Release of soluble CD40L from platelets is regulated by glycoprotein IIb/IIIa and actin polymerization.

OBJECTIVES: The purpose of this study was to examine the effects of glycoprotein (GP) IIb/IIIa antagonists (abciximab, eptifibatide, and tirofiban) and other inhibitors on translocation of CD40L from intraplatelet stores to the platelet surface and on the release of soluble CD40L (sCD40L) from platelets. BACKGROUND: CD40L is a proinflammatory and prothrombotic ligand in the tumor necrosis factor family. METHODS: Platelet surface CD40L was measured by flow cytometry, and sCD40L was measured by enzyme-linked immunosorbent assay. RESULTS: Translocation of CD40L from intraplatelet stores to the platelet surface was not inhibited by GP IIb/IIIa antagonists. However, release of sCD40L from the surface of activated platelets was inhibited by GP IIb/IIIa antagonists in a dose-dependent manner, in concert with inhibition of PAC1 binding to platelets (a surrogate marker for fibrinogen binding). Release of sCD40L from activated platelets was also markedly reduced in Glanzmann platelets (deficient in GP IIb/IIIa). Ethylenediaminetetraacetic acid was an effective inhibitor of sCD40L release, but only when added before platelet activation. Both cytochalasin D (an inhibitor of actin polymerization) and GM6001 (an inhibitor of matrix metalloproteinases [MMPs]) inhibited the release of sCD40L from platelets when added before, as well as 3 min after, platelet activation. However, neither cytochalasin D nor GM6001 affected translocation of CD40L to the platelet surface. CONCLUSIONS: The GP IIb/IIIa antagonists inhibit release of sCD40L from activated platelets. Release of sCD40L from platelets is regulated, at least in part, by GP IIb/IIIa, actin polymerization, and an MMP inhibitor-sensitive pathway. In addition to their well-characterized inhibition of platelet aggregation, GP IIb/IIIa antagonists may obviate the proinflammatory and prothrombotic effects of sCD40L.

Identification of human antigen-specific T cells using MHC class I and class II tetramers.

Major histocompatibility complex (MHC) tetramers typically consist of a fluorophore-streptavidin complex and biotinylated soluble MHC molecules carrying a peptide of interest. Tetramers bind to T cell receptors (TCR) that recognize the MHC molecule/peptide combination with high specificity. Native MHC molecules are expressed as cell-surface glycoproteins capable of binding a variety of peptides generated from the degradation of self and non-self proteins for display to T cells. The human MHC gene locus is highly polymorphic, with >800 class I and >500 class II alleles currently identified. This heterogeneity contributes to the uniqueness of each person's immune system. This unit describes procedures for labeling CD8(+) T cells with MHC class I tetramers and CD4(+) T cells with MHC class II tetramers. The protocols can be used for detecting and enumerating human antigen-specific T cells. Both CD8(+) and CD4(+) antigen-specific T cells are rare events and require that sufficient numbers of cells be evaluated. To minimize nonspecific tetramer binding contributed by irrelevant cell populations, a cumulative gating strategy using positive selection and/or exclusion gating is described.

Combined effects of mild hypothermia and glycoprotein IIb/IIIa antagonists on platelet-platelet and leukocyte-platelet aggregation.

Endovascular cooling was assessed as a potential treatment for percutaneous coronary intervention in patients with acute myocardial infarction. Here we show that mild hypothermia: (1) inhibits platelet aggregation; (2) augments eptifibatide- and tirofiban- but not abciximab-induced inhibition of platelet aggregation; (3) increases the formation of adenosine diphosphate-induced leukocyte-platelet aggregates; and (4) diminishes the glycoprotein IIb/IIIa antagonist-induced decrease in leukocyte-platelet aggregates.

Quantification of abciximab-induced platelet inhibition is assay dependent: a comparative study in patients undergoing percutaneous coronary intervention.

