The tumour microenvironment and immune milieu of cholangiocarcinoma

Abstract

Tumour microenvironment is a complex, multicellular functional compartment that, particularly when assembled as an abundant desmoplastic reaction, may profoundly affect the proliferative and invasive abilities of epithelial cancer cells. Tumour microenvironment comprises not only stromal cells, mainly cancer-associated fibroblasts, but also immune cells of both the innate and adaptive system (tumour-associated macrophages, neutrophils, natural killer cells, and T and B lymphocytes), and endothelial cells. This results in an intricate web of mutual communications regulated by an extensively remodelled extracellular matrix, where the tumour cells are centrally engaged. In this regard, cholangiocarcinoma, in particular the intrahepatic variant, has become the focus of mounting interest in the last years, largely because of the lack of effective therapies despite its rising incidence and high mortality rates worldwide. On the other hand, recent studies in pancreatic cancer, which similarly to cholangiocarcinoma, is highly desmoplastic, have argued against a tumour-promoting function of the tumour microenvironment. In this review, we will discuss recent developments concerning the role of each cellular population and their multifaceted interplay with the malignant biliary epithelial counterpart. We ultimately hope to provide the working knowledge on how their manipulation may lead to a therapeutic gain in cholangiocarcinoma.

Keywords: cancer associated fibroblasts; extracellular matrix; immune cells; immunotherapy; tumor associated macrophages; tumor reactive stroma.

Figure 1.. Spatial relationships of different cell types in the tumor microenvironment of CCA.

Immunohistochemistry for α-SMA (A), CD45 (B), CD34 (C), and dual immunohistochemistry for α-SMA (brown) and podoplanin (blue) (D) of formalin-fixed paraffin-embedded tissue sections obtained from surgical specimens of a patient with intrahepatic CCA undergoing liver resection. A) Cancer-associated fibroblasts identified by their immunoreactivity for α-SMA form a tight shell around the malignant bile ducts. B) Innate inflammatory cells expressing CD45 (neutrophils, macrophages, NK cells) are located in close vicinity to a large vascular space (*) consistent with their recruitment into the tumor microenvironment from the circulating compartment. C) Blood endothelial cells positive for CD34 are rarely observed nearby the neoplastic bile ducts, compared with D) the numerous podoplanin⁺ lymphatic endothelial cells laying strictly adjacent to α-SMA⁺ cancer-associated fibroblasts in between the tumoral ducts. A-C counterstained with DAPI. Original magnification: 200x.

Figure 2.. Main features enabling the tumor microenvironment proficient to tumor growth and invasion in CCA.

Compared with the normal stroma (upper side of the cartoon), the tumor microenvironment undergoes significant structural changes which profoundly affect the proliferative and invasive abilities of tumor cholangiocytes. The rupturing of the basement membrane (BM) allows invasive cholangiocytes to get access to a dense infiltrate comprising, among many cell types, activated cancer-associated fibroblasts (CAF), M2-polarized tumor-associated macrophages (TAM), regulatory T lymphocytes (Treg), and newly assembled lymphatic vessels through which tumor cells can early disseminate. A stiff extracellular matrix (ECM) provides support for the reciprocal interactions of the multiple cell types populating the tumor microenvironment.