Adenoviral-mediated gene transfer restores plasma ADAMTS13 antigen and activity in ADAMTS13 knockout mice

ADAMTS13 is a plasma metalloprotease that regulates the size of the von Willebrand factor (VWF) multimers. Genetic or acquired deficiency of ADAMTS13 causes thrombotic thrombocytopenic purpura (TTP) in humans. Plasma infusion is the treatment of choice for patients with congenital ADAMTS13 deficiency. However, this practice exposes patients to the risk of infections, allergies and fluid volume overload. The search for alternative treatments is required. Here, we tested the ability of systemically administered adenovirus encoding human ADAMTS13 to restore the deficient protein in the circulation of Adamts13(-/-) mice. Injection of the adenovirus efficiently transduced the liver, kidney, lung, heart and spleen, resulting in the secretion of ADAMTS13 into plasma. A reduced area of thrombi was observed when blood from Ad-ADAMTS13-treated mice was perfused over a collagen-coated surface in a parallel plate flow chamber compared with blood of Ad-betaGal-treated controls. The secreted ADAMTS13 protein was functionally active even after 2 months from injection. The data provide the proof of principle for developing a novel therapy for the correction of ADAMTS13 deficiency in patients with hereditary TTP.

Rituximab as pre-emptive treatment in patients with thrombotic thrombocytopenic purpura and evidence of anti-ADAMTS13 autoantibodies

Thrombotic thrombocytopenic purpura (TTP) is a rare and severe disease characterized by thrombocytopenia, microangiopathic haemolytic anemia, neurological and renal involvement associated with deficiency of the von Willebrand factor-cleaving protease, ADAMTS13. Persistence of high titers of anti-ADAMTS13 autoantibodies predisposes to relapsing TTP. Since relapses are associated with high morbidity and mortality rates, the optimal therapeutic option should be a pre-emptive treatment able to deplete anti-ADAMTS13 autoantibodies and avoid relapses. Five patients who presented with persistence of undetectable ADAMTS13 activity and high titers of autoantibodies, were treated with rituximab as pre-emptive therapy during remission. Four of them were affected by relapsing TTP and one was treated after the first episode. ADAMTS13 activity ranging from 15% to 75% with disappearance of inhibitors was achieved after three months in all patients, and persisted >20% without inhibitors at six months. In three patients disease-free status is still ongoing after 29, 24 and six months, respectively. Relapses were documented in two patients during follow-up: in one patient remission lasted 51 months; while in the other patient relapse occurred after 13 months. Results demonstrated that rituximab used as pre-emptive treatment may be effective in maintaining a sustained remission in patients with anti-ADAMTS13 antibodies in whom other treatments failed to limit the production of inhibitors, and suggests that re-treatment with rituximab should be considered when ADAMTS13 activity decreases and inhibitors reappear into the circulation, to avoid a new relapse.

Involvement of renal tubular Toll-like receptor 9 in the development of tubulointerstitial injury in systemic lupus

OBJECTIVE

Toll-like receptor 9 (TLR-9), a receptor for CpG DNA, has been implicated in the activation of immune cells in lupus. We undertook this study to determine whether the expression of TLR-9 in resident renal cells in lupus nephritis is related to the development of tubulointerstitial injury.

METHODS

TLR-9 was analyzed in selectively retrieved renal tissue from (NZB x NZW)F1 mice at different stages of disease by laser capture microdissection combined with real-time quantitative reverse transcriptase-polymerase chain reaction, and in renal biopsy specimens from lupus nephritis patients by immunohistochemistry. We investigated for the molecular component responsible for TLR-9 activation by cultured proximal tubular cells in serum from patients with lupus.

RESULTS

Renal tissue from NZB x NZW mice displayed robust TLR-9 expression localized to proximal tubular cells. TLR-9 levels correlated with proteinuria and tubulointerstitial injury to the extent that a cyclin-dependent kinase inhibitor, while reducing proteinuria and renal structural damage, prevented tubular TLR-9 generation in lupus mice. Consistently, exaggerated TLR-9 staining was found in proximal tubular cells of lupus patients, which correlated with tubulointerstitial damage. DNA-containing immune complexes purified from sera of patients with lupus induced TLR-9 in cultured proximal tubular cells. This was prevented by CCGG-rich short oligonucleotides, specific antagonists of CpG DNA, indicating that the DNA component of immune complexes was required for TLR-9 stimulation.

