Species Specificity of Anti-β2 Glycoprotein I Autoantibodies and Its Relevance to Anticardiolipin Antibody Quantitation

Abstract

SummarySome patients suspected of having antiphospholipid antibody syndrome (APS) were found to be positive for anti-β2 glycoprotein I (β2GPI) antibodies despite negative results for antibodies to cardiolipin (ACA). Since the major source of β2GPI in the ACA assay is animal (usually bovine) serum, we studied the influence on ACA quantitation of the species specificity of anti-β2GPI antibodies from patients with various autoimmune disorders, mostly systemic lupus erythematosus and primary APS. Ninety-seven sera were selected based on IgG (n = 76) or IgM (n = 64) positivity by ELISA using γ-irradiated plates coated with human or bovine purified β2GPI. A higher proportion of IgM (43.7%) than IgG (7.9%) reacted to human, but not bovine, β2GPI. Furthermore, from the samples reactive to both proteins, the ratio of antibody level against bovine to that against human β2GPI was 1.08 ± 0.58 for IgG and 0.58 ± 0.3 for IgM (p <10−5). IgG and IgM ACA were detected in 78 and 40 sera, respectively; concordance between the two ELISAs for ACA and anti-β2GPI antibodies was 94% for IgG and 75% for IgM. Out of 28 IgM showing recognition restricted to human β2GPI, 21 were missed by the ACA assay, possibly because of lower concentrations of β2GPI in those patients’ sera. The antibody reactivity pattern towards human and bovine β2GPI of individual sera showed no variation with time and was related to the relative antibody avidity for each protein. A murine anti-human β2GPI monoclonal antibody, 9G1, that cross-reacts with bovine β2GPI, competed to a large extent with the patients’ anti-β2GPI antibody binding sites whatever isotype involved or protein recognized. Therefore, anti-β2GPI antibodies of IgM isotype display a marked preference for human compared to bovine β2GPI responsible for frequent inconsistencies in the ACA assay.