Defective platelet beta-N-acetyl hexosaminidase content and release in chronic myeloproliferative disorders.

BACKGROUND AND OBJECTIVES: Abnormalities of platelet function or structure are a hallmark of chronic myeloproliferative disorders (MPD). In vivo platelet activation with the release of alpha- and delta-granules in the circulation is one of the most frequently described alterations in MPD. Platelets contain and release upon activation also lysosomes, and in particular beta-N-acetylhexosaminidase (Hex). We have assessed whether the content and in vivo release of Hex of platelets from MPD patients is altered. DESIGN AND METHODS: Twenty-three MPD patients were compared with 19 age- and sex-matched healthy controls. The activity of platelet beta-N-acetylhexosaminidase was measured in plasma, serum and in the capillary blood emerging from the skin wound inflicted for the measurement of the bleeding time. Lysosome integral membrane protein (LIMP or CD63), lysosome-associated membrane protein (LAMP-2 or CD107b) and P-selectin were evaluated by flow cytometry. Platelet aggregation in vitro and the release of beta-N-acetylhexosaminidase, ATP and beta-thromboglobulin were performed to study platelet reactivity. RESULTS: Hex levels in plasma were significantly higher in MPD than in controls while the release of Hex in the bleeding time blood, i.e. at a localized site of in vivo platelet plug formation, was lower in MPD and the platelet content of Hex was reduced. These changes were accompanied by in vivo platelet activation. Finally, the isoenzymatic pattern of Hex was altered in platelets of MPD patients, with a reduced amount of the Hex A isoform as compared with controls.b INTERPRETATIONS AND CONCLUSIONS: MPD patients present an altered platelet Hex content and release; prospective studies to assess whether altered platelet Hex is related to thrombotic/hemorrhagic complications and/or tissue fibrosis in MPD are warranted.

Platelets are essential for leukocyte recruitment in allergic inflammation.

BACKGROUND: The role of platelets in inflammation is recognized but poorly characterized, and little is known of their interaction with leukocytes. However, platelet-leukocyte interactions have been demonstrated in cardiovascular disease, culminating in enhanced leukocyte recruitment. OBJECTIVES: This study was undertaken to assess the possibility and potential role of similar phenomena occurring in asthmatic patients, a murine model of allergic inflammation, and in vitro adhesion studies. METHODS: Asthmatic patients had blood taken at various time points to document the degree of leukocyte activation and the presence of platelet-leukocyte aggregates through FACS analysis before and after allergen exposure. Similar studies were carried out in mice exposed to allergen after previous sensitization, with some groups being selectively depleted of platelets through both an immunologic (antiplatelet antiserum) and nonimmunologic (busulfan) method. Additionally, lavage fluid and airway tissue were analyzed to assess the degree of pulmonary leukocyte recruitment. The importance of platelets on leukocyte adhesion to the endothelium was then assessed with in vitro incubation of radiolabeled leukocytes in the presence of activated platelets on cultured human vascular endothelial cells. RESULTS: We have observed circulating platelet-leukocyte aggregates in the blood of allergic asthmatic patients during the allergen-induced late asthmatic response and in sensitized mice after allergen exposure. In platelet-depleted mice infiltration of leukocytes into airways after allergen challenge was significantly reduced and could be restored by means of infusion of platelets from allergic animals, indicating an essential role for platelets in leukocyte recruitment. CD11b expression on leukocytes involved in aggregates with platelets, although not on free leukocytes, was upregulated. Furthermore, the presence of autologous platelets augmented the adhesion of human polymorphonuclear leukocytes to cultured vascular endothelial cells, an effect that was found to be endothelial cell dependent and to involve platelet activation. CONCLUSION: These results suggest that platelet participation in cell recruitment occurs at the level of the circulation and might involve the priming of leukocytes for subsequent adhesion and transmigration into tissues.

Acute, short-term hyperglycemia enhances shear stress-induced platelet activation in patients with type II diabetes mellitus.

OBJECTIVES: The aim of our study was to assess whether acute, short-term hyperglycemia affects platelet reactivity in patients with Type II diabetes mellitus (T2DM). BACKGROUND: Hyperglycemic spikes are thought to precipitate ischemic events in T2DM. Previous studies have shown in vivo platelet activation in diabetes; however, no studies have assessed whether acute in vivo hyperglycemia induces further activation of platelets. METHODS: In a cross-over, randomized, double-blind study, 12 patients with T2DM underwent 4 h of either acute hyperglycemia (13.9 mmol/l, 250 mg/dl) or euglycemia (5.5 mmol/l, 100 mg/dl). Shear stress-induced platelet activation, P-selectin and lysosomal integral membrane protein (LIMP) expression on platelets in the bleeding-time blood, urinary 11-dehydro-thromboxane B(2) (TxB(2)) excretion, von Willebrand factor:antigen (vWF:Ag), and von Willebrand factor:activity (vWF:activity) were measured before and after hyperglycemia or euglycemia. RESULTS: Shear stress-induced platelet activation, P-selectin and LIMP expression on platelets in the bleeding-time blood, and urinary 11-dehydro-TxB(2) excretion increased significantly after hyperglycemic clamping, whereas no changes were observed after euglycemic clamping. Plasma vWF:Ag and vWF:activity increased strikingly in parallel fashion after hyperglycemic clamping, whereas no changes were observed after euglycemic clamping. CONCLUSIONS: Our data demonstrate that acute, short-term hyperglycemia induces an increased activation of platelets exposed to high shear stress conditions in vitro (filtration method) or in vivo (bleeding time). In vivo platelet activation is reflected by an increased urinary excretion of 11-dehydro-TxB(2). The increased levels of vWF in the circulation correlate with the increase in platelet activation markers and may indicate some degree of causation. Acute, short-term hyperglycemia in T2DM may precipitate vascular occlusions by facilitating platelet activation.