The galactoside-specific lectin from mistletoe as biological response modifier.

Experience with herbal extracts can guide the isolation of substances that effectively amend aspects of the host's defence system against tumor growth and spread. This capacity appears to be conferred to mistletoe (Viscum album) extract by a rather small dose range of the galactoside-specific lectin (VAA). It is a biochemically characterized dimer, consisting of a toxic subunit that acts as a RNA N-glycosidase (depurination of A-4324 in 28S rRNA) and a carbohydrate-binding B-chain. The binding of this subunit to galactose-exposing glycoligands on mononuclear cells elicits cytokine secretion in vitro and antitumoral/antimetastatic capacity in vivo in lymphosarcoma/sarcoma model systems. Increases in NK cell number, the activity of peritoneal macrophages and NK cells as well as the response of splenic T cells to mitogens are detectable. Immunophenotyping of blood samples from lectin-treated patients reveals increases in the number of helper/inducer T cells, NK cells and CD25-positive cells. Since the response to the treatment is assumed to be dependent on the presence of lectin-specific ligands on inflammatory host cells, histopathological monitoring of tumor specimens with labelled lectin is conceivable to be one factor of relevance to predict a response to the treatment in animal models or within clinical trials.