CONCLUSION

These findings suggest that tubular TLR-9 activation has a pathogenetic role in tubulointerstitial inflammation and damage in experimental and human lupus nephritis, and they indicate a novel target for future therapies.

In-vitro and in-vivo consequences of mutations in the von Willebrand factor cleaving protease ADAMTS13 in thrombotic thrombocytopenic purpura

Thrombotic thrombocytopenic purpura (TTP) is a disease characterized by microvascular thrombosis, often associated with deficiency of the vonWillebrand factor (VWF) cleaving protease ADAMTS13. We investigated the spectrum of ADAMTS13 gene mutations in patients with TTP and congenital ADAMTS13 deficiency to establish the consequences on ADAMTS13 processing and activity. We describe five missense (V88M, G1239V, R1060W, R1123C and R1219W), 1 nonsense (W1016Stop) and 1 insertion (82_83insT) mutations. In two patients no mutation was identified despite undetectable protease activity. Expression in HEK293 mammalian cells (V88M, G1239V, R1123C and R1219W) documented that three missense mutants were not secreted, whereas theV88M was secreted at low levels and with reduced activity. We also provide evidence that impaired secretion of ADAMTS13 mutants observed in vitro translates into severely reduced ADAMTS13 antigen levels in patients in vivo. To evaluate whether the small amounts of mutant protease present in the circulation of patients had VWF cleaving activity, WT and mutant rADAMTS13 were stably expressed in Drosophila S2 cells under the influence of the Drosophila BiP protein signal sequence, which allows protein secretion. Drosophila expression system showed a 40-60% protease activity in the mutants. Several single nucleotide polymorphisms (SNPs) within exons and intron boundaries were found in patients, suggesting that the interplay of SNPs could at least in part account for ADAMTS13 functional abnormalities in patients without mutations. In conclusion, defective secretion and impaired activity of the mutants concur to determine an almost complete deficiency of ADAMTS13 activity in patients with a homozygous or two heterozygous ADAMTS13 mutations.

Thrombotic Thrombocytopenic Purpura-Then and Now

Thrombotic thrombocytopenic purpura (TTP) is a potentially life-threatening disease characterized by microangiopathic hemolytic anemia, thrombocytopenia, and formation of microthrombi in several organs. The disease may manifest once in a lifetime or may relapse after complete recovery of the initial episode; in these recurrent cases, death or neurological sequelae are common final outcomes. Accumulation of unusually large (UL) von Willebrand factor (VWF) multimers was described in the plasma of patients with TTP. Such ULVWF multimers are synthesized in endothelial cells and megakaryocytes and are secreted into the blood upon stimulation. However, in healthy individuals ULVWF multimers do not normally circulate because they are rapidly reduced into smaller multimers soon after their secretion due to cleavage by a plasma metalloprotease, ADAMTS13. Deficiency of ADAMTS13 has been reported consistently in patients with TTP. Such defect may be constitutive, due to homozygous or double heterozygous mutations in the corresponding gene, or acquired, due to the presence of circulating inhibitory antibodies. It follows that in TTP patients, the absent or severely depressed plasma ADAMTS13 activity limits the cleavage of ULVWF multimers, which remain anchored to the endothelial cells in long strings. Particularly under conditions of high shear stress, the multimers may promote the adhesion of circulating platelets, initiating thrombus formation. The clinical implications of these findings to the diagnosis and treatment of TTP are discussed.

Rituximab prevents recurrence of thrombotic thrombocytopenic purpura: a case report

Thrombotic thrombocytopenic purpura (TTP) is a rare disorder of small vessels that is associated with deficiency of the von Willebrand factor–cleaving protease, ADAMTS13. The presence of anti-ADAMTS13 autoantibodies is considered a factor predisposing to relapses. Despite close monitoring and intensive plasma treatment, in these patients acute episodes are still associated with substantial morbidity and mortality rates, and the optimal therapeutic option should be prevention of relapses. This study was conducted in a patient with recurrent TTP due to high titers of ADAMTS13 inhibitors, who used to have 2 relapses of TTP a year. The study compared the standard treatment plasma exchange with rituximab. Results documented that plasma exchange had only a small transient effect on ADAMTS13 activity and inhibitors; on the contrary, prophylaxis with rituximab was associated with disappearance of anti-ADAMTS13 antibodies, a progressive recovery of protease activity, and it allowed the patient to maintain a disease-free state during a more than 2-year follow-up.

