Interleukin-6 and its receptor are expressed by human megakaryocytes: in vitro effects on proliferation and endoreplication.
 Interleukin-6 (IL-6) is a pleiotropic cytokine that plays an important role in the megakaryocytic differentiation.
 Recently, we have observed that IL-6 is synthesized by several human cell lines with megakaryocytic features.
 In this study, we have investigated whether a similar phenomenon occurs during normal megakaryocytic differentiation.
 Human megakaryocytes (MK) were obtained by culturing normal marrow in liquid culture with aplastic plasma (AP).
 First, an IL-6 secretion in bone marrow culture enriched in MK as well as in purified MK populations was demonstrated by a biologic assay.
 Second, IL-6 mRNA was detected in a purified population of MK by the polymerase chain reaction and dot blot analysis.
 IL-6 mRNA and protein were undetectable in platelets.
 Third, in situ hybridization procedure demonstrated the presence of IL-6 mRNA in individual immature MK.
 Fourth, IL-6 protein was detected in MK at the unicellular level by an immunoalkaline phosphatase technique using a monoclonal antibody against IL-6.
 Furthermore, the presence of IL-6 receptor (IL-6-R) on MK was demonstrated by in situ hybridization using an IL-6-R probe and in situ autoradiography after binding with [125I]-labeled recombinant IL-6.
 The IL-6 endogenously produced in liquid cultures containing normal human plasma or AP was subsequently neutralized.
 This resulted in a 50% decrease of the MK growth with a minor shift in the ploidy distribution toward lower values.
 In semisolid cultures the addition of anti-IL-6 antibodies led to a 42% decrease in colony number in cultures stimulated by IL-3 but not in other conditions of culture.
 These results suggest that normal human megakaryocytopoiesis might be regulated in part by an IL-6 autocrine loop.
