Highly Increased Plasma Concentrations of the Nicked Form of  2 Glycoprotein I in Patients with Leukemia and with Lupus Anticoagulant: Measurement with a Monoclonal Antibody Specific for a Nicked Form of Domain V

Abstract

beta(2)-Glycoprotein I (beta(2)GPI) consists of five tandem repeated domains (I, II, III, IV, and V). The nicked form of beta(2)GPI (N-beta(2)GPI ) which was cleaved by plasmin in vitro at Lys 317-Thr 318 in domain V, showed reduced affinity for the negatively charged phospholipids, especially cardiolipin (CL). Recently, the N-beta(2)GPI was detected in the plasma of patients with disseminated intravascular coagulation syndrome (DIC) by an immunological method. In the present study, we prepared monoclonal antibodies for the nicked form, and demonstrated that the concentrations of this form of beta(2)GPI, which were analyzed by a sandwich ELISA using two specially prepared monoclonal antibodies, were significantly increased in the plasma of patients with leukemia (n = 51, mean +/- SD: 162.0 +/- 118.3 ng/ml) and with lupus anticoagulant (LA) (n =40, mean +/- SD: 3,041.5 +/- 16,579.7 ng/ml), compared to the normals (n = 33, mean +/- SD: 1.04 +/- 1.54 ng/ml). We found a significant correlation between the concentrations of N-beta(2)GPI and those of typical molecular markers for a fibrinolytic state such as plasmin-alpha(2) plasmin inhibitor complex (PIC) and D-dimer in patients with leukemia, but not in patients with LA. These results suggested that the generation of N-beta(2)GPI was caused by plasmin in the patients with leukemia, and by unknown proteases in the patients with LA. In the patients with LA, the levels of N-beta(2)GPI tended to be higher in those without thrombosis than in those with thrombosis.

Domain V of β2-Glycoprotein I Binds Factor XI/XIa and Is Cleaved at Lys317-Thr318

“…In addition, FXIa cleaves ␤ 2 GPI at Lys 317 -Thr 318 within DV. The cleavage site on ␤ 2 GPI for FXIa is the same as that for plasmin (13)(14)(15). Functional studies demonstrate the significance of maintaining the integrity of the C-terminal loop in DV, because cleavage of DV at Lys 317 -Thr 318 retained the binding of ␤ 2 GPI with FXI but attenuated or abolished the ability of ␤ 2 GPI to inhibit activation of FXI by thrombin (27).…”

“…Heparin greatly enhances the plasmin-mediated cleavage of the Lys 317 -Thr 318 site in ␤ 2 GPI at concentrations that can be achieved in vivo during anticoagulation therapy (11). The cleavage at Lys 317 -Thr 318 is generated in vitro by plasmin and at extremely low efficiency by factor Xa (13) and in vivo in pathological states of increased fibrinolysis (13)(14)(15).…”

“…Increased levels of clipped ␤ 2 GPI (c␤ 2 GPI) have been reported in the plasma of patients with lupus anticoagulant and in patients with disseminated intravascular coagulation (13). Moreover, increased fibrinolysis and thrombin generation occurs in patients with disseminated intravascular coagulation and antiphospholipid antibodies (13)(14)(15).…”

Heparin Inhibits the Binding of β2-glycoprotein I to Phospholipids and Promotes the Plasmin-mediated Inactivation of This Blood Protein

“…The first domain within ␤2-GPI contains a positively charged region that includes Arg 39 , Arg 43 , and Lys 44 . Because it has been reported that this domain binds negatively charged phospholipids, an expression/site-directed mutagenesis approach was used to evaluate the possible importance of this positively charged domain in heparin interaction.…”

“…Thus, it has been proposed that the cleaved form of ␤2-GPI is probably generated during normal fibrinolysis. More recently, the presence of the processed form of ␤2-GPI was demonstrated in patients with leukemia and with lupus anticoagulant activity (39). It was proposed that plasmin is the likely protease that generates the proteolytically processed form of ␤2-GPI.…”