Increased von Willebrand factor binding to platelets in single episode and recurrent types of thrombotic thrombocytopenic purpura

Partial amino acid sequence of purified von Willebrand factor-cleaving protease

von Willebrand factor-cleaving protease (vWF-cp) is responsible for the continuous degradation of plasma vWF multimers released from endothelial cells. It is deficient in patients with thrombotic thrombocytopenic purpura, who show unusually large vWF multimers in plasma. Purified vWF-cp may be useful for replacement in these patients, who are now treated by plasma therapy. In this study, vWF-cp was purified from normal human plasma by affinity chromatography on the IgG fraction from a patient with autoantibodies to vWF-cp and by a series of further chromatographic procedures, including affinity chromatography on Protein G, Ig-TheraSorb, lentil lectin, and heparin. Four single-chain protein bands, separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis under nonreducing conditions, showed M(r) of 150, 140, 130, and 110 kd and were found to share the same N-terminal amino acid sequence, suggesting that they were derived from the same polypeptide chain that had been partially degraded at the carboxy-terminal end. A hydrophobic sequence (Ala-Ala-Gly-Gly-Ile-Leu-His-Leu-Glu-Leu-Leu-Val-Ala-Val-Gly) of the first 15 residues was established. The protease migrates in gel filtration as a high-molecular-weight complex with clusterin, a 70-kd protein with chaperonelike activity. vWF-cp bound to clusterin is dissociated by the use of concentrated chaotropic salts. vWF-cp in normal human plasma or serum is not associated with clusterin, suggesting that the observed complex is due to vWF-cp denaturation during the purification procedure. Activity of vWF-cp is unusually stable during incubation at 37 degrees C; its in vitro half-life in citrated human plasma, heparin plasma, or serum is longer than 1 week. There was even a temporary increase in protease activity during the first 3 days of incubation.

New aspects in the pathogenesis and treatment of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

The thrombotic microangiopathy (TM) syndromes, thrombotic thrombocytopenic purpura and the hemolytic uremic syndrome, are a rare and heterogeneous group of disorders characterized by widespread microvascular thrombosis and end organ injury. Decades of descriptive studies have defined clinical subsets of TM syndromes by clinical and laboratory features. Despite many advances, however, progress towards understanding of the etiology and pathogenesis of TM disorders remains limited. The rarity of occurrence and lack of natural animal models of TM syndromes have hampered progress in experimental and clinical studies. Treatment remains essentially empirical and options are limited. However, recent advances in the genetic and molecular understanding of subsets of TM disorders and the development of relevant animal models offer new resources to explore the pathogenic mechanisms. With these new advances more effective and individualized treatments for TM syndromes can be developed.

Cryoprecipitate poor plasma does not improve early response in primary adult thrombotic thrombocytopenic purpura (TTP)

Thrombotic thrombocytopenic purpura (TTP) is a potentially fatal disease that is treated with plasma exchange and typically with replacement with fresh frozen plasma (FFP). This approach results in an approximate 50% response rate following 1 week of therapy and 80% survival. Cryoprecipitate poor plasma (CPP) is plasma from which the cryoprecipitate fraction is removed. CPP has been reported to be successful as salvage therapy in refractory TTP and has been suggested to be superior to FFP in retrospective studies. The present report compares initial therapy of TTP with exchange using replacement with either FFP or CPP in a multi-institutional prospective randomized study performed by the North American TTP Group (NATG Group) from 1993 to 1995. Initial therapy also included corticosteroids. Antiplatelet drugs or vinca alkaloids were not employed. A severity score index, response score, and individual clinical parameters (platelet count, LDH x upper limit of normal, hemoglobin level, and creatinine) were compared at their nadir or peak values, baseline, and days +6 and +13 of therapy. Thirteen patients were randomized to FFP exchange and 14 to CPP exchange. Results were equivalent for all parameters. Survival was equal with three deaths in each group. These data indicate that the efficacy of FFP and CPP are the same in the initial treatment of TTP in adults.

