The in vivo effects of interleukin-3 (IL-3), interleukin-6 (IL-6), and a combination of IL-3 plus IL-6 on murine megakaryocytopoiesis and thrombopoiesis were examined. Human recombinant IL-6 was administered subcutaneously as 14 equal injections of 5,000 units each during a 102- hour period. Murine recombinant IL-3 was given as 8 injections of 80,000 units each during the first 54 hours. Megakaryopoiesis and thrombopoiesis were evaluated 120 hours after initial administration of the cytokines. Platelet levels increased by 20% following IL-3 alone, 35% following IL-6 alone and 61% after administration of both IL-3 and IL-6. Platelet production, as measured by 75Se-selenomethionine incorporation, increased by approximately 120% in animals that had received IL-6 or IL-3 plus IL-6. Megakaryocyte ploidy analysis by two- color flow cytometry showed a shift in the modal ploidy class from 16N to 32N and a significant increase in the frequency of 64N cells only in IL-6 treated animals. Both bone marrow and splenic megakaryocyte colony- forming cells were significantly increased following either IL-3 or IL- 6. Bone marrow megakaryocyte size increased 18%, 43%, and 38%, respectively, after administration of IL-3, IL-6, or the combination of IL-3 plus IL-6. Leukocyte counts and hematocrits were unaffected by either cytokine. Additional groups of mice received the same injection schedule as above and the serial effects on peripheral blood cell levels were assessed for 30 days. Platelet levels, which had been elevated by IL-3 or IL-6, fell to control values within 4 days following the last injection. Animals given IL-6 or IL-3 plus IL-6 were subsequently thrombocytopenic relative to controls on days 7 through 9 following cessation of treatment. Temporary ‘cycling’ of platelet levels was observed for 3 weeks following treatment with IL-6 or the combination of IL-3 plus IL-6. We conclude that IL-6 and to a lesser extent IL-3 stimulate platelet production in vivo and that their combined effects on platelet levels are approximately additive. Following discontinuation of IL-3 or IL-6, the effects are rapidly reversed, presumably by negative feedback mechanisms, resulting in a period of ‘rebound thrombocytopenia’ in mice that had received IL-6.

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