Activation of porcine endothelium in response to xenogeneic serum causes thrombosis independently of platelet activation

BACKGROUND

Endothelial cell activation and microvascular thrombosis are hallmarks of hyperacute xenograft rejection. However, the molecular determinants of platelet-endothelial interaction and thrombus formation are poorly understood. This study investigated whether: (i) xenogeneic human serum (HS), as a source of xenoreactive antibodies and complement, activates porcine aortic endothelial cells (PAEC) to promote thrombus formation under high shear stress; (ii) the endothelial adhesive proteins vitronectin receptor and P-selectin are involved in the von Willebrand factor (VWF)-platelet interaction during the thrombotic process under flow; (iii) reactive oxygen species (ROS) are activated by complement and served as intracellular signals for adhesive protein up-regulation.

METHODS

The PAEC were pre-exposed for 90 min in static conditions to medium plus 10, 20, and 50% HS or 20% porcine serum (PS), as control, then cells were perfused at 50 dynes/cm2 in a parallel plate flow chamber with human blood and area occupied by thrombi was measured. The role of complement in HS-induced thrombus formation was assessed by incubating PAEC with 20% HS in the presence of soluble complement receptor type 1 (sCR1) before blood perfusion. The effect of platelet activation was assessed using human blood treated or not with ADP and then flowed over PAEC pre-exposed to 20% HS or 20% PS as control. To identify the endothelial adhesive proteins involved in thrombus formation PAEC treated with 20% HS were then incubated with anti-vitronectin receptor antibody, anti-P-selectin antibody or P-selectin glycoprotein ligand-1 (PSGL-1), the soluble ligand of P-selectin, before the adhesion assay. Confocal microscopy was used to detect changes in endothelial adhesive protein expression. VWF interaction with platelet receptors GPIb and alphaIIbbeta3 was assessed adding aurin tricarboxylic acid (ATA) and anti-alphaIIbbeta3 antibody to blood before perfusion. The ROS involvement in xenogeneic serum-induced thrombus formation was determined studying the intracellular production of hydrogen peroxide (H2O2). The effect of antioxidants and metal chelators on HS-induced thrombus formation was evaluated treating PAEC with pyrrolidine dithiocarbamate (PDTC) or 1,3-dimethyl-2-thiourea (DMTU) before and during incubation with 20% HS followed by blood perfusion. The effect of antioxidants and sCR1 on ROS generation was investigated treating PAEC with PDTC or DMTU before and during incubation with 20% HS. Intracellular ROS generation was measured by fluorescence spectroscopy using the probe dihydrorhodamine 123 (DHR-123).

RESULTS

Human serum but not PS caused thrombus formation on PAEC under high shear stress. Blockade of complement activation by sCR1 prevented xenogeneic serum-induced thrombus formation. Activated platelets did not promote thrombus formation on resting endothelium, and did not further increase platelet deposition on xenogeneic serum-treated PAEC. Vitronectin receptor and P-selectin were up-regulated on the endothelial surface by HS. Their functional blockade by specific antibodies prevented platelet deposition and thrombus formation. H2O2 production significantly increased when PAEC were exposed to the xenogeneic condition. Antioxidants and sCR1 completely prevented thrombus formation by reducing excessive ROS production and the expression of vitronectin receptor and P-selectin.

CONCLUSIONS

Xenogeneic complement induces endothelial cell activation and thrombosis which is independent of platelet activation. Complement deposition elicits a rapid generation of ROS that lead to overexpression of endothelial adhesive molecules instrumental for platelet deposition.

In response to protein load podocytes reorganize cytoskeleton and modulate endothelin-1 gene: implication for permselective dysfunction of chronic nephropathies

