Thrombotic Thrombocytopenic Purpura Plasma Research Articles

Immune-mediated thrombotic thrombocytopenic purpura plasma induces calcium- and IgG-dependent endothelial activation: correlations with disease severity.

Immune-mediated thrombotic thrombocytopenic purpura (iTTP) is characterized by a severe ADAMTS13 deficiency due to the presence of anti-ADAMTS13 auto-antibodies, with subsequent accumulation of circulating ultra-large von Willebrand factor (VWF) multimers. The role of endothelial cell activation as a trigger of the disease has been suggested in animal models but remains to be demonstrated in humans. We prospectively obtained plasma from the first plasma exchange of 25 patients during iTTP acute phase. iTTP but not control plasma, induced a rapid VWF release and P-selectin exposure on the surface of dermal human micro-vascular endothelial cell (HMVEC-d), associated with angiopoietin-2 and endothelin-1 secretion, consistent with Weibel-Palade bodies exocytosis. Calcium (Ca2+) blockade significantly decreased VWF release, whereas iTTP plasma induced a rapid and sustained Ca2+ flux in HMVEC-d which correlated in retrospect, with disease severity and survival in 62 iTTP patients. F(ab)'2 fragments purified from the immunoglobulin G fraction of iTTP plasma mainly induced endothelial cell activation with additional minor roles for circulating free heme and nucleosomes, but not for complement. Furthermore, two anti-ADAMTS13 monoclonal antibodies purified from iTTP patients' B cells, but not serum from hereditary TTP, induced endothelial Ca2+ flux associated with Weibel-Palade bodies exocytosis in vitro, whereas inhibition of endothelial ADAMTS13 expression using small intering RNA, significantly decreased the stimulating effects of iTTP immunoglobulin G. In conclusion, Ca2+-mediated endothelial cell activation constitutes a "second hit" of iTTP, is correlated with the severity of the disease and may constitute a possible therapeutic target.

Shrinking Weibel‐Palade bodies prevents high platelet recruitment in assays using thrombotic thrombocytopenic purpura plasma

BackgroundThrombotic thrombocytopenic purpura (TTP), caused by a genetic or autoimmune‐driven lack of ADAMTS‐13 activity, leads to high levels of the ultra‐large von Willebrand factor (VWF) multimers produced by endothelial cells, causing excess platelet recruitment into forming thrombi, often with mortal consequences. Treatments include plasma infusion or replacement to restore ADAMTS‐13 activity, or prevention of platelet recruitment to VWF. ObjectivesWe tested a different approach, exploiting the unique cell biology of the endothelium. Upon activation, the VWF released by exocytosis of Weibel‐Palade bodies (WPBs), transiently anchored to the cell surface, unfurls as strings into flowing plasma, recruiting platelets. Using plasma from patients with TTP increases platelet recruitment to the surface of cultured endothelial cells under flow. WPBs are uniquely plastic, and shortening WPBs dramatically reduces VWF string lengths and the recruitment of platelets. We wished to test whether the TTP plasma‐driven increase in platelet recruitment would be countered by reducing formation of the longest WPBs that release longer strings. MethodsEndothelial cells grown in flow chambers were treated with fluvastatin, one of 37 drugs shown to shorten WPBs, then activated under flow in the presence of platelets and plasma of either controls or patients with TTP. ResultWe found that the dramatic increase in platelet recruitment caused by TTP plasma is entirely countered by treatment with fluvastatin, shortening the WPBs. ConclusionsThis potential approach of ameliorating the endothelial contribution to thrombotic risk by intervening far upstream of hemostasis might prove a useful adjunct to more conventional and direct therapies. [Display omitted]

Microangiopathic and Vascular Disorders

The thrombotic microangiopathies are characterized by microangiopathic hemolytic anemia and thrombocytopenia and can be classified as autoimmune, drug induced, complement mediated, and infectious/other. Reaching a definitive diagnosis for these disorders can be challenging due to the similarity of presenting symptoms and laboratory findings. Specific disorders described in this review include thrombotic thrombocytopenic purpura, the hemolytic-uremic syndrome, thrombotic microangiopathies of pregnancy (including preeclampsia and HELLP syndrome), disseminated intravascular coagulation, and antiphospholipid syndrome. Vascular disorders that lead to hematologic abnormalities are also discussed. Figures show the major classifications of the thrombotic microangiopathies; ADAMTS13 activity in normal and thrombotic thrombocytopenic purpura plasma; a fragmented red blood cell (arrow), also known as a schistocyte or helmet cell; major considerations in the initial treatment of thrombotic thrombocytopenic purpura and options for refractory patients as well as treatment considerations after discontinuation of plasma exchange; and a diagram of the complement pathway showing regulatory proteins as well as the site of action for the monoclonal antibody eculizumab. Tables list medications associated with thrombotic thrombocytopenia purpura, diagnostic criteria for HELLP, major classifications and examples of the causes of disseminated intravascular coagulation, diagnostic criteria for the antiphospholipid syndrome, vascular purpuras, and criteria for diagnosing hereditary hemorrhagic telangiectasia. This review contains 5 highly rendered figures, 10 tables, and 74 references Key words: anemia, hereditary hemorrhagic telangiectasia,thrombotic microangiopathies, thrombotic Thrombocytopenic Purpura, von Willebrand factor

Defibrotide Inhibits Endothelial Cell Injury Induced By Plasmas of Patients with Thrombotic Microangiopathies

