Cytotoxic activity of invertebrate hemocytes with preliminary findings on the tunicateCiona intstinalis

Cytotoxic activity of Holothuria tubulosa (Echinodermata) coelomocytes

The immune system of marine invertebrates, in particular that of holothurians, still requires further study. Our research showed that coelomocyte cells contained in the coelomic fluid of the sea cucumber, Holothuria tubulosa, are able to lyse, in vitro, red blood cells in rabbits and sheep. A plaque-forming assay showed spherule cells to be the effector cells, able to release cytotoxic molecules after xenogenic cell contact. The coelomocyte lysate supernatant, analysed by polyacrylamide gel electrophoresis overlay technique, using rabbit and sheep erythrocytes, showed two different haemolytic protein patterns: one calcium dependent and the other calcium independent. The fractions of each pattern were resolved on a polyacrylamide gel and calcium-dependent and independent coelomocyte lysate patterns were compared.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

A lytic mechanism based on soluble phospholypases A2 (sPLA2) and β-galactoside specific lectins is exerted by Ciona intestinalis (ascidian) unilocular refractile hemocytes against K562 cell line and mammalian erythrocytes

Hemocytes from the ascidian Ciona intestinalis exert in vitro Ca²+-dependent cytotoxic activity toward mammalian erythrocytes and K562 cells. To examine the lytic mechanism, hemocyte populations were separated (B1-B6 bands) through a Percoll discontinuous density gradient, the hemocyte cytotoxic activity (HCA) and the lytic activity of the hemocyte lysate supernatant (HLS) were assayed. In addition the separated hemocytes were cultured and the cell-free culture medium (CFM) assayed after 3 h culture. Results support that unilocular refractile hemocytes (URGs), enriched in B5, are cytotoxic. The B5-HLS contains lysins and the activity of B5-CFM shows that lysins can be released into a culture medium. The B5 activity was blocked by D-galactose, α-lactose, lactulose, LacNAc, thiodigalactoside (TDG), L-fucose, D-mannose, D-glucose, sphingomyelin (SM), and soluble phospholipase A2 (sPLA2) inhibitors (dibucain, quinacrine). Accordingly, HLS chemico-physical properties (alkaline medium, high thermostability, Ca²+-dependence, trypsin treatment, protease inhibitors) and SEM observations of the affected targets suggested that sPLA2 could be responsible for changes and large alterations of the target cell membrane. An apoptotic activity, as recorded by a caspase 3, 7 assay, was found by treating K562 cells with very diluted HLS. A lytic mechanism involving sPLA2 and lectins promptly released by URGs and morula cells respectively is suggested, whereas target cell membrane SM could be a modulator of the enzyme activity.

Tunicate cytokine-like molecules and their involvement in host defense responses

Tunicates (ascidians or sea squirts) are a large group of invertebrate chordates that are closely related to vertebrates. Their critical phylogenetic position has stimulated substantial interest in their host defense ("immune") responses. Whilst this interest has generated a wealth of knowledge regarding the humoral and cellular mechanisms that undertake defensive responses, there is less known about the regulation of those reactions. This chapter focuses on three cellular responses (cell proliferation, phagocytosis and chemotaxis) that are known to be regulated by cytophilic humoral molecules. Some of the humoral factors that affect these responses have functional and physicochemical similarities to vertebrate cytokines, like interleukin-1. However, the only regulatory molecules that have been characterized at a molecular level bear far greater similarity to C-type lectins or complement components.

