Characterisation of the conformational changes in platelet factor 4 induced by polyanions: towards in vitro prediction of antigenicity

Utilization of 4T score to determine the pretest probability of heparin-induced thrombocytopenia in a community hospital in upstate New York

Background

Thrombocytopenia is common in hospitalized patients. Heparin-induced thrombocytopenia (HIT) is a life-threatening condition which can lead to extensive thrombosis. Diagnosis of HIT relies on clinical suspicion determined by 4T score and immunoassays through testing for anti-PF4/heparin antibodies. Clinical practice guidelines published by the American Society of Hematology in 2013 recommended use of the 4T score before ordering the immunoassays as a measure of pretest probability. The purpose of this study was to evaluate the utilization of 4T score before ordering anti-PF4/heparin antibodies at Unity Hospital.

Methods

We did a retrospective chart review for patients who are 18 years or older, admitted to Unity Hospital between July 1, 2013, and December 31, 2014, and had anti-PF4/heparin antibodies ordered. Subjects who had prior history of HIT or had end-stage renal disease on hemodialysis were excluded. After calculating 4T score retrospectively, we calculated the proportion of patients who had 4T score documented prior to ELISA testing and proportion of ELISA tests, which were not indicated due to a 4T score less than or equal to 3 using Minitab 16.

Results

Review of 123 patients, with an average age of 69.4 years, showed that testing was indicated in 18 patients. Six subjects had positive results, and testing was indicated in all of them. 4T score was documented in three patients. This quality improvement study showed that 4T score documentation rate at Unity Hospital is 2.4%. Anti-PF4/heparin antibody testing was indicated in 14.6%. This test is being overused in thrombocytopenia work up at Unity Hospital, costing $9,345. The topic was reviewed for residents. A prompt and calculator for 4T score were added to electronic medical records before ordering the test as a step to improve high value care.

The platelet Fc receptor, FcγRIIa

Human platelets express FcγRIIa, the low-affinity receptor for the constant fragment (Fc) of immunoglobulin (Ig) G that is also found on neutrophils, monocytes, and macrophages. Engagement of this receptor on platelets by immune complexes triggers intracellular signaling events that lead to platelet activation and aggregation. Importantly these events occur in vivo, particularly in response to pathological immune complexes, and engagement of this receptor on platelets has been causally linked to disease pathology. In this review, we will highlight some of the key features of this receptor in the context of the platelet surface, and examine the functions of platelet FcγRIIa in normal hemostasis and in response to injury and infection. This review will also highlight pathological consequences of engagement of this receptor in platelet-based autoimmune disorders. Finally, we present some new data investigating whether levels of the extracellular ligand-binding region of platelet glycoprotein VI which is rapidly shed upon engagement of platelet FcγRIIa by autoantibodies, can report on the presence of pathological anti-heparin/platelet factor 4 immune complexes and thus identify patients with pathological autoantibodies who are at the greatest risk of developing life-threatening thrombosis in the setting of heparin-induced thrombocytopenia.

Immune pathogenesis of heparin-induced thrombocytopenia

The immune response to heparin is one of the most common drug-induced allergies, and yet, atypical for a drug hypersensitivity reaction. Whereas most drug-induced allergies are rare, idiosyncratic and life-long, the allergic response to heparin is common, predictable in certain clinical settings and transient. Advances in the last decade with regards to structural characterisation of the PF4/heparin antigenic complex, contributions of innate immunity and development of animal models have provided insights into the distinctive features of the HIT immune response. Recent descriptions of the crystal structure of the PF4/heparin complex, alongside other biophysical studies, have clarified the structural requirements for immunogenicity and heparin-dependency of antibody formation. Studies of interactions of PF4 with bacterial cell walls as well as epidemiologic associations of anti-PF4/heparin antibody formation and infection suggest a role for immune priming and explain the rapid evolution of an isotype-switched immune response in sensitised patients. Murine models have greatly facilitated investigations of cellular basis of the HIT response and identified a major role for T-cells and marginal zone B-cells, but key findings have yet to be validated in human disease. This chapter will summarise recent investigations of the HIT immune response in the context of major pathways of immune activation and identify areas of uncertainty.

