BackgroundCancer vaccines are designed to activate and enhance cancer-antigen-targeted T cells that are suppressed through multiple mechanisms of immune tolerance in cancer-bearing hosts. T regulatory cell (Treg) suppression of tumor-specific T cells is one barrier to effective immunization. A second mechanism is the deletion of high avidity tumor-specific T cells, which leaves a less effective low avidity tumor specific T cell repertoire available for activation by vaccines. Treg depleting agents including low dose cyclophosphamide (Cy) and antibodies that deplete CD25-expressing Tregs have been used with limited success to enhance the potency of tumor-specific vaccines. In addition, few studies have evaluated mechanisms that activate low avidity cancer antigen-specific T cells. Therefore, we developed high and low avidity HER-2/neu-specific TCR transgenic mouse colonies specific for the same HER-2/neu epitope to define the tolerance mechanisms that specifically affect high versus low avidity tumor-specific T cells.Methodology/Principal FindingsHigh and low avidity CD8+ T cell receptor (TCR) transgenic mice specific for the breast cancer antigen HER-2/neu (neu) were developed to provide a purified source of naïve, tumor-specific T cells that can be used to study tolerance mechanisms. Adoptive transfer studies into tolerant FVB/N-derived HER-2/neu transgenic (neu-N) mice demonstrated that high avidity, but not low avidity, neu-specific T cells are inhibited by Tregs as the dominant tolerizing mechanism. High avidity T cells persisted, produced IFNγ, trafficked into tumors, and lysed tumors after adoptive transfer into mice treated with a neu-specific vaccine and low dose Cy to deplete Tregs. Analysis of Treg subsets revealed a Cy-sensitive CD4+Foxp3+CD25low tumor-seeking migratory phenotype, characteristic of effector/memory Tregs, and capable of high avidity T cell suppression.Conclusion/SignificanceDepletion of CD25low Tregs allows activation of tumor-clearing high avidity T cells. Thus, the development of agents that specifically deplete Treg subsets should translate into more effective immunotherapies while avoiding autoimmunity.

Regulatory T cells (Tregs) that suppress tumor-specific T cell-mediated immune responses are the subject of an intense wave of investigation. We recently reported that a subset of Tregs, namely effector/memory CD25low cells, are responsible for suppressing high avidity tumor-specific T cells in mouse mammary tumors. Here, we discuss additional findings that clarify this mechanism of Treg-mediated immunosuppression.

Cancer vaccines are one treatment modality that can prevent the development of some cancers. The advantage of cancer vaccines is their ability to target cancer proteins specifically, thereby resulting in minimal toxicities to normal tissue. Unfortunately, immune tolerance mechanisms provide a formidable barrier to effective treatment of cancer when vaccines are used as a single agent. One way to overcome these barriers is to develop vaccination strategies that combine vaccines with tolerance-reducing agents that allow the recruitment and activation of the most potent tumor-specific T cells. In fact, accumulating data suggest that the availability and activation of a diverse high avidity T cell repertoire is a key factor in the effectiveness of cancer vaccines. To further understand the role of high avidity T cells in the treatment of cancer, we developed TCR transgenic mice that are high avidity and specific for the immunodominant CD8+ T cell epitope, RNEU420–429, that derives from the HER-2/neu breast cancer antigen. By adoptively transferring these high avidity T cells into the tolerant environment of the neu-N mouse, we have studied their proliferation, trafficking, and tumor-clearing capacity. High avidity T cell responses were evaluated following vaccination using a neu-expressing, GM-CSF-secreting, whole cell vaccine. Vaccine was given either alone or in combination with two T regulatory cell (Treg) depleting agents: Cyclophosphamide (Cy) and the anti-CD25 antibody, PC61. Our data demonstrate that the Cy specifically targets a subset of Tregs and the depletion of this subset allows for more effective tumor clearance by high avidity T cells. While most tolerance reduction strategies focus on the depletion of CD4+CD25+ T regulatory cells, our studies reveal the existence of a population of CD25low effector Tregs that can effectively traffic to the tumor microenvironment and mediate tolerance. Through targeted removal of this CD25low effector Treg population, high avidity T cells undergo increased activation, T cell trafficking, and tumor clearance. Thus, we demonstrate that vaccination in combination with targeted, effector Treg depletion can activate higher avidity T cell populations to eradicate pre-existing tumor burdens. Through an in-depth understanding of the tolerance mechanisms that mediate high avidity T cells suppression, it will be possible to design more effective vaccination strategies to activate the small but potent high avidity T cell populations present in cancer patients. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1923.