Rational design and characterization of platelet factor 4 antagonists for the study of heparin-induced thrombocytopenia

Quality control, safety assessment and preparation approaches of low molecular weight heparin

Low Molecular Weight Heparins (LMWHs) are well-established for use in the prevention and treatment of thrombotic diseases, and as a substitute for unfractionated heparin (UFH) due to their predictable pharmacokinetics and subcutaneous bioavailability. LMWHs are produced by various depolymerization methods from UFH, resulting in heterogeneous compounds with similar biochemical and pharmacological properties. However, the delicate supply chain of UFH and potential contamination from animal sources require new manufacturing approaches for LMWHs. Various LMWH preparation methods are emerging, such as chemical synthesis, enzymatic or chemical depolymerization and chemoenzymatic synthesis. To establish the sameness of active ingredients in both innovator and generic LMWH products, the Food and Drug Administration has implemented a stringent scientific method of equivalence based on physicochemical properties, heparin source material and depolymerization techniques, disaccharide composition and oligosaccharide mapping, biological and biochemical properties, and in vivo pharmacodynamic profiles. In this review, we discuss currently available LMWHs, potential manufacturing methods, and recent progress for manufacturing quality control of these LMWHs.

Platelet factor 4 limits neutrophil extracellular trap- and cell-free DNA-induced thrombogenicity and endothelial injury

Plasma cell-free DNA (cfDNA), a marker of disease severity in sepsis, is a recognized driver of thromboinflammation and a potential therapeutic target. In sepsis, plasma cfDNA is mostly derived from neutrophil extracellular trap (NET) degradation. Proposed NET-directed therapeutic strategies include preventing NET formation or accelerating NET degradation. However, NET digestion liberates pathogens and releases cfDNA that promote thrombosis and endothelial cell injury. We propose an alternative strategy of cfDNA and NET stabilization with chemokine platelet factor 4 (PF4, CXCL4). We previously showed that human PF4 (hPF4) enhances NET-mediated microbial entrapment. We now show that hPF4 interferes with thrombogenicity of cfDNA and NETs by preventing their cleavage to short-fragment and single-stranded cfDNA that more effectively activates the contact pathway of coagulation. In vitro, hPF4 also inhibits cfDNA-induced endothelial tissue factor surface expression and von Willebrand factor release. In vivo, hPF4 expression reduced plasma thrombin-antithrombin (TAT) levels in animals infused with exogenous cfDNA. Following lipopolysaccharide challenge, Cxcl4-/- mice had significant elevation in plasma TAT, cfDNA, and cystatin C levels, effects prevented by hPF4 infusion. These results show that hPF4 interacts with cfDNA and NETs to limit thrombosis and endothelial injury, an observation of potential clinical benefit in the treatment of sepsis.

Pathogenetic mechanisms and treatment targets in cerebral malaria

Malaria, the most prevalent mosquito-borne infectious disease worldwide, has accompanied humanity for millennia and remains an important public health issue despite advances in its prevention and treatment. Most infections are asymptomatic, but a small percentage of individuals with a heavy parasite burden develop severe malaria, a group of clinical syndromes attributable to organ dysfunction. Cerebral malaria is an infrequent but life-threatening complication of severe malaria that presents as an acute cerebrovascular encephalopathy characterized by unarousable coma. Despite effective antiparasite drug treatment, 20% of patients with cerebral malaria die from this disease, and many survivors of cerebral malaria have neurocognitive impairment. Thus, an important unmet clinical need is to rapidly identify people with malaria who are at risk of developing cerebral malaria and to develop preventive, adjunctive and neuroprotective treatments for cerebral malaria. This Review describes important advances in the understanding of cerebral malaria over the past two decades and discusses how these mechanistic insights could be translated into new therapies.

Modulation of ultralarge immune complexes in heparin-induced thrombocytopenia

BACKGROUND

Heparin-induced thrombocytopenia (HIT) is a serious thrombotic disorder caused by ultralarge immune complexes (ULICs) containing platelet factor 4 (PF4) and heparin that form the HIT antigen, together with a subset of anti-PF4 antibodies. ULICs initiate prothrombotic responses by engaging Fcγ receptors on platelets, neutrophils, and monocytes. Contemporary anti-thrombotic therapy for HIT is neither entirely safe nor entirely successful and acts downstream of ULIC formation and Fcγ receptor-initiated generation of thrombin.

