Identification of a Novel Protein Adjuvant Isolated from Abrus precatorius

Comparative study on modeccin- and phytohemagglutinin (PHA)-induced secretion of cytokines and nitric oxide (NO) in RAW264.7 cells

The effects of cytotoxic lectins, modeccin and phytohemagglutinin (PHA) on mouse macrophage cell line RAW264.7 was studied by detecting the induction of inflammatory mediators. Results showed that modeccin induced the release of tumor necrosis factor-α (TNF-α) from RAW264.7 cells with a bell-shape concentration-dependent profile. PHA that showed no significant cytotoxicity on RAW264.7 cells up to 100,000 ng/ml induced much higher level of TNF-α than modeccin. PHA simultaneously induced the secretion of granulocyte colony stimulation factor (G-CSF) from RAW264.7 cells with even much higher level than that of TNF-α, whereas modeccin did not. Furthermore, PHA induced the secretion of nitric oxide (NO) in RAW264.7 cells, while no significant level of NO was detected in the modeccin-treated cells. NH₄Cl (a lysomotoropic agent) and cycloheximide (a ribosome inhibitor) strongly inhibited modeccin-induced TNF-α secretion, but no significant inhibitory effects of these reagents on the PHA-induced TNF-α secretion were observed. Contrary to modeccin-induced TNF-α secretion, even slightly increased TNF-α secretion was observed in PHA-treated cells in the presence of 10 mM NH₄Cl. In addition, the inhibition profiles of modeccin-induced TNF-α secretion by various kinase inhibitors were different from those of PHA. These results suggested that the action mode of modeccin to stimulate RAW264.7 cells leading to the secretion of inflammatory molecules, including TNF-α, is distinct from that of PHA. On the other hand, significantly increased translocation of activator protein-1 (AP-1), a crucial transcription factor involved in expression of inflammatory molecules, into nucleus was observed in RAW264.7 cells treated with PHA and modeccin.

CEL-I, an N-acetylgalactosamine (GalNAc)-specific C-type lectin, induces nitric oxide production in RAW264.7 mouse macrophage cell line

We found that CEL-I, a GalNAc-specific C-type lectin isolated from the marine invertebrate Holothuroidea (Cucumaria echinata), induces inducible nitric oxide synthase (iNOS) expression and NO production in RAW264.7 cells. The NO production was inhibited by an iNOS inhibitor, L-NAME, but was not by a lipopolysaccharide (LPS) inhibitor, polymyxin B. In the presence of 0.1-M GalNAc, increased NO production by CEL-I-treated RAW264.7 cells was observed rather than the inhibition. Bovine serum albumin (BSA) significantly inhibited the CEL-I-induced NO production as well as the binding of FITC-labelled CEL-I on RAW264.7 cells. Three MAP kinase inhibitors (specific to extra-cellular regulated kinase, c-jun NH(2)-terminal kinase and p38 MAP kinase) inhibited CEL-I-induced NO production with different extents. Heat-treatment of CEL-I resulted in a decreased activity of CEL-I depending on the temperature. These results suggest that CEL-I induces NO production in RAW264.7 cells through the protein-cell interaction rather than the binding to the specific carbohydrate chains on the cell surface.

Immunomodulatory and anti-tumor activities of native and heat denatured Abrus agglutinin

Abrus agglutinin (AAG), a hetero tetrameric gal beta (1-3) NAc gal specific lectin, is isolated from seeds of Abrus precatorius. In our previous studies we found that the protein could act as an immunomodulator and immunoadjuvant in native (NA) and heat denatured (HDA) conditions. An anticancer effect of the lectin is reported, but its mode of action is not clearly known. In the present study, the anti-tumor activity of AAG (NA, HDA) has been evaluated in a murine Dalton's lymphoma (DL) ascites tumorogenic model. We found that treatment with both NA and HDA were able to decrease the tumor cell number in vivo and significantly increased median survival time. In vitro studies showed that AAG (NA, HDA) treatment of Dalton's lymphoma ascites cells (DLAC) resulted in growth inhibition at the concentration of 1 microg/ml and above. Whereas, AAG (NA, HDA) at much lower concentrations (approximately 1 ng/ml) can stimulate peritoneal macrophage and spleen derived NK cells in vitro demonstrating cytotoxicity against DLAC. Cell cycle analysis showed an increased number of cells in Sub-G0/G1 phase for in vitro and in vivo treatments. In summary, AAG (NA, HDA) at non-toxic concentration was able to elicit anti-tumor effects in DL bearing mice by stimulating the innate immune system and Th1 type immunomodulation.

Immunomodulatory role of native and heat denatured agglutinin from Abrus precatorius

Lectins are known as polyclonal activators of lymphocytes and work through the induction of battery of cytokines, which vary from lectin to lectin. Most widely used biological response modifier Mistletoe lectin (ML-1) in therapy stimulates lymphocytes, macrophages, and natural killer cells and induces both TH1 and TH2 type cytokines. Abrus agglutinin, similar to ML-1 with respect to carbohydrate specificity [gal (beta1-->3) gal/Nac], was studied both in native (NA) and heat denatured (HDA) condition for murine splenocyte proliferation, cytokine secretion, NK-cell activation, and thymocyte proliferation in vitro with a view to assess its potential as an immunomodulator. Both NA and HDA activate splenocytes and induce production of cytokines like IL-2, IFN-gamma and TNF-alphabeta indicating a TH1 type of immune response. Native agglutinin and HDA induced conditioned media of adherent splenocytes could stimulate non-adherent splenocytes and vice versa. Heat denatured agglutinin was able to induce NK-cell activation at much lower concentration than that of NA, but the extent of NK-cell activation was higher for NA. Proliferation of thymocytes by NA and HDA was also observed. This study indicates that Abrus agglutinin could be a potential immunomodulator both in native as well as in heat denatured form.

Immunostimulatory Role of Tryptic Digest ofAbrusAgglutinin

Many bioactive peptides are in therapeutic use as immunomodulators at present. The origin of these bioactive peptides is diverse. Such bioactive peptides are reported to be present in enzymatic digest of food and milk proteins. In our previous work we have reported that Abrus agglutinin retains its bioactivity even after heat denaturation but loses its haemagglutination properties. This leads to the supposition that immunostimulatory regions in the protein might be responsible for its in vivo and in vitro stimulatory properties. Thus the bioactivity of tryptic digest of Abrus agglutinin (TDA) was checked in vitro to ascertain the presence of some bioactive region in the protein, which will lead to the discovery of certain immunostimulatory peptides which might be of use in nonspecific immunotherapy. In this study it is observed that TDA stimulates macrophage increasing the phagocytic and bactericidal activity as well as hydrogen peroxide production. TDA also proliferates splenocytes leading to Th1 response and NK cell activation.

Stimulation of murine macrophages by native and heat-denatured lectin from Abrus precatorius

We have evaluated the immunostimulant activity of native agglutinin (NA) and heat-denatured agglutinin (HDA) obtained from Abrus precatorius seed kernels on murine macrophages. Activated macrophages play an important role in nonspecific immunostimulation in turn by activating the other immune cells in the cascade. Significant alterations are observed in the macrophage functions both by native and heat-denatured Abrus agglutinin. Increased production of nitric oxide and hydrogen peroxide and a high phagocytic and bactericidal activity is potentiated by both NA and HDA. It is also observed that activated macrophages also released interleukin-1. These results suggest that both NA and HDA act as immunostimulants in vitro.