High titers of inhibitors of von Willebrand factor-cleaving metalloproteinase in a fatal case of acute thrombotic thrombocytopenic purpura

Acute thrombotic thrombocytopenic purpura is a disease with diffuse platelet thrombi in the microcirculation. Despite plasma exchange therapy, approximately 20 percent of the patients succumb to the disease. A significant number of the survivors suffer one or more relapses. This study investigated the activity of von Willebrand factor-cleaving metalloproteinase and the titers of IgG inhibitors to the proteinase in serial plasma samples from a patient who died during a relapse of the disease despite continued plasma exchanges. A deficiency in the von Willebrand factor-cleaving metalloproteinase activity, due to inhibitory IgG, was detected at presentation. After initiation of plasma exchange, a transient increase in proteinase activity coincided with clinical remission. When thrombocytopenia relapsed, the activity of the proteinase also declined to undetectable levels. Toward the end of her course, proteinase levels raised by plasma exchange were not sustained because the proteinase activity was quickly neutralized by rising titers of inhibitors. High titers of inhibitors of von Willebrand factor-cleaving metalloproteinase cause refractoriness to standard plasma therapy. These findings suggest that intensive plasma exchanges and measures that suppress antibody titers may prevent death and promote remission in refractory cases.

von Willebrand factor and thrombotic thrombocytopenic purpura

Recent advances in the understanding of platelet-dependent hemostasis and von Willebrand factor (vWF) functional regulation offer new insights into the pathogenesis of thrombotic microangiopathic disorders. The discovery of vWF-cleaving protease activity in normal plasma, and its deficiency in thrombotic thrombocytopenic purpura (TTP) patients, provides additional support for a pathologic role of ultra-large vWF in TTP. Although vWF-cleaving protease deficiency is highly prevalent among TTP patients, the defect has also been detected in individuals without active TTP. Therefore, vWF-cleaving protease deficiency appears to be an important risk factor for thrombotic microangiopathy rather than a specific diagnostic marker of TTP. Recent data indicate that vWF-cleaving protease activity correlates with clinical parameters in thrombotic microangiopathy patients. Therefore, determination of vWF-cleaving protease activity might prove useful in the future care of thrombotic microangiopathy patients and might be a rational basis for future classification of thrombotic microangiopathic disorders.

Increased von Willebrand factor (vWf) binding to platelets associated with impaired vWf breakdown in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disorder of systemic platelet aggregation. Evidence has accumulated that the aggregating agonist in TTP of all types is likely to be von Willebrand factor (vWf), especially unusually large vWf multimers derived from endothelial cells. Recent evidence indicates that a metalloproteinase involved in vWf breakdown is produced in inadequate amounts in children with chronic relapsing TTP. Chronic relapsing TTP is, therefore, likely to be a congenital enzyme deficiency. In adults with single episode or intermittent types of TTP, the vWf metalloproteinase is inhibited by autoantibodies that are present either transiently or intermittently in patient blood. Single episode and intermittent types of TTP in adults are likely to be short-term or recurrent autoimmune processes, respectively.

Thrombotic thrombocytopenic purpura and the hemolytic-uremic syndrome

Large and unusually large von Willebrand factor (vWf) multimers may be responsible for systemic platelet aggregation in thrombotic thrombocytopenic purpura (TTP). This possibility is supported by studies that show deficient vWf-cleaving metalloproteinase and increased platelet-vWf binding during TTP episodes. In acute idiopathic TTP, decreased vWf metalloproteinase is the result of autoantibodies against the enzyme. In familial and acquired hemolytic-uremic syndrome, vWf-cleaving metalloproteinase activity is normal. A deficiency or defect in factor H, which normally dampens the activation of C3 via the alternative complement pathway, has been seen in some patients with familial hemolytic-uremic syndrome. Ticlopidine therapy is an important risk factor for TTP.

Close relationship between systemic lupus erythematosus and thrombotic thrombocytopenic purpura in childhood

OBJECTIVE

To determine the association between childhood-onset thrombotic thrombocytopenic purpura (TTP) and systemic lupus erythematosus (SLE).

METHODS

The charts of all 5 patients diagnosed with idiopathic TTP at the Hospital for Sick Children (HSC) in Toronto from 1975 to 1998, and all cases of childhood-onset TTP (ages 6-20 years) reported in the literature over the same period were reviewed. Fourteen of the 44 patients identified in the literature were excluded from the analysis for lack of clinical and laboratory information. The remaining 35 patients were grouped into either an SLE/TTP group or a TTP only group, according to the presence or absence of the American College of Rheumatology (ACR) classification criteria for SLE. The groups were compared for differences in clinical or laboratory features.

