Scientific Report 2008

Immunology & Microbial Science

Consequences of T-Cell Recognition of Self-Antigens and Tumor Antigens in Normal and Diabetes-Prone Mice

L.A. Sherman, C.H. Wei, E. Hamilton-Williams, G. Verdeil, R. Bos, J.A. Biggs, K.L. Marquardt

The consequence of antigen recognition by naive CD8⁺ T cells can be either tolerance or immunity, depending on the activation status of the antigen-presenting dendritic cells and the duration of exposure to antigen. Understanding the signals that result in either T-cell deletion or immunity is important in preventing autoimmunity, which is a failure to control self-destructive T lymphocytes. This understanding is also important in promoting tumor immunity, in which the goal is to promote the autoimmune destruction of tumor cells. We are comparing the consequence of the interaction of naive CD8⁺ T lymphocytes with a self-antigen expressed by the insulin-producing beta cells in the pancreatic islets in 3 different types of mice: normal mice; nonobese diabetic (NOD) mice, which are diabetes prone; and mice in which the beta cells express an oncogene that promotes spontaneous transformation and production of tumors.

Mechanisms of Protection From Type 1 Diabetes by Genetic Polymorphisms

The spontaneous diabetes that develops in NOD mice is similar to type 1 diabetes in humans. The disease process involves destruction of the insulin–producing beta cells in the pancreas by CD8⁺ T lymphocytes. In both humans and mice, genetic regions have been identified in which allelic polymorphism predisposes individuals to type 1 diabetes. We are studying the effects such allelic polymorphism, designated insulin-dependent diabetes (Idd) loci, has on the establishment of CD8⁺ T-cell tolerance.

Congenic mice that express protective alleles at Idd3/5 have normal abortive activation of islet antigen-specific CD8⁺ T cells in the pancreas, suggesting that tolerance is restored at the earliest time when naive CD8⁺ T cells first encounter antigen. In contrast, in NOD mice, such CD8⁺ T cells accumulate in the pancreatic lymph nodes and then enter the islets. This difference in the accumulation of CD8⁺ T cells in the pancreatic lymph nodes occurs in the absence of all CD4⁺ T cells. Production of radiation bone marrow chimeras suggests that Idd3/5 genes that determine tolerance are expressed by a nonlymphoid bone marrow–derived cell, possibly the antigen cross-presenting dendritic cells.

Another congenic NOD strain expresses protective alleles at the Idd9 locus. Islet-specific CD8⁺ T cells accumulate in the pancreatic lymph nodes of Idd9 mice, similar to the situation in other NOD mice. The survival of activated self-specific CD8⁺ T cells in Idd9 mice is limited, however, compared with the survival of such cells in NOD mice, indicating that compared with Idd3/5 genes, Idd9 genes correct a distinct checkpoint of tolerance.

Enhancement of Tumor Immunotherapy by Synergy Between Adjuvants Targeting Innate and Adaptive Immunity

Self-tolerance is a major barrier to effective immunotherapy because the tolerance results in the elimination of most self-reactive CD8⁺ T cells, including a subset of cells specific for tumor-expressed antigens. The tumor-specific CD8⁺ cells that remain escape tolerance because they have low avidity for tumor cells. Strategies that enhance the effector capabilities and longevity of tumor-specific CD8⁺ cells would greatly enhance tumor immunotherapy. Cytokine complexes composed of IL-2 and antibodies to IL-2 are a highly effective reagent for augmenting CD8⁺ T-cell activity. However, the cells only survive as long as they are in the presence of the cytokine complexes. When the complexes are removed, the cells soon die of cytokine withdrawal. By combining IL-2 complexes with an inflammatory adjuvant, polyinosinic acid–polycytidylic acid, we have been able to both greatly enhance the effector function of low-avidity tumor-specific T cells and promote survival of the cells long enough to achieve tumor eradication. This strategy presents many of the benefits of whole-body irradiation, including the provision of high levels of homeostatic cytokines, enhanced expansion of effector cells relative to regulatory T cells, and provision of inflammatory cytokines, and is therefore likely to be a strategy for both tumor vaccines and adoptive immunotherapy of cancer.

Role Of CD4⁺ Helper Cells in Promoting Tumor Cell Destruction by CD8⁺ Cells

CD4⁺ helper T cells can enhance the performance of CD8⁺ T cells in different ways, including enhanced clonal expansion during activation of CD8⁺ T cells, enhanced tissue infiltration by the activated effector CD8⁺ cells, and enhanced survival of the effector cells. We are assessing the effects of CD4⁺ helper T cells at various times after activation of CD8⁺ T cells to evaluate the ability of the helper cells to promote destruction of tumor cells by CD8⁺ cells. One way CD4⁺ T cells help tumor-specific CD8⁺ cells is by facilitating the entry of the CD8+ cells into the tumor tissue. This process is much less efficient if no CD4⁺ helper T cells are present within the tumor environment. In addition, we found that the lytic activity of CD8⁺ cells in the tumor is greatly augmented by the presence of CD4⁺ helper cells. The cellular and molecular interactions that result in such enhancements are currently under investigation.

Publications

Redmond, W.L., Wei, C.H., Kreuwel, H.T.C., Sherman, L.A. The apoptotic pathway contributing to the deletion of naive CD8 T cells during induction of peripheral tolerance to a cross-presented self-antigen. J. Immunol. 180:5275, 2008.

Wei, C.H., Sherman, L.A. N-terminal trimer extension of nominal CD8 T cell epitopes is sufficient to promote cross-presentation to cognate CD8 T cells in vivo. J. Immunol. 179:8280, 2007.

Wong, S.B.J., Bos, R., Sherman, L.A. Tumor-specific CD4⁺ T cells render the tumor environment permissive for infiltration by low-avidity CD8⁺ T cells. J. Immunol. 180:3122, 2008.