OBJECTIVES

To determine whether HIT antigen and ULIC formation and stability could be modified favorably by inhibiting PF4-heparin interactions with fondaparinux, together with blocking formation of PF4 tetramers using a humanized monoclonal anti-PF4 antibody (hRTO).

METHODS

Results: The combination of fondaparinux and hRTO inhibited HIT antigen formation, promoted antigen dissociation, inhibited ULIC formation, and promoted ULIC disassembly at concentrations below the effective concentration of either alone and blocked Fcγ receptor-dependent induction of factor Xa activity by monocytic THP1 cells and activation of human platelets in whole blood. Combined with hRTO, fondaparinux inhibited HIT antigen and immune complex formation and activation through Fcγ receptors at concentrations at or below those used clinically to inhibit FXa coagulant activity.

CONCLUSIONS

HIT antigen and immune complexes are dynamic and amenable to modulation. Fondaparinux can be converted from an anticoagulant that acts at a downstream amplification step into a rationale, disease-specific intervention that blocks ULIC formation. Interventions that prevent ULIC formation and stability might increase the efficacy, permit use of lower doses, shorten the duration of antithrombotic therapy, and help prevent this serious thrombotic disorder.

Neutrophil extracellular trap stabilization by platelet factor 4 reduces thrombogenicity and endothelial cell injury

Neutrophil extracellular traps (NETs) are abundant in sepsis, and proposed NET-directed therapies in sepsis prevent their formation or accelerate degradation. Yet NETs are important for microbial entrapment, as NET digestion liberates pathogens and NET degradation products (NDPs) that deleteriously promote thrombosis and endothelial cell injury. We proposed an alternative strategy of NET-stabilization with the chemokine, platelet factor 4 (PF4, CXCL4), which we have shown enhances NET-mediated microbial entrapment. We now show that NET compaction by PF4 reduces their thrombogenicity. In vitro, we quantified plasma thrombin and fibrin generation by intact or degraded NETs and cell-free (cf) DNA fragments, and found that digested NETs and short DNA fragments were more thrombogenic than intact NETs and high molecular weight genomic DNA, respectively. PF4 reduced the thrombogenicity of digested NETs and DNA by interfering, in part, with contact pathway activation. In endothelial cell culture studies, short DNA fragments promoted von Willebrand factor release and tissue factor expression via a toll-like receptor 9-dependent mechanism. PF4 blocked these effects. Cxcl4-/- mice infused with cfDNA exhibited higher plasma thrombin anti-thrombin (TAT) levels compared to wild-type controls. Following challenge with bacterial lipopolysaccharide, Cxcl4-/- mice had similar elevations in plasma TAT and cfDNA, effects prevented by PF4 infusion. Thus, NET-stabilization by PF4 prevents the release of short fragments of cfDNA, limiting the activation of the contact coagulation pathway and reducing endothelial injury. These results support our hypothesis that NET-stabilization reduces pathologic sequelae in sepsis, an observation of potential clinical benefit.

Heparin-Induced Thrombocytopenia: A Focus on Thrombosis

Heparin-induced thrombocytopenia is an immune-mediated disorder caused by antibodies that recognize complexes of platelet factor 4 and heparin. Thrombosis is a central and unpredictable feature of this syndrome. Despite optimal management, disease morbidity and mortality from thrombosis remain high. The hypercoagulable state in heparin-induced thrombocytopenia is biologically distinct from other thrombophilic disorders in that clinical complications are directly attributable to circulating ultra-large immune complexes. In some individuals, ultra-large immune complexes elicit unchecked cellular procoagulant responses that culminate in thrombosis. To date, the clinical and biologic risk factors associated with thrombotic risk in heparin-induced thrombocytopenia remain elusive. This review will summarize our current understanding of thrombosis in heparin-induced thrombocytopenia with attention to its clinical features, cellular mechanisms, and its management.

Platelet chemokines in health and disease

In recent years, it has become clear that platelets and platelet-derived chemokines, beyond their role in thrombosis and haemostasis, are important mediators affecting a broad spectrum of (patho)physiological conditions. These biologically active proteins are released from α-granules upon platelet activation, most probably even during physiological conditions. In this review, we give a concise overview and an update on the current understanding of platelet-derived chemokines in a context of health and disease.

Note: The review process for this manuscript was fully handled by G. Y. H. Lip, Editor in Chief.