Effacement of podocyte foot processes occurs in many proteinuric nephropathies and is accompanied by rearrangement of the actin cytoskeleton. Here, we studied whether protein overload affects intracellular pathways, leading to cytoskeletal architecture changes and ultimately to podocyte dysfunction. Mouse podocytes bound and endocytosed both albumin and IgG via receptor-specific mechanisms. Protein overload caused redistribution of F-actin fibers instrumental to up-regulation of the prepro-endothelin (ET)-1 gene and production of the corresponding peptide. Increased DNA-binding activity for nuclear factor (NF)-kappaB and Ap-1 nuclear proteins was measured in nuclear extracts of podocytes exposed to excess proteins. Both Y27632, which inhibits Rho kinase-dependent stress fiber formation, and jasplakinolide, an F-actin stabilizer, decreased NF-kappaB and Ap-1 activity and reduced ET-1 expression. This suggested a role for the cytoskeleton, through activated Rho, in the regulation of the ET-1 peptide. Focal adhesion kinase (FAK), an integrin-associated nonreceptor tyrosine kinase, was phosphorylated by albumin treatment via Rho kinase-triggered actin reorganization. FAK activation led to NF-kappaB- and Ap-1-dependent ET-1 expression. These data suggest that reorganization of the actin cytoskeletal network in response to protein load is implicated in modulation of the ET-1 gene via Rho kinase-dependent FAK activation of NF-kappaB and Ap-1 in differentiated podocytes. Increased ET-1 generation might alter glomerular permselectivity and amplify the noxious effect of protein overload on dysfunctional podocytes.

ADAMTS13 autoantibodies in patients with thrombotic microangiopathies and other immunomediated diseases

Autoantibodies neutralizing human ADAMTS13 (a disintegrin-like and metalloproteinase with thrombospondin type 1 motif), the metalloprotease that physiologically cleaves von Willebrand factor, are a major cause of severe deficiency of the protease and of acquired thrombotic thrombocytopenic purpura (TTP). We evaluated prevalence of anti-ADAMTS13 antibodies in 59 patients with thrombotic microangiopathies (TMAs) and in 160 patients with immunologic or thrombocytopenic diseases different from TTP, using an enzyme-linked immunosorbent assay (ELISA). Immunoglobulin G (IgG) antibodies directed against ADAMTS13 were found in 97% of untreated patients with acute acquired TMA who had plasma levels of ADAMTS13 activity below 10%. The corresponding prevalence of IgM antibodies was 11%. In contrast, anti-ADAMTS13 antibodies of G or M isotypes were detected in 20% of patients with TMA with ADAMTS13 activity above 10%. The ELISA was more sensitive than the standard functional inhibitor assay for detecting antibodies against ADAMTS13. Patients with thrombocytopenia from various causes (n = 50), systemic lupus erythematosus (SLE; n = 40), and the antiphospholipid antibody syndrome (APS; n = 55) had prevalences of IgG antibodies of 8%, 13%, and 5% respectively, only slightly higher than the prevalence in 111 healthy donors (4%). A rather high prevalence of anti-ADAMTS13 IgM antibodies was found in patients with SLE and APS (18% each). The clinical significance of IgM antibodies in these groups is unclear. In conclusion, the ELISA method detected anti-ADAMTS13 IgG antibodies in a very large proportion of patients with acquired TMA associated with severe ADAMTS13 deficiency, and was more sensitive than the inhibitor assay.

Complement factor H mutation in familial thrombotic thrombocytopenic purpura with ADAMTS13 deficiency and renal involvement

Thrombotic thrombocytopenic purpura is a rare disorder of small vessels that is associated with deficiency of the von Willebrand factor-cleaving protease ADAMTS13, which favors platelet adhesion and aggregation in the microcirculation. The disease manifests mainly with central nervous system symptoms, but cases of renal insufficiency have been reported. Presented are findings of the genetic basis of phenotype heterogeneity in thrombotic thrombocytopenic purpura in two sisters within one family. The patients had ADAMTS13 deficiency as a result of two heterozygous mutations (causing V88M and G1239V changes). In addition, a heterozygous mutation (causing an S890I change) in factor H of complement was found in the patient who developed chronic renal failure but not in her sister, who presented with exclusive neurologic symptoms.

Platelet dysfunction in renal failure

Patients with end-stage renal disease suffer from complex hemostatic disorders. Uremic patients show a bleeding diathesis that is mainly due to abnormalities of primary hemostasis; in particular, platelet dysfunction and impaired platelet-vessel wall interaction. However, despite decreased platelet function, these patients have a high prevalence of cardiovascular and thrombotic complications. Platelet dysfunction in uremic patients is partially due to uremic toxins present in circulating blood. Dialysis improves platelet abnormalities and reduces, but does not eliminate, the risk of hemorrhage. Hemodialysis can even contribute to the bleeding through the continuous platelet activation induced by the interaction between blood and artificial surfaces. Thrombocytopenia, glomerular thrombosis, and thrombi in small arteries and glomerular capillaries are common pathological features in many renal diseases. Platelets are also involved directly in the pathogenesis of glomerular diseases through a variety of mechanisms, including release of active molecules, by enhancing immune complex deposition, and by altering glomerular permeability.

