Pathogenicity of human anti-platelet factor 4 (PF4)/heparin in vivo: generation of mouse anti-PF4/heparin and induction of thrombocytopenia by heparin

Galactosaminoglycans: Medical Applications and Drawbacks

Galactosaminoglycans (GalAGs) are sulfated glycans composed of alternating N-acetylgalactosamine and uronic acid units. Uronic acid epimerization, sulfation patterns and fucosylation are modifications observed on these molecules. GalAGs have been extensively studied and exploited because of their multiple biomedical functions. Chondroitin sulfates (CSs), the main representative family of GalAGs, have been used in alternative therapy of joint pain/inflammation and osteoarthritis. The relatively novel fucosylated chondroitin sulfate (FCS), commonly found in sea cucumbers, has been screened in multiple systems in addition to its widely studied anticoagulant action. Biomedical properties of GalAGs are directly dependent on the sugar composition, presence or lack of fucose branches, as well as sulfation patterns. Although research interest in GalAGs has increased considerably over the three last decades, perhaps motivated by the parallel progress of glycomics, serious questions concerning the effectiveness and potential side effects of GalAGs have recently been raised. Doubts have centered particularly on the beneficial functions of CS-based therapeutic supplements and the potential harmful effects of FCS as similarly observed for oversulfated chondroitin sulfate, as a contaminant of heparin. Unexpected components were also detected in CS-based pharmaceutical preparations. This review therefore aims to offer a discussion on (1) the current and potential therapeutic applications of GalAGs, including those of unique features extracted from marine sources, and (2) the potential drawbacks of this class of molecules when applied to medicine.

Two patients with heparin-induced thrombocytopenia followed by idiopathic (immune) thrombocytopenic purpura: case report

Heparin-induced thrombocytopenia (HIT) and idiopathic thrombocytopenic purpura (ITP) are separate and well-recognized clinical syndromes, 10both having potential for disastrous outcomes. Sequential occurrence of these two diseases has not been reported in the literature. We report herein two patients who were initially diagnosed as having HIT but whose later clinical course was similar to ITP. Although dysfunction of immune system seems to play a role, the precise mechanisms for the development of these diseases are unclear. As both conditions have different natural histories and require different treatment modalities, it is important to recognize that these two diseases may be seen sequentially.

Anti-beta2-glycoprotein I antibodies recognizing platelet factor 4-heparin complex in antiphospholipid syndrome in patient substantiated with mouse model

The antiphospholipid syndrome is defined by the presence of antiphospholipid antibodies associated with arterial and/or venous thrombosis, and recurrent abortion accompanied often by thrombocytopenia. These antibodies are heterogeneous and react against phospholipid-binding proteins such as beta2-glycoprotein I (beta2GPI) and prothrombin. The recognition of anti-beta2-glycoprotein I (anti-beta2GPI) by platelet factor 4-heparin complex (PF4-Hc) has been previously evoked and partially confirmed by the present inhibition studies. Further, the anti-beta2-glycoprotein I antibodies were purified from a patient with primary antiphospholipid syndrome using Affi-gel-10-beta2GPI immunoaffinity chromatography. The purified anti-beta2GPI IgM as well as patient serum equally recognized PF4-Hc in ELISA mode. In order to substantiate this data and to better understand we studied an animal model using mouse active immunization with the purified human anti-beta2GPI. The mice showed a significant decrease in their platelet count. In addition the ELISA responses of the immunized mice sera were positive against both beta2GPI and PF4-Hc, substantiating the double recognition. Despite many previous reported animal model studies, this is the first time we have shown the specific recognition of anti-beta2GPI antibodies by PF4-Hc, the results in the induced mice correlating the data observed with some patients.

Insights from mouse models of heparin-induced thrombocytopenia and thrombosis

Heparin-induced thrombocytopenia/thrombosis (HIT/T) is the most frequent cause of drug-induced antibody-mediated thrombocytopenia, a common cause of life-and limb-threatening platelet activation and thrombosis. Although antibodies to heparin-platelet factor 4 (PF4) complexes are found in essentially all patients with HIT/T, it is unclear how antibody formation is initiated, why only a small subset of these antibodies cause disease, or the mechanism by which they initiate thrombosis. The recent development of a transgenic mouse model of HIT/T showed, for the first time in vivo, that heparin, PF4, antibodies to the heparin-PF4 complex, and Fc gamma RIIA are necessary and sufficient to recapitulate the severe thrombocytopenia and thrombosis seen in patients. This model can be expanded to systematically study individual factors important to HIT/T development in vivo, to determine their contribution to the spectrum of human disease. Several recent papers have examined the role of other contributors, such as monocyte tissue factor, endothelial cell activation, and leukocyte-platelet aggregates in the pathogenesis of the disease. Mouse models will provide a means to test new diagnostic and therapeutic approaches.

Heparin-induced thrombocytopenia with thrombotic sequelae: a review

Heparin-induced thrombocytopenia (HIT) occurs in 1-5% of patients treated with heparin. The pathogenesis involves the formation of antibodies to heparin-platelet factor 4 complexes, and the major clinical sequelae are thrombotic. Diagnosis is based on a combination of clinical and laboratory data. Treatment consists of stopping heparin, but, insofar as the risk of thrombosis remains high, treatment by alternative antithrombotic agents is indicated. Most clinical experience has been with danaparoid sodium and hirudin. The use of low-molecular-weight heparins (LMWH) in subsequent HIT episodes has been described, but is not recommended, especially with the introduction of new agents, such as oral thrombin inhibitors and pentasaccharides, which are hoped to reduce the use of heparins and the occurrence of HIT.

