C. HESDORFFER 4/93 
hematopoietic cell lineages (red cells, granulocytes, platelets, 
macrophages, and lymphocytes) , and are also a self-renewing 
population. Anti-CD34 has been used to enrich for BM-SC and is 
capable of reconstituting ablated human marrows (64) . In these 
recent experiments, a CellPro Corporation CEPRATE™ Stem Cell 
Concentrator has been used (64,65). This system provides a 
closed system in which bone marrow cells are passed through a 
column of beads containing biotinylated anti-CD34 antibody 12.8 
(65). Several other antibodies including anti-CD33, anti-CD38, 
anti-DR, and anti-Thyl have also been used (66-69) . This 
enrichment for hematopoietic precursor cells containing BM-SC is 
desirable and necessary because it concentrates the BM-SC into a 
small volume facilitating optimal transduction with supernatants 
of MDR retroviral producer cells and incubation with growth 
factors. These conditions also: 1) Favor BM-SC replication 
required for retroviral gene transfer; 2) Increase the 
virus: target cell ratio; and 3) May increase the number of stem 
cells by replication. 
Lethally irradiated mice given both mouse and human bone marrows 
depleted of B and T lymphocytes survive, and support the growth 
of human B and T cells for 6-8 months (70) . Immune-deficient 
(SCID) mice also support the survival and growth of human bone 
marrow cells (71-75) . The most successful model to date has been 
SCID mice given PIXY321, a fusion protein of IL-3 and GM-CSF in 
combination with SCF and GM-CSF (74). In these mice: 1) Large 
numbers of human myeloid and erythroid precursors are present in 
marrow over 1-2 months; 2) Human marrow stem cells or very early 
hematopoietic precursors are present; and 3) Repopulation with 
human marrow is dependent on PIXY321. 
Transfer of neo R (76,77) and adenosine deaminase (ADA) genes (78) 
into the lymphocytes of human subjects have recently been 
reported. These experiments have been approved in humans by the 
RAC and the FDA. ABMT in humans is routinely used in association 
with intensive cancer chemotherapy to prevent life threatening 
bone marrow toxicity (5-8) . The high-level expression of the MDR 
gene, normally expressed at very low levels in bone marrow, 
should provide Resistance of treated marrow to the toxic effects 
of subsequent chemotherapy with MDR-responsive drugs (79) . 
Another approach to gene therapy is by correcting a gene defect 
in the involved cells by homologous recombination (80-82) . In 
the case of MDR, this procedure could conceivably be used to add 
a promoter or enhancer to the normal relatively inactive MDR gene 
in bone marrow cells. However, in experiments to date, the 
frequency of appropriate homologous recombination leading to gene 
correction is 1/10 -10 7 treated cells (80-82) , and it is not yet 
possible to select and grow rare hematopoietic cells with the 
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Recombinant DNA Research, Volume 18 
