The goal of this project is to test the hypothesis that the tumor microenvironment plays a critical role in influencing T cell responses to tumor antigens. This microenvironment is comprised of a complex interaction between tumor cells, lymphocytes, myeloid cells, and stromal cells. This project has historically used TRAMP mice which express the transforming SV40 T antigen (TAg) under the transcriptional control of a prostate-specific promoter, which causes the development of murine prostate cancer. Naive TAg-specific T cells are transferred into TRAMP mice. We previously reported that CD8+ T cells become tolerized when they enter the tumor microenvironment, acquire suppressive activity, and the induction of tolerance and suppressor activity is dependent on infiltration into the tumor. These regulatory cells have the capacity to suppress proliferation of other T cells. We also reported that transfer of TAg-specific CD4+ T cells also undergo transient activation before deletion and trafficking to the prostate. Taking advantage of this transient activation, we further demonstrated that co-transfer of both CD4+ and CD8+ T cells delays tolerization of the CD8+ T cells. Continuous administration of the tumor-specific CD4+ T cells prevented T cell tolerance and reduced tumor growth. Our on-going and future studies will identify the mechanism(s) by which T cells become tolerized in the tumor microenvironment. We are currently studying TRAMP tumor-associated macrophages (TAMs). It is well-accepted that TAMs contribute to both tumor development directly as well as tumor-associated immune suppression. However, the mechanisms by which TAMs suppress T cells are diverse and incompletely understood. Our preliminary studies suggest that TRAMP TAMs express an alternatively-activated phenotype, which includes arginase, IL-4 receptor, and possibly IL-13. Interestingly, TRAMP mice that are also IL-13-deficient develop tumors at a significantly slower rate than wild-type TRAMP mice. Therefore, we will study the function, phenotype, and gene expression profiles of TAMs from IL-13-deficient TRAMP mice. This will enable us to identify potentially novel targets that will reduce immune suppression and enhance immunity to tumor antigens. In a new project, we are also studying the role of complement in tumor-induced immune suppression. Historically, a role for complement in tumor development has been reported, although the connection remains complex. In addition, recent studies suggest that complement may be associated with regulatory T cell development. We have observed that complement levels rise as tumors develop in TRAMP mice. Specifically, C5a levels are elevated in mice bearing low-grade prostate lesions, and continue to rise until mice have large tumors. We propose to study the role of C5a in tumor-induced immune suppression. We initiated a collaboration with Dr. Wenchao Song, an expert in complement biology, at the University of Pennsylvania. We will cross TRAMP mice on to the C5-deficient background as well as the DAF-deficient background (which results in elevated C5 levels) and study tumor development and function of lymphoid and myeloid cells that infiltrate the tumors of these mice.
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