Biophysical tools to assess the interaction of PF4 with polyanions

The antigen in heparin-induced thrombocytopenia (HIT) is expressed on platelet factor 4 (PF4) when PF4 complexes with polyanions. In recent years, biophysical tools (e. g. circular dichroism spectroscopy, atomic force microscopy, isothermal titration calorimetry, x-ray crystallography, electron microscopy) have gained an important role to complement immunological and functional assays for better understanding the interaction of heparin with PF4. This allowed identification of those features that make PF4 immunogenic (e. g. a certain conformational change induced by the polyanion, a threshold energy of the complexes, the existence of multimeric complexes, a certain number of bonds formed by PF4 with the polyanion) and to characterize the morphology and thermal stability of complexes formed by the protein with polyanions. These findings and methods can now be applied to test new drugs for their potential to induce the HIT-like adverse drug effect by preclinical in vitro testing. The methods and techniques applied to characterize the antigen in HIT may also be helpful to better understand the mechanisms underlying other antibody-mediated disorders in thrombosis and hemostasis (e. g. acquired hemophilia, thrombotic thrombocytopenic purpura). Furthermore, understanding the mechanisms making the endogenous protein PF4 immunogenic may help to understand the mechanisms underlying other autoimmune disorders.

Autoimmune heparin-induced thrombocytopenia

Autoimmune heparin-induced thrombocytopenia (aHIT) indicates the presence in patients of anti-platelet factor 4 (PF4)-polyanion antibodies that are able to activate platelets strongly even in the absence of heparin (heparin-independent platelet activation). Nevertheless, as seen with serum obtained from patients with otherwise typical heparin-induced thrombocytopenia (HIT), serum-induced platelet activation is inhibited at high heparin concentrations (10-100 IU mL-1 heparin). Furthermore, upon serial dilution, aHIT serum will usually show heparin-dependent platelet activation. Clinical syndromes associated with aHIT include: delayed-onset HIT, persisting HIT, spontaneous HIT syndrome, fondaparinux-associated HIT, heparin 'flush'-induced HIT, and severe HIT (platelet count of < 20 × 109  L-1 ) with associated disseminated intravascular coagulation (DIC). Recent studies have implicated anti-PF4 antibodies that are able to bridge two PF4 tetramers even in the absence of heparin, probably facilitated by non-heparin platelet-associated polyanions (chondroitin sulfate and polyphosphates); nascent PF4-aHIT-IgG complexes recruit additional heparin-dependent HIT antibodies, leading to the formation of large multimolecular immune complexes and marked platelet activation. aHIT can persist for several weeks, and serial fibrin, D-dimer, and fibrinogen levels, rather than the platelet count, may be helpful for monitoring treatment response. Although standard anticoagulant therapy for HIT ought to be effective, published experience indicates frequent failure of activated partial thromboplastin time (APTT)-adjusted anticoagulants (argatroban, bivalirudin), probably because of underdosing in the setting of HIT-associated DIC, known as 'APTT confounding'. Thus, non-APTT-adjusted therapies with drugs such as danaparoid and fondaparinux, or even direct oral anticoagulants, such as rivaroxaban or apixaban, are suggested therapies, especially for long-term management of persisting HIT. In addition, emerging data indicate that high-dose intravenous immunoglobulin can interrupt HIT antibody-induced platelet activation, leading to rapid platelet count recovery.

Heparin-Induced Thrombocytopenia: A Comprehensive Clinical Review

Heparin-induced thrombocytopenia is a profoundly dangerous, potentially lethal, immunologically mediated adverse drug reaction to unfractionated heparin or, less commonly, to low-molecular weight heparin. In this comprehensive review, the authors highlight heparin-induced thrombocytopenia's risk factors, clinical presentation, pathophysiology, diagnostic principles, and treatment. The authors place special emphasis on the management of patients requiring procedures using cardiopulmonary bypass or interventions in the catheterization laboratory. Clinical vigilance of this disease process is important to ensure its recognition, diagnosis, and treatment. Misdiagnosis of the syndrome, as well as misunderstanding of the disease process, continues to contribute to its morbidity and mortality.