Measurement of von Willebrand factor cleaving protease (ADAMTS-13): results of an international collaborative study involving 11 methods testing the same set of coded plasmas

BACKGROUND

ADAMTS-13 is a von Willebrand factor (VFW)-cleaving protease. Its congenital or acquired deficiency is associated with thrombotic thrombocytopenic purpura (TTP) and more rarely with the hemolytic uremic syndrome. We report on a survey evaluating 11 methods for ADAMTS-13 measurement performed in different labs.

DESIGN

Two plasmas, one normal and one from a patient with familial TTP, were mixed at the co-ordinating center to prepare 6 plasmas with 0%, 10%, 20%, 40%, 80% and 100% ADAMTS-13 levels. Each plasma was aliquoted and assembled into sets of 60 (coded from 1 to 60), each containing 10 copies of the original 6 plasmas. Plasmas were frozen and shipped in dry ice to 10 labs with a common frozen reference plasma. Laboratories were asked to measure ADAMTS-13 with their methods. Results were sent to the coordinating center for statistical analysis.

RESULTS

Of the 10 methods performed under static conditions 9 were quantitative and one was semiquantitative. One method performed under flow conditions evaluated the extent of cleavage of endothelial cell-derived ultralarge VWF string-like structures and expressed results as deficient, normal, or borderline. Linearity (expected-vs-observed levels), assessed as the squared correlation coefficient, ranged from 0.98 to 0.39. Reproducibility, expressed as the coefficient of variation for repeated measurements, ranged from < 10% to 83%. The majority of methods were able to discriminate between different ADAMTS-13 levels. The majority were able to detect the plasma with 0% level and some of them to discriminate between 0% and 10%. Overall the best performance was observed for three methods measuring cleaved VWF by ristocetin cofactor, collagen binding, and immunoblotting of degraded multimers of VWF substrate, respectively. The poor interlaboratory agreement of results was hardly affected by the use of the common standard. The method performed under flow conditions identified the plasmas with 0%, 10%, 20% and 40% activity as deficient in 7, 5, 1 and 3 of the 10 replicate measurements. The plasmas with 80% and 100% were identified as normal in all of the 10 replicate measurements.

CONCLUSIONS

The survey shows varied performance, but supports an optimistic view about the reliability of current methods for ADAMTS-13.

von Willebrand factor cleaving protease (ADAMTS13) is deficient in recurrent and familial thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Whether measurement of ADAMTS13 activity may enable physicians to distinguish thrombotic thrombocytopenic purpura (TTP) from hemolytic uremic syndrome (HUS) is still a controversial issue. Our aim was to clarify whether patients with normal or deficient ADAMTS13 activity could be distinguished in terms of disease manifestations and multimeric patterns of plasma von Willebrand factor (VWF). ADAMTS13 activity, VWF antigen, and multimeric pattern were evaluated in patients with recurrent and familial TTP (n = 20) and HUS (n = 29). Results of the collagen-binding assay of ADAMTS13 activity were confirmed in selected samples by testing the capacity of plasma to cleave recombinant VWF A1-A2-A3. Most patients with TTP had complete or partial deficiency of ADAMTS13 activity during the acute phase, and in some the defect persisted at remission. However, complete ADAMTS13 deficiency was also found in 5 of 9 patients with HUS during the acute phase and in 5 patients during remission. HUS patients with ADAMTS13 deficiency could not be distinguished clinically from those with normal ADAMTS13. In a subgroup of patients with TTP or HUS, the ADAMTS13 defect was inherited, as documented by half-normal levels of ADAMTS13 in their asymptomatic parents, consistent with the heterozygous carrier state. In patients with TTP and HUS there was indirect evidence of increased VWF fragmentation, and this occurred also in patients with ADAMTS13 deficiency. In conclusion, deficient ADAMTS13 activity does not distinguish TTP from HUS, at least in the recurrent and familial forms, and it is not the only determinant of VWF abnormalities in these conditions.

