Immune Suppression Mediated by Myeloid and Lymphoid Derived Immune Cells in the Tumor Microenvironment Facilitates Progression of Thyroid Cancers Driven by Hras and Pten Loss.

Thyroid cancer is the most common endocrine malignancy and is predicted to be the 4 most commonly diagnosed cancer by 2030. Approximately one-half of follicular thyroid carcinomas (FTC) contain genetic alterations in family members. Furthermore, Cowden's disease, which is characterized by loss of , predisposes for the development of FTC in humans. We have shown that thyroid specific expression of at endogenous levels and inactivation ( mice) leads to the development of FTCs that closely recapitulate human disease, with complete penetrance at one year. In patients, FTCs metastasize via the bloodstream to distant sites, frequently the lungs, bones and brain. The first objective of the study was to determine if these mice developed metastasis to relevant sites. Indeed, spontaneous metastasis to the lungs was observed in 56% of mice. We next sought to identify the cellular components within the tumor microenvironment (TME) of FTC that contribute to tumor progression and metastasis via FACS analysis. Surprisingly, a large amount of immune infiltrate was observed. thyroid tumors were comprised of 68.5 ± 11.79% CD45 cells, in stark contrast to wild-type (WT) thyroids which were comprised of 17.6% CD45 cells. Further, 53.1 ± 10.9% of the CD45 cells from thyroid tumors were of myeloid-lineage (CD11b), consisting of macrophages (F4/80Gr-1) and myeloid-derived suppressor cells (F4/80Gr-1). Further, tumors contained Arginase-1 positive cells as determined by immunohistochemical analysis, supporting an immunosuppressive TME in thyroid tumors. We next evaluated whether or not cytotoxic (CD8) or helper T cells (CD4) were recruited to tumors. The majority of T cells in these tumors were double positive for CD4 and CD25, markers of immune suppressive regulatory T cells (T). Additionally, we identified Foxp3 positive cells by immunohistochemical analysis of tumor sections, indicating a functional suppressive T phenotype . tumor cell lines displayed increased secretion of SDF-1, I-TAC, CCL9/10, and MCP5, cytokines that have been reported to play a direct role in the chemotaxis of immune cells and thus could contribute to the increased recruitment of myeloid and lymphoid derived cells in tumors. These studies are the first to identify and implicate the interaction between tumor cells and immune cells in Ras-driven thyroid cancer progression, which we hope will lead to the development of more effective therapeutic approaches for aggressive forms of thyroid cancer that target the TME.