Effect of irradiation-induced intercellular adhesion molecule-1 expression on natural killer cell-mediated cytotoxicity toward human cancer cells

Background aims: Irradiation enhances the adhesion between natural killer (NK) cells and target cells by up-regulating intercellular adhesion molecule-1 (ICAM-1) on target cells. Therefore, we investigated the effect of irradiation-induced ICAM-1 expression on human cancer cells on NK cell–mediated cytotoxicity. Methods: Expression levels of ICAM-1 on the target cell surface before and after irradiation of six human cancer cell lines (HL60, SKBR-3, T47D, HCT-116, U937 and U251) were analyzed by flow cytometry. Ex vivo expansion of NK cells from human peripheral blood mononuclear cells was performed by co-culture with irradiated K562 cells. The related adhesion molecule lymphocyte function–associated antigen 1 (LFA-1) on NK cells was analyzed by flow cytometry. An enzyme-linked immunosorbent assay was used to detect interferon-γ (IFN-γ), and WST-8 assays were performed to check NK cell cytotoxicity. Finally, blocking assays were performed using monoclonal antibodies against ICAM-1 or LFA-1. Results: LFA-1 expression increased on NK cells after expansion (P < 0.001). The expression of ICAM-1 was significantly upregulated by irradiation after 24 h in various cell lines, including HL60 (P < 0.001), SKBR-3 (P < 0.001), T47D (P < 0.001) and U937 (P < 0.001), although the level of expression depended on the cell line. ICAM-1 expression was extremely low before and after irradiation in U251 cells. NK cell–mediated cytotoxicity increased after irradiation of HL60 (P < 0.001), SKBR-3 (P < 0.001), T47D (P = 0.003), and U937 (P = 0.004) cells, in which ICAM-1 expression was significantly increased after irradiation. IFN-γ production by NK cells in response to HL60 (P < 0.001) and T47D (P = 0.011) cells significantly increased after irradiation. NK cell–mediated cytotoxicity against irradiated SKBR-3 (P < 0.001) and irradiated T47D cells (P = 0.035) significantly decreased after blocking of ICAM-1. Blocking of LFA-1 on NK cells resulted in reduced cytotoxicity against irradiated HL60 (P < 0.001) and irradiated SKBR-3 (P < 0.001). Conclusions: Irradiation upregulates ICAM-1 expression on the surface of human cancer cells and enhances activated NK cell–mediated cytotoxicity. Therefore, irradiation combined with NK cell therapy may improve the antitumor effects of NK cells.