Antiphospholipid antibodies targeting b2-glycoprotein I (b2GPI) are a hallmark of antiphospholipid syndrome (APS), associated with an increased risk of thrombosis and pregnancy morbidity. Among these, antibodies targeting Domain I (DI) are common in individuals at higher risk; however, their epitopes and prevalence among APS phenotypes remain unclear. Here, we employ a large collection of 29 structurally and functionally validated b2GPI variants to provide new insights into the molecular mechanisms of autoantibody recognition in APS. Using the prototypic human-derived monoclonal anti-DI antibody MBB2, we identified positively charged residue R39 as the key driver for MBB2 binding, followed by residues R43, N56, and T57. Structural analyses revealed that, while R39 is solvent-exposed, R43 is not, as it is caged by residues N56 and T57. The narrow epitope footprint explains why MBB2 exhibits a modest affinity for soluble b2GPI. The cage structure accounts for the epitope being conformational rather than linear. Mutational analyses of IgG anti-b2GPI antibodies from 52 triple-positive APS patients, 37 with a history of thrombosis and 15 non-vascular obstetric patients, confirmed significant reactivity against DI and showed signatures of two conformational epitopes: one similar to MBB2 (epitope I), in which the presence of R39 is essential, and another that does not require R39 (epitope II). While less frequent than epitope-II in our cohort, epitope-I reactivity was notably enriched in thrombotic obstetric patients. Varying epitope specificities for DI may therefore aid in identifying different APS phenotypes and predicting clinical outcomes.

Lupus anticoagulant (LAC) is a well-known laboratory test used to explore potential reasons for the prolongation of phospholipid-dependent coagulation tests. An extended clotting time in a coagulation test typically suggests a bleeding tendency, as the plasma takes longer to clot. However, a positive LAC result, defined as normalization of prolonged clotting time by adding anionic phospholipids in the system, does not necessarily imply this. In fact, quite the opposite is true: a positive LAC often strongly correlates with an increased risk of thromboembolic events. Therefore, despite being conceptually counterintuitive, LAC remains extremely valuable in routine clinical practice for identifying individuals at risk for thromboembolic events. Over the years, various factors have been recognized as potential inducers of LAC, with antiphospholipid antibodies associated with antiphospholipid syndrome (APS) playing a significant role. Today, research indicates that, among antiphospholipid antibodies, those targeting plasma proteins β-glycoprotein I and prothrombin are central to LAC. This article offers a historical perspective on LAC, emphasizing recent developments in antiprothrombin antibodies, their connection to LAC, and novel detection methods. Our premise is that a deeper understanding of how antiprothrombin antibodies contribute to LAC and the identification of subpopulations of these antibodies potentially responsible for it in thrombotic APS patients could lead to transformative advancements, offering new strategies for risk stratification and personalized treatments for patients with APS and beyond.

Antiphospholipid syndrome (APS) is an autoimmune disorder characterized by recurrent life-threatening blood clots and pregnancy complications in individuals with antiphospholipid antibodies. Among these antibodies, those targeting the plasma glycoprotein β-glycoprotein I (βGPI) hold particular clinical significance. Despite extensive research, controversies persist regarding the structure of βGPI, which has substantial implications for understanding autoantibody reactivity and APS development. This article critically examines recent advancements in the structural biology of βGPI and its relevance to the recognition of antiphospholipid antibodies. Additionally, it introduces a new structure-based theory to explain how autoantibodies interact with βGPI and the functional consequence of this interaction. Finally, it identifies potential areas for future research that could enhance approaches to diagnosing and treating APS.