Emerging roles of adjunct therapies in acquired thrombotic thrombocytopenia purpura

• Acquired thrombotic thrombocytopenia purpura (aTTP) is caused by autoantibody‐mediated severe deficiency of the von Willebrand factor (vWF) cleaving protease ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 repeats, member 13), with subsequent accumulation of ultra‐large vWF‐multimers that spontaneously form platelet‐VWF complexes and microthrombi within the microcirculation.
• Therapeutic plasma exchange (TPE), by removing autoantibodies and excess ultra‐large vWF multimers and replenishing ADAMTS13 activity, remains the urgent primary initial treatment. Although heterogeneity in treatment exists, most centers add upfront immunosuppression with steroids, and many also add upfront rituximab. Refractoriness, exacerbation and relapse are commonly treated with adjunct rituximab.
• Despite adjunct steroids and rituximab, TTP refractoriness, exacerbation, relapse, morbidity, and mortality remain problematic. Newer adjunct therapies include suppression of ADAMTS13 autoantibody production via plasma cell depletion, inhibition of vWF‐platelet interaction, and replenishment of ADAMTS13 function with recombinant ADAMTS13 protein.

     

External validation of the PLASMIC score: a clinical prediction tool for thrombotic thrombocytopenic purpura diagnosis and treatment

Essentials

• Severe ADAMTS‐13 deficiency is key to thrombotic thrombocytopenic purpura (TTP) diagnosis.
• PLASMIC score predicts ADAMTS‐13 deficiency in suspected TTP with high discrimination.
• PLASMIC score is more generalizable with fewer missing data than alternative clinical scores.
• PLASMIC score identifies a subgroup of patients lacking significant response to plasma exchange.

Summary

Background

The PLASMIC score was recently published to distinguish patients with severe ADAMTS‐13 deficiency from those without for early identification of thrombotic thrombocytopenia purpura (TTP).

Objective

We performed an independent external validation of the PLASMIC score for clinical prediction of severe ADAMTS‐13 deficiency.

Patients/Methods

We studied an independent cohort of 112 consecutive hospitalized patients with suspected thrombotic microangiopathy and appropriate ADAMTS‐13 testing (including 21 patients with TTP diagnosis).

Results

The PLASMIC score model predicted severe ADAMTS‐13 deficiency with a c statistic of 0.94 (0.88–0.98). When dichotomized at high (score 6–7) vs. low‐intermediate risk (score 0–5), the model predicted severe ADAMTS‐13 deficiency with positive predictive value of 72%, negative predictive value of 98%, sensitivity of 90% and specificity of 92%. In the low‐intermediate risk group (score 0–5) there was no significant improvement in overall survival associated with plasma exchange.

Conclusions

The PLASMIC score model had excellent applicability, discrimination and calibration for predicting severe ADAMTS‐13 deficiency. The clinical algorithm allowed identification of a subgroup of patients who lacked a significant response to empiric treatment.     

Platelet rescue by macrophage depletion in obese ADAMTS‐13‐deficient mice at risk of thrombotic thrombocytopenic purpura

Essentials

• Obesity is a potential risk factor for development of thrombotic thrombocytopenic purpura (TTP).
• Obese ADAMTS‐13‐deficient mice were triggered with von Willebrand factor (VWF).
• Depletion of hepatic and splenic macrophages protects against thrombocytopenia in this model.
• VWF enhances phagocytosis of platelets by macrophages, dose‐dependently.

Summary

Background

Thrombotic thrombocytopenic purpura (TTP) is caused by the absence of ADAMTS‐13 activity. Thrombocytopenia is presumably related to the formation of microthrombi rich in von Willebrand factor (VWF) and platelets. Obesity may be a risk factor for TTP; it is associated with abundance of macrophages that may phagocytose platelets.

Objectives

To evaluate the role of obesity and ADAMTS‐13 deficiency in TTP, and to establish whether macrophages contribute to thrombocytopenia.

Methods

Lean or obese ADAMTS‐13‐deficient (Adamts‐13^?/?) and wild‐type (WT) mice were injected with 250 U kg^?1 of recombinant human VWF (rVWF), and TTP characteristics were evaluated 24 h later. In separate experiments, macrophages were depleted in the liver and spleen of lean and obese WT or Adamts‐13^?/? mice by injection of clodronate‐liposomes, 48 h before injection of rVWF.

