Scuticociliate proteinases may modulate turbot immune response by inducing apoptosis in pronephric leucocytes

In Vitro Antiprotozoal Activity of Hibiscus sabdariffa Extract against a Ciliate Causing High Mortalities in Turbot Aquaculture

Philasterides dicentrarchi is an histophagous parasite that infects flatfish, namely turbot (Scophthalmus maximus), and cause significant losses in aquaculture units. The available measures for P. dicentrarchi control have limited efficiency, and some cause harm to fish. Hence, sustainable and natural control strategies are urgently needed. This study evaluated the in vitro bioactivity of the ethanol extract of Hibiscus sabdariffa calyces on P. dicentrarchi population growth rate (PGR), oxidative stress biomarkers (glutathione-S-transferases (GST), glutathione reductase (GR), glutathione peroxidase (GPx), total glutathione (TG) and catalase (CAT), neurotoxicity (acetylcholinesterase, AChE), activity and gene expression of proteases as major virulence factors. H. sabdariffa extract inhibited parasite PGR (IC₅₀ = 1.57 mg mL^-1), and caused significant changes in the activity of antioxidant enzymes (LOEC = 0.22 mg mL^-1), especially GPx, TG, and CAT. The activity of proteases was also severely inhibited (IC₅₀ = 0.76 mg mL^-1), and gene expression of catepsin 90 and leishmanolysin proteases was downregulated. Organic acids and phenolic phytochemicals in hibiscus extract are potentially responsible for the antiprotozoal bioactivity herein determined. Therefore, H. sabdariffa extract can be a promising disease-control alternative against the ciliate proliferation, cellular defense mechanisms and pathogenicity. Still, its applicability in aquaculture settings, and potential effects on farmed fish, should be further elucidated.

Scuticociliatosis caused by Philasterides dicentrarchi

The ciliate Philasterides dicentrarchi has been previously identified as a new agent of scuticociliatosis in marine fish. The parasite can cause high mortalities in fish reared on farms or kept in aquariums. P. dicentrarchi is usually a free-living protozoan but can become an opportunistic histophagous parasite causing rapid lethal systemic infections in cultured fish. This review provides information about the morphology and biology of the scuticociliate P. dicentrarchi, as well as information about the pathological and immunological reactions of the host in response to the infection with the parasite. The epidemiology and the control strategies of the disease are also reviewed.

In vivo and in vitro assessment of host-parasite interaction between the parasitic copepod Brachiella thynni (Copepoda; Lernaeopodidae) and the farmed Atlantic bluefin tuna (Thunnus thynnus)

The Atlantic bluefin tuna (ABFT; Thunnus thynnus) today represents one of the economically most important species for Croatian fisheries industry. Although the most diverse and abundant parasitofauna is usually found in the largest specimens of wild ABFT, the opposite was observed in captivity where parasite populations significantly decline by the end of the farming cycle. Copepod Brachiella thynni, is a skin parasite frequently parasitizing tuna, whose population also decreases in number throughout the rearing process. In order to better understand the immunity mechanisms underlying ABFT reaction to B. thynni infection, we studied expression profiles of immunity related genes; interleukin 1β (il1β), tumour necrosis factors (tnfα1, tnfα2), complement component 4 (c4) and caspase 3 (casp3), in peripheral blood leukocytes (PBLs) during in vitro stimulation by B. thynni protein extracts (i.e. antigens) and in infected tissues at B. thynni parasitation site. Finally, a histopathological analysis of semi-thin and ultra-thin sections of tissues surrounding B. thynii attachment site was performed to evaluate the severity of parasite-induced lesions and identify involved cell lineages. In vitro stimulation of ABFT PBLs with B. thynii antigens caused a dose-depended upregulation of selected genes, among which tnfα1 showed the highest induction by both concentrations of B. thynni protein extract. However, targeted genes were not significantly upregulated in the infected tissue. Also, no significant alterations in ultrastructure of epithelial layers surrounding B. thynii attachment site were noticed, except local tissue erosion, necrosis of squamous epithelium and proliferation of rodlet and goblet cells. Our results suggest that B. thynii has evolved strategies to successfully bypass both innate immune response and the connective-tissue proliferation processes. Therefore, the observed disappearance of this copepod by the end of the rearing process is more likely related to its limited lifespan on the host and its inability to complete the life cycle in the rearing cages, rather than host's reaction.