Introduction: Defibrotide is a polydisperse mixture of predominantly single-stranded polydeoxyribonucleotide sodium salts derived from porcine intestinal mucosa. It is currently the only therapy FDA-approved to treat hematopoietic stem cell transplant (HSCT)-associated hepatic veno-occlusive disease (VOD) with multi-organ dysfunction. Its mechanisms of action are complex and incompletely understood. In vitro, defibrotide can: promote endothelial cell (EC) mitogenesis via interaction with basic fibroblast growth factor; inhibit platelet adhesion and aggregation; reduce calcineurin inhibitor-induced EC apoptosis; and block pro-inflammatory cytokine activity, in the absence of anticoagulant effects (Blood Adv 2018;2:1495-1509). Given the critical role of EC injury with impaired regulation of thrombo-fibrinolytic pathways in both transplant-associated VOD and thrombotic microangiopathies (TMA), defibrotide has been used off-label to treat TMAs post-allogeneic HSCT. Significant clinical responses occurred in the majority of cases not complicated by renal failure (Bone Marrow Transplant 2019;54:142-145). It is known that conditioning regimens for autologous HSCT involve activation of pro-inflammatory pathways in EC which can be blocked by defibrotide (Biol Blood Marrow Transplant 2011;17:497-506). We sought to determine whether clinically relevant concentrations of defibrotide could block activation of caspase 8 in EC, an early step in apoptotic injury, as induced by plasmas from patients with TMAs secondary to thrombotic thrombocytopenic purpura (TTP), cancer/chemotherapy, and allogeneic HSCT. Methods: We utilized plasmas from individuals with acute TMAs: 13 with TTP; 5 with an atypical hemolytic-uremic syndrome (HUS)-type of TMA in the setting of cancer chemotherapy, persisting after withdrawal of chemotherapy; and 10 with TMAs following alloHSCT, persisting after withdrawal of GvHD prophylaxis. The in vitro model system was previously published by our lab, documenting the ability of plasmas from acute TTP patients to activate caspase 8 in EC, followed by apoptotic EC death (Blood 2008;112:340-349). This effect was dependent on plasma-based proinflammatory cytokines capable of down-regulating EC cytoprotective factors. Briefly, 5 x 105 primary human neonatal microvascular endothelial cells of dermal origin were plated in 1ml culture medium/well of 12 well polystyrene plates (3.6cm2/well) pre-coated with 1% fibronectin. 2% (v/v) patient or control plasma was added, alone or with varying concentrations of defibrotide (0.5-10μg/ml), for 3hrs at 37ºC. Cells were then harvested and a commercial colorimetric kit (Millipore) was used to assess caspase 8 enzyme activity. Results: Initial dose-response curves showed a plateau effect for defibrotide-mediated inhibition of caspase 8 activation at 5μg/ml; subsequent experiments utilized that dose. To control for the possibility of direct inhibition of caspase 8 by defibrotide, we found that caspase 8 activity initiated by staurosporine (1μM) was unaffected by defibrotide (Fig.) >50% inhibition of caspase 8 was seen for 3/5 cancer/chemotherapy-linked TMA patient plasmas, 7/13 TTP plasmas, and 0/10 TA-TMA plasmas. >20% suppression of such activation was seen for 5/5 cancer/chemotherapy TMA plasmas, 12/13 TTP plasmas, and 4/10 TA-TMA plasmas. Representative examples are shown in the Fig., including defibrotide blocking caspase 8 activity induced by plasmas from a patient with cancer/chemotherapy linked TMA (patient 18) or TTP (patient 41), but not from another TTP patient (patient 17). Conclusions: Interventions with high degrees of therapeutic success are available for TMAs related to TTP and primary aHUS. However, TMAs complicate 20% of alloHSCT, and though half may resolve when GvHD immunoprophylaxis is discontinued, three year survival rates for those in whom the TMA persists are as low as 11%. Based on multiple case reports, defibrotide may represent an important resource in those cases, and in other TMAs refractory to conventional treatment. Further studies are required to see if defibrotide-mediated blockade of plasma-induced EC activation and injury which we demonstrate in vitro correlates with in vivo responses, and if the mechanism involves suppression of pro-inflammatory cytokine-mediated cell activation. Disclosures No relevant conflicts of interest to declare.

Domain Specificity and Mechanism of Action of Anti-ADAMTS13 Antibodies at Presentation and Relapse in Acquired Thrombotic Thrombocytopenic Purpura (TTP)