BACKGROUND: The best method for measuring the degree of platelet inhibition with glycoprotein (GP) IIb-IIIa antagonists during percutaneous coronary intervention (PCI) and the optimal degree of periprocedural inhibition is uncertain. Low molecular weight heparins have been reported to cause less platelet activation than unfractionated heparin. Therefore, compared with unfractionated heparin (UHF), a low molecular weight heparin could enhance measured platelet inhibition. In this study, we compared 3 methods of measuring platelet inhibition and investigated the effects of half doses of abciximab in combination with either UFH or the low molecular weight heparin dalteparin in patients undergoing PCI with planned abciximab administration. METHODS: Abciximab-induced platelet inhibition was measured serially by means of 3 assays: 1) GP IIb-IIIa receptor occupancy, 2) binding of the activated GP IIb-IIIa-specific monoclonal antibody PAC1, and 3) agglutination of platelets with fibrinogen-coated beads (RPFA). Forty patients were randomly allocated to receive either UFH (70 U/kg) or dalteparin (60 IU/kg), followed by a half dose of abciximab (0.125 mg/kg) administered twice at 10-minute intervals. Assays were obtained 10 minutes after each half dose of abciximab and 8 to 10 and 24 hours after abciximab administration. RESULTS: No differences between UFH and dalteparin were observed. At each time-point measured, the mean percent platelet inhibition as determined by means of the receptor occupancy assay and PAC1 binding assay was less than the degree of inhibition determined by means of the RPFA. CONCLUSIONS: The results of targeted levels of platelet inhibition cannot be extrapolated between different clinical trials of GP IIb-IIIa antagonists unless the same assay is used. Dalteparin, compared with UFH, does not enhance platelet inhibition or receptor occupancy by abciximab, as demonstrated by means of 3 separate assays.

The cleaved peptide of PAR1 is a more potent stimulant of platelet-endothelial cell adhesion than is thrombin.

PURPOSE: Platelet-endothelial cell adhesion is an important pathologic response to vessel injury or inflammation. On binding to its endothelial or platelet G protein-linked seven-transmembrane domain receptor, protease-activated receptor-1 (PAR1), thrombin releases a 41-amino acid peptide (TR(1-41)). We examined the effect of TR(1-41) on platelet activation and on platelet-endothelial cell adhesion. METHODS: A monolayer of confluent human saphenous vein endothelial cells was incubated with washed human platelets. Platelets were stimulated with either TR(1-41), TR(21-41), scrambled TR(1-41), adenosine diphosphate (ADP)-epinephrine (EPI), thrombin, or thrombin receptor activating peptide (TRAP). Platelet activation was identified with flow cytometry. The magnitude of platelet-endothelial cell adhesion was determined with a laser scanning cytometer that scanned the monolayer of endothelial cells and identified fluorescently bound platelets. RESULTS: Maximal thrombin stimulation (0.1 U/mL) induced a threefold increase in platelets bound to endothelial cells compared with buffer alone. Stimulation with TR(1-41) (20 mmol/L) tripled the number of platelets bound to endothelial cells compared with thrombin. Scrambled sequence of TR(1-41) (20 mmol/L) and TR(21-41) (20 mmol/L), neither of which induces platelet activation, had minimal effect on platelet adhesion. Both TRAP (20 mmol/L) and ADP-EPI (20 mmol/L) induced less platelet-endothelial cell adhesion than did thrombin. TR(1-41)-induced platelet-endothelial cell adhesion was partially blocked by glycoprotein (GP)IIb-IIIa-specific monoclonal antibody, 10E5 (10 mg/mL). CONCLUSIONS: TR(1-41), the cleaved peptide of PAR1, is a more potent stimulant of platelet-endothelial cell adhesion than is thrombin, TRAP, or ADP-EPI, and this adhesion is at least in part mediated by the platelet GPIIb-IIIa receptor.

Whole blood analysis of leukocyte-platelet aggregates.

In inflammatory and thrombotic syndromes, platelets aggregate with circulating leukocytes, especially monocytes and neutrophils. The platelet binding is initiated primarily through platelet surface expression of P-selectin (CD62P) following activation-dependent degranulation. The levels of P-selectin involved can be low enough to make direct measurement difficult, but detection of leukocyte-platelet aggregates is relatively simply by whole-blood flow cytometry. Light scatter and at least one leukocyte-specific antibody are used to gate the desired population, and the presence of associated platelets is detected by immunostaining for abundant platelet-specific markers.

Immunophenotypic analysis of platelets.

Platelets are the smallest cellular component in the peripheral circulation. Their primary role is maintenance of hemostasis. The evaluation of platelets by flow cytometry has proven beneficial in the investigation of many disease states, including inherited defects, cardiovascular disease, stroke, and many other inflammatory processes. In the absence of an added exogenous platelet agonist, the activation state of in vivo circulating platelets can be measured. Inclusion of an exogenous agonist in the assay enables analysis of the reactivity of circulating platelets in vitro. This unit describes protocols to evaluate platelet surface glycoproteins, platelet activation status, and platelet responsiveness to agonist. Detailed discussion of appropriate sample handling, reagent preparation, flow cytometric set-up, and data interpretation are provided for three independent assays.