Differential inhibition of adenosine diphosphate- versus thrombin receptor-activating peptide-stimulated platelet fibrinogen binding by abciximab due to different glycoprotein IIb/IIIa activation kinetics

Activated thrombin-activatable fibrinolysis inhibitor (TAFIa) attenuates fibrin-dependent plasmin generation on thrombin-activated platelets

BACKGROUND

Thrombin-activated platelets can promote fibrinolysis by binding plasminogen in a fibrinogen-dependent manner and enhancing its activation by tissue-type plasminogen activator (t-PA). Whether t-PA also binds to activated platelets and the mechanism for regulation of platelet-dependent fibrinolysis remain unknown.

OBJECTIVES

Determine the mechanism of plasminogen and t-PA binding on thrombin-activated platelets and its regulation by activated thrombin-activatable fibrinolysis inhibitor (TAFIa).

METHODS

Plasminogen and t-PA binding with or without TAFIa treatment was quantified using flow cytometry. Plasmin generation on platelets was quantified using a plasmin-specific substrate. Mass spectrometry analyses identified fibrinogen as a potential target of TAFIa. Thrombus formation was studied in mice lacking fibrinogen (Fg-/- ) using intravital microscopy.

RESULTS

Plasminogen and t-PA bind to platelets activated by thrombin but not by other agonists, including protease-activated receptor agonists (PAR-AP). Plasminogen binds via its kringle domains because ε-aminocaproic acid eliminates binding, whereas t-PA binds via its finger and kringle domains. Plasminogen binding is fibrinogen-dependent because it is abolished on (a) Fg-/- platelets, and (b) thrombi in Fg-/- mice. Binding requires thrombin-mediated fibrinogen modification because addition of batroxobin to PAR-AP activated platelets has no effect on plasminogen binding but induces t-PA binding. TAFIa reduces plasminogen and t-PA binding to thrombin-activated platelets and attenuates plasmin generation in a concentration-dependent manner. Mass spectrometry identified K556 on the fibrinogen alpha-chain as a potential thrombin cleavage site that generates a TAFIa sensitive C-terminal lysine residue.

CONCLUSION

These findings provide novel mechanistic insights into how platelets activated by thrombin at sites of vascular injury can influence fibrinolysis.

How to improve the concept of individualised antiplatelet therapy with P2Y12 receptor inhibitors – is an algorithm the answer?

Within the past decade, high on-treatment platelet reactivity (HTPR) on clopidogrel and its clinical implications have been frequently discussed. Although it has been previously assumed that HTPR is a phenomenon occurring only in patients treated with clopidogrel, recent data show that HTPR might also occur during treatment with prasugrel or ticagrelor in the acute phase of ST-elevation myocardial infarction. Moreover, it has been postulated that there is a therapeutic window for P2Y12 receptor blockers, thus indicating that HTPR is associated with thrombotic events whereas low on-treatment platelet reactivity (LTPR) is associated with bleeding events. The current paper focuses on tools to identify risk factors for HTPR (pharmacogenomic testing, clinical scoring and drug-drug interactions) and on the use of platelet function testing in order to identify patients who might not respond adequately to clopidogrel. The majority of recent clinical randomised trials have not supported the hypothesis that platelet function testing and tailored antiplatelet therapy are providing a favourable clinical outcome. These trials, mainly performed in low-to-moderate risk patients, will be reviewed and discussed. Finally, an algorithm based on current knowledge is suggested, which might be of use for design of clinical trials.

Measurement of platelet aggregation, independently of patient platelet count: a flow-cytometric approach

Essentials Platelet function may influence bleeding risk in thrombocytopenia, but useful tests are needed. A flow cytometric platelet aggregation test independent of the patient platelet count was made. Platelet aggregation was reduced in thrombocytopenic patients with hematological cancer. High platelet aggregation ruled out bleeding tendency in thrombocytopenic patients.