Acquired thrombotic thrombocytopenic purpura (TTP) is an autoimmune disease due to autoantibodies (Auto-Ab) against ADAMTS13, resulting in a severe deficiency in enzyme activity (<10%). Little is known about the pathogenicity of anti-ADAMTS13 IgG directed against different domains of the enzyme, or how domain specificity alters between presentation, remission or TTP relapse. We developed novel assays to explore domain specificity and inhibitory potential of anti-ADAMTS13 IgG using samples from the UK TTP Registry.Full-length ADAMTS13 and ADAMTS13 fragments (MD, MDTC and MDTCS - N-terminal domains; TSR2-8 and CUB1/2 - C-terminal domains) were expressed in HEK293T cells and purified. Microtitre plates were coated with ADAMTS13 or ADAMTS13 fragments, incubated with TTP plasmas and bound IgG detected. Domain specificity was further analysed by preincubation of patient samples with ADAMTS13 fragments, assessing the ability of fragments to compete for full-length ADAMTS13 binding on the plate. From a cohort of 92 acquired TTP patients at presentation, 100% (92/92) contained auto-Ab that recognised ADAMTS13 N-terminal domains (MDTCS). Of these, 39% (36/92) had auto-Ab recognising the N-terminal domains alone, 22% (20/92) also had anti-TSR2-8, and 39% (36/92) had anti-CUB1/2 auto-Abs. There was no correlation between auto-Ab domain specificity and mortality/disease severity (anti-ADAMTS13 IgG titre, plasma exchanges to remission, platelet count and cardiac involvement).There was no evidence for auto-Ab recognising MD or MDTC domains in 25 patients tested, suggesting immunoreactivity for MDTCS was primarily mediated through the spacer domain. Consistent with this, TTP IgG inhibited proteolysis of VWF115 (a recombinant VWF A2 domain fragment), but failed to inhibit VWF106 proteolysis (VWF A2 domain fragment lacking the spacer domain binding exosite).To examine the inhibitory potency of anti-ADAMTS13 auto-Ab, IgG isolated from TTP patient samples were titrated in reactions measuring MDTCS-mediated proteolysis of FRETS-VWF73. Twenty eight out of 43 presentation samples tested had inhibitory IgG (IC50 ranging from <0.2-5μM total IgG). Addition of excess of MDTC to 19/19 of these samples did not alter the IC50, confirming inhibition in these samples is mediated solely by anti-spacer antibodies. Intriguingly, 15/43 patients had no evidence of inhibitory auto-Ab, suggesting the pathogenic mechanism in these cases may be enhanced clearance of ADAMTS13. We therefore determined ADAMTS13 antigen concentrations by ELISA. All patients at presentation exhibited markedly reduced ADAMTS13 antigen levels (n=91, median 58ng/ml (range 0-450) vs. normal n=67, median 951ng/ml (515-1829); p<0.0001). Interestingly, patients with predominantly C terminal auto-Ab and no evidence of inhibitory auto-Ab had significantly lower ADAMTS13 antigen levels at first presentation than the rest of the cohort (median 2ng/ml (0-141) vs. 57ng/ml (0-356); p=0.006).Cases where rituximab failed to normalise ADAMTS13 activity despite clinical remission were associated with the persistence of inhibitory anti-N-terminal auto-Ab. Two patients with persistently high anti-ADAMTS13 IgG but normal ADAMTS13 activity and antigen levels during remission had non-pathogenic anti-TSR2-8 antibodies that seemingly had no effect upon ADAMTS13 function or clearance. Nine out of 16 patients exhibited altered auto-Ab domain specificity upon relapse, with either loss or development of anti-C-terminal auto-Ab.In conclusion, from our functional studies ADAMTS13 inhibition appears to be mediated exclusively by anti-spacer antibodies, with no evidence for the presence of auto-Ab that recognise/inhibit other N terminal domains. Importantly, we demonstrate ADAMTS13 antigen depletion is a crucial pathogenic mechanism in acquired TTP, accounting for a large proportion of the reduced ADAMTS13 activity detected in patient samples. Our results suggest that auto-Ab directed against the C-terminal domains of ADAMTS13 may be particularly important in mediating this clearance. There is continual development of the autoimmune response during treatment of TTP patients that can result in altered domain specificity at relapse. DisclosuresNo relevant conflicts of interest to declare.

Complement Activation Associated with ADAMTS13 Deficiency in Human and Murine Thrombotic Microangiopathy

This study addressed the contribution of ADAMTS13 deficiency to complement activation in thrombotic thrombocytopenic purpura (TTP). Renal tissue and blood samples were available from 12 TTP patients. C3 and C5b-9 deposition were demonstrated in the renal cortex of two TTP patients, by immunofluorescence and immunohistochemistry, respectively. C3 was also demonstrated in the glomeruli of Shiga toxin-2-treated Adamts13(-/-) mice (n = 6 of 7), but less in mice that were not Shiga toxin-2 treated (n = 1 of 8, p < 0.05) or wild-type mice (n = 0 of 7). TTP patient plasma (n = 9) contained significantly higher levels of complement-coated endothelial microparticles than control plasma (n = 13), as detected by flow cytometry. Exposure of histamine-stimulated primary glomerular endothelial cells to platelet-rich plasma from patients, or patient platelet-poor plasma combined with normal platelets, in a perfusion system, under shear, induced C3 deposition on von Willebrand factor-platelet strings (on both von Willebrand factor and platelets) and on endothelial cells. Complement activation occurred via the alternative pathway. No C3 was detected when cells were exposed to TTP plasma that was preincubated with EDTA or heat-inactivated, or to control plasma. In the perfusion system, patient plasma induced more release of C3- and C9-coated endothelial microparticles compared with control plasma. The results indicate that the microvascular process induced by ADAMTS13 deficiency triggers complement activation on platelets and the endothelium, which may contribute to formation of thrombotic microangiopathy.

Recent Therapeutic Advances in Thrombotic Thrombocytopenic Purpura

Whole plasma infusion, in our experience, has been highly effective in the management of patients with thrombotic thrombocytopenic purpura. The effectiveness of plasma infusion in the treatment of this severe disorder implies the deficiency of a factor in the patient's plasma. Furthermore, we have observed that when platelets are suspended in plasma obtained during active, untreated thrombotic purpura, aggregation occurs. This effect is neutralized by preincubation of the thrombotic thrombocytopenic purpura plasma with normal plasma. Thus, there is both a correlation with the clinical pathogenic mechanism, disseminated platelet aggregation, and with the therapeutic response to plasma infusion. Based upon our experience and the concept that thrombotic thrombocytopenic purpura is a plasma factor deficiency state, we recommend initial infusion of a full plasma volume equivalent over the first 24 hours. This should be done under an intensive care setting. After this initial plasma infusion, we advise the infusion of 3 units of plasma daily until a full remission is obtained. When the clinical situation has stabilized, we stop the daily plasma infusions and cautiously observe for recurrence of the manifestations of thrombotic thrombocytopenic purpura. If there is a recurrence, plasma is again infused in substantial quantity during the first 24 hours and then 3 units daily until a full remission is again evident. Whether the plasma requirement might be attenuated or the course of the disease shortened by the concomitant use of antiplatelet agents, corticosteroids or other means remains to be determined.

