Hematology 2006 415

Thrombotic Thrombocytopenic Purpura:A Moving TargetJ. Evan Sadler

Almost 80 years after Eli Moschcowitz published thefirst description of the disease, most patients withidiopathic thrombotic thrombocytopenic purpura (TTP)were found to have acquired autoantibody inhibitors ofthe ADAMTS13 metalloprotease. Plasma ADAMTS13normally cleaves von Willebrand factor within nascentplatelet-rich thrombi, and ADAMTS13 deficiencyallows unchecked thrombus growth to causemicroangiopathic hemolysis, thrombocytopenia, andtissue infarction. At present, ADAMTS13 deficiencywith a high-titer inhibitor level appears to be associ-

ated with an increased risk of early death and subse-quent relapse. Thus, acquired ADAMTS13 deficiencyidentifies a specific mechanism of TTP and is apotential biomarker of disease activity or risk. Atpresent, two major clinical questions in the field maybe summarized as follows. First, by emphasizing TTPcaused by ADAMTS13 deficiency, are we in danger ofneglecting other causes that should be treated withplasma exchange? Second, should we treat asymp-tomatic patients who have severe ADAMTS13 defi-ciency to prevent future disease, and if so, how?

The last few years have set the stage for a new approach tothe management of thrombotic thrombocytopenic purpura(TTP). The criteria for idiopathic TTP have remained ap-proximately constant, and plasma exchange is still the stan-dard therapy. But the discovery of the ADAMTS13metalloprotease has revolutionized our understanding ofTTP and initiated a period of experimentation with regardto diagnosis and treatment. The management of TTP hasbeen reviewed recently,1 and I will not discuss it compre-hensively. Instead, I will try to integrate the new knowl-edge about ADAMTS13 into a model for the pathogenesisof TTP, relate it to our experience in caring for these pa-tients, and suggest where we may look for further advancesduring the next few years.

I. Plasma Exchange for Idiopathic TTPThe modern era in the management of TTP began with arandomized trial, published in 1991, which established thatplasma exchange is superior to plasma infusion for the treat-ment of TTP.2 During the decade before these results werereported, anecdotal experience indicated that plasmatherapy could cure patients who otherwise had an expectedsurvival of < 10%. By demonstrating that plasma exchangeincreased survival to > 80%, this key trial simultaneouslyestablished a standard of care and an enduring clinical defi-nition of idiopathic TTP.2

The entry criteria for this study included micro-angiopathic hemolytic anemia (Coombs negative), throm-bocytopenia (< 100,000/L), and no alternative (second-ary) explanation such as cancer, disseminated intravascu-lar coagulation (DIC), or eclampsia. There was no require-ment for neurological deficits, fever, or renal involvement.In fact, subjects with oliguria were excluded because theycould not tolerate plasma infusion.2

Today we still diagnose idiopathic TTP using essen-tially the same criteria (Table 1): unexplained micro-angiopathic hemolytic anemia and thrombocytopenia.However, the spectrum of associated symptoms has changed.In contrast to case series from the era before plasma ex-change, patients often do not have fever, significant renalimpairment, or obvious neurological involvement at diag-nosis.2 This shift toward earlier diagnosis follows naturallyfrom the desire to provide plasma exchange therapy to ev-ery patient who may benefit. However, there has been littlefurther improvement in prognosis. Approximately 20% ofpatients with idiopathic TTP still die during the first monthof acute illness, and at least 30% experience one or morerelapses within 2 years.3-5 Practically speaking, will we everbe able to predict who will respond to plasma exchange, orwho will relapse? If so, can we offer effective alternativetherapy?

