in response to very high concentrations of IL-3. 35/36 Post- 
chemotherapy IL-3 given in a Phase I study in at the NCI has 
not produced accelerated tumor growth in 15 patients with 
metastatic breast cancer .( personal communication, Joyce 
O'Shaughnessy ) CML progenitors do respond to these growth 
factors, but as discussed above, culture of CML marrow cells 
appears to selectively favor normal progenitor cells, and 
our preclinical data thus far shows no selective stimulation 
of bcr/abl + progenitors during transduction procedures. 
Plasma cells from some patients with multiple myeloma 
proliferate in response to IL6, 37 thus we have elected to 
use IL3 and SCF alone during transduction of cells from the 
multiple myeloma patients. However, there is evidence that 
the true neoplastic "stem cell" in myeloma is not stimulated 
by in vitro culture with IL6. 38 C-kit, the receptor for 
stem cell factor, is not found on plasma cells, and neither 
is the receptor for interleukin 3. 39/40 We have not observed 
stimulation of myelomatous plasma cells using our 
transduction conditions. 
3 . 4 Feasibility of gene transfer to hematopoietic 
progenitors and stem cells : 
There is extensive evidence in murine model systems that 
retroviral vectors can introduce foreign genes at high 
efficiency into hematopoietic progenitors and reconstituting 
stem cells . 9/10/30/41 A number of variables appear to be 
critical to effect efficient gene transfer to primitive 
progenitor and stem cells, such as 5-f luorouracil 
- pretreatment of the donor animal prior to marrow harvest, 
co-culture of the target marrow cells with the retroviral 
producer line instead of with purified viral-containing 
supernatant, use of very high titer viruses, and inclusion 
of hematopoietic growth factors in the media during 
transduction . 30/31/42/43 Culture in the presence of growth 
factors for at least 48 hours appears to be necessary for 
efficient retroviral gene transfer to murine stem cells. 30 ' 41 
This is presumably due to the increased likelihood of 
proviral integration in actively cycling cells. The use of 
5FU pre-treatment of the donor and in vitro culture in IL3 
and IL6 produces long-term engraftment with marked cells in 
up to 90% of recipient mice, with up to 20% of circulating 
cells positive for the provirus. 15/30/31 
Experience in larger animals is less extensive. Two groups 
workings with dogs have shown gene transfer in 0.1-10% of 
circulating cells or progenitor cells of multiple lineages 
up to 2 years post- transplantation . 44/45 Early primate 
studies using short exposure to low-titer viral supernatant 
without addition of growth factors could not demonstrate 
transduction of long-term repopulating cells despite 
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Recombinant DNA Research, Volume 16 
