IntroductionMultiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) that is histologically characterized by focal mononuclear cell infiltration with a varying extent of demyelination (1, 2). It is postulated that autoimmune T cells recognizing myelin autoantigens such as myelin basic protein (MBP) or proteolipid protein (PLP) play a central role in the pathogenesis of MS. This postulate is based on substantial evidence such as increased frequency of activated MBP-or PLP-specific T cells in peripheral blood and cerebrospinal fluid of MS patients (3-6), and significant complementarity-determining region 3 (CDR3) homology between T-cell infiltrates within MS plaques and MBP-or PLP-specific T-cell clones generated from MS patients (7,8). Moreover, previous studies have identified the peptides MBP84-102 and PLP95-116 as immunodominant and possible encephalitogenic epitopes in MS patients with the HLA-DR2 haplotype (9, 10). However, the role of such autoreactive T cells remains unclear. In fact, it is possible that some (but not all) autoreactive T cells may protect against CNS tissue damage by producing neurotrophic factors (11-13).To address the issue regarding the function of autoimmune T cells, we have introduced HLA-DR2 (DRB1*1502) transgenic mice that are able to mount a T-cell response to DR2-related epitopes in the context of HLA-DR2 molecules. To explore the potential pathogenicity of PLP95-116 in HLA-DR2 + MS, we challenged the HLA-DR2 (DRB1*1502) transgenic mice with PLP95-116 and generated 2 peptide-specific T-cell lines (TCLs) from the mice. These TCLs were HLA-DR2-restricted and produced T-helper type 1 (Th1) cytokine in response to the peptide in the presence of mouse or human antigen-presenting cells (APCs) expressing HLA-DR2 (DRB1*1502) molecules. It was remarkable that transfer of 1 of the TCLs induced atypical autoimmune encephalitis involving the deep cere- In multiple sclerosis (MS) patients who carry the Class II major histocompatibility (MHC) type HLA-DR2, T cells specific for amino acids 95-116 in the proteolipid protein (PLP) are activated and clonally expanded. However, it remains unclear whether these autoreactive T cells play a pathogenic role or, rather, protect against the central nervous system (CNS) damage. We have addressed this issue, using mice transgenic for the human MHC class II region carrying the HLA-DR2 (DRB1*1502) haplotype. After stimulating cultured lymph node cells repeatedly with PLP95-116, we generated 2 HLA-DR2-restricted, PLP95-116-specific T-cell lines (TCLs) from the transgenic mice immunized with this portion of PLP. The TCLs were CD4 + and produced T-helper 1 (Th1) cytokines in response to the peptide. These TCLs were adoptively transferred into RAG-2 -/-mice expressing HLA-DR2 (DRB1*1502) molecules. Mice receiving 1 of the TCLs developed a neurological disorder manifested ataxic movement without apparent paresis on day 3, 4, or 5 after cell transfer. Histological examination revealed inflammatory foci primarily restricted to th...