Newer insights on the mechanism of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) type II is a complex clinical syndrome. It is an immune reaction to heparin in which the formation of antibodies targeted against the heparin-platelet factor 4 complex results in platelet activation. Platelet activation plays a central role in HIT; however, platelet activation does not occur as an isolated physiologic response. To elucidate further the mechanism of thrombogenesis in HIT, we undertook studies to determine the effect of heparin antibodies on endothelial cells, leukocytes, and the inflammatory state. We summarize our previous and new findings. For endothelial cells: Antiheparin antibodies bind to and directly activate microvascular endothelial cells, whereas binding to and activating macrovascular endothelial cells requires preactivation by platelets or tumor necrosis factor alpha (TNFalpha). Increased circulating levels of hemostatic activation factors as observed with thrombosis, particularly soluble P-selectin, plasminogen activator inhibitor type 1 (PAI-1), tissue factor, and thrombomodulin, were associated with endothelial cell activation and were also found in the blood circulation of patients with HIT. For the inflammatory state: Neutrophils and monocytes (but not lymphocytes) bind to and form complexes with platelets in the presence of HIT antibodies. Activated monocytes bind to endothelial cells and produce a procoagulant state. Patients with HIT have an increased level of cytokines in their blood circulation. For HIT antibodies: Only heparin fractions larger than 5 kd interacted with HIT antibodies, explaining why low-molecular-weight heparin (LMWH) usually does not generate antibodies. HIT antibodies are heterogeneous in structure, affinity, and specificity. These data suggest that, in addition to the platelet component, several other mechanisms are associated with the pathophysiology of HIT. These include an inflammatory state, endothelial cell remodeling, and the known procoagulant state. Differences between patients in the levels of the inflammatory markers may relate to various stages of the inflammatory/procoagulant state that exists in patients with HIT. The variations within the HIT antibodies may influence their ability to activate platelets, endothelial cells, and leukocytes, and thus contribute further to the variations in the pathogenicity of HIT.     

Fundamental concepts in the pathobiology of heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is among the most common causes of drug-related immune-mediated thrombocytopenia. It is a unique syndrome, in that despite the fact that thrombocyto-penia is the major laboratory manifestation of HIT, its major complication is a highly morbid (and commonly fatal) thrombotic diathesis, known as the HIT with thrombosis syndrome (HITTS). The pathogenesis of HIT and HITTS has been recently elucidated, and involves an immune response against epitopes within circulating heparin-platelet factor-4 (PF4) complexes. This leads to cross-linking and activation of platelets, increasing the risk for thromboses. Furthermore, significant immunological cross-reactivity occurs between endothelial-cell bound PF4 and the HIT antibody, which may lead to endothelial damage, activation, and hyperplasia. This complex process leads to a hypercoagulable state, which may lead to overt thromboses.     

Risk factors for thrombosis in patients with immune mediated heparin-induced thrombocytopenia

