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modifications in the vector and the 
packaging cell lines aimed at preventing 
recombination appear successful since 
replication virus has been detected only 
rarely after long periods of incubation. 
Therefore, each batch of supernate 
intended for clinical use is extensively 
screened for replication-competent virus. 
While extensive testing is done to insure 
that no replication-competent virus 
contaminates the vector preparation, data 
are now available in primates suggesting 
that MLV replicates poorly in primates in 
vivo and cells producing MLV are cleared 
efficiently by the primate immune system. 
These conclusions are based on a study in 
which five rhesus monkeys were injected 
with a large inoculum of retrovirus 
(reprint provided in Appendix 5) . Four 
animals were injected intravenously and 
no replicating virus could be detected in 
the blood, peripheral blood lymphocytes 
or lymph node lymphocytes at any time 
after the initial clearance of virus 
(clearance time 12 min or less) , although 
viral integration had occurred in 
lymphocytes in a small percentage of 
cells (Southern blot negative, but PCR 
positive) . Only when an immunosuppressed 
monkey was transplanted with virus 
producing-autologous fibroblasts, along 
with intraperitoneal injection of virus, 
could virus replication be detected in 
vivo. In this animal virus production was 
detected for 22 days post-exposure. After 
22 days, virus production was no longer 
detected despite the persistence of viral 
sequences. The disappearance of viral 
replication was coincident with the 
discontinuation of immunosuppression and 
development of anti-viral antibodies. To 
date no clinical illness or sequelae has 
developed in these animals. 
Additional information exists in four 
animals exposed to high titer 
replication-competent retrovirus along 
with retroviral vector at the time of 
severe immunosuppression, and autologous 
bone marrow transplant (reprint provided 
in Appendix 6) . The transplanted monkeys 
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Recombinant DNA Research, Volume 15 
