Studies of retroviral neo-gene transfer in primate in models and 
human studies 
Hematopoietic stem cells are rare in crude bone marrow. Attention 
has focused on the use of recombinant retroviruses as efficient 
vehicles to introduce new genes into these rare cells. Several 
laboratories have documented experimental infection with a 
recombinant retrovirus of a cell contributing to the long-term • 
reconstitution of hematopoiesis in mice. A number of variables 
appear to influence the efficiency of retroviral transduction of 
hematopoietic stem cells. Pre-treatment of bone marrow donors 
with cytotoxic agents such as 5-fluorouracil (5-FU) and incubation 
of bone marrow cells with hematopoietic growth factors such as 
Interleukin 3 (IL-3) and Interleukin 6 (IL-6) during in vitro exposure 
to the retrovirus improve both gene transfer efficiency and graft 
survival (33, 34). Co-culture of bone marrow cells with virus- 
producing cell lines for extended periods and the use of high titer 
producer cell lines also appear to improve the probability of 
successful stem cell infection (35). Experimental designs that 
incorporate these modifications are capable of achieving long-term 
retroviral gene transfer into greater than 90 percent of transplanted 
mice, with 20 percent of circulating cells in each animal containing 
the recombinant proviral genome (34). 
Retroviral marking of multi-lineage and single lineage human 
progenitors can be achieved at an efficiency of 20 - 25% (40, 41). A 
few laboratories have extended these studies by examining long- 
term culture-initiating cells (LTC-IC). Again, retroviral marking in 
these cells has been achieved with an estimated frequency of 10 - 
20%. This is determined by the ability to recover retrovirally- 
marked clonogenic progenitors from long-term cultures 5-6 weeks 
after retroviral infection and initiation of culture (42, 43). The 
assay for the LTC-IC is considered the best surrogate for the human 
stem cell at present. Various combinations of hematopoietic growth 
factors have been used to achieve infection of primitive human 
hematopoietic cells; IL-3, IL-6 and/or stem cell factor (SCF) appear 
to be as effective as any combination reported to date (43, 44). Co- 
Recombinant DNA Research, Volume 16 
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