Lisa Nannizzi-Alaimo

Platelet-Derived CD40L The Switch-Hitting Player of Cardiovascular Disease

Studies focusing on cellular and molecular mechanisms that regulate atherosclerosis have fed scientific journals for decades, nearly as long as it takes an atherosclerotic plaque to grow, rupture, and eventually induce vascular occlusive events. See p 981 Three closely linked lines of research have now merged. In the 1980s, concepts crystallized on the role of lipids (eg, oxidized LDL, elevated cholesterol) in the genesis of atherosclerotic plaque. In the 1990s, use of antiplatelet agents proved that platelet aggregation caused thrombotic ischemic events resulting from the rupture of plaques in advanced lesions and from the vascular injuries inflicted by percutaneous interventions (PCI). Now, atherosclerosis is recognized as an inflammation-mediated disease involving multiple interactions between leukocytes, cells of the vessel wall, and platelets. Indeed, recent studies of predictors of cardiovascular risk rank markers of inflammation (eg, high-sensitive C-reactive protein) as comparable to markers of cholesterol (eg, total cholesterol/HDL cholesterol). Emerging data suggest that CD40L may be at the heart of the atherosclerotic process. What makes CD40L so unique? Its localization and its multifunctionality (Figure 1). CD40L is a surprisingly abundant protein in platelets and may have roles in the inflammatory aspects of atherosclerotic lesion progression, thrombosis, and now, as implied by the work of Urbich et al1 in this issue of Circulation , in restenosis. Figure 1. Three functions of sCD40L released from platelets during thrombosis. sCD40L is released from platelet-rich thrombi and contributes to various steps in atherosclerotic lesion progression: (1) Inflammation. sCD40L induces the production and release of proinflammatory cytokines from vascular cells and matrix metalloproteinases from resident cells in the atheroma. (2) Thrombosis. sCD40L stabilizes platelet-rich thrombi. (3) Restenosis. sCD40L inhibits the reendothelialization of the injured vessel, potentially leading to the activation and proliferation of smooth muscle cells. CD40L is a trimeric, transmembrane protein of the tumor necrosis …

Effect of Ca2+ on GP IIb-IIIa Interactions With Integrilin Enhanced GP IIb-IIIa Binding and Inhibition of Platelet Aggregation by Reductions in the Concentration of Ionized Calcium in Plasma Anticoagulated With Citrate

BACKGROUND Integrilin (eptifibatide), a potent inhibitor of the fibrinogen binding function of GP IIb-llla, has been shown to reduce the thrombotic complications of angioplasty and of acute coronary syndromes. The present study was designed to determine whether the reduced Ca2+ concentrations in plasma anticoagulated with citrate affect Integrilin binding to GP IIb-IIIa and the ex vivo pharmacodynamic measurements for this drug. METHODS AND RESULTS Lower concentrations of Integrilin were found to inhibit platelet aggregation in plasma anticoagulated with citrate (for ADP, mean+/-SD IC(50)=140+/-40 nmol/L, n=6; Ca2+ =40 to 50 micromol/L) than with PPACK (IC(50)=570+/-70 nmol/L, P<.0001, n=6; Ca2+ approximately 1 mmol/L). Chelation of Ca2+ with EDTA or citrate caused a similar degree of enhancement in the inhibitory activity of Integrilin. Measurements of D3 LIBS epitope expression showed that the enhanced inhibitory activity was caused by enhanced GP IIb-IIIa occupancy by Integrilin. Citrate anticoagulation decreased the amounts of Integrilin required to inhibit the binding of PAC1, a monoclonal antibody that mimics the GP IIb-IIIa binding activity of fibrinogen. Reduced Ca2+ also increased Integrilin inhibition of the binding of biotinylated fibrinogen to purified, immobilized GP IIb-IIIa. CONCLUSIONS These data suggest that citrate anticoagulation removes Ca2+ from GP IIb-IIIa and enhances the apparent inhibitory activity of Integrilin. This finding indicates that the inhibitory activity of Integrilin is overestimated in blood samples collected with citrate, suggesting that it may be possible to achieve greater antithrombotic efficacy beyond that observed in clinical trials to date with Integrilin.

