Reconsidering the immunological aspects of solid-phase assays for antiphospholipid antibodies detection

Solid‑phase assays for the detection of antiphospholipid antibodies (aPL) are central to the laboratory evaluation of antiphospholipid syndrome (APS). Growing insights into the molecular immunology underlying these assays challenge traditional interpretations. It is now evident that the antigenic drivers are phospholipids (PL)‑binding proteins - primarily β2-glycoprotein I (β2GPI) and prothrombin (PT). β2-glycoprotein I‑dependent aPL represent the dominant subset, recognizing discontinuous conformational epitopes spanning domains I-II (DI-DII). Solid matrices coated with anionic PL or partially oxidized polystyrene amplify DV anchoring through its lysine‑rich cationic loop, thereby increasing local antigen density and molecule orientation, ultimately enhancing functional avidity of intrinsically low‑affinity aPL. These antibodies contribute to reactivity across all three APS laboratory criteria assays: lupus anticoagulant (LA), anticardiolipin, and anti‑β2GPI. Beyond β2GPI, PT‑reactive aPL, detected by phosphatidylserine/prothrombin (aPS/PT) ELISAs, bind epitopes unveiled only when PT engages anionic PL in a Ca²⁺‑dependent conformational transition. Distinct antigenic specificities - type I (open‑conformation fragment‑1 epitope) and type II (C‑terminal Gla domain) - exhibit divergent affinities, matrix‑dependent binding behaviors, and heterogeneous LA activity. Antibodies detected through aPS/PT assays are associated with APS manifestations and often with LA (range 54.8-82.0%). Conversely, a smaller population of 'pure' lipid‑binding aPL, independent of β2GPI or PT, appears to target lysobisphosphatidic acid (LBPA) within the endothelial protein C receptor/LBPA complex, activating pro‑inflammatory/pro‑coagulant pathways. Although these antibodies may contribute to APS pathogenesis in a minority of patients, their detection remains technically challenging and is currently confined to research settings. Understanding the immunological characteristics of aPL subsets is essential for interpreting solid‑phase assays and improving APS management.