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Background Coronavirus disease 2019 vaccine ChAdOx1 nCov-19 may rarely lead to vaccine-induced thrombotic thrombocytopenia (VITT). Antibody-mediated, platelet factor 4 (PF4)-dependent platelet activation appears to resemble a key mechanism in VITT, partially comparable to heparin-induced thrombocytopenia. The use of PF4/heparin immunoassays has been proposed as part of a diagnostic approach, but their sensitivity has not been established. Methods Sera from 12 well-defined VITT patients were first studied by two different laboratories in functional assays. Sera where then used for an interlaboratory comparison, in which five different PF4/heparin immunoassays were used by four laboratories. Results Results for functional testing were highly concordant. VITT antibodies were also reliably detected by PF4/heparin enzyme-linked immunosorbent assays (ELISAs) (92–100%). In contrast, only 25% of VITT antibodies were reactive in a particle gel immunoassay (PaGIA), and 8% in a lateral flow assay (LFA). An automated chemiluminescence immunoassay (CLIA) was negative for all sera tested (0%). Conclusion It seems feasible to establish functional antibody testing for the confirmation of VITT. For the initial screening of suspected VITT cases, PaGIA, LFA, and CLIA are useless when applied as single tests. Only ELISA-based PF4/heparin immunoassays are sensitive enough to be incorporated in the diagnostic workup. However, a combination of a positive ELISA and a negative CLIA may be useful to identify VITT antibodies in the absence of confirmatory functional assays.

Introduction/Objectives Extracorporeal membrane oxygenation (ECMO) provides circulatory support in patients with severe heart failure, but the frequent use of unfractionated heparin exposes patients to high risk of heparin-induced thrombocytopenia (HIT). We prospectively evaluated the development and clinical impact of platelet factor 4 (PF4)-specific antibodies (Abs) during ECMO and whether specific biological characteristics could predict HIT. Materials and Methods From 2014 to 2018, we studied 57 adults who underwent an ECMO for at least 5 days. The plasma samples collected daily were tested for PF4-specific Abs using immunoassays to detect immunoglobulin (Ig) G, A, and M isotypes or only IgG. Serotonin release assay was performed without and with PF4 to detect pathogenic Abs. Results Twenty-nine patients (50%) were positive for PF4-specific Abs (IgG, A, M), with IgG in 17/57 (30%) and 16 of them (94%) were immunized within 10 days. PF4-specific IgG Abs did not affect the clinical or biological course of most patients. HIT was suspected in only two patients with ECMO circuit dysfunction and unexpected platelet count decrease after day 5. High levels of PF4-specific IgG were detected in both patients, and HIT was confirmed by a serotonin release assay, which was also more sensitive when exogenous PF4 was present. Conclusion PF4-specific Abs are common during ECMO but are mostly non-pathogenic and not associated with a less favorable prognosis. However, an abnormal platelet count evolution, in particular if associated with ECMO circuit dysfunction, should prompt the search for pathogenic PF4-specific IgG.

Background Heparin-induced thrombocytopenia (HIT) in infants is a rare disorder, and the diagnosis and management of HIT still remains challenging. Argatroban is a synthetic direct thrombin inhibitor (DTI) that is widely used for treating HIT. However, little is known about the efficacy of the activated clotting time (ACT) test in monitoring DTI treatment as an alternative to the routinely used activated partial thromboplastin time (aPTT). Methods Between July 2013 and January 2015, four infants were diagnosed with HIT after surgical correction of congenital anomalies. In all cases, heparin was used during cardiopulmonary bypass (CPB). Diagnosis of HIT was based on the “4 Ts” pretest clinical scoring system, and platelet factor 4 (PF4) antibody was detected using enzyme-linked immunosorbent assay. Argatroban was used in treating HIT. When argatroban was infused, anticoagulation tests (aPTT, prothrombin time [PT], thrombin time [TT], and fibrinogen) were performed every 4 to 12 hours. ACT was used in addition to monitor the anticoagulation effect of argatroban. The target ACT was 1.5 to 3.0 times the baseline. ACT was measured every 2 to 4 hours and remeasured 1 hour after each dosage adjustment. Results Thrombocytopenia (defined as a 50% decrease in platelet count) occurred during the 3rd to 6th day postoperatively. After the diagnosis of HIT, argatroban was started immediately, and platelet counts stabilized and gradually increased. Anticoagulation effect of argatroban was successful monitored by ACT and aPTT. Poor correlation between the ACT test and aPTT test (R = 0.270, p = 0.092) was noted in one patient. ACT values increased rapidly after 3 to 7 days on argatroban treatment. In most cases, low dosage of argatroban was given ranging from 0.04 to 5.00 μg/kg/min. Conclusion Argatroban may be an effective medicine in treating HIT in infants, in a reduced dosage. The great fluctuation in argatroban dosage during the course of HIT treatment necessitates close monitoring. ACT test may be reliable and convenient for monitoring HIT treatment and may contribute to positive clinical outcomes in infants. The efficacy of argatroban and the use of ACT monitoring in the management of HIT infants needs further study.

Mechanisms linking deep vein thrombosis (DVT) and subclinical atherosclerosis and risk of cardiovascular events are poorly understood. The aim of this study was to investigate the potential impact of CX3CR1/CX3CL1 axis in DVT-associated endothelial dysfunction. The study included 22 patients (age: 37.5 ± 8.2 years) with a history of idiopathic DVT and without known cardiovascular risk factors and 23 aged-matched control subjects (age: 34 ± 7.8 years). Flow cytometry was used to evaluate peripheral markers of platelet activation, leukocyte immunophenotypes and CX3CR1/CX3CL1 expression in both groups. A flow chamber assay was employed to measure leukocyte arrest under dynamic conditions. Platelet activation and the percentage of circulating CX3CR1-expressing platelets, CX3CR1-expressing platelet-bound monocytes and CD8+ lymphocytes were higher in patients with DVT than in controls. Additionally, patients with DVT had increased plasma levels of CX3CL1, soluble P-selectin and platelet factor 4/CXCL4. Interestingly, this correlated with enhanced platelet–leukocyte interaction and leukocyte adhesion to TNFα-stimulated arterial endothelial cells, which was partly dependent on endothelial CX3CL1 upregulation and increased CX3CR1 expression on platelets, monocytes and lymphocytes. In conclusion, increased CX3CR1 expression on circulating platelets may constitute a prognostic marker for long-term adverse cardiovascular events in patients with DVT. Blockade of CX3CL1/CX3CR1 axis may represent a new therapeutic strategy for the prevention of cardiovascular comorbidities associated with DVT.

