The β2-Glycoprotein I IgG test is a chemiluminescent immunoassay (CLIA), for use on IDS automated analyzers. It is used for the quantitative determination of the specific IgG antibodies directed against β2-glycoprotein I in human samples of serum or plasma (EDTA and Heparin).
The presence of antiphospholipid (aPL) antibodies in patients with venous and/or arterial thrombosis or in patients with pregnancy-related complications is the essential laboratory marker (together with LAC [lupus anticoagulant] testing) for diagnosis of antiphospholipid antibody syndrome (or antiphospholipid syndrome) (APS or APLS).1
In accordance with the Sapporo criteria, updated in 2006,1 APS can be definitively diagnosed in the presence of at least one clinical criterion and one laboratory criterion.
The laboratory criteria are persistent positivity (12 weeks) with an average/high titer for anti-cardiolipin (aCL) and/or anti-β2-glycoprotein I (a-β2GPI) antibodies and/or “lupus anticoagulant” (LAC) antibodies.
The aCL and a-β2GPI antibody isotype may be G or M and the antibodies may be present in titers in excess of 40 U/mL.
Antiphospholipid antibodies were first noted in 1941 in samples of patients with serologic diagnoses of syphilis.2 It was shown that the serum of these patients inter-reacted with the cardiolipin phospholipid contained in the beef heart extract used in the VDRL (Venereal Disease Research Laboratory) test, which is considered specific for diagnosis of syphilis.
The specificity of the VDRL assay was challenged by the numerous false positive results in samples from patients with systemic autoimmune diseases in the absence of venereal diseases. In 1983, Harris et al.,3 applying a highly-sensitive method for detection of anticardiolipin antibodies, found high concentrations of aCL in 61% of patients with SLE (systemic lupus erythematosus), thus demonstrating a significant correlation between antibody levels and venous and arterial thrombosis, “lupus anticoagulant,” and thrombocytopenia.
In 1990, two independent groups of researchers4,5 demonstrated that the presence of β2-glycoprotein I is indispensable for detecting anticardiolipin antibodies.
β2-glycoprotein I has a molecular weight of 50 kDa ca., a plasma concentration of 0.15-0.30 mg/mL ca., and a biological function that is still not fully understood (although it would seem that it can modulate lipoprotein metabolism, interfere with some coagulation reactions, and act as a platelet anti-aggregant6-9). Recent crystallographic studies have defined the three-dimensional structure of the protein and its 5-domain organization,10-11 providing useful information about the way this molecule works.
In detail, the fifth domain shows numerous lysine residues, which are responsible for the electrostatic interaction of β2-glycoprotein I with the anionic phospholipids of the cell membranes.12 The same mechanism is responsible for in vitro bonding between β2-glycoprotein and cardiolipin adsorbed to a solid phase. It has been amply demonstrated that the anticardiolipin antibodies of patients affected with antiphospholipid antibody syndrome recognize a modified portion of the β2-glycoprotein I molecule; these autoantibodies cannot recognize cardiolipin or native β2-glycoprotein not bound to solid phases or to other structures.4,5,13-15
Knowledge acquired to date permits us to define the anticardiolipin antibodies as antibodies that can bind to neoepitopes generated by binding of β2-glycoprotein and cardiolipin adsorbed to a solid phase.
It was later shown4,16 that the anticardiolipin antibodies in patients with autoimmune diseases can recognize the β2-glycoprotein I directly adsorbed on UV-treated or irradiated polystyrene microtiter plates. In this case as well, recognition of the molecule by the autoantibodies is determined by the structural modifications caused when the protein binds to the solid phase.
- Miyakis S, Lockshin MD, Atsumi T, Branch DW, Brey RL, Cervera R, et al. International consensus statement on an update of the classification criteria for definite antiphospholipid syndrome. J Thromb Haemost 2006; 4:295-306.
- Pangborn MC. A new serologically active phospholipid from beef heart. Proc Soc Exp Biol & Med (NY) 1941; 48:484-486.
- Harris EN, Gharavi AE, Boey ML, Patel BM, Mackworth-Young CG, Loizou S, and Hughes GR. Anticardiolipin antibodies: detection by radioimmunoassay and association with thrombosis in systemic lupus erythematosus. Lancet 1983; 2:1211-1214.
- Galli M, Comfurius P, Maassen C, Hemker HC, de Baets MH, van Breda-Vriesman PJC, et al. Anticardiolipin antibodies (ACA) directed not to cardiolipin but to a plasma protein cofactor. Lancet 1990; 335:1544-1547.
- McNeil HP, Simson RJ, Chesterman CN, and Krilis SA. Antiphospholipid antibodies are directed against a complex antigen that includes a lipid-binding inhibitor of coagulation : β2-glicoprotein I (apolipoprotein H). Proc Natl Acad Sci USA 1990; 87:4120-4127.
- Wurm H, Beubler E, Polz E, Holasek A, and Kostner G. Studies on the possible function of beta 2-glycoprotein-I: influence in the triglyceride metabolism in the rat. Metabolism 1982; 31:484-486.
- Nimpf J, Wurm H, Kostner GM. Interaction of beta 2-glycoprotein-I with human blood platelets: influence upon the ADP-induced aggregation. Thromb Haemost 1985; 54:397-401.
- Nimpf J, Bevers EM, Boman PH, Till U, Wurm H, Kostner GM, et al.. Prothrombinase activity of human platelets is inhibited by beta 2-glycoprotein-I. Biochim Biophys Acta 1986; 884:142-149.
- Balasubramanian K, Chandra J, Schroit AJ. Immune clearance of phosphatidylserine-expressing cells by phagocytes. The role of beta 2-glycoprotein I in macrophage recognition. J Biol Chem 1997; 272: 31113-31117.
- Bouma B, de Groot PG, van den Elsen JM, Ravelli RBG, Schouten A, Simmelink M, Derksen RHWM, Kroon J, and Gros P. Adhesion mechanism of human β2-Glicoprotein I to phospholipids based on its crystal structure. EMBO J 1999; 18:5166-5174.
- Schwarzenbacher R, Zeth K, Diederichs K, Gries A, Kostner GM, Laggner P, and Prassi R. Crystal structure of human β2-glycoprotein I: Implications for phospholipid binding and the antiphospholipid syndrome. EMBO J 1999; 18:6228-6239.
- Hunt J and Krilis S. The fifth domain of beta 2-glycoprotein I contains a phospholipid binding site (Cys281-Cys 288) and a region recognized by anticardiolipin antibodies. J Immunol 1994; 152:653-659.
- Matsuura E, Igarashi Y, Fujimoto M, Ichikawa K, and Koike T. Anticardiolipin cofactor(s) and differential diagnosis of autoimmune disease. Lancet, 1990; 336:177-178.
- Koike T and Matsuura E. What is the “true” antigen for anticardiolipin antibodies? Lancet 1991; 337:671-672.
- Matsuura E, Igarashi Y, Fujimoto M, Ichikawa K, Suzuki T, Sumida T, Yasuda T, and Koike T. Heterogeneity of anticardiolipin antibodies defined by the anticardiolipin cofactor. J Immunol, 1992; 148:3885-3891.
- Viard JP, Amoura Z, and Bach JF. Association of anti-β2-glycoprotein I antibodies with lupus-type circulating anticoagulant and thrombosis in systemic anticoagulant and thrombosis in systemic lupus erythematosus. Am J Med, 1992; 93:181-186.