Viscum album agglutinin-I (VAA-I) is a plant lectin that possesses interesting potential therapeutic properties and immunomodulatory activities. We have recently found that VAA-I is a potent inducer of human neutrophil apoptosis, but the mechanism(s) involved require further elucidation. In this study, we found that VAA-I alters mitochondrial transmembrane potential and increases intracellular levels of reactive oxygen species (ROS). Despite these observations, treatment with the mitochondrial stabilizer, bongkrekic acid, or with catalase, known to degrade H2O2, fails to reverse VAA-I-induced apoptosis. Moreover, VAA-I was found to induce apoptosis in PLB-985 cells deficient in gp91phox, indicating that the lectin acts via an ROS-independent mechanism. Pretreatment of neutrophils with brefeldin A, an inhibitor of vesicular transport, was found to reverse VAA-I-induced apoptosis. Protein expression of Mcl-1 was decreased by VAA-I. The role of caspases in the degradation of cytoskeletal proteins during both spontaneous and VAA-I-induced neutrophil apoptosis was also investigated. Paxillin and vimentin were markedly degraded by VAA-I when compared with neutrophils that undergo spontaneous apoptosis, but not vinculin or α- and β-tubulin. Caspases were involved in cytoskeletal protein degradation because preincubation with the pan-caspase inhibitor N-benzyloxycarbonyl-V-A-D-O-methylfluoromethyl ketone was found to reverse protein cleavage. We conclude that VAA-I needs to be internalized to mediate apoptosis and that its activity is not dependent on a cell surface receptor-mediated pathway. Also, we conclude that VAA-I induces apoptosis by ROS-independent and Mcl-1-dependent mechanisms and that caspases are involved in cytoskeletal protein degradation in both spontaneous and VAA-I-induced neutrophil apoptosis.

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Background: Mistletoe is often used as a complementary approach in oncology. Despite experimental anti-tumour effects and several reviews there remains controversy about its clinical role. Patients and Methods: Potentially relevant trials were identified to perform a systematic review (databases: e.g. EMBASE, MEDLINE; hand search: e.g. bibliographies; search terms: e.g. mistletoe). To be included, randomised or comparative clinical trials at least had to examine mistletoe preparations standardized according to manufacturing process and to describe interventions explicitly. Additionally, cohort studies were included for reasons of external validity. Results were summarised in tables. Results: 18 clinical trials (>6,800 participants) were included. Their internal quality was mostly low. Due to heterogeneity between trials a meta-analysis was impossible. Regarding efficacy, findings were inconsistent regarding life expectancy, relation to tumour entity, dosing and treatment duration. Yet, studies indicate that quality of life (QoL) is improved. As these findings do not seem to be limited to one of the different parenteral mistletoe preparations reviewed the treatment may be summarised under the umbrella term ‘mistletoe therapy’. Regarding safety, 1 serious adverse event (AE) related to mistletoe was described; non-serious AEs were local reactions at injection site. Allergic reactions were rare. Conclusion: Supportive ‘mistletoe therapy’ seems safe and beneficial for QoL in adult patients with solid tumours. But there is an urgent need to confirm its efficacy in patient-centred care in a complex oncological setting. This has to be evaluated systematically in prospective observational trials with validated, multidimensional patient-rated QoL questionnaires and comparisons of different preparations and dosages.

A plant lectin, Viscum album agglutinin-I (VAA-I) has been shown to increase the number and cytotoxic activity of natural killer (NK) cells in animal models, but the mechanisms underlying these effects are poorly understood. We investigated the effects of the recombinant form of this lectin (rVAA) on secretion of interleukin (IL)-12 and on NK-mediated cytotoxicity against K562 target cells in cultures of human peripheral blood mononuclear cells (PBMC) as well as against YAC-1 target cells in cultured rat spleen cells. In 24-hour cultures of PBMC, 10 ng/ml plant VAA-I and 50 ng/ml rVAA induced significant increases in the secretion of total IL-12. Its biologically active heterodimeric form, p70, was also significantly induced by rVAA. Preincubation of PBMC or splenocytes for 48 h with rVAA in concentrations ranging between 10 pg/ml and 100 pg/ml resulted in moderate enhancements of NK-mediated cytotoxicity. However, coincubation of a low dose of rVAA (100 pg/ml) together with IL-2 and IL-12 (60 U/ml and 2 U/ml, respectively) led to additive stimulation of NK activity. In in vivo experiments, rVAA showed an enhancing effect on NK activity with a bell-shaped curve of efficacy. Forty-eight hours after a single intravenous injection of its most effective doses, 0.5 and 1 ng/kg, into Wistar rats, the NK cytotoxicity of splenocytes against YAC-1 targets doubled, and the frequency of large granular lymphocytes in peripheral blood showed 2.1- and 3-fold increases as compared to control animals. Twenty-four hours following these low lectin doses, the number of large granular lymphocytes was also significantly elevated. After 48 h, 0.5 ng/kg rVAA induced a significant augmentation in the percentage of peripheral Mac-1+ mononuclear cells, including activated monocytes and NK cells. The present results suggest that rVAA augments the secretion of an active form of IL-12 and potentiates the cytokine-induced NK activation. These effects of rVAA may be related to its stimulatory effects on MHC-unrestricted cytotoxicity in vivo.