II. ADAMTS13 Changed the LandscapeBefore 1998, our ability to make such predictions was ham-pered by a fundamental lack of knowledge about the patho-physiology of TTP. In that year, adults with idiopathic TTPwere reported to have acquired autoantibodies that inhibita von Willebrand factor (VWF) cleaving protease, which isnormally present in plasma.6,7 In 2001, the VWF cleavingprotease was purified and cloned by several groups, shownto be a new member of the a disintegrin and metallo-protease with thrombospondin type 1 repeats family, and

Departments of Medicine and Biochemistry and MolecularBiophysics, and Howard Hughes Medical Institute, WashingtonUniversity School of Medicine, St. Louis, MO

Correspondence: J. Evan Sadler, Howard Hughes MedicalInstitute, Washington University School of Medicine, 660 SouthEuclid Avenue, Box 8022, St. Louis, MO 63110; Phone 314-362-9029, Fax 314-454-3012; Email esadler@im.wustl.edu.

416 American Society of Hematology

named ADAMTS13.8-12 Thanks to these discoveries, we nowhave a plausible model for the pathogenesis of TTP (Fig-ure 1): autoantibodies inhibit the activity of ADAMTS13.The absence of ADAMTS13 allows VWF and platelets toaccumulate unchecked in microvascular thrombi, whichleads to platelet consumption, hemolysis, and microvascu-lar occlusion. Plasma exchange is efficacious because itremoves pathogenic autoantibodies and replenishes themissing ADAMTS13 protease, restoring the normal regula-tion of VWF-dependent platelet adhesion. As we will see,this is a reasonable model but it may not explain every-thing about idiopathic TTP.

III. Clinical Correlations of ADAMTS13 Deficiency

III.A. Specificity and sensitivity for idiopathic TTPSubsequent studies have begun to demonstrate the poten-tial and also the limitations of ADAMTS13 testing. Mod-est decreases in plasma ADAMTS13 activity occur in avariety of acute and chronic illness, but rarely are the val-ues < 25% of normal. ADAMTS13 is synthesized mainly inthe liver, and severe ADAMTS13 deficiency (< 5%) hasoccurred with hepatic failure from various causes. SevereADAMTS13 deficiency appears to be more frequent in sep-sis-induced DIC. In a retrospective study of 109 patients,ADAMTS13 levels were < 5% in 17 patients and < 20% in51 patients.13

Table 1. Classification and characteristics of thrombotic microangiopathy syndromes.

Category Clinical Features* Mechanism TreatmentIdiopathic TTP Coombs negative, without DIC or Autoimmune ADAMTS13 deficiency > 80% response to plasma

other conditions associated with in a majority of patients. exchange. May benefit fromsecondary TTP. Severe renal failure immunosuppression.is uncommon. Specific conditions**that may coexist with idiopathic TTPare discussed in the text.

Secondary TTP Associated conditions include Mechanisms are mostly unknown. With few exceptions, responsescancer, infection, hematopoietic ADAMTS13 deficiency is rare. to plasma exchange are unlikely.stem cell transplantation, solid organ Treatment and prognosis aretransplantation, chemotherapy, dictated by the specific associ-certain drugs. ated conditions.

Diarrhea- Acute renal failure, often oliguric, Endothelial damage by Shiga toxin No demonstrated efficacy ofassociated HUS preceded by bloody diarrhea. producing E. coli. ADAMTS13 plasma exchange.(D+HUS) deficiency is rare.Atypical HUS Acute renal failure, often oliguric, Complement regulatory protein No demonstrated efficacy of

without a prior diarrheal illness. defects in at least 50% of patients; plasma exchange, exceptADAMTS13 deficiency is rare. possibly for factor H deficiency.

*Clinical features listed are in addition to microangiopathic hemolytic anemia and thrombocytopenia.**Conditions associated with autoimmune ADAMTS13 deficiency and idiopathic TTP include certain autoimmune disorders, preg-nancy, and ticlopidine.

Figure 1. Pathogenesis of idiopathicthrombotic thrombocytopenic purpura(TTP) caused by ADAMTS13 deficiency.Multimeric von Willebrand factor (VWF)adheres to endothelial cells or to connectivetissue exposed in the vessel wall. Plateletsadhere to the VWF through plateletmembrane glycoprotein GPIb. In flowingblood, VWF in the platelet-rich thrombus isstretched and cleaved by themetalloprotease ADAMTS13, limitingthrombus growth. If ADAMTS13 is absent,VWF-dependent platelet accumulationcontinues, eventually causing microvascu-lar thrombosis and TTP.

