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The recent success of the anti-CLTA-4 antibody, ipilimumab, for late stage metastatic melanoma, provides proof of principle that stimulating the immune system can have profound effects on patient outcome. However, a detailed understanding of the role of immune-modulating cells within the tumor microenvironment has been confounded by the lack of a suitable model system that would allow for direct visualization of cells within the intact tumor microenvironment. Previous work has demonstrated that the zebrafish, Danio Rerio, can be genetically manipulated to produce melanomas that recapitulate BRAFV600E;P53-/- human tumors. Due to its translucency, ease of genetic manipulation and high fecundity, the zebrafish is an attractive model system that allows for in vivo characterization of cells within intact tissue. Using this model system, we have begun investigating the adaptive immune response to melanoma in zebrafish. We have focused our initial investigations on determining whether transcript expression of key adaptive immune-modulatory proteins are present in the zebrafish and to what extent they are expressed intratumorally . Our studies have found that orthologs of CLTA-4, FOXP3, and CD8 are all expressed in wild-type zebrafish, but not in rag-/- zebrafish. Furthermore, transcript levels of CTLA-4 were greatest in the melanoma tumor relative to other regions of the zebrafish, which is consistent with the constitutive expression of CTLA-4 in human melanomas. Additionally, we have cloned the promoters for zebrafish FOXP3 and CD8 and are developing fluorescent reporter lines that will enable us to visualize and ablate subpopulations of T cells within the tumor. These lines, along with previously described reporter lines for MHC II and lck, will enable us to determine the role of adaptive immune cells during melanoma onset and progression in the zebrafish. Our research supports the notion that there is conservation between the human and zebrafish adaptive immune systems, and that similar immune evasion mechanisms may be contributing to melanoma growth in zebrafish. These data position the zebrafsh as a powerful model system for immunological investigations that focus on the adaptive immune response to melanoma. Citation Format: Corrie A. Painter, Craig Ceol. Adaptive immunity in a zebrafish model of melanoma. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1273. doi:10.1158/1538-7445.AM2013-1273

SummaryThe zebrafish is a useful model for understanding normal and cancer stem cells, but analysis has been limited to embryogenesis due to the opacity of the adult fish. To address this, we have created a transparent adult zebrafish in which we transplanted either hematopoietic stem/progenitor cells or tumor cells. In a hematopoiesis radiation recovery assay, transplantation of GFP-labeled marrow cells allowed for striking in vivo visual assessment of engraftment from 2 hr–5 weeks posttransplant. Using FACS analysis, both transparent and wild-type fish had equal engraftment, but this could only be visualized in the transparent recipient. In a tumor engraftment model, transplantation of RAS-melanoma cells allowed for visualization of tumor engraftment, proliferation, and distant metastases in as little as 5 days, which is not seen in wild-type recipients until 3 to 4 weeks. This transparent adult zebrafish serves as the ideal combination of both sensitivity and resolution for in vivo stem cell analyses.

Melanoma is the most lethal form of skin cancer with high mortality rates. Most melanoma cases have activating mutations in BRAF (V600E) and the selective inhibitors of BRAF(V600E) have been successfully used in patients. However, after initial tumor regression, the majority of patients develop drug resistance resulting in tumor regrowth. It is therefore important to understand the mechanisms underlying these processes. We have recently described the role of the master melanocyte transcription factor MITF in tumor growth, regression, and recurrence. Here, we describe protocols to study regression and recurrence in vivo, as well as for histology and immunohistochemistry, using a temperature-sensitive zebrafish model of human melanoma.