Heterogeneity of Cholangiocarcinoma Immune Biology

Abstract

Cholangiocarcinomas (CCAs) are aggressive tumors arising along the biliary tract epithelium, whose incidence and mortality are increasing. CCAs are highly desmoplastic cancers characterized by a dense tumor microenvironment (TME), in which each single component plays a fundamental role in shaping CCA initiation, progression and resistance to therapies. The crosstalk between cancer cells and TME can affect the recruitment, infiltration and differentiation of immune cells. According to the stage of the disease and to intra- and inter-patient heterogeneity, TME may contribute to either protumoral or antitumoral activities. Therefore, a better understanding of the effect of each immune cell subtype may open the path to new personalized immune therapeutic strategies for the management of CCA. In this review, we describe the role of immune cells in CCA initiation and progression, and their crosstalk with both cancer-associated fibroblasts (CAFs) and the cancer-stem-cell-like (CSC) niche.

Keywords: cancer stem cells; cancer-associated fibroblasts; cholangiocarcinoma; heterogeneity; immune cells; liver; tumor microenvironment.

Figure 1

Traditional classification of monocytes. Traditionally, monocytes are classified as classical, intermediate and non-classical. Intermediate and non-classical monocytes can differentiate from the classical subset. The figure shows markers, molecules and activities of each subset.

Figure 2

Monocyte subclasses in CCA. Tie-expressing monocytes and TNF-α-producing monocytes are identified as subgroups of “antitumoral monocytes” (right, green) in CCA, whereas immunosuppressive monocytes and chemotactic-molecule-producing monocytes are classified as subsets of “protumoral monocytes” (left, purple).

Figure 3

Classification, recruitment and functional activity of myeloid-derived suppressor cells (MDSC). (A) Subclassification of MDSC. MDSC are subgrouped into granulocitic (PMN-MDSC), representing the 80% of total MDSC, and monocytic (M-MDSC). (B) Recruitment and functional activity of MDSC. MDSC are recruited into CCA tissue mainly through the FAP–STAT3–CCL2 pathway and they cover their protumoral activity mainly by inhibiting T cells, by affecting angiogenesis and by sustaining the cancer stem cell niche.

Figure 4

Neutrophil recruitment to the tumor site. TAN are recruited to the tumor site by interaction with different molecules produced by other cell types such as GM-CSF, granulocyte colony-stimulating factor (G-CSF), VEGF and IL-1β produced by CAFs; IL-6 and IL-8 produced by TAMs; and CXCL1, CXCL2, IFN-γ and TNF-α produced by T cells. Moreover, cancer cells also recruit neutrophils to the tumor site by releasing CXCL5. Once they reach the tumor site, TAN release different factors described to be highly involved in CCA biology such as MMP-8, MMP-9, CXCL1, CXCL2, CXCL6, CXCL8, CCL7 and VEGF.

Figure 5

The crosstalk between immune cells and cancer-associated fibroblast (CAFs). CAFs are in continuous interaction with tumor cells and with other components of the tumor microenvironment, promoting tumor progression and metastasis. CAFs can directly induce CCA proliferation, survival and invasion through Notch, Hh, HGF, PDGF-BBB, HB-EGF, SDF-1, MMP1, MMP2, MMP9, periostin and tenascin-c release. Moreover, CAFs can activate endothelial cells via VEGF-C and VEGF-A release, and, hence, promote metastasis. Furthermore, CAFs can induce an immunosuppressive microenvironment by directly producing TGF-β, which is a potent inhibitor of antitumor immunity, as well as by releasing thrompodin-1, which subsequently activates TGF-β. Additionally, FAP⁺ CAFs, which are characterized by an inflammatory phenotype and are the main cells responsible for CCL2 release, enhance MDSC recruitment into the TME to exert their immunosuppressive function. Additionally, CAFs can also recruit immune cells from the bloodstream to the tumor site by releasing VEGF, FGF, HGF, IGF, CCL2, CXCL12, CXCL14, SDF-1 and MMP.

Figure 6

The crosstalk between immune cells and cancer stem cells. Cancer stem cells (CSCs) are involved in a circular network of connection with tumor cells and the other components of the tumor microenvironment (TME). Protumoral features and sustainment of CSC are supported by the dysregulation of Wnt/β, Notch, Hedgehog, MAPK/ERK and TGF-β which leads to proliferation, self-renewal and aggressiveness. CSC sustainment is also promoted by macrophages through the release of IL-6 and TGF-β. On the other hand, CSCs themselves can shape immune cell behavior, affecting monocyte recruitment and differentiation, by secreting IL-13, IL-34 and OA. Furthermore, periostin released by CSCs and CAFs can guide macrophage recruitment and infiltration in CCA tissue.