Verotoxin-1-induced up-regulation of adhesive molecules renders microvascular endothelial cells thrombogenic at high shear stress

Verotoxin-1 (VT-1)-producing Escherichia coli is the causative agent of postdiarrheal hemolytic uremic syndrome (D+HUS) of children, which leads to renal and other organ microvascular thrombosis. Why thrombi form only on arterioles and capillaries is not known. This study investigated whether VT-1 directly affected endothelial antithrombogenic properties promoting platelet deposition and thrombus formation on human microvascular endothelial cell line (HMEC-1) under high shear stress. Human umbilical vein endothelial cells (HUVECs) were used for comparison as a large-vessel endothelium. HMEC-1 and HUVECs were pre-exposed for 24 hours to increasing concentrations of VT-1 (2-50 pM) and then perfused at 60 dynes/cm(2) with heparinized human blood prelabeled with mepacrine. Results showed that VT-1 significantly increased platelet adhesion and thrombus formation on HMEC-1 in comparison with unstimulated control cells. An increase in thrombus formation was also observed on HUVECs exposed to VT-1, but to a remarkably lower extent. The greater sensitivity of HMEC-1 to the toxin in comparison with HUVECs was at least in part due to a higher expression of VT-1 receptor (20-fold more) as documented by FACS analysis. The HMEC-1 line had a comparable susceptibility to the thrombogenic effect of VT-1 as primary human microvascular cells of the same dermal origin (HDMECs). The adhesive molecules involved in VT-induced thrombus formation were also studied. Blocking the binding of von Willebrand factor to platelet glycoprotein Ib by aurintricarboxylic acid (ATA) or inhibition of platelet alpha(IIb)beta(3)-integrin by chimeric 7E3 Fab resulted in a significant reduction of VT-1-induced thrombus formation, suggesting the involvement of von Willebrand factor-platelet interaction at high shear stress in this phenomenon. Functional blockade of endothelial beta(3)-integrin subunit, vitronectin receptor, P-selectin, and PECAM-1 with specific antibodies was associated with a significant decrease of the endothelial area covered by thrombi. Confocal microscopy studies revealed that VT-1 increased the expression of vitronectin receptor and P-selectin and redistributed PECAM-1 away from the cell-cell border of HMEC-1, as well as of HDMECs, thus indicating that the above endothelial adhesion molecules are directly involved and possibly determine the effect of VT-1 on enhancing platelet adhesion and thrombus formation in microvascular endothelium. These results might help to explain why thrombi in HUS localize in microvessels rather than in larger ones and provide insights on the molecular events involved in the process of microvascular thrombosis associated with D+HUS.

A novel interpretation of the role of von Willebrand factor in thrombotic microangiopathies based on platelet adhesion studies at high shear rate flow

Clinical manifestations of thrombotic microangiopathies (TMA) are secondary to platelet aggregation and thrombotic occlusion of the microvasculature of the affected organs. Abnormalities in von Willebrand factor (vWF) in these patients were considered instrumental in promoting the process leading to microvascular thrombosis. We evaluated the capacity of plasma in these patients to induce adhesion of normal platelets and thrombus formation under conditions of controlled fluid shear stress. We also studied vWF multimeric distribution to establish whether abnormalities of this glycoprotein correlate with platelet adhesion and thrombus formation. Plasma from patients in the acute phase and remission showed the same capacity to induce platelet adhesion and thrombus formation at a low level of shear rate (600 sec(-1)) as plasma from control subjects. At a high shear rate (1,500 sec(-1)), platelet adhesion and thrombus dimensions were significantly increased (P: < 0.05) by plasma from patients with TMA compared with controls. The capacity to enhance thrombus formation at high shear stress was present during the acute phase and disease remission and did not correlate with the presence of unusually large vWF multimers. Increased thrombus formation with patient plasma is completely normalized by blocking the interaction of vWF with the platelet receptors, glycoprotein (GP)Ib and GPIIb-IIIa, suggesting that the phenomenon is completely mediated by vWF. Our results suggest the possibility of an intrinsically altered vWF molecule in these patients that is probably more effective than normal vWF in mediating platelet adhesion and thrombus formation.