Defining a second epitope for heparin-induced thrombocytopenia/thrombosis antibodies using KKO, a murine HIT-like monoclonal antibody

Heparin-induced thrombocytopenia/thrombosis (HIT/T) is a common complication of heparin therapy that is caused by antibodies to platelet factor 4 (PF4) complexed with heparin. The immune response is polyclonal and polyspecific, ie, more than one neoepitope on PF4 is recognized by HIT/T antibodies. One such epitope has been previously identified; it involves the domain between the third and fourth cysteine residues in PF4 (site 1). However, the binding sites for other HIT/T antibodies remain to be defined. To explore this issue, the binding site of KKO, an HIT/T-like murine monoclonal antibody, was defined. KKO shares a binding site with many HIT/T antibodies on PF4/heparin, but does not bind to site 1 or recognize mouse PF4/heparin. Therefore, the binding of KKO to a series of mouse/human PF4 chimeras complexed with heparin was examined. KKO recognizes a site that requires both the N terminus of PF4 and Pro34, which immediately precedes the third cysteine. Both regions lie on the surface of the PF4 tetramer in sufficient proximity (within 0.74 nm) to form a contiguous antigenic determinant. The 10 of 14 HIT/T sera that require the N terminus of PF4 for antigen recognition also require Pro34 to bind. This epitope, termed site 2, lies adjacent to site 1 in the crystal structure of the PF4 tetramer. Yet sites 1 and 2 can be recognized by distinct populations of antibodies. These studies further help to define a portion of the PF4 tetramer to which self-reactive antibodies develop in patients exposed to heparin.

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.

Pathophysiology of heparin-induced thrombocytopenia. Clinical and diagnostic implications--a review

OBJECTIVE

This review of heparin-induced thrombocytopenia (HIT), the most frequent and dangerous side effect of heparin exposure, covers the epidemiology, pathophysiology, clinical presentation, diagnosis, and treatment of this disease syndrome.

DATA SOURCES AND STUDY SELECTION

Current consensus of opinion is given based on literature reports, as well as new information where available. A comprehensive analysis of the reasons for discrepancies in incidence numbers is given. The currently known mechanism is that HIT is mediated by an antibody to the complex of heparin->platelet factor 4, which binds to the Fc receptor on platelets. New evidence suggests a functional heterogeneity in the anti-heparin-platelet factor 4 antibodies generated to heparin, and a "superactive" heparin-platelet factor 4 antibody that does not require the presence of heparin to promote platelet activation or aggregation has been identified. Up-regulation of cell adhesion molecules and inflammatory markers, as well as preactivation of platelets/endothelial cells/leukocytes, are also considered to be related to the pathophysiology of HIT. Issues related to the specificity of currently available and new laboratory assays that support a clinical diagnosis are addressed in relation to the serotonin-release assay. Past experience with various anticoagulant treatments is reviewed with a focus on the recent successes of thrombin inhibitors and platelet GPIIb/IIIa inhibitors to combat the platelet activation and severe thrombotic episodes associated with HIT.

CONCLUSIONS

The pathophysiology of HIT is multifactorial. However, the primary factor in the mediation of the cellular activation is due to the generation of an antibody to the heparin-platelet factor 4 complex. This review is written as a reference for HIT research.

Thromboembolism in a patient with transient eosinophilia and thrombocytopenia

A 24-year-old woman with an unremarkable medical history who developed bilateral deep venous thrombosis and pulmonary emboli is presented. Associated findings were severe eosinophilia and moderate thrombocytopenia. Since the major acquired and hereditary thrombogenic disorders were ruled out in this case (including antiphospholipid syndrome and heparin-induced thrombocytopenia), we believe that the severe eosinophilia per se could be the pro-coagulant factor leading to thrombosis and embolism in our patient. The role of eosinophilia in thrombosis is discussed.

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 IgG2bκ 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 FcγRIIA. 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.

Heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia with or without thrombosis has been recognized increasingly as a serious complication of heparin use. This article reviews type II heparin-induced thrombocytopenia, which is mediated by an antibody that in most cases has specificity for a complex between heparin and platelet factor 4, a secreted platelet alpha-granule protein. The antibody-heparin-platelet factor 4 complex can activate platelets and endothelial cells, thereby initiating thrombosis. Clinical thrombosis in this syndrome may be arterial or venous. Treatment of the syndrome requires discontinuation of heparin and institution of an alternative anticoagulant.

High-dose intravenous gamma-globulins for heparin-induced thrombocytopenia: a prompt response

Heparin-induced thrombocytopenia (HIT) is a common drug induced autoimmune condition. The thrombocytopenia is caused in most cases by an antibody directed against the complex PF4/heparin. Recently, we have induced an experimental model of HIT by idiotypic manipulation. To confirm further the idiotypic involvement of HIT, we have treated successfully three patients with HIT with high-dose intravenous gamma-globulin (IVIG). Our three patients joint other two cases previously reported who were treated with IVIG and point to the efficacy of this type of therapy with minimal side effects. IVIG suppression of the anti-PF4/heparin autoantibody may support the idiotypic etiology of HIT.