Inhibitory Effects of Glycoprotein IIb/IIIa Antagonists and Aspirin on the Release of Soluble CD40 Ligand During Platelet Stimulation

Background—Glycoprotein (GP) IIb/IIIa antagonists inhibit platelet aggregation, an activity attributed to the clinical benefits of these drugs in settings that involve acute coronary thrombosis. However, platelet activation and subsequent aggregation are now known to cause the release of a soluble form of CD40 ligand (sCD40L), a prothrombotic and proinflammatory protein with GP IIb/IIIa binding activity and an established role in atherosclerotic lesion progression. The present study was designed to determine what effect GP IIb/IIIa antagonists have on the release of sCD40L. Methods and Results—Doses of eptifibatide, abciximab, and tirofiban that inhibited platelet aggregation by at least 80% also inhibited sCD40L release in vitro (by 85%, 57%, and 80%, respectively). When platelets were stimulated with a thrombin receptor agonist, inhibition by GP IIb/IIIa antagonists occurred without affecting the release of &bgr;TG, an &agr;-granule protein. Unexpectedly, concentrations of the 3 antagonists that blocked aggregation by only 20% to 50% potentiated the release of sCD40L (by 19% to 26%). Platelets from aspirin-treated individuals were partially protected from sCD40L release, but only when the agonist was collagen, an affect augmented by the addition of GP IIb/IIIa antagonists. Conclusions—These studies suggest that the mechanisms responsible for the clinical benefits of GP IIb/IIIa antagonists (at doses that optimally inhibit aggregation) and of aspirin may not be limited to the inhibition of thrombosis through their blockade of platelet aggregation but may also involve the inhibition of inflammation and thrombosis through their blockade of sCD40L release. These studies also provide a mechanism by which suboptimal doses of GP IIb/IIIa antagonists may be proinflammatory.

Integrin tyrosine phosphorylation in platelet signaling.

The beta 3 integrin cytoplasmic tyrosine (ICY) motif of alpha IIb beta 3 becomes tyrosine phosphorylated during platelet aggregation, causing Shc and myosin to interact with the beta-integrin cytoplasmic domain. Platelets from mice lacking beta 3 ICY motif tyrosines formed defective aggregates and poorly retracted clots, establishing integrin tyrosine phosphorylation as a key mediator of beta 3-integrin signals.

Cardiopulmonary bypass induces release of soluble CD40 ligand.

Background—Cardiopulmonary bypass (CPB) is known to induce platelet activation, thrombosis, thrombocytopenia, and a systemic inflammatory response. It is known that CD40 ligand (CD40L) exists in platelets, that a soluble form of this protein (sCD40L) is released on platelet activation, that platelets are the primary source of sCD40L in blood, and that sCD40L is involved in thrombosis and inflammation. The present study was designed to determine whether sCD40L is released during CPB. Methods and Results—Blood was obtained from patients undergoing CPB-requiring surgery and analyzed for sCD40L, interleukin-6, and platelet factor 4 and &bgr;-thromboglobulin (markers of platelet activation). Platelets were also isolated and analyzed for their levels of CD40L. Plasma levels of sCD40L increased >1.7-fold (from 0.29 to 0.51 ng/mL, P =0.001) within 1 hour on CPB and increased further to 3.7-fold (to 1.08 ng/mL, P =0.03) 2 hours after the procedure. Half of the released sCD40L was cleared in 2 hours, which allowed the sCD40L to return to approximately baseline levels 8 hours after the procedure. The platelet content of CD40L was decreased by 40% (2.675 to 1.64 ng/108 platelets, P =0.001) 1 hour after initiation of CPB and was similar to that observed for platelet factor 4 and &bgr;-thromboglobulin. Interleukin-6, a marker of inflammation, also increased during CPB. Conclusions—The present study demonstrates that CPB causes an increase in the concentration of plasma sCD40L. The corresponding decrease in platelet CD40L suggests that this prothrombotic and proinflammatory protein was derived primarily from platelets and may contribute to the thrombotic and inflammatory complications associated with CPB.

GP IIb/IIIa inhibition with eptifibatide lowers levels of soluble CD40L and RANTES after percutaneous coronary intervention

Platelets are the most abundant blood source of CD40L, a proinflammatory and prothrombotic costimulatory molecule implicated in atherosclerosis. Agonist stimulation results in the secretion of a soluble form of CD40L (sCD40L) and GP IIb/IIIa receptor inhibition blocks secretion of sCD40L in vitro. However, the effect of GP IIb/IIIa inhibition on sCD40L levels in humans is unknown. Plasma sCD40L and inflammatory markers were measured (t = 0, 0.5, 2, and 24 hr post‐PCI) in a cohort of patients receiving abciximab (n = 15), eptifibatide (n = 15), or no GP IIb/IIIa inhibitor (n = 15). PCI in the absence of GP IIb/IIIa inhibitor was associated with a small but measurable rise in sCD40L and the platelet‐derived chemokine RANTES. In contrast, eptifibatide significantly lowered baseline sCD40L (P = 0.018) and RANTES (P = 0.006) levels. This effect was not observed with abciximab. GP IIb/IIIa inhibition with eptifibatide lowers levels of sCD40L and RANTES post‐stenting, possibly conferring anti‐inflammatory as well as antithrombotic effects. Catheter Cardiovasc Interv 2004;61:185–189.