Interactions between the Parasite Philasterides dicentrarchi and the Immune System of the Turbot Scophthalmus maximus. A Transcriptomic Analysis

The present study analyses the interactions between Philasterides dicentrarchi (a ciliate parasite that causes high mortalities in cultured flatfish) and the peritoneal cells of the turbot Scophthalmus maximus during an experimental infection. The transcriptomic response was evaluated in the parasites and in the fish peritoneal cells, at 1, 2 and 4 h post-infection (hpi) in turbot injected intraperitoneally (ip) with 10⁷ ciliates and at 12 and 48 hpi in turbot injected ip with 10⁵ ciliates. Numerous genes were differentially expressed (DE) in P. dicentrarchi, relative to their expression in control ciliates (0 hpi): 407 (369 were up-regulated) at 1 hpi, 769 (415 were up-regulated) at 2 hpi and 507 (119 were up-regulated) at 4 hpi. Gene ontology (GO) analysis of the DE genes showed that the most representative categories of biological processes affected at 1, 2 and 4 hpi were biosynthetic processes, catabolic processes, biogenesis, proteolysis and transmembrane transport. Twelve genes of the ABC transporter family and eight genes of the leishmanolysin family were DE at 1, 2 and 4 hpi. Most of these genes were strongly up-regulated (UR), suggesting that they are involved in P. dicentrarchi infection. A third group of UR genes included several genes related to ribosome biogenesis, DNA transcription and RNA translation. However, expression of tubulins and tubulin associated proteins, such as kinesins or dyneins, which play key roles in ciliate division and movement, was down-regulated (DR). Similarly, genes that coded for lysosomal proteins or that participate in the cell cycle mitotic control, glycolysis, the Krebs cycle and/or in the electron transport chain were also DR. The transcriptomic analysis also revealed that in contrast to many parasites, which passively evade the host immune system, P. dicentrarchi strongly stimulated turbot peritoneal cells. Many genes related to inflammation were DE in peritoneal cells at 1, 2 and 4 hpi. However, the response was much lower at 12 hpi and almost disappeared completely at 48 hpi in fish that were able to kill P. dicentrarchi during the first few hpi. The genes that were DE at 1, 2 and 4 hpi were mainly related to the apoptotic process, the immune response, the Fc-epsilon receptor signalling pathway, the innate immune response, cell adhesion, cell surface receptors, the NF-kappaB signalling pathway and the MAPK cascade. Expression of toll-like receptors 2, 5 and 13 and of several components of NF-κB, MAPK and JAK/STAT signalling pathways was UR in the turbot peritoneal cells. Genes expressing chemokines and chemokine receptors, genes involved in prostaglandin and leukotriene synthesis, prostaglandins, leukotriene receptors, proinflammatory cytokines and genes involved in apoptosis were strongly UR during the first four hours of infection. However, expression of anti-inflammatory cytokines such as Il-10 and lipoxygenases with anti-inflammatory activity (i.e., arachidonate 15-lipoxygenase) were only UR at 12 and/or 48 hpi, indicating an anti-inflammatory state in these groups of fish. In conclusion, the present study shows the regulation of several genes in P. dicentrarchi during the early stages of infection, some of which probably play important roles in this process. The infection induced a potent acute inflammatory response, and many inflammatory genes were regulated in peritoneal cells, showing that the turbot uses all the protective mechanisms it has available to prevent the entry of the parasite.

