A UNIQUE POPULATION OF CD34(-BLOOD() CELLS IN CORD)

Human umbilical cord blood (CB) is a rich source of hematopoietic stem cells for both research and stem cell transplantation. In clinical st udies, it appears that recovery from myeloablative therapy using CB re quires significantly fewer cells than a typical allogeneic marrow tran splant. This suggests that CB may be enriched for early hematopoietic progenitors. The present studies were undertaken to determine the pres ence of CD34(+) cells in CB with the phenotypic characteristics of mul tipotential stem cells. In 22 CB harvests, the average percentage of C D34(+) 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-stimulat ing factor (G-CSF) mobilized peripheral blood stem cell harvests. CB c ontained 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 b een shown to be CD34-bright. Eighty five to ninety percent were negati ve for three antigens expressed at an early stage of stem cell maturat ion: CD38, HLA-DR and LFA-1. Fifty-five percent of these CD34 (bright) cells did not express the CD45RA isoform, an additional marker of imm aturity. The antigen-bright cells also lacked lineage-specific antigen s including CD33, CD56, CD19, CD10 and CD7 as well as CD71. Approximat ely 46% were Thy-(1+), and 40% expressed c-kit receptors, These data s uggest that, by phenotypic criteria, CB may be a particularly enriched source of primitive hematopoietic precursors.