I 2.2 Cytokine Gene Therapy in Animal Tumor Models 
E Cytokine gene transfer has resulted in significant anti-tumor immune responses in several animal tumor 
[ models (10-13). In these studies, the transfer of cytokine genes into tumor cells has reduced or 
li abrogated the tumorigenicity of the cells after implantation into syngeneic hosts. The transfer of genes 
» for R.-2 (10,11), gamma interferon (IFN) (12), or IL-4 (13) significantly reduced or eliminated the 
I growth of several different histological types of murine tumors. No toxicity associated with expression of 
i the cytokine transgenes was reported in these animal tumor studies (10-13). In the studies employing IL- 
j 2 gene transfer, the treated animals also developed systemic anti-tumor immunity and were protected 
; against subsequent tumor challenges with unmodified parental tumor (10,1 1). Similar inhibition of tumor 
growth and protective immunity were also demonstrated when immunizations were performed with a 
mixture of unmodified parental tumor cells and genetically modified tumor cells engineered to express the 
IL-2 gene. We have successfully induced anti-tumor immunity in a model of colorectal carcinoma by 
immunization with IL-2 transduced cells. Immunization with a mixture of irradiated tumor cells and IL-2 
transduced cells induced systemic anti-tumor immunity capable of rejecting a subsequent live tumor celt 
challenge. Repeated immunizations with a mixture of irradiated tumor cells and IL-2 transduced 
fibroblasts abolished established, visible tumors in a subset of the treated animals (Appendix 12.7). 
Glioblastomas are known to secrete immunosupressive factors which compromise host anti-tumor 
immune responses and glioblastoma patients have depressed CD4+ helper T cells (19). Several studies, 
including our own have indicated the ability of IL-2 transduced cells to bypass the need for CD4+ helper 
T cells in generating efficacious cellular immunity (10,1 1, Appendix 12.7). 
2.3 Immunotherapy of Gliomas 
The potential utility of active immunotherapy for the treatment of gliomas was suggested by a study 
performed Mahaley et. al (16). In this investigation, patients received monthly subcutaneous 
immunizations with human glioma tissue culture cell lines. Patients were also treated with levamisole and 
received radiotherapy and BCNU chemotherapy. Patients inoculated with the U-251MG cell line had 
significantly longer survival compared to non-immunized historical control patients treated with 
levamisole, radiotherapy and chemotherapy. There was no clinical evidence of allergic encephalomyelitis 
in the immunized patients and no pathological findings of this disorder on gross or light microscopic 
evaluations performed at autopsy in a subset of patients. 
Since the performance of these studies nearly a decade ago, there have been significant advances in our 
knowledge of the immune system and in our ability to modulate immune responses. This has led to the 
development of novel experimental immunotherapies for cancer. In more recent animal studies of 
gliomas, immune rat cytotoxic T cells primed ex vivo against a rat glioma were given intravenously to rats 
with intracerebral glioma and induced regression of these brain tumors (17). In the C6 rat glioma model, 
effective treatment of intracranial tumors was observed following immunizations with tumor cells 
genetically modified to inhibit insulin like growth factor (IGF-1) gene expression by plasmids encoding 
IGF-1 anti-sense nucleotides (18). The results of these studies document the ability of systemically 
generated immune effector cells to inhibit the growth of intracranial glioma tumors (17,18). Furthermore, 
adoptive immunotherapy studies with primed lymphocytes in rodent glial tumor models have 
demonstrated the requirement for IL-2 to produce efficacious anti-tumor responses (17,20). These 
findings further support the proposed use of IL-2 transduced tumor cells. 
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