Hematology 2006 417

In contrast, studies of patients with thromboticmicroangiopathy have confirmed the specificity of severeADAMTS13 deficiency (undetectable, or < 5%) for idio-pathic TTP, although the sensitivity remains controversial.Severe ADAMTS13 deficiency rarely occurs in second-ary TTP associated with cancer, hematopoietic stem cellor solid organ transplantation, preeclampsia, systemic in-fections, drug toxicity, or other predisposing conditions.Also, severe ADAMTS13 deficiency is almost unheard ofin diarrhea-associated hemolytic uremic syndrome causedby Shiga toxinproducing E. coli (D+HUS) or other throm-botic microangiopathy accompanied by oliguric renal fail-ure (atypical HUS). The Oklahoma TTP-HUS registryprovides representative data: none of 92 patients with thesevarieties of secondary TTP or D+HUS had severeADAMTS13 deficiency.4 Similarly, a major referral labora-tory in Switzerland reported that 3 of 188 subjects (1.6%)with secondary TTP and none of 130 subjects with HUShad severe ADAMTS13 deficiency.14

On the other hand, when patients with idiopathic TTPare stratified by plasma ADAMTS13 activity level, the in-cidence of severe ADAMTS13 deficiency (< 5% of normalpooled plasma) has varied from 33%4 to 100%,6,7,15 withintermediate values of 52% to 94% in other studies.3,14,16-18Some of this variation probably reflects differences in casedefinitions for idiopathic TTP or assay methodology. Inany case, some patients diagnosed with idiopathic TTP donot have severe ADAMTS13 deficiency, at least in vitro.Whether the short-term prognosis differs for idiopathic TTPwith or without ADAMTS13 deficiency is not yet clear.One study suggests that the initial response to plasma ex-change is similar for the two groups.4

As an aside, the recent progress in understanding fa-milial and idiopathic TTP has been matched by similaradvances regarding atypical HUS. In a study of 156 pa-tients, mutations in regulators of the alternative comple-ment pathway (factor H, factor I, or MCP) were identified in74 of them.19 Thus, the clinical distinction between TTP(mild renal insufficiency) and HUS (severe renal insuffi-ciency) often correlates with distinct pathophysiologicmechanisms. Because the features of idiopathic TTP andatypical HUS can overlap, laboratory testing forADAMTS13 and complement function could be important,because familial TTP caused by ADAMTS13 deficiencyresponds to simple plasma infusion, whereas atypical HUSresponds poorly if at all to intensive plasma exchange andhas a much worse long term prognosis.19

III.B. Exceptional causes of autoimmune TTPIn general, secondary TTP (Table 1) is not associated withsevere ADAMTS13 deficiency and rarely responds to plasmaexchange, but there are a few interesting exceptions.

Autoimmune diseases of various kinds have been de-scribed in association with severe ADAMTS13 deficiencyand thrombotic microangiopathy that is indistinguishablefrom idiopathic TTP.20-22 Conversely, patients with idio-

pathic TTP and severe ADAMTS13 deficiency (< 5%) of-ten have manifestations of systemic lupus erythematosusor other autoimmune diseases, such as antinuclear anti-bodies, polyarthritis, malar rash, extramembranous glom-erulonephritis, discoid lupus, or autoimmune thyroiditis.23Many autoimmune diseases can cause hemolytic anemia,thrombocytopenia and organ dysfunction by severalmechanisms, and ADAMTS13 testing may facilitate therecognition of idiopathic TTP in these complex cases.

