Differential binding of toxic lectins from Viscum album L., ML I and ML III, to human lymphocytes

The killing capacity of extracts from Viscum album L., winery used as an ad juvant in complementary cancer therapy, is dependent on the content of toxi c proteins, especially the mistletoe lectins (ML). Although one may expect a homogenous distribution of 'receptors' for these proteins on the cell sur face, the sensitivity of cells to the ML mediated cytotoxicity obviously di ffers, as the galNAc-binding ML III in contrast to the gal-binding ML I sel ectively killed CD8(+) lymphocytes with a 'memory' phenotype (CD62L(lo)), w hile 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 SI 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 trea tment with ML I and less effectively by ML III, while in the leukaemic T ce ll 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 extr acts ii? the treatment of B cell neoplasia has to be carefully investigated .