Polyphosphate/platelet factor 4 complexes can mediate heparin-independent platelet activation in heparin-induced thrombocytopenia

Heparin-induced thrombocytopenia (HIT) is a thrombotic disorder initiated by antibodies to complexes between platelet factor 4 (PF4) and heparin. The risk of recurrent thromboembolism persists after heparin is cleared and platelet activation leading to release of PF4 has dissipated. We asked whether antigenic complexes between polyphosphates and PF4 released from activated platelets might intensify or sustain the prothrombotic phenotype of HIT. PF4 forms stable, ultralarge complexes with polyphosphates of various sizes, including those released from platelets, which are recognized by the HIT-like monoclonal KKO, an immunoglobulin G2bκ monoclonal heparin/PF4 binding antibody, and by human HIT antibodies. KKO helps to protect PF4/polyphosphate complexes from degradation by phosphatases. Complement is activated when HIT antibodies bind to PF4/polyphosphate complexes and PF4 reverses the inhibition of complement by polyphosphates. Polyphosphates and PF4 are stored primarily in separate granules in resting platelets, but they colocalize when the cells are activated. Platelets activated by subaggregating doses of thrombin receptor activating peptide release polyphosphates and PF4, which form antigenic complexes that allow KKO to further activate platelets in the absence of heparin and exogenous PF4. These studies suggest that thrombin- or immune complex-mediated release of endogenous antigenic PF4/polyphosphate complexes from platelets may augment the prothrombotic risk of HIT and perpetuate the risk of thrombosis after heparin has been discontinued.

Platelet Factor 4 Inhibits and Enhances HIV-1 Infection in a Concentration-Dependent Manner by Modulating Viral Attachment

Platelet factor 4 (PF4) has been recently shown to inhibit infection by a broad range of human immunodeficiency virus type 1 (HIV-1) isolates in vitro. We found that the inhibitory effects of PF4 are limited to a defined concentration range where PF4 exists largely in a monomeric state. Under these conditions, PF4 bound the HIV-1 envelope protein and inhibited HIV-1 attachment to the cell surface. However, as concentrations increased to the point where PF4 exists largely in tetrameric or higher-order forms, viral infection in vitro was enhanced. Enhancement could be inhibited by mutations in PF4 that shift the oligomeric equilibrium toward the monomeric state, or by using soluble glycosaminoglycans (GAGs) to which tetrameric PF4 avidly binds. We conclude that at physiologically relevant concentrations, oligomeric PF4 enhances infection by HIV-1 by interacting with the viral envelope protein as well as cell surface GAGs, enhancing virus attachment to the cell surface. This effect was not specific to HIV-1, as enhancement was seen with some but not all other viruses tested. The biphasic effects of PF4 on HIV-1 infection suggest that native PF4 will not be a useful antiviral agent and that PF4 could contribute to the hematologic abnormalities commonly seen in HIV-infected individuals by enhancing virus infection in the bone marrow.

Management of the multiple phases of heparin-induced thrombocytopenia

The clinical course of heparin-induced thrombocytopenia (HIT) may be separated into five sequential phases: 1. suspected HIT, 2. acute HIT, 3. subacute HIT A, 4. subacute HIT B, and 5. remote HIT. Each phase confronts the clinician with a unique set of management questions. In this review, the phases of HIT are defined and key management questions associated with each phase are discussed. Among patients with Suspected HIT, I use the 4Ts score to determine which patients have a sufficiently high probability of HIT to justify discontinuation of heparin and initiation of a non-heparin parenteral anticoagulant. An algorithm for selecting an appropriate non-heparin anticoagulant based on the patient's clinical stability, renal and hepatic function, drug availability, and physician comfort is provided. In patients with Acute HIT, I generally avoid prophylactic platelet transfusion and inferior vena cava filter insertion because of a potential increased risk of thrombosis. I perform 4-limb screening compression ultrasonography. In patients with symptomatic thromboembolism or asymptomatic proximal deep-vein thrombosis, I treat with anticoagulation for three months. In patients without thrombosis, I discontinue anticoagulation upon platelet count recovery. I do not transition patients to an oral anticoagulant until platelet count recovery (i. e. Subacute HIT A). I increasingly choose direct oral anticoagulants over vitamin K antagonists in this setting because of their greater convenience and safety. In Subacute HIT B and Remote HIT, I use heparin for cardiovascular surgery, whereas I use bivalirudin in patients with Acute HIT and Subacute HIT A in whom surgery cannot be delayed.

Intramedullary megakaryocytes internalize released platelet factor 4 and store it in alpha granules

BACKGROUND

Megakaryocytes express and store platelet factor 4 (PF4) in alpha granules. In vivo, PF4 is a clinically relevant, negative regulator of megakaryopoiesis and hematopoietic stem cell replication. These findings would suggest a regulated source of free intramedullary PF4.

OBJECTIVES

Define the source of free intramedullary PF4 and its intramedullary life cycle.

