Analysis by NMR Spectroscopy of the Structural Homology between the Linear and the Cyclic Peptide Recognized by Anti-human Leukocyte Antigen Class I Monoclonal Antibody TP25.99*

Abstract

The anti-human leukocyte antigen (HLA) class I monoclonal antibody (mAb) TP25.99 has a unique specificity since it recognizes both a conformational and a linear determinant expressed on the ␤ 2 --associated and ␤ 2 --free HLA class I heavy chains, respectively. Previously, we reported the identification of a cyclic and a linear peptide that inhibits mAb TP25.99 binding to the ␤ 2 --associated and ␤ 2 --free HLA class I heavy chains (S. A. Desai, X. Wang, E. J. Noronha, Q. Zhou, V. Rebmann, H. Grosse-Wilde, F. J. Moy, R. Powers, and S. Ferrone, submitted for publication). The linear X 19 and cyclic LX-8 peptides contain sequence homologous to residues 239 -242, 245, and 246 and to residues 194 -198, respectively, of HLA class I heavy chain ␣ 3 domain. Analysis by twodimensional transfer nuclear Overhauser effect spectroscopy of the induced solution structures of the linear X 19 and cyclic LX-8 peptides in the presence of mAb TP25.99 showed that the two peptides adopt a similar structural motif despite the lack of sequence homology. The backbone fold is suggestive of a short helical segment followed by a tight turn, reminiscent of the determinant loop region (residues 194 -198) on ␤ 2 --associated HLA class I heavy chains. The structural similarity between the linear X 19 and cyclic LX-8 peptides and the lack of sequence homology suggests that mAb TP25.99 predominantly recognizes a structural motif instead of a consensus sequence. HLA1 class I antigens present peptides to cytotoxic lymphocytes and thus play a crucial role in allograft rejection and tumor surveillance. Like their counterparts in other animal species, HLA class I antigens are comprised of a polymorphic heavy chain non-covalently associated with ␤ 2 -. The association of ␤ 2 -to HLA class I heavy chains causes marked changes in their conformation and in their antigenic profile. This finding, with a few exceptions (2, 3), accounts for the selective reactivity of monoclonal and polyclonal allo-and xenoantibodies with either ␤ 2 --free or ␤ 2 --associated HLA class I heavy chains. In the course of the analysis of the fine specificity of a panel of anti-HLA class I mAb, we have found that mAb TP25.99 has the unusual characteristic to react with both ␤ 2 --free and ␤ 2 --associated HLA class I heavy chains (4, 5). As described in a related paper, 2 this reactivity is mediated by the recognition of distinct and spatially distant antigenic determinants located in HLA class I heavy chain ␣ 3 domains. One expressed on ␤ 2 --associated HLA class I heavy chains has been mapped to amino acid residues 194 -198, and the other one expressed on ␤ 2 --free HLA class I heavy chains has been mapped to amino acid residues 239 -242, 245, and 246. The two antigenic determinants have no homology in their amino acid sequence.Since only the x-ray structure of ␤ 2 --associated HLA class I heavy chain (6, 7) has been described, the structural relationship between the two antigenic determinants in the ␤ 2 --free and ␤ 2 --associated HLA class I heavy chains is not known. To obt...