Identification of an immunodominant region on the I-A beta chain using site-directed mutagenesis and DNA-mediated gene transfer

To identify which polymorphic residues determine the allospecific antibody binding sites on A beta polypeptides, mutant Ak beta genes were constructed encoding single or multiple amino acids of the d allele at 14 polymorphic positions in the beta 1 domain. Cell lines expressing these genes were analyzed by quantitative immunofluorescence using 16 mAbs reactive to Ak beta or Ad beta. Substitution of d allele residues at positions 63 and 65-67 in the Ak beta polypeptide resulted in the loss of binding of all Ak beta-reactive antibodies and the gain of binding of most Ad beta-reactive antibodies. Two Ad beta-reactive mAbs bound to the mutant Ak beta polypeptide containing d allele-characteristic residue at position 40. In contrast, substitution of the other polymorphic residues in the NH2-terminal and COOH-terminal regions of the beta 1 domain did not alter antibody binding.