Combined antiparasitic and anti-inflammatory effects of the natural polyphenol curcumin on turbot scuticociliatosis

The histiophagous scuticociliate Philasterides dicentrarchi is the aetiological agent of scuticociliatosis, a parasitic disease of farmed turbot. Curcumin, a polyphenol from Curcuma longa (turmeric), is known to have antioxidant and anti-inflammatory properties. We investigated the in vitro effects of curcumin on the growth of P. dicentrarchi and on the production of pro-inflammatory cytokines in turbot leucocytes activated by parasite cysteine proteases. At 100 μm, curcumin had a cytotoxic effect and completely inhibited the growth of the parasite. At 50 μm, curcumin inhibited the protease activity of the parasite and expression of genes encoding two virulence-associated proteases: leishmanolysin-like peptidase and cathepsin L-like. At concentrations between 25 and 50 μm, curcumin inhibited the expression of S-adenosyl-L-homocysteine hydrolase, an enzyme involved in the biosynthesis of the amino acids methionine and cysteine. At 100 μm, curcumin inhibited the expression of the cytokines tumour necrosis factor-alpha (TNF-α) and interleukin-1 beta (IL-1β) produced in turbot leucocytes activated by parasite proteases. Results show that curcumin has a dual effect on scuticociliatosis: an antiparasitic effect on the catabolism and anabolism of ciliate proteins, and an anti-inflammatory effect that inhibits the production of proinflammatory cytokines in the host. The present findings suggest the potential usefulness of this polyphenol in treating scuticociliatosis.

Presence of an isoform of H+-pyrophosphatase located in the alveolar sacs of a scuticociliate parasite of turbot: physiological consequences

H+-pyrophosphatases (H+-PPases) are integral membrane proteins that couple pyrophosphate energy to an electrochemical gradient across biological membranes and promote the acidification of cellular compartments. Eukaryotic organisms, essentially plants and protozoan parasites, contain various types of H+-PPases associated with vacuoles, plasma membrane and acidic Ca+2storage organelles called acidocalcisomes. We used Lysotracker Red DND-99 staining to identify two acidic cellular compartments in trophozoites of the marine scuticociliate parasitePhilasterides dicentrarchi: the phagocytic vacuoles and the alveolar sacs. The membranes of these compartments also contain H+-PPase, which may promote acidification of these cell structures. We also demonstrated for the first time that theP. dicentrarchiH+-PPase has two isoforms: H+-PPase 1 and 2. Isoform 2, which is probably generated by splicing, is located in the membranes of the alveolar sacs and has an amino acid motif recognized by the H+-PPase-specific antibody PABHK. The amino acid sequences of different isolates of this ciliate are highly conserved. Gene and protein expression in this isoform are significantly regulated by variations in salinity, indicating a possible physiological role of this enzyme and the alveolar sacs in osmoregulation and salt tolerance inP. dicentrarchi.

Reprint of "fish immunity to scuticociliate parasites"

Some species of scuticociliates (Ciliophora) behave as facultative parasites and produce severe mortalities in cultured fish. Pathogenic scuticociliates can cause surface lesions and can also penetrate inside the body, where they feed on tissue and proliferate in the blood and most internal organs, killing the host in a few days. In this review, we describe the current knowledge on the protective role of fish cellular and humoral immune responses against these parasites. Immune humoral factors, especially complement, are of particular importance in defending fish against these ciliates. However, knowledge about how the fish immune system responds to scuticociliates is scant, and the cellular and molecular events that occur during the response are not known. We also describe the possible mechanisms used by scuticociliates to avoid or resist the defensive reaction of the host. For example, the release of proteases can help parasites enter fish tissues and impair the fish cellular and humoral responses. Several vaccine formulations containing scuticociliates have induced a good antibody response and protection in fish immunized and challenged with homologous strains of particular species. However, protection was not achieved in fish immunized and challenged with heterologous strains, and the antigens involved in protection and the antigenic differences between heterologous strains have not yet been determined.

