Interactions between platelets and neutrophils in essential thrombocythaemia. Effects on neutrophil chemiluminescence and superoxide anion generation

Essential thrombocythaemia (ET) is frequently associated with neutrophil and platelet dysfunction, and with increased incidence of vascular complications (thrombosis, haemorrhage). Several interactions between platelets and neutrophils have been reported, and the reciprocal actions between these cells may have an important role both in thromboregulation and in diseases such as those caused by uncontrolled neutrophil activation. In the current paper the authors studied 15 patients affected by ET and 10 normal subjects as controls. Circulating neutrophils and platelets were purified and were recombined in constant ratios (50: 1, 100: 1 and 200: 1) and the individual platelet to neutrophil ratio. Superoxide anion (O-2(-)) generation and luminol-enhanced chemiluminescence (CL) were studied after neutrophil stimulation with fMLP. In normal subjects both O-2(-) generation and CL were inhibited by autologous platelets in a dose-dependent manner. In ET patients, on the contrary, platelet-dependent inhibition of O-2(-) generation did not occur, while a dose-dependent inhibition of CL was observed. Two groups of ET patients were found: patients with neutrophil O-2(-) generation and CL within the normal range, and patients with significantly reduced neutrophil respiratory burst. However, no differences were found between these two groups of patients in terms of platelet effects towards fMLP-stimulated neutrophils. Therefore, platelets from ET patients were not able to exert the homeostatic control towards neutrophil O-2(-) generation shown by platelets from normal subjects, and this phenomenon may have a role in the clinical setting. In fact, O-2(-) has been shown to be a very strong direct platelet activator, is able to inactivate nitric oxide (which is a powerful inhibitor of platelet aggregation and adhesion to endothelium), and is directly involved in neutrophil-mediated tissue damage.