High-affinity antibodies against β₂-glycoprotein I

Background

Antibodies against β₂-glycoprotein I (anti-β2GPI) are believed to be of low affinity and thus unable to bind to the free antigen in a solution.

Objective

The aim of our study was to determine the affinity of IgG anti-β2GPI, isolated by affinity chromatography.

Methods

A β2GPI affinity column was prepared by CNBr-activated agarose without spacer arms and human purified unnicked β2GPI. The IgG fraction from the protein G column was applied to the column and bound antibodies were eluted with various solutions: A) 0.1 M glycine / 0.5 M NaCl / 0.1% Tween 20 pH 2.5; B) 0.1 M glycine / 4 M NaCl / 0.1% Tween 20 pH 2.5; C) 0.1 M sodium borate pH 10 and D) 25% ethylene glycol. Eluted fractions containing anti-β2GPI antibodies were neutralised and analysed by ELISA using various binding buffers. The level of anti-β2GPI antibodies in each sample was derived from the standard curve according to the defined dilutions of monoclonal antibodies (AUG are arbitrary units of IgG monoclonals).

Results

Increased concentrations of sodium ions in the binding solution from 0.15, 0.25, 0.50, 1.11, 2.07 and 4.0 M NaCl did not completely prevent the binding between isolated antibodies and β₂-GPI (79.8, 65.3, 36.1, 19.9, 12.0 and 8.1 AUG, respectively).

Conclusion

In contrast to the common opinion that all anti-β2GPI autoantibodies are of low affinity, we clearly showed that at least one subset among them was of high affinity.