Expression and characterization of cathepsin L-like cysteine protease from Philasterides dicentrarchi

Philasterides dicentrarchi is a causative agent of scuticociliatosis in olive flounder Paralichthys olivaceus, aquaculture in Korea. In this study, a cDNA encoding a cathepsin L-like cysteine protease (PdCtL) of P. dicentrarchi (synonym Miamiensis avidus) was identified. To express the PdCtL recombinant protein in a heterologous system, 10 codons were redesigned to conform to the standard eukaryotic genetic code using polymerase chain reaction (PCR)-based site-directed mutagenesis. The recombinant P. dicentrarchi procathepsin L (proPdCtL) was expressed at high levels in E. coli Rosetta (DE3) pLysS with a pPET21a vector, and successfully refolded, purified, and activated into a functional and enzymatically active form. The optimal pH for protease activity was 5. Similar to other cysteine proteases, enzyme activity was inhibited by E64 and leupeptin. Immunogenicity of recombinant PdCtL was assessed by enzyme-linked immunosorbent assay, western blot, and specific anti-recombinant PdCtL antibodies were detected. Our results suggest that the biochemical characteristics of the recombinant ciliate proPdCtL protein are similar to those of the cathepsin L-like cysteine protease, that the PCR-based site-direct mutated ciliate gene was successfully expressed in a biochemically active form, and that the recombinant PdCtL acted as a specific epitope in olive flounder.

Fish immunity to scuticociliate parasites

Some species of scuticociliates (Ciliophora) behave as facultative parasites and produce severe mortalities in cultured fish. Pathogenic scuticociliates can cause surface lesions and can also penetrate inside the body, where they feed on tissue and proliferate in the blood and most internal organs, killing the host in a few days. In this review, we describe the current knowledge on the protective role of fish cellular and humoral immune responses against these parasites. Immune humoral factors, especially complement, are of particular importance in defending fish against these ciliates. However, knowledge about how the fish immune system responds to scuticociliates is scant, and the cellular and molecular events that occur during the response are not known. We also describe the possible mechanisms used by scuticociliates to avoid or resist the defensive reaction of the host. For example, the release of proteases can help parasites enter fish tissues and impair the fish cellular and humoral responses. Several vaccine formulations containing scuticociliates have induced a good antibody response and protection in fish immunized and challenged with homologous strains of particular species. However, protection was not achieved in fish immunized and challenged with heterologous strains, and the antigens involved in protection and the antigenic differences between heterologous strains have not yet been determined.

Hydrogenosome metabolism is the key target for antiparasitic activity of resveratrol against Trichomonas vaginalis

Metronidazole (MDZ) and related 5-nitroimidazoles are the recommended drugs for treatment of trichomoniasis, a sexually transmitted disease caused by the protozoan parasite Trichomonas vaginalis. However, novel treatment options are needed, as recent reports have claimed resistance to these drugs in T. vaginalis isolates. In this study, we analyzed for the first time the in vitro effects of the natural polyphenol resveratrol (RESV) on T. vaginalis. At concentrations of between 25 and 100 μM, RESV inhibited the in vitro growth of T. vaginalis trophozoites; doses of 25 μM exerted a cytostatic effect, and higher doses exerted a cytotoxic effect. At these concentrations, RESV caused inhibition of the specific activity of a 120-kDa [Fe]-hydrogenase (Tvhyd). RESV did not affect Tvhyd gene expression and upregulated pyruvate-ferredoxin oxidoreductase (a hydrogenosomal enzyme) gene expression only at a high dose (100 μM). At doses of 50 to 100 μM, RESV also caused overexpression of heat shock protein 70 (Hsp70), a protective protein found in the hydrogenosome of T. vaginalis. The results demonstrate the potential of RESV as an antiparasitic treatment for trichomoniasis and suggest that the mechanism of action involves induction of hydrogenosomal dysfunction. In view of the results, we propose hydrogenosomal metabolism as a key target in the design of novel antiparasitic drugs.

Molecular cloning and expression analysis of peptidase genes in the fish-pathogenic scuticociliate Miamiensis avidus

Background

Parasite peptidases have been actively studied as vaccine candidates or drug targets for prevention or treatment of parasitic diseases because of their important roles for survival and/or invasion in the host. Like other parasites, the facultative histophagous ciliate Miamiensis avidus would possess peptidases that are closely associated with the invasion into the host tissue and survival in the host.

