EFFECTS OF MISTLETOE LECTIN-I ON HUMAN BLOOD-CELL LINES AND PERIPHERAL-BLOOD CELLS - CYTOTOXICITY, APOPTOSIS AND INDUCTION OF CYTOKINES

The effects of mistletoe lectin I (ML I) on the human T-cell leukemia line MOLT-4, the monocytic line THP-1 and on human peripheral blood mo nonuclear cells (PBMC) were investigated with regard to general cell v iability and induction of apoptosis. Using a sensitive serum-free cyto toxicity assay, the time-and concentration-dependent direct toxicity t owards MOLT-4 cells was determined with IC50-values ranging from 20-40 pg/ml (300-600 fmol/l). Investigations on the time course of the toxi c effect using selected concentrations of ML I revealed distinct respo nse curves for concentrations of high, low and intermediate toxicity, respectively. The ratio of apoptotic to viable MOLT-4 cells was determ ined after treatment with ML I for 24 h. Apoptosis and cytotoxicity we re correlated at low and intermediate concentrations, whereas at long intervals and high concentrations of ML I mostly necrotic effects were observed. The data showed that in the concentration range of low cyto toxicity ML I-induced cell death is quantitatively due to apoptotic pr ocesses. The immuno-modulatory activity of ML I was investigated in vi tro by measuring cytokine release. At concentrations of low cytotoxici ty ML I showed immunostimulatory activity on PBMC and THP-1. RT-PCR wi th THP-1 cells confirmed that cytokine induction by ML I is regulated on the transcriptional level. These findings suggest that in the blood cells investigated both apoptosis and cellular signalling are induced by the same concentration range of ML I.