Peptide‐binding motifs of the mixed haplotype Aβ z /Aα d major histocompatibility complex class II molecule: a restriction element for auto‐reactive T cells in (NZB×NZW)F 1 mice

We previously showed that the mixed haplotype Aβ z /Aα d major histocompatibility complex (MHC) class II molecules function as restricting element for autoreactive T‐cell clones derived from autoimmune prone (NZB×NZW)F 1 (B/WF 1 ) mice. Subsequent analysis revealed that some of these Aβ z /Aα d ‐restricted autoreactive T‐cell clones were pathogenic upon transfer to pre‐autoimmune B/WF 1 mice. In this paper, we analysed the peptide‐binding motif of Aβ z /Aα d class II molecules. Amino acid‐sequencing analysis of peptides eluted from purified Aβ z /Aα d molecules revealed several sequences, including one that corresponds to murine l‐plastin 588–601. Synthetic 18‐mer l‐plastin 588–605 peptide (SMARKIGARVYALPEDLV, as described by the amino acid single letter code) was demonstrated to bind to Aβ z /Aα d MHC class II molecules on transfectant B lymphoma cells (TAβ z ). A competitive binding inhibition assay using truncation peptides revealed the core sequence for binding resides in 591Arg to 601Pro. Binding inhibition assay using substitution peptides, each having substitution to the other 19 residues at positions from 590Ala to 601Pro, revealed four major anchor sites 592Lys (p1), 594Gly (p3), 595Ala (p4), 597Val (p6) and one minor anchor site 600Leu (p9). Positively charged residues are not allowed at p3 and negatively charged residues are not allowed at p4 and p6. Relatively large hydrophobic residues (Leu, Ile) are not tolerated at p3 and p4. Met and Trp are not tolerated at p6. Based on these findings, the characteristics of peptides recognized by autoreactive T cells in B/WF 1 mice are discussed.