TRANSPLANTATION OF HUMAN UMBILICAL-CORD BLOOD-CELLS IN MACROPHAGE-DEPLETED SCID MICE - EVIDENCE FOR ACCESSORY CELL INVOLVEMENT IN EXPANSIONOF IMMATURE CD34(-) CELLS()CD38()

In vivo expansion and multilineage outgrowth of human immature hematop oietic cell subsets from umbilical cord blood (UCB) were studied by tr ansplantation into hereditary immunodeficient (SCID) mice. The mice we re preconditioned with Cl2MDP-liposomes to deplete macrophages and 3.5 Gy total body irradiation (TBI). As measured by immunophenotyping, th is procedure resulted in high levels of human CD45(+) cells in SCID mo use bone marrow (BM)5 weeks after transplantation, similar to the leve ls of human cells observed in NOD/SCID mice preconditioned with TBI. G rafts containing approximately 10(7) unfractionated cells, approximate ly 10(5) purified CD34(+) cells, or 5 x 10(3) purified CD34(+)CD38(-) cells yielded equivalent numbers of human CD45(+) cells in the SCID mo use BM, which contained human CD34(+) cells, monocytes, granulocytes, erythroid cells, and B lymphocytes at different stages of maturation. Low numbers of human GpA(+) erythroid cells and CD41(+) platelets were observed in the peripheral blood of engrafted mice. CD34(+)CD38(+) ce lls (5 x 10(4)/mouse) failed to engraft, whereas CD34(-) cells (10(7)/ mouse) displayed only low levels of chimerism, mainly due to mature T lymphocytes. Transplantation of graded numbers of UCB cells resulted i n a proportional increase of the percentages of CD45(+) and CD34(+) ce lls produced in SCID mouse BM. In contrast, the number of immature, CD 34(+)CD38(-) cells produced in vivo showed a second-order relation to CD34(+) graft size, and mice engrafted with purified CD34(+)CD38(-) gr afts produced 10-fold fewer CD34(+) cells without detectable CD34(+)CD 38(-) cells than mice transplanted with equivalent numbers of unfracti onated or purified CD34(+) cells. These results indicate that SCID rep opulating CD34(+)CD38(-) cells require CD34(+)CD38(+) accessory cell s upport for survival and expansion of immature cells, but not for produ ction of mature multilineage progeny in SCID mouse BM. These accessory cells are present in the purified, non-repopulating CD34(+)CD38(+) su bset as was directly proven by the ability of this fraction to restore the maintenance and expansion of immature CD34(+)CD38(-) cells in viv o when cotransplanted with purified CD34(+)CD38(-) grafts. The possibi lity to distinguish between maintenance and outgrowth of immature repo pulating cells in SCID mice will facilitate further studies on the reg ulatory functions of accessory cells, growth factors, and other stimul i. Such information will be essential to design efficient stem cell ex pansion procedures for clinical use. (C) 1998 by The American Society of Hematology.