BACKGROUND: As reported by major clinical series in the literature, about 2% of patients receiving unfractionated heparin (UFH) develop immune-mediated (type II) heparin-induced thrombocytopenia (HIT) that may be complicated in 30-75% of cases by a paradoxical thrombotic syndrome (HITTS), either arterial or venous. HITTS carries relevant rates of mortality and morbidity, amongst which cerebral and/or myocardial infarction and limb amputations. It is unclear as yet why some patients suffer from isolated thrombocytopenia (HIT), whilst others have HITTS. The aim of the present study was to look for clinical and laboratory features related to the occurrence of HITTS. PATIENTS AND METHODS: We retrospectively analysed the clinical records of 56 patients with proven HIT, as diagnosed on clinical grounds and by in vitro demonstration of immunoglobulin (IgG)/IgM against the PF4/heparin complex. Thirty-four patients (61%) had HITTS (19 venous thrombosis, seven arterial thrombosis, five arterial and venous thrombosis, two skin necrosis, one diffuse intravascular coagulation), whereas 22 had uncomplicated HIT. Amongst HITTS patients, two had limb amputation, five had recurrent thrombosis and seven died. Amongst HIT patients three died from causes unrelated to HIT. RESULTS: No significant difference in sex, age, previous exposure to heparin, UFH route of administration or dose, duration of therapy, time of onset of thrombocytopenia and platelet count recovery, nor antiheparin/PF4 antibodies subtype (IgG or IgM) was detected when comparing HIT and HITTS. In contrast, in the HITTS group a higher prevalence of orthopaedic surgery (15 of 34 vs. 2/22; P=0.01), a significantly lower platelet count nadir (43 +/- 32 vs. 75 +/- 63 x 109/L; P=0.01) and a significantly higher titre of antiheparin/PF4 antibodies, expressed as optical density of enzyme-linked immunosorbent assay (ELISA); (1989 +/- 1024 vs. 1277 +/- 858; P=0.009), were observed in comparison with the HIT group. Amongst HITTS patients, the prevalence of venous thrombosis was significantly higher in orthopaedic patients and in those being treated for venous thromboembolism (18/24 vs. 1/9 patients, chi2 8.4, P=0.004), whilst arterial thrombosis (ART) occurred more often in heparin treatment for arterial disease (3/4 vs. 4/29 patients, chi2 4.6, P=0.03). CONCLUSIONS: Orthopaedic surgery, the severity of thrombocytopenia and high antiheparin/PF4 antibodies titre are adverse prognostic or concurrent factors in the development of HITTS.     

Heparin-induced thrombocytopenia and thrombosis syndrome: in vivo cross-reactivity with danaparoid and successful treatment with r-Hirudin

Heparin-induced thrombocytopenia and thrombosis syndrome (HITTS) is an immune-mediated drug reaction that occurs 5-14 d after initiation of heparin therapy and is a potentially life-threatening thrombotic complication. The antibody-heparin-PF4 complexes cause platelet activation and generation of platelet microparticles. The need for anticoagulant treatment in asymptomatic thrombocytopenia is uncertain. However, treatment is warranted in HITTS, as illustrated in the case reported here. Danaparoid, r-Hirudin and argatroban are effective drugs. Danaparoid has a 10-50% in vitro cross-reactivity rate with the HIT antibodies, but has been proven to be clinically efficacious even in these cases. Here, we report a case of in vivo cross-reactivity with danaparoid, the patient showed an excellent recovery with r-Hirudin.     

Characterization of a murine monoclonal antibody that mimics heparin-induced thrombocytopenia antibodies

Antibodies to PF4/heparin can be demonstrated in almost all patients with heparin-induced thrombocytopenia/thrombosis (HIT/HITT) and in some persons exposed to heparin who do not have clinical manifestations. The role of anti-PF4/heparin antibodies in the pathogenesis of HIT/HITT has been difficult to establish because the antibodies found in serum are generally polyclonal and polyspecific. To circumvent this problem, we developed a murine monoclonal antibody (mAb) to human (h) PF4/heparin complexes. A monoclonal IgG(2bkappa )antibody (designated KKO) was identified that bound specifically to hPF4/heparin complexes. Maximal binding of KKO to hPF4/heparin complexes occurred at similar molar ratios of PF4:heparin observed for HIT/HITT antibodies. KKO also bound to hPF4 in association with other glycosaminoglycans. Platelet activation by KKO required heparin and was abrogated by blockade of FcgammaRIIA. In the presence of PF4, KKO bound to endothelial cells, but not to CHO cells lacking heparan sulfate proteoglycans. Variants of PF4 complexed to heparin were recognized equally well by KKO and HIT/HITT sera. KKO competes for binding with a subset of HIT/HITT antibodies that are relatively spared by mutations in the 3rd domain of PF4. The nucleotide and predicted amino acid sequences of KKO and RTO, a murine anti-hPF4 mAb that does not require heparin for binding, revealed no obvious relationship in either the heavy- or the light-chain immunoglobulin variable regions. These studies suggest that KKO recapitulates the antigenic and functional specificity of a subset of HIT/HITT antibodies and may, therefore, provide insight into the pathogenesis of thrombocytopenia and thrombosis in affected persons. (Blood. 2000;95:1533-1540)     

