Pathologic high shear stress induces apoptosis events in human platelets

Dendritic cells of immune thrombocytopenic purpura (ITP) show increased capacity to present apoptotic platelets to T lymphocytes

OBJECTIVE

Altered self-antigen processing/presentation of apoptotic cells by DCs and/or modifications of autoantigens may lead to the development of autoantibodies. Increasing evidence indicates that platelets may undergo apoptosis. Therefore, in the present study we investigated whether platelet apoptosis and/or dendritic cells (DCs) may play a role in the stimulation of the immuno-mediated anti-platelet response in chronic immune thrombocytopenic purpura (ITP).

METHODS AND RESULTS

Twenty-nine patients with active ITP and 29 healthy adult volunteers were enrolled into the study. Freshly washed platelets and platelets aged in a plasma-free buffer for 72 hours at 37 degrees C were assessed by flow cytometry for phosphatidylserine exposure using annexin V-FITC, caspase activation, and platelet activation markers. CD14-derived DCs were characterized by immunophenotyping, cytokine production, and ability to present fresh and aged platelets to T lymphocytes. We demonstrated that platelets from ITP patients, either fresh or in vitro aged, show increased apoptosis (with low levels of activation) in comparison to their normal counterparts. We also found that immature DCs readily ingest apoptotic platelets. Furthermore, in ITP patients DCs, prepulsed with autologous/allogeneic fresh and aged platelets, are highly efficient in stimulating autologous T-cell proliferation as compared to DCs derived from healthy donors. This finding may be related to the upregulated expression of CD86 in DCs from ITP patients and not to higher phagocytic activity.

CONCLUSION

These results suggest that DC dysfunction, together with increased propensity of platelets to undergo apoptosis, may play a role in the stimulation of the immune system in ITP.

Intravenous immunoglobulin inhibits anti-glycoprotein IIb-induced platelet apoptosis in a murine model of immune thrombocytopenia

We have previously shown that injection of anti-glycoprotein (GP) IIb induces murine immune thrombocytopenia (ITP) and that intravenous immunoglobulin (IVIg) ameliorates ITP. We hypothesise that murine ITP may be associated with platelet apoptosis, which is upregulated by anti-GPIIb and downregulated by IVIg. The current study demonstrated that anti-GPIIb injection induced three critical apoptosis manifestations in platelets: (i) mitochondrial inner transmembrane potential (delta psi m) depolarisation; (ii) caspase-3 activation; and (iii) phosphatidylserine (PS) exposure. IVIg administration inhibited caspase-3 activation and PS exposure, but not delta psi m-depolarisation, in anti-GPIIb-treated platelets, demonstrating that IVIg ameliorates thrombocytopenia concomitantly with inhibiting late, but not early mechanisms of platelet apoptosis.

Thrombin-induced caspases 3 and 9 translocation to the cytoskeleton is independent of changes in cytosolic calcium in human platelets

Apoptosis has been shown to be associated with changes in cytosolic free calcium concentration ([Ca(2+)](c)). Here we show that the agonist thrombin induces activation of caspases 9 and 3 and translocation of the caspase active forms and procaspases to the cytoskeleton in human platelets. Dimethyl-BAPTA loading did not affect thrombin-induced caspase 9 and 3 activation or translocation suggesting that these responses are independent of increases in [Ca(2+)](c). Treatment with thapsigargin plus ionomycin, to induce extensive Ca(2+) store depletion and subsequent increase in [Ca(2+)](c), stimulates caspase activation although it was unable to induce caspase translocation to the cytoskeleton. Similar results were observed in cells loaded with dimethyl-BAPTA, suggesting that activation of caspases 9 and 3 by thapsigargin plus ionomycin does not require rises in [Ca(2+)](c). These findings suggest that thrombin-induced caspase 9 and 3 activation and translocation are independent on rises in [Ca(2+)](c) but might require store depletion in human platelets.

Fibrin Affinity of Erythrocyte-Coupled Tissue-Type Plasminogen Activators Endures Hemodynamic Forces and Enhances Fibrinolysis in Vivo

Plasminogen activators (PAs; e.g., tissue-type, tPA) coupled to red blood cells (RBCs) display in vivo features useful for thromboprophylaxis: prolonged circulation, minimal extravasation, and preferential lysis of nascent versus preexisting clots. Yet, factors controlling the activity of RBC-bound PAs in vivo are not defined and may not mirror the profile of soluble PAs. We tested the role of RBC/PA binding to fibrin in fibrinolysis. RBC/tPA and RBC/tPA variant with low fibrin affinity (rPA) bound to and lysed plasminogen-containing fibrin clots in vitro comparably. In contrast, when coinjected in mice with fibrin emboli lodging in pulmonary vasculature, only RBC/tPA accumulated in lungs, which resulted in a more extensive fibrinolysis versus RBC/rPA (p < 0.01). Reconciling this apparent divergence between in vitro and in vivo behaviors, RBC/tPA, but not RBC/rPA perfused over fibrin in vitro at physiological shear stress bound to fibrin clots and caused greater fibrinolysis versus RBC/rPA (p < 0.001). These results indicate that because of high fibrin affinity, RBC/tPA binding to clots endures hemodynamic stress, which enhances fibrinolysis. Behavior of RBC/PAs under hemodynamic pressure is an important predictor of their performance in vivo.

Vitronectin stabilizes thrombi and vessel occlusion but plays a dual role in platelet aggregation

The role of vitronectin (Vn) in thrombosis is currently controversial; both inhibitory and supportive roles have been reported. To monitor directly the function of Vn in thrombotic events at the site of vascular injury, we studied Vn-deficient (Vn-/-) and wild-type (WT) control mice with two real-time intravital microscopy thrombosis models. In the mesenteric arteriole model, vessel injury was induced by ferric chloride. We observed unstable thrombi and a significantly greater number of emboli in Vn-/- mice. Vessel occlusion was also delayed and frequent vessel re-opening occurred. In the cremaster muscle arteriole model, vessel injury was induced by a nitrogen dye laser. We observed significantly fewer platelets, lower fibrin content, and unstable fibrin within the thrombi of Vn-/- mice. To define further the role of Vn in thrombus growth, we studied platelet aggregation in vitro. Consistent with our in vivo data, the second wave of thrombin-induced aggregation of gel-filtered platelets was abolished at a low concentration of thrombin in Vn-/- platelets. Interestingly, adenosine diphosphate (ADP)-induced platelet aggregation was significantly increased in Vn-/- platelet-rich plasma (PRP) and this effect was attenuated by adding purified plasma Vn. We also observed increased platelet aggregation induced by shear stress in Vn-/- whole blood. These data demonstrate that Vn is a thrombus stabilizer. However, in contrast to released platelet granule Vn which enhances platelet aggregation, plasma Vn inhibits platelet aggregation.