Anti-mistletoe lectin antibodies are produced in patients during therapy with an aqueous mistletoe extract derived from Viscum album L. and neutralize lectin-induced cytotoxicity in vitro.

The humoral response components of an aqueous mistletoe extract (HM) was evaluated in 23 tumor patients who had been treated from 2 months up to 6 years with increasing dosages of HM. IgG antibodies against mistletoe lectin and other components of this extract were detected by ELISA, immunodiffusion, and blotting technique, using either the aqueous extract (HM) or a purified lectin preparation (ML). Their activity depended upon dosage of HM and length of therapy. No anti-HM/ML antibodies of the IgM type could be detected. Immunoblotting revealed lectin-specific antigens at 62 kD, 33k D, and 29 kD. In the presence of ML or HM, PHA-induced proliferation of normal lymphocytes was decreased in a dose-dependent manner; this effect was neutralized by adding the IgG fraction from pooled anti-HM-antibody-positive sera, indicating that the cytotoxic effect of lectins was eliminated by these specific antibodies. In view of these findings, it is questionable whether exposing tumor cells to mistletoe extracts in vivo exerts the same direct effect on tumor cells that is observed in vitro.