Antibodies from patients with heparin-induced thrombocytopenia stimulate monocytic cells to express tissue factor and secrete interleukin-8

Thrombosis is a life-threatening complication that occurs in a subset of patients with heparin-induced thrombocytopenia (HITT). The pathogenic mechanisms underlying the variable occurrence of thrombosis in HITT is poorly understood. It was hypothesized that monocyte activation leading to tissue factor expression may play a role in promoting a thrombogenic state in HITT. This study demonstrates that a human platelet factor 4 (PF4)/heparin-specific murine monoclonal antibody (KKO) binds to peripheral blood-derived human monocytes in a PF4-dependent manner. KKO and antibodies from patients with HITT induce monocytes to synthesize and secrete interleukin-8 and induce cell-surface procoagulant activity, which is abrogated following treatment with antihuman tissue factor antibody. The findings suggest a novel mechanism by which PF4/heparin antibodies may promote a hypercoagulable state in patients with HITT. (Blood. 2001;98:1252-1254)     

肝素诱导的血小板减少症研究进展

肝素诱导的血小板减少症是一种特异性自身免疫性疾病,是由于抗体对肝素-血小板因子4复合物的识别,损伤了血小板以及内皮细胞而导致,以血小板减少及血栓形成为主要表现,通过4T评分以及实验室检查可有助于诊断,治疗上需要立即停用肝素及其相关制剂并开始替代抗凝治疗。     

The laboratory diagnosis and clinical management of patients with heparin-induced thrombocytopenia: an update

Heparin-induced thrombocytopenia (HIT) is a serious adverse effect of heparin exposure that can progress to severe thrombosis, amputation, or death. HIT is an immune response in which antibodies cause platelet activation, platelet aggregation, the generation of procoagulant platelet microparticles, and activation of leukocytes and endothelial cells. Early diagnosis based on a comprehensive interpretation of clinical and laboratory information is important to improve clinical outcomes. However, limitations of the laboratory assays and atypical clinical presentations can make the diagnosis difficult. Clinical management of patients with HIT is with a non-heparin anticoagulant such as a direct thrombin inhibitor or danaparoid followed by a vitamin K antagonist for long-term treatment. The new anti-factor Xa drugs (fondaparinux, rivaroxaban, apixaban) and other non-heparin antithrombotic agents can potentially be used for the treatment of HIT if clinically validated. Important drug-specific limitations and dosing and monitoring guidelines must be respected for patient safety. Issues still exist regarding the optimal clinical management of HIT.     

Argatroban anticoagulation in patients with heparin-induced thrombocytopenia

BACKGROUND: Heparin-induced thrombocytopenia (HIT) is an intensely prothrombotic syndrome managed by discontinuation of heparin therapy and substitution of an alternative inhibitor of thrombin. We describe our experience with argatroban, a direct thrombin inhibitor, in patients with HIT or HIT with thrombosis (HITTS). METHODS: In this multicenter, nonrandomized prospective study, 418 patients with HIT were administered intravenous argatroban, 2 micro g/kg per minute, adjusted to maintain the activated partial thromboplastin time at 1.5 to 3 times the baseline value for a mean of 5 to 7 days. Comparisons were made with a historical control cohort (n = 185). The prospectively defined, primary efficacy end point was a composite of all-cause death, all-cause amputation, or new thrombosis in 37 days. Other end points included the components of the composite, death due to thrombosis, increased platelet count, and bleeding. RESULTS: In the HIT arm, the composite end point was significantly reduced in argatroban-treated patients vs controls (28.0% vs 38.8%; P =.04). In the HITTS arm, the composite end point occurred in 41.5% of argatroban-treated patients vs 56.5% of controls (P =.07). By time-to-event analysis of the composite end point, argatroban therapy was significantly better than historical control therapy in HIT (P =.02) and HITTS (P =.008). Argatroban therapy also significantly reduced new thrombosis in HIT and HITTS and death due to thrombosis in HITTS. There were no significant between-group differences in all-cause death or amputation. Platelet counts recovered more rapidly in argatroban-treated patients than in controls. Bleeding rates were similar between groups. CONCLUSION: Argatroban therapy, compared with historical control, improves outcomes, particularly new thrombosis and death due to thrombosis, in patients with heparin-induced thrombocytopenia.     

