Visualization of myelin basic protein (MBP) T cell epitopes in multiple sclerosis lesions using a monoclonal antibody specific for the human histocompatibility leukocyte antigen (HLA)-DR2-MBP 85-99 complex

Susceptibility to multiple sclerosis (MS) is associated with the human hist ocompatibility leukocyte antigen (HLA)-DR2 haplotype, suggesting that major histocompatibility complex class II-restricted presentation of central ner vous system-derived antigens is important in the disease process. Antibodie s specific for defined HLA-DR2-peptide complexes may therefore be valuable tools for studying antigen presentation in MS. We have used phage display t echnology to select HLA-DR2-peptide-specific antibodies from HLA-DR2-transg enic mice immunized with HLA-DR2 molecules complexed with an immunodominant myelin basic protein (MBP) peptide (residues 85-99). Detailed characteriza tion of one clone (MK16) demonstrated that both DR2 and the MBP peptide wer e required for recognition. Furthermore, MK16 labeled intra- and extracellu lar HLA-DR2-MBP peptide complexes when antigen-presenting cells (APCs) were pulsed with recombinant MBP. In addition, MK16 inhibited interleukin 2 sec retion by two transfectants that expressed human MBP-specific T cell recept ors. Analysis of the structural requirement for MK16 binding demonstrated t hat the two major HLA-DR2 anchor residues of MBP 85-99 and the COOH-termina l part of the peptide, in particular residues Val-96, Pro-EX, and Arg-99, w ere important for binding. Based on these results, the antibody was used to determine if the HLA-DR2-MBP peptide complex is presented in MS lesions. T he antibody stained APCs in MS lesions, in particular microglia/macrophages but also in some cases hypertrophic astrocytes. Staining of APCs was only observed in MS cases with the HLA-DR2 haplotype but not in cases that carri ed other haplotypes. These results demonstrate that HLA-DR2 molecules in MS lesions present a myelin-derived self-peptide and suggest that microglia/m acrophages rather than astrocytes are the predominant APCs in these lesions .