A unique population of cd34⁺ cells in cord blood

Human umbilical cord blood (CB) is a rich source of hematopoietic stem cells for both research and stem cell transplantation. In clinical studies, it appears that recovery from myeloablative therapy using CB requires significantly fewer cells than a typical allogeneic marrow transplant. This suggests that CB may be enriched for early hematopoietic progenitors. The present studies were undertaken to determine the presence of CD34⁺ cells in CB with the phenotypic characteristics of multi‐potential stem cells. In 22 CB harvests, the average percentage of CD34⁺ cells was 1.33 ± 0.21% (SE), a value similar to that in adult normal bone marrows (BM). However, the distribution of CD34⁺ cells was distinctly different from either BM or granulocyte colony‐stimulating factor (G‐CSF) mobilized peripheral blood stem cell harvests. CB contained a defined population of brightly staining CD34⁺ cells with low side scatter. These CD34 (bright) cells comprised a mean of 14.5 ± 2.5% of the CB CD34⁺ cells, whereas <1% of BM CD34⁺ cells has been shown to be CD34‐bright. Eighty‐five to ninety percent were negative for three antigens expressed at an early stage of stem cell maturation: CD38, HLA‐DR and LFA‐1. Fifty‐five percent of these CD34 (bright) cells did not express the CD45RA isoform, an additional marker of immaturity. The antigen‐bright cells also lacked lineage‐specific antigens including CD33, CD56, CD19, CD10 and CD7 as well as CD71. Approximately 46% were Thy‐1⁺, and 40% expressed c‐kit receptors. These data suggest that, by phenotypic criteria, CB may be a particularly enriched source of primitive hematopoietic precursors.