HIGH-LEVEL EXPRESSION OF A NOVEL EPITOPE OF CD59 IDENTIFIES A SUBSET OF CD34(-MARROW CELLS HIGHLY ENRICHED FOR PLURIPOTENT STEM-CELLS() BONE)

To further define the hierarchy of human hematopoietic progenitor cell s, we have attempted to identify antibodies to cell-surface molecules expressed on CD34(+) progenitor cell subsets. Herein we describe the u tility of a new monoclonal antibody, HCC-1, which binds to a novel epi tope of CD59 differentially expressed among Cd34(+) progenitor cells. HCC-1 subdivides the adult marrow CD34(+) population into HC-1(high) a nd HCC-1(low/-) fractions of approximately equal size. Cobblestone are a-forming cells (CFAC) in long-term bone marrow culture were enriched 10-30-fold in CD34(+)HCC-1(high) cells compared with CD34(+)HCC-1(low/ -) cells and two-fold compared with CD34(+) cells. When injected into fetal human bone fragments implanted in SCID mice, the CD34(+)HCC-1(hi gh) population of myeloid, lymphoid, and erythroid elements, as well a s the retention of progenitor cell phenotype. These studies demonstrat e that the CD34(+)HCC-1(high) population contains primitive pluripoten t hematopoietic stem cells. No hematopoietic engrafting activity was d etected in the CD34(+)HCC-1(low/-) population. Consistent with this fi nding, simultaneous five-color flow cytometric analysis revealed that HCC-1(high) cells include virtually all CD34(+)Thy-1(+)Lin(-) cells, a cell population previously characterized as highly enriched for primi tive pluripotent hematopoietic stem cells. The ability of CD34(+) cell s divided into subsets by HCC-1 to produce T cells was assessed by tra nsplantation of sorted cells into human fetal thymus implanted into SC ID mice. A higher frequency of thymus-engrafting activity was observed in the CD34(+)HCC-1(high) limited ability to engraft in the SCID-hu t hymus model, the CD34(+)HCC-1(low/-) population was shown to contain a low frequency of CD34(+)CD10(+) lymphoid progenitor cells. We conclud e that the HCC-1 epitope is expressed at high levels on a subset of CD 34(+) cells that contains virtually all primitive pluripotent hematapo ietic stem cells and that the population of CD59 molecules expressed o n CD34(+) cells is not homogeneous.