CD34+/CD105+cells are enriched in primitive circulating progenitors residing in the GO phase of the cell cycle and contain all bone marrow and cord blood CD34+/CD38(low/-) precursors

A subset of circulating CD34+ cells was found to express CD105 antigen. Sor ting experiments showed that most granulocyte-macrophage colony-forming uni ts (GM-CFU) and burst-forming units - erythroid (BFU-E) were retained in th e CD34+/CD105- fraction, whereas rare GM-CFU/BFU-E were generated from CD34 +/CD105+ cells. Megakaryocytic aggregates were entirely retained in the CD3 4+/CD105+ fraction. Neutralizing doses of an anti-TGF-beta 1 antibody demon strated CD34+/CD105+ cells capable of colony-forming activity without any s ignificant effect on CD34+/CD105- cells. Cloning of secondary colonies reve aled that CD34+/CD105+ cells had a significantly higher secondary cloning e fficiency than CD34+/CD105- cells. CD34+/CD105+ cells had a significantly h igher long-term culture-initiating cell (LTC-IC) frequency than CD34+/CD105 - cells. Kinetic analysis showed that 75% of CD34+/CD105+ cells consisted o f DNA 2n G0Ki-67- cells whereas 82% of CD34+/CD105- were DNA 2n G1Ki-67+ ce lls, and this latter subset showed a RNA content consistently higher than C D34+/CD105+ cells. CD34+/CD105+ progenitors were CD25+, whereas CD34+/CD105 - contained a small CD25+ subset. Three-colour analysis of bone marrow and cord blood CD34+ cells demonstrated that all the CD34+/CD38(low/-) primitiv e precursors were contained in CD34+/CD105+ cells. Extensive characterizati on of these CD105+ precursors indicated that they have biological propertie s associated with primitive haematopoietic precursors.