Effects of recombinant human granulocyte-macrophage colony-stimulating factor on platelet survival and activation using a nonhuman primate model.

In humans and nonhuman primates, the in vivo administration of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) consistently results in marked increase of megakaryocyte ploidy and size similar to that observed with interleukin-6 (IL-6). However, whereas the administration of IL-6 also results in an increase in circulating platelets, there is no predictable corresponding increase in peripheral blood platelets following treatment with rhGM-CSF. To determine whether the failure of rhGM-CSF to produce thrombocytosis is secondary to cytokine-related increased platelet activation and consumption in vivo, we quantified autologous platelet survival time and in vivo platelet activation before and during 5 days of administration of rhGM-CSF to two rhesus monkeys. Platelet survival was measured using autologous platelets labeled with 111Indium-oxine. Platelet activation was assessed by flow cytometric determination of the expression of the major platelet membrane glycoprotein (GP) IIb/IIIa complex, and an activation-dependent epitope on GPIIb/IIIa (recognized by monoclonal antibodies [MABs] LJ-P4 and PAC1, respectively). Platelet activation was also assessed by dose-response aggregometry using adenosine diphosphate (ADP). While megakaryocyte ploidy increased during rhGM-CSF administration, peripheral platelet counts were 418 x 10(9)/L and 525 x 10(9)/L before and 402 x 10(9)/L and 508 x 10(9)/L during cytokine treatment in animals 1 and 2, respectively. No changes were observed in the mean platelet volume. 111Indium-labeled platelet recovery in circulation was similar before (94.7%, 91.8%) and during (92.9%, 92.8%) rhGM-CSF administration, which indicates that cytokine-related in vivo sequestration of platelets does not occur. Autologous platelet survival was 5.6 and 6.2 days before and 5.0 and 5.4 days during the rhGM-CSF treatment (p = 0.07), without significant change in the corresponding platelet turnover rate (derived from the platelet count and survival time). The flow cytometric analysis showed no increase in the binding of either LJ-P4 or PAC1 MABs to the platelet membrane during rhGM-CSF administration. The aggregometry studies demonstrated similar concentrations of ADP inducing half-maximal aggregation (ED50). Overall, the above data indicate that treatment with rhGM-CSF is not associated with in vivo activation, sequestration, or increased consumption of platelets. The data suggest that the failure of rhGM-CSF-stimulated megakaryocytes to increase peripheral platelet count is a manifestation of ineffective megakaryocytopoiesis resulting from inability to increase platelet delivery to the circulation.