We describe a rare case of autoimmune heparin-induced thrombocytopenia after transcatheter aortic valve implantation in which antibodies against platelet factor 4/heparin have led to platelet activation even after heparin cessation, causing a delayed drop in platelet count to below 20 × 109/L. Most interestingly, platelet count rapidly improved after intravenous immunoglobulin treatment and no new thromboembolic complications were observed with further anticoagulation with rivaroxaban.

SummaryOne of the standard distinctions between type 1 (non-immune) and type 2 (immune-mediated) heparin-induced thrombocytopenia (HIT) is the transience of thrombocytopenia: type 1 HIT is viewed as early-onset and transient thrombocytopenia, with platelet count recovery despite continuing heparin administration. In contrast, type 2 HIT is viewed as later-onset (i. e., 5 days or later) thrombocytopenia in which it is generally believed that platelet count recovery will not occur unless heparin is discontinued. However, older reports of type 2 HIT sometimes did include the unexpected observation that platelet counts could recover despite continued heparin administration, although without information provided regarding changes in HIT antibody levels in association with platelet count recovery. In recent years, some reports of type 2 HIT have confirmed the observation that platelet count recovery can occur despite continuing heparin administration, with serological evidence of waning levels of HIT antibodies (“seroreversion”). We now report two additional patient cases of type 2 HIT with platelet count recovery despite ongoing therapeutic-dose (1 case) or prophylactic-dose (1 case) heparin administration, in which we demonstrate concomitant waning of HIT antibody levels. We further review the literature describing this phenomenon of HIT antibody seroreversion and platelet count recovery despite continuing heparin administration. Our observations add to the concept that HIT represents a remarkably transient immune response, including sometimes even when heparin is continued.

SummaryPlatelets actively participate in inflammatory processes and drive diseases such as atherosclerosis, rheumatoid arthritis and cancer metastasis. However, platelets also have anti-inflammatory and anti-infective properties, which have received less consideration in the past. In this review, we highlight recent findings on the role of platelets in host defense and describe regulatory pathways modulating immuneresponses. Furthermore, we discuss the role of platelets for the resolution of inflammation and tissue repair. These conceptual changes contribute to our understanding of platelet biology in disease.

SummaryThe 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 lifelong, 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.

SummaryThe 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.

SummaryHeparin-induced thrombocytopenia (HIT) is an autoimmune disorder characterised by thrombocytopenia and thrombosis. The mechanisms leading to platelet destruction are complex and the thrombotic complications of HIT appear to be due to multiple different intravascular targets. The dual binding of HIT antibodies to platelet surface PF4/GAG complexes and to FcγRIIA likely leads to both platelet clearance and to their direct activation. Monocytes and endothelial cells bind PF4 with higher avidity than platelets and are more resistant to competitive removal of surface-bound PF4 in the presence of heparin. Binding of HIT antibodies to PF4/glycosaminoglycan complexes on the surface on these cells leads to their activation and increased procoagulant activity. Binding of higher levels of PF4 released from activated platelets to the endothelium may lead to changes of the anticoagulant properties of the glycocalyx and target the endothelial cells for HIT antibodies. Pathogenic antibodies bound to endothelial cells further promote prothrombotic conditions by a mechanism that is independent of FcγR activation, yet not completely understood. A more detailed understanding of the role of monocytes and endothelium may identify new targets for intervention to mitigate the risk of thrombosis with less impact on systemic haemostasis than current approaches to treatment for this serious disorder.

SummaryPlatelet activation and thrombin generation are crucial steps in primary and secondary haemostasis. However, both also play major roles in intravascular thrombus formation and therefore in the development of adverse cardiovascular events. In the current study, we first sought to investigate the associations of the platelet biomarkers platelet factor (PF)-4, thrombospondin (TSP)-1, soluble CD40 ligand (sCD40L), and soluble P-selectin (sP-selectin) with each other and with monocyte-platelet aggregate (MPA) formation in 316 patients undergoing angioplasty and stenting. To better understand the interplay between platelet activation and thrombin generation, we subsequently investigated the associations of the platelet biomarkers with thrombin generation potential. The mostly platelet-specific markers PF-4, TSP-1 and sCD40L correlated strongly with each other (all p < 0.001), and the best correlation was observed between PF-4 and TSP-1 (r=0.91). In contrast, sP-selectin, which derives from platelets and endothelial cells, correlated rather poorly with TSP-1 (r=0.12, p=0.04), and did not correlate with PF-4 and sCD40L. While PF-4, TSP-1 and sP-selectin correlated significantly with in vivo MPA formation (all p < 0.001), no such association was found between sCD40L and MPA formation. PF-4, TSP-1 and sCD40L correlated strongly with peak thrombin generation (all p < 0.001) with the best correlation between PF-4 and peak thrombin generation (r=0.55), whereas sP-selectin did not correlate with peak thrombin generation. Likewise, PF-4, TSP-1 and sCD40L correlated significantly with the area under the thrombin generation curve (AUC; all p< 0.01), whereas sP-selectin did not correlate with the AUC. In conclusion, platelet-specific markers are associated with MPA formation and thrombin generation potential in patients with advanced atherosclerosis.

