RECOMBINANT HUMAN INTERLEUKIN-11 DIRECTLY PROMOTES MEGAKARYOCYTOPOIESIS IN-VITRO

We have investigated the mechanism of action of the thrombopoietic cyt okine, recombinant human interleukin-11 (rhlL-17), on megakaryocytopoi esis in vitro. We have shown that rhlL-11-induced murine and human meg akaryocytopoiesis are not mediated by thrombopoietin (Tpo). Murine meg akaryocytes (MKs) were produced from bone marrow (BM) mononuclear cell s cultured with rhlL-11, IL-3, and a combination of the two cytokines. Conditioned media (CM) were collected and assayed for the presence of biologically active Tpo. Tpo activity was not detected in any of the CMs tested. Next, human BM CD34(+) cells were cultured in serum-free f ibrin clot medium with rhlL-11, IL-3, or rhlL-11 plus IL-3 and an anti body that neutralizes human Tpo activity. No inhibition of either burs t-forming unit-MK- or colony-forming unit-MK-derived colony formation was observed. The antibody did partially inhibit steel factor-induced MK-colony formation, suggesting that the actions of this cytokine are mediated, in part, by Tpo. We determined that MKs can be direct target s of rhIL-11 by showing the expression of functional IL-11 receptor on these cells. Total RNA was prepared from cultured human BM CD41(+)CD1 4(-) cells (MKs) and IL-11 receptor alpha chain mRNA was detected in t he MKs by reverse transcription-polymerase chain reaction. Analysis of single-sorted CD41(+)CD14(-) cells confirmed that the observed IL-11 receptor expression was not due to contaminating CD41(-) cells in the pool. The presence of rhlL-11 receptor alpha chain protein in the cell s was established by Western blot analysis. After a short exposure of purified BM MKs to rhlL-11, enhanced phosphorylation of both its signa l transduction subunit, gp130, and the transcription factor, STAT3 was detected, showing a direct activation of receptor signaling by the cy tokine. Consistent with the lack of effect of rhIL-11 on platelets in vivo, IL-11 receptor alpha chain mRNA and protein were not detected in isolated human platelets. These data indicate that rhlL-11 acts direc tly on MKs and MK progenitors but not on platelets. (C) 1997 by The Am erican Society of Hematology.