MOdeling of human anti-GBM antibody-α3(IV)NC1 interactions Predicts antigenic cross-linking through contact of both heavy chains with repeating epitopes on α3(IV)NC1

Background/Aims: Patients with anti-glomerular basement membrane diseases produce pathogenic autoantibodies (autoAb) that deposit in the kidney and initiate severe inflammation. Restricted antigenic specificity of the autoAb against 2 regions (with related sequences) within α3(IV)NC1, along with shared idiotypes (i.e. structural determinants), among pathogenic human autoAb suggested that common genetic elements encode the autoAb. The aim of this study was to determine whether the idiotypic relatedness of the autoAb was due to the fact that unique and similar genes were used to encode them, divergent genes were used to produce Ab with similar Ag-binding properties and conformation, or if other mechanisms were operative. Methods: The encoding V gene sequences of pathogenic human anti-α3(IV)NC1 Ab, derived following immunization of XenoMice® which produce human but not murine IgG, with α3(IV)NC1 were determined. Predicted conformations of autoAb-α3(IV)NC1 interactions were derived using the Ab sequences and molecularmodels of the α3(IV)NC1 structure. Results: The pathogenic Ab were encoded by multiple, common V _H and V_L gene families indicating that they were not encoded by a unique subset of genes and that normal individuals have the capacity to produce them. However, modeling of the Ag-Ab interactions suggested that although the contact regions varied for individual Ab, the optimized energy constraints facilitate interaction of both Ab-binding regions with pathogenically relevant epitopes on α3(IV)NC1. Conclusions: The results suggest that the repetitive nature and relatedness of the α3(IV)NC1 antigenic epitopes facilitate cross-linking of pathogenic Ab, in vivo, by allowing both IgG Fab to bind to the basement membrane. This most likely accounts for the high-affinity Ab binding we and others observed among human anti-α3(IV)NC1 Ab. Based on these observations, we postulate that this interaction provides for the stability of the Ab interaction, resulting in a high-affinity interaction that serves as an ideal scaffold for optimal FcR engagement and complement activation, thereby accelerating inflammation and contributing to the rapidly progressive nature of this disease.