Increased fragmentation of von Willebrand factor, due to abnormal cleavage of the subunit, parallels disease activity in recurrent hemolytic uremic syndrome and thrombotic thrombocytopenic purpura and discloses predisposition in families. The Italian Registry of Familial and Recurrent HUS/TTP

We investigated here the changes in von Willebrand factor (vWF) multimers in recurrent, sporadic and familial forms of hemolytic uremic syndrome (HUS)/thrombotic thrombocytopenic purpura (TTP) to see whether they are actually proteolyzed in vivo in these patients. Molecular determinants of fragments in vWF were also characterized to identify possible sites of cleavage of the subunit. Unusually large vWF multimers were found in blood of 8 of 10 patients with recurrent HUS/TTP, both in the acute phase and in remission, but never in familial and sporadic cases. Instead, all of the groups showed evidence of enhanced fragmentation of vWF multimers during the acute phase. Increased fragmentation was also shown by decrease in native 225-kD vWF subunit. In recurrent and sporadic HUS/TTP, enhanced fragmentation normalized at remission, but the abnormality persisted in familial HUS/TTP patients. The latter findings suggest that patients with familial HUS/TTP may have a congenital abnormality in vWF processing. Analysis with specific monoclonal antibodies showed the presence of the normal vWF fragments with apparent molecular mass of 189, 176, and 140 kD in all patients; however, in 6 recurrent and in 5 familial cases, novel fragments that differed in size from normal ones were found. The size of these abnormal fragments differed from one patient to another and none of them was ever found in normal plasma. These results documented, for the first time in HUS/TTP, an abnormal cleavage of the vWF subunit that might account for the increased fragmentation observed in these patients.

Unrecognized pattern of von Willebrand factor abnormalities in hemolytic uremic syndrome and thrombotic thrombocytopenic purpura

Heterogeneous abnormalities in multimeric structure and fragmentation of endothelial-derived von Willebrand factor (vWF) have been reported in hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP). This study was conducted to establish whether different patterns of vWF abnormalities were associated with different clinical syndromes. Plasmatic levels of vWF antigen (vWF:Ag), vWF release from endothelial cells (EC) exposed to patient sera, and vWF multimeric pattern were studied during episodes and again in remission in three groups of patients with severe forms of HUS and TTP paradigmatic of the most common clinical patterns of disease presentation: (1) plasma-responsive; (2) plasma-resistant; and (3) frequently relapsing. Plasma vWF:Ag and serum-induced vWF release from EC were increased in the acute phase of either plasma-responsive and plasma-resistant HUS and TTP, but normalized at remission only in plasma-responsive cases. Both indices were persistently normal in the relapsing forms. Enhanced vWF fragmentation as defined by disappearance of high molecular weight and increase in low molecular weight forms was a consistent finding of the acute phases, and always normalized in remission in all three groups. Unusually large vWF multimers were found exclusively in plasma of relapsing forms of HUS and TTP both during and between relapses. Enhanced levels of vWF:Ag and serum capability to induce vWF release in vitro are markers of disease activity and may reflect systemic endothelial injury and consequent activation. Their presence discriminates acute single-episode cases from relapsing forms and, when failing to normalize with plasma therapy, predicts plasma resistance. Enhanced low molecular weight multimers that closely paralleled disease activity suggest a permissive role of fragmented vWF in the formation of microvascular thrombi. Finally, finding of unusually large multimers exclusively in relapsing forms of HUS and TTP even between relapses, when no other clinical signs of disease activity could be detected, suggests that they cannot be the only factor in microvascular thrombosis.

Neutrophil activation and hemostatic changes in healthy donors receiving granulocyte colony-stimulating factor

Granulocyte colony-stimulating factor (G-CSF) enhances neutrophil functions in vitro and in vivo. It is known that neutrophil-derived products can alter the hemostatic balance. To understand whether polymorphonuclear leukocyte (PMN) activation, measured as PMN degranulation and phenotypical change, may be associated to hemostatic alterations in vivo, we have studied the effect of recombinant human G-CSF (rHuG-CSF) administration on leukocyte parameters and hemostatic variables in healthy donors of hematopoietic progenitor cells (HPCs). Twenty-six consecutive healthy donors receiving 10 micrograms/kg/d rHuG-CSF subcutaneously for 5 to 7 days to mobilize HPCs for allogeneic transplants were included in the study. All of them responded to rHuG-CSF with a significant white blood cell count increase. Blood samples were drawn before therapy on days 2 and 5 and 1 week after stopping rHuG-CSF treatment. The following parameters were evaluated: (1) PMN activation parameters, ie, surface CD11b/CD18 antigen expression, plasma elastase antigen levels and cellular elastase activity; (2) plasma markers of endothelium activation, ie, thrombomodulin (TM) and von Willebrand factor (vWF) antigens; (3) plasma markers of blood coagulation activation, ie, F1+2, TAT complex, D-dimer; and (4) mononuclear cell (MNC) procoagulant activity (PCA) expression. The results show that, after starting rHuG-CSF, an in vivo PMN activation occurred, as demonstrated by the significant increment of surface CD11b/CD18 and plasma elastase antigen levels. Moreover, PMN cellular elastase activity, which was significantly increased at 1 day of treatment, returned to baseline at day 5 to 6, in correspondence with the elastase antigen peak in the circulation. This change was accompanied by a parallel significant increase in plasma levels of the two endothelial and the three coagulation markers. The PCA generated in vitro by unstimulated MNC isolated from rHuG-CSF-treated subjects was not different from that of control cells from untreated subjects. However, endotoxin-stimulated MNC isolated from on-treatment individuals produced significantly more PCA compared with both baseline and control samples. All of the parameters were decreased or normal 1 week after stopping treatment. These data show that rHuG-CSF induces PMN activation and transiently affects some hemostatic variables in healthy HPC donor subjects. The clinical significance of these findings remains to be established.

