Gene Therapy Using Tet-Repressor System to Modulate Prostate Tumor Microenvironment

Prostate cancer is the most commonly diagnosed malignancy in men in the United States and is projected to be the third most frequent cause of male cancer-related deaths in 2007 after lung and skin cancers. The initial treatment for prostate cancer at early stages is prostatectomy or radiation, which usually is curative. However, approximately 20% of patients are not cured by such treatments and their cancer recurs, sometimes with long latencies. In other patients prostate cancer is diagnosed only after the cancer has metastasized and there are no effective therapies at this stage. Therefore immunotherapy seems to be a promising approach to treat metastatic prostate cancer through enhancing tumor-specific T cell responses. In this regard, there is a growing interest in the generation of fully competent dendritic cells (DCs) that are known to be potent antigen presenting cells and capable of activating naïve T cells. While DCs need to acquire a mature phenotype to induce T cell activation, it is known that the microenvironment of many tumors including prostate tumors is immunosuppressive and prevents DC maturation. We used the transgenic adenocarcinoma of mouse prostate (TRAMP) model to show that DCs infiltrating prostate tumors are phenotypically immature and using an in vitro assay we showed that TRAMPC2 cells but not granulocytes are the major inducers of this phenotype. We used a well-defined orthotopic prostate cancer model to study chemokine/cytokine vaccines. Expression of secondary lymphoid tissue chemokine (SLC), granulocyte macrophage-colony stimulating factor (GM-CSF) or CD40 ligand (CD4OL) in the TRAMP tumor microenvironment (TME) was chosen to induce co-localization of T cells and DCs and their interaction, expand DCs and induce their maturation. In order to make a clinically relevant model in this study we took advantage of the tetracycline inducible expression system that enabled us to control the expression of the chemokines and cytokines during the course of study. We showed that expression of SLC in the TRAMP TME inhibited tumor growth, decreased metastasis and increased survival of tumor bearing mice. Although CD4OL transfected TRAMPC2 cells did not grow in vivo and GMCSF transfected TRAMPC2 cells failed to grow tumors after in vitro passages, using in vitro assays we showed that these molecules reversed the inhibition of DC maturation induced by TRAMPC2 cells