A. Bondanza et al., Anti-beta 2 glycoprotein I antibodies prevent the de-activation of platelets and sustain their phagocytic clearance, J AUTOIMMUN, 15(4), 2000, pp. 469-477
Exposure to phosphatidylserine (PS) tags dying and senescent cells for remo
val and identifies activated platelets. In this study we followed the fate
of PS-exposing platelets in the presence of antibodies purified from System
ic Lupus Erythematosus (SLE) and primary Anti-phospholipid Syndrome (APS) p
atients' sera by beta 2GPI affinity chromatography. Thrombin-activated plat
elets exposed PS and associated to beta 2GPI. 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 60min at 37 degreesC. Phagocytosis did not increase
after longer incubations (4.65 +/- 0.26 platelets internalized by each mac
rophage after 300 min). Recognition of platelets by anti-beta 2GPI antibodi
es significantly increased phagocytosis (P < 0.01). Upon withdrawal of thro
mbin, platelets downregulated PS (PS exposure t(1/2): 242 min) and the abil
ity to be recognized by macrophages. Purified <beta>2GPI bound to PS-exposi
ng platelets (association t(1/2): 250min). Phosphatidyl serine exposure and
beta 2GPI association had virtually identical kinetics. Antibody binding p
rolonged the exposure of the beta 2GPI/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 mi
n and 9.4 +/- 0.3 after 300 min). Anti-beta 2GPI antibodies therefore poise
activated platelets in a PS-exposing status, preventing the recycling of t
heir function and favoring their phagocytic clearance. (C) 2000 Academic Pr
ess.