Ligand motifs of HLA-DRB5*0101 and DRB1*1501 molecules delineated from self-peptides.

Antigenic peptides are presented to CD4+ T cells by MHC class II molecules via a highly polymorphic peptide-binding groove. The two HLA-DR alleles isotypically expressed on HLA-DR15Dw2-positive cells, DRB11501 (DR2b) and DRB50101 (DR2a) molecules, show a number of differences in polymorphic residues of the beta-chain, including the Gly-Val-dimorphism at position beta 86. Therefore, different requirements for interaction of peptides with these alleles must be expected. In this study, naturally processed self-peptides were eluted from purified HLA-DR15Dw2 molecules and related to DRB11501 or DRB50101 molecules by binding assays. An alignment of self-peptides and foreign peptides allowed the delineation of putative anchor motifs. N- and C-terminally truncated and alanine-substituted derivatives of the DR15Dw2 restricted myelin basic protein epitope MBP(85-105) confirmed their validity. Thus, DRB50101 requires a bulky hydrophobic residue (F or Y) at position i as a primary anchor, and Q or an aliphatic residue, such as V, I, or M, at position i + 3; positively charged residues at positions i + 7 and i + 8 are secondary anchors. For DRB11501, a nonaromatic, hydrophobic anchor (L, V, or I) at position i is supplemented by a bulky hydrophobic residue (F or Y) at position i + 3 as primary anchor; an additional hydrophobic side chain represented by M, I, V, or F occurs at position i + 6. Therefore, MBP(85-105) seems to contain two MHC interaction sites for DRB11501 and DRB50101, respectively, that may contribute to its immunodominance. Because HLA-DR15 Dw2 is associated with susceptibility to develop multiple sclerosis, the delineation of ligand motifs of the two DR2 alleles may help to study the interaction between potential autoantigenic peptides and these molecules in the future.