The role of tumor microenvironment in cholangiocarcinoma

Cholangiocarcinoma (CCA) is an extremely aggressive neoplasia, mostly because of diagnostic delay and lack of effective therapies. CCA is typically surrounded by a peculiar microenvironment that includes abundant desmoplastic stroma and various cell types, which support and enhance CCA development. Among the tumor microenvironment (TME) cells, there are tumor infiltrating lymphocytes (TILs), such as CD8+ and CD4+ cells, Tregs, natural killers (NKs) and B lymphocytes. TILs contribute to an immunosuppressive microenvironment that leads to tumor immune escape. Dendritic cells (DCs) may lead to immunotolerance by maturation or antigen-presentation deficiency. Hepatic stellate cells (HSCs) are one of the major precursors of cancer-associated fibroblast (CAFs), which are distinguished in various subpopulations, each with different functions and interactions with other TME cells. CAFs can promote lymphangiogenesis, early lymph-node metastasis and proinflammatory environment, but they can also provide a physical and chemical barrier to protect CCA. Tumor-associated macrophages (TAMs) could be differentiated between two phenotypes, pro- and anti-inflammatory, and they may sustain invasiveness and immunosuppression. Myeloid-derived suppressor cells (MDSCs) impair cytotoxic T lymphocytes (CTLs) function, stimulating tumor proliferation and angiogenesis. Tumor-associated neutrophils (TANs) function is influenced by the TME, leading to tumor-suppressing or tumor-promoting functions. This paper aims to provide an overview of the CCA microenvironment cells, their role in tumor progression and possible correlated diagnostic, therapeutic and prognostic implications.