infection similar to that associated with measles virus and subacute sclerosing 
panencephalitis in human beings. The disease is due to a paramyxovirus. 
Multiple samples from different organs including brain were analyzed by PCR for 
the presence of retrovirus vector DNA or replication-competent amphotropic 
proviral genomes. Neo gene sequences were only detected in peripheral blood 
granulocytes and marrow cells (Figure 5). None of the samples contained 
amphotropic envelope gene sequences. It is therefore unlikely that the 
development of encephalitis was related to gene transduction into transplanted 
peripheral blood cells. 
3. Safety of retrovirus-mediated gene transduction in Rhesus monkeys and humans 
Five rhesus monkeys were exposed to replication-competent murine amphotropic 
retrovirus under various conditions to determine the potential clinical significance 
of an in vivo exposure to these viruses for primates (37). Retrovirus was infused 
intravenously into three normal animals and into one animal which was 
immunosuppressed by pretreatment with prednisone (5mg/kg/day) and 
cyclosporine A (10 mg/kg/day) for 26 days prior to virus infusion. Virus was 
cleared rapidly (within 15 minutes after the infusion) from the circulation and 
subsequent viremia has not been detected (mean follow-up of 27.4 months). A 
fifth monkey was immunosuppressed and received an intraperitoneal injection of 
replication-competent virus as well as a subcutaneous implant of virus-producing 
autologous fibroblasts. This animal developed viremia for two days post- 
inoculation. Replicating virus was recovered from peripheral blood mononuclear 
cells and lymph node cells for up to 22 days after virus-inoculation. Subsequent 
analysis has been negative for up to 5 years post-inoculation (38). No clinical 
illness was detected any time after virus exposure in the five animals studied. In 
a separate experiment, four monkeys received an otherwise lethal dose of total 
body irradiation followed by infusion of autologous marrow' which had been 
exposed to a replication-defective retrovirus vector which was contaminated with 
replication-competent virus at a high titer (1 x 10 6 cfu/ml) (39). Animals were 
studied for up to 7 years after transplantation and remained alive and well (38). 
No replicating virus or amphotropic envelope sequences were detected in 
peripheral blood or marrow although antibodies directed against the p30 and 
gp70 viral antigens were found, indicating that the animals had been exposed to 
replication-competent retrovirus. These findings suggest that even during a time 
of severe immunosuppression after total body irradiation and bone marrow 
transplantation, primates can clear replication-competent murine amphotropic 
retroviruses. These results are in contrast to a recent observation that three of 
ten rhesus monkeys developed T-cell lymphomas 6 to 7 months after they had 
received marrow transduced with retrovirus vectors which were contaminated 
with replication-competent virus (40). Tumor cells contained the replication- 
competent virus, but lacked the retrovirus vector genome, suggesting that the 
spread of replication-competent virus through the regenerating hematopoietic 
tissue caused tumor development by insertional mutagenesis. These findings 
indicate the need for stringent screening of vector-producing packaging cells to 
exclude contamination with replication-competent virus. 
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Recombinant DNA Research, Volume 18 