Morphological analysis of microparticle generation in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) with thrombosis is a serious complication of heparin use. HIT sera can generate platelet-derived microparticles, which are produced in a heparin-dependent manner and are hypothesized to be important initial pathological participants because they promote vascular occlusion. To date, microparticles have been studied using flow cytometric techniques. However, it is uncertain whether the small-sized material seen in flow cytometric studies represents true platelet microparticles shed from activated platelets or whether they are platelets that have contracted after releasing their internal components. This report describes a morphological investigation of platelet-derived microparticles in HIT using, among other techniques, confocal, scanning electron, and transmission electron microscopy. Following incubation with HIT sera, the existence of small membrane-bound vesicles in the milieu of activated platelets was demonstrated. A population of microparticles, expressing platelet-specific glycoproteins, was separated from platelets by centrifugation over a sucrose layer. These microparticles had identical flow cytometric profiles, size heterogeneity, and GPIb(alpha) and GPIIb/IIIa staining intensity as the microparticle population in unfractionated samples. When microparticles were generated in situ and fixed onto grids, they were demonstrated to be distinct membrane-bound vesicles that originated near the platelet body and terminal ends of pseudopods on activated platelets. These microparticles appeared to be generated by localized swelling, budding, and release. Collectively, these morphological studies document the existence of true microparticles in platelets activated by HIT sera. The microparticles may play an important role in the pathogenesis of HIT.     

The management of heparin-induced thrombocytopenia

The Haemostasis and Thrombosis Task Force of the British Committee for Standards in Haematology has produced a concise practical guideline to highlight the key issues in the management of heparin-induced thrombocytopenia (HIT) for the practicing physician in the UK. The guideline is evidence-based and levels of evidence are included in the body of the article. All patients who are to receive heparin of any sort should have a platelet count on the day of starting treatment. For patients who have been exposed to heparin in the last 100 d, a baseline platelet count and a platelet count 24 h after starting heparin should be obtained. For all patients receiving unfractionated heparin (UFH), alternate day platelet counts should be performed from days 4 to 14. For surgical and medical patients receiving low-molecular-weight heparin (LMWH) platelet counts should be performed every 2-4 d from days 4 to 14. Obstetric patients receiving treatment doses of LMWH should have platelet counts performed every 2-4 d from days 4 to 14. Obstetric patients receiving prophylactic LMWH are at low risk and do not need routine platelet monitoring. If the platelet count falls by 50% or more, or falls below the laboratory normal range and/or the patient develops new thrombosis or skin allergy between days 4 and 14 of heparin administration HIT should be considered and a clinical assessment made. If the pretest probability of HIT is high, heparin should be stopped and an alternative anticoagulant started at full dosage unless there are significant contraindications while laboratory tests are performed. Platelet activation assays using washed platelets have a higher sensitivity than platelet aggregation assays but are technically demanding and their use should be restricted to laboratories experienced in the technique. Non-expert laboratories should use an antigen-based assay of high sensitivity. Only IgG class antibodies need to be measured. Useful information is gained by reporting the actual optical density, inhibition by high concentrations of heparin, and the cut-off value for a positive test rather than simply reporting the test as positive or negative. In making a diagnosis of HIT the clinician's estimate of the pretest probability of HIT together with the type of assay used and its quantitative result (enzyme-linked immunosorbent assay, ELISA, only) should be used to determine the overall probability of HIT. Clinical decisions should be made following consideration of the risks and benefits of treatment with an alternative anticoagulant. For patients with strongly suspected or confirmed HIT, heparin should be stopped and full-dose anticoagulation with an alternative, such as lepirudin or danaparoid, commenced (in the absence of a significant contraindication). Warfarin should not be used until the platelet count has recovered. When introduced in combination with warfarin, an alternative anticoagulant must be continued until the International Normalised Ratio (INR) is therapeutic for two consecutive days. Platelets should not be given for prophylaxis. Lepirudin, at doses adjusted to achieve an activated partial thromboplastin time (APTT) ratio of 1.5-2.5, reduces the risk of reaching the composite endpoint of limb amputation, death or new thrombosis in patients with HIT and HIT with thrombosis (HITT). The risk of major haemorrhage is directly related to the APTT ratio, lepirudin levels and serum creatinine levels. The patient's renal function needs to be taken into careful consideration before treatment with lepirudin is commenced. Severe anaphylaxis occurs rarely in recipients of lepirudin and is more common in previously exposed patients. Danaparoid in a high-dose regimen is equivalent to lepirudin in the treatment of HIT and HITT. Danaparoid at prophylactic doses is not recommended for the treatment of HIT or HITT. Patients with previous HIT who are antibody negative (usually so after >100 d) who require cardiac surgery should receive intraoperative UFH in preference to other anticoagulants that are less validated for this purpose. Pre- and postoperative anticoagulation should be with an anticoagulant other than UFH or LMWH. Patients with recent or active HIT should have the need for surgery reviewed and delayed until the patient is antibody negative if possible. They should then proceed as above. If deemed appropriate early surgery should be carried out with an alternative anticoagulant. We recommend discussion of these complex cases requiring surgery with an experienced centre. The diagnosis must be clearly recorded in the patient's medical record.     