Bakterielle Pneumonien sind weltweit eine der haufigsten Todesursachen und S. pneumoniae ist das haufigste klinische Isolat. Neutrophile Granulozyten gehoren zur Klasse der myeloiden Zellen und sind eine wichtige Komponente der angeborenen Immunitat gegen bakterielle Infektionen. Kruppel-like factor 4 (KLF4) spielt dabei nicht nur eine Rolle in der Differenzierung der Zellen des Immunsystems, sondern auch wahrend der Infektion bei der Vermittlung inflammatorischer Signale in unterschiedlichen Zelltypen. Diese Studie zeigt zum ersten Mal in vivo, dass myeloides KLF4 Einfluss auf den Krankheitsverlauf hat und die mit einer bakteriellen Pneumonie einhergehende Entzundungsreaktion reguliert. Die hier aufgefuhrten Ergebnisse demonstrieren, dass der Transkriptionsfaktor KLF4 wahrend einer Pneumokokken Pneumonie in humanen und murinen neutrophilen Granulozyten induziert wird. Diese Induktion ist Zeit- und Dosisabhangig. Auserdem wird die Expression von myeloidem KLF4 durch die Autolyse von S. pneumoniae reguliert, aber nicht uber Toll-like Rezeptor 2 (TLR2), TLR4 oder TLR9 vermittelt. Studien in einem Maus-Pneumonie Modell zeigen, dass myeloides KLF4 einen proinflammatorischen Phanotyp bewirkt. Mause mit einem KLF4 knockout (KLF4-/-) in myeloiden Zellen haben im Vergleich zu Wildtyp (KLF4+/+) Mausen eine hohere Bakterienlast in Lunge, Blut und Milz. Obwohl die Produktion proinflammatorischer Zytokine (wie TNF-α, IL-1β und KC) in BALF und Plasma von KLF4-/- Mausen geringer war, gab es keine Unterschiede bei der Zellrekrutierung in der BALF von KLF4-/- und KLF4+/+ Mausen. Allerdings war die Zellrekrutierung im Blut der KLF4-/- Mause geringer als bei den KLF4+/+ Mausen. Auserdem wurde eine erhohte vaskulare Permeabilitat verbunden mit perivaskularen Odemen und Pleuritis bei KLF4-/- Mausen wahrend der S. pneumoniae-induzierten Infektion beobachtet. Diese Mause erreichten auch eher die humanen Endpunkte als die vergleichbaren KLF4+/+ Mause.%%%%Bacterial pneumonia is one of the leading causes of death worldwide. Streptococcus pneumoniae is the most frequently isolated pathogen from clinical pneumonia samples. Neutrophils belong to the class of myeloid cells and forms an important component of this innate immune system against bacterial infections. Kruppel-like factor 4 (KLF4) has been reported to not only play a role in differentiation of cells of the immune system but also in mediating inflammatory signals in different kinds of host cells during infection. This study shows myeloid KLF4 has an impact on pneumococcal pneumonia outcome and regulates the inflammation associated with bacterial pneumonia in vivo in mice. The results presented in the work show that the transcription factor KLF4 is induced in human and mice neutrophils during pneumococcal pneumonia. The induction of KLF4 is time and dose dependent. Additionally, the expression of myeloid KLF4 is regulated by the autolysis of S. pneumoniae but is not mediated via Toll-like receptor (TLR) 2, TLR4 or TLR9. Studies using a mouse pneumonia model showed that myeloid KLF4…

SummaryShort chain polyphosphates (polyP) are pro-coagulant and pro-inflammatory platelet released inorganic polymers. The platelet chemokine platelet factor 4 (PF4) binds to lipid A on bacteria, inducing an antibody mediated host defense mechanism, which can be misdirected against PF4/heparin complexes leading to the adverse drug reaction heparin-induced thrombocytopenia (HIT). Here, we demonstrate that PF4 complex formation with soluble short chain polyP contributes to host defense mechanisms. Circular dichroism spectroscopy and isothermal titration calorimetry revealed that PF4 changed its structure upon binding to polyP in a similar way as seen in PF4/heparin complexes. Consequently, PF4/polyP complexes exposed neoepitopes to which human anti-PF4/heparin antibodies bound. PolyP enhanced binding of PF4 to Escherichia coli, hereby facilitating bacterial opsonisation and, in the presence of human anti-PF4/polyanion antibodies, phagocytosis. Our study indicates a role of polyP in enhancing PF4-mediated defense mechanisms of innate immunity.

SummaryWe report a strategy to generate by electron beam lithography high fidelity micropatterned arrays to assess the interaction of single platelets with immobilised ligands. As a proof-of-principle we functionalised the microarrays with platelet factor 4 (PF4)-heparin-IgG complexes. We embedded biotinylated water-soluble quantum dots into polyethylene glycol (PEG)-coated micropatterned arrays and functionalised them via streptavidin to bind biotinylated ligands, here biotinylated-PF4/heparin complexes. The integrity of the PF4/heparin-complexes was shown by binding of anti-PF4/heparin antibodies. Ligand density was quantified by immunofluorescence and immunogold antibody labelling. Real-time calcium imaging was employed for read-out of single platelets activated on micropatterned surfaces functionalised with PF4/heparin-IgG complexes. With the smallest micropatterns (0.5x0.5 µm) we show that single platelets become strongly activated by binding to surface-immobilised PF4/heparin-IgG, while on larger micropatterns (10x10 µm), platelet aggregates formed. These findings that HIT antibodies can cause platelet activation on microarrays illustrate how this novel method opens new avenues to study platelet function at single cell level. Generating functionalized microarray surfaces to which highly complex ligands can be bound and quantified has the potential for platelet and other cell function assays integrated into high-throughput microfluidic microdevices.

SummaryHeparin-induced thrombocytopenia (HIT) is the most frequent drug-induced immune reaction affecting blood cells. Its antigen is formed when the chemokine platelet factor 4 (PF4) complexes with polyanions. By assessing polyanions of varying length and degree of sulfation using immunoassay and circular dichroism (CD)-spectroscopy, we show that PF4 structural changes resulting in antiparallel β-sheet content >30% make PF4/polyanion complexes antigenic. Further, we found that polyphosphates (polyP-55) induce antigenic changes on PF4, whereas fondaparinux does not. We provide a model suggesting that conformational changes exposing antigens on PF4/polyanion complexes occur in the hairpin involving AA 32–38, which form together with C-terminal AA (66–70) of the adjacent PF4 monomer a continuous patch on the PF4 tetramer surface, explaining why only tetrameric PF4 molecules express “HIT antigens”. The correlation of antibody binding in immunoassays with PF4 structural changes provides the intriguing possibility that CD-spectroscopy could become the first antibody-independent, in vitro method to predict potential immunogenicity of drugs. CD-spectroscopy could identify compounds during preclinical drug development that induce PF4 structural changes correlated with antigenicity. The clinical relevance can then be specifically addressed during clinical trials. Whether these findings can be transferred to other endogenous proteins requires further studies.

