Prevention of platelet-polymorphonuclear leukocyte interactions: new clues to the antithrombotic properties of parnaparin, a low molecular weight heparin

Abstract

BACKGROUND AND OBJECTIVES: Heparin might possess anti-thrombotic properties other than anticoagulation. The aim of the present study was to test the effects of a low-molecular weight heparin, parnaparin, on adhesive molecule-mediated platelet-polymorphonuclear (PMN) leukocyte interactions and on PMN function. DESIGN AND METHODS: Platelets and PMN were isolated from citrated blood from healthy subjects. Pre-activated platelets incubated with PMN under dynamic conditions formed mixed cell aggregates. In previous experiments PMN were stimulated in vitro by purified P-selectin or formyl-methionyl-leucyl-phenylalanine (fMLP). Dual color flow cytometry was used to detect the formation of platelet-PMN mixed cell aggregates, and PMN activation was tested for by measuring L-selectin shedding, tissue factor expression and PMN degranulation. The effect of parnaparin was compared to that of unfractionated heparin. RESULTS: Parnaparin, at a concentration of 0.3-0.8 IUaXa/mL, inhibited the formation of mixed cell aggregates (48.8+/-9.7% of total PMN population) by up to 60% in a concentration-dependent manner, while heparin inhibited aggregation up to 40%. Parnaparin, (0.3-0.8 IUaXa/mL), prevented L-selectin shedding from PMN, which was induced by purified P-selectin (5 mg/mL) or fMLP (0.5 mmol/L) by 65% and 67%, respectively. Inhibition was independent of incubation time (5-20 min). Parnaparin (0.8 IUaXa/mL) also inhibited tissue factor expression on PMN (% of positive cells), which was induced by P-selectin or fMLP (185+/-10 and 241+/-80% of basal value, respectively). Parnaparin protected PMN from degranulation after challenge with either stimulus (>95% inhibition). All the effects of parnaparin were observed with heparin at similar concentrations, although to a lesser extent and were often not significantly different from events in controls. INTERPRETATION AND CONCLUSIONS: In conclusion, the process of depolymerization of heparin to obtain low molecular weight parnaparin resulted in an increased, anticoagulant-independent effect on PMN function. Thus, the overall anti-thrombotic properties of parnaparin may be partly due to a leukocyte-mediated anti-inflammatory effect.