Antiphospholipid antibodies (Abs) are associated with thrombosis and are a risk factor for recurrent pregnancy loss and obstetric complications in patients with the antiphospholipid syndrome. It is generally accepted that the major autoantigen for aPL Abs is β₂ glycoprotein I, which mediates the binding of aPL Abs to target cells (i.e., endothelial cells, monocytes, platelets, trophoblasts, etc.) leading to thrombosis and fetal loss. This article addresses molecular events triggered by aPL Abs on endothelial cells, platelets, and monocytes and complement activation, as well as a review of the current knowledge with regard to the putative receptor(s) recognized by aPL Abs on target cells as well as novel mechanisms that involve fibrinolytic processes. A section is devoted to the description of thrombotic and inflammatory processes that lead to obstetric complications mediated by aPL Abs. Based on experimental evidence using in vitro and in vivo models, new targeted therapies for treatment and/or prevention of thrombosis and pregnancy loss in antiphospholipid syndrome are proposed.

Key Points: A recombinant antibody recognizing the D1 domain of β2 glycoprotein I induces fetal loss and clot formation in animal models. The CH2-deleted antibody fails to activate complement and prevents the procoagulant and proabortive effects of patient antibodies.

In vitro studies have documented β2 glycoprotein I (β2GPI) binding to endothelial cells (ECs) and trophoblast using antiphospholipid antibodies. The in vivo binding of β2GPI to these cells and the conditions that favor their interaction have not been investigated. We analyzed the in vivo distribution of cyanine 5.5-labeled β2GPI in mice and evaluated the effect of pregnancy and circulating antibodies on its tissue localization. The signal was detected in the liver by whole body scan and ex vivo analysis. The β2GPI failed to bind to the vascular endothelium and reacted only with the ECs of uterine vessels. In pregnant mice the protein was localized on ECs and trophoblast at the embryo implantation sites. Immunized mice showed a similar β2GPI biodistribution to naive mice but the immunized pregnant animals exhibited a significant increase in fetal loss associated with C3 and C9 deposition at the implantation sites. Treatment of mice with LPS after β2GPI-Cy5.5 injection promoted protein localization on gut and brain ECs associated with IgG, C1q, and C9 deposition in immunized mice. These findings indicate that β2GPI binding to EC requires priming with pro-inflammatory factors which is not needed for uterine and placental localization probably dependent on hormonal changes.

Objective: It has been suggested that only antibodies against domain 1 (D1) of β₂‐glycoprotein I (β₂GPI) are pathogenic and diagnostic. The role of antibodies against other β₂GPI domains is still debated. This study was undertaken to evaluate the clinical relevance of domain specificity profiling of anti‐β₂GPI IgG antibodies in antiphospholipid syndrome (APS) patients and in control groups of patients with systemic autoimmune rheumatic diseases and in asymptomatic antiphospholipid antibody (aPL) carriers.Methods: We evaluated 159 subjects with persistently positive, medium or high‐titer anti‐β₂GPI IgG, including 56 patients with thrombotic (obstetric or nonobstetric) primary APS, 31 women with obstetric primary APS, 42 aPL‐positive patients with systemic autoimmune rheumatic diseases, and 30 asymptomatic aPL carriers. One hundred healthy donors were included. Anti‐β₂GPI D1 and D4/5 IgG were tested on research enzyme‐linked immunosorbent assays containing recombinant β₂GPI domains.Results: As compared to other groups, aPL carriers displayed higher frequency/titer of anti‐D4/5 IgG. Unlike anti‐D4/5, anti‐D1 IgG antibodies were more frequent and at higher titer in triple than in single or double aPL‐positive subjects. An anti‐D1 to anti‐D4/5 ratio of ≥1.5 was predictive of systemic autoimmunity (odds ratio 3.25 [95% confidence interval 1.45–7.49], P = 0.005). Neither anti‐D1 nor anti‐D4/5 antibodies were associated with APS clinical criteria.Conclusion: Anti‐D1 IgG is the preferential specificity not only in vascular and obstetric primary APS, but also in patients with systemic autoimmune rheumatic disease with no clinical features of APS. Conversely, aPL carriers do not have a polarized profile toward D1. Combined testing for anti‐β₂GPI IgG with different domain specificity allows a more accurate aPL profiling, with polarization toward anti‐D1 IgG as a possible fingerprint of systemic autoimmunity.