Evidence for perforin-like activity associated with earthworm leukocytes

Earthworm (Eisenia fetida) coelomic fluid contains several leukocytes (coelomocytes): basophils, acidophils and neutrophils as well as chloragocytes. Small coelomocytes and coelomocyte lysate are cytotoxic for the tumor cell target K562. The expression of a lytic factor was investigated by immunocytochemistry using light and transmission electron microscopy. A rat-anti-mouse-perforin-mAb labeled mainly small coelomocytes (nearly 20%) as visualized by light microscopy. TEM analysis using immunogold showed a homogenous labeling in the cytoplasm of small coelomocytes. The highest number of immunogold particles was estimated in coelomocytes with many small cytoplasmic granules. Coelomocytes with large lysosomal granules were also labeled but less intensely. No antibody binding was observed for chloragocytes either in light or electron microscopy. This suggests that the perforin-like activity is associated with only one cell type and that chloragocytes are responsible for other lytic activities. MALDI-MS revealed calreticulin usually associated with perforin in mammalian cells that mediate lysis (e.g. NK, CTL). Together, results strongly suggest the presence of putative perforin in earthworms. This in turn supports the hypothesis that perforin is a conserved component important in immune defense during evolution.

Tributyltin affects phagocytic activity of Ciona intestinalis hemocytes

Organotin compounds have been used in marine anti-fouling paints as biocides. Because tunicates are vulnerable to these compounds in their natural habitats, we used Ciona intestinalis to establish an assay for phagocytosis in vitro of yeast by hemocytes after exposure to different concentrations (0.0015, 0.015, 0.15 and 1.5 microM) of four organotin compounds: tributyltin (TBT), triphenyltin (TPT), dibutyltin (DBT) and diphenyltin (DPT). To evaluate the phagocytic activity, we used a method based on fluorescence excitation of yeast pre-treated with eosin-Y. The percentage of phagocytosis decreased from 45.1 +/- 3.49 to 22.4 +/- 5.14 at 1.5 microM of TBT (P < 0.001); it was significantly reduced in presence of the ionophore A23187. TPT, DPT and DBT did not show significant effects on phagocytosis. Because the effect of TBT was irreversible, differences between the inhibitory mechanisms of ionophore and TBT are suggested. These results indicate that for future analyses, tunicates should become excellent models for dissecting events such as phagocytosis that are associated with immunosuppression after exposure to xenobiotics.

Sphingomyelin inhibition of Ciona intestinalis (Tunicata) cytotoxic hemocytes assayed against sheep erythrocytes

Hemocytes from the ascidian, Ciona intestinalis, are capable of lysing erythrocytes in vitro following cell membrane contact. With the aim of examining the mechanism of cytotoxicity, we performed inhibition experiments with lipid components of erythrocyte membranes. Cholesterol is not an inhibitor, whereas, among the phospholipids tested, (sphingomyelin, phosphatidylcholine, phosphatidylserine, phosphatidylethanolamine) sphingomyelin inhibits the hemolytic activity of hemocytes. However, thin layer chromatography showed that sphingomyelinase activity was not contained in the chloroform-methanol extracts from hemocyte debris. The inhibition capacity of the components ceramide and phosphorylcholine suggests that the entire sphingomyelin molecule is involved in binding cytolysins. The lysin-lipid interactions probably cause changes in erythrocyte membrane permeability, leading to lysis.

Cytotoxic activity of Ciona intestinalis (Tunicata) hemocytes: properties of the in vitro reaction against erythrocyte targets

Hemocytes (effectors) of Ciona intestinalis showed a natural cytotoxic capacity (HCA) when assayed in vitro against erythrocytes (targets). Cytotoxic cells lysed, to a variable extent, rabbit (RE), human (A, B, O), guinea pig, and sheep (SE) erythrocytes. Hemocyte cytotoxic activity (HCA) assayed against SE is a calcium-dependent reaction, occurs rapidly (15-30 min), at 25-37 degrees C over a wide range of pH (5.4-8.0). Assays were carried out using: 1) the medium in which hemocytes were maintained, 2) the soluble portion of hemocyte lysates, and 3) debris prepared from hemocyte lysates. Results suggest that HCA is a cell-mediated process that requires effector-target cell contacts. Anti-SE (calcium-dependent) and anti-RE (calcium-independent) agglutinins were also found in the reaction medium, probably released by hemocytes as a consequence of the in vitro experiments. The occurrence of HCA was independent of any allogeneic reaction between mixed hemocytes. Various levels of cytotoxic activity reveal hemocyte specificity.