Complement activation: the missing link between ADAMTS-13 deficiency and microvascular thrombosis of thrombotic microangiopathies

Abstract

Endothelial injury is the central factor in the events leading to thrombotic microangiopathy (TMA); however, the mechanisms involved are not fully understood. Here we investigate the role of neutrophils (PMNs) and of complement activation in inducing microvascular damage and loss of thromboresistance in TMA associated with ADAMTS-13 deficiency. PMNs isolated during the acute phase of the disease released excessive amounts of reactive-oxygen species (ROS), N-derived oxidants and proteinases and induced damage and thromboresistance loss in human microvascular endothelial cell line (HMEC-1) ex vivo. Endothelial cytotoxicity and thromboresistance loss was also induced by TMA serum. Complement-derived products were responsible for the above effects: in fact, TMA serum caused C3 and Membrane Attack Complex (MAC) deposition on HMEC-1 and its cytotoxic effect was abolished by complement inhibition. TMA serum caused surface expression of P-selectin on HMEC-1 which may promote PMN adhesion and resulted in increased PMN cytotoxicity, indicating that complement may have a role in PMN activation. In addition, TMA serum stimulated control PMNs to release ROS and proteinases, and to cause endothelial cell cytotoxicity. All of the above effects were abrogated by complement inactivation. These data document for the first time that complement-initiated PMN activation and endothelial injury may have a crucial role in microvascular thrombosis of TMA associated with ADAMTS-13 deficiency.

Human neutrophil peptides and complement factor Bb in pathogenesis of acquired thrombotic thrombocytopenic purpura

“…For instance, serum from patients with TMA caused C3 and membrane attack complex (MAC) deposition on human microvascular endothelial cells (HMEC)-1 and its cytotoxic effect was abolished by complement inhibition. 12 Additionally, plasma levels of C3a and C5a were significantly elevated in patients during acute TTP as compared with those in remission. 13 Most importantly, complement factor H mutations were identified in 5 out of 6 patients with ticlopidine (anti-platelet drug)-associated TTP with severe deficiency of plasma ADAMTS13 activity.…”

Complement activation associated with ADAMTS13 deficiency in human and murine thrombotic microangiopathy

“…Previous studies have suggested that complement is activated during acute episodes of TTP with reduced levels of serum C3 or elevated platelet-bound C3 (16,(36)(37)(38), with the reservation that older studies did not apply ADAMTS13 diagnostics. A more recent study demonstrated high levels of C3a and sC5b-9 during the acute phase of TTP, normalizing at remission (15), although this study did not find a parallel decrease in serum C3.…”

“…Thus, when ADAMTS13 is deficient and larger platelet thrombi are allowed to form on damaged endothelium, complement deposition could occur on these cells. TTP sera allowed C3 and C5b-9 deposition to occur on microvascular endothelial cells and upregulated P selectin on the cells thus enhancing the cells capacity to bind neutrophils (16). TTP sera also increased neutrophil degranulation inducing a damaging effect on the cells (16), effects that were attributed to complement activation (40).…”

“…TTP sera allowed C3 and C5b-9 deposition to occur on microvascular endothelial cells and upregulated P selectin on the cells thus enhancing the cells capacity to bind neutrophils (16). TTP sera also increased neutrophil degranulation inducing a damaging effect on the cells (16), effects that were attributed to complement activation (40). In the current study we showed that C3 deposition on VWF strings is abolished by heat-inactivation of plasma as well as by addition of EDTA but not EGTA, the latter suggesting complement activation via the alternative pathway.…”

“…Serum from patients with ADAMTS13 deficiency induced C3 deposition and membraneattack-complex formation on microvascular endothelial cells (16). The aim of this study was to demonstrate complement activation in renal tissue (human TTP, or murine after challenge with Stx2) and on circulating endothelial microparticles in TTP, and to study how ADAMTS13 deficiency promotes complement activation on the glomerular endothelium and endothelial microparticles as well as on VWF-platelet strings.…”