Pregnancy-associated thrombotic microangiopathyhas many causes that can be difficult to diagnose correctly.The features of preeclampsia or HELLP syndrome may over-lap those of idiopathic TTP, but these patients do not haveADAMTS13 deficiency.24 However, pregnancy can triggerTTP in women who do have congenital or acquiredADAMTS13 deficiency.25 ADAMTS13 testing may be use-ful for discriminating among the varieties of thromboticmicroangiopathy during pregnancy and postpartum.

Ticlopidine causes thrombotic microangiopathy with afrequency of 1 case per 1600 to 5000 patients treated, usuallyafter 2 to 12 weeks of drug use, usually by inducing the for-mation of autoantibodies to ADAMTS13.26 The mechanismof autoantibody induction is unknown. Ticlopidine-associ-ated autoimmune TTP responds to plasma exchange, whichreduces the mortality from 60% without treatment to 14-20%with treatment.27 TTP is much less common with clopidogrel,a related thienopyridine antagonist of platelet ADP receptorsignaling,28 and in many cases has not been associated withautoantibodies to ADAMTS13.

III.C. Prognostic significanceIn three studies that included both idiopathic and second-ary TTP (excluding D+HUS), patients with severeADAMTS13 deficiency had a good response rate to plasmaexchange (89-100%) and low mortality (8-19%), whereaspatients without ADAMTS13 deficiency had a lower re-sponse rate (54-82%) and higher mortality (18-56%).3-5Unfortunately, knowledge of the ADAMTS13 level addedlittle because most of the differences in response rate andmortality were captured by the clinical distinction betweenidiopathic and secondary TTP.3,4

Focusing specifically on idiopathic TTP, however,ADAMTS13 assays may provide useful prognostic infor-mation. In the Oklahoma TTP-HUS registry, 6 of 14 pa-tients (43%) with severe ADAMTS13 deficiency later re-lapsed, compared to 2 of 25 patients (8%) without severeADAMTS13 deficiency.4 In studies of idiopathic TTP withsevere ADAMTS13 deficiency, patients with detectableinhibitors had a delayed response to plasma exchange18,29and an increased risk of relapse.3,17 Deaths occurred only inpatients with detectable inhibitors, for whom the death ratewas 17% to 25%.3,17,18 Although relatively few patients havebeen studied to date, the results suggest that severeADAMTS13 deficiency with a significant inhibitor titer atdiagnosis confers an increased risk of early death and re-lapsing disease.

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III.D. A biomarker of diseaseFor most patients, a complete response to plasma exchangeis accompanied by normalization of ADAMTS13 activityand disappearance of ADAMTS13 inhibitors, if present.3,17Interestingly, one-fourth to one-third of responders has per-sistent total ADAMTS13 deficiency, and many such pa-tients also have unchanged or increased inhibitor titers dur-ing remission.3,17 Why does their disease remit in the firstplace, and what is their risk of relapse?

The resolution of thrombotic microangiopathy with-out improvement in ADAMTS13 deficiency also occurs infamilial TTP caused by ADAMTS13 mutations. Some sub-jects with congenital ADAMTS13 deficiency require con-tinuous prophylactic treatment with plasma to preventthrombotic microangiopathy, whereas others have longdisease-free intervals that may last years without treatment,but develop acute thrombotic microangiopathy in associa-tion with infections, surgery, pregnancy, or other stress. Avery few adults with total ADAMTS13 deficiency havenever had thrombotic microangiopathy at all.30 Thus, in-flammatory stress exacerbates TTP in congenital ADAMTSdeficiency, and the stress of surgery31 or pregnancy25 doesthe same in acquired idiopathic TTP. In both familial andacquired idiopathic TTP, the resolution of stress may re-store a condition of compensated microvascular thrombo-sis that is insufficient to cause overt disease, thereby ac-counting for remissions with plasma therapy despite per-sistent ADAMTS13 deficiency, as well as relapses with laterepisodes of inflammatory stress.

