The killing capacity of extracts from Viscum album L., widely used as an adjuvant in complementary cancer therapy, is dependent on the content of toxic proteins, especially the mistletoe lectins (ML). Although one may expect a homogeneous distribution of 'receptors' for these proteins on the cell surface, the sensitivity of cells to the ML-mediated cytotoxicity obviously differs, as the galNAc-binding ML III in contrast to the gal-binding ML I selectively killed CD8+ lymphocytes with a 'memory' phenotype (CD62Llo), while CD19+ B cells remained almost unaffected. B cells hardly bind ML III but did bind the gal-specific ML I. In accordance with these observations, in leukaemic B cells from patients with B chronic lymphocytic leukaemia and the human IgE-secreting myeloma cell line U-266 a strong induction of apoptosis-associated mitochondrial Apo2.7 molecules was observed after treatment with ML I and less effectively by ML III, while in the leukaemic T cell line Molt-4 both ML were strong inductors of apoptosis. In the light of these findings, the possible impact of ML I- and ML III-rich mistletoe extracts in the treatment of B cell neoplasia has to be carefully investigated.
