Olive Leaf Extract Enhances Natural Killer Cell-Mediated Cytotoxicity against Colorectal Cancer Cells.

Background: Natural Killer (NK) cells are innate lymphoid cells that eliminate malignant cells via perforin/granzyme-mediated cytotoxicity. This study investigates whether Olive Leaf Extract (OLE), rich in oleuropein and hydroxytyrosol, can enhance NK-cell cytotoxicity against colorectal cancer (CRC) cells. Although OLE exhibits direct anticancer effects, its therapeutic utility is constrained by poor bioavailability, requiring supraphysiological concentrations for direct cytotoxicity. Objective: To evaluate the effect of OLE on the cytotoxic activity of NK-92 cells against HT-29 colorectal cancer cells within a co-culture model. Methods: Olive leaves obtained from Balıkesir, Türkiye were dried and extracted with 70% methanol. The effects of OLE on the viability of HT-29 cells and NK-92 cells were evaluated using MTT and ATP assays. Additionally, the cytotoxic activity of NK-92 cells against HT-29 cells was assessed in a direct co-culture system. Granzyme B and perforin levels were measured using ELISA kits. Results: OLE inhibited the proliferation of HT-29 cells in a dose-dependent manner, with an IC₅₀ values of 548 µg/mL. In NK-92 cells, low concentrations of OLE (100-200 µg/mL) promoted cell proliferation, whereas higher concentrations exerted cytotoxic effects. In co-culture experiments, NK-92-mediated cytotoxicity against HT-29 cells was significantly enhanced by the addition of OLE at non-toxic concentrations (100 and 200 µg/mL). This enhanced cytotoxicity was further supported by a significant increase in granzyme B and perforin levels following OLE treatment. Conclusion: Our findings suggest that OLE can elicit a potent anticancer response via NK cells at lower, physiologically achievable doses. These results highlight a promising therapeutic strategy for CRC, leveraging OLE's immunomodulatory effects to enhance innate antitumor defenses.