Synergistic interactions between interferon-γ and TRAIL modulate c-FLIP in endothelial cells, mediating their lineage-specific sensitivity to thrombotic thrombocytopenic purpura plasma–associated apoptosis

AbstractMicrovascular endothelial cell (MVEC) injury coupled to progression of platelet microthrombi facilitated by ADAMTS13 deficiency is characteristic of idiopathic and HIV-linked thrombotic thrombocytopenic purpura (TTP). Cytokines capable of inducing MVEC apoptosis in vitro are up-regulated in both TTP and HIV infection. However, the concentrations of these cytokines required to elicit EC apoptosis in vitro are 2- to 3-log–fold greater than present in patient plasmas. We report that clinically relevant levels of tumor necrosis factor–related apoptosis-inducing ligand (TRAIL) and interferon (IFN)–γ act in synergy to induce apoptosis in dermal MVECs, but have no effect on large-vessel ECs or pulmonary MVECs. This reflects the tissue distribution of TTP lesions in vivo. Sensitivity to TTP plasma or TRAIL plus IFN-γ is paralleled by enhanced ubiquitination of the caspase-8 regulator cellular FLICE-like inhibitory protein (c-FLIP), targeting it for proteasome degradation. c-FLIP silencing with anti-FLIP short interfering RNA (siRNA) in pulmonary MVECs rendered them susceptible to TTP plasma– and cytokine-mediated apoptosis, while up-regulation of c-FLIP by gene transfer partially protected dermal MVECs from such injury. TTP plasma–mediated apoptosis appears to involve cytokine-induced acceleration of c-FLIP degradation, sensitizing cells to TRAIL-mediated caspase-8 activation and cell death. Suppression of TRAIL or modulation of immunoproteasome activity may have therapeutic relevance in TTP.

Increased Endothelial Colony-Forming Cells (ECFC) and Endothelial Microparticles (EMP) and Their Interaction in Thrombotic Thrombocytopenic Purpura (TTP).

BACKGROUND. Endothelial damage is a prominent feature of TTP, evidenced by elevated EMP levels during acute episodes of TTP. However, endothelial repair in TTP remains to be investigated. ECFC are key players in endothelial regeneration, and their assay in blood has been employed to monitor endothelial remodeling. We investigated circulating ECFC in TTP and their relationship with EMP.METHODS. Six patients with acquired TTP were studied. All were treated with therapeutic plasma exchange (TPE). All but one underwent complete remission (CR). The one who failed to achieve CR with TPE suffered multiple strokes and developed chronic renal failure. We measured the following in acute phase and remission, as well as pre- and post-TPE:i.EMP defined by CD31+/CD42b- (EMP31) or CD62E+ (EMP62) by flow cytometry;ii.ECFC in blood by the method of Ingram et al [Blood 104:2752, 2004];iii.ADAMTS13 activity in blood as well as in supernatants of ECFC-derived EC by the FRETS-vWF73 fluorogenic method.iv.In addition, we investigated effects of microparticles (MP) from TTP plasma on normal ECFC.In separate experiments, EC cultured from TTP patients were assayed for surface expression of CD106 and CD54, and EMP released in response to TNF-α.RESULTS.i.In the acute phase, EMP were elevated: EMP31 (1.89 ± 0.65 × 106 counts/mL vs. 0.28 ±0.1 for controls, p=0.001) and EMP62 (5.7 ±1.3 × 106 counts/mL vs. 0.40 ±0.12 for controls, p<0.001). After the first TPE, EMP fell significantly.ii.In acute episodes of TTP, ECFC colonies were significantly increased in 5 responders (4.1 ±0.5 vs. 1.0 ±0.03 for controls, p<0.01). However, no elevation in ECFC was observed in the non-responder. Upon remission, ECFC colonies returned to normal in responders.iii.ADAMTS13 activity was <10% in acute phase and rose after TPE.iv.A positive correlation was found between initial EMP31 or EMP62 levels and ECFC (R=0.60 and R=0.71, respectively; p<0.001).To test whether this correlation might be due to ECFC-EMP interaction, we studied the MP fraction from the 5 responders on ECFC. A significant increase in colony counts was observed upon treatment with 1 × 108 MP/mL (1.2 ±0.04 ECFC colonies vs. 2.0 ±0.5 MP-treated, p=0.01). Thus, MP apparently stimulate ECFC colony formation. We have also tested the functional activities of ECFC from TTP patients. No significant difference was observed in ADAMTS13 activity secreted by EC from TTP compared to control or remission EC. In addition, EC cultures derived from ECFC colonies from TTP patients did not differ from controls in response to TNF-α stimulation, neither in EMP released or expression of surface markers.CONCLUSIONS. TPE decreases plasma EMP in all responders. The benefit of TPE may be attributable in part to removal of EMP and other procoagulant MP. The increased ECFC in acute TTP, demonstrated for the first time in this study, apparently reflects enhanced endothelial repair in the face of injurious processes among responders. Failure of this regeneration may entail irreversible damage, as seen in the non-responder. Finally, EMP or other MP apparently stimulate ECFC and may play a role in endothelial repair through interaction with ECFC.

