Anti-β2-glycoprotein I paratopes and β2-glycoprotein I epitopes characterization using random peptide libraries

Studies concerning interactions between anti-beta 2-glycoprotein I antibodies (anti-beta 2GPI) and beta 2-glycoprotein I (beta 2GPI) suggest relevance of charge interactions and hydrogen bonds. However, paratope of diagnostically and clinically relevant anti-beta 2GPI and epitope characteristics of beta 2GPI, still remain unclear. The aim of our study was to determine paratope characteristics of various anti-beta 2GPI antibodies and epitope characteristics of beta 2GPI using phage display. Monoclonal IgG anti-beta 2GPI, purified polyclonal high avidity and low avidity IgG anti-beta 2GPI derived from plasma of APS patients were used to screen phage display libraries. The affinity and competition ability of selected clones were evaluated. Various heptapeptides presenting putative paratopes of anti-beta 2GPI and specific heptapeptides presenting putative epitopes of beta 2GPI were determined. Epitope presenting peptides bind to the respective anti-beta 2GPI and consequently interrupt antibody-antigen interaction. The amino acid composition of selected peptides confirmed the importance of hydrogen bonds and charge interactions in the binding of anti-beta 2GPI to the antigen. Epitopes recognized by high avidity anti-beta 2GPI predominately contain hydrogen bond forming side chains, while in low avidity anti-beta 2GPI epitope the charged side chains prevail. The alignment of selected sequences to three-dimensional antigen structure revealed that polyclonal high avidity anti-beta 2GPI recognize native epitopes that are accessible regardless of beta 2GPI's conformation whereas the epitope recognized by low avidity anti-beta 2GPI is cryptic and cannot be accessed when beta 2GPI takes the closed plasma conformation.