Mistletoe therapy for human cancer: the role of the natural killer cells

Extracts produced from Viscum album L. (mistletoe) as well as certain isolated components are able to stimulate different functions of the immune system. The natural killer cells have been suggested as one of the candidates for direct tumour cell destruction. These cells are defined by their ability to mediate non-major histocompatibility complex (MHC) restricted cytotoxicity without prior sensitization against a specific antigen. However, their effectiveness in tumor defence in vivo is unclear. In general, natural killer cells are unable to lyse fresh autologous tumour cells in vitro unless activated by interleukin-2-preincubation. The results of clinical studies are contradictory, but there is evidence that they may contribute to the prevention of the development of recidives and metastases. In this regard it is interesting that mistletoe extracts are able to stimulate natural killer cell-mediated cytotoxicity in vitro directly as well as indirectly in a cytokine-like manner, with the active components being carbohydrates rather than lectins. Clinical application of mistletoe extracts or isolated lectins is reported to induce augmentation of both number and activity of natural killer cells in peripheral blood in a dose-dependent manner; however, non-responders also have been described. In future work it has to be clarified whether a mistletoe-derived modulation of the natural killer system is of benefit in the tumour defence of cancer patients.