SUMMARY

Background Methods for testing platelet aggregation in thrombocytopenia are lacking. Objective To establish a flow-cytometric test of in vitro platelet aggregation independently of the patient's platelet count, and examine the association of aggregation with a bleeding history in thrombocytopenic patients. Patients/methods We established a flow-cytometric assay of platelet aggregation, and measured samples from healthy individuals preincubated with antiplatelet drugs, and samples from two patients with inherited platelet disorders. Then, we included 19 healthy individuals and 20 patients with platelet counts of ≤ 50 × 109 L-1 , diagnosed with acute myeloid leukemia or myelodysplastic syndrome. We measured platelet aggregation and platelet activation by platelet surface expression of activated glycoprotein IIb-IIIa, P-selectin and CD63 after addition of agonists: collagen-related peptide, thrombin receptor-activating peptide (TRAP), and ADP. Results The platelet aggregation assay showed a low intraserial coefficient of variation of ≤ 3%. Similar results were obtained for platelet-rich plasma and isolated platelets at platelet counts of > 10 × 109 L-1 ; otherwise, platelet isolation was required. The platelet aggregation percentage decreased with increasing antiplatelet drug concentration. Platelet aggregation in patients was reduced as compared with healthy individuals: 42% (interquartile range [IQR] 27-58) versus 66% (IQR 60-67) for TRAP; 41% (IQR 25-48) versus 70% (IQR 69-72) for collagen-related peptide; and 44% (IQR 30-53) versus 65% (IQR 46-72) for ADP. Platelet activation after stimulation was reduced in patients and correlated with platelet aggregation (e.g. r = 0.78-0.81 when stimulated with collagen-related peptide). Platelet aggregation had a negative predictive value of 100% for a bleeding tendency among patients. Conclusion The established platelet aggregation assay was applicable for thrombocytopenic patients, and improved the identification of bleeding risk.

Polyclonal and monoclonal antibodies in clinic

Immunoglobulins (Ig) or antibodies are heavy plasma proteins, with sugar chains added to amino-acid residues by N-linked glycosylation and occasionally by O-linked glycosylation. The versatility of antibodies is demonstrated by the various functions that they mediate such as neutralization, agglutination, fixation with activation of complement and activation of effector cells. Naturally occurring antibodies protect the organism against harmful pathogens, viruses and infections. In addition, almost any organic chemical induces antibody production of antibodies that would bind specifically to the chemical. These antibodies are often produced from multiple B cell clones and referred to as polyclonal antibodies. In recent years, scientists have exploited the highly evolved machinery of the immune system to produce structurally and functionally complex molecules such as antibodies from a single B clone, heralding the era of monoclonal antibodies. Most of the antibodies currently in the clinic, target components of the immune system, are not curative and seek to alleviate symptoms rather than cure disease. Our group used a novel strategy to identify reparative human monoclonal antibodies distinct from conventional antibodies. In this chapter, we discuss the therapeutic relevance of both polyclonal and monoclonal antibodies in clinic.

Preliminary evidence for a matrix metalloproteinase-2 (MMP-2)-dependent shedding of soluble CD40 ligand (sCD40L) from activated platelets

Platelets are the major source of soluble CD40 ligand (sCD40L) in the blood. It has been demonstrated that CD40L is cleaved from the surface of activated platelets to release sCD40L. However, the enzyme involved in sCD40L shedding has not been identified yet. Using a panel of pharmacological inhibitors of serine, cysteine, aspartate, or metalloproteinases, preliminary evidence is presented for the hypothesis that matrix metalloproteinase-2 (MMP-2) might be the protease, primarily responsible for CD40L cleavage from platelet surface.

Effects of selective cyclooxygenase isoform inhibition on systemic prostacyclin synthesis and on platelet function at rest and after exercise in healthy volunteers

To test the hypothesis that selective inhibition of cyclooxygenase (COX)-2 would result in exercise-induced platelet activation by causing a shift in the endogenous thromboxane (TX)/prostacyclin balance, a double blind, randomized study comparing aspirin (300 mg/d) with rofecoxib (25 mg/d) (cross-over design, 14 days washout between treatments) in n = 10 trained healthy volunteers was carried out. Physical exercise resulted only in a minor platelet activation, as reflected by the expression of basal or ADP-stimulated platelet activation markers or basal plasma concentrations of TXB(2). Aspirin significantly reduced TXB(2) in plasma while rofecoxib significantly increased TXB(2) in urine. Although no increase in systemic prostacyclin concentration was observed, there was a significant exercise-related increase in both platelet cAMP and cGMP without any drug-related effects. It is concluded that, in trained healthy volunteers, selective inhibition of COX-1 (aspirin) or COX-2 (rofecoxib) does not affect systemic prostacyclin synthesis after physical exercise. However, our data do not exclude the possibility that in subjects at risk for atherothrombotic complications (e.g. patients with advanced atherosclerotic disease) COX-2 inhibitors may result in platelet activation by inhibiting endothelial prostacyclin formation.