SummaryA new ELISA (Zymutest HIA®), based on incubation of diluted plasma with protamine/heparin (PRT/H) complexes without and with platelet factor 4 (PF4) provided by a platelet lysate, was used to detect heparindependent antibodies in a cohort of 232 cardiac surgery (CS) patients and in 47 patients with heparin-induced thrombocytopenia (HIT). Significant binding of IgG/A/M to PRT/H complexes was demonstrated in 59 CS patients (25.4%), with similar absorbances whether platelet lysate was added to the plasma or not, and significant reactivity to PF4/H in 29 of them. Antibodies to PRT or heparin alone were present in 15 and two of these patients, respectively. Importantly, antibodies to PRT/H were detected in only three of the 47 HIT patients, who had also undergone recent CS. The Zymutest HIA® was positive in another 41 CS patients (17%), but only or mainly when their plasma was tested with platelet lysate, with significant levels of antibodies to PF4/H in 40 of them without detectable reactivity to PRT or heparin alone. Slight antibody binding to PRT/H complexes was also measured in six of these 41 patients. Therefore, a total of 35 CS patients exhibited dual antibody reactivity towards PRT/H and PF4/H complexes. Serotonin release assay performed with PRT alone was positive in 17 CS patients with antibodies to PRT/H, but all had normal platelet count evolution without thrombosis postoperatively. In conclusion, antibodies to PRT/H are frequently present in CS patients postoperatively (25.4%), and can activate platelets in vitro, but their clinical impact remains questionable.

SummaryRAD001 is currently used as an immunosuppressant and anticancer drug. Megakaryocyte (MK) differentiation includes development from pluripotent stem cells to proliferation and differentiation toward MK formation and platelet maturation. Our preliminary assay showed that RAD001 might stimulate MK differentiation; however, the exact regulatory mechanisms needed to be elucidated. By the ex vivo assay, RAD001 induced MK differentiation in human haematopoietic stem cells, with both the stimulation of CFU-GM colony formation and CD61 surface marker expression. Then, BALB/c mice were orally administrated with or without agrylin and/or RAD001 for 15 days. The platelet count and bone marrow CFU-MK colony formation were eliminated by agrylin, but unchanged in RAD001 and RAD001 plus agrylin mice. An ex vivo assay of bone marrow-derived stem cells demonstrated that RAD001 increased the number of CFU-MK colonies. The MK count in bone section indicated the decreased effect by agrylin and then recovered by RAD001. The level of plasma thrombopoietin was also enhanced in RAD001-treated mice. The effect of RAD001 on human leukaemic K562 and HEL cells showed the growth inhibition and MK differentiation activities; including morphological observation, CD41 and CD61 expression, and platelet factor 4 secretion. In RAD001-treated HEL cells, p-STAT3 expression, STAT3 translocation, and STAT3-DNA binding activity were up-regulated. Furthermore, STAT3 siRNA decreased the p-STAT3 and CD61 expression, as well as the CD61 fluorescence intensity, indicating that STAT3 may be critical in RAD001-mediated MK differentiation. Conclusion, the present study demonstrated that RAD001 might have the capacity to induce MK differentiation through the up-regulation of STAT3 signalling.

SummaryHeparin-induced thrombocytopenia (HIT) remains a challenge, with diagnosis confirmed only by functional assays. The gold standard 14C-se-rotonin release assay (SRA) is highly sensitive but technically challenging and unsuitable for routine use. We conducted a large study to validate whole blood impedance aggregometry (WBIA) as a suitable diagnostic tool for HIT. WBIA and SRA were used to test 181 samples positive for H-PF4 antibodies by PaGIA or ELISA. Using the same high responder donor, 77 samples were positive by WBIA (aggregation with low-dose but not high-dose heparin). Using the strict definition for SRA positivity, 72 samples were true HIT. In nine samples, serotonin release with high-dose heparin dropped by > 50% but was still >20%; these were retested after a one-half dilution and 8/9 became positive. Ten other samples were discrepant between the two assays: one strongly positive (89% release) and six weakly positive samples by SRA (average release 56%) were WBIA negative. When these samples were retested using a random donor, only two remained SRA positive. Three samples were strongly WBIA positive but SRA negative; two were retested by SRA with 0.5IU/ml heparin and one became positive. Under controlled conditions, using the same selected high-responder donor, WBIA and SRA performed similarly with slightly increased sensitivity of the WBIA when using the strict definition of SRA positivity. WBIA is easy to perform with rapid turn-around time and warrants a multi-laboratory trial to complete its validation as a confirmatory assay for platelet-activating HIT antibodies.

SummaryIt was the objective of this study to obtain best estimates of the prevalence of anti-PF4/heparin antibodies in patients not suspected to have clinical heparin-induced thrombocytopenia (HIT) prior to undergoing cardiac surgery and to determine whether preoperative antibody status and antibody class is predictive of postoperative thromboembolic outcomes, non-thromboembolic outcomes, length of stay, and mortality. PubMed and EMBASE online databases were searched up to July 2011, and we included studies involving adults undergoing cardiac surgery examining the relationship between preoperative anti-PF4/heparin antibodies (ELISA) and postoperative clinical outcomes. Five studies involving a combined total of 2,332 patients met our inclusion criteria. Preoperative anti-PF4/heparin antibodies were detected in 5–22% of patients. No study demonstrated an association between preoperative anti-PF4/heparin antibodies and postoperative thromboembolic outcomes or mortality. Three studies demonstrated a statistically significant association between preoperative anti-PF4/heparin antibodies and length of stay while two showed an association with non-thromboembolic complications. In the one study that examined outcomes by anti-PF4/heparin antibody class, IgM antibodies predicted non-thromboembolic complications and length-of-stay. None of the studies reported prior heparin exposure, and most studies did not examine the relationship of the absolute value of antibody titres (ELISA OD) and risk, nor the incidence of true/clinical HIT in preoperative positive or negative patients. In conclusion, pre-formed anti-PF4/heparin antibodies are common in patients undergoing cardiac surgery, but the available literature does not support that they predict postoperative thromboembolic complications or death. There does appear to be an association between anti-PF4/heparin antibodies and non-thromboembolic adverse events, but a causal relationship is unlikely.