Results

Obese Adamts‐13^?/? mice had a lower platelet count than their lean counterparts, suggesting that they might be more susceptible to TTP development. Lean Adamts‐13^?/? mice triggered with a threshold dose of rVWF did not develop TTP, whereas typical TTP symptoms developed in obese Adamts‐13^?/? mice, including severe thrombocytopenia and higher lactate dehydrogenase (LDH) levels. Removal of hepatic and splenic macrophages by clodronate injection in obese Adamts‐13^?/? mice before treatment with rVWF preserved the platelet counts measured 24 h after the trigger. In vitro experiments with cultured macrophages confirmed a VWF dose‐dependent increase of platelet phagocytosis.

Conclusions

Obese Adamts‐13^?/? mice are more susceptible to the induction of TTP‐related thrombocytopenia than lean mice. Phagocytosis of platelets by macrophages contributes to thrombocytopenia after rVWF injection in this model.

     

Pathophysiology of thrombotic thrombocytopenic purpura and hemolytic uremic syndrome

Summary

Thrombotic microangiopathies are rare disorders characterized by the concomitant occurrence of severe thrombocytopenia, microangiopathic hemolytic anemia, and a variable degree of ischemic end‐organ damage. The latter particularly affects the brain, the heart, and the kidneys. The primary forms, thrombotic thrombocytopenic purpura (TTP) and hemolytic uremic syndrome (HUS), although their clinical presentations often overlap, have distinctive pathophysiologies. TTP is the consequence of a severe ADAMTS‐13 deficiency, either immune‐mediated as a result of circulating autoantibodies, or caused by mutations in ADAMTS‐13. HUS develops following an infection with Shiga‐toxin producing bacteria, or as the result of excessive activation of the alternative pathway of the complement system because of mutations in genes encoding complement system proteins.

     

Inactivation of ADAMTS13 by plasmin as a potential cause of thrombotic thrombocytopenic purpura

Summary. Background: ADAMTS13 deficiency causes accumulation of unusually large von Willebrand factor molecules, which cross‐link platelets in the circulation or on the endothelial surface. This process of intravascular agglutination leads to the microangiopathy thrombotic thrombocytopenic purpura (TTP). Most TTP patients have acquired anti‐ADAMTS13 autoantibodies that inhibit enzyme function and/or clear it from the circulation. However, the reason for ADAMTS13 deficiency is not always easily identified in a subset of patients. Objectives: To determine the origin of ADAMTS13 deficiency in a case of acquired TTP. Methods: Western blotting of ADAMTS13 in plasmas from acute and remission phases was used. Results: The ADAMTS13 deficiency was not caused by mutations or (detectable) autoantibodies; however, an abnormal ADAMTS13 truncated fragment (100 kDa) was found in acute‐phase but not remission‐phase plasma. This fragment resulted from enzymatic proteolysis, as recombinant ADAMTS13 was also cleaved when in the presence of acute‐phase but not remission‐phase plasma. Inhibitor screening showed that ADAMTS13 was cleaved by a serine protease that could be dose‐dependently inhibited by addition of exogenous α₂‐antiplasmin. Examination of the endogenous α₂‐antiplasmin antigen and activity confirmed deficiency of α₂‐antiplasmin function in acute‐phase but not remission‐phase plasma. To investigate the possibility of ADAMTS13 cleavage by plasmin in plasma, urokinase‐type plasminogen activator was added to an (unrelated) congenital α₂‐antiplasmin‐deficient plasma sample to activate plasminogen. This experiment confirmed cleavage of endogenous ADAMTS13 similar to that observed in our TTP patient. Conclusion: We report the first acquired TTP patient with cleaved ADAMTS13 and show that plasmin is involved.     