RESULTS

The clinical presentation and initial disease course of pediatric patients with TTP were similar to those observed in adults. Of the 35 patients with childhood-onset TTP included in this review, 9 (26%) fulfilled > or = 4 ACR criteria for SLE and 8 (23%) were found to have incipient SLE. Of the 5 patients initially diagnosed with idiopathic TTP at the HSC, 3 were diagnosed with SLE within 3 years, and the other 2 patients fulfilled 3 ACR classification criteria for SLE within 4 years of disease onset. The clinical syndrome of pediatric TTP presenting with proteinuria, especially with high-grade proteinuria, was significantly associated with the development or coexistence of childhood-onset SLE.

CONCLUSION

TTP in childhood is a rare, but life-threatening, disease. Unlike in adults, TTP in childhood is commonly associated with SLE. High-grade proteinuria at diagnosis of TTP is the best predictor for the presence or subsequent development of SLE.

Thrombotic thrombocytopenic purpura today

Thrombotic thrombocytopenic purpura (TTP) usually responds to plasma exchange and plasma infusion. However, until recently, it was not clear how the treatment works. New understanding of the pathophysiology of TTP has clarified that issue and suggested new therapies that may eliminate plasma infusion and its risks.

Laboratory abnormalities in thrombotic thrombocytopenic purpura. Canadian Apheresis Group

Thrombotic thrombocytopenic purpura is an uncommon disorder that requires prompt recognition and intervention to prevent death. To date, information regarding the classic laboratory abnormalities in the disease has been derived from small numbers of patients whose laboratory tests have been done at many different sites. We report the laboratory findings in 135 patients who presented with thrombotic thrombocytopenic purpura to 17 Canadian centres. 50 men and 85 women had a mean platelet count of 25.3+/-19.4x10(9)/l. The initial platelet count correlated with mortality; 32% of patients with a platelet count of 20x10(9)/l or less died compared with 18% of patients with a platelet count >20x10(9)/l (P=0.058). The platelet-associated IgG was elevated in 88% at presentation whereas the indirect platelet suspension immunofluorescence test was positive in only 18%, 93% of the sera showed reactivity against platelets following protein blotting. All sera tested also showed reactivity against endothelial cells. Immune complexes were seen in all patients, whereas the platelet aggregating factor was detected in 59%. Although the von Willebrand factor was elevated in the majority of patients at entry, the multimer pattern was variable and showed no predictive pattern. Renal dysfunction was common (18%).

von Willebrand factor-cleaving protease in thrombotic thrombocytopenic purpura and the hemolytic-uremic syndrome

BACKGROUND

Thrombotic thrombocytopenic purpura and the hemolytic-uremic syndrome are severe microvascular disorders of platelet clumping with similar signs and symptoms. Unusually large multimers of von Willebrand factor, capable of agglutinating circulating platelets under high shear stress, occur in the two conditions. We investigated the prevalence of von Willebrand factor-cleaving protease deficiency in patients with familial and nonfamilial forms of these disorders.

METHODS

Plasma samples were obtained from 53 patients with thrombotic thrombocytopenic purpura or hemolytic-uremic syndrome. Von Willebrand factor-cleaving protease was assayed in diluted plasma samples with purified normal von Willebrand factor as the substrate. The extent of the degradation of von Willebrand factor was assessed by electrophoresis in sodium dodecyl sulfate-agarose gels and immunoblotting. To determine whether an inhibitor of von Willebrand factor-cleaving protease was present, we measured the protease activity in normal plasma after incubation with plasma from the patients.

RESULTS

We examined 30 patients with thrombotic thrombocytopenic purpura and 23 patients with the hemolytic-uremic syndrome. Of 24 patients with nonfamilial thrombotic thrombocytopenic purpura, 20 had severe and 4 had moderate protease deficiency during an acute event. An inhibitor found in 20 of these patients was shown to be IgG in five of five tested plasma samples. Of 13 patients with nonfamilial hemolytic-uremic syndrome, 11 had normal levels of activity of von Willebrand factor-cleaving protease during the acute episode, whereas in 2 patients, the activity was slightly decreased. All 6 patients with familial thrombotic thrombocytopenic purpura lacked von Willebrand factor-cleaving protease activity but had no inhibitor, whereas all 10 patients with familial hemolytic-uremic syndrome had normal protease activity. In vitro proteolytic degradation of von Willebrand factor by the protease was studied in 5 patients with familial and 7 patients with nonfamilial hemolytic-uremic syndrome and was normal in all 12 patients.