The relationship between high triglyceride values and alterations of the fibrinolytic system in cardiovascular heart disease (CHD) patients is well known. The aim of the study was to evaluate the effect of moderate hypertriglyceridemia on fibrinolysis, lipoprotein(a) [(Lp(a)], and platelet activation indexes in asymptomatic patients. To this end, 46 nondiabetic, dyslipidemic patients (age 51±8 years), with no associated pathologies and 23 normolipidemic, homogeneus patients (in terms of age, sex, and smoke habits) were studied. The dyslipidemic patients were split up into two groups: 23 type IIb hyperlipoproteinemia patients (Group I), and 23 type IIa hyperlipoproteinemia patients (Group 2). The control group had total cholesterol values of

The authors studied changes in platelets and oxygen radicals after arterial reconstruction or hyperbaric oxygenation (HBO) in 41 patients with chronic arterial occlusion of the extremities who were treated in their department. The patients ranged in age from twenty-three to seventy-nine years, with a mean of 61.3 years. Twenty-nine patients had arteriosclerosis obliterans, and 12 had Buerger’s disease. Arterial reconstruction was performed in 29, and HBO was used in 12. The parameters measured were platelet count, β-thromboglobulin (βTG), platelet factor 4 (PF4), white blood cell (WBC) count lipid peroxide, and superoxide dismutase (SOD). Blood samples were collected from the cubital vein of the arm without arterial occlusion. Determinations were made before surgery, three days after surgery, and at discharge from hospital in the group treated by arterial reconstruction; and before HBO, immediately after the first HBO, and on the day after the final HBO in the group treated by HBO. On admission, the patient’s mean levels of βTG and PF4 were as high as 67.8 ng/mL and 28.7 ng/mL, respectively. In the arterial reconstruction group, PF4 was significantly lower and WBC count was significantly higher three days after surgery as compared with those before arterial reconstruction. The platelet count was significantly higher and PF4 was significantly lower at discharge as compared with those before reconstruction. On the other hand, there were no HBO-related changes in platelet count, βTG, PF4, WBC count, lipid peroxide, or SOD. Platelet activation in patients with chronic arterial occlusion was suppressed by arterial reconstruction.

The aim of the study was to evaluate the direct influence of cholesterol and triglycerides on the rheologic-coagulative pattern. Blood rheologic properties (blood plasmatic and seric viscosity, whole blood and red cell filterability), some coagulative factors (fibrinogen, levels of clotting factors VII and VIII activity), and beta-thromboglobulin and platelet factor 4 were studied in 156 men aged 40–54 years, 87 with type II hyperlipoproteinemia (46 type IIa and 41 type IIb) and 69 normolipemic controls. Smokers and patients with arterial hypertension, diabetes mellitus, or cardiovascular clinical manifestations were excluded. Increased blood viscosity (shear rate 225 sec-1,p

SummaryThe poor accuracy of the enzyme immune assay (EIA) contributes to the diagnostic challenge of heparin-induced thrombocytopenia (HIT) following cardiac surgery. We sought to determine if adjusting the threshold optical density (OD) defining a positive EIA improves the test's accuracy in subjects with an OD>0.40. We retrospectively analysed the results from both EIA and confirmatory serotonin release assays (SRAs) in cardiac surgery patients with EIA OD of >0.4. Employing the SRA as the standard, we compared the area under the receiver-operating characteristic (AUROC) curves of various OD measurements for identifying HIT. We examined baseline clinical variables associated with a positive SRA in the setting of a positive HIT EIA (OD >0.4). We then used logistic regression to identify baseline clinical variables independently associated with a positive SRA given a positive EIA. The cohort included 99 subjects with positive EIAs and 35% had positive SRAs. An OD>0.40 had moderate utility as a screening test for a positive SRA (AUROC: 0.68; 95% CI: 0.55–0.80). Increasing the OD threshold did not improve the HIT EIA's screening utility. Clinical variables independently associated with a positive SRA if the EIA were positive included female gender, absence of diabetes, and use of cardiopulmonary bypass. A relatively modest elevation in the OD measurement, when it is already known to be greater than 0.4, does not reliably exclude the potential for a positive SRA in this setting.

SummaryNearly 15% of patients with non-valvular atrial fibrillation (NVAF) have left atrial appendage thrombus (LAAT) by transesophageal echocardiography (TEE) and yet the annual stroke rate averages 5%. The aim of this study was to identify variables influencing embolic propensity of LAAT. Platelet RNA was extracted from platelet-rich regions within formalin- fixed, paraffin-embedded specimens obtained from NVAF patients during cardiac surgery (26 LAAT from 23 patients) or peripheral embolectomy (51 thrombi from 41 patients). Platelet RNA was also assessed from whole blood from 40 NVAF patients. Expression of six platelet- predominate genes: H2A histone family, A1 domain of factor XIII, integrin α2bβ3; glycoprotein IX, platelet factor 4, glycoprotein Ib, was performed using TaqMan MGB-probe based quantitative real-time polymerase chain reaction. Platelet factor XIII subunit A gene expression was significantly lower in embolised compared to non-embolised thrombi as determined by normalised cycle threshold values (4.0 ± 1.2 v 2.8 ± 1.8, p=0.02). Expression of other genes did not differ by embolic status. In conclusion, RNA extracted from formalin-fixed, paraffin-embedded platelet-rich tissues can be used for analysis of platelet- predominate gene expression. Variable factor XIII gene expression in thrombi generated during NVAF may in part explain the propensity to embolisation.

Atherosclerosis is not a risk factor for anti-platelet factor 4/heparin antibody formation after cardiopulmonary bypass surgery

SummaryHeparin-induced thrombocytopenia (HIT) is a life-threatening complication of heparin treatment; the prognosis depends on early and accurate diagnosis, and prompt start of alternative anticoagulants. Because of high sensitivity, the commercially available immunologic assays are widely used, though not suited to be run on single samples and with a turnaround time of 2–3 hours. We evaluated two new, rapid, automated, semi-quantitative chemiluminescent immunoassays in HIT suspected patients: HemosIL® AcuStar HIT-IgG(PF4-H) (specific for IgG anti- PF4/heparin antibodies) and HemosIL® AcuStar HIT-Ab(PF4-H) (detecting IgG, IgM and IgA anti-PF4/heparin antibodies) (both from Instrumentation Laboratory). A total of 102 patients with suspected HIT were included; HIT was diagnosed in 17 (16.7%). No false negative cases were observed using either the HemosIL AcuStar HIT-IgG(PF4-H) or the HITAb(PF4-H) assay (sensitivity and negative predictive values = 100%; negative likelihood ratios

Contribution of platelet microparticles generation assay to the diagnosis of type II heparin-induced thrombocytopenia