ADAMTS13 phenotype in plasma from normal individuals and patients with thrombotic thrombocytopenic purpura

The activity of ADAMTS13, the von Willebrand factor cleaving protease, is deficient in patients with thrombotic thrombocytopenic purpura (TTP). In the present study, the phenotype of ADAMTS13 in TTP and in normal plasma was demonstrated by immunoblotting. Normal plasma (n = 20) revealed a single band at 190 kD under reducing conditions using a polyclonal antibody, and a single band at 150 kD under non-reducing conditions using a monoclonal antibody. ADAMTS13 was not detected in the plasma from patients with congenital TTP (n = 5) by either antibody, whereas patients with acquired TTP (n = 2) presented the normal phenotype. Following immunoadsorption of immunoglobulins, the ADAMTS13 band was removed from the plasma of the patients with acquired TTP, but not from that of normal individuals. This indicates that ADAMTS13 is complexed with immunoglobulin in these patients. The lack of ADAMTS13 expression in the plasma from patients with hereditary TTP may indicate defective synthesis, impaired cellular secretion, or enhanced degradation in the circulation. This study differentiated between normal and TTP plasma, as well as between congenital and acquired TTP. This method may, therefore, be used as a complement in the diagnosis of TTP.

Mechanisms of the interaction between twoADAMTS13 gene mutations leading to severe deficiency of enzymatic activity

The inherited deficiency of the von Willebrand factor-cleaving protease ADAMTS13 is associated with rare forms of thrombotic thrombocytopenic purpura (TTP). We investigated a woman with a family history of chronic recurrent TTP and undetectable plasma levels of ADAMTS13 activity. Genetic analysis revealed two missense mutations in the heterozygous state: p.Val88Met substitution in the metalloprotease domain and p.Gly1239Val substitution in the first CUB domain of ADAMTS13. To explore the mechanism of ADAMTS13 deficiency in this patient, the wild type (WT; ADAMT13(WT)) and each mutant construct (ADAMTS13(Val88Met), ADAMTS13(Gly1239Val)) were transiently expressed in HEK 293 and COS-7 cells. To recapitulate the compound heterozygous state of the patient, both mutant ADAMTS13 proteins were also expressed together. The p.Val88Met mutation led to a defect of secretion of the protease associated with a reduction of enzymatic activity, the p.Gly1239Val mutation led to a secretion defect causing intracellular accumulation of the protease. The mechanistic effects of the mutations were further explored by means of differential immunofluorescence, that demonstrated an homogeneous distribution of ADAMTS13(WT) in the Cis-Golgi and endoplasmic reticulum (ER) compartments, a reduction of ADAMTS13(Val88Met) in both compartments, while ADAMTS13(Gly1239Val) failed to reach the Cis-Golgi compartment and remained in the ER.

ADAMTS-13 regulates platelet adhesion under flow

ADAMTS-13 cleaves large and ultra-large von Willebrand factor multimers normally secreted by endothelial cells. Severe deficiency of this enzyme leads to thrombotic thrombocytopenic purpura (TTP). We applied the Impact-R system [Cone and plate(let) Analyzer, CPA] to determine optimal conditions for ADAMTS-13 function, to assess it's activity in TTP patients and to distinguish inherited TTP (inTTP) from acquired TTP (acTTP). The role of ADAMTS-13 in platelet adhesion under different conditions was investigated applying recombinant forms of VWF and ADAMTS-13. rVWF was first treated by rADAMTS-13 either in solution or when immobilized on the surface of the well, under static or flow conditions, in the presence or absence of BaCl2. The resulting cleaved VWF fragments were then immobilized and served to assess type 3 von Willebrand disease whole blood platelet adhesion under flow. Maximal effect of the rADAMTS-13 (decrease of platelet adhesion in the absence compared to the presence of BaCl2), was observed when the rVWF was pre-immobilized and the cleavage step took place under flow (85%). Mixing plasma ofTTP patients with normal blood (1:3) yielded a 1.6- to 2-fold increase of platelet adhesion under flow compared to mixing normal plasma with normal blood, at shear rate range of 1,800 - 2,500 s(-1) . Maximal increase of platelet adhesion was observed under 2,050 s(-1) . Under these conditions, the extent of adhesion was similarly higher in patients with inTTP and acTTP versus control [surface coverage (SC) 14.5 +/- 2.8% and 14.6 +/- 2.5% vs.7.4 +/- 1.7%, respectively]. ADAMTS-13 activity measured by collagen-binding test was similarly low (4.2 +/- 3.8% and 3.5 +/- 2.4% vs. 72.2 +/- 8.0%, respectively). An inverse correlation between SC and ADAMTS-13 activity was observed in a patient with inTTP assayed eight times during plasma infusion treatment. Addition of BaCl2 to the mixture of TTP plasma and normal whole blood yielded a decrease in platelet adhesion in inTTP (by 51%) but not in acTTP. The lack of reduction of platelet adhesion in acTTP could presumably be due to the presence of ADAMTS-13 inhibitor in these patients. These results suggest that VWF immobilization and high shear flow yielded optimal conditions for ADAMTS-13 activity, and that introduction of BaCl2 in the Impact-R (CPA) test may be useful for differentiation between inherited and acquired TTP.