Results

The 17 genes encoding peptidases, including seven cathepsin-like cysteine peptidases, four serine carboxypeptidases, a eukaryotic aspartyl protease family protein, an ATP-dependent metalloprotease FtsH family protein, three leishmanolysin family proteins and a peptidase family M49 protein were identified from a Miamiensis avidus cDNA library by BLAST X search. Expression of genes encoding two cysteine peptidases, three leishmanolysin-like peptidases and a peptidase family M49 protein was up-regulated in the cell-fed ciliates compared to the starved ciliates. Especially, one cysteine peptidase (MaPro 4) and one leishmanolysin-like peptidase (MaPro 14) were transcribed more than 100-folds in the cell-fed ciliates.

Conclusions

The genetic information and transcriptional characteristics of the peptidases in the present results would be helpful to elucidate the role of peptidases in the invasion of scuticociliates into their hosts.

Gene expression profiles of spleen, liver, and head kidney in turbot (Scophthalmus maximus) along the infection process with Philasterides dicentrarchi using an immune-enriched oligo-microarray

We evaluated the expression profiles of turbot in spleen, liver, and head kidney across five temporal points of the Philasterides dicentrarchi infection process using an 8x15K Agilent oligo-microarray. The microarray included 2,176 different fivefold replicated gene probes designed from a turbot 3' sequenced EST database. We were able to identify 221 differentially expressed (DE) genes (8.1% of the whole microarray), 113 in spleen, 83 in liver, and 90 in head kidney, in at least 1 of the 5 temporal points sampled for each organ. Most of these genes could be annotated (83.0%) and functionally categorized using GO terms (69.1%) after the additional sequencing of DE genes from the 5' end. Many DE genes were related to innate and acquired immune functions. A high proportion of DE genes were organ-specific (70.6%), although their associated GO functions showed notable similarities in the three organs. The most striking difference in functional distribution was observed between the up- and downregulated gene groups. Upregulated genes were mostly associated to immune functions, while downregulated ones mainly involved metabolism-related genes. Genetic response appeared clustered in a few groups of genes with similar expression profiles along the temporal series. The information obtained will aid to understand the turbot immune response and will specifically be valuable to develop strategies of defense to P. dicentrarchi to achieve more resistant broodstocks for turbot industry.

Poly D,L-lactide-co-glycolic acid (PLGA)-encapsulated vaccine on immune system in Epinephelus bruneus against Uronema marinum

We investigate the efficacy of poly D,L-lactide-co-glycolic acid (PLGA)-encapsulated vaccine on innate and adaptive immune response in kelp grouper (Epinephelus bruneus) against Uronema marinum at weeks 1, 2, and 4. The respiratory burst (RB) activity, complement activity, and α2-macroglobulin were significantly enhanced in fish immunization with vaccine on week 4 whereas vaccine and PLGA-encapsulated vaccine from weeks 1 to 4. The serum lysozyme activity, antiprotease activity, and antibody level were significantly enhanced in fish immunized with vaccine and PLGA-encapsulated vaccine on weeks 2 and 4. The cumulative mortality was low in PLGA-encapsulated vaccine with 20% whereas high in PLGA and vaccine with 40% and 30%. The results from the present study suggest that PLGA-encapsulated vaccine is useful for further design of immunoprophylatic nano formulation against scuticociliatosis.

