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

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.

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.

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.

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.

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.