While early studies suggested that there were only """"""""a few"""""""" cancers susceptible to immune based therapy, emerging data has demonstrated that a wide array of cancer can be approached with immunotherapy. Indeed, in 2011, immunotherapy has documented efficacy in malignant melanoma, renal cell carcinoma, squamous cell carcinoma, prostate cancer and leukemia. This project seeks to determine the extent to which pediatric tumors can be treated with immune based therapies and to develop new immunotherapies targeting childhood cancer. The first accomplishment of this project in FY11 was publication of our results on the development of a new mouse model to study the immunobiology of childhood cancer and to model new therapies. Specifically, we developed a mouse model of embryonal rhabdomyosarcoma in C57Bl/6 mice that we used to evaluate immune responsiveness in this disease. We demonstrated that embryonal rhabdomyosarcoma is immunogenic, shows diminished growth when animals were pretreated with a whole cell vaccine, and grew more slowly or shrunk when adoptive immunotherapy was administered with and without dendritic cell vaccines. We also demonstrated that regulatory T cells contribute to immune escape in rhabdomyosarcoma. This work, for the first time, provided clear evidence that immunotherapy was a viable therapeutic option in rhabdomyosarcoma (Meadors et al, Ped Blood and Cancer, 2011). Ongoing studies have demonstrated that in addition to use of FOXP3+ regulatory T cells, rhabdomyosarcoma also mediated immune escape via expansion of myeloid derived suppressor cells. We have demonstrated that depletion of these cells diminished rhabdomyosarcoma growth and are currently involved in studies aimed at identifying the factor (or factors) produced by rhabdomyosarcoma bearing mice that induce the dramatic expansions of these immunosuppressive cells. In addition, we have demonstrated that rhabdomyosarcoma expresses an immunosuppressive ligand called PDL1, which induces """"""""tolerance"""""""" or """"""""exhaustion"""""""" in tumor reactive T cells. Treatment of mice with anti-PD1 antibodies induces tumor regression and prevents tumor growth. This is an important observation because early clinical trials are currently underway with anti-PD1 antibodies and these preclinical results provide a foundation for clinical development of these agents in pediatric solid tumors. We are currently pursuing these possibilities with pharmaceutical companies engaged in these studies. A second ongoing accomplishment in this project has been the development of a genetically engineered mouse that accurately models pediatric tumors in mice. These findings were a discovery in the true sense because we did not expect these mice to develop cancer, yet they reproducibly develop cancers that closely resemble T cell acute lymphoblastic leukemia, thus providing potential new insights into the oncogenesis of this disease. Specifically we developed a T cell transgenic mouse whereby the vast majority of T cells have specificity for """"""""survivin"""""""" an anti-apoptotic molecule considered to be a universal tumor antigen. We hypothesized that these mice would be immune to tumor growth as most tumors express high levels of survivin. Unfortunately and remarkably, expression of this T cell receptor did not prevent tumor growth but surprisingly induced T cell lymphoblastic leukemia in essentially all mice. This was not due to insertional mutagenesis because it occurred in 3 separate founders but rather appears to be due to self-reactivity with survivin antigens expressed in the thymus that led to expansion of early thymic progenitors followed by acquisition of NOTCH mutations. We saw similar development of T-cell ALL in mice with TCR transgenes specific for gp100 and for WT1, thus implying that gene therapies aimed at expressing TCRs specific for tumor antigens early in thymopoiesis carry a substantial risk for secondary leukemia. These insights provide novel clues to potential dangers of genetic engineering that involves targeting developing thymocytes toward self antigens. This work is being prepared for publication. A third ongoing accomplishment in this project is embodied by the substantial progress our laboratory has made during FY11 in developing chimeric antigen receptors targeting childhood tumors. Specifically, during this period, we have developed chimeric antigen receptors targeting CD19, CD22 on pediatric leukemia, GD2 on pediatric solid tumors, B7H3, ALK and FGFR4 on pediatric solid tumors. These chimeric antigen receptors (also known as CARs) are genetically engineered into a retroviral vector and expressed in human T cells. In in vitro and in xenograft models, they demonstrate antitumor activity, which varies based upon differing costimulatory domains and different antigenic targets. We are currently seeking to elucidate the principles that dictate potent vs. less potent activity as a means for optimize antitumor effects and prioritizing those receptors which should be tested in early phase triails. We anticipate initiation of clinical trials within FY12 using these new therapeutic agents.
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