Immunological Tolerance to Luciferase and Fluorescent Proteins Using Tol Mice Enables Development of Improved Tumor Models for Investigating Immunity and Metastasis

Abstract There is a continuing need for improved preclinical mouse models of cancer that more accurately predict therapy outcomes for future clinical translation. Luciferase and bioluminescence have long been utilized to generate models conducive to noninvasive imaging to monitor tumor growth, disease progression, and response to therapy. However, luciferase, as well as fluorescent reporter proteins, are highly immunogenic, limiting their use in some syngeneic tumor models in immunocompetent mice. In this study, we described the utility of transgenic mice engineered to have tolerance to luciferase and several other reporter proteins, known as Tol mice, in cancer immunology research. Some tumor cell lines expressing both luciferase and GFP were completely rejected in wild-type mice but maintained robust growth in Tol mice. Additionally, Tol mice allowed the development of an experimental brain metastasis model and a postsurgical resection spontaneous metastasis model. Importantly, even when certain cell lines carrying reporter proteins successfully formed tumors in immunocompetent wild-type mice, underlying immunity existed that could be reinvigorated by immune checkpoint inhibitors. Therefore, caution is needed when using such models in wild-type mice, as exaggerated effects may be induced by immunotherapy. Tol mice circumvent this problem and will likely widen the number of orthotopic and metastatic tumor models that can be used in immunotherapy studies in both C57Bl/6 and BALB/c mice. Significance: Tol transgenic mice, tolerant to reporter proteins like luciferase and GFP, can be used to develop improved tumor models for studying metastasis and immunotherapy, avoiding immune rejection issues in immunocompetent mice. See related commentary by Grzelak and Ghajar, p. 2143