Molecular profiling of the gilthead sea bream (Sparus aurata L.) response to chronic exposure to the myxosporean parasite Enteromyxum leei

The aim of the present work was to investigate the transcriptome response of gilthead sea bream (Sparus aurata) after challenge with the myxosporean Enteromyxum leei, a wide-spread enteric parasite causing heavy economic losses in Mediterranean sparid farms. This parasite causes severe desquamative enteritis which usually leads to death of the fish, and there are no preventative or curative treatments for this enteromyxosis. After 113 days of exposure to parasite-contaminated effluent, fish were classified into three cohorts: control fish not exposed to parasite, those that were exposed and infected, and those that were exposed but not infected. In order to detect target genes that may be candidates for infective status or resistance, a cDNA microarray containing 18,490 cDNA clones enriched in genes differentially expressed after infection was hybridised with head kidney and intestine samples. In infected fish, 371 and 373 genes were differentially regulated at the >1.5-fold level in intestine and head kidney respectively, whereas in non-infected fish 175 and 501 genes were differentially regulated in these tissues, respectively. A global marked gene down-regulation was evident in infected fish, mainly in genes involved in the immune and acute phase response particularly complement and mannose binding lectin. Microarray analysis demonstrated a complex interplay between host and/or parasite derived proteases and protease inhibitors, apoptosis, cell proliferation and antioxidant defence genes in exposed fish. In the head kidney of non-infected fish a marked depression of genes involved in the acute phase response was evident. By contrast, in the intestine of non-infected fish, interferon-stimulated and MHC class II genes involved in antigen processing and presentation were up-regulated, possibly indicating that an active immune response at the local level is important to avoid infection with or proliferation of the parasite.

Turbot resistance to Philasterides dicentrarchi is more dependent on humoral than on cellular immune responses

Philasterides dicentrarchi is a ciliate that causes high mortalities in cultured turbot, Psetta maxima (L.). This pathogen displays high phagocytic activity and after entering the body it multiplies and feeds on host cells and tissue components. In previous studies, we found that complement, activated through the classical pathway, is a potent killer of P. dicentrarchi. Here, we compared the killing activity of turbot leucocytes and humoral factors against two virulent isolates of P. dicentrarchi, in order to determine the importance of leucocytes in the defence against this pathogen. Components of P. dicentrarchi (ciliary and membrane) stimulated turbot leucocytes, and increased the respiratory burst, degranulation and the expression of pro-inflammatory cytokines. We tested the susceptibility of ciliates to reactive oxygen and nitrogen species, by incubating them with different oxidative systems (H(2)O(2), Fe/ascorbate, which induces lipid peroxidation, an O(2)(-) donor (XOD/HX), an NO donor (SNAP) and an ONOO(-) donor (SIN-1)), for 24h. Both isolates were susceptible to high concentrations of H(2)O(2,) Fe/ascorbate, XOD/HX, and SIN-1 but were resistant to incubation with SNAP. Leucocytes became strongly activated when they were in contact with or were phagocytosed by the ciliate. Incubation of P. dicentrarchi with a combination of fresh serum and specific antibodies killed most of the ciliates, but the addition of leucocytes to ciliate cultures did not increase the toxicity to the ciliates. On the contrary, the number of ciliates increased when leucocytes were added to the culture because the ciliates fed on them. Despite being activated, leucocytes did not produce sufficiently high concentrations of toxic substances to kill the parasite. The most virulent isolate was that which induced greatest activation of leucocytes but was least susceptible to complement. We concluded that humoral factors such as complement (activated through the classical pathway) are critical for fish defence against P. dicentrarchi and that cellular responses appear to play a minor role, if any, in defence against this ciliate.

Scuticociliatosis and its recent prophylactic measures in aquaculture with special reference to South Korea Taxonomy, diversity and diagnosis of scuticociliatosis: Part I Control strategies of scuticociliatosis: Part II

