Overexpression of natural killer group 2 member D ligands predicts favorable prognosis in cholangiocarcinoma

Abstract

The natural killer group 2 member D (NKG2D) receptor and its ligands are important mediators of immune responses to tumors. NKG2D ligands are overexpressed in several malignant tumor types; however, the prognostic value of these ligands is unclear. Here, we aimed to elucidate the role of NKG2D ligands in extrahepatic cholangiocarcinoma (EHCC). We therefore investigated the expression of the NKG2D receptor and its ligands MHC class I chain-related proteins A and B (MICA/B), unique long 16 binding protein (ULBP) 1, and ULBP2/5/6 in resected specimens from 82 patients with EHCC. All NKG2D ligands were highly expressed in EHCC. High expression of MICA/B or ULBP2/5/6 correlated with overall and disease-free survival. In contrast, high expression of ULBP1 was significantly associated with improved overall survival, but not disease-free survival. Concurrent high expression of multiple NKG2D ligands revealed significantly better overall and disease-free survival than that observed with the overexpression of any one NKG2D ligand. Co-expression of multiple NKG2D ligands was an independent prognostic indicator of improved survival. Furthermore, co-overexpression of multiple NKG2D ligands was significantly correlated with high expression of the NKG2D receptor. Inhibiting interactions between multiple NKG2D ligands and the NKG2D receptor might be a promising approach for controlling cancer progression and improving patient prognosis in EHCC.

Keywords: Cholangiocarcinoma; MHC class I chain-related proteins A and B; immunohistochemistry; natural killer group 2 member D; unique long 16 binding protein.

Figure 1

Immunohistochemical staining of natural killer group 2 member D (NKG2D) ligands in human extrahepatic cholangiocarcinoma tissues. Representative images of immunohistochemical staining of specimens are shown. Resected extrahepatic cholangiocarcinoma specimens were stained using standard streptavidin–biotin–peroxidase complex methods and 3,3′‐diaminobenzidine tetrahydrochloride and counterstained with Mayer's hematoxylin. The expression of NKG2D ligands was evaluated according to the staining intensity observed and scored as follows: 0, negative; 1, weak expression; and 2, strong expression. Cases with scores of 0 and 1 were defined as the low‐expression group, and cases with scores of 2 were defined as the high‐expression group. MHC class I chain‐related proteins A and B (MICA/B) (a), unique long 16 binding protein 1 (ULBP1) (b), and ULBP2/5/6 (c) low/high expression.

Figure 2

Kaplan–Meier plots showing overall survival in patients with extrahepatic cholangiocarcinoma tumors expressing MHC class I chain‐related proteins A and B (MICA/B) (a), unique long 16 binding protein 1 (ULBP1) (b), ULBP2/5/6 (c), and natural killer group 2 member D (NKG2D) ligand (d). (a) MICA/B overexpression was significantly associated with overall survival (P = 0.0031). (b) High expression of ULBP1 was significantly associated with improved overall survival (P = 0.0041). (c) ULBP2/5/6 overexpression was significantly associated with overall survival (P = 0.0142). (d) High expression of multiple NKG2D ligands was significantly associated with overall survival when compared to high expression of a single ligand or of no ligands (P < 0.0001).

Figure 3

Expression of the natural killer group 2 member D (NKG2D) receptor in human extrahepatic cholangiocarcinoma tissues. Immunohistochemical staining of the NKG2D receptor (a) and the NKG2D ligand, unique long 16 binding protein 1 (b), from the same patient. NKG2D receptor‐stained slides were examined for the presence of NKG2D within the cancer stroma. (c) Correlation between NKG2D ligands and NKG2D receptor expression. Co‐overexpression of multiple NKG2D ligands was significantly correlated with high expression of the NKG2D receptor (P = 0.0006).