Modulation of cytotoxicity and enhancement of cytokine release induced by Viscum album L. extracts or mistletoe lectins.

Cytotoxic effects of Viscum album L. (mistletoe) extracts and mistletoe lectins were studied by light and electron microscopy. The first events observed were membrane perforation and protrusions typical for apoptosis. Inhibition of Molt 4 cell growth was obtained with lectin concentrations in the pg/ml range as long as cells were cultured in serum-free medium. Under this condition, mistletoe lectin-III was about 10 times more cytotoxic than mistletoe lectin-I; mistletoe lectin-II was in between. Lectin cytotoxicity was modulated by human serum from donors who had never been treated with mistletoe preparations and lectin-specific carbohydrates, added at the mmol/l range, particularly D-galactose (or beta-lactose) for mistletoe lectin-I and N-acetyl-galactosamine for mistletoe lectin-II and -III. In addition, at subtoxic concentrations, mistletoe lectin-I, -II and -III enhanced the production of cytokines (tumour necrosis factor-alpha, interleukin-1 alpha) by isolated human monocytes. The experimental results are discussed in relation to the treatment of cancer patients administered with mistletoe extracts.