Scuticociliatosis caused by about 20 species belonging to the Phylum Ciliophora has been recognized as an emerging problem inflicting significant economic loss in aquaculture industry in the world. Among these Philasterides dicentrarchi, Miamiensis avidus, and Uronema marinum are the three species responsible for scuticociliatosis in olive flounder farms of South Korea. Some of the parasites living or scavenger ciliates also have become parasites of aquaculture fish. The major clinico-pathological manifestations of scuticociliatosis infected fishes are anemia, weight loss, dark coloration, enteritis, excessive body mucus, yellowish intestinal mucus, loss of scales, hemorrhagic and/or bleached spots on the skin, and dermal necrotic lesions that finally destroy tissues lead to high mortalities. Affected fish exhibit organ-specific pathological changes in the brain, eyes, muscle, gills, liver, kidney, intestine, and stomach that lead to severe mortality. At present, farmers in South Korea manage scuticociliatosis by using therapeutic measures, such as application of antibiotics like oxytetracycline, gentamycine, tetracycline, amoxycililin, and cefazolin and chemicals, such as formalin, hydrogen peroxide, malachite green, and jenoclean at a concentration of 350 +/- 150 ppm. However till date, no systematic scientific study has been conducted under field condition on the efficacy of these management measures. Under laboratory condition the ciliate can be effectively controlled with the antibiotics and chemicals while on the host, but on entering the host no systemic chemotherapeutic treatment has been yet proven effective. Furthermore the indiscriminate uses of harmful chemicals in aquaculture are increasingly becoming a cause of concern. Recently formalin and malachite green, the most widely used chemicals have been banned in food fish production by FDA as not consumer friendly and being carcinogenic respectively. Vaccines and immunostimulants can induce good immune response and protect against scuticociliatosis as it has been proved in the case of freshwater Ich. Now a days a number of probiotics and herbal formulations are in use against freshwater bacterial and fungal diseases, while, little information is available regarding the different prophylactic measures against marine scuticociliatosis. This review attempts to provide information on the various prophylaxic measures practiced against scuticociliatosis with special reference to olive flounder farms in South Korea.

Tetrahymena sp. infection in guppies, Poecilia reticulata Peters: parasite characterization and pathology of infected fish

Tetrahymena sp. infection was diagnosed in guppies imported from Singapore. The parasite was isolated (Tet-NI) and optimally cultured in vitro in RM-9 medium. Cytological analyses [silver-staining and scanning electron microscopy (SEM)] revealed a pyriform-shaped, 64 x 41-microm holotrich ciliate without caudal cilium, containing a macro-nucleus (18.25 x 16.83 microm) and micro-nucleus (5.73 x 5.40 microm). Wet-mount examination and histological analyses of fish exposed to the parasite by co-habitation, immersion and infection by i.p. (intra-peritoneal) and i.m. (intra-muscular) injection revealed numerous ciliates on the skin, and in the gill and caudal fin blood vessels. Ciliates surrounded internal organs, the peri-orbital region of the eye, and were observed inside developing guppy embryos. Some muscle necrosis was associated with infection, but little or no inflammatory response. Immersion, co-habitation and i.m. injection caused relatively high infection rates and levels in the skin and tail, and lower infection in the gill blood vessels and internal organs; i.p. injection caused higher infection in the gill blood vessels and internal organs. Co-habited fish had relatively high infection levels in the hind-gut sub-mucosa. This is the first report of controlled systemic infection by Tetrahymena sp.

Living off a fish: a trade-off between parasites and the immune system

Research in fish immune system and parasite invasion mechanisms has advanced the knowledge of the mechanisms whereby parasites evade or cope with fish immune response. The main mechanisms of immune evasion employed by fish parasites are reviewed and considered under ten headings. 1) Parasite isolation: parasites develop in immuno-privileged host tissues, such as brain, gonads, or eyes, where host barriers prevent or limit the immune response. 2) Host isolation: the host cellular immune response isolates and encapsulates the parasites in a dormant stage without killing them. 3) Intracellular disguise: typical of intracellular microsporidians, coccidians and some myxosporeans. 4) Parasite migration, behavioural and environmental strategies: parasites migrate to host sites the immune response has not yet reached or where it is not strong enough to kill them, or they accommodate their life cycles to the season or the age in which the host immune system is down-regulated. 5) Antigen-based strategies such as mimicry or masking, variation and sharing of parasite antigens. 6) Anti-immune mechanisms: these allow parasites to resist innate humoral factors, to neutralize host antibodies or to scavenge reactive oxygen species within macrophages. 7) Immunodepression: parasites either suppress the fish immune systems by reducing the proliferative capacity of lymphocytes or the phagocytic activity of macrophages, or they induce apoptosis of host leucocytes. 8) Immunomodulation: parasites secrete or excrete substances which modulate the secretion of host immune factors, such as cytokines, to their own benefit. 9) Fast development: parasites proliferate faster than the ability of the host to mount a defence response. 10) Exploitation of the host immune reaction. Knowledge of the evasion strategies adopted by parasites will help us to understand host-parasite interactions and may therefore help in the discovery of novel immunotherapeutic agents or targeted vaccines, and permit the selection of host-resistant strains.