SummaryThe diagnosis of heparin-induced thrombocytopenia (HIT) is based on clinical criteria and biological assays. Most immunoassays detect antibodies (either IgG alone or additionally IgA and IgM) against PF4 immobilised in wells of microtiter plates with stoichiometric concentrations of polyanion (heparin or polyvinylsulfonate). We studied whether diagnostic sensitivity and/or specificity for HIT could be improved using a novel assay in which unfractionated heparin is immobilised alone to the microwells, with PF4 (and, potentially, other heparin-dependent antigen proteins) provided by adding platelet lysate during the procedure. Samples from 101 patients with suspected HIT and from 101 controls (including 50 with antiphospholipid antibodies) were tested. The global assay (Zymutest HIA IgG/A/M®, Hyphen BioMed) was positive for 39 of 40 patients with definite HIT (positive PF4-specific ELISA and positive serotonin release assay). It was positive in only two of the 101 control patients studied and also in 14 of the 61 patients with suspected HIT for whom the disease was excluded (specificity (sp): 77%). On the other hand, Zymutest HIA IgG®, an IgG-specific assay, was positive in only six patients without HIT (Sp: 90%). Heparin-dependent IgG antibodies were present at higher levels in patients with definite HIT than in those for whom the diagnosis of HIT was ruled out. A single ELISA that detects IgG antibodies is more effective for the diagnosis of HIT in clinical practice. These results also support the hypothesis that heparin-dependent antibodies of IgG class have a major role in the pathogenesis of HIT.

Heparin is the second most widely used anticoagulant/antithrombotic agent besides warfarin and is commonly used for various purposes such as treatment and surgical indications. A significant adverse effect of heparin treatment can occur when heparin binds platelet factor 4 (H-PF4) to form a complex that results in formation of H-PF4 antibodies, which in turn leads to platelet/endothelial cell activation followed by heparin-induced thrombocytopenia (HIT). Based on the heparin-induced platelet aggregation and enzyme-linked immunosorbent assay tests, the H-PF4 antibody (HIT antibody) was diagnosed in 6% of Indian patients undergoing cardiovascular surgery who received unfractionated heparin, but the frequency of occurrence rose to 15% when the patients were tested with the (14)C-serotonin release assay. This highlights some methodological variations in the diagnosis of HIT antibodies. It was also found that all the HIT-positive patients were either homozygous or heterozygous for the FcgammaRIIa polymorphism, which also highlights the role of this polymorphism in the occurrence of HIT. Very recent studies show that increases in HIT antibody production may also be due to heparin contaminants. Although these antibodies do not result in thrombocytopenia, contaminants in heparin may be capable of triggering a differential immunogenic response in comparison with contaminant-free heparin. Here we discuss the methodological differences in diagnosing HIT, the potential impact of contaminants in heparin, as well as future considerations.

Heparin-induced thrombocytopenia (HIT), sometimes complicated by the occurrence of thrombosis (HITT), is a rare but severe complication of heparin therapy (both unfractionated and low molecular weight heparin). It is induced by the generation of antibodies targeted to complexes of platelet factor (PF) 4 and heparin (H), mainly IgG isotypes with the highest avidity. Laboratory studies and clinical surveys help elucidate the mechanisms of HIT/HITT. The presence of stoichiometric complexes of H-PF4 is probably the immunogenic stimulus that induces the generation of antibodies, via a T-cell response. In pathologies, where a large extent of platelet activation occurs, especially at the vicinity of pathological sites, large amounts of H-PF4 complexes can be formed that bind to platelet surfaces (mainly activated platelets), but also to endothelial cells and other blood cells such as monocytes. This induces cell-cell interactions and the release of microparticles, which can amplify to a hypercoagulable state resulting from release of tissue factor, microparticles, and expression of procoagulant phospholipids. The clinical consequence is the development of thrombocytopenia, which can be complicated by a rapid evolution to thrombosis that becomes life threatening. The present understanding of the mechanisms of HIT/HITT, the advances in clinical investigations, and the availability of alternative anticoagulants have progressively introduced new tools for a better diagnosis and management of patients with this disease.

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.

SummaryActivated platelets, which release platelet factor 4 (PF4) are present in patients with atherosclerosis. To date, no direct invivo evidence exists for the involvement of PF4 in atherogenesis. In the current study, we tested the hypothesis that PF4 is atherogenic, and that genetic elimination of PF4 would protect mice from atherosclerosis. We have bred PF4-/- mice onto two athero-susceptible backgrounds, WT-C57Bl/6(WT) and apoE-/- to examine the importance of PF4 in atherogenesis. In order to induce atherosclerosis, WT and PF4-/- mice were fed an atherogenic diet for 30 weeks, while apoE-/- and apoE-/- PF4-/- mice were fed a high-fat Western-style diet for 10 weeks. Examination of lesions in the aortic roots of atherogenic diet fed mice demonstrated reduced atherosclerosis in PF4-/- (20% compared to WT). Examination of apoE-/- mice demonstrated similar changes, with apoE-/- PF4-/- mice demonstrating 37% of the aortic atherosclerotic burden compared to apoE-/- mice. Although we found similar levels of total and non-HDL cholesterol inWT and PF4-/- mice, HDL-cholesterol levels were increased in PF4-/- on both backgrounds. These data demonstrate, for the first time, that the platelet specific chemokine PF4 promotes atherosclerotic lesion development in vivo.

The Ca(2+)-dependence of protein C activation by thrombin in complex with thrombomodulin (TM) containing chondroitin sulfate (CS) exhibits saturation at approximately 0.5-1 mM Ca(2+), but with TM lacking CS, it has a distinct optimum at approximately 0.1 mM Ca(2+). Since the substrate protein C has multiple Ca(2+)-binding sites, and the cofactor TM also interacts with Ca(2+), the basis for differences in Ca(2+) effect on protein C activation by thrombin in complex with TM containing or lacking CS is not known. In this study, by using full-length and Gla-domainless mutants of protein C whose activation by thrombin is independent of either Ca(2+) or both Ca(2+) and TM, we demonstrate that i) the Ca(2+) occupancy of a high-affinity binding site in TM is essential for the high-affinity interaction of the cofactor with thrombin, ii) the Ca(2+) occupancy of a binding site (K(D) approximately 50 microM) in the catalytic domain of protein C is required for the substrate recognition by the thrombin-TM complex, however, at this concentration of Ca(2+) the Gla domain of protein C is not folded properly and thus interacts with exosite-2 of thrombin in complex with TM that lacks CS but not with TM that contains CS, and finally iii) platelet factor 4 can nonspecifically interact with the Gla domain of protein C and other coagulation factors to influence their activation only at subphysiological concentrations of Ca(2+).