Is it quinine TTP/HUS or quinine TMA? ADAMTS13 levels and implications for therapy

Thrombocytopenia with or without microangiopathy following quinine is often referred to as quinine “hypersensitivity.” When schistocytes are present it is frequently termed “quinine‐associated TTP/HUS.” A severe deficiency of the vWF‐cleaving protease, ADAMTS13, is associated with idiopathic TTP. A previous study of patients with “quinine‐associated TTP/HUS” found that ADAMTS13 activities were not abnormal in 12/12 patients. A retrospective review of TTP patients with quinine‐associated thrombotic microangiopathy (TMA) for whom ADAMTS13 was measured before plasma exchange was performed. Six patients were identified. All were females (age range: 43 to 73, mean = 61.7 years) and had taken quinine for leg cramps. Four of the six experienced renal failure requiring dialysis. Five of the patients had D‐Dimers levels measured, all were elevated. In four patients the levels were ≥18 times the upper limit of normal. ADAMTS13 was normal in four patients and mildly decreased in two patients. We conclude that while thrombocytopenia and schistocytosis can be seen in quinine‐associated TTP/HUS, the pathophysiology seems to be distinct from that seen in most cases of idiopathic TTP (i.e., severely decreased ADAMTS13 with an inhibitor). We recommend that a TMA in association with quinine be consistently referred to as quinine‐associated thrombotic microangiopathy (quinine‐TMA) to better distinguish this entity from idiopathic TTP. The use of plasma exchange in quinine‐TMA is called into question. J. Clin. Apheresis, 2009. © 2009 Wiley‐Liss, Inc.     

Congenital and acquired ADAMTS13 deficiency: Two mechanisms,one patient

Thrombotic thrombocytopenic purpura (TTP) is a life‐threatening microangiopathy with a heterogeneous and largely unpredictable course. It is caused by ADAMTS13 deficiency, that can be either congenital or due to anti‐ADAMTS13 autoantibodies development. ADAMTS13 deficiency is necessary but not always sufficient to cause acute clinical manifestations and trigger factors may be needed. We report the case of a woman diagnosed with congenital TTP in her adulthood, presenting with anti‐ADAMTS13 autoantibodies in acute phase during ticlopidine consumption. Noteworthy, the two ADAMTS13 mutations identified in this patient are novel: one is a splice‐site mutation located in intron 11 (c.1308+2_5delTAGG) and the other is a point missense mutation in exon 29 (c.4184T>C leading to p.Leu1395Pro substitution). Since congenital TTP is an extremely rare disease and drug‐induced TTP is an uncommon side effect of treatment with ticlopidine, the simultaneous occurrence of both mechanisms of disease in one patient is exceptional. This case represents TTP as a multifactorial disease, with ADAMTS13 genetic abnormality and environmental exposures acting together in determining individual clinical phenotype. J. Clin. Apheresis 30:252–256, 2015. © 2014 Wiley Periodicals, Inc.     

Rituximab pharmacokinetics during the management of acute idiopathic thrombotic thrombocytopenic purpura

Summary. Background: Increasingly, patients with acute, idiopathic, antibody mediated thrombotic thrombocytopenic purpura (TTP) are being treated with rituximab to achieve a durable remission, however, there is the potential that it is removed by plasma exchange (PEX). Objectives: To look at the pharmacokinetics and pharmacodynamics of rituximab in patients with acute idiopathic TTP undergoing PEX. Patients and methods: Patients who received rituximab for acute idiopathic TTP (group 1, n = 30) and a control group (group 2, n = 3) of TTP patients in remission receiving rituximab electively as maintenance were included. Rituximab levels were measured before/after each infusion, before/after PEX and in follow‐up. ADAMTS‐13 activity, anti‐ADAMTS‐13 IgG and CD19% were measured to assess response. Results: The median number of PEX to remission after rituximab was 10 (range 4–25). In group 1 there was no significant incremental rise in the peak serum rituximab level until dose 4. Trough levels were lower in patients who had had PEX since their last rituximab infusion. In the control group, there was an incremental rise in the peak serum rituximab level and all patients had detectable trough levels. The median fall in rituximab per PEX was 65%. All patients achieved CD19 < 1%. In group 1, the median time to undetectable rituximab was 5 months (range 0–12 months) and to B cell return was 7 months (range 3–24 months). ADAMTS‐13 increased and anti‐ADAMTS‐13 fell after therapy. There were three deaths and two relapses in group 1. Relapse was not temporally related to B cell return.     

