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 recombi nant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) consistently results in marked increase of megakaryocyte ploidy and s ize similar to that observed with interleukin-6 (IL-6). However, where as the administration of IL-6 also results in an increase in circulati ng platelets, there is no predictable corresponding increase in periph eral blood platelets following treatment with rhGM-CSF. To determine w hether the failure of rhGM-CSF to produce thrombocytosis is secondary to cytokine-related increased platelet activation and consumption in v ivo, we quantified autologous platelet survival time and in vivo plate let activation before and during 5 days of administration of rhGM-CSF to two rhesus monkeys. Platelet survival was measured using autologous platelets labeled with (111)Indium-oxine. Platelet activation was ass essed by flow cytometric determination of the expression of the major platelet membrane glycoprotein (GP) IIb/IIIa complex, and an activatio n-dependent epitope on GPIIb/IIIa (recognized by monoclonal antibodies [MABs] LJ-P4 and PAC1, respectively). Platelet activation was also as sessed by dose-response aggregometry using adenosine diphosphate (ADP) . While megakaryocyte ploidy increased during rhGM-CSF administration, peripheral platelet counts were 418x10(9)/L and 525x10(9)/L before an d 402x10(9)/L and 508x10(9)/L during cytokine treatment in animals 1 a nd 2, respectively. No changes were observed in the mean platelet volu me. (111)Indium-labeled platelet recovery in circulation was similar b efore (94.7%, 91.8%) and during (92.9%, 92.8%) rhGM-CSF administration , which indicates that cytokine-related in vivo sequestration of plate lets 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), w ithout significant change in the corresponding platelet turnover rate (derived from the platelet count and survival time). The flow cytometr ic analysis showed no increase in the binding of either LJ-P4 or PAC1 MABs to the platelet membrane during rhGM-CSF administration. The aggr egometry studies demonstrated similar concentrations of ADP inducing h alf-maximal aggregation (ED(50)). Overall, the above data indicate tha t treatment with rhGM-CSF is not associated with in vivo activation, s equestration, or increased consumption of platelets. The data suggest that the failure of rhGM-CSF-stimulated megakaryocytes to increase per ipheral platelet count is a manifestation of ineffective megakaryocyto poiesis resulting from inability to increase platelet delivery to the circulation.