Patients with essential thrombocythemia (ET) and polycythemia vera (PV), complicated by microvascular ischemic or thrombotic events, have shortened platelet survival, increased β-thromboglobulin, platelet factor 4, and thrombomodulin levels, and increased urinary thromboxane B2 excretion. These are all reversible by inhibition of platelet cyclooxygenase 1 with aspirin, and are therefore indicative of platelet activation and platelet-mediated thrombotic processes. The thrombotic tendency persists as long as platelet counts are above the upper limit of normal (400 × 109/L). Despite strong evidence of in vivo platelet activation, the ex vivo platelet function tests are impaired. Platelet dysfunction in ET and PV typically is characterized by a missing second-wave adrenaline aggregation, an increased adenosine diphosphate aggregation threshold, and reduced secretion products, but a normal arachidonic acid or collagen-induced aggregation. The proposed concept is that platelets in thrombocythemia (ET and PV) are hypersensitive. Due to the existing high shear stress in the microvasculature (end-arterial circulation), platelets spontaneously activate, secrete their products, form aggregates mediated by von Willebrand factor (vWF) that transiently plug the microcirculation, deaggregate, and then recirculate as exhausted defective platelets with secondary storage pool disease on ex vivo analysis. At increasing platelet counts from below to above 1000 × 109/L, the thrombotic condition changes into an overt spontaneous bleeding tendency as a result of a functional vWF deficiency that is caused by proteolysis of large vWF multimers. This is consistent with acquired type 2 von Willebrand syndrome (AvWS). AvWS is reversible by reduction of the platelet count to normal. The acquired JAK2 V617F gain of function mutation is the cause of trilinear myeloproliferative disease with the sequential occurrence of ET and PV. Heterozygous JAK2 V617F mutation with slightly increased kinase activity is enough for the induction of spontaneous megakaryopoiesis and erythropoiesis, and an increase of hypersensitive platelets is the cause of aspirin-sensitive, platelet-mediated microvascular ischemic and thrombotic complications in ET and early PV mimicking ET. Homozygous JAK2 mutation with pronounced increase of kinase activity is associated with pronounced trilinear megakaryocyte, erythroid, and granulocytic myeloproliferation, with the most frequent clinical picture of classical PV complicated by major thrombosis, in addition to the platelet-mediated microvascular thrombotic syndrome of thrombocythemia. Copyright

SummaryPlatelet recruitment crucially depends on amplification systems provided by autocrine and paracrine factors such as adenosine diphosphate. In inflammatory states, consumption of coagulation proteins, such as antithrombin aggravates the procoagulant state. In this study, we report that platelets express synde-can-4, an antithrombin-binding cell surface heparan sulphate proteoglycan, whose ligation with antithrombin inhibits activated platelet-dependent superoxide anion release from neut-rophils by the limitation of adenosine diphosphate and adeno-sine triphosphate secretion in activated platelets. Adenosine triphosphate-induced platelet aggregation is reduced after treatment of platelets with antithrombin, which is reversed by blockade of syndecan-4. We further observed that antithrombin limits CD40 ligand expression in adenosine diphosphate-activated platelets and inhibits the shedding of syndecan-4 from activated platelets. Syndecan-4 appears to be directly involved in regulating platelet aggregation as anti-syndecan-4 antibody augments platelet aggregation. We suggest that antithrombin might exert beneficial effects in disseminated intravascular coagulation by reducing platelet activation, observed as inhibited CD40 ligand expression, syndecan-4 shedding, and adenosine diphosphate- and adenosine triphosphate-release from activated platelets with subsequent inhibition of neutrophil respiratory burst. From these data it is concluded that syndecan-4 may play important roles in the regulation of inflammatory effects of platelets.

SummaryInappropriate platelet activation is a feature of acute and chronic diseases such as disseminated intravascular coagulation (DIC) and atherosclerosis. Since proinflammatory microbial-derived agonists can be involved in the pathogenesis of these diseases, we examined the potential role ofTLR4 (mediating responses to LPS) andTLR2 (which responds to bacterial lipopeptides) in platelet activation. Our data suggested low-level expression of TLR2 andTLR4 on platelets, determined by flow cytometry, and we also observed expression of TLR4 on a megakaryocytic cell line by both flow cytometry and immunohistochemistry. Stimulation of the platelets with the TLR4 agonist LPS, and the synthetic TLR2 agonist Pam3CSK4, resulted in no platelet aggregation, no increase in CD62P surface expression and no increase in the cytosolic concentration of Ca2+. The TLR agonists were also unable to directly activate platelets primed with epinephrine, or pretreated with a low concentration ofADP or PAF. Pretreatment of platelets with LPS or Pam3CSK4 also failed to modulate the platelet response to submaximal concentrations of the classical platelet agonists ADP and PAF. We conclude that theTLR agonists LPS and Pam3CSK4 have no direct effect on platelet activation and that platelet TLRs may be a remnant from megakaryocytes. TLR2 and TLR4 agonists are thought to have a significant role in diseases such as atherosclerosis and DIC, but our research suggests that this is through a mechanism other than direct platelet activation or by modification of platelet responses to other agonists.

SummaryPlatelets and neutrophils constitute a high source of metalloproteinases (MMPs), and their interactions via P-selectin and Pselectin- glycoprotein-ligand-1 (PSGL-1) are involved in thrombosis, vascular remodelling, and restenosis. We investigated the impact of these interactions on platelet MMP-2 secretion and function in platelet and neutrophil aggregation. The secretion of MMP-2 from human platelets was significantly increased threefold after thrombin activation, and enhanced two-fold in the presence of neutrophils. Neutrophil supernatant had no effect on platelet MMP-2 secretion. While no MMP-2 was detected in the supernatant of neutrophils, a high amount of MMP-9 was released by neutrophils, and remained unchanged upon thrombin activation or in the presence of platelets. Platelet P-selectin, which increased significantly after activation, triggered platelet binding to neutrophils that was completely inhibited by P-selectin or PSGL-1 antagonists, and was reduced by 50% with a GPIIb/ IIIa antagonist. P-selectin or PSGL-1 antagonism abolished the enhanced secretion of platelet MMP-2 in the presence of neutrophils and reduced platelet-neutrophil aggregation. Platelet activation and binding to neutrophils enhance the secretion of platelet MMP-2 via an adhesive interaction between P-selectin and PSGL-1, which contribute to increase platelet-neutrophil aggregation.

