Besides tropism, the frequency of transformation must be considered 
when attempting to assess risk of malignancy. Unfortunately, the transformation 
frequency is difficult to estimate, even in the mouse, but existing data suggest 
that it is a very uncommon event. Animals miist be infected by the retrovirus as 
newborns, a time when persistent retroviremia develops. Viral expression 
continues in infected tissues with the majority of thymocytes expressing viral 
sequences throughout their lifetime. Despite this continued exposure to 
retrovirus the tumors that develop are clonal and occur only after a relatively 
long latency (months) . 
While the malignant potential of retroviral vectors has not been 
studied in primates we have attempted retroviral gene transfer in 21 primates 
using an autologous bone marrow transplant protocol. To date no animal has 
displayed evidence of malignancy, with animals alive 2 1/2 years post transplant. 
The probability of malignancy secondary to infection of TIL with the N2 
retroviral vector appears, therefore, to be very low. To summarize: the N2 
vector is no longer capable of establishing a retroviremia so that only a small 
number of cells (10^) would receive gene transfer. The transformation frequency 
of MoMLV appears to be very low and the vector may not have a tropism for human T 
cells. We are able to study TIL prior to reinfusion to assess if retroviral 
infection has altered the cells' phenotype or if contaminating retroviruses are 
present. Additionally, malignancy has not been observed in primates exposed to 
murine retroviral vectors or to replication competent murine retroviruses. 
B . Risk from Murine Retrovirus . 
A po^tential complication of retroviral-mediated gene transfer is the 
exposure of patient calls to replication competent murine retrovirus. This can 
occur by a recombination event between vector sequences and sequences within the 
packaging cell line. What the clinical effects might be of such an exposure is 
currently being investigated. All studies to data (19) suggest that murine 
ampho tropic virus is not pathologic for primates. In addition, however, the 
current protocol for TIL infection provides the opportvinity to test for the 
presence of murine helper virus after infection prior to infusion of the cells 
back into the patient. Thus, the absence of replication competent murine 
retrovirus can be assured. 
C. Recombination with Human Endogenous Retroviral Sequences . 
A theoretical concern is recombination between the retroviral vector 
and human endogenous retroviral sequences leading to the production of a 
replication competent human retrovirus. The probability of this occurring appears 
very small. The retroviral sequences within the N2 vector, specifically the LTR 
and a small portion of gag, have no homology with known human retroviral 
sequences (20). In addition, the retroviral sequences needed to retxim 
replicative function to our vector, (namely gag, pol, and env) , are defective in 
human endogenous retroviral sequences (21-23) . The deletion and frame-shift 
mutations found in human endogenous sequences are thought to explain the »• 
inability to detect a replication competent human endogenous retrovirus. 
While the likelihood of producing a replication competent virus is very 
Recombinant DNA Research, Volume 14 
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