A study on the significance of plasma thrombospondin1 in thrombotic thrombocytopenic purpura and the relationship between thrombospondin1 and von Willebrand factor cleaving protease (ADAMTS13)

To measure plasma thrombospondin1 (TSP1) in thrombotic thrombocytopenic purpura (TTP) and other diseases such as idiopathic thrombocytopenic purpura (ITP), systemic lupus erythematosus (SLE), myocardial infarction, brain infarction, and malignant tumor et al. and 8 patients after bone marrow transplantation (BMT) were also investigated. Then to study on the relationship between TSP1 and von Willebrand factor cleaving protease (ADAMTS13); and to identify the significance of plasma TSP1 in TTP. TSP1 was measured by a commercial kit and the activity of ADAMTS13 was evaluated by residue collagen binding assay. TSP1 in TTP plasma before plasma exchange or plasma infusion was 6.49 mg/L, and stepping up to 13.02 mg/L after therapy, but still significantly lower than 18.34 mg/L in normal control. While the decrease in different degree of ADAMTS13 activity was observed from 0%-52%, and there were 8 samples whose activity of ADAMTS13 were no more than 10%; the different extent of increase in those patients after therapy was demonstrated to be 2.9%-93.4%, only one patient's ADAMTS13 activity was below 10%. The activity of ADAMTS13 in some ITP and SLE patients were mildly decreased (63% +/- 16% and 70% +/- 14% respectively), TSP1 were also decreased (16 mg/L +/- 8 mg/L). TSP1 in patients of myocardial infarction and brain infarction were increased (24.0 mg/L +/- 2.9 mg/L), while ADAMTS13 activity had no significant change (72% +/- 16%). Same things happened in BMT patients. There was some concordance between the decrease of ADAMTS13 activity and TSP1 in plasma of TTP patients. And the change of TSP1 was restricted to TTP. TSP1 may contribute to the episode of TTP in a still unclear fashions.

Endothelial Microparticles (EMP) Inhibit ADAMTS13 Activity: Implications in Thrombotic Thrombocytopenic Purpura.

Deficiency of ADAMTS13 is implicated in the pathogenesis of congenital and some acquired idiopathic thrombotic thrombocytopenic purpura (TTP). However, injury to endothelial cells (EC) is also believed to play a major role. Endothelial microparticles (EMP), markers of EC injury, are elevated in acute TTP, and subsets of EMP have been shown to carry ULvWF. The aim of this study was to further characterize the role of EMP in TTP by assessing their effect on ADAMTS13 activity.METHODS. EMP derived from activated renal microvascular EC (RMVEC) and coronary artery EC (CAEC) were depleted of phenotypic subsets by negative panning with immunomagnetic beads against CD62E, CD31, or CD54. For clinical studies, platelet-poor plasmas (PPP) were selectively depleted of PMP and LMP by beads against CD41 and CD45, respectively. Antigenic phenotypes of EMP were measured by flow cytometry. vWF multimer size was analyzed by Western blotting after 0.8% argarose gel electrophoresis. Proaggregatory activity of vWF (free in plasma or on EMP) was assessed by incubation of washed platelets with EMP (some are vWF+) or soluble plasma vWF, and ristocetin for 10min, then measuring platelet aggregation by flow cytometry. To measure interaction of ADAMTS13 with EMP, PPP was incubated with EMP (1x105 – 1x108 EMP/mL) for 15min – 3hr, then centrifuged to remove EMP (15,000xg, 1hr), and ADAMTS13 activity remaining in the plasma was assayed by the method of Furlan et al [Blood, 1997, 89:3097].RESULTS. (1) RMVEC released >2-fold more EMP than CAEC and the proportion of vWF+ EMP was 2-fold higher with RMVEC (65 ±17% of total) than with CAEC (30±12%), p<0.01. (2) We next selectively depleted total EMP of CD31+, CD62E+ or CD54+ subsets, then incubated with normal plasma (defined as 100% ADAMTS13 activity) and measured activity remaining after centrifugation. Maximal reduction of activity was seen with non-depleted (total) EMP, which was nearly the same as with the CD54-depleted preparations (all experiments at 1x108 EMP/mL for 3hr), resulting in a 71 ±23% reduction after centrifugation (p<0.01). However, depletion of EMP positive for CD31 or CD62E resulted in significantly less removal or inactivation of ADAMTS13 (55 ±17%, 35 ±8% inhibition, respectively. Similar results were observed with EMP from CAEC except that the effect was about 25% less. (3) To determine whether EMP from TTP patients can also negatively regulate ADAMTS13 activity, we isolated total MP by centrifugation from plasma of 3 acute TTP patients, and removed MP of platelet and leukocyte origin by immunomagnetic beads. The resulting EMP were incubated with normal plasma. As observed with in vitro-generated EMP, similar concentrations of patient EMP removed or inactivated ADAMTS13 activity in plasma, by 66 ±18% in these experiments (p<0.02). (4) To further test the ability of EMP to adsorb ADAMTS13, we found that preincubation of plasma with EMP, resulted in 57 ±10% increased platelet aggregate formation relative to absence of EMP preincubation, (p<0.01).CONCLUSION. EMP released from EC culture and EMP isolated from TTP plasma inactivated ADAMTS13 activity and enhanced formation of platelet aggregates. These findings further support the hypothesis that EMP play an important role in TTP by downregulating ADAMTS13 and promoting the formation of platelet rich microthrombi in microvasculature.

Technozym® ADAMTS-13 ANTIGEN ELISA - A New Assay for Specific Quantification of the vWF Cleaving Protease ADAMTS-13.