METHODS

We interrogated both murine and human bone marrow-derived cells during megakaryopoiesis in vitro by using confocal microscopy and enzyme-linked immunosorbent assay. With immunohistochemistry, we examined in vivo free PF4 in murine bone marrow before and after radiation injury and in the setting of megakaryocytopenia and thrombocytopenia.

RESULTS

Exogenously added human PF4 is internalized by murine megakaryocytes. Human megakaryocytes similarly take up murine PF4 but not the related chemokine, platelet basic protein. Confocal microscopy shows that internalized PF4 colocalizes with endogenous PF4 in alpha granules and is available for release on thrombin stimulation. Immunohistochemistry shows free PF4 in the marrow, but not another alphagranule protein, von Willebrand factor. Free PF4 increases with radiation injury and decreases with megakaryocytopenia. Consistent with the known role of low-density lipoprotein receptor-related protein 1 in the negative paracrine effect of PF4 on megakaryopoiesis, PF4 internalization is at least partially low-density lipoprotein receptor-related protein 1 dependent.

CONCLUSIONS

PF4 has a complex intramedullary life cycle with important implications in megakaryopoiesis and hematopoietic stem cell replication not seen with other tested alpha granule proteins.

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.

Distinct specificity and single-molecule kinetics characterize the interaction of pathogenic and non-pathogenic antibodies against platelet factor 4-heparin complexes with platelet factor 4

Heparin-induced thrombocytopenia (HIT) is a thrombotic complication of heparin therapy mediated by antibodies to complexes between platelet factor 4 (PF4) and heparin or cellular glycosaminoglycans. However, only a fraction of patients with anti-PF4-heparin antibodies develop HIT, implying that only a subset of these antibodies is pathogenic. The basis for the pathogenic potential of anti-PF4-heparin antibodies remains unclear. To elucidate the intrinsic PF4-binding properties of HIT-like monoclonal antibody (KKO) versus non-pathogenic antibody (RTO) at the single-molecule level, we utilized optical trap-based force spectroscopy to measure the strength and probability of binding of surface-attached antibodies with oligomeric PF4 to simulate interactions on cells. To mimic the effect of heparin in bringing PF4 complexes into proximity, we chemically cross-linked PF4 tetramers using glutaraldehyde. Analysis of the force histograms revealed that KKO-PF4 interactions had ∼10-fold faster on-rates than RTO-PF4, and apparent equilibrium dissociation constants differed ∼10-fold with similar force-free off-rates (k(off) = 0.0031 and 0.0029 s(-1)). Qualitatively similar results were obtained for KKO and RTO interacting with PF4-heparin complexes. In contrast to WT PF4, KKO and RTO showed lower and similar binding probabilities to cross-linked PF4(K50E), which forms few if any oligomers. Thus, formation of stable PF4 polymers results in much stronger interactions with the pathogenic antibody without a significant effect on the binding of the non-pathogenic antibody. These results suggest a fundamental difference in the antigen-binding mechanisms between model pathogenic and non-pathogenic anti-PF4 antibodies that might underlie their distinct pathophysiological behaviors.

Emphasis on the Role of PF4 in the Incidence, Pathophysiology and Treatment of Heparin Induced Thrombocytopenia

Heparin Induced Thrombocytopenia (HIT) is caused by antibodies that recognize platelet factor 4 (PF4) associated with polyanionic glycosaminoglycan drugs or displayed on vascular cell membranes. These antibodies are elicited by multimolecular complexes that can occur when heparin is administered in clinical settings associated with abundant PF4. Heparin binding alters native PF4 and elicits immune recognition and response. While the presence of heparin is integral to immunogenesis, the HIT antibody binding site is within PF4. Thus HIT antibodies develop and function to cause thrombocytopenia and/or thrombosis only in the presence of PF4. Future emphasis on understanding the biology, turnover and regulation of PF4 may lead to insights into the prevention and treatment of HIT.

