Viscum album agglutinin-induced aggregation of blood cells and the lectin effects on neutrophil function

Extracts from mistletoe enjoy a large popularity in central Europe as an unconventional treatment modality for cancer, warranting scientific efforts with defined components to delineate any potential benefit. The galactose-specific lectin fromViscum album (VAA), known to exhibit immunomodulatory and ensuing antitumoral capacities in animal model systems, was shown to aggregate human blood cells in the following order: neutrophils, mononuclear cells - thrombocytes and erythrocytes. To contribute to the analysis of lectin effects on individual aspects of the host defense system, two parameters of neutrophils were quantitatively assessed, namely the aggregating activity of VAA as a measure of strength of interaction with cell surface ligands and the effect of lectin on oxidative metabolism (H2O2 release) of these cells. It was found that whole lectin and its carbohydrate-binding B-subunit possessed the capacity to induce cell aggregation and H2O2 release, which were blocked by D-galactose and lactose. Both effects displayed similar dependence on the lectin concentration in the range 0.1-25 μg/ml. The toxic A-subunit displayed detectable activity only in high doses (50 μg/ml) while the bovine heart galaptin (14 kDa; galectin-1) failed to affect neutrophils. The role of oxidative metabolism in regulation of neutrophil aggregation induced by VAA was studied using metabolic inhibitors and controlled heating at 46°C leading to inhibition of plasma membrane NADPH-oxidase system. Trifluoperazine and menadione inhibited the neutrophil aggregation in a dose-dependent manner in comparison with such inhibitors as amiloride and theophylline. The treatment of cells by heating at 46°C elicited a complete inhibition of H2O2 release in 5 minutes, whereas the aggregation was observed ad least after 30 minute incubation at the elevated temperature. These findings indicated that VAA-induced aggregation of neutrophils is necessary, but not sufficient for H2O2 release from cells. Finally, both VAA capacities were found to be inhibited by blood plasma that may play an important role in impairing of the lectin biological activity upon injection to cancer patients