Human Gene Therapy Subcommittee - July 29-30, 1991 
the most potent of biologic inducers, and the use of an HLA antigen is another 
advantage of the protocol. The introduction of the HLA antigen gene into tumors 
would elicit an immune response to this specific cell surface molecule. This in turn 
would induce degradation and lysis of the cells. The normal tumor-associated antigens 
(that otherwise would not be recognized as being abnormal) would now be presented 
to the immune system with a fairly vigorous cytokine response. This allows the 
immune system to respond to cells that otherwise would not be recognized. Data were 
presented showing the presence and expression of the HLA-B7 gene in the cells and 
the immune response to cells expressing the gene in the arterial wall tissue of pigs. 
The immune response was significant. Four independent parameters used to assess 
recombinant gene expression in vivo were described: (1) the measurement of the 
DNA, (2) the measurement of the histologic response in the tissue, (3) the direct assay 
of the protein, and (4) the determination of the immune response to that protein. The 
immune response is largely cell-mediated in this system. 
Dr. Nabel presented data in which a comparable vector was constructed with the 
allele and introduced into CT-26, a mouse colon carcinoma cell line. When these 
transduced lines were introduced into mice, all of the parental tumors grew in mice 
that were of the same haplotype as the tumor. No tumors grew if they expressed the 
inappropriate MHC gene. In the best case scenario, incompatibility should lead to 
total rejection of the tumor. To determine what tissues take up the DNA, 20 mice 
were analyzed. Every animal had DNA in the tumor at the injection site. 
Occasionally, the DNA was found in heart and/or kidney tissues. There does not 
appear to be a rampant spread of the DNA to other tissues; it appears to be confined 
primarily to the tumor. Cytolytic T cells obtained from these animals, with either 
retrovird vectors or with liposomes, were capable of lysing the parental unmodified cell 
line. Therefore, the treatment provides a mechanism for boosting the responsiveness 
of the immune system to the unmodified cell. The treatment is not curative, but the 
ability of the immune system to fight the tumor is enhanced. Therefore, the treatment 
could be used as an adjuvant to current existing approaches. 
Dr. Nabel summarized the effects of the treatment in terms of the therapeutic efficacy 
in mice. In a series of experiments using 14 control animals and 12 animals that 
received the vector, neither group responded to treatment. Four of six animals 
responded to injection of the gene. Of the pre-immunized animals, none responded 
to control vectors, and five of six responded to the gene. Clearly, more dramatic 
effects are observed with the pre-immunization. Subsequent modifications of the 
protocol would build in pre-immunization of the patients. 
Dr. Nabel addressed some of the safety issues. In animal experiments vectors carrying 
the histocompatibility antigen gene were injected into liposomes. The investigators 
generated PCR expression data. Organ toxicity data was obtained by monitoring serum 
enzymes. These enzymes are often indicative of autoimmune response. In more than 
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Recombinant DNA Research, Volume 14 
