EVALUATION OF THE IN-VITRO BEHAVIOR OF PHENOTYPICALLY DEFINED POPULATIONS OF UMBILICAL-CORD BLOOD HEMATOPOIETIC PROGENITOR CELLS

Umbilical cord blood (CB) has been identified as a potential source of hematopoietic stem cells suitable for clinical transplantation. We used long-term cord blood cultures (LTCBC) to evaluate the hematopoietic potential of populations of umbilical CB cells phenotypically defined and isolated by flow cytometry. LTCBC initiated with CD34(+)HLA-DR(+) and CD34(+)HLA-DR(-) CB cells were examined over a period of 8 weeks for the production of assayable burst-forming units-erythroid (BFU-E), colony-forming units-granulocyte/macrophage (CFU-GM), and colony-forming units-mixed (CFU-GEMM) in response to repeated additions of stem cell factor (SCF), interleukin-3 (IL-3), IL-6, and either erythropoietin (Epo) or granulocyte-macrophage colony-stimulating factor (GM-CSF). The LTCBC-initiating cell (LTCBC-IC) appeared to be present among CD34(+)HLA-DR(+) cells, in contrast to our previous findings in adult bone marrow (BM), where the long-term culture initiating cells were shown to be CD34(+)HLA-DR(-). In addition, production of BFU-E, CFU-GM, and CFU-GEMM in CB CD34(+)HLA-DR(+) cells displaying low uptake of the supravital dye rhodamine 123 (Rh123) exceeded those detected in the fraction of cells with high uptake of Rh123. Furthermore, on day 21 of LTCBC, the production of the high proliferative potential colony-forming units (HPP-CFC) by CB CD34(+)HLA-DR(+)Rh123(dull) cells was five-fold greater than that detected in cultures initiated with their Rh123(bright) counterparts. Collectively, these data show that, contrary to what has been documented in adult human BM, LTCBC-IC and presumably CB cells capable of in vivo engraftment reside in the CD34(+)HLA-DR(+)Rh123(dull) fraction of CB. Although the functional significance of these differences between the in vitro behavior of phenotypically defined populations of CB and BM remains to be determined, these findings constitute an objective parameter with which the suitability of CB for clinical transplantation may be assessed.