Doctor of Philosophy
Anatomy & Neurobiology
J. L. Haar
Previous studies have determined adenovirus mediated herpes simplex thymidine kinase gene transfer (AV-TK) to be effective for the treatment of experimental gliomas. In this study we report three distinct phenomenon. First, animals with complete regression of subcutaneous tumors upon intratumoral injections of AV-TK with concomitant Ganciclovir® (GCV) administration developed tumor immunity. These animals had the ability to reject a subsequent inoculum of lethal doses of tumor cells. This tumor immunity was long standing and protective as far as 6 months from the time of initial tumor ablation. Of interest, adoptive transfer of splenocytes from AV-TK treated-tumor ablated animals to naive animals conferred resistance to tumor formation upon injection of lethal doses of tumor cells. This data strongly indicated the mechanism of tumor immunity was cell mediated. Further analysis of the anti-tumor immune response implicated CD8a83 cytotoxic T-lymphocytes as the effector cell. Animals with complete tumor regression survived over 300 days and showed no signs of tumor relapse. Therefore, treatment of solid unifocal tumors with AV-TK and GCV may be able to prevent tumor relapse through the generation of an anti-tumor immune response.
Secondly, we determined that AV-TK and GCV treatment efficacy was dependent on tumor antigenicity. Two different subcutaneous tumor models were utilized; the weakly immunogenic 9L and the strongly immunogenic RT2. For the same dose of intratumorally injected AV-TK, a greater percentage of RT2 tumor were eliminated as compared to 9L. Final survival efficacy was dependant on the tumor type and the initial tumor size.
In studying the importance of host immunity in tumor progression, we have determined that in vivo, the GCV mediated bystander effect was not sufficient to result in tumor eradication without involvement of a host immune response. In athymic rats, 9L tumors failed to regress upon AV-TK and GCV treatment. In contrast, tumors of similar size were ablated upon treatment in immunocompetent animals.
Of related significance, adenovirus mediated gene transfer facilitated generation of tumor immunity. Animals with tumors ablated by AV-TK and GCV treatment developed an anti-tumor immune response which was protective against further tumor engraftment. In contrast, alternative treatments such as surgical excision of subcutaneous gliomas or tumor vaccination was not sufficient to protect against secondary tumor challenge. Injection of adenovirus altered the amount and phenotypes of tumor infiltrating lymphocytes from the NK phenotype towards tumor specific CD8+ CTL cells. This immunomodulatory property was potentially responsible for generation of the immune response.
Therefore, AV-TK was effective through two mechanisms. Transfer of the HSVTK gene conferred GCV sensitivity resulting in substantial tumor regression and the adenovirus backbone served as an immune adjuvant to augment generation of host tumor immunity. This immunomodulatory property of adenovirus vectors is an added advantage to their use for cancer gene therapy.
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