title: Opposite beta2-glycoprotein I requirement for the binding of infectious and autoimmune antiphospholipid antibodies to cardiolipin liposomes is associated with antibody avidity.
abstract: The aim of this study was to investigate the interaction of antiphospholipid antibodies (aPL) from two different populations (patients with autoimmune or infectious disorders) with cardiolipin (CL) arranged in a defined bilayer. beta2-Glycoprotein I (beta2GPI), an apolipoprotein that plays a critical role in the aPL binding to phospholipids, was quantified by dot blot in purified IgG-aPL samples, further classified according to apparent avidity to CL. In solid-phase assays, beta2GPI increased, preferentially, the binding of low-avidity autoimmune aPL to CL but inhibited the binding of low-avidity syphilitic aPL. In the absence of beta2GPI, both autoimmune and infectious aPL induced the leakage of the entrapped fluorescent probe, carboxyfluorescein (CF), from small unilamellar vesicles containing CL. aPL-induced probe leakage was protein concentration-dependent and characterized by a lag-phase onset of 100-120 min. beta2GPI increased the leakage rate induced by low-avidity autoimmune aPL only and inhibited the leakage induced by all syphilitic aPL. The following conclusions were provided: (1) in the absence of beta2GPI, autoimmune and infectious aPL bind to CL in a bilayer, inducing liposome leakage; (2) the leakage mechanism induced by aPL is suggested to be intravesicular; (3) beta2GPI requirement for phospholipid binding in both solid and fluid phase is associated to aPL avidity; (4) CL alone or the CL-beta2GPI complex are the most likely epitopes for autoimmune aPL; (5) aPL from syphilis patients can only form the CL-aPL complex, supporting that beta2GPI is not (part of) the target epitope.
authors.affiliation: Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, SP 05508-900, Brazil. clmcelli@hotmail.com
authors.forename: C M
authors.initials: CM
authors.lastname: Celli
authors.forename: A E
authors.initials: AE
authors.lastname: Gharavi
authors.forename: H
authors.initials: H
authors.lastname: Chaimovich
mesh_terms: D017153:Antibodies, Anticardiolipin / Q000097:blood / Q000276:immunology; D017152:Antibodies, Antiphospholipid / Q000097:blood / Q000276:immunology*; D000915:Antibody Affinity; D001327:Autoimmune Diseases / Q000097:blood; D002308:Cardiolipins / Q000032:analysis / Q000276:immunology*; D000939:Epitopes / Q000032:analysis; D005452:Fluoresceins; D005456:Fluorescent Dyes; D006023:Glycoproteins / Q000032:analysis / Q000276:immunology*; D006801:Humans; D007074:Immunoglobulin G / Q000032:analysis; D008081:Liposomes / Q000737:chemistry; D010539:Permeability; D013587:Syphilis / Q000097:blood; D053482:beta 2-Glycoprotein I
chemical_list: D017153:Antibodies, Anticardiolipin; D017152:Antibodies, Antiphospholipid; D002308:Cardiolipins; D000939:Epitopes; D005452:Fluoresceins; D005456:Fluorescent Dyes; D006023:Glycoproteins; D007074:Immunoglobulin G; D008081:Liposomes; D053482:beta 2-Glycoprotein I; C024098:6-carboxyfluorescein