Complex effects of different green tea catechins on human platelets

Epigallocatechin gallate (EGCG), a major component of green tea, has been previously shown to inhibit platelet aggregation. The effects of other green tea catechins on platelet function are not known. Pre-incubation with EGCG concentration-dependently inhibited thrombin-induced aggregation and phosphorylation of p38 mitogen-activated protein kinase and extracellular signal-regulated kinases-1/2. In contrast EGCG stimulated tyrosine phosphorylation of platelet proteins, including Syk and SLP-76 but inhibited phosphorylation of focal adhesion kinase. Other catechins did not inhibit platelet aggregation. Interestingly, when EGCG was added to stirred platelets, a tyrosine kinase-dependent stimulation of platelet aggregation was observed. The two other catechins containing a galloyl group in the 3' position (catechin gallate, epicatechin gallate) also stimulated platelet aggregation, while catechins without a galloyl group (catechin, epicatechin) or the catechin with a galloyl group in the 2' position (epigallocatechin) did not.

The C-terminal peptide of thrombospondin-1 stimulates distinct signaling pathways but induces an activation-independent agglutination of platelets and other cells

A peptide from the C-terminal domain of thrombospondin-1 (4N1-1) has been proposed to stimulate platelet aggregation by a novel mechanism involving both an activation-independent agglutination and an activation-dependent, glycoprotein (GP) IIb/IIIa-mediated aggregation which involves GPVI signaling but does not involve CD47. The present study demonstrates that 4N1-1 stimulated a different pattern of signal transduction pathways than the GPVI agonist convulxin. Furthermore, 4N1-1-induced platelet aggregation was activation-independent and not dependent on GPVI or GPIIb/IIIa. Interestingly, 4N1-1 also stimulated activation-independent agglutination of different megakaryocytic and non-megakaryocytic cells. 4N1-1-induced cell agglutination but not platelet signaling was inhibited by anti-CD47 antibodies.

No evidence for an influence of the human platelet antigen-1 polymorphism on the antiplatelet effects of glycoprotein IIb/IIIa inhibitors

This study investigated the hypothesis that the human platelet antigen-1 (HPA-1) polymorphism may influence the antiplatelet effects of glycoprotein (GP)IIb/IIIa inhibitors. Adenosine diphosphate (30 micro mol)-induced fibrinogen binding was measured by flow cytometry. Abciximab (0.03-3 micro g/ml), tirofiban (0.3-30 nmol/l) or eptifibatide (0.01-1 micro g/ml) were incubated for 15 min with the samples prior to stimulation. IC(50) values for the inhibition of fibrinogen binding were determined from each experiment. All subjects were genotyped by GALIOS and automated fluorescence correlation spectroscopy. Although a marked variability in the inhibitory effects of all three GPIIb/IIIa inhibitors was confirmed, there were no significant differences between the genotypes with respect to the inhibition of fibrinogen binding. Thus, the present study does not provide evidence for an effect of HPA-1 polymorphism on the inter-individual variability in the platelet inhibitory effects of the three GPIIb/IIIa inhibitors approved for clinical use.

Functional impairment of human T-lymphocytes following PHA-induced expansion and retroviral transduction: implications for gene therapy

The immune function of retrovirus-mediated gene modified (GM) T cells is critical for a beneficial effect to follow their adoptive transfer into patients. Recent clinical data show that GM T cells expanded with PHA have reduced function in vivo. However, little functional analysis of PHA stimulation is available. Our results show that expansion of T cells with PHA impairs their ability to respond (proliferation, cytotoxicity and IFN gamma and perforin expression) to allogeneic stimulation or viral antigens in vitro. Conversely, CD3/CD28-based protocols can preserve this immune function. Retroviral transduction did not alter the functional profile induced by polyclonal stimulation. We investigated the mechanisms leading to this functional effect, and identified differential effects of PHA and CD3/CD28 on the distribution of CCR7/CD45RA T cell functional subsets, which may explain the functional differences observed. While CD3/CD28 stimulation parallels the lineage differentiation pattern induced by antigens in physiological conditions, PHA induces a skewed distribution of the CCR7/CD45RA functional T cell subsets, with near disappearance of the subpopulations that display the effector phenotype. Overall, this study demonstrates a functional disadvantage for transduction protocols based on PHA, uncovers mechanisms that may explain this functional effect, and provides us with information to design and select transduction protocols with an improved functional outcome.

