Human Leukocyte Antigen B-Cell Epitope Tertiary Structure and Surface Electrostatic Potential Reveals the Molecular Basis for Alloantibody Binding and Epitope Immunogenicity.

Abstract

Introduction The potential of donor HLA B-cell epitopes to induce recipient humoral alloimmunity depends on their structural and physiochemical properties. We determined the three-dimensional structure and electrostatic potential of two widely-expressed HLA B-cell epitopes and examined the impact of amino acid mutations on HLA antibody reactivity. Methods The structure of 24 HLA-B alleles, expressing either the Bw4 or Bw6 epitope (defined by sequence motifs at positions 77-83), were predicted using homology modelling (MODELLER) based on the structure of crystallographically-resolved HLA molecules. Multiple conformers were sampled from a 1 ns molecular dynamics simulations using GROMACS. The electrostatic potential in three-dimensional space encompassing the Bw4/Bw6 epitope, for each HLA-B molecule, was computed by solving the Poisson-Boltzmann equation and quantitatively compared with one another to form dendrograms that cluster epitopes with similar electrostatics properties. Amino acid mutations within the 77-83 sequence motifs were also examined. Results Comparison of the electrostatic potential in the space surrounding residues 77-83 allowed tight clustering of HLA-B molecules according to Bw4 or Bw6 epitope expression, independent of epitope amino acid composition and variability in the structural context of epitope expression. This provides a molecular basis for known patterns of serological cross-reactivity. Critical amino acid mutations that abrogated antibody binding to Bw6-expressing HLA-B*07:02 (G83R, R79G, R82L) induced distinct electrostatic potential changes displacing the mutants from the Bw6 epitope cluster; in contrast, mutation N80T did not affect antibody binding and had negligible physiochemical effect. Conclusion This study suggests that HLA B-cell epitopes are characterized by distinct topographic patterns of electrostatic potential explaining HLA-specific antibody binding and enabling novel insights into HLA immunogenicity.