A small number of residues in the class II molecule I-Au confer the ability to bind the myelin basic protein peptide Ac1-11

Abstract

The N-terminal peptide Ac1-11 of myelin basic protein induces experimental autoimmune encephalomyelitis in H-2(u) and (H-2(u) x H-2(s)) mice but does not in H-2(s) mice. Ac1-11 binds weakly to the class II major histocompatibility complex (MHC) molecule I-Au but not at all to I-As. We have studied the interaction of Ac1-11 and I-Au as a model system for therapeutic intervention in the autoimmune response seen in experimental autoimmune encephalomyelitis. Two polymorphic residues that differ between I-Au and I-As, Y26beta and T28beta, and one conserved residue, E74beta, confer specific binding of Ac1-11 to I-Au. A fourth residue, R70beta in I-Au, affects both peptide binding and T cell recognition. These results are consistent with a model that places arginine at position five of Ac1-11 in pockets 4 and 7 of the MHC groove, which is formed in part by residues 26, 28, 70, and 74 of Abetau and places lysine at position four of Ac1-11, previously shown to be a major MHC contact, in hydrophobic pocket 6. The data indicate that the primary region of I-Au that confers specific binding of Ac1-11 lies in the center of the peptide binding groove rather than in the region that contacts the N terminus of the peptide, as has been shown for HLA DR and the homologous I-E molecules.

Figure 1

The positions of residues mutated are indicated on the DR1 structure. The amino acid in Aβ^s is noted on the right of the residue number, and the amino acid in Aβ^u is noted on the left of the residue number. Residues E74β, L35α, F28α, and T86β were mutated singly. Polymorphic residues that differ between Aβ^s and Aβ^u were grouped into HVRs and were mutated on the Aβ^s backbone to those of Aβ^u, either as a single HVR or as combinations of HVRs. HVR1, residues 8β, 9β, 12β, 13β, and 14β; HVR2, residues 26β and 28β; HVR3, residue 70β; and HVR4, residues 81β, 85β, 86β, 88β, and 89β.

Figure 2

Sequences of the β1 domains of I-A^u and I-A^s. Nine of the HVR mutant class II molecules are shown. The residues included in each HVR are underlined in the last mutant listed for the β chains.

Figure 3

Residues in pockets 4, 6, and 7 that are predicted to interact with Ac1-11 as outlined on the DR1 structure. E74β is predicted to interact with arginine at position five in Ac1-11. V9β is predicted to interact with lysine, alanine, or tyrosine at position 4 in Ac1-11, Ac1-11[4A], or Ac1-11[4Y], respectively. The regions that compose pockets 1, 4, 6, 7, and 9 have been circled and labeled accordingly.