PLATELET ABNORMALITIES IN IDIOPATHIC MYELOFIBROSIS - FUNCTIONAL, BIOCHEMICAL AND IMMUNOMORPHOLOGICAL CORRELATIONS

Background. An extensive study of platelet function was performed on 1 8 consecutive patients affected by idiopathic myelofibrosis (IM). Mate rials and methods. Clinical hematological and morphofunctional paramet ers were studied in IM patients and control subjects. Platelet tests, ultrastructural data, immunocytochemical von Willebrand factor detecti on, freeze fracturing results and free cytosolic calcium level were ev aluated. Results. Bleeding time was frequently found to be prolonged, but it never reached levels which could give any cause for concern. Ag gregation by ADP, collagen and epinephrine was always altered, sometim es profoundly;on the contrary, agglutination by ristocetin was almost always normal, albeit occasionally increased. Plasma beta-TG and PF4 l evels were found to be elevated in 11 and 12 patients, respectively. T his indicated an abnormal release from platelet alpha-granules. Deplet ion of alpha-granules was also confirmed by the intraplatelet von Will ebrand factor (VWF) labelling with colloidal gold particles bound to p olyclonal antibodies against human vWE. In fact: 1) the number of posi tive alpha-granules/mu m(2) and per single platelet was reduced; 2) th e intensity of the immunocytochemical reaction for single positive alp ha-granules and for each platelet was significantly reduced. Freeze-fr acturing studies showed an increase in the number of intra-membrane pa rticles (IMP) on the P face of the platelet membrane with respect to n ormal platelets preincubated with ADP. However, no differences in thei r distribution or diameter were observed. High concentrations of free cytosolic calcium were always found and Ca(++)ATPase activity was incr eased. Conversely, Na+/K+ ATPase activity was always reduced. Conclusi ons. We can hypothesize that the platelet membrane is altered in IM, r esulting in facilitated activation, even by subliminal stimuli, and th at this continuous platelet activation ultimately leads to alpha-granu le depletion.