Antigenic structures recognized by anti-β2-glycoprotein I auto-antibodies

β2-Glycoprotein I (β2-GPI) is a major antigen for anti-cardiolipin antibodies and their epitopes are cryptic. Conformation of each domain of β2-GPI was optimized from its crystal structure by energy minimization and by molecular dynamics simulation. Three electrostatic interactions, i.e. D¹⁹³-K²⁴⁶, D²²²-K³¹⁷ and E²²⁸-K³⁰⁸, were observed between domains IV and V in the optimized structure that was constructed based on the consensus sequences obtained by the phage-displayed random peptide library. Antigenic structures determined by the epitope mapping mainly consisted of hydrophobic amino acids located on two discontinuous sequences in domain IV. These amino acid clusters, as an epitope, were covered by domain V and were of a hidden nature. A similar but incomplete counterpart to the epitopic clusters was found in domain I but was not in domains II or III. Binding of anti-β2-GPI auto-antibodies to solid-phase β2-GPI was significantly reduced either by L replacement for W²³⁵, a common amino acid component for the epitopes, or by V replacement for all of D¹⁹³, D²²² and E²²⁸. Structural analysis indicated a hypothesis that these electrostatic interactions between domains IV and V retained exposure to W²³⁵ and that epitope spreading occurred in the region surrounding W²³⁵. Thus, epitopic structures recognized by anti-β2-GPI auto-antibodies are cryptic and inter-domain electrostatic interactions are involved in their in exposure.