Interaction of anti-phospholipid antibodies with late endosomes of human endothelial cells

Anti-phospholipid antibodies (APLAs) are associated with thrombosis and/or recurrent pregnancy loss. APLAs bind to anionic phospholipids directly or i ndirectly via a cofactor such as beta(2)-glycoprotein 1 (beta(2)GPI), The l ipid target of APLA is not yet established. Recently, we observed that APLA s in vitro can bind lysobisphosphatidic acid (LBPA), The internal membranes of late endosomes are enriched in this phospholipid, The current study was undertaken to determine to what extent binding of APLA to LBPA is correlat ed with binding to cardiolipin and to beta(2)GPI and to determine whether p atient antibodies interact with late endosomes of human umbilical vein endo thelial cells (HUVECs) and thus modify the intracellular trafficking of pro teins. Binding of patient immunoglobulin G (n=37) to LBPA was correlated si gnificantly with binding to cardiolipin. Although LBPA binding was correlat ed to a lesser extent with beta(2)GPI binding, we observed that beta(2)GPI binds with high affinity to LBPA, Immunofluorescence studies showed that la te endosomes of HUVECs contain LBPA. Patient but not control antibodies rec ognized late endosomes, but not cardiolipin-rich mitochondria, even when we used antibodies that were immunopurified on cardiolipin, Incubation of HUV ECs with patient plasma samples immunoreactive toward LBPA resulted in an a ccumulation of the antibodies in late endosomes and led to a redistribution of the insulinlike growth factor 2/mannose-6-phosphate receptor from the G olgi apparatus to late endosomes. Our results suggest that LBPA is an impor tant lipid target of APLA in HUVECs, These antibodies are internalized by t he cells and accumulate in late endosomes. By modifying the intracellular t rafficking of proteins, APLA could contribute to several of the proposed pa thogenic mechanisms leading to the antiphospholipid syndrome.