Antibody-mediated effects on parasite behavior: Evidence of a novel mechanism of immunity against a parasitic protist

Transcriptomic analysis of Nibea albiflora skin in response to infection by Cryptocaryon irritans

Massive infection caused by Cryptocaryon irritans is detrimental to the development of marine aquaculture. Recently, our lab found that Nibea albiflora has low sensitivity and low mortality to C. irritans infection. The present study was designed to investigate the mechanisms of the N. albiflora response to C. irritans infection by analyzing transcriptome changes in the skin. Skin samples of control and experimental groups with C. irritans infection were collected at 24 and 72 h (24 h control, 24 h post-infection, 72 h control, and 72 h post-infection). Three parallels were set for each group and sample time, and a total of 12 skin samples were collected for sequencing. Overall, 297,489,843 valid paired-end reads and 48,817 unigenes were obtained with an overall length of 59,010,494 nt. In pairwise comparisons, changes in expression occurred in 1621 (764 upregulated and 857 downregulated), 285 (180 upregulated and 105 downregulated), 993 (489 upregulated and 504 downregulated), and 37 (8 upregulated and 29 downregulated) genes at 24 h control vs 24 h post-infection, 72 h control vs 72 h post-infection, 24 h post-infection vs 72 h post-infection, and 24 h control vs 72 h control, respectively. Gene Ontology (GO) analysis of differentially expressed genes (DEGs) indicated that the number of genes enriched in GO sub-categories were ordered 24 h control vs 24 h post-infection > 24 h post-infection vs 72 h post-infection >72 h control vs 72 h post-infection > 24 h control vs 72 h control. Further analysis showed that immune-related GO terms (including immune system process, complement activation, and humoral immunity) were significantly enriched at both 72 h control vs 72 h post-infection and 24 h post-infection vs 72 h post-infection, but no immune-related GO terms were significantly enriched in the 24 h control vs 72 h control and at 24 h control vs 24 h post-infection, indicating that C. irritans infection mainly affected the physiological metabolism of N. albiflora at an early stage (24 h), and immune-related genes play an important role at a later stage (72 h) of infection. In KEGG pathway analysis, the complement and coagulation cascade pathway are involved in early infection. Hematopoietic cell lineage, natural killer (NK) cell-mediated cytotoxicity, and the intestinal immune network for IgA production are involved in later infection. Further analysis showed that the alternative pathway of complement and coagulation cascades plays an important role in the resistance of N. albiflora to early C. irritans infection. During late infection, CD34, IgM, and IgD were significantly upregulated in the hematopoietic cell lineage pathway. CCR9 was significantly downregulated, and IGH and PIGR were significantly upregulated in the intestinal immune network for IgA production. GZMB and IGH were significantly downregulated in NK cell-mediated cytotoxicity. These findings indicate that acquired immunity at the mRNA level was initiated during later infection. In addition, the IL-17 signaling pathway was enriched by downregulated DEGs at 24 h post-infection vs 72 h post-infection, suggesting the inflammatory response at 24 h was stronger than at 72 h and the invasion of the parasite has a greater impact on the host.

Convergent Evolution of Mucosal Immune Responses at the Buccal Cavity of Teleost Fish

The buccal mucosa (BM) is a critical first line of defense in terrestrial animals. To gain further insights into the evolutionary origins and primordial roles of BM in teleosts here we show that rainbow trout, a teleost fish, contains a diffuse mucosal associated lymphoid tissue (MALT) within its buccal cavity. Upon parasite infection, a fish immunoglobulin specialized in mucosal immunity (sIgT) was induced to a high degree, and parasite-specific sIgT responses were mainly detected in the buccal mucus. Moreover, we show that the trout buccal microbiota is prevalently coated with sIgT. Overall our findings revealed that the MALT is present in the BM of a non-tetrapod species. As fish IgT and mucus-producing cells are evolutionarily unrelated to mammalian IgA and salivary glands, respectively, our findings indicate that mucosal immune responses in the BM of teleost fish and tetrapods evolved through a process of convergent evolution.

