Binding motifs of copolymer 1 to multiple sclerosis- and rheumatoid arthritis-associated HLA-DR molecules

Copolymer 1 (Cop 1, poly (Y, E, A, K)) is a random synthetic amino acid cop olymer effective in the treatment of relapsing forms of multiple sclerosis (MS), Cop 1 binds promiscuously, with high affinity and in a peptide-specif ic manner to purified MS-associated HLA-DR2 (DRB1*1501) and rheumatoid arth ritis-associated HLA-DR1 (DRB1*0101) or HLA-DR4 (DRB1*0401) molecules, In t he present work at least 95% of added Cop 1 could be bound to recombinant " empty" HLA-DR1 and -DR4, and 80% could be bound to HLA-DR2 proteins. Amino acid composition, HPLC profiles, and sequencing patterns of Cop 1 eluted by acid extraction from HLA-DR molecules were similar to those of the unsepar ated Cop 1, Protruding N-terminal ends of Cop 1 bound to HLA-DR1, -DR2, or -DR4 molecules were then treated with aminopeptidase I, followed by elution , HPLC, and pool sequencing. In contrast to untreated or unbound Cop I, thi s material exhibited distinct motifs at some positions with increases in le vels of E at the first and second cycles, of Ii at the second and third cyc les, and of Y (presumably at P1 of the bound peptide) at the third to fifth cycles, regardless of the HLA-DR molecule employed. No preference was seen at the following cycles that were mainly A. These first pooled HLA-DR bind ing epitopes provide clues to the components of Cop 1 that are biologically active in suppressing MS and possibly rheumatoid arthritis.