Nearly all patients with relapsing idiopathic TTP haveADAMTS13 deficiency at the time of relapse,3,17 althoughit is difficult to predict when or whether a relapse will oc-cur. However, relapsing TTP is a catastrophic and some-times fatal illness that strikes at least 30% of patients withintwo years of surviving an acute episode of TTP. It is worthconsidering whether the endpoint of treatment should re-main the achievement of complete clinical response, orwhether the goal should include the eradication of autoan-tibody inhibitors and restoration of a normal ADAMTS13level.

IV. Rituximab for Idiopathic TTPThe discovery that idiopathic TTP usually is an autoim-mune disease has encouraged the routine use of immuno-suppressive agents. For example, the efficacy of cortico-steroids has not been demonstrated conclusively, but manyof us combine prednisone with plasma exchange, expect-ing that it may reduce autoantibody production. Even so,many patients do not respond satisfactorily, and during thepast few years immunosuppression with rituximab has be-come popular as salvage therapy for refractory or relapsingTTP. Rituximab is a humanized monoclonal antibodyagainst CD20, which is expressed on B cells, and it rapidlyclears B cells from the circulation. Most antibody-produc-ing plasma cells live only a few days, and destruction of Bcells can prevent the replenishment of these pathological

plasma cells. Several case reports and small series suggestthat rituximab induces complete responses in the majorityof patients with TTP refractory to plasma exchange, corti-costeroids, and other treatments such as vincristine or sple-nectomy. Responses to rituximab correlate with disappear-ance of ADAMTS13 inhibitors and a rise of the ADAMTS13level into the normal range.32,33

Success with refractory disease suggests that rituximabcould be useful for newly diagnosed patients who achieveremission but are at high risk of relapse.33 Assessing theefficacy of rituximab in this setting may be difficult be-cause the majority of patients do not experience relapses,and the utility of ADAMTS13 data for predicting the riskof relapse is still uncertain. These issues will be addressedin a proposed randomized trial of plasma exchange, with orwithout rituximab, as primary therapy for idiopathic TTP.34

V. Autoimmune ADAMTS13 Deficiency andThrombosis Without TTPIn principle, ADAMTS13 deficiency might cause devastat-ing thrombosis in a critical organ such as the brain or heart,without enough microvascular thrombosis to cause micro-angiopathic hemolysis and thrombocytopenia. In fact, 3patients have been described who presented with acutecerebrovascular events but normal platelet counts, normalor trivially elevated LDH, and no schistocytes. Becausethese patients had a prior history of TTP, ADAMTS13 ac-tivity was checked and found to be low, with a detect-able inhibitor, and they responded to plasma exchange orrituximab.35,36 Such cases raise the frightening possibilitythat ADAMTS13 deficiency may cause stroke or other throm-bosis in patients without any history of TTP. If this provesto occur at a significant frequency, then ADAMTS13 test-ing would be appropriate for patients with acute throm-botic events who have no obvious risk factors, especially ifaccompanied by thrombocytopenia.

VI. The Evolving Approach to ThromboticMicroangiopathyRecent discoveries have provided new insight into themechanism of idiopathic TTP but have had a relativelymodest impact on the clinical approach to diagnosing andtreating thrombotic microangiopathy. This may change asthe results of ongoing clinical research become available.

VI.A. Clinical strategy for todayEmpiric plasma exchange for all idiopathic TTP. For now,plasma exchange remains the primary treatment for idio-pathic TTP, reducing the acute mortality from > 90% to< 20%. Therefore, any patient who reasonably may haveidiopathic TTP should receive plasma exchange as soon aspossible, typically at 1.0 or 1.5 volumes of plasma per day.Prednisone can be included at 1 or 2 mg/kg per day, individed doses. The criteria for selecting these patients in-clude microangiopathic hemolytic anemia, thrombocytope-nia, and the absence of an obvious alternative explanation

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such as cancer, sepsis, disseminated intravascular coagula-tion, tissue transplantation, certain drugs, and recent bloodydiarrhea. Significant renal insufficiency at presentation issubstantially more common in HUS but is not a sufficientreason to withhold plasma exchange because it does occursometimes in idiopathic TTP that is caused by ADAMTS13deficiency. However, a rapidly rising creatinine, especiallywith oliguria, should prompt an aggressive search for sec-ondary causes of TTP, for infection by Shiga toxinpro-ducing organisms, and in some cases for complement regu-latory defects.

