culture of human bone marrow cells with autologous stroma 
increases the efficiency of retroviral marking of human 
hematopoietic progenitors (45). 
Several studies have shown gene transfer into the hematopoietic 
cells of large animals after autologous BMT. Kantoff et al. 
demonstrated that the neo R and adenine deaminase ( ADA ) genes were 
present in three cynomalogous monkeys 2 months after 
transplantation (46). A study in dogs found 0.03 to 0.14% of CFU-GM 
progenitors resistant to drug selection after infection and BMT (47). 
A recent study used PCR to detect vector genome in Rhesus monkeys 
at 32, 54, and 99 days post-BMT (35). These studies show that 
multipotential myeloid progenitor cells have been transduced 
because there is convincing evidence that clonogenic progenitors 
sampled from the animals 1 to 2 months after transplantation are 
resistant to selective drugs or their progeny positive by PCR 
analysis (45). The use of longer-term supernatant infections in the 
presence of hematopoietic growth factors will hopefully improve 
the efficiency of gene transfer into primates and other large 
animals. 
Recently, three monkeys that had been transplanted with BM cells 
bearing the neo gene in NHLB1 by Nienhuis, et al. developed T-cel! 
leukemia/lymphoma. These monkeys' bone marrow cells had been 
infected with a very high titer producer cell line which produces not 
only the neo retroviral vector (N 2 ), but also replication competent 
helper virus at a titer of 10 4 -106 pfu/ml. Appendix III is a letter to 
the FDA describing the evaluation of these three monkeys. It is 
believed that these three lymphomas were caused by chronic wild- 
type amphotrophic helper retrovirus infections. The animals were 
chronically viremic and the tumor cells contained multiple (20-30) 
copies of the helper virus genome. Other monkeys transplanted with 
BM cells infected with the helper virus-free LNL6 vector have shown 
no evidence of lymphoma or any other malignancy. It is believed that 
these malignancies were not related to retroviral vector gene 
insertion. 
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Recombinant DNA Research, Volume 16 
