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 cytotoxic in various eukaryotic cells, in vitro above a 10 ng/ml concentration. Noncytotoxic concentrations of VAA-I induced mRNA expression and enhanced secretion of proinflammatory cytokines in cultures of human peripheral blood mononuclear cells. In an animal model VAA-I has been shown to stimulate natural killer cells and granulocytes. In this study, human peripheral blood lymphocytes (PBL), human peripheral blood monocytes (PBM), murine thymocytes and human monocytic THP-1 cells were incubated for 24 h in the presence of various concentrations of VAA-I. The apoptotic effect of VAA-I was analyzed by flow cytometry following 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 end-labeling (TUNEL) assay. In cultures of all types of investigated cells, a dose-dependent VAA-I concentrations above 10 ng/ml in PBL and at 1 ng/ml VAA-I concentration in PBM, thymocytes and THP-1 cells, a lectin-induced increase of the apoptotic nuclei was observed. In 24-hour cultures of PBL and thymocytes, the ratios between apoptotic and nonapoptotic cells were enhanced 10 times and 8 times, respectively, by 100 ng/ml VAA-I compared to the negative control. The concentration of 100 micrograms/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 cytokine 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 density, both characteristic features of apoptosis.