Megakaryocyte induced fibroblast proliferation is enhanced by costimulation with IL-6/IL-3 and dependent on secretory and adhesion events.

Experimental data are in keeping with the finding that megakaryocytes isolated from normal human bone marrow may promote fibroblast growth. This effect can be significantly enhanced by interleukin (IL)-3. In this context it has been demonstrated that IL-3 induces the release of platelet-derived growth factor (PDGF) and transforming growth factor beta1 (TGFbeta1) from megakaryocytes, factors known to enhance fibroblast proliferation. The present in vitro study was performed to elucidate the action of several other cytokines which are able to influence the different steps of megakaryocyte maturation and function like stem cell factor (SCF), IL-6, and leukemia inhibitory factor (LIF) as well. Following an appropriate experimental design we were able to show that none of the mentioned cytokines enhanced megakaryocyte dependent fibroblast proliferation in the coculture assays. On the other hand, IL-6 in combination with IL-3 surpassed the IL-3 dependent action significantly. However, the combined IL-3/IL-6 effect was not explainable by an increased PDGF/TGF-beta secretion of the megakaryocytes. In transwell experiments the inhibition of cell-to-cell contact via tissue culture inserts generated a conspicuous impairment of fibroblast growth in the IL-3/IL-6 treated cocultures. This reversal surpassed even the effect on the untreated and IL-3 stimulated cocultures. Hence, a direct contact of both cell types probably inducing adhesion phenomenons and warranting a certain threshold of local PDGF/TGF-beta concentration is a prerequisite for the proliferative effect on fibroblasts in the costimulation experiments. These results are of special interest regarding the evolution of myelofibrosis in chronic myeloproliferative disorders (CMPDs) because (1) various progenitor cells including the megakaryocytic lineage are hypersensitive towards IL-3 and (2) an abnormal secretion of IL-6 is described for megakaryocytes in these disorders.