Immune responses of fish to Ichthyophthirius multifiliis (Ich): A model for understanding immunity against protozoan parasites

The parasitic ciliate Ichthyophthirius multifiliis (Ich), which infects almost all freshwater fish species, provides an optimal model for the study of immunity against extracellular protozoa. Ich invades the epithelia of mucosal tissues, forms white spots covering the whole body, and induces high mortality, while survivor fish develop both innate and adaptive immunity against Ich attack in systemic and mucosal tissues. Besides the protective roles of the Toll-like receptor (TLR)-mediated innate immune response, the critical immune functions of novel IgT in the skin, gut, gill, and olfactory organ of teleosts have been demonstrated in recent years, and all this information contributes to the ontogeny of the mucosal immune response in vertebrates. Especially in rainbow trout, Ich-infected fish exhibited higher IgT concentrations and titers in the mucosa and increased IgT⁺ B-lymphocyte proliferation in mucosal tissues. IgM mainly functions in the adaptive immune response in the systemic tissues of rainbow trout, accompanied with increased IgM⁺ B-lymphocyte proliferation in the head kidney of Ich-infected trout. However, little is known about the interaction between these mucosal tissues and systemic immune organs and the interaction between the inductive immune organs and functional immune organs. Immobilization antigens (Iags), located on the parasite cell and ciliary membranes, have been characterized to be targeted by specific antibodies produced in the host. The crosslinking of antigens mediated by antibodies triggers either an escape response or the immobilization of Ich. With more knowledge about the Iags of Ich and the immunity of teleosts, a more targeted vaccine, even a DNA vaccine, can be developed for the immune control strategy of Ich. Due to the high frequency of clinical fish ichthyophthiriasis, the study of fish immune responses to Ich provides an optimal experimental model for understanding immunity against extracellular protozoa.

Evaluation of salinomycin isolated from Streptomyces albus JSY-2 against the ciliate, Ichthyophthirius multifiliis

The present study was undertaken to investigate the antiparasitic activity of extracellular products of Streptomyces albus. Bioactivity-guided isolation of chloroform extracts affording a compound showing potent activity. The structure of the compound was elucidated as salinomycin (SAL) by EI-MS, 1H NMR and 13C NMR. In vitro test showed that SAL has potent anti-parasitic efficacy against theronts of Ichthyophthirius multifiliis with 10 min, 1, 2, 3 and 4 h (effective concentration) EC50 (95% confidence intervals) of 2.12 (2.22-2.02), 1.93 (1.98-1.88), 1.42 (1.47-1.37), 1.35 (1.41-1.31) and 1.11 (1.21-1.01) mg L-1. In vitro antiparasitic assays revealed that SAL could be 100% effective against I. multifiliis encysted tomonts at a concentration of 8.0 mg L-1. In vivo test demonstrated that the number of I. multifiliis trophonts on Erythroculter ilishaeformis treated with SAL was markedly lower than that of control group at 10 days after exposed to theronts (P < 0.05). In the control group, 80% mortality was observed owing to heavy I. multifiliis infection at 10 days. On the other hand, only 30.0% mortality was recorded in the group treated with 8.0 mg L-1 SAL. The median lethal dose (LD50) of SAL for E. ilishaeformis was 32.9 mg L-1.

Mucosal immunoglobulins protect the olfactory organ of teleost fish against parasitic infection

The olfactory organ of vertebrates receives chemical cues present in the air or water and, at the same time, they are exposed to invading pathogens. Nasal-associated lymphoid tissue (NALT), which serves as a mucosal inductive site for humoral immune responses against antigen stimulation in mammals, is present also in teleosts. IgT in teleosts is responsible for similar functions to those carried out by IgA in mammals. Moreover, teleost NALT is known to contain B-cells and teleost nasal mucus contains immunoglobulins (Igs). Yet, whether nasal B cells and Igs respond to infection remains unknown. We hypothesized that water-borne parasites can invade the nasal cavity of fish and elicit local specific immune responses. To address this hypothesis, we developed a model of bath infection with the Ichthyophthirius multifiliis (Ich) parasite in rainbow trout, Oncorhynchus mykiss, an ancient bony fish, and investigated the nasal adaptive immune response against this parasite. Critically, we found that Ich parasites in water could reach the nasal cavity and successfully invade the nasal mucosa. Moreover, strong parasite-specific IgT responses were detected in the nasal mucus, and the accumulation of IgT⁺ B-cells was noted in the nasal epidermis after Ich infection. Strikingly, local IgT⁺ B-cell proliferation and parasite-specific IgT generation were found in the trout olfactory organ, providing new evidence that nasal-specific immune responses were induced locally by a parasitic challenge. Overall, our findings suggest that nasal mucosal adaptive immune responses are similar to those reported in other fish mucosal sites and that an antibody system with a dedicated mucosal Ig performs evolutionary conserved functions across vertebrate mucosal surfaces.