Defects in activity and/or antigen levels of ADAMTS-13, the von Willebrand Factor (vWF) cleaving protease, are viewed as the main cause inducing the microvascular thrombotic disorder TTP (thrombotic thrombocytopenic purpura) that is in more than 90% of cases fatal if not treated early and appropriately. Malfunction of ADAMTS-13 with respect to cleave multimeric vWF can be caused by auto-antibodies directed against ADAMTS-13, by decreased presence of ADAMTS-13 in the circulation or by defective activity of ADAMTS-13. Therefore rapid and reliable diagnosis of ADAMTS-13 parameters is a clinical need. We present here an assay for quantification of ADAMTS-13 antigen levels. The assay is a regular double sandwich ELISA employing a monoclonal antibody for capturing ADAMTS-13 from the sample by binding to the CUB domains. The bound ADAMTS-13 is detected by a polyclonal antibody conjugate. The sensitivity of the assay is below 10% of the normal antigen level. In normal samples a wide range (50% to 200%) of the mean level was observed. In idiopathic TTP plasmas also a wide range of antigen levels was observed but with a slighltly lower mean value than in normal samples. Samples from hereditary TTP mostly showed lower antigen levels than normal samples. Thus we could show that this assay provides a useful tool for measuring ADAMTS-13 antigen levels. In combination with our activity assay, which is being developed in parallel, it will be possible to establish ratios of activity and antigen which could provide more insight into the mechanisms of ADAMTS-13 deficiencies than the individual values for antigen and activity.

Distinction of Inherited from Acquired Thrombotic Thrombocytopenic Purpura Applying the Cone and Plate(Let) Analyzer.

Measurement of von Willebrand factor-cleaving protease (VWF-CP) activity is useful for diagnosis of thrombotic thrombocytopenic purpura (TTP), but the current methods are cumbersome. We recently introduced a simple method of TTP detection using the Cone and Plate(let) Analyzer (CPA), that was based on the observation that mixing of acute TTP plasma (50 μL) with normal whole blood (150 μL) induces a significant increase in platelet deposition (surface coverage, %SC) on polystyrene surface under flow condition (1800 s−1) (Brit J Haematol , 120: 597, 2003). The test was specific for TTP, since plasma from patients with HUS, APLS, ITP or HIT did not affect normal platelet deposition. In this study, we further explored the potential use of the CPA method in differential diagnosis of inherited and acquired TTP by increasing the shear rate and by inducing the VWF-CP activity with BaCl2. TTP or normal plasma was mixed with normal blood (type O+) and the surface covered by platelets was tested under shear rates ranging between 1800 s−1 to 2500 s−1. Maximal difference in SC between TTP and normal plasma was observed at a shear rate of 2050 s−1. Under these conditions, plasmas of 5 patients with inherited TTP (ITTP), and 11 patients with acquired TTP (ATTP) yielded comparable increase in SC of normal platelets in whole blood, i.e. 77±19% and 78±17%, respectively. However, experiments in which BaCl2 was added to allow activation of VWF-CP resulted in a substantial distinction between inherited and acquired TTP plasma. While addition of BaCl2 to normal or inherited TTP plasma, both mixed with normal blood, yielded similar reductions of SC by 41±12% and 51±19%, only 11±4% reduction of SC was observed when acquired TTP plasma was used. These results suggest that when normal or inherited TTP plasma is mixed with normal blood, activation of VWF-CP cleaves large VWF multimers giving rise to substantial reduction in SC, whereas in the case of acquired TTP plasma the inhibitor of VWF-CP abolishes BaCl2-induced activation of VWF-CP and the ensuing reduction in SC. We conclude that introduction of BaCl2 in the CPA at high shear conditions may be useful for differentiation between inherited and acquired TTP.

Circulating CD62E Occurs in Soluble Form as Well as Bound to Endothelial Microparticles (EMP): Functional Differences in Platelet Aggregation in Thrombotic Thrombocytopenic Purpura (TTP).

Injured endothelial cells (EC) are believed to play a critical role in the pathophysiology of TTP. Soluble markers of endothelial disturbance measured by enzyme-linked immunoassay (ELISA) have been found elevated in TTP. We have recently demonstrated an increase in the release of CD31/42b- EMP, and CD62E+ EMP. Moreover, we have observed that CD62E+ EMP also express vWF. The aim of this study was to quantitate soluble (s) vs. EMP-bound CD62E (bCD62E) in vitro and in vivo, in relation to the functional activity of vWF+ EMP.METHODS: Brain and renal microvascular endothelial cells (MVEC) were cultured and treated with 10ng/mL TNF-α to induce activation, or deprived of serum and growth factors (GFD) to induce apoptosis. Culture supernatants were collected and evaluated in a time-dependent manner. For in vivo studies, platelet-poor plasma was obtained from 4 TTP patients during the acute phase and upon remission. Filtration through 0.1μm, which retains most EMP, was employed to discriminate between (s) and bCD62E. sCD62E was measured by ELISA post-filtration and bCD62E by ELISA pre-filtration. Additionally, CD62E+ and CD62E+/vWF+ EMP were measured by flow cytometry. To assess pro-aggregatory function, EMP were added to washed platelets in the presence of 1 mg/mL ristocetin and aggregates were measured by flow cytometry.RESULTS:In vitro: Activation did not induce release of sCD62E at 3 hours, although bCD62E was present (1.5±0.5X106 EMP/mL). At 6 hours, some sCD62E was detected in the filtrate (0.09±0.02 ng/mL), but most was present in the unfiltered medium (3.5±0.85 ng/mL), signifying that the majority was bCD62E, confirmed by a doubling of CD62E+ EMP (3.0±0.6X106/mL). Subsequently, sCD62E levels were 1.0±0.2 ng/mL at 12 hr, 3.5±0.7 ng/mL at 18 hr, and 5±0.9 ng/mL at 24 hr. In contrast, EMP counts at 12, 18 and 24 hours were 4.6±1, 7±1.3 and 9±1.8 X106/mL (p=0.01, p=0.01, p=0.02, respectively). For all time periods, 40-60% of CD62E were positive for vWF. In control or GFD cultures, there was not a significant increase in sCD62E or CD62E+ EMP at any time period. MVEC from renal gave similar results. In acute TTP plasma samples, CD62E measured by ELISA was significantly increased (65±22 ng/mL) vs. remission (30±6 ng/mL). bCD62E accounted for 50% in acute and 15% in remission. CD62E+/vWF+ EMP were significantly elevated in plasma from acute TTP patients vs. remission (15±4.5 vs. 3±0.5, p=0.01). Sample filtration resulted in a decrease of >95% EMP in both acute and remission TTP plasma. MVEC-derived CD62E+/vWF+ EMP resulted in a dose-dependent increase in platelet aggregation. Additionally, plasma from 4 TTP patients with elevated CD62E+/vWF+ EMP obtained during the acute phase enhanced the formation of platelet aggregates by 48±12% (p=0.02) above remission plasma with low EMP counts.CONCLUSIONS: The results demonstrate that CD62E heretofore regarded as a soluble marker of endothelial dysfunction, in reality exists in both a soluble and EMP-bound form. Indeed, this distinction is highly relevant because CD62E+ EMP also express vWF and are pro-aggregatory to platelets. These EMP have been shown to be elevated during the acute phase of TTP and decrease upon remission. Thus, CD62E+/vWF+ EMP may be active participants in the formation of platelet-rich thrombi in TTP.