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.

Fluid shear stress modulates von Willebrand factor release from human vascular endothelium

Fluid shear stress generated by blood flow on arterial wall may play a role in the process of atherosclerosis, not only affecting the mass transport phenomena that take place in blood, but also by modulation of synthesis and secretion of humoral factors released by vascular endothelium that mediate platelet-vessel wall interactions. The present study was designed to investigate whether shear stress, induced by laminar flow, modulates von Willebrand factor (vWF) release from cultured human umbilical vein endothelial cells (HUVEC) and whether this physical stimulation can affect vWF synthesis. Monolayers of HUVEC were exposed to laminar flow of varying magnitude (from 2 to 12 dynes/cm2) using a cone-and-plate device. The release of vWF in cell supernatant and in extracellular matrix by cells exposed to flow or maintained in static conditions was evaluated by enzyme-linked immunosorbent assay. HUVEC exposed to laminar flow released higher amounts of vWF into the cell supernatant within few hours of exposure and vWF secretion was dependent on shear stress magnitude. vWF released in extracellular matrix was also higher in cell monolayers exposed to shear than in static controls. vWF mRNA expression in HUVEC was not affected by exposure of cells to laminar flow, indicating that shear-induced vWF release reflected enhanced secretion without de novo protein synthesis. Immunofluorescence studies showed that the release of vWF is due to exocytosis from Weibel-Palade bodies, the storage organelles of vWF. These data indicate a novel mechanism by which local hemodynamic shear forces modulate endothelial cell function and may play a role in development of arterial thrombotic events.

Bilateral nephrectomy stopped disease progression in plasma-resistant hemolytic uremic syndrome with neurological signs and coma

Four women were admitted over three years because of anemia and renal failure. They had evidence of hemolytic uremic syndrome (HUS) with severe vascular involvement and glomerular collapse. Despite intensive plasma exchange, all patients developed neurologic signs (with seizures and coma in 2) and papilledema. Three developed refractory hypertension and three required dialysis. All patients had abnormal von Willebrand factor (vWF) fragmentation as reflected by decreased high molecular weight and increased low molecular weight vWF multimers in the circulation. Assuming that the disease was sustained by shear stress-induced abnormal vWF fragmentation in damaged renal microvasculature, bilateral nephrectomy was done. Surgery was followed within two weeks by complete hematologic and clinical remission consistently associated with the restoring of vWF fragmentation pathway to normal. We speculate that in HUS resistant to plasma exchange or infusion, removing the kidneys eliminates a major site of vWF fragmentation, which would limit platelet activation and protect patients from the further spreading of microvascular lesions.

alpha 1-Antitrypsin therapy in a case of thrombotic thrombocytopenic purpura

In thrombotic thrombocytopenic purpura (TTP), activated leucocytes release elastase which mobilises unusually large von Willebrand factor (vWF) multimers from the endothelium. We investigated the effect of an inhibitor of leucocyte elastase (alpha 1-antitrypsin) on circulating vWF and platelet count in a patient with chronic relapsing TTP. alpha 1-antitrypsin consistently normalised vWF multimeric composition but failed to increase platelet count and induce remission. Plasma cryosupernatant, which never normalised vWF, always induced laboratory and clinical remission within 96 hours. In chronic relapsing TTP, unusually large vWF multimers are not the main cause of intravascular platelet aggregation.