Relapsing or refractory idiopathic thrombotic thrombocytopenic purpura‐hemolytic uremic syndrome: the role of rituximab

Idiopathic thrombotic thrombocytopenic purpura–hemolytic uremic syndrome (TTP‐HUS) is a rare disease responsive to treatment with plasma exchange (PE) but with a high percentage of relapse or refractory patients. A severe deficiency of ADAMTS‐13 (<5% of normal activity), congenital or caused by an autoantibody, may be specific for TTP and it has been proposed that severe ADAMTS‐13 deficiency now defines TTP. B cells play a key role in both the development and the perpetuation of autoimmunity, suggesting that B‐cell depletion could be a valuable treatment approach for patients with idiopathic TTP‐HUS. This review of the literature focuses on the role of rituximab, a chimeric monoclonal antibody directed against CD20 antigen expressed by B lymphocytes, in patients with relapsing or refractory TTP‐HUS with or without ADAMTS‐13 deficiency, suggesting that rituximab may produce clinical remission in a significant proportion of patients. Rituximab therapy reduces plasma requirement and avoids complications related to salvage‐immunosuppressive therapy. In conclusion, rituximab provides an effective, well‐tolerated, and safe treatment option for patients with idiopathic TTP‐HUS, thus giving an alternative approach to the current treatment based on PE.     

Secondary thrombotic microangiopathy with severely reduced ADAMTS13 activity in a patient with Capnocytophaga canimorsus sepsis: a case report

BACKGROUND

The Gram‐negative bacillus Capnocytophaga canimorsus may cause a severe illness resembling thrombotic thrombocytopenic purpura (TTP). The pathogenesis and optimal therapy of this secondary thrombotic microangiopathy (TMA) remain uncertain.

CASE REPORT

A 63‐year‐old Caucasian man was admitted with suspicion for TTP, but blood cultures grew C. canimorsus. Initial investigations revealed severe thrombocytopenia, a disintegrin and metalloproteinase with a thrombospondin type 1 motif, member 13 (ADAMTS13) activity level of less than 1%, and strongly elevated D‐dimer and lactate dehydrogenase levels. He made a full recovery with antibiotics and plasma infusion for 3 days. Plasmapheresis was not performed. Retrospective determination of serial ADAMTS13 activity levels revealed that ADAMTS13 activity had already increased to 25% at the start of plasma infusion.

CONCLUSION

This case highlights that a C. canimorsus sepsis may cause a secondary TMA with a severe ADAMTS13 deficiency. It also illustrates that the adjunctive role of plasma exchange or plasma infusion is doubtful as ADAMTS13 activity levels increased with antibiotics alone.     

Role of von Willebrand factor and ADAMTS‐13 in early brain injury after experimental subarachnoid hemorrhage

Essentials

• von Willebrand Factor (VWF) and ADAMTS13 may affect early injury after subarachnoid hemorrhage (SAH).
• Early brain injury was assessed in VWF^?/?, ADAMTS13^?/? and recombinant (r) ADAMTS13 treated mice.
• VWF^?/? and rADAMTS13 treated mice had less brain injury than ADAMTS13^?/? and wild‐type mice.
• Early administration of rADAMTS13 may improve outcome after SAH by reducing early brain injury.

Summary

Background

Early brain injury is an important determinant of poor functional outcome and case fatality after aneurysmal subarachnoid hemorrhage (SAH), and is associated with early platelet aggregation. No treatment exists for early brain injury after SAH. We investigated whether von Willebrand factor (VWF) is involved in the pathogenesis of early brain injury, and whether ultra‐early treatment with recombinant ADAMTS‐13 (rADAMTS‐13) reduces early brain injury after experimental SAH.

Methods

Experimental SAH in mice was induced by prechiasmatic injection of non‐anticoagulated blood from a littermate. The following experimental SAH groups were investigated: C57BL/6J control (n = 21), VWF^?/? (n = 25), ADAMTS‐13^?/? (n = 23), and C57BL/6J treated with rADAMTS‐13 (n = 26). Mice were killed at 2 h after SAH. Primary outcome measures were microglial activation (IBA‐1 surface area) and neuronal injury (number of cleaved caspase‐3‐positive neurons).

