism for anti-MOGinduced demyelination, and suggest in vitro tools for determining autoimmune antibody pathogenicity in multiple sclerosis patients. multiple sclerosis ͉ experimental autoimmune encephalomyelitis ͉ lipid rafts ͉ B cell-deficient mice ͉ encephalitogenicity M ultiple sclerosis (MS) is a chronic demyelinating disease of the central nervous system (CNS) in which both T cells and antibodies against myelin antigens are routinely detected (1, 2). B cell responses in MS pathogenesis are implicated by the presence of Ig deposits and myelin debris in demyelinating lesions (3-8), and the observation that plasma exchange dramatically reduces clinical disease in a subset of patients (9). Of particular interest to the present study, antibodies to myelin oligodendrocyte glycoprotein (MOG) are detected in the sera and plaques of MS patients (10), and thus are possible predictors of disease progression (11). However, because some control subjects can also harbor anti-myelin antibodies (1, 2, 12), their contribution to MS pathogenesis has been controversial and difficult to identify in individual patients. Further complicating the issue, MS may be several diseases of differing etiologies (5), whereby anti-myelin antibodies may be pathogenic in some forms of MS but merely a reflection of tissue damage in others. Thus, an understanding of whether anti-myelin antibodies are in fact pathogenic, and if so, by what mechanisms they operate, could provide important information for novel diagnosti...