Human Gene Therapy Subcommittee - 4/5/91 
(IL-2) to allow the gene-modified cells to survive in circulation and that continued 
expression of the ADA gene in vivo was evident for several weeks. Also, he presented 
preliminary data showing that there was an apparent improvement in the patient's 
immune function with gene therapy. 
In response to a question by Dr. Gellert as to whether reinfusing cultured but 
untransduced T cells would provide any advantage, Dr. Blaese said there had been some 
suggestion that non-gene corrected cells had a relatively limited survival. The gradual 
increase in ADA level in the first patient shows that gene-modified cells are experiencing 
a survival advantage in vivo and is further evidenced by a continued rise in ADA levels 
during periods between treatments in which lymphocyte counts were decreasing. 
Dr. Parkman asked Dr. Blaese to present an estimate of the frequency of ADA- 
containing cells in the T cell population. Dr. Blaese responded that this experiment was 
performed. Since the PCR signal of the patient samples was greater than that of the 
control, it was impossible to calculate the frequency. This experiment will be repeated at 
a later date with a different control. 
Dr. Epstein asked how the investigators planned to proceed in light of the fact that the 
absolute T cell count does not seem to reflect what is occurring immunologically in the 
patients. Dr. Blaese said that, due to the technical difficulties with the culture 
conditions, they expect to do eight infusions in the first patient and then to evaluate 
before proceeding. Recent success in increasing the transduction efficiency of the vector 
to 30% has eliminated the need to select in G418. However, until the Food and Drug 
Administration (FDA) certifies this vector, they will expand the treatments on the first 
patient to once every 5-6 weeks for the next couple of infusions. 
Mr. Capron questioned whether the fact that there was an apparent survival advantage 
to the gene-modified lymphocytes and a constant rise in ADA levels meant that some 
stem cells had been infected. Dr. Blaese said this was possible. However, a better 
interpretation is that the gene modification is occurring in some T cells that are 
immunologically competent, that are surviving in the circulation, and that are potentially 
capable of responding by proliferation when exposed to antigen, which results in an in 
vivo expansion. 
Dr. Mclvor said one of his students had suggested that since the cells coming from the 
patient were ADA deficient, they could be stained for ADA activity using enzyme 
histochemistry and then scored microscopically. Dr. Blaese said he had used published 
methods to accomplish this and had not been successful. Any suggestions in this regard 
would be welcomed by the investigators. 
Dr. Walters thanked Dr. Blaese for the presentation and called on Dr. Mclvor to present 
Recombinant DNA Research, Volume 14 
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