Flow cytometry analysis of platelet cyclooxygenase-2 expression: induction of platelet cyclooxygenase-2 in patients undergoing coronary artery bypass grafting

There are conflicting reports about the expression of cyclooxygenase (COX)-2 in human platelets. The present study describes a flow cytometric method for the measurement of platelet COX. Both COX-1 and COX-2 were shown to be expressed in platelets from patients undergoing a coronary artery bypass graft. There was a significant increase in COX-2 expression at day 5 as compared with pre-surgery values (mean fluorescence 12.31 +/- 0.88 versus 9.15 +/- 0.88; means +/- SEM, n = 7, P < 0.05), whereas COX-1 levels did not change (13.45 +/- 1.11 versus 12.38 +/- 1.41; n = 7, P > 0.05).

CD40 ligand (CD40L) does not stimulate proliferation of vascular smooth muscle cells

The present study investigates the effects of CD40 ligand (CD40L) on mitogenic signalling, proliferation, and migration of cultured bovine coronary artery smooth muscle cells (SMC). A time- and concentration-dependent phosphorylation of the extracellular signal-regulated kinases-1/2 (ERK-1/2) and the mitogen-activated protein kinase p38 (p38-MAPK) was observed upon stimulation with soluble CD40L (sCD40L). This phosphorylation was inhibited by neutralizing antibodies against the CD40 and CD40L, respectively. Activation of the phosphatidylinositol-3-phosphate (PI-3) kinase pathway by sCD40L, as determined by the measurement of Akt phosphorylation, was not detected. However, there was evidence for direct activation of the NFkappaB system (degradation of IkappaBalpha and nuclear translocation of the p65 NFkappaB subunit) by sCD40L. Accordingly, sCD40L caused a small but significant increase in DNA synthesis. However, sCD40L-induced DNA synthesis was not followed by proliferation (increase in cell number). Furthermore, sCD40L did not potentiate SMC mitogenesis induced by known mitogens such as platelet-derived growth factor-BB, thrombin or serum. The lack of cell proliferation was not caused by a concomitant induction of SMC apoptosis by sCD40L. The possible role of membrane-bound CD40L in SMC mitogenesis was also studied using different membrane preparations (platelets, lymphocytes). However, no mitogenic effects of membrane-bound CD40L were detected. Finally, sCD40L did not induce SMC migration. From these data it is concluded that CD40L activates mitogenic signalling and DNA synthesis but does not contribute to proliferation or migration of vascular SMC.

Low incidence of paradoxical platelet activation by glycoprotein IIb/IIIa inhibitors

The human platelet antigen-1 (HPA-1, Pl(A)) polymorphism has been proposed to influence the inhibitory actions of abciximab. Thus, we hypothesized that this polymorphism might also be the cause for paradoxical activation of platelets by GPIIb/IIIa inhibitors. The effects of abciximab (1-10 microg/ml), tirofiban (3-30 nM), or eptifibatide (0.3-3 microg/ml) on basal and ADP (3 microM)-induced CD62P externalization were measured in n=62 healthy blood donors and n=177 patients with stable coronary artery disease. All subjects were genotyped for the human platelet antigen-1 (HPA-1, Pl(A)) polymorphism by GALIOS(R) and fluorescence correlation spectroscopy. Although a significant platelet hyperreactivity was observed in the patients, the HPA-1 genotype did not influence basal or ADP-induced CD62P expression. A moderate (twofold) stimulation of CD62P expression by abciximab but not by tirofiban or eptifibatide was observed in one patient. Interestingly, this patient carried the HPA-1 b/b genotype. In no other subject any activation of platelets by GP IIb/IIIa inhibitors was observed and there were no statistically significant differences between HPA-1 genotypes with respect to the effects of GP IIb/IIIa inhibitors on basal or ADP-stimulated CD62P expression. It is concluded that paradoxical platelet activation by abciximab is a rare (<2%) phenomenon. HPA-1 b/b genotype might be a contributing factor but clearly does not predict platelet activation by GP IIb/IIIa inhibitors.