Novel diagnostic assays for heparin-induced thrombocytopenia

Laboratory testing for heparin-induced thrombocytopenia (HIT) has important shortcomings. Immunoassays fail to discriminate platelet-activating from nonpathogenic antibodies. Specific functional assays are impracticable due to the need for platelets and radioisotope. We describe 2 assays that may overcome these limitations. The KKO-inhibition test (KKO-I) measures the effect of plasma on binding of the HIT-like monoclonal antibody KKO to platelet factor 4 (PF4)/heparin. DT40-luciferase (DT40-luc) is a functional test comprised of a B-cell line expressing FcγRIIa coupled to a luciferase reporter. We compared these assays to polyspecific and immunoglobulin (Ig)G-specific PF4/heparin enzyme-linked immunosorbent assays (ELISAs) in samples from 58 patients with suspected HIT and circulating anti-PF4/heparin antibodies. HIT was defined as a 4Ts score ≥ 4 and positive (14)C-serotonin release assay. HIT-positive plasma demonstrated greater mean inhibition of KKO binding than HIT-negative plasma (78.9% vs 26.0%; P < .0001) and induced greater luciferase activity (3.14-fold basal vs 0.96-fold basal; P < .0001). The area under the receiver-operating characteristic curve was greater for KKO-I (0.93) than for the polyspecific (0.82; P = .020) and IgG-specific ELISA (0.76; P = .0044) and for DT40-luc (0.89) than for the IgG-specific ELISA (P = .046). KKO-I and DT40-luc showed better discrimination than 2 commercially available immunoassays, are simple to perform, and hold promise for improving the specificity and feasibility of HIT laboratory testing.

Investigation of a potential protective mechanism against heparin-induced thrombocytopenia in patients on chronic intermittent hemodialysis

BACKGROUND

Heparin-induced thrombocytopenia (HIT) develops as a result of platelet (PLT) activation by anti-platelet factor 4 (PF4)/heparin complex antibodies. Despite repeated exposure to heparin, patients undergoing chronic intermittent hemodialysis (HD) rarely develop HIT. We investigated the possibility that HD decreases/removes PF4 from PLT surfaces and/or plasma, thereby disfavoring immune complex formation as a mechanism of protection against HIT.

MATERIALS AND METHODS

We enrolled 20 patients undergoing chronic HD at the Penn Presbyterian Medical Center. Blood samples were drawn before, during and after treatment in the presence and absence of heparin. PF4, anti-PF4/heparin antibody, heparin, and P-selectin levels were measured.

RESULTS

No patients demonstrated clinical symptoms of HIT. PLT surface PF4 levels decreased and plasma PF4 levels increased concurrently with the increase in plasma heparin concentration. In the absence of heparin, PLT surface and plasma PF4 levels were unchanged. Anti-PF4/heparin antibodies, which were non-functional by the serotonin release assay, were detectable in 8 patients. PLT surface P-selectin levels did not change during treatment.

CONCLUSIONS

Removal of PLT surface and/or plasma PF4 as a mechanism of protection against HIT in patients undergoing HD is not supported by the results of our study, although the transient decrease in PLT surface PF4 in the presence of large amounts of heparin remains a candidate mechanism. The small sample size, single type of dialyzer membrane, and early sampling time points may have led to the inability to detect changes in PF4 levels. Future studies should explore other potential protective mechanisms.

"Inactivating" PF4: a new approach to HIT treatment?

Current methods of treating heparin-induced thrombocytopenia (HIT) focus on treatment and prevention of thrombotic complications. In this issue of Blood, Sachais et al describe a novel therapeutic approach: pharmacologic disruption of PF4 tetramers essential for formation of immune complexes that are central to the pathogenesis.

Antigen and substrate withdrawal in the management of autoimmune thrombotic disorders

Prevailing approaches to manage autoimmune thrombotic disorders, such as heparin-induced thrombocytopenia, antiphospholipid syndrome and thrombotic thrombocytopenic purpura, include immunosuppression and systemic anticoagulation, though neither provides optimal outcome for many patients. A different approach is suggested by the concurrence of autoantibodies and their antigenic targets in the absence of clinical disease, such as platelet factor 4 in heparin-induced thrombocytopenia and β(2)-glycoprotein-I (β(2)GPI) in antiphospholipid syndrome. The presence of autoantibodies in the absence of disease suggests that conformational changes or other alterations in endogenous protein autoantigens are required for recognition by pathogenic autoantibodies. In thrombotic thrombocytopenic purpura, the clinical impact of ADAMTS13 deficiency caused by autoantibodies likely depends on the balance between residual antigen, that is, enzyme activity, and demand imposed by local genesis of ultralarge multimers of von Willebrand factor. A corollary of these concepts is that disrupting platelet factor 4 and β(2)GPI conformation (or ultralarge multimer of von Willebrand factor oligomerization or function) might provide a disease-targeted approach to prevent thrombosis without systemic anticoagulation or immunosuppression. Validation of this approach requires a deeper understanding of how seemingly normal host proteins become antigenic or undergo changes that increase antibody avidity, and how they can be altered to retain adaptive functions while shedding epitopes prone to elicit harmful autoimmunity.