Epithelial‐mesenchymal transition may be involved in the immune evasion of circulating gastric tumor cells via downregulation of ULBP1

BACKGROUND: Increasing numbers of studies have demonstrated that circulating tumor cells (CTCs) undergo a phenotypic change termed epithelial‐mesenchymal transition (EMT), and researchers have proposed that EMT might provide CTCs with increased potential to survive in the different microenvironments encountered during metastasis through various ways, such as by increasing cell survival and early colonization. However, the exact role of EMT in CTCs remains unclear. METHODS: In this study, we identified CTCs of 41 patients with gastric cancer using Cyttel‐CTC and im‐FISH (immune‐fluorescence in situ hybridization) methods, and tested the expression of EMT markers and ULBP1 (a major member of the NKG2D—natural killer [NK] group 2 member D—ligand family) on CTCs. Moreover, we investigated the relationship between the expression of EMT markers and ULBP1 on CTCs and gastric cancer cell lines. RESULTS: Our results showed that the CTCs of gastric cancer patients exhibited three EMT marker subtypes, and that the expression of ULBP1 was significantly lower on mesenchymal phenotypic CTCs (M(+)CTCs) than on epithelial phenotypic CTCs (E(+)CTCs). EMT induced by TGF‐β in vitro produced a similar phenomenon, and we therefore proposed that EMT might be involved in the immune evasion of CTCs from NK cells by altering the expression of ULBP1. CONCLUSIONS: Our study indicated that EMT might play a vital role in the immune invasion of CTCs by regulating the expression of ULBP1 on CTCs. These findings could provide potential strategies for targeting the immune evasion capacity of CTCs.