Results

As compared with controls, microglial activation was decreased in VWF^?/? mice (mean difference of ? 20.0%, 95% confidence interval [CI] ? 4.0% to ? 38.6%), increased in ADAMTS‐13^?/? mice (mean difference of + 34.0%, 95% CI 16.2–51.7%), and decreased in rADAMTS‐13‐treated mice (mean difference of ? 22.1%, 95% CI ? 3.4% to ? 39.1%). As compared with controls (185 neurons, interquartile range [IQR] 133–353), neuronal injury in the cerebral cortex was decreased in VWF^?/? mice (63 neurons, IQR 25–78), not changed in ADAMTS‐13^?/? mice (53 neurons, IQR 26–221), and reduced in rADAMTS‐13‐treated mice (45 neurons, IQR 9–115).

Conclusions

Our findings suggest that VWF is involved in the pathogenesis of early brain injury, and support the further study of rADAMTS‐13 as a treatment option for early brain injury after SAH.

     

Successful management of three patients with autoimmune thrombotic thrombocytopenic purpura with paradigm-changing therapy: Caplacizumab,steroids, plasma exchange,rituximab, and intravenous immunoglobulins (CASPERI)

Autoimmune thrombotic thrombocytopenic purpura (aTTP) is a severe disease caused by the production of autoantibodies against von Willebrand factor (vWF)-cleaving ADAMTS13 (a disintegrin and metalloproteinase with thrombospondin-1 motifs; 13th member of the family). In 2018, caplacizumab was approved for the treatment of patients with acute aTTP in conjunction with plasma exchange (PE) and immunosuppressive therapy. Immunosuppressive standard of care includes mainly steroids whereas rituximab is usually reserved for refractory cases. We report three patients with a first acute episode of aTTP who were successfully treated with a paradigm-changing scheme including standard of care (caplacizumab, steroids and PE) plus upfront therapy with rituximab and intravenous immunoglobulins (CASPERI). Rituximab was added 1–4 days after diagnosis, when ADAMTS13 autoantibodies were detected and intravenous immunoglobulins were administered after performing PE using albumin as replacement solution. Successful outcome was observed in all three patients: platelet recovery (>150 × 10⁹/L) was observed after 3, 4, and 5 days from diagnosis; ADAMTS13 activity >5% and ADAMTS13 autoantibodies were negative after 14, 15, and 21 days from diagnosis. In conclusion, caplacizumab, steroids, PE (using fresh frozen plasma or albumin as replacement solution and adding intravenous immunoglobulins) plus upfront rituximab therapy was a safe and efficient combination to induce remission in case of acute aTTP.     

Amplified endogenous plasmin activity resolves acute thrombotic thrombocytopenic purpura in mice

Essentials

• Plasmin is able to proteolyse von Willebrand factor.
• It was unclear if plasmin influences acute thrombotic thrombocytopenic purpura (TTP).
• Plasmin levels are increased during acute TTP though suppressed via plasmin(ogen) inhibitors.
• Allowing amplified endogenous plasmin activity in mice results in resolution of TTP signs.

     

Humoral immune response to ADAMTS13 in acquired thrombotic thrombocytopenic purpura

Summary. The apparently spontaneous development of autoantibodies to ADAMTS13 in previously healthy individuals is a major cause of thrombotic thrombocytopenic purpura (TTP). Epitope mapping studies have shown that in most patients antibodies directed towards the spacer domain of ADAMTS13 are present. A single antigenic surface comprising Arg⁶⁶⁰, Tyr⁶⁶¹ and Tyr⁶⁶⁵ that contributes to the productive binding of ADAMTS13 to unfolded von Willebrand factor is targeted by anti‐spacer domain antibodies. Antibodies directed to the carboxyl‐terminal CUB1–2 and TSP2–8 domains have also been observed in the plasma of patients with acquired TTP. As yet it has not been established whether this class of antibodies modulates ADAMTS13 activity. Inspection of the primary sequence of human monoclonal anti‐ADAMTS13 antibodies suggests that the variable heavy chain germline gene segment VH1–69 is frequently incorporated. We suggest a model in which ‘shape complementarity’ between the spacer domain and residues encoded by the VH1–69 gene segment explain the preferential use of this variable heavy chain gene segment. Finally, a model is presented for the development of anti‐ADAMTS13 antibodies in previously healthy individuals that incorporates the recent identification of HLA DRB1*11 as a risk factor for acquired TTP.     