Heparin-induced thrombocytopenia

BACKGROUND AND OBJECTIVE: There are two types of heparin-induced thrombocytopenia (HIT). HIT I is characterized by a transitory, slight and asymptomatic reduction in platelet count, occurring in the first 1-2 days of therapy, that resolves spontaneously; in contrast, HIT II, which has an immunologic origin, is characterized by a significant thrombocytopenia generally after the fifth day of therapy that usually resolves in 5-15 days only after therapy withdrawal. HIT II is the most frequent and dangerous side-effect of heparin therapy; in fact, in spite of thrombocytopenia, it can be complicated by venous and arterial thrombosis. Therefore, the recognition of HIT II may be difficult due to the underlying thrombotic symptoms for which heparin is administered. The aim of this article is to review the most recent advances in the field and to give critical guidelines for the clinical diagnosis and treatment of HIT II. STATE OF THE ART: The prevalence of HIT II, as confirmed by laboratory tests, seems to be about 2% in patients receiving unfractionated heparin (UH), while it is much lower in those receiving low molecular weight heparin (LMWH). The immunologic etiology of HIT II is largely accepted. Platelet factor 4 (PF4) displaced from endothelial heparan sulphate or directly from the platelets, binds to the heparin molecule to form an immunogenic complex. The anti-heparin/ PF4 IgG immunocomplexes activate platelets and provoke an immunologic endothelial lesion with thrombocytopenia and/or thrombosis. The IgG anti-heparin/PF4 immunocomplex activates platelets mainly through binding with the FcgRIIa (CD32) receptor. The onset of thrombocytopenia is independent of the dosage, schedule and route of administration of heparin. Orthopedic and cardiovascular surgery patients receiving post-surgical prophylaxis or treatment for deep venous thrombosis are at higher risk of HIT II. Besides thrombocytopenia, cutaneous allergic manifestations and skin necrosis may be present. Hemorrhagic events are not frequent, while the major clinical complications in 30% of patients are both arterial and venous thromboses which carry a 20% mortality. The diagnosis of HIT II should be formulated on the basis of clinical criteria and in vitro demonstration of heparin-dependent antibodies. Functional tests, such as platelet aggregation and (14)C-serotonin release assay and immunologic tests, such as the search for anti-PF4/heparin complex antibodies by an ELISA method are available. If HITT II is probable, heparin must be immediately suspended and an alternative anticoagulant therapy should be initiated before resolution of thrombocytopenia and the following treatment with a vitamin K antagonist. The general opinion is to administer low molecular weight heparin (in the absence of in vitro cross-reactivity with the antibodies), heparinoids such as Orgaran or direct thrombin inhibitors such as hirudin. PERSPECTIVES: Further studies are required to elucidate the pathogenesis of HIT II and especially to discover the clinical and immunologic factors that induce the occurrence of thrombotic complications. The best therapeutic strategy remains to be confirmed in larger clinical trials.     

