Antibodies to Domain I β₂-Glycoprotein 1 in Patients with Antiphospholipid Syndrome and Systemic Lupus Erythematosus

The role of antiphospholipid antibodies (aPL), which are not included in the Sydney diagnostic criteria, in antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE) is poorly understood. The aim of this study was to determine the clinical significance of IgG antibodies for domain 1 of β₂-glycoprotein 1 (β₂-GP1), IgG anti-β₂-GP1DI, in patients with APS with and without SLE. The study included 187 patients with APS with or without SLE, 49 patients formed the comparison group, and 100 apparently healthy individuals formed the control group. IgG/IgM antibodies to cardiolipin (aCL) and IgG/IgM anti-β₂-GP1 were determined by enzyme immunoassay (ELISA) in patients with or without APS, and IgG anti-β₂-GP1DI was determined by chemiluminescence assay (CLA) in all patients and controls. IgG anti-β₂-GP1DI was detected in 37 (71%) of 52 patients with primary APS (PAPS), in 6 (50%) of 12 patients with probable APS, in 42 (71%) of 59 patients with SLE + APS, in 17 (26%) of 64 patients with SLE, in 1 (2%) of the comparison group, and in none of the control group. IgG anti-β₂-GP1DI was significantly associated with PAPS and SLE + APS compared with the patients with SLE (p = 0.0002 and 0.0001, respectively). The association of IgG anti-β₂-GP1DI with clinical manifestations of APS (thrombosis (p = 0.001) and obstetric pathology (p = 0.04)) was detected. There was a significant association of IgG anti-β₂-GP1DI with arterial thrombosis (p = 0.002) and with late gestational obstetric pathology (p = 0.01). High specificity of IgG anti-β₂-GP1DI depending on the diagnosis and clinical manifestations of APS despite low sensitivity was noted: specificity was 84% for thrombosis, 94% for obstetric pathology, and 89% for APS. Isolated IgG anti-β₂-GP1DI positivity was reported in 2% of 50 aPL-negative patients and was not associated with APS manifestations. The frequency of IgG anti-β₂-GP1DI detection was higher in the patients with APS compared to the patients with SLE, comparison group, and control (p < 0.05). Positive IgG anti-β₂-GP1DI values were significantly associated with thrombotic complications and with obstetric pathology (p = 0.002 and p = 0.01, respectively). Specificity of IgG anti-β₂-GP1DI for APS and its clinical manifestations (thrombosis and obstetric pathology) was higher than sensitivity (89, 94, and 84%, respectively).