Pro-Inflammatory Cytokine IL-1b on Megakaryopoiesis: A Possible Explanation of Thrombocytosis in Inflammation

Pro-inflammatory cytokine interleukin-1b (IL-1b) is multifunctional and exerts its effects on a wide variety of cell types. Among many functions, IL-1b enhances the hematopoietic system by inducing T and B cell proliferation and works in combination with other cytokines on early stem and progenitor cell expansion and differentiation. Megakaryopoiesis and platelet production is maily controlled by thrombopoietin (TPO, Mpl-ligand), which is a growth factor for the megakaryocyte/ platelet lineage. Other early acting cytokines including IL-1, IL-3, IL-6 and IL-11 are also involved in the regulation of this lineage at different degree. Thrombocytosis is observed in various inflammation diseases. IL-1b, as a highly pro-inflammatory cytokine, induces thrombocytosis in mice. However, its mechanism on megakaryopoiesis is still not well understood. Our hypothesis is that IL-1b possesses a direct stimulatory effect on megakaryocytes via its receptors and the transcription factor NF-E2. The expression of IL-1 type I (IL-1 RI) and type II receptors was studied in megakaryotic cell lines: Meg-01, Dami, CHRF-288-11 and M-07e by RT-PCR, southern blot hybridization, flow cytometry or immuno-fluorescent staining. The effect of IL-1b on the formation of megakaryocytic colonies from human and murine stem and progenitor cells was investigated by the plasma clot culture method. The expression of the transcription factor p45 NF-E2, in response to IL-1b, was analyzed by Western blot. Our results show that IL-1 RI and IL-1 RII were expressed in the four human megakaryocytic cells. IL-1b alone or in combination with TPO induced megakaryocyte colony forming units (CFU-MK) in the culture system. The mitogenic effect of IL-1b on the enhancement of TPO-induced megakaryopoiesis was stronger than that of IL-3, IL-6, Flt-3 ligand and stem cell factor. The expression of NF-E2 was demonstrated in all four cell lines. In Meg-01 cells, the expression of NF-E2 was increased at both transcriptional and translational level after treatment with IL-1b for 4 hours. These observations strengthen the evidence of the direct effect of IL-1b on megakaryopoiesis. Our findings also provide a possible explanation of thrombocytosis during inflammation conditions.