title: Anti-beta2 glycoprotein I antibodies prevent the De-activation of platelets and sustain their phagocytic clearance.
abstract: Exposure to phosphatidylserine (PS) tags dying and senescent cells for removal and identifies activated platelets. In this study we followed the fate of PS-exposing platelets in the presence of antibodies purified from Systemic Lupus Erythematosus (SLE) and primary Anti-phospholipid Syndrome (APS) patients' sera by beta2GPI affinity chromatography. Thrombin-activated platelets exposed PS and associated to beta2GPI. Both events were required for recognition by antibodies. Human monocyte-derived macrophages phagocytosed activated platelets only. Each macrophage internalized an average of 3.16+/-0.2 platelets after 60 min at 37 degrees C. Phagocytosis did not increase after longer incubations (4.65+/-0.26 platelets internalized by each macrophage after 300 min). Recognition of platelets by anti-beta2GPI antibodies significantly increased phagocytosis (P< 0.01). Upon withdrawal of thrombin, platelets downregulated PS (PS exposure t(1/2): 242 min) and the ability to be recognized by macrophages. Purified beta2GPI bound to PS-exposing platelets (association t(1/2): 250 min). Phosphatidyl serine exposure and beta2GPI association had virtually identical kinetics. Antibody binding prolonged the exposure of the beta2GPI/PS complex (t(1/2): >1200 min). The ability to phagocytose opsonized platelets was accordingly sustained (5.3+/-0.2 opsonized platelets were internalized by each macrophage after 60 min and 9.4+/-0.3 after 300 min). Anti-beta2GPI antibodies therefore poise activated platelets in a PS-exposing status, preventing the recycling of their function and favoring their phagocytic clearance.
authors.affiliation: Clinical Immunology and Rheumatology Unit and Cancer Immunotherapy and Gene Therapy Program, Via Olgettina 60, Milano, 20132, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Bondanza
authors.forename: M G
authors.initials: MG
authors.lastname: Sabbadini
authors.forename: F
authors.initials: F
authors.lastname: Pellegatta
authors.forename: V S
authors.initials: VS
authors.lastname: Zimmermann
authors.forename: A
authors.initials: A
authors.lastname: Tincani
authors.forename: G
authors.initials: G
authors.lastname: Balestrieri
authors.forename: A A
authors.initials: AA
authors.lastname: Manfredi
authors.forename: P
authors.initials: P
authors.lastname: Rovere
mesh_terms: D000906:Antibodies / Q000276:immunology*; D006023:Glycoproteins / Q000276:immunology*; D006801:Humans; D007074:Immunoglobulin G / Q000276:immunology; D008264:Macrophages / Q000502:physiology; D010587:Phagocytosis; D010718:Phosphatidylserines / Q000494:pharmacology; D015539:Platelet Activation; D013917:Thrombin / Q000494:pharmacology; D053482:beta 2-Glycoprotein I
chemical_list: D000906:Antibodies; D006023:Glycoproteins; D007074:Immunoglobulin G; D010718:Phosphatidylserines; D053482:beta 2-Glycoprotein I; D013917:Thrombin