Scuticociliate cysteine proteinases modulate turbot leucocyte functions

The effects exerted by cysteine proteinases isolated from the histiophagous ciliate Philasterides dicentrarchi on the phagocytic functions of turbot pronephric leucocytes (PL) were investigated. The enzymes were tested at concentrations of 125, 250 and 500 microg ml(-1), and it was found that the viability of the leucocytes was not affected after treatment for 24h. Leucocyte migration was inhibited by the cysteine proteinases in a dose-dependent manner, whereas the ascitic fluid obtained from turbot experimentally infected with P. dicentrarchi induced high chemotactic activity in the turbot PL. The proteinases did not affect yeast cell phagocytosis but increased intracellular production of the superoxide anion (O2(-)). Stimulation with the proteinases did not alter the PGE2 levels in supernatants from 24-h cultures of PL, however, beta-glucans (100 microg ml(-1)) provoked a large increase in PGE2 levels, which were inhibited after addition of 10 microg ml(-1) of indomethacin, a non-selective inhibitor of COX2 enzymatic activity. The mean PGE2 level in ascitic fluid from turbot, experimentally infected with P. dicentrarchi, was 500 pg ml(-1), and the addition of low levels of PGE2 (62.5 pg ml(-1)) to PL cultures stimulated O2(-) production, although addition of PGE2 at concentrations higher than 250 pg ml(-1) blocked the increase in stimulation. Addition of cysteine proteinases to 24-h cultures of PL also increased mRNA levels in the pro-inflammatory cytokine interleukin-1beta. The results revealed the capacity of cysteine proteinases isolated from P. dicentrarchi to modulate the innate immune response of turbot, which together with the inflammation mediators produced during infection, may play an important role in pathogenesis of the disease and in the survival of the parasite.

In vitro activity of the nonsteroidal anti-inflammatory drug indomethacin on a scuticociliate parasite of farmed turbot

The scuticociliatosis produced by the endoparasite Philasterides dicentarchi is a severe parasitic infection of farmed turbot (Scophthalmus maximus) characterized by several histopathological effects including extensive inflammation. Indomethacin is a nonsteroidal anti-inflammatory drug that specifically inhibits synthesis of the proinflammatory mediator prostaglandins. The effect of indomethacin on the in vitro growth of P. dicentrarchi was investigated. In vitro growth of the scuticociliate was significantly inhibited by treatment with 100 microM indomethacin for 48 h. Higher concentrations of indomethacin (mM levels) did not affect the gelatinolytic activity of the cysteine proteinases of P. dicentrarchi. In vitro treatment with 25, 50 or 100 microM indomethacin for 3 days did not significantly affect the enzymatic activity of cysteine proteinases, as assayed with p-nitroanilide as substrate. Immunoblot analysis with anti-cysteine proteinase antibodies revealed an increase in proteinase expression (molecular weights of 80, 32 and 40-45 kDa) in parasite lysates originating from in vitro cultures incubated with 25 microM indomethacin for 72 h. Degradation of genomic DNA of the ciliates was observed in cultures incubated with 100 microM indomethacin for 1, 3 and 7 days. The results suggest that indomethacin is capable of inhibiting in vitro growth of the scuticociliate P. dicentrarchi by a mechanism related to the induction of programmed cell death, without affecting the enzymatic activation of parasite proteinases, which demonstrates the potential therapeutic use of this drug in the control of turbot scuticociliatosis.