ADAMTS13 testing and difficult decisions. If rapidtesting is available, a finding of severe ADAMTS13 defi-ciency can help to justify the continuation of plasma ex-change when the clinical presentation is not typical of id-iopathic TTP, especially when thrombotic microangiopathyoccurs in association with preexisting autoimmune diseaseor during pregnancy. Also, patients with other potentialcauses of secondary TTP do rarely acquire antibodies toADAMTS13. Appropriate testing can identify this minor-ity with coincidental TTP caused by ADAMTS13 deficiencythat will, in fact, respond to plasma exchange.

ADAMTS13 as a biomarker in relapsing TTP. Relaps-ing idiopathic TTP almost always is caused by persistentor recurrent autoimmune ADAMTS13 deficiency, and im-munosuppression can prevent subsequent relapses by re-storing normal ADAMTS13 activity. Therefore, testing canidentify refractory or relapsing patients with severeADAMTS13 deficiency, often with detectable inhibitors,who may benefit from immunosuppression. ADAMTS ac-tivity and inhibitor titer also serve as biomarkers to moni-tor the response to treatment.

VI.B. Questions for tomorrowRoutine ADAMTS 13 assays. The possibility of usingADAMTS13 data to manage thrombotic microangiopathyobviously is premised on the widespread availability ofADAMTS13 assays that are rapid, robust, and feasible formost clinical laboratories. Several such assays are underdevelopment. As one example, a potentially suitablefluorogenic ADAMTS13 substrate has been described.37 As-says using this substrate can be adapted to measureADAMTS13 inhibitors, but more sensitive inhibitor assaysstill are needed. Some pathogenic antibodies appear to pro-mote the clearance of ADAMTS13 from blood without in-hibiting its activity,38 and assays for these non-neutraliz-ing antibodies and for ADAMTS13 antigen39 could be use-ful as well.

Primary versus secondary immunosuppression. Thesuccessful treatment of relapsing TTP with rituximab sug-gests that combining rituximab with plasma exchange couldimprove the long-term prognosis of newly diagnosed idio-pathic TTP, especially if high-risk patients could be identi-fied. Whether ADAMTS13 data will be useful for selectingpatients for immediate immunosuppressive treatment is un-known. Whether such treatment will actually reduce the

rate of future relapses also is unknown. A related issue iswhether asymptomatic patients with persistent ADAMTS13deficiency should be treated while in remission to preventfuture relapses. These questions will need to be answeredby clinical trials.34

Idiopathic TTP without ADAMTS13 deficiency. Theability to routinely measure ADAMTS13 level will enablecloser study of patients with idiopathic TTP despite hav-ing normal ADAMTS13 activity. Either their thromboticmicroangiopathy is caused by defective ADAMTS13 func-tion that cannot be detected with our current in vitro as-says, or it is caused by another mechanism that needs to becharacterized.

Thrombosis without idiopathic TTP. The clinical usesof ADAMTS13 assays may prove to extend beyond theevaluation of patients with idiopathic TTP. Patients withsevere autoimmune ADAMTS13 deficiency can presentwith acute neurological deficits, in the absence of overtthrombotic microangiopathy. This phenomenon could beinvestigated as a potential cause of stroke in persons wholack traditional risk factors and also have no history ofTTP. Finally, non-immune ADAMTS13 deficiency occursin some patients with liver failure or sepsis-induced DIC,and has been proposed to contribute to microvascularthrombosis and renal injury. If prospective studies confirma proposed relationship between ADAMTS13 deficiencyand tissue injury in these settings,13 then replacementtherapy to increase ADAMTS13 levels could be evaluated.

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