Heparin-induced thrombocytopenia: a review

Immune heparin-induced thrombocytopenia (HIT) is a relevant adverse drug reaction consisting in a hypercoagulable state caused by an anticoagulant agent. The incidence is approximately 6.5% in patients receiving unfractionated heparin after orthopedic surgery, and is equal to or lower than 1% in other settings. HIT occurrence is a function of heparin type, duration of heparin administration, patient population, and gender. The pathogenesis is due to an antibody response to the complex heparin/platelet factor 4 in most cases, with secondary activation of platelets and coagulation, and finally increased thrombin generation. Thrombocytopenia, venous or arterial thrombosis, heparin-induced skin necrosis, adrenal hemorrhage, and transient amnesia can characterize the clinical course of HIT. Platelet monitoring in patients receiving heparin is indicated to early detect HIT. A thrombotic event can be the first manifestation of HIT. Laboratory demonstration of heparin-dependent platelet activation confirms the clinical diagnosis; antigenic or functional assays are available. Once HIT is highly likely or confirmed serologically, immediate heparin cessation is mandatory and an alternative therapeutic anticoagulant is needed due to the high risk (or the presence) of thrombotic events. The available nonheparin anticoagulants aim to reduce thrombin generation. Lepirudin, argatroban, and bivalirudin (direct thrombin inhibitors) and danaparoid and fondaparinux (factor Xa inhibitors) represent the current treatment options for HIT. Vitamin K antagonists can be used safely only after a stable platelet count has been obtained.     

PF4/heparin-antibody complex induces monocyte tissue factor expression and release of tissue factor positive microparticles by activation of FcγRI

Heparin-induced thrombocytopenia (HIT) is a potentially devastating form of drug-induced thrombocytopenia that occurs in patients receiving heparin for prevention or treatment of thrombosis. Patients with HIT develop autoantibodies to the platelet factor 4 (PF4)/heparin complex, which is termed the HIT Ab complex. Despite a decrease in the platelet count, the most feared complication of HIT is thrombosis. The mechanism of thrombosis in HIT remains poorly understood. We investigated the effects of the HIT Ab complex on tissue factor (TF) expression and release of TF-positive microparticles in peripheral blood mononuclear cells and monocytes. To model these effects ex vivo, we used a murine mAb specific for the PF4/heparin complex (KKO), as well as plasma from patients with HIT. We found that the HIT Ab complex induced TF expression in monocytes and the release of TF-positive microparticles. Further, we found that induction of TF is mediated via engagement of the FcγRI receptor and activation of the MEK1-ERK1/2 signaling pathway. Our data suggest that monocyte TF may contribute to the development of thrombosis in patients with HIT.     

Successful Coronary Interventions Performed with Argatroban Anticoagulation in Patients with Heparin-Induced Thrombocytopenia and Thrombosis Syndrome

Patients with heparin-induced thrombocytopenia and thrombosis syndrome (HITTS) frequently have coincident vascular disease. Patients with HITTS who require vascular procedures have often been excluded from vascular intervention because intravascular procedures require heparin. Re-exposure to heparin places these patients at great risk for reactivation of thrombosis related to HIT antibody. We present our initial experience with an alternative anticoagulant to heparin, argatroban in patients with HIT antibody who underwent 14 coronary interventions. All 14 coronary lesions were treated successfully and no patient suffered an HITTS-related or an argatroban-related complication.     