Atheroprotective Kruppel-like factor 4 is downregulated in monocyte subsets of patients with coronary artery disease

The biochemical mechanisms by which activated platelets participate in exposing receptors for the assembly of enzyme-cofactor-substrate complexes at all stages of the blood coagulation cascade are reviewed. Information derived from studies conducted during the last 30 years supports the concept that the initiation of blood coagulation is triggered by exposure of tissue factor at injury sites, leading to the generation of minute quantities of thrombin (limited by tissue factor pathway inhibitor), sufficient to activate platelets, factors XI, VIII, and V, and trigger the consolidation pathway (i.e., the sequential activation of factors XI, IX, X, and prothrombin on the activated platelet surface), leading to the generation of sufficient thrombin to convert fibrinogen to fibrin and effect hemostasis. Platelets localize coagulation to the hemostatic thrombus and protect coagulation enzymes from inhibition by both plasma and platelet inhibitors (e.g., protease nexin 2), thus preventing disseminated intravascular coagulation.

Platelet factor 4 (PF4) is a platelet-specific protein that is stored in platelet alpha granules and released following platelet activation. PF4 was the first chemokine that was isolated, but unlike other chemokines, it may not have a clear role in inflammation. Gathering evidence suggests that unlike other chemokines that bind to specific receptors, PF4's biology depends on its unusually high affinity for heparan sulfates and other negatively charged molecules at concentrations attained in the immediate vicinity of activated platelets. There has been one report that PF4 binds to CXCR3B, a chemokine receptor isoform that may be present in some vascular beds, but the biological relevance of this single observation is not clear. We propose that the main biological role of PF4 and the basis for its presence in the alpha granules of all known mammalian platelets is to neutralize surface heparan sulfate side-chains of glycosaminoglycans and to optimize thrombus development at sites of vascular injury. In addition, the binding of PF4 to surface glycosaminoglycans may also underlie its angiostatic and proatherogenic properties. Additionally, PF4 binds to several other proteins that are central to thrombosis, angiogenesis, and atherogenesis. These interactions may also contribute to its biological and pathobiological effects. Certainly, future studies using in vivo models to test biological relevance of each of these proposed mechanisms by which PF4 interacts with the vasculature are needed, as are studies to define the importance of PF4 binding to CXCR3B.

We postulated that patients with recent acute coronary syndromes and antibodies to the platelet factor 4/heparin complex would have an increased risk of myocardial infarction (MI), even in the absence of thrombocytopenia. We analyzed sera from patients enrolled in the placebo/unfractionated heparin arm of the GUSTO IV-ACS trial who had a high likelihood of prior heparin exposure. We selected 109 patients without thrombocytopenia with the 30-day primary endpoint (death, MI, or revascularization) and 109 age-, gender-, and race-matched controls who did not achieve the primary endpoint. Twenty-three of 218 patients (10.6%) had anti-PF4/heparin antibodies. Patients with anti-PF4/heparin were more likely to have death or MI (30.4% vs. 11.3%, p = 0.011) or MI (21.7% vs. 6.2%, p = 0.008) than patients who were negative for the antibody. Antibody-positive patients had higher levels of sVCAM-1 (892 +/- 263 microg/L vs. 780 +/- 228 microg/L; p = 0.04) and sICAM-1 (246 +/- 50 microg/L vs. 222 +/- 71 microg/L; p = 0.02) than antibody-negative patients. In a multiple logistic regression model that included inflammatory markers and clinical risk factors, antibodies to PF4/heparin were a strong predictor of 30-day MI (odds ratio, 9.0; 95% confidence intervals, 2.1 to 38.6; p < 0.01), with IL-6 being the only other predictor (odds ratio, 1.1; 95% confidence intervals, 1.0 to 1.2; p = 0.03). Antibodies to the platelet factor 4/heparin complex are a novel, independent predictor of MI at 30 days in patients presenting with acute coronary ischemic syndromes. Antibodies to PF4/heparin are a stronger predictor of MI than clinical characteristics or inflammation markers.

Platelet factor 4 (PF4) is an antiangiogenic ELR-negative chemokine. PF4 inhibits endothelial cell proliferation and migration and angiogenesis in vitro and in vivo. Three different mechanisms have been proposed to explain PF4's antiangiogenic effects. First, PF4 may bind proteoglycans and interfere with the proteoglycan-bystander effect on growth factor activity. Second, PF4 is able to interact directly with angiogenesis growth factors such as fibroblast growth factors or vascular endothelial growth factors and inhibits their interaction with cell surface receptors. Third, PF4 may activate cell surface receptors on endothelial cells and induce inhibitory signals. Recently, one such receptor, CXCR3-B, was identified. In cardiovascular disease, PF4 may possibly intervene in collateral vessel formation, plaque neovascularization, heparin-induced thrombocytopenia and stent endothelialization. Several PF4 fragments such as PF4-CTF and modified molecules have been made that exhibit antiangiogenesis properties and may serve as leads for further therapeutic development.

Heparin-induced thrombocytopenia (HIT) is a potentially catastrophic complication of heparin therapy. The syndrome is the result of the production of an antibody to the complex that forms between heparin and platelet factor 4 (H-PF4) released from activated platelets. IgG antibodies bind to platelet Fc receptors and cause platelet activation, resulting in thrombocytopenia and greatly increased risk of thrombosis. Tests for the H-PF4 antibody can be classified into functional assays (which rely on the demonstration of platelet activation) and immunoassays (which detect the presence of an antibody without regard for its functional ability). The functional assays have a greater specificity for clinical HIT, but require normal donor platelets and are relatively un-standardized. The immunoassays have the advantage of better standardization and do not require normal platelets, but may give positive results in the absence of clinical HIT. The choice of test is usually dictated by what is possible for a given laboratory in terms of instrumentation, expertise, and interest. For most institutions this will be a commercially available enzyme-linked immunosorbent assay. Although HIT is a true clinicopathological syndrome, there are several reasons why its diagnosis still rests primarily on clinical grounds. First, laboratory tests may not be available locally. Second, they may not be available in a sufficiently timely manner. Finally, available tests are not completely sensitive or specific for the condition. There is, therefore, a continuing need to develop more rapid testing strategies with greater specificity for clinical HIT.