Evolution of the HLA-DR1 gene family. Structural and functional analysis of the new allele "DR-BON".

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Abstract
The molecules of the MHC are highly polymorphic and involve in Ag presentation; their striking genetic polymorphism allows probable interactions with a large variety of antigenic fragments when these are presented to the TCR. It is therefore of interest to explore the extent of this polymorphism and the mechanisms of its generation. We have studied the class II HLA-DR blank allele DR-BON that has been previously defined by MLR, restriction fragment length polymorphism, and two-dimensional gel electrophoresis. A cDNA library was constructed from a DR-BON homozygous typing cell and cDNA corresponding to DR alpha- and DR beta-chains were sequenced. By comparison with other known alpha- and beta-chain sequences it is shown that the alpha-chain is invariant and the beta-chain differs from DR1 by only six nucleotides, clustered in the third variable region with three amino acid changes at position 67, 70, and 71. The short DNA stretch of sequence encoding the 67-71 region is also present in other DR alleles: DR4/Dw10, DRw13, and some DRw11 specificities. Therefore we propose that a gene conversion-like event has occurred between the DRB1 *0101 (DR1/Dw1) gene and one of these three DRB1 genes. Extensive typing has been performed with a DR-BON-specific 17-mer oligonucleotide. Cross-hybridization with other genes than the ones expected from DNA sequence comparison was not observed. A selected panel of DR-BON reactive T cell clones shows three patterns of reactivity. Some clones are strictly DR-BON specific; some cross-reacted with DRw13 and a few cross-reacted with other haplotypes. The role of different epitopes of the third variable region of HLA-DR beta chain in allo-reaction is discussed.

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