Heparin-induced thrombocytopenia/thrombosis in a transgenic mouse model requires human platelet factor 4 and platelet activation through FcgammaRIIA

Heparin-induced thrombocytopenia/thrombosis (HIT/HITT) is a severe, life-threatening complication that occurs in 1% to 3% of patients exposed to heparin. Interactions between heparin, human platelet factor 4 (hPF4), antibodies to the hPF4/heparin complex, and the platelet Fc receptor (FcR) for immunoglobulin G, FcgammaRIIA, are the proposed primary determinants of the disease on the basis of in vitro studies. The goal of this study was to create a mouse model that recapitulates the disease process in humans in order to understand the factors that predispose some patients to develop thrombocytopenia and thrombosis and to investigate new therapeutic approaches. Mice that express both human platelet FcgammaRIIA and hPF4 were generated. The FcgammaRIIA/hPF4 mice and controls, transgenic for either FcgammaRIIA or hPF4, were injected with KKO, a mouse monoclonal antibody specific for hPF4/heparin complexes, and then received heparin (20 U/d). Nadir platelet counts for KKO/heparin-treated FcgammaRIIA/hPF4 mice were 80% below baseline values, significantly different (P <.001) from similarly treated controls. FcgammaRIIA/hPF4 mice injected with KKO and 50 U/d heparin developed shock and showed fibrin-rich thrombi in multiple organs, including thrombosis in the pulmonary vasculature. This is the first mouse model of HIT to recapitulate the salient features of the human disease and demonstrates that FcgammaRIIA and hPF4 are both necessary and sufficient to replicate HIT/HITT in an animal model. This model should facilitate the identification of factors that modulate disease expression and the testing of novel therapeutic interventions.     

Identification, Diagnosis and Treatment of Heparin-induced Thrombocytopenia and Thrombosis: A Registry of Prolonged Heparin Use and Thrombocytopenia among Hospitalized Patients with and without Cardiovascular Disease

Background: Heparin-induced thrombocytopenia (HIT) is estimated to occur in 1–5% of all patients receiving heparin, and 25–50% of such cases develop heparin-induced thrombocytopenia with thrombosis (HITT) A conservative estimate based only on cardiovascular patients suggests that in the United States approximately 100,000 patients develop thrombocytopenia, and 25–50,000 develop HITT annually. Both HIT and HITT are associated with high morbidity and mortality and represent substantial worldwide public health concerns.Registry Design: The objective of the Complication After Thrombocytopenia Caused by Heparin (CATCH) Registry is to identify the incidence of HIT and/or HITT in patients treated with systemic heparin (unfractionated or low molecular weight heparin) in contemporary practice. Additional objectives include to: (1) provide a comprehensive database of patients with suspected HIT or HITT, (2) monitor and define clinical events, including thrombocytopenia, thrombosis, and mortality among patients treated with prolonged (> 96 hours) heparin, (3) describe the incidence and outcomes of HIT and HITT in patients who are treated with heparin and who develop thrombocytopenia in the Coronary Care Unit setting, and (4) document and characterize current diagnostic and therapeutic strategies of suspected HIT. The unblinded registry will record approximately 5,000 patients at 60–80 US hospitals with either prolonged systemic heparin administration or thrombocytopenia and those with suspected HIT or HITT. Enrollment began in the first quarter 2003 and was completed at the end of 2004.Implications: The registry will provide valuable insights to the incidence and consequences of HIT and HITT that will enable improvements in diagnosis and treatment.The CATCH Registry is supported by a grant from Berlex Laboratories, Inc.     

