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TITLE: SEROLOGIC EPITOPES OF HLA–DR8 AND HLA–DRw52

Christina L. Newman, Donna P. Lucas, Brian A. Iglehart, Mary S. Leffell, Andrea A. Zachary.

Department of Medicine, Johns Hopkins University, Baltimore, MD

It has been proposed that the gene encoding the DR8 molecule arose through a deletion that juxtaposed the 5` end of a DRB1 gene to the 3` end of a DRB3 gene leaving only one functional DRB locus on DR8–bearing haplotypes. DRw52 epitopes on DR8 molecules most likely account for the frequent occurrence of antibodies that react with both DR8 and DRw52. In tests of antibodies by ELISA, we have observed three antibody patterns: [1]antibodies that react with both DR8 and DRw52, produced by individuals lacking both antigens; [2] antibodies that react only with DR8, produced by individuals who are DR8–DRw52+; and [3] antibodies reactive only with DRw52, arising in DR8+DRw52– individuals. We have performed absorptions to characterize the nature of the antibodies and, by deduction, of the epitopes. Abrogation of reactivity of antibodies reactive with both DR8 and DRw52 by absorption with cells positive for only DR8 or DRw52 indicates the presence of at least one epitope common to both molecules. Conversely, we observed no change in reactivity when either anti–DR8 antibody was absorbed with DR8–DRw52+ cells or anti–DRw52 antibody was absorbed with DR8+DRw52– cells. We sequenced the DRB1 and DRB3 alleles of individuals making antibodies to DRw52 and DR8, respectively. There was no consistency in the DRB1*08xx alleles with *0801, *08032, and *08041 all being found. In contrast, all DRw52+ individuals making anti–DR8 antibody possessed the DRB3*0202 allele which, in our population, is the less frequent DRB3 allele, compared to DRB3*0101. Comparisons of the nucleic acid sequences of the DRB1*0801 and DRB3 alleles revealed 5 codon differences, three of which, encoding residues at positions 28, 30 and 51, distinguish DRB3*02021 from both DRB1*0801 and DRB3*0101. Residue 51 is located on the &bgr;1 alpha helix and would be most accessible to antibody. The substitution at position 51 replaces a threonine on DRB1*0801 and DRB3*0101 with an arginine on DRB3*02021. This amino acid change substitutes a non–charged residue with a charged one and may result in a serologically–defined epitope that differentiates certain DR8 and DRw52 molecules.