Effect of fetal calf serum on the cytotoxic activity of mistletoe (Viscum albumL.) lectins in cell culture

Mistletoe (Viscum album L.) contains three cytotoxic lectins, ML I, ML II and ML III. The interaction between these lectins and the fetal calf serum (FCS) commonly added to cell culture media was studied on human leukaemia Molt 4 cells. When these cells were grown in RPMI 1640 medium supplemented with 10% FCS, the lectins were about 40 times less toxic than when the former were grown in serum‐free Ultraculture medium. An addition of FCS to the serum‐free Ultraculture medium produced a dose‐dependent inhibition of the cytotoxic activities of the lectins, by up to 50 fold. Moreover, FCS increased by 2 to 3 fold the protective action of anti‐ML I antibodies against the toxicity of the lectins. The data suggest that FCS contains glycoproteins or glycolipids which interact with the lectins.