The olfactory organ is a vitally important chemosensory organ in vertebrates but it is also continuously stimulated by pathogenic microorganisms in the external environment. In mammals and birds, nasopharynx-associated lymphoid tissue (NALT) is considered one of the first lines of immune defense against inhaled antigens and in bony fish, protecting against water-borne infections. However, although B-cells and immunoglobulins (Igs) have been found in teleost NALT, the defensive mechanisms of parasite-specific immune responses after pathogen challenge in the olfactory organ of teleost fish remain poorly understood. Considering that the NALT of all vertebrates has been subjected to similar evolutionary forces, we hypothesize that mucosal Igs play a critical role in the defense of olfactory systems against parasites. To confirm this hypothesis, we show the local proliferation of IgT⁺ B-cells and production of pathogen-specific IgT within the nasal mucosa upon parasite infection, indicating that parasite-specific IgT is the main Ig isotype specialized for nasal-adaptive immune responses. From an evolutionary perspective, our findings contribute to expanding our view of nasal immune systems and determining the fate of the host–pathogen interaction.

Can community-based integrated vector control hasten the process of LF elimination?

Community-based integrated vector control (IVC) using polystyrene beads (EPS) and pyrethroid impregnated curtains (PIC) as an adjunct to mass drug administration (MDA) was implemented for lymphatic filariasis elimination, in the filaria endemic villages of Tirukoilur, south India. In all the villages, MDA was carried out by the state health machinery, as part of the national filariasis elimination programme. Thirty-six difficult-to-control villages were grouped as, viz, MDA alone, MDA + EPS and MDA + EPS + PIC arms. Implementation and monitoring of IVC was carried out by the community. After 3 years of IVC, higher reductions in filariometric indices were observed in both the community and vector population. Decline in antigenaemia prevalence was higher in MDA + IVC as compared to MDA alone arm. Vector density dropped significantly (P < 0.05) in both the IVC arms, and nil transmission was observed during post-IVC period. Almost 53.8 and 75.8 % of the cesspits in MDA + EPS and MDA + EPS + PIC arms were closed by the householders, due to the enhanced awareness on vector breeding. The paper presents the key elements of IVC implementation through social mobilization in a LF prevalent area. Thus, community-based IVC strategy can hasten LF elimination, as it reduced the transmission and filariometric indices significantly. Indices were maintained at low level with nil transmission, by the community through IVC tools.

Antiparasitic efficacy of Gracillin and Zingibernsis newsaponin from Costus speciosus (Koen ex. Retz) Sm. against Ichthyophthirius multifiliis

The present study aims to evaluate the antiparasitic activity of active components from Costus speciosus against Ichthyophthirius multifiliis. Bioassay-guided fractionation was employed to identify active compounds from C. speciosus yielding 2 bioactive compounds: Gracillin and Zingibernsis newsaponin. In-vitro assays revealed that Gracillin and Zingibernsis newsaponin could be 100% effective against I. multifiliis at concentrations of 0·8 and 4·5 mg L−1, with median effective concentration (EC50) values of 0·53 and 3·2 mg L−1, respectively. All protomonts and encysted tomonts were killed when the concentrations of Gracillin and Zingibernsis newsaponin were 1·0 and 5·0 mg L−1. In-vivo experiments demonstrated that fish treated with Gracillin and Zingibernsis newsaponin at concentrations of 1·0 and 5·0 mg L−1 carried significantly fewer parasites than the control (P<0·05). Mortality of fish did not occur in the treatment group (Zingibernsis newsaponin at 5·0 mg L−1) during the trial, although 100% of untreated fish died. Acute toxicities (LD50) of Gracillin and Zingibernsis newsaponin for grass carp were 1·64 and 20·7 mg L−1, respectively. These results provided evidence that the 2 compounds can be selected as lead compounds for the development of new drugs against I. multifiliis.

