Purpose: Immunotherapy for cancer has been most well developed for malignant melanoma, renal cell carcinoma and chronic myelogenous leukemia where interleukin-2 and cell therapies have been shown to have antitumor activity. It remains unclear whether the vast array of other cancers are equally susceptible to immune based therapies. This project seeks to determine whether pediatric tumors are immunogenic and if so, to identify specific targets that can be used to develop effective immune based therapies. Materials and Methods: In order to answer this question, new materials needed to be generated that accurately model pediatric tumors in mice. We therefore spent considerable effort to create novel models of pediatric cancer on the C57BL/6 background in mice and have made substantial progress in this regard. These models address a critical barrier to progress in this field since animal models provide the essential first step in understanding host:tumor interactions and immune studies are much more efficient when undertaken in the BL/6 model where many reagents and mice are available with genetic modifications in specific immune pathways and are critical reagents for identifying nodel points in the host:tumor interactions. Major accomplishments of this project are the development of three pediatric tumor models. First, an acute lympoblastic leukemia model derived from a C57BL/6 mouse with transgenic expression of the E2A-PBX translocation was generated and cells derived from this mouse have been adapted to serial transfer in mice and can readily induce leukemia in immunocompetent mice. Second, C57BL/6 mice which were transgenic for hepatocyte growth factor and knock-out for p53 developed an embryonal rhabdomyosarcoma, which was adapted for growth in culture and induces local and metastatic rhabdomyosarcoma in C57BL/6 mice. Third, C57BL/6 mice, which were transgenic for the N-myc oncogene, developed a neuroblastoma which grows in culture and induces local and metastatic neuroblastoma in C57BL/6 mice. These models form the basis for a series of studies to investigate the immunobiology of these three pediatric tumors. Additional accomplishments thus far include the demonstration that whole cell tumor vaccines protect from tumor growth in all models. This was surprising since pediatric tumors, especially acute lymphoblastic leukemia, was believed not to be immunogenic, yet protection from tumor challenge after administration of irradiated tumor is the classical model for assessing immunogenicity. In addition, peptide based vaccine targeting an immunodominant epitope of murine survivin also induced some protection in all models, but this is not as complete as the whole cell vaccine. Similarly, peptide based vaccine targeting an immunodominant epitope of WT-1 was also somewhat effective in the leukemia model. In all cases, depletion of regulatory or suppressive T cells prior to vaccination improves effectiveness. . Importantly, CD8 depletion inhibits the effect of the tumor vaccine, thus implicating antigen specific CD8+ T cells in the antitumor immune response. Despite this clear evidence for immunogenicity and immune mediated protection, we have found it much more challenging to induce tumor regression of established disease using tumor vaccines. In no case have we seen tumor regression with whole cell vaccine in the presence of established tumor. Remarkably however, treatment of mice with PT-100, a modulator of the immune response induces regression of rhabdomyosarcoma in this model. In summary, this work provides clear evidence that these pediatric tumors are immunogenic, that they are susceptible to immune based killing, and that regulatory T cells provide one mechanism of tumor immune escape. They also provide a model system to begin to molecularly identify the tumor antigens. Future work will isolate antigen specific, tumor reactive CD8+ T cells and seek to identify tumor antigens using reverse cloning. It will also further interrogate the potential for survivin, as a candidate tumor antigen, to be an effective target of immune based therapies in these tumors.
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