P-selectin in arterial thrombosis

P-selectin is a transmembrane protein present in the alpha granules of platelets and the Weibel-Palade bodies of endothelial cells. Following activation, it is rapidly translocated to the cell surface. P-selectin expression in platelets has been shown to be elevated in disorders associated with arterial thrombosis such as coronary artery disease, acute myocardial infarction, stroke, and peripheral artery disease. P-selectin mediates rolling of platelets and leukocytes on activated endothelial cells as well as interactions of platelets with leukocytes. Platelet P-selectin interacts with P-selectin glycoprotein ligand-1 (PSGL-1) on leukocytes to form platelet-leukocyte aggregates. Furthermore, this interaction of P-selectin with PSGL-1 induces the upregulation of tissue factor, several cytokines in leukocytes and the production of procoagulant microparticles, thereby contributing to a prothrombotic state. P-selectin is also involved in platelet-platelet interactions, i. e. platelet aggregation which is a major factor in arterial thrombosis. P-selectin interacts with platelet sulfatides, thereby stabilizing initial platelet aggregates formed by GPIIb/IIIa-fibrinogen bridges. Inhibtion of the P-selectin-sulfatide interaction leads to a reversal of platelet aggregation. Thus, P-selectin plays a significant role in platelet aggregation and platelet- leukocyte interactions, both important mechanisms in the development of arterial thrombosis.     

Procoagulant State of Sleep Apnea Depends on Systemic Inflammation and Endothelial Damage

IntroductionGrowing evidence shows a hypercoagulable state in obstructive sleep apnea (OSA) that could be a risk factor for thromboembolic disease.ObjectivesWe aimed to elucidate mechanisms involved in the procoagulant profile observed in patients with OSA and to investigate the potential utility of global tests in its characterization.MethodsThirty-eight patients with severe OSA without previous history of thrombosis and nineteen healthy age- and sex-matched controls were included.Kinetic of clot formation was determined using rotational thromboelastometry.Haemostatic capacity of plasma and microparticles was determined by Calibrated Automated Thrombinography.Platelet surface receptors, activation markers and formation of platelet/leukocytes aggregates were analyzed by flow cytometry.ResultsThromboelastometry showed a procoagulant state in patients with OSA that did not seem to be related to a basal activation of platelets but by the increased existence of platelet/leukocyte aggregates.Patients with OSA presented many signs of endothelial damage such as increased plasma levels of E-selectin and cfDNA and enhanced thrombin generation due to the presence of microparticles rich in tissue-factor, which is related to OSA severity.ConclusionsOSA induces an enhancement in the dynamics of clot formation which appears to be caused by at least two pathological mechanisms. First, a greater formation of platelet-leukocyte aggregates; secondly, endothelial damage which provokes a greater procoagulant potential due to the increase in tissue factor-rich microparticles. Moreover, this study has identified thromboelastometry and thrombin generation assay as useful tools to evaluate the prothrombotic state in these patients.     

Generation of platelet-derived microparticles and procoagulant activity by heparin-induced thrombocytopenia IgG/serum and other IgG platelet agonists: a comparison with standard platelet agonists

Heparin-induced thrombocytopenia (HIT) is a relatively common, immunoglobulin-mediated adverse drug reaction associated with in vivo thrombin generation and both venous and arterial thrombosis. Serum and purified IgG from patients with HIT induce normal platelets to generate procoagulant platelet-derived microparticles, but the magnitude of this response in comparison with other IgG and standard platelet agonists is unknown. We describe a comparison of IgG platelet agonists, including HIT-IgG/serum, heat-aggregated IgG, and platelet-activating murine monoclonal antibodies, with standard 'strong' and 'weak' platelet agonists, and have determined their relative ability to generate platelet procoagulant activity. Using washed normal platelets as targets, we observed that HIT sera as well as other IgG agonists produced similar or even greater numbers of microparticles and procoagulant activity than the standard strong platelet agonists (thrombin, collagen, and thrombin receptor agonist peptide). The only exception was the non-physiological platelet agonist, calcium ionophore, which consistently produced a platelet procoagulant response even greater than the IgG agonists. We conclude that the IgG class of platelet agonists (including pathogenic HIT antibodies) is an effective trigger of the platelet procoagulant response comparable at least to strong physiological platelet agonists. These results help to explain the association between HIT, in vivo thrombin generation, and thrombosis.