CONCLUSIONS

Nonfamilial thrombotic thrombocytopenic purpura is due to an inhibitor of von Willebrand factor-cleaving protease, whereas the familial form seems to be caused by a constitutional deficiency of the protease. Patients with the hemolyticuremic syndrome do not have a deficiency of von Willebrand factor-cleaving protease or a defect in von Willebrand factor that leads to its resistance to protease.

Antibodies to von Willebrand factor-cleaving protease in acute thrombotic thrombocytopenic purpura

BACKGROUND

Thrombotic thrombocytopenic purpura is a potentially fatal disease characterized by widespread platelet thrombi in the microcirculation. In the normal circulation, von Willebrand factor is cleaved by a plasma protease. We explored the hypothesis that a deficiency of this protease predisposes patients with thrombotic thrombocytopenic purpura to platelet thrombosis.

METHODS

We studied the activity of von Willebrand factor-cleaving protease and sought inhibitors of this protease in plasma from patients with acute thrombotic thrombocytopenic purpura, patients with other diseases, and normal control subjects. We also investigated the effect of shear stress on the ristocetin cofactor activity of purified von Willebrand factor in the cryosupernatant fraction of the plasma samples.

RESULTS

Thirty-nine samples of plasma from 37 patients with acute thrombotic thrombocytopenic purpura had severe deficiency of von Willebrand factor-cleaving protease. No deficiency was detected in 16 samples of plasma from patients with thrombotic thrombocytopenic purpura in remission or in 74 plasma samples from normal subjects, randomly selected hospitalized patients or outpatients, or patients with hemolysis, thrombocytopenia, or thrombosis from other causes. Inhibitory activity against the protease was detected in 26 of the 39 plasma samples (67 percent) obtained during the acute phase of the disease. The inhibitors were IgG antibodies. Shear stress increased the ristocetin cofactor activity of von Willebrand factor in the cryosupernatant of plasma samples obtained during the acute phase, but decreased the activity in cryosupernatant of plasma from normal subjects.

CONCLUSIONS

Inhibitory antibodies against von Willebrand factor-cleaving protease occur in patients with acute thrombotic thrombocytopenic purpura. A deficiency of this protease is likely to have a critical role in the pathogenesis of platelet thrombosis in this disease.

Thrombotic thrombocytopenic purpura: understanding a disease no longer rare

Serial studies of plasma samples from patients during episodes of thrombotic thrombocytopenic purpura (TTP) have often shown either the presence of unusually large (UL) von Willebrand factor (vWf) multimers or, alternatively, absence of the largest plasma vWf forms. The presence of ULvWf multimers in TTP patient plasma may reflect impaired processing of the ULvWf forms released from endothelial cells. The disappearance of ULvWf and large vWf multimers in some TTP patient plasma samples during acute TTP episodes may be predominantly because these ULvWf forms, along with the largest vWf multimers, bind to platelets and cause aggregation. Serial flow cytometry studies of EDTA-whole blood samples from patients with initial episode, intermittent, and chronic relapsing types of TTP confirm that vWf is the likely aggregating agent, perhaps in association with fluid shear stress. The amount of vWf bound to single platelets has been found to be significantly increased during TTP relapses relative to remission periods in patients with all types of TTP. A substance in normal platelet-poor plasma and the cryoprecipitate-depleted fraction of normal plasma (cryosupernatant) is capable in vitro of reversibly reducing the size of ULvWf multimeric forms released by endothelial cells into the somewhat smaller vWf multimers ordinarily in circulation. This activity has characteristics of a limited disulfide bond reductase. The process of ULvWf breakdown may be made irreversible by the tandem proteolysis, catalyzed by a vWf metalloproteinase, of partially reduced vWf multimers. Several patients with chronic relapsing TTP have decreased or absent plasma vWf metalloproteinase activity, apparently on a congenital basis. Adult initial episode and intermittent TTP patients have been found to have vWf metalloproteinase activity inhibited by an autoantibody during, but not after, TTP epidsodes.