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TITLE: POLYMORPHIC RESIDUES LOCATED WITHIN THE GROOVE OF HLA CLASS I ARE NOT SEROLOGICALLY SILENT

Howard M. Gebel,¹ Karen L. Pierce,² Robert A. Bray.<sup>1

1</sup>Pathology, Emory University Hosptial, Atlanta, GA; ²Blood Center of Southeastern Wisconsin, Milwaukee, WI

Antibodies to HLA antigens/alleles are considered to recognize epitopes present on the alpha helices and/or beta loops of HLA molecules. For example, the Matchmaker algorithm (Duquesnoy,. Hum Immunol 63:339,2002), specifically excludes residues residing within the peptide–binding groove based on the assumption that these residues cannot make direct contact with alloantibodies. In this study, we report that serological detection of several antigens is influenced by polymorphic residues located on the floor of the MHC groove. Situation 1: Antisera to HLA–B47 reacted with cells molecularly typed as HLA–B*2705. Applying the concepts of Matchmaker, polymorphic residues on the alpha helices and beta loops shared between HLA–B*2705 and HLA–B47 were excluded as epitopes responsible for the serological activity. None of the HLA–B47 antisera reacted with other antigens (e.g., HLA–B* 1301,4001, 4002, 5501) displaying the same residues. The polymorphic residues shared exclusively between HLA–B2705 and HLA–B47 are residues 113, 114 and 116, all located on the floor of the groove. Situation 2: A patient typed as HLA–A*6802, A*2301 developed an antibody to A*6801. There are five residues that discriminate HLA–A*6801 from 6802, two located on the alpha helix and three on the floor of the groove. Applying Matchmaker, the *6801 antibody should not have been produced. It`s presence suggests that epitopes on the floor of the groove must contribute to the final antigenic conformation. Situation 3: HLA–B*0801, 3801 and 4201 are each recognized by a single antibody when the antigens are attached to a solid phase matrix. In contrast, the antibody is negative on cells expressing these antigens. The single shared residue is position 114. Collectively, our data suggest polymorphic sites accessible to antibody are influenced by the three dimensional configuration of any given HLA molecule. We speculate that polymorphic residues (particularly position 114) on the floor of certain class I HLA molecules alters their three dimensional conformation. Our data indicate that polymorphic residues assumed to be inaccessible to antibodies may not necessarily be serologically innocuous.