Immunity to Ichthyophthirius infections in fish: a synopsis

Ichthyophthirius multifiliis is a ciliated protozoan parasite that infects freshwater fish. It has been the subject of both applied and basic research for over 100years, which can be attributed to its world-wide distribution and its significant economic impact on both food and aquarium fish production. I. multifiliis serves as a model for studies in fish on innate and acquired immunity, as well as on mucosal immunity. Although an obligate parasite, I. multifiliis is relatively easily passaged from infected to naïve fish in laboratory aquaria, and is easily observed and manipulated under laboratory conditions. It parasitizes the epithelia of the skin and gills, which facilitates in vivo experimentation and quantification of challenge. This review provides a description of both mucosal and systemic innate and adaptive immune responses to parasite infection, a synopsis of host-parasite immunobiology, vaccine research, and suggested areas for future research to address critical remaining questions. Studies in carp and rainbow trout have shown that extensive tissue damage occurs when the parasite invades the epithelia of the skin and gills and substantial focal and systemic inflammatory responses are elicited by the innate immune response. The adaptive immune response is initiated when phagocytic cells are activated by antigens released by the parasite. It is not known whether activated T and B cells proliferate locally in the skin and gills following infection or migrate to these sites from the spleen or anterior kidney. I. multifiliis infection elicits both mucosal and systemic antibody production. Fish that survive I. multifiliis infection acquire protective immunity. Memory B cells provide long-term humoral memory. This suggests that protective vaccines are theoretically possible, and substantial efforts have been made toward developing vaccines in various fish species. Exposure of fish to controlled surface infections or by intracoelomic injection of live theronts provides protection. Vaccination with purified immobilization antigens, which are GPI-anchored membrane proteins, also provides protection under laboratory conditions and immobilization antigens are currently the most promising candidates for subunit vaccines against I. multifiliis.

A short history of research on immunity to infectious diseases in fish

This review describes the history of research on immunity to infectious diseases of fish in the period between 1965 and today. Special attention is paid to those studies, which are dealing with the interaction between immune system and invading pathogens in bony fish. Moreover, additional biographic information will be provided of people involved. In the 1960s and 1970s the focus of most studies was on humoral (Ig, B-cell) responses. Thorough studies on specific cellular (T-cell) responses and innate immunity (lectins, lysozyme, interferon, phagocytic cells) became available later. In the period between 1980 and today an overwhelming amount of data on regulation (e.g. cell cooperation, cytokines) and cell surface receptors (e.g. T-cell receptor; MHC) was published. It became also clear, that innate responses were often interacting with the acquired immune responses. Fish turned out to be vertebrates like all others with a sophisticated immune system showing specificity and memory. These basic data on the immune system could be applied in vaccination or in selection of disease resistant fish. Successful vaccines against bacterial diseases became available in the 1970s and 1980s. Effective anti-viral vaccines appeared from the 1980s onwards. There is no doubt, that Fish Immunology has become a flourishing science by the end of the 20th century and has contributed to our understanding of fish diseases as well as the success of aquaculture.

Approaches towards DNA vaccination against a skin ciliate parasite in fish

Rainbow trout (Oncorhynchus mykiss) were immunized with plasmid DNA vaccine constructs encoding selected antigens from the parasite Ichthyophthirius multifiliis. Two immobilization antigens (I-ags) and one cysteine protease were tested as genetic vaccine antigen candidates. Antigenicity was evaluated by immunostaining of transfected fish cells using I-ag specific mono- and polyclonal antibodies. I. multifiliis specific antibody production, regulation of immune-relevant genes and/or protection in terms of parasite burden or mortality was measured to evaluate the induced immune response in vaccinated fish. Apart from intramuscular injection, needle free injection and gene gun delivery were tested as alternative administration techniques. For the I-ags the complement protein fragment C3d and the termini of the viral haemorrhagic septicaemia virus glyco(G)protein (VHSV G) were tested as opsonisation and cellular localisation mediators, respectively, while the full length viral G protein was tested as molecular adjuvant. Expression of I-ags in transfected fish cells was demonstrated for several constructs and by immunohistochemistry it was possible to detect expression of a secreted form of the Iag52B in the muscle cells of injected fish. Up-regulations of mRNA coding for IgM, MHC I, MHC II and TCR β, respectively, were observed in muscle tissue at the injection site in selected trials. In the spleen up-regulations were found for IFN-γ and IL-10. The highest up-regulations were seen following co-administration of I-ag and cysteine protease plasmid constructs. This correlated with a slight elevation of an I. multifiliis specific antibody response. However, in spite of detectable antigen expression and immune reactions, none of the tested vaccination strategies provided significant protection. This might suggest an insufficiency of DNA vaccination alone to trigger protective mechanisms against I. multifiliis or that other or additional parasite antigens are required for such a vaccine to be successful.

