EFFECTS OF THROMBOPOIETIN ON THE PROLIFERATION AND DIFFERENTIATION OFPRIMITIVE AND MATURE HEMATOPOIETIC PROGENITOR CELLS IN CORD-BLOOD

Thrombopoietin (TPO) is considered to be the primary growth factor for regulating megakaryopoiesis and thrombopoiesis. In this study we inve stigated the in vitro effect of TPO on relatively immature and mature CD34(+) progenitor cells in cord blood. Cells were cultured in both li quid and semi-solid cultures containing 50 ng/ml TPO. The CD34(+)/CD45 RA(-) and CD34(+)/CD38(-) subfractions in cord blood were both enriche d for megakaryocyte progenitors as determined in a semisolid CFU-meg a ssay. Progenitor cells derived from the CD34+/CD45RA(-) and CD34(+)/CD 38(-) subfractions showed high proliferative capacity in liquid cultur es. We observed a mean 19-fold expansion of the total CD34+ cell fract ion, whereas in the CD34+/CD45RA(-) and CD34+/CD38(-) subfractions the mean expansion was 23- and 50-fold respectively. The expansion of the immature progenitor cell subfractions resulted in a highly purified m egakaryocyte suspension containing > 80% megakaryocytes after 14d in c ulture. However, these expanded megakaryocytes remained in a diploid ( 2N) and tetraploid (4N) state. Maturation could not be further induced by low concentration of TPO (0.1 ng/ml). The majority of the cells we re 2N (80%) and 4N (15%) and only 5% of the cells had a ploidy of more than 4N. These results indicate that megakaryocyte progenitor cells i n cord blood residing in the immature stem cell fraction exhibit a hig h proliferative capacity when cultured in the presence of TPO as the s ingle growth factor, without maturation to hyperploid megakaryocytes.