517. Anti-Tumor Immunity Is Elicited by a Conditionally Replicative Adenovirus in Canine Osteosarcoma

Abstract

Conditionally replicative viruses have been proposed as a potential therapy for a wide variety of tumors. Initially, these viruses were thought to exert their anti-tumor properties through the lysis of infected tumor cells. However, there is the potential that oncolytic viruses may also elicit anti-tumor immune responses in both the context of viral antigens and potentially tumor antigens. Canine osteosarcoma is an excellent intermediate animal model in which the ability of a conditionally replicative adenovirus (CRAd) to induce anti-tumor immunization can be uniquely ascertained. This tumor exhibits many of the same characteristics of the human disease, including metastasis to the lungs. A CRAd based on canine adenovirus type 2 with the E1 gene driven by the osteocalcin promoter was administered to canine patients with confirmed osteosarcoma of the appendicular skeleton. Cellular and humoral immune responses, as well as cytokine profiles and regulatory T-cells were assessed before and after treatment. The number of dogs surviving past one year was approximately 20%. No changes were identified in levels of cytotoxic T-cells after therapy, when using autologous tumor cells as targets. Likewise, no significant changes in a panel of cytokines were observed. However, evaluation of humoral immune responses by western blot and flow cytometry, using autologous tumor, indicated that all dogs in the study had pre-existing antibodies to their own tumors and that these antibodies increased post treatment. This increase consisted of both an increase in the amount of antibody present and the recognition of new antigenic determinants. Serum from treated dogs also cross-reacted with tumor cells from unrelated dogs as well as a canine melanoma cell line. These results indicate that treatment with a CRAd does result in active immunization against the tumor, however it is unclear if the observed immune responses play any role in patient outcomes. Based upon these findings, we can now hypothesize that manipulation of these immune responses by including immune modulating genetic constructs in the CRAd may serve to alter the type and efficacy of the patient's immune response and enhance survival.