The recommended dose for further clinical trials is 5600 ng/kg twice weekly. Based on the short half-life of the recombinant protein observed in this trial, the exploration of prolonged infusion schedules of aviscumine is warranted.

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.

Mistletoe lectin I (MLI) is the major active constituent of mistletoe extracts, which are widely used for adjuvant tumour therapy. The 66-kDa heterodimeric disulphide-linked glycoprotein is classified as type II ribosome-inactivating protein (RIP) due to the rRNA-cleaving enzyme activity of the A-subunit, also referred to as toxic entity. MLI and the close relative ricin both belong to the family of the two-chain plant type II RIP proteins. Isolation of the glycosylated proteins from plant material yield inhomogeneous material probably due to post-translational modifications. The aim of this study was to prepare pure and homogeneous protein as a prerequisite for structural and mechanistic studies in order to gain insight into the mode of action of this cytotoxic plant protein on tumour and immune cells. Of particular interest was to explain whether the differences in toxicity of ML and ricin are the result of variations of their enzymatic activities. By investigating the sequence homologies between the active sites of different RIPs we were able to deduce a set of primers which were suitable for specific amplification of the mistletoe lectin gene. Applying this PCR strategy the full-length 1923 nucleotide DNA sequence coding for the prepro-protein was obtained showing the existence of a single intron-free gene. In order to elucidate the molecular basis for the observed differences in cytotoxicity within the family of RIP the enzymatic A-subunit was expressed in a heterologous system. Expression of the A-chain in E. coli BL21/pT7 resulted in production of insoluble inclusion bodies constituting 20-30% of total protein. Refolding led to a pure and homogeneous protein species with an apparent molecular mass of 27 kDa and a pI value of 6.4. The ribosome-inactivating activity of the unglycosylated recombinant A-chain (IC50 20.5 pM) protein was in the same range as that of the glycosylated plant-derived ML A-chain (IC50 3.7 pM), which was very similar to that of ricin A-chain (IC50 4.9 pM). Thus, the higher cytotoxicity of ricin cannot be accountable for differences in the enzymatic activities of the type II RIP A-chains.

Mistletoe lectin I (pML) and its isoforms ML II and III constitute the active principle in extract preparations from mistletoe, commonly used as immunomodulator in adjuvant tumour therapy. The heterodimeric disulfide-linked cytotoxic protein is classified as type II ribosome inactivating protein (RIP). Recently, the sequence coding for the mistletoe lectin prepro-protein was identified and the existence of a single intron-free gene was shown [Eck, J., Langer, M., Mo Èckel, B., Baur, A., Rothe, M., Zinke, H. & Lentzen, H. (1999) Eur. J. Biochem. 264, 775± 784].The aim of this study was to prepare pure and homogeneous rMLB-chain as well as rML heterodimer for studying the carbohydrate binding specificity of recombinant versus natural protein and its contribution to the observed cytotoxic effect. Expression in E. coli resulted in the production of insoluble protein (inclusion bodies). A procedure for generating correctly folded, biochemically and biologically active rMLB was established starting from the insoluble single chain. Carbohydrate binding and specificity of pMLB and rMLB were analysed by a competitive enzyme linked lectin assay (ELLA). Asialofetuin was able to compete with binding of both chains (50% at 0.8 mm). The specificity of the B-chains to lactose was more distinct with halfmaximal competition at 4.9 mm (pMLB) and . 90 mm (rMLB), respectively.Furthermore, in a coassociation process rMLA-and rMLB inclusion bodies were associated in one step by defined dilution yielding active rML-heterodimer. The activities of recombinant (rML) and plant derived mistletoe lectin (pML) were compared. Cytotoxicity was determined using MOLT-4 cells and enzymatic rRNA N-glycosidase activity was measured in a coupled transcription/translation assay. The IC 50 values of the two heterodimers were similar in both assays; rMLB-chain did not show any cytotoxic effect. In the ELLA with lactose as a competitor 50% competition of binding to asialofetuin was achieved at 1.6 mm (rML) and 1.8 mm (pML). Hence, using three different assays we found no significant differences between the recombinant protein and the glycosylated form of ML. Comparing the biological activities of the single chains with those of the heterodimer we conclude, that both, lectin activity and the rRNA N-glycosidase activity, are prerequisites for the cytotoxic effects on target cells. Mistletoe lectins type I, II and III are a group of glycosylated, 56±64 kDa cytotoxic proteins, which consist of two covalently cystine linked subunits. The A-chain mediates the enzymatic inactivation of eucaryotic ribosomes and the B-chain is responsible for carbohydrate binding [17] and therefore mistletoe lectin belongs to the type II ribosome inactivating proteins (RIP II) [18]. Interestingly, the specificity of carbohydrate binding varies between the three species with ML I being mainly specific for d-galactose and ML III for N-acetyl-d-galactosamine [19,20]. Both, ML I and ML III mediated cytotoxicity can be strongly reduced by b-d-lactose as a competitor [21]. ML III, ho...

Mistletoe lectin I (ML-I) is a type II ribosome-inactivating protein, which inhibits the protein biosynthesis at the ribosomal level. ML-I is composed of a catalytically active A-chain with rRNA N-glycosidase activity and a B-chain with carbohydrate binding specificities. Using comparative solid-phase binding assays along with electrospray ionization tandem mass spectrometry, ML-I was shown to preferentially bind to terminally alpha2-6-sialylated neolacto series gangliosides from human granulocytes. IV(6)Neu5Ac-nLc4Cer, VI(6)Neu5Ac-nLc6Cer, and VIII(6)Neu5Ac-nLc8Cer were identified as ML-I receptors, whereas the isomeric alpha2-3-sialylated neolacto series gangliosides were not recognized. Only marginal binding of ML-I to terminal galactose residues of neutral glycosphingolipids with a Galbeta1-4Glc or Galbeta1-4GlcNAc sequence was determined, whereas a distal Galalpha1-4Gal, GalNAcbeta1-3Gal, or GalNAcbeta1-4Gal disaccharide did not bind at all. Among the glycoproteins investigated in Western blot and microwell adsorption assays, only those carrying Neu5Acalpha2-6Galbeta1-4GlcNAc residues, exclusively, predominantly, or even as less abundant constituents in an assembly with Neu5Acalpha2-3Galbeta1-4GlcNAc-terminated glycans, displayed high ML-I binding capacity. From our data we conclude that (i) ML-I has to be considered as a sialic acid- and not a galactose-specific lectin and (ii) neolacto series gangliosides and sialoglycoproteins with type II glycans, which share the Neu5Acalpha2-6Galbeta1-4GlcNAc terminus, are true ML-I receptors. This strict preference might help to explain the immunostimulatory potential of ML-I toward certain leukocyte subpopulations and its therapeutic success as a cytotoxic anticancer drug.