β2-glycoprotein I-dependent Anticardiolipin Antibodies Preferentially Bind the Amino Terminal Domain of β2-glycoprotein I

Abstract

SummaryMany of the autoantibodies in antiphospholipid syndrome (APS) are directed against β2-glycoprotein I (β2-GPI). Recent studies from our laboratories have indicated that the immunodominant binding epitope(s) for high titer, affinity purified antibodies from 11 APS patients are localized to the amino terminal domain (domain 1) of β2-GPI. The present study employed surface plasmon resonance to localize the immunodominant domain in serum samples from a large cohort of patients with GPL values ranging from 21 to 230 units (n = 106 patients). Eighty-eight percent of patients showed ≥ threefold selectivity for β2-GPI containing domain 1 relative to the domain deletion mutant that lacked domain 1. The domain 1 binding activity in patient serum was abolished by removing the IgG fraction from the serum and the binding activity could be fully reconstituted with the IgG fraction. Thus, analysis of serum samples from a large cohort of APS patients indicates that the immunodominant binding epitope(s) for anti- 2 antibodies are localized to the amino terminal domain of β2-GPI.

Pathogenic anti-β2-glycoprotein I antibodies recognize domain I of β2-glycoprotein I only after a conformational change

“…24 The purified ␤ 2 -GPI was analyzed with gel filtration and showed a single peak at the expected molecular weight. 25 The recombinant full-length ␤ 2 -GPI was analyzed with gel filtration and showed a single peak at the expected molecular weight. On SDS-PAGE electrophoresis the molecular weight of recombinant ␤ 2 -GPI was slightly lower than the molecular weight of plasma-purified ␤ 2 -GPI.…”

Molecular Pathogenesis of the Antiphospholipid Syndrome

“…24 It seems that most aPL antibodies recognize domain I of ␤ 2 GPI. 25,26 The crystal structure of the protein has been solved 27,28 ; its structure suggests that the protein binds to phospholipid membranes via the cationic portion of its fifth SCR domain ( Figure 1) and that aPL antibody binding to domains I and II promotes the increased binding of the protein to membrane phospholipid, 27 perhaps as a consequence of the increased affinity of the divalent IgG-␤ 2 GPI complexes. 29 The physiological function of ␤ 2 GPI has not been established; it has been suggested that the protein may play a scavenging role for exposed anionic phospholipid after apoptosis.…”

IgG antibodies that recognize epitope Gly40-Arg43 in domain I of β2–glycoprotein I cause LAC, and their presence correlates strongly with thrombosis

“…Interestingly, studies using only anti-␤ 2 GPI antibodies purified from patients concluded that domain I is the major domain of ␤ 2 GPI involved in the binding of anti-␤ 2 GPI antibodies. [18][19][20] Recently it was suggested that the positively charged epitope R39-R43 on domain I is important in binding of anti-␤ 2 GPI antibodies. 21 Here we further investigate that suggestion by testing the specificity of anti-␤ 2 GPI antibodies in patients with systemic lupus erythematosus, lupuslike disease, and primary antiphospholipid syndrome by using deletion mutants of ␤ 2 GPI, recombinant full-length ␤ 2 GPI, point-mutated recombinant full-length ␤ 2 GPI, and both hydrophilic and hydrophobic enzyme-linked immunosorbent assay (ELISA) plates.…”