Binding of high-avidity anti-β2-glycoprotein I antibodies

Objectives. We studied the influence of different binding conditions on the interaction between high- or low-avidity IgG anti-beta(2)-glycoprotein I antibodies (anti-beta(2)-GPI) and beta(2)-GPI, which was either free in solution or bound to microtitre plates or nitrocellulose membranes. Methods. Sera from 30 patients with antiphospholipid syndrome and/or systemic lupus erythematosus were selected on the basis of anti-beta(2)-GPI positivity. Avidity of IgG anti-beta(2)-GPI was determined by chaotropic ELISA, using increased NaCl concentration during the antibody binding. Immunodetection on nitrocellulose membrane followed reducing or non-reducing sodium dodecyl sulphate-polyacrylamide gel electrophoresis (PAGE) of beta(2)-GPI. In converted, non-reducing PAGE, the preparation with high-affinity Fab fragments (obtained by papain digestion) was subjected to electrophoresis, while purified beta(2)-GPI was used as the sample in immunodetection. Results. Anti-beta(2)-GPI antibodies of high, low or heterogeneous (low and high) avidity were found in 5/30, 9/30 and 16/30 sera, respectively. The density of beta(2)-GPI, which was 20-30 times higher on the nitrocellulose membrane than on the surface of ELISA plates, was not sufficient for the recognition of the antigen by anti-beta(2)-GPI: 2/5 high-avidity samples reacted only with non-reduced beta(2)-GPI, 3/9 low-avidity samples recognized only denatured and reduced beta(2)-GPI, and 1/16 samples with heterogeneous-avidity antibodies reacted with reduced and non-reduced beta(2)-GPI. Conclusions. Our results suggest that neither high density of the antigen nor high avidity of the antibodies (or Fab fragments) alone was sufficient for the binding of anti-beta(2)-GPI to beta(2)-GPI. Some conformational modifications and, consequently, exposed neo-epitopes are required for the recognition of beta(2)-GPI by polyclonal anti-beta(2)-GPI antibodies.