**2. The Thyroid Cancer Microenvironment**

Over the last 20 years, the role of the microenvironment in tumorigenesis of thyroid cancer has been well-defined [15]. The components of the thyroid cancer microenvironment (stromal cells, ST, and extracellular matrix components (ECM)) surround and support cancer cells, interacting with them by direct cell-cell and cell-extracellular matrix components interaction. Moreover, a plethora of cytokines and growth factors are produced and released in the cancer microenvironment by cancer cells as well as by ST [16]. It has been demonstrated that cancer-associated fibroblasts (CAFs) surround the tumor cells and play a role in tumor initiation and promotion, tumor cell growth, spreading, and metastasization [17]. Moreover, CAFs are involved in inflammation, metabolism, drug response, and immune surveillance [17]. There is evidence that the expression of CAF-related proteins in stromal and cancer cells varies on the basis of histologic subtype of thyroid carcinomas, BRAFV600E mutation, and subtype of stroma, and an association has been reported between CAF-related proteins and poor survival [18]. It has also been demonstrated that CAFs are involved in the lymphatic spread of thyroid cancers [19], and that, in poorly differentiated thyroid cancer driven by BRAFV600E mutations and loss of Pten, there is a close association between CAF infiltration and collagen I deposition in the tumor microenvironment [20]. Moreover, CAFs produce and release many angiogenic cytokines that contribute to the angiogenic process in the tumor microenvironment [21].

Tumor-associated macrophages (TAMs) are another pivotal component of the thyroid tumor microenvironment associated with tumor cell growth, spread, and poor progno-

sis [22,23]. In the tumor microenvironment, TAMs contribute to the anti-inflammatory status because of their high expression of interleukin-10 (IL-10) and mannose receptor (MR, CD206), and low expression of IL-12 [24,25]. It has been shown that TAMs present an M2-like activated status, differently from the M1-like circulating macrophages [26,27]. TAMs contribute to angiogenesis of tumors with an increased production of proangiogenic factors, such as vascular endothelial growth factor (VEGF), platelet derived growth factor (PDGF), and basic fibroblast growth factor (bFGF), and produce a high amount of matrix metalloproteases (MMPs), which are responsible for ECM remodeling and facilitate tumor cells spread and invasion [28,29]. The relationship between positive tryptase mast cell (MCs) infiltration and thyroid cancer invasiveness or extrathyroidal extension has been demonstrated. A significantly more abundant presence and distribution of MCs in the intratumoral and peritumoral areas of thyroid cancer has also been shown with respect to adenoma [30,31]. MCs also produce a broad spectrum of chemokines (CXCL8/IL-8, CCL25/TECK, CXCL10/IP-10, CXCL1/GRO-α), interleukins (IL-6), and other molecules (TNF-α) that are involved in the epithelial-to-mesenchymal transition (EMT) activation of thyroid cancer cells [32,33]. Moreover, MCs recruited by several tumor-derived chemotactic factors, such as stem cell factor, VEGF, chemokines, and cytokines, histologically are localized close to epithelia, fibroblasts, and blood and lymphatic vessels and are involved in wound healing, angiogenesis, and lymphangiogenesis [32–34]. Neutrophils are recruited by thyroid cancer cells by releasing CXCL8/IL-8 and granulocyte colony-stimulating factor [35]. In the tumor microenvironment, neutrophils produce and release angiogenic cytokines (onconstatin-M, VEGF-A) and their granule proteins (elastase), which induce cancer cell proliferation, invasiveness, and angiogenesis [36–38]. Our group's complete gene expression profile study demonstrated that the microenvironment components surrounding the thyroid cancer cells express a genomic profile different from that of normal ST [39]. The results of this study indicate that interactions between tumor cells and ST induce in the ST the modulation of genes involved in the control of apoptosis, metabolism, cell movement, cell response to hypoxia, and cell proliferation [39].