Experimental evidence for direct in situ binding of IgM and IgT to early trophonts of Ichthyophthirius multifiliis (Fouquet) in the gills of rainbow trout, Oncorhynchus mykiss (Walbaum)

Freshwater fish are able to mount a protective immune response against the parasite Ichthyophthirius multifiliis (Ich) following a non-lethal exposure. Factors involved in immunity comprise cellular and humoral factors, but antibodies have been suggested to play a prominent role in protection. However, host antibodies have not yet been demonstrated to bind to the parasite in situ. By the use of immunohistochemical techniques, this study demonstrated that IgT and IgM bind to surface structures, including cilia, on the early feeding stage of the parasite in the gills of immune rainbow trout, Oncorhynchus mykiss, shortly (2 h) after invasion. No binding of IgT and no or only a weak binding of IgM was observed on the parasites in the gills of similarly exposed but naïve rainbow trout. This study indicates that antibodies play an important part in the protection of immune fish against Ich although additional humoral and cellular factors may contribute to this reaction.

Cellular and humoral factors involved in the response of rainbow trout gills to Ichthyophthirius multifiliis infections: molecular and immunohistochemical studies

The parasitic ciliate Ichthyophthirius multifiliis infecting skin, fins and gills of fish induces a protective immune response in rainbow trout (Oncorhynchus mykiss) surviving the infection and a similar protection can be conferred by i.p. injection of live theronts. A combined molecular and immunohistochemical approach has been used in this work for pinpointing cellular and humoral immune factors in gill tissue involved in the response and indicating interactions between the systemic and local responses. Fish were immunized by intra-peritoneal injection of live I. multifiliis theronts, control fish were injected with PBS and subgroups were treated with the immuno-suppressant hydrocortisone before fish were challenged with live theronts. Significant up-regulations of genes encoding IgM, IgT, C3, SAA, IL-8, IL-22 and IFN-γ were induced by immunization and challenge. Hydrocortisone treatment had a significant down-regulating effect on genes incoding IgT, IgM, CD4, CD8, IFN-γ, IL-8 and IL-22 in all groups. Immunohistochemistry, using monoclonal antibodies to detect cellular markers, demonstrated active involvement of CD8, MHC II, IgT and IgM positive cells in gill tissue. Putative T-cells (CD8 positive cells) were detected in the intraepithelial lymphoid tissue located at the base of gill filaments and in hyperplastic gill tissue but following infection a clear efflux of these cells was detected. MHC II positive cells were distributed across the gill filaments and accumulated in hyperplastic tissue but hydrocortisone treatment affected their density negatively in both immunized and non-immunized fish. IgT positive cells were present in the epithelial lining of the gill lamellae (suggesting a primary role of this protein in the mucosal defence against the ciliate) whereas IgM positive cells were found only in gill arterioles and the lamellar capillaries. The present work indicates an intensive activity and specialized function of immune cells (B-cells, T-cells and macrophages) and humoral elements such as immunoglobulins IgT and IgM which are orchestrated by cytokines in gill tissue reacting against I. multifiliis.

Effects of ethanol extract of Artemisia annua L. against monogenean parasites of Heterobranchus longifilis

Ethanol extract of Artemisia annua was effective in the dislodgement and mortality of monogenean parasites of juvenile Heterobranchus longifilis at concentrations ranging from 50 to 200 mg/l. Five hundred 1-week-old juvenile fish were stocked in hapa in earthen pond for 7 days to accumulate parasites. The approximate number of parasites per fish was confirmed by counting the number of parasites attached to body surfaces and the gills with a stereo-microscope before being exposed to the extract under in vivo conditions. The bioactivity of the extract was conducted in plastic Petri dishes with three replications and controls. The results obtained from A. annua extract were matched against those produced by pure artemisinin and artesunate powder, respectively, under similar experimental conditions. There was a faster effect of pure artemisinin crystals on the parasites