RELATIVE REACTIVITY OF PLATELETS FROM THROMBOPOIETIN-TREATED AND INTERLEUKIN-6 TREATED DOGS

Previous reports have shown that interleukin-6 (IL-6) enhances the res ponsiveness of platelets to thrombin stimulation and has modest thromb ocytopoietic effects in vivo. Thrombopoietin (TPO; mpl ligand) has bee n shown to have dramatic thrombocytopoietic effects in vivo, but littl e is known of its capacity to alter platelet function. In this study, a direct comparison of the effects of IL-6 and TPO on platelet functio n in dogs has been performed, with modest doses of TPO (1 mu g/kg/d) c hosen to match or moderately exceed the platelet counts achieved with IL-6 (40 mu g/kg/d) for 10 days. Platelet responsiveness to thrombin s timulation was assessed in TPO-treated, IL-6-treated, and control dogs by flow cytometric measurement of P-selectin expression. On day 5, th e dose of thrombin promoting half maximal stimulation (EC(50)) of plat elets was not significantly changed in TPO-treated dogs, whereas in IL -6-treated dogs the EC(50) decreased to 73.1% +/- 6.1% (mean +/- 1 SD; n = 5) of control values (P < 0.01). These experiments were performed on both gel-filtered platelets and washed whole blood, indicating tha t the observed changes in EC(50) were caused by cytokine-mediated alte ration of platelets rather than plasma components. Because it has been shown that thiazole orange specifically labels a subpopulation of dog platelets that is less than 24 hours old, the thrombin responsiveness of these young, newly synthesized platelets was determined. The EC(50 ) of thiazole orange-positive platelets from IL-6-treated dogs decreas ed dramatically by day 5 to 46.5% +/- 13.1% (n = 4) of control values (P < 0.001), whereas TPO-treated dogs did not significantly change. Wh en TPO was directly incubated with platelets ex vivo, no effects on ei ther thrombin-mediated P-selectin expression or adenosine diphosphate- induced fibrinogen binding were observed. These data show that IL-6 al ters platelet function, as measured by reactivity to thrombin, whereas TPO does not. This divergence in function is observed even though TPO is equally, or more, effective at promoting platelet production under these experimental conditions. (C) 1996 by The American Society of He matology.