SURFACE BLEBS ON APOPTOTIC CELLS ARE SITES OF ENHANCED PROCOAGULANT ACTIVITY - IMPLICATIONS FOR COAGULATION EVENTS AND ANTIGENIC SPREAD IN SYSTEMIC LUPUS-ERYTHEMATOSUS

The restriction of phosphatidylserine (PtdSer) to the inner surface of the plasma membrane bilayer is lost early during apoptosis, Since Ptd Ser is a potent surface procoagulant, and since there is an increased incidence of coagulation events in patients with systemic lupus erythe matosus (SLE) who have anti-phospholipid antibodies, we addressed whet her apoptotic cells are procoagulant and whether anti-phospholipid ant ibodies influence this, Apoptotic HeLa cells, human endothelial cells, and a murine pre-B-cell line were markedly procoagulant in a modified Russell viper venom assay, This procoagulant effect was entirely abol ished by addition of the PtdSer-binding protein, annexin V, confirming that it was PtdSer-dependent. The procoagulant effect was also abolis hed by addition of IgG purified from the plasma of three patients with anti-phospholipid antibody syndrome, but not IgG from normal controls , Confocal microscopy of apoptotic cells stained with fluorescein-isot hiocyanate-conjugated-annexin V demonstrated Ca2+-dependent binding to the surface of membrane blebs on apoptotic tells, but not to intracel lular membranes, Recent data indicate that the surface blebs of apopto tic cells constitute an important immunogenic particle in SLE, We prop ose that the PtdSer exposed on the outside of these blebs can induce t he production of antiphospholipid antibodies, which might also enhance the immunogenicity of the bleb contents. When apoptosis occurs in a m icroenvironment in direct contact with circulating plasma, the unique procoagulant consequences of the apoptotic surface may additionally be expressed, This might explain the increased incidence of pathological intravascular coagulation events that occur in some lupus patients wh o have anti-phospholipid antibodies.