ENGRAFTMENT OF EX-VIVO EXPANDED AND CYCLING HUMAN CORD-BLOOD HEMATOPOIETIC PROGENITOR CELLS IN SCID MICE

The ability of human hematopoietic cells to engraft SCID mice provides a useful model in which to study the efficiency of retroviral gene tr ansfer and expression in primitive stem cells. In this regard, it is n ecessary to determine whether SCID mice can be engrafted by cycling hu man hematopoietic progenitor cells. Human cord blood cells from 12 dif ferent donors were cultured in vitro for 6 days with interleukin-3 and stem cell factor. Phenotypic analysis indicated that hematopoietic ce lls were induced to cycle and the number of progenitors was expanded, thus making them targets for retroviral gene transfer. The cells were then transferred to SCID mice. Human hematopoietic progenitor cell eng raftment was assessed up to 7 weeks later by growth of human progenito r cells in soft agar. After in vitro culture under conditions used for retroviral gene transfer, human cord blood hematopoietic cells engraf ted the bone marrow and spleen of SCID mice. Interestingly, cultured c ord blood cells engrafted after intraperitoneal but not after intraven ous injection. Furthermore, engraftment of cord blood cells was observ ed in mice receiving no irradiation before transfer of the human cells , suggesting that competition for space in the marrow is not a limitin g factor when these cells have been cultured. Administration of human cytokines after transfer of human cord blood cells to SCID mice was al so not required for engraftment. Thus, engraftment of SCID mice with h uman hematopoietic cells cultured under conditions suitable for gene t ransfer may provide an in vivo assay for gene transfer to early human hematopoietic progenitor cells.