Gastric cancer (GC) is the third most common cause of cancer death worldwide. Natural killer cells play an important role in the immune defense against transformed cells. They express the activating receptor NKG2D, whose ligands belong to the MIC and ULBP/RAET family. Although it is well established that these ligands are generally expressed in tumors, the association between their expression in the tumor and gastric mucosa and clinical parameters and prognosis of GC remains to be addressed. In the present study, MICA and MICB expression was analyzed, by flow cytometry, in 23 and 20 pairs of gastric tumor and adjacent non-neoplasic gastric mucosa, respectively. Additionally, ligands expression in 13 tumors and 7 gastric mucosa samples from GC patients were evaluated by immunohistochemistry. The mRNA levels of MICA in 9 pairs of tumor and mucosa were determined by quantitative PCR. Data were associated with the clinicopathological characteristics and the patient outcome. MICA expression was observed in 57% of tumors (13/23) and 44% of mucosal samples (10/23), while MICB was detected in 50% of tumors (10/20) and 45% of mucosal tissues (9/20). At the protein level, ligand expression was significantly higher in the tumor than in the gastric mucosa. MICA mRNA levels were also increased in the tumor as compared to the mucosa. However, clinicopathological analysis indicated that, in patients with tumors >5 cm, the expression of MICA and MICB in the tumor did not differ from that of the mucosa, and tumors >5 cm showed significantly higher MICA and MICB expression than tumors ≤5 cm. Patients presenting tumors >5 cm that expressed MICA and MICB had substantially shorter survival than those with large tumors that did not express these ligands. Our results suggest that locally sustained expression of MICA and MICB in the tumor may contribute to the malignant progression of GC and that expression of these ligands predicts an unfavorable prognosis in GC patients presenting large tumors.

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Gastric cancer (GC) is the third most common cause of cancer‐related death worldwide. Invariant natural killer T (iNKT) cells are innate‐like cytotoxic T lymphocytes involved in tumor immune surveillance. They can be activated either through CD1d‐presented glycolipid antigens recognized by their invariant T‐cell receptor, cytokines or by sensing tumor‐associated stress‐induced ligands through the natural killer group 2, member D (NKG2D) receptor. Although the number and functionality of iNKT cells may be decreased in several types of cancer, here we show that GC patients presented a mild increase in iNKT cell frequencies and numbers in the blood compared with healthy donors. In GC patients, iNKT cells, expanded in vitro with α‐galactosyl ceramide and stimulated with phorbol 12‐myristate 13‐acetate and ionomycin, produced higher levels of interleukin‐2 and transforming growth factor‐beta, while their capacity to degranulate remained preserved. Because tumor‐derived epithelial cell adhesion molecule‐positive epithelial cells did not display surface CD1d, and NKG2D ligands (NKG2DLs) were detected in the gastric tumor milieu, we envisioned a role for NKG2D in iNKT cell functions. Peripheral iNKT cells from GC patients and controls presented similar levels of NKG2D; nevertheless, the percentages of interferon‐γ‐producing and CD107a‐positive iNKT cells from patients were reduced upon challenge with CD1d‐negative, NKG2DL‐positive K562 cells, suggesting a compromised response by iNKT cells in GC patients, which may not result from impaired NKG2D/NKG2DL signaling. The decreased response of iNKT cells may explain the fact that higher frequencies of circulating iNKT cells did not confer a survival benefit for GC patients. Therefore, functional impairment of iNKT cells in GC may contribute to tumor immune escape and favor disease progression.

Background: Natural killer (NK) cells are paramount for immunity against infectious agents and tumors. Their cytokine and cytolytic responses can be mediated by natural killer group 2, member D (NKG2D), an activating receptor whose ligands (NKG2DL) expression is induced in conditions of cell stress and malignant transformation. Since sustained expression of NKG2DL MICA is related to lower survival rates in gastric adenocarcinoma patients, and Helicobacter pylori infection contributes to tumorigenesis; we asked whether H. pylori stimulus could promote NKG2DL expression on human gastric adenocarcinoma cells.Methods: Heat-killed H. pylori (HKHP) was used to stimulate MKN45 cells before analysis of NKG2DL and Toll-like receptor 4 (TLR4) protein levels by flow cytometry and transcripts by real-time PCR. LPS from Rhodobacter sphaeroides and inhibitory peptide Pepinh MYD were used to inhibit TLR4/MyD88 signaling pathway to assess its participation on NKG2DL expression. NK cell-mediated cytotoxicity was measured by lactate dehydrogenase (LDH) and CD107a mobilization assays.Results: Stimulation of MKN45 cells with HKHP increased MICA, ULBP4 (another NKG2DL), and TLR4 at the protein and transcriptional levels. MICA, but not ULBP4 expression, was upregulated in a TLR4/MyD88-dependent manner. Furthermore, the presence of NKG2DL on the surface of HKHP-stimulated MKN45 cells enabled NK cell cytotoxic activation. Conclusions:Our data indicate that induction of NKG2DL expression on gastric adenocarcinoma cells by H. pylori promotes an immune response that may ultimately contribute to either gastric tissue damage, as a consequence of persistent activation of immunity, or tumor immune evasion due to chronic NKG2DL expression.