A NATURAL IMMUNITY-ACTIVATING PLANT LECTIN, VISCUM-ALBUM AGGLUTININ-I, INDUCES APOPTOSIS IN HUMAN-LYMPHOCYTES, MONOCYTES, MONOCYTIC THP-1 CELLS AND MURINE THYMOCYTES

A galactoside-specific plant lectin, Viscum album agglutinin-I (VAA-I) with protein synthesis-inhibiting properties, has been shown to be cy totoxic in various eukaryotic cells, in vitro above a 10 ng/ml concent ration. Noncytotoxic concentrations of VAA-I induced mRNA expression a nd enhanced secretion of proinflammatory cytokines in cultures of huma n peripheral blood mononuclear cells. In an animal model VAA-I has bee n shown to stimulate natural killer cells and granulocyles. In this st udy, human peripheral blood lymphocytes (PBL), human peripheral blood monocytes (PBM), murine thymocytes and human monocytic THP-1 cells wer e incubated for 24 h in the presence of various concentrations of VAA- I. The apoptotic effect of VAA-I was analyzed by flow cytometry follow ing staining of the apoptotic nuclei in the cells with PI in hypotonic buffer and quantitative detection of DNA breaks were analyzed by the terminal deoxynucleotidyl transferase-mediated dUTP-digoxigenin nick e nd-labeling (TUNEL) assay. In cultures of all types of investigated ce lls, a dose-dependent VAA-I concentrations above 10 ng/ml in PBL and a t 1 ng/ml VAA-I concentration in PBM, thymocytes and THP-1 cells, a le ctin-induced increase of the apoptotic nuclei was observed. In 24-hour cultures of PBL and thymocytes, the ratios between apoptotic and nona poptotic cells were enhanced 10 times and 8 times, respectively, by 10 0 ng/ml VAA-I compared to the negative control. The concentration of 1 00 mu g/ml VAA-I only caused necrosis. The isolated A chain of the VAA -I induced apoptosis in PBL and thymocytes. In the culture of PBL the isolated B chain of the VAA-I was not effective indicating that cytoki ne induction by VAA-I is probably not involved in its apoptotic effect . On CD4+8+ thymocytes, VAA-I resulted in a reduced expression of CD8 molecules that could be related to a loss of volume and increase of d ensity, both characteristic features of apoptosis.