Role of β2-Glycoprotein I

β2 Glycoprotein I (β2-Glycoprotein I, β2-GP1) is an important plasma protein that is synthesized mainly by the liver. β2-GP1 circulates in the blood in a bound form with plasma lipoproteins such as very low density lipoproteins and high density lipoproteins, so it is also known as apolipoprotein H (ApoH), which has a very important role in triglyceride metabolism. β2-GP1 has anticoagulant and binding properties to various anionic substances, and is a functional protein that dually regulates the complement and coagulation systems.

β2-GP1 Structure

β2-GP1 is a single-chain glycoprotein consisting of 326 amino acids with a molecular weight of approximately 50 kDa. The nucleotide sequence and amino acid sequence of β2-GP1 were obtained from the cDNA of a human stem cell-derived hepatocellular carcinoma cell line (HepG2), which consists of five domains, of which domains I to IV are composed of 60 amino acids, and each of which contains highly conserved cystine, proline, and tryptophan. Four conserved cysteines are interconnected to form two internal disulfide bonds. Unlike the first four domains, domain V contains six cysteines.

In the free state, the interaction between domains I and V of β2-GP1 forms a closed "O" shape. When encountering the exposed surface of the negative phospholipid, due to the strong affinity of domain V for the negative phospholipid, the closed loop is opened and forms a "J" structure that anchors to the surface of the negative phospholipid. At this point domain I is fully exposed and the "J" type may be the active form of β2-GP1 in plasma. It is currently believed that when β2-GP1 binds to negative substances, its structure is altered and the antigenic determinants hidden within it are exposed and recognized by anti-β2-GP1 antibodies (aβ2-GP1), which in turn form immune complexes. Studies of mutant proteins and in vitro synthesized peptides of β2-GP1 and the resolution of crystalline constructs revealed the presence of a lipid-binding region on domain V of β2-GP1. The structural properties of β2-GP1 allow it to interact with negatively charged phospholipids on cell membranes in the bloodstream, thus playing an important role in coagulation and immune responses.

Figure 1. Domains of β2-glycoprotein I
(Source: Sorice M, et al. 2020)

β2-GP1-Related Diseases

β2-GP1 is a highly conserved plasma protein that is the major autoantigen in primary systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). APS is an autoimmune disease characterized by recurrent thrombosis and pregnancy complications, and testing for β2-GP1 has been included in the laboratory diagnostic criteria for APS since the identification of β2-GP1 as a key autoantigen in APS. Anti-β2-GP1 positivity was significantly higher in SLE patients than in healthy controls.

β2-GP1 binds to a variety of coagulation- and complement-related biomolecules, such as PLs, C3 components, annexin A2, PF4, fibrinogen, and fibronectin. This suggests that β2-GP1 regulates the hemostatic and complement systems, thereby modulating the pathogenesis of APS. On the other hand, apoptotic cells have long been recognized as a source of SLE autoantigens, and the physical interaction of β2-GP1 with these cells provides a "carrier protein"-like connection to a large number of cellular autoantigens. Thus, β2-GP1-reactive T cells have the potential to promote autoantibody production to a wide range of autoantigens expressed by dead cells. It was found that β2-GP1-reactive CD4 T-cell lines from APS and SLE patients primarily recognize epitopes 244-264 within β2-GP1 domain V. They recognize in the context of a variety of HLA class II molecules and exhibit Th0 or Th2-like responses. In contrast, T-cell lines from healthy controls exhibited a Th0 or Th1-like phenotype. Immunization of mice with β2-GP1 and lipopolysaccharide (LPS) resulted in dendritic cells presenting β2-GP1-derived peptides to T cells and activation and proliferation of β2-GP1-reactive CD4 T cells. In the second phase of the response, β2-GP1-reactive CD4 T cells can provide help not only to β2-GP1-specific B cells but also to other autoantigen-specific B cells.

Figure 3. β2-GP1-reactive T cells promote autoantibody production and pathology in APS and SLE
(Source: Rauch J, et al. 2018)

The "J" type of β2-GP1 exposes the hidden epitope G40-R43 in domain I and allows anti-DI β2-GP1 autoantibodies to bind to it. Antibody recognition against domain I has been found to correlate with APS pathogenesis and clinical manifestations, and therefore detection of anti-DI β2-GP1 antibodies can help in the diagnosis of APS.

Table 1. Overview of assays used for the detection of anti-domain I antibodies (anti-DI)

(Source: Yin D, et al. 2018)

β2-GP1 ELISA Kits

β2-GP1 Antigens

β2-GP1 Antibodies