title: Thrombopoietin responsiveness reflects the number of doublings undergone by megakaryocyte progenitors.
abstract: To assess the variation of thrombopoietin (TPO) responsiveness associated with megakaryocyte (MK) progenitor amplification, TPO dose-response curves were obtained for normal human, single-cell plated CD34(+)CD41(+) cells. The number of MKs per well was determined in situ and expressed as number of doublings (NbD). Dose-response curves of the mean frequency of clones of each size versus log TPO concentration showed highly significant differences in the TPO concentration needed for half-maximum generation of clones of different sizes (TPO(50)): 1.89 +/- 0.51 pg/mL for 1 MK clones; 7.75 +/- 0.81 pg/mL for 2 to 3 MK clones; 38.5 +/- 5.04 pg/mL for 4 to 7 MK clones, and 91.8 +/- 16.0 pg/mL for 8 to 15 MK clones. These results were consistent with a prediction of the generation-age model, because the number of previous doublings in vivo was inversely correlated with the number of residual doublings in vitro. TPO responsiveness decreased in vitro by a factor of 3.5 per doubling, reflecting the recruitment of progressively more ancestral progenitors. In support of this hypothesis, the more mature CD34(+)CD41(+)CD42(+) cell fraction had a lower TPO(50) (P < .001), underwent fewer NbD (P < .001), and expressed a 2.8-fold greater median Mpl receptor density (P < .001) than the CD34(+)CD41(+)CD42(-) fraction. Progenitors that have completed their proliferative program have maximum factor responsiveness and are preferentially induced to terminal differentiation.
authors.affiliation: Laboratory of Hematology, Hôpital du Sart-Tilman, 4000 Liège, Belgium. jmpaulus@ulg.ac.be
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Paulus
authors.forename: Najet
authors.initials: N
authors.lastname: Debili
authors.forename: Frédéric
authors.initials: F
authors.lastname: Larbret
authors.forename: Jack
authors.initials: J
authors.lastname: Levin
authors.forename: William
authors.initials: W
authors.lastname: Vainchenker
mesh_terms: D015703:Antigens, CD / Q000378:metabolism; D002452:Cell Count; D002454:Cell Differentiation / Q000187:drug effects; D002455:Cell Division / Q000187:drug effects; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D006801:Humans; D008533:Megakaryocytes / Q000166:cytology* / Q000187:drug effects* / Q000378:metabolism; D013234:Stem Cells / Q000166:cytology* / Q000187:drug effects* / Q000378:metabolism; D013926:Thrombopoietin / Q000494:pharmacology*
chemical_list: D015703:Antigens, CD; D013926:Thrombopoietin