Atomic features of an autoantigen in heparin-induced thrombocytopenia (HIT)

Autoantigen development is poorly understood at the atomic level. Heparin-induced thrombocytopenia (HIT) is an autoimmune thrombotic disorder caused by antibodies to an antigen composed of platelet factor 4 (PF4) and heparin or cellular glycosaminoglycans (GAGs). In solution, PF4 exists as an equilibrium among monomers, dimers and tetramers. Structural studies of these interacting components helped delineate a multi-step process involved in the pathogenesis of HIT. First, heparin binds to the 'closed' end of the PF4 tetramer and stabilizes its conformation; exposing the 'open' end. Second, PF4 arrays along heparin/GAG chains, which approximate tetramers, form large antigenic complexes that enhance antibody avidity. Third, pathogenic HIT antibodies bind to the 'open' end of stabilized PF4 tetramers to form an IgG/PF4/heparin ternary immune complex and also to propagate the formation of 'ultralarge immune complexes' (ULCs) that contain multiple IgG antibodies. Fourth, ULCs signal through FcγRIIA receptors, activating platelets and monocytes directly and generating thrombin, which transactivates hematopoietic and endothelial cells. A non-pathogenic anti-PF4 antibody prevents tetramer formation, binding of pathogenic antibody, platelet activation and thrombosis, providing a new approach to manage HIT. An improved understanding of the pathogenesis of HIT may lead to novel diagnostics and therapeutics for this autoimmune disease.

Effects of a Foot Pump on the Incidence of Deep Vein Thrombosis After Total Knee Arthroplasty in Patients Given Edoxaban: A Randomized Controlled Study

We conducted a randomized clinical trial to compare the effectiveness of the A-V Impulse System foot pump for reducing the incidence of deep-vein thrombosis (DVT) after total knee arthroplasty (TKA) in patients under edoxaban thromboprophylaxis. Patients undergoing primary TKA at our institution between September 2013 and March 2015 were enrolled after obtaining informed consent. The patients were randomized to use the foot pump (n = 58) and not to use the foot pump (n = 62). Both groups were given prophylactic edoxaban. Primary outcomes were any DVT as detected by bilateral ultrasonography up to postoperative day 10 (POD10) and pulmonary embolism (PE) up to POD28. The safety outcomes were bleeding and death of any cause up to POD28. Plasma D-dimer levels were measured before TKA and on POD10 after TKA. Immunoglobulin G (IgG)-class anti-PF4/heparin antibodies were measured using an IgG-specific enzyme-linked immunosorbent assay. The incidences of any DVT up to POD28 were 31.0% and 17.7% in patients with or without the foot pump, respectively. The incidences of major bleeding up to POD28 were 5.1% and 4.8% in patients with or without the foot pump, respectively. Foot pump use did not significantly reduce the incidence of DVTs in patients undergoing TKA under edoxaban thromboprophylaxis. Although seroconversion of anti-PF4/heparin antibodies was confirmed in one-fourth of patients, the seroconversion rates did not differ between patients with (20.7%) or without (25.8%) foot pump use. This study shows that the A-V Impulse system foot pump did not affect the incidence of DVT under edoxaban thromboprophylaxis in patients undergoing TKA. Seroconversion of anti-PF4/heparin antibodies was detected in a significant number of patients who underwent TKA under antithrombotic prophylaxis using edoxaban.

