EFFECT OF AMINO-ACID SUBSTITUTIONS WITHIN THE REGION-62-76 OF I-A-BETA(B) ON BINDING WITH AND ANTIGEN PRESENTATION OF TORPEDO ACETYLCHOLINE-RECEPTOR ALPHA-CHAIN PEPTIDE-146-162

Previous study has shown that reduced T cell response to peptide alpha 146-162 of Torpedo californica acetylcholine receptor (tAChR) in B6.C -H-2b(bm12) (bm12) mice, a mutant of C57BL/6 (B6) mice, correlated wit h its nonsusceptibility to experimental autoimmune myasthenia gravis. There are three amino acid differences between the I-A beta(b) of the two strains (positions 67, 70, and 71). We synthesized peptides I-A be ta(b)62-76 (peptide b6), I-A beta(bm12)62-76 (peptide bm), and three a dditional peptides, b6(67F), b6(70Q), and b6(71K), and determined thei r ability to bind peptide alpha 146-162 and the dissociation constants (K-d) of the binding. Peptide alpha 146-162 bound with a significantl y higher affinity to peptide b6 than to peptides bm or b6(71 K), sugge sting that the lower affinity of peptide alpha 146-162 to I-A(bm12) is a factor in the reduced response to this peptide by bm12 T cells. Thi s was confirmed by measurement of the K-d values of the binding of pep tide alpha 146-162 to the I-A molecules of B6 and bm12. Furthermore, A PC of bm12 presented the peptide, or tAChR, poorly to peptide-specific or to t4ChR-specific B6 T cells. The major effect is caused by the ch ange of Thr-71 in I-A beta(b) to lysine in I-A beta(bm12). However, AP C of B6 also presented peptide alpha 146-162 much less efficiently to peptide-specific T cells of bm12. This demonstrated that these three a mino acid changes also influence the T cell receptor recognition of pe ptide-MHC complex and that both B6 and bm12 T cells recognizing peptid e alpha 146-162 or tAChR are under a high H-2 restriction.