A phenotype–genotype correlation of ADAMTS13 mutations in congenital thrombotic thrombocytopenic purpura patients treated in the United Kingdom

Summary. Background: ADAMTS13 mutations play a role in thrombotic thrombocytopenic purpura (TTP) pathogenesis. Objectives: To establish a phenotype–genotype correlation in a cohort of congenital TTP patients. Patients/Methods: Clinical history and ADAMTS13 activity, antigen and anti‐ADAMTS13 antibody assays were used to diagnose congenital TTP, and DNA sequencing and in vitro expression were performed to identify the functional effects of the ADAMTS13 mutations responsible. Results: Seventeen (11 novel) ADAMTS13 mutations were identified in 17 congenital TTP patients. All had severely reduced ADAMTS13 activity and antigen levels at presentation. Six patients with pregnancy‐associated TTP and six patients with childhood TTP were homozygous or compound heterozygous for ADAMTS13 mutations located in the metalloprotease (MP), cysteine‐rich, spacer and/or distal thrombospondin type 1 domains. The adults had TTP precipitated by pregnancy, and had overall higher antigen levels (median, 30 ng mL^?1; range, < 10–57 ng mL^?1) than the children (median, 14 ng mL^?1; range, < 10–40 ng mL^?1). Presentation in the neonatal period was associated with more intensive treatment requirements. The two neonates with the most severe phenotype had mutations in the first thrombospondin type 1 motif of ADAMTS13 (p.R398C, p.R409W, and p.Q436H). Using transfected HEK293T cells, we have shown that p.R398C and p.R409W block ADAMTS13 secretion, whereas p.Q436H allows secretion at reduced levels. Conclusions: This study confirms the heterogeneity of ADAMTS13 defects and an association between ADAMTS13 genotypes and TTP phenotype.     

ADAMTS‐13 activity and autoantibodies classes and subclasses as prognostic predictors in acquired thrombotic thrombocytopenic purpura

Summary. Background: Thrombotic thrombocytopenic purpura (TTP) is a rare life‐threatening disease. Of surviving patients, 45% develops an exacerbation or a late recurrence. Severe ADAMTS‐13 deficiency, both during the acute episode and remission, is a well‐established predictor of recurrence. The predictive value of anti‐ADAMTS‐13 antibodies, their inhibitory activity and Ig class subtype for disease recurrence is still to be established. Objectives: To analyze ADAMTS‐13‐related biomarkers (ADAMTS‐13 and anti‐ADAMTS‐13 immunoglobulins, classes and subclasses) and their potential relationship with prognosis. Patients/Methods: In 115 patients with TTP, we assessed the association between levels of these biomarkers and the severity of acute episodes; we analysed also the hazard ratio (HR) and 95% confidence interval (CI) of recurrence in association with biomarkers levels retrieved at the previous acute episode or during remission, using Cox regression models. Results: During the acute phase, higher IgA, IgG1 and IgG3 titers showed the strongest association with acute episode severity. In the survival analyzes, the only biomarker significantly associated with a high hazard of recurrence after an acute episode was the presence of IgG. Conversly, low ADAMTS‐13 activity or antigen levels (< 10%), the presence of ADAMTS‐13 inhibitor or IgG during remission were all significantly associated with a higher hazard of recurrence. Conclusions: Both the Ig class and subclass are of predictive value for acute episode severity in patients with TTP. Although markers that could predict the risk of recurrence in the acute phase are limited, a thorough assessment of ADAMTS‐13‐related parameters during remission is warranted.     

