Tumor Microenvironment and its Implications for Antitumor Immunity in Cholangiocarcinoma: Future Perspectives for Novel Therapies

Abstract

The incidence of cholangiocarcinoma (CCA) has been increasing over the past few years. Although there are surgery, chemotherapy and other conventional treatment methods, the effect is not as expected. At present, immunotherapy has become the research frontier of cancer treatment, and CCA tumor microenvironment (TME) is becoming a hot exploration direction of immunobiology. TME can affect tumor progression through changes in metabolism, secretion and immunity. Accordingly, understanding the role played by immune cells and stromal cells in TME is important for the study of CCA immunotherapy. This review will discuss the interactions between immune cells (including CD8⁺ T cells, CD4⁺ T cells, macrophages, natural killer cells, dendritic cells, myeloid suppressor cells, mast cells, and neutrophils) and stromal cells (including cancer-associated fibroblasts, endothelial cells) in the TME of CCA. In addition, we will also discuss current research results on TME of CCA and recent advances in immunotherapy.

Keywords: cholangiocarcinoma; immune mechanism; immunotherapy; prognostic markers; targeted therapy; tumor microenvironment.

Figure 1

According to the relative anatomical location with the liver, CCA is divided into three types: ICC, pCCA and dCCA. ICC is defined as bile duct cancer located inside the liver, and the histological types are classified as mass-forming, periductal infiltrating, and intraductal growing. pCCA is defined as a secondary branch located in the hepatic duct (from the common bile duct above the cystic duct to the liver) and histologically as a Klatskin tumor. dCCA refers to distal cholangiocarcinoma located extrahepatically, and the histological type is mainly adenocarcinoma.

Figure 2

CCA TME refers to an intrinsic environment for the interaction between various cells and tumor cells including CD4 ⁺ T cells, CD8 ⁺ T cells, Tregs, DCs, NK cells, vascular endothelial cells, macrophages, neutrophils, CAFs, mast cells and MDSCs as well as body tissues.

Figure 3

CAFs has diverse sources and is differentiated from epithelial cells, mesenchyme stem cells, resident fibrohlast, HSCs, pericyte, adipocyte, vascular smooth muscle cells, and bone marrow fibrocytes. CAFs from hepatic stellate cells (HSCs) mediate the release of hepatocyte growth factor from inflammatory CAFs through direct interaction of the hsc-cafa-tumor pathway and promote the proliferation of ICC through tumor-expressed MET. VCAFs (vascular carcinoma-associated fibroblasts) secrete IL-6 (interleukin-6) to enhance the malignancy of ICC cells through the interaction of the IL-6/IL-6R axis with tumor cells, while exosomal miR-95p of ICC cells can induce IL-6 expression in vCAFs. High expression of miR-34c in tumor-derived exosomes can target and inhibit Wnt1, allowing it to activate the Wnt signaling pathway in CCA and slow the conversion of fibrocytes to CAFs. Nintedanib can treat refractory CCA by inhibiting the activation of CAFs.

Figure 4

DCs activate naive T cells by presenting antigens by phagocytosis, initiate immune responses, secrete chemotactic cytokines, chemotactic T/B cells, and present antigens to CD8 ⁺ T cells/CD4 ⁺ T cells by MHC class I/II cells. Upon activation of CD8 ⁺ T cells, perforin is released, and granzymes and FAS/FASL transmembrane glycoproteins kill tumors. mlDH1 inhibited the recruitment of CD8 ⁺ T cells and the expression of IFN-γ, whereas mlDH1 inhibitor could contact this inhibition. At the same time, HLA class I is expressed and presents tumor antigen-derived peptides to the immune system, which ultimately stimulate CD8 ⁺ T cells to show anti-tumor effects. Inhibition of B7-H4 by lentiviral transcription encoding shRNA could enhance CD8 ⁺ T cell-mediated cytotoxicity. The response of CD4 ⁺ T cells to mutated ERBB2IP antigen can be used to mediate the degeneration of metastatic epithelial cell carcinoma tissues, trametinib can lead to the up-regulation of MHC-I and PD-L1 on tumor cells in vitro, and the combination of trametinib with anti-PD-L1 drugs can enhance the anti-tumor toxicity of hepatic effector memory CD4 ⁺ T cells. The increased expression of TGF-β1 in tumor cells induces Tregs heterogeneity in TME, forms an environment conducive to tumor cell proliferation, anti-apoptosis and angiogenesis, and promotes tumor progression, a mechanism that can be inhibited by the combination of GCA. LAIR2 expressed by Tregs blocks the binding of LAIR1 by competing ligands, interferes with platelet activation and adhesion, and inhibits the classical pathway of the complement system and the lectin pathway to kill pathogens.

Figure 5

MDSCs are generated from bone marrow progenitor cellsimmature myeloid cells in the bone marrow and subsequently differentiate from blood vessels into M-MDSCs and G-MDSCs. Negative bacteria secrete CXCL1 to promote M-MDSCs transformation and generation, and M-MDSCs are converted into M0 Macrophages. Stimulation with cytokines such as IFN-γ, TNF-α, GM-CSF, or bacterial endotoxin induced the conversion of M0 Macrophages into M1 Macrophages, and IL-4, IL-10, IL-13, and IC induced the conversion of M0 Macrophages into M2 Macrophages. M2 macrophages induce the production of EMT through IL-10/STAT3 and AKT3/PRAS40 pathways. TWEAK produced by macrophages interacts with the Fn14 receptor on the surface of CCA cells and affects CCA progression. Tumor-derived exosome miR-183-5p up-regulates macrophage PD-L1 expression in TME through 315 miR-183-5p/PTEN/AKT/PD-L1 pathway, which promotes the development of immunosuppression in ICC. TAMs can interact with TANs, activate OSM/IL-11/STAT3 signaling pathway, and promote ICC progression. G-MDSCs were converted to N0 TANs and subsequently IFNs induced TANs to exhibit an anti-tumor N1 phenotype, whereas TGF-β could modulate TANs to exhibit a pre-malignant N2 phenotype. CXCL5 is highly expressed in CCA cells and promotes CCA progression by inducing neutrophil recruitment in tumor tissues via PI3K-Akt and ERK1/2-MAPK.

Figure 6

DCs capture antigens, present antigen information to effector T cells, and effector T cells receiving APC antigen presentation are activated and CTLs proliferate. CTLs move from blood vessels to tumor tissue, recognize tumor cells and infiltrate tumor tissue, attack tumor cells, induce programmed cell death (apoptosis) of tumor cells, and subsequently apoptotic tumor cells release antigens to form a dynamic cycle. PD-1 is an important immunosuppressive molecule. It prevents the immune system from killing cancer cells by regulating the immune system's response to human cells downward, as well as regulating the immune system and promoting self-tolerance by suppressing T-cell inflammatory activity. PD-L1 is a ligand expressed on the surface of tumor cells. PD-L1 is up-regulated in a variety of tumor cells. It binds to PD-1 on CTLs and inhibits CTLs proliferation and activation, so that CTLs are in an inactivated state and finally induce immune escape. ICIs can block the binding of PD-1 and PD-L1, up-regulate the growth and proliferation of CTLs, enhance the recognition of CTLs to tumor cells, activate their attack and killing function, and achieve anti-tumor effect by mobilizing the body 's own immune function.