Atomic description of the immune complex involved in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is an autoimmune thrombotic disorder caused by immune complexes containing platelet factor 4 (PF4), antibodies to PF4 and heparin or cellular glycosaminoglycans (GAGs). Here we solve the crystal structures of the: (1) PF4 tetramer/fondaparinux complex, (2) PF4 tetramer/KKO-Fab complex (a murine monoclonal HIT-like antibody) and (3) PF4 monomer/RTO-Fab complex (a non-HIT anti-PF4 monoclonal antibody). Fondaparinux binds to the ‘closed' end of the PF4 tetramer and stabilizes its conformation. This interaction in turn stabilizes the epitope for KKO on the ‘open' end of the tetramer. Fondaparinux and KKO thereby collaborate to ‘stabilize' the ternary pathogenic immune complex. Binding of RTO to PF4 monomers prevents PF4 tetramerization and inhibits KKO and human HIT IgG-induced platelet activation and platelet aggregation in vitro, and thrombus progression in vivo. The atomic structures provide a basis to develop new diagnostics and non-anticoagulant therapeutics for HIT.

Heparin-induced thrombocytopenia (HIT) is an autoimmune thrombotic disease with limited treatment options. Here the authors present crystallographic data on the disease-causing immune complex, providing the structural basis for the development of new diagnostic and therapeutic approaches to HIT.

Quantitative description of thermodynamic and kinetic properties of the platelet factor 4/heparin bonds

Heparin is the most important antithrombotic drug in hospitals. It binds to the endogenous tetrameric protein platelet factor 4 (PF4) forming PF4/heparin complexes which may cause a severe immune-mediated adverse drug reaction, so-called heparin-induced thrombocytopenia (HIT). Although new heparin drugs have been synthesized to reduce such a risk, detailed bond dynamics of the PF4/heparin complexes have not been clearly understood. In this study, single molecule force spectroscopy (SMFS) is utilized to characterize the interaction of PF4 with heparins of defined length (5-, 6-, 8-, 12-, and 16-mers). Analysis of the force-distance curves shows that PF4/heparin binding strength rises with increasing heparin length. In addition, two binding pathways in the PF4/short heparins (≤8-mers) and three binding pathways in the PF4/long heparins (≥8-mers) are identified. We provide a model for the PF4/heparin complexes in which short heparins bind to one PF4 tetramer, while long heparins bind to two PF4 tetramers. We propose that the interaction between long heparins and PF4s is not only due to charge differences as generally assumed, but also due to hydrophobic interaction between two PF4s which are brought close to each other by long heparin. This complicated interaction induces PF4/heparin complexes more stable than other ligand-receptor interactions. Our results also reveal that the boundary between antigenic and non-antigenic heparins is between 8- and 12-mers. These observations are particularly important to understand processes in which PF4-heparin interactions are involved and to develop new heparin-derived drugs.

Heparin/Heparan sulfate proteoglycans glycomic interactome in angiogenesis: biological implications and therapeutical use

Angiogenesis, the process of formation of new blood vessel from pre-existing ones, is involved in various intertwined pathological processes including virus infection, inflammation and oncogenesis, making it a promising target for the development of novel strategies for various interventions. To induce angiogenesis, angiogenic growth factors (AGFs) must interact with pro-angiogenic receptors to induce proliferation, protease production and migration of endothelial cells (ECs). The action of AGFs is counteracted by antiangiogenic modulators whose main mechanism of action is to bind (thus sequestering or masking) AGFs or their receptors. Many sugars, either free or associated to proteins, are involved in these interactions, thus exerting a tight regulation of the neovascularization process. Heparin and heparan sulfate proteoglycans undoubtedly play a pivotal role in this context since they bind to almost all the known AGFs, to several pro-angiogenic receptors and even to angiogenic inhibitors, originating an intricate network of interaction, the so called "angiogenesis glycomic interactome". The decoding of the angiogenesis glycomic interactome, achievable by a systematic study of the interactions occurring among angiogenic modulators and sugars, may help to design novel antiangiogenic therapies with implications in the cure of angiogenesis-dependent diseases.

Advances in the pathophysiology and treatment of heparin-induced thrombocytopenia

PURPOSE OF REVIEW

To review the recent developments in understanding the pathophysiology of heparin-induced thrombocytopenia (HIT) and in applying this knowledge to the treatment of patients with suspected and proven HIT.

