Antibodies to beta 2-glycoprotein-I: Urea resistance, binding specificity,and association with thrombosis

The aim of the present study was to evaluate the urea resistance and bindin g characteristics of anti-beta 2-glycoprotein I (anti-beta 2GPI) antibodies using standard anticardiolipin (aCL) and anti-beta 2GPI enzyme immunosorbe nt assays (ELISAs). Sera from patients with antiphospholipid syndrome (APS) (n = 22) and non-APS (n = 24), positive in a standard aCL ELISA, were test ed in an anti-beta 2GPI ELISA performed in polystyrene-irradiated ELISA pla tes. Urea resistance aCL and anti-beta 2GPI ELISAs were performed by measur ing the ability of antibodies to recognize antigen in the presence of 2 M u rea. The serum dilution after urea treatment (D) expressed as a percentage of the serum dilution without urea treatment (D-o) corresponding to the sam e optical density was defined as residual activity (RA = 100 D/D-o). The hi gher the RA, the higher the resistance of the antibodies to urea. APS compa red to non-APS sera had higher aCL binding (absorbance values ranging betwe en 0.180 and 1.400; median, 0.717 vs 0.120-1.273; median, 0.250, respective ly; P < 0.004). Six APS patients' sera had low aCL levels but they expresse d RA greater than or equal to 30%. Anti-beta 2GPI antibodies were detected in 15 of 22 APS vs 3 of 24 non-APS patients (P < 0.03); RA greater than or equal to 30% was detected in 15 of 22 APS vs 1 of 23 non-APS patients (P < 0.004). Using a CL affinity column, antibodies were purified from three APS anti-beta 2GPI negative and three non-APS anti-beta 2GPI-positive patients and tested in a aCL ELISA, using highly purified bovine serum albumin (BSA ) as a blocking agent (modified ELISA); reactivity was not detected in two APS and one non-APS sera. On the contrary, the reactivity of the purified a ntibodies was high when beta 2GPI was incubated with CL in the ELISA plates ; thus some anti-beta 2GPI negative sera from APS patients recognized the C L/beta 2GPI complex, rather than CL or beta 2GPI alone. In conclusion, anti -beta 2GPI antibodies are common in the APS patients, but a number of such patients recognize the CL/beta 2GPI complex and not CL or beta 2GPI. Antibo dies to either beta 2GPI or the CL/beta 2GPI complex derived from APS sera present a high resistance to urea. Anti-beta 2GPI antibodies of low urea re sistance exist in a minority of non-APS patients with autoimmune disease.