Defective processing of unusually large von Willebrand factor multimers and thrombotic thrombocytopenic purpura

Defective processing of unusually large von Willebrand factor multimers and thrombotic thrombocytopenic purpura

Epitope mapping of ADAMTS13 autoantibodies in acquired thrombotic thrombocytopenic purpura

AbstractSevere deficiency of the von Willebrand factor (VWF)-cleaving protease ADAMTS13 can lead to thrombotic thrombocytopenic purpura (TTP), a disease associated with the widespread formation of platelet-rich thrombi in many organs. Autoantibodies that inactivate ADAMTS13 are the most frequent cause of acquired TTP. Little is known about epitope specificity and reactivity of anti-ADAMTS13 antibodies. In this study, a series of ADAMTS13 domains were expressed in Escherichia coli, and the reactivity of purified recombinant fragments with anti-ADAMTS13 auto-antibodies from 25 patients with severe ADAMTS13 deficiency was evaluated in vitro. All TTP plasmas contained antibodies directed against the cysteine-rich spacer (cys-rich/spacer) domain of ADAMTS13. In the plasma of 3 patients, antibodies were detected that reacted exclusively with the cys-rich/spacer domain, underscoring the importance of this region for functional activity of ADAMTS13. In 64% of the plasmas, antibodies reacted with the 2 CUB domains, and in 56% they reacted with the isolated first thrombospondin type 1 (TSP-1) repeat and with the compound fragment consisting of the catalytic, the disintegrin-like, and the TSP1-1 domain. Less frequently, in 28% of the plasmas, antibodies reacted with the TSP1 repeats 2 to 8. Unexpectedly, antibodies reacted with the propeptide region in 20% of the plasmas. In conclusion, this study shows that even though anti-ADAMTS13 autoantibodies react with multiple domains of the protease, the cys-rich/spacer domain is consistently involved in antibody reactivity. (Blood. 2004;103:4514-4519)

Thienopyridine-linked thrombotic microangiopathy: association with endothelial cell apoptosis and activation of MAP kinase signalling cascades.

The thienopyridine platelet antagonist ticlopidine is associated with development of thrombotic thrombocytopenic purpura (TTP) but the pathophysiology of this link is unclear. Severe deficiency of disintegrin and metalloproteinase with thrombospondin motif-13 (ADAMTS13), described in familial cases and a significant fraction of idiopathic TTP, has been reported in only a few ticlopidine-linked cases. As ticlopidine can disrupt production of extracellular matrix (ECM) components critical to microvascular endothelial cell (MVEC) integrity in vitro, we explored the hypotheses that ticlopidine and ticlopidine-linked TTP plasmas induce MVEC apoptosis in a manner similar to that of idiopathic TTP plasmas, and that ECM components and related mitogen-activated protein kinase (MAPK) signalling cascades may be involved in this process. Replicating the activity of plasmas from patients with idiopathic TTP, plasma from five ticlopidine-linked TTP patients induced apoptosis of primary human dermal, glomerular and hepatic MVEC, but had no effect on pulmonary MVEC or large vessel endothelial cells (EC). Pharmacological levels of ticlopidine initiated apoptosis with similar EC lineage restriction. In parallel, ticlopidine and plasmas from idiopathic and ticlopidine-TTP patients decreased transcripts for the ECM component thrombospondin-1 in MVEC, but not in large vessel EC. These changes were accompanied by prolonged induction of MAPKs extracellular signal-related kinase (ERK)-1/2 and p38 only in TTP susceptible MVEC. Induction of apoptosis by ticlopidine and TTP plasma was abrogated by inhibitors of ERK-1/2 and p38 phosphorylation. In conclusion, MVEC apoptosis related to altered ECM-MVEC interactions may be a key part of the pathology of ticlopidine-linked and idiopathic TTP.