RECENT FINDINGS

HIT pathophysiology is dynamic and complex. HIT pathophysiology is initiated by four essential components--heparin (Hep), platelet factor 4 (PF4), IgG antibodies against the Hep-PF4 complex, and platelet FcγRIIa. HIT is propagated by activated platelets, monocytes, endothelial cells, and coagulation proteins. Insights into the unique HIT antibody response continue to emerge, but without consensus as to the relative roles of B cells, T cells, and antigen-presenting cells. Platelet activation via FcγRIIa, the sine qua non of HIT, has become much better appreciated. Therapy remains challenging for several reasons. Suspected HIT is more frequent than proven HIT, because of the widespread use of Hep and the inadequacies of current diagnostic tests and scoring systems. In proven HIT, approved treatments reduce but do not eliminate thrombosis, and have substantial bleeding risk. Rational novel therapeutic strategies, directed at the initiating steps in HIT pathophysiology and with potential combinations staged over time, are in various phases of development.

SUMMARY

Progress continues in understanding the breadth of molecular and cellular players in HIT. Translation to improved diagnosis and treatment is needed.

Evaluation of Immunostimulatory Potential of Branded and US-Generic Enoxaparins in an In Vitro Human Immune System Model

Low-molecular-weight heparins (LMWHs) have several positive therapeutic effects and can also form immunostimulatory complexes with plasma proteins, such as platelet factor 4 (PF4). We compared the innate response and functional profiles of branded and US-generic enoxaparins from 2 manufacturers in either native or PF4-bound forms in an in vitro model of human immunity. In an analysis of 2 product lots from each manufacturer and multiple separate batches of protein–heparin complexes, branded enoxaparin was shown to be consistently nonstimulatory for innate responses, whereas US-generic enoxaparins generated variable immunostimulatory profiles depending on the enoxaparin lot used to prepare the PF4–LMWH complexes. Production of tissue factor pathway inhibitor (TFPI), a physiologic heparin-induced inhibitor of tissue factor-induced coagulation that was used as a functional readout of biological activity of enoxaparins in these assays, was heightened in the presence of branded enoxaparin complexes, but its levels were variable in cultures treated with complexes containing US-generic enoxaparins. Analytical analyses suggest that the heightened immunostimulatory potential of some of the US-generic enoxaparin product lots could be tied to their capacity to form ultra-large and/or more stable complexes with PF4 than the other LMWHs included in this study. Although these distinct biological and analytical profiles might be related to the composition and/or consistency of branded and US-generic enoxaparins included in our data set, further studies are warranted to elucidate the pathophysiological relevance of these in vitro findings.

Heparin-independent, PF4-dependent binding of HIT antibodies to platelets: implications for HIT pathogenesis

Antibodies specific for platelet factor 4 (PF4)/heparin complexes are the hallmark of heparin-induced thrombocytopenia and thrombosis (HIT), but many antibody-positive patients have normal platelet counts. The basis for this is not fully understood, but it is believed that antibodies testing positive in the serotonin release assay (SRA) are the most likely to cause disease. We addressed this issue by characterizing PF4-dependent binding of HIT antibodies to intact platelets and found that most antibodies testing positive in the SRA, but none of those testing negative, bind to and activate platelets when PF4 is present without any requirement for heparin (P < .0001). Binding of SRA-positive antibodies to platelets was inhibited by chondroitinase ABC digestion (P < .05) and by the addition of chondroitin-4-sulfate (CS) or heparin in excess quantities. The findings suggest that although all HIT antibodies recognize PF4 in a complex with heparin, only a subset of these antibodies recognize more subtle epitopes induced in PF4 when it binds to CS, the major platelet glycosaminoglycan. Antibodies having this property could explain "delayed HIT" seen in some individuals after discontinuation of heparin and the high risk for thrombosis that persists for weeks in patients recovered from HIT.