Complement activation in thrombotic thrombocytopenic purpura

Summary. Background: Ultra‐large von Willebrand factor and deficiency of its cleaving protease are important factors in the events leading to thrombotic microangiopathy; however, the mechanisms involved are only partly understood. Whereas pathological activation of the alternative complement pathway is linked to atypical hemolytic uremic syndrome, the role of complement activation in thrombotic thrombocytopenic purpura (TTP) is unknown. The aim of this study was to investigate whether signs of complement activation are characteristic of TTP. Patients and methods: Twenty‐three patients with TTP (18 women, median age 38 years) and 17 healthy controls (13 women, median age 38 years) were included. Complement parameters (C3, Factors H, I, B and total alternative pathway activity) together with complement activation fragments (C3a) or complexes (C1rs‐INH, C3bBbP, sC5b9) were measured by ELISA or RID. ADAMTS13 activity and anti‐ADAMTS13 inhibitory antibodies were measured by the VWF‐FRET73 assay. Results: Increased levels of C3a, and SC5b9 were observed in TTP during acute episodes, as compared with healthy controls. Decreased complement C3 levels indicative of complement consumption occurred in 15% of acute TTP patients. Significant decrease of complement activation products C3a and SC5b9 was observed during plasma exchange (PEX). The sustained presence of anti‐ADAMTS13 inhibitory antibodies in complete remission was associated with increased complement activation. Conclusion: These data document in an observational study the presence of complement activation in TTP. Further investigation is needed to determine its potential pathogenetic significance.     

A humanized monoclonal antibody that inhibits platelet‐surface ERp72 reveals a role for ERp72 in thrombosis

Essentials

• ERp72 is a thiol isomerase enzyme.
• ERp72 levels increase at the platelet surface during platelet activation.
• We generated a humanized monoclonal antibody which blocks ERp72 enzyme activity (anti‐ERp72).
• Anti‐ERp72 inhibits platelet functional responses and thrombosis.

Summary

Background

Within the endoplasmic reticulum, thiol isomerase enzymes modulate the formation and rearrangement of disulfide bonds in newly folded proteins entering the secretory pathway to ensure correct protein folding. In addition to their intracellular importance, thiol isomerases have been recently identified to be present on the surface of a number of cell types where they are important for cell function. Several thiol isomerases are known to be present on the resting platelet surface, including PDI, ERp5 and ERp57, and levels are increased following platelet activation. Inhibition of the catalytic activity of these enzymes results in diminished platelet function and thrombosis.

Aim

We previously determined that ERp72 is present at the resting platelet surface and levels increase upon platelet activation; however, its functional role on the cell surface was unclear. We aimed to investigate the role of ERp72 in platelet function and its role in thrombosis.

Methods

Using HuCAL technology, fully humanized Fc‐null anti‐ERp72 antibodies were generated. Eleven antibodies were screened for their ability to inhibit ERp72 activity and the most potent inhibitory antibody (anti‐ERp72) selected for further testing in platelet functional assays.

Results and conclusions

Anti‐ERp72 inhibited platelet aggregation, granule secretion, calcium mobilisation and integrin activation, revealing an important role for extracellular ERp72 in the regulation of platelet activation. Consistent with this, infusion of anti‐ERp72 into mice protected against thrombosis.     

Rituximab maintenance for relapsed refractory thrombotic thrombocytopenic purpura

BACKGROUND: Rituximab, an anti‐CD20 chimeric monoclonal antibody, has been used successfully to treat patients with relapsed or refractory thrombotic thrombocytopenic purpura (TTP); however, the optimal dose and frequency and the role of rituximab maintenance remain uncertain. STUDY DESIGN AND METHODS: We describe a 45‐year‐old woman with chronic relapsing immune thrombocytopenia who responded to rituximab retreatment administered in four doses over the course of 12 months. Previously, she had received four doses of rituximab and sustained a remission for 19 months. During her latest TTP relapse, multiple treatments were administered including rituximab retreatment. After the first dose (375 mg/m²), she developed serum sickness requiring further doses to be deferred. Three subsequent doses were administered at 4‐month intervals over the course of 12 months. ADAMTS13 activity was measured by von Willebrand factor (VWF) digestion. ADAMTS13 inhibition was measured by a modification of the VWF digestion assay and anti‐ADAMTS13 antibodies were measured by enzyme‐linked immunoassay (enzyme‐linked immunosorbent assay, American Diagnostica). RESULTS: Clinical and laboratory remission were achieved after one dose of rituximab, with normalization of ADAMTS13 activity and disappearance of ADAMTS13 inhibitor. Three subsequent doses of rituximab were given without incident and the patient remained in remission after 3.5 years of follow‐up (2.5 years since her last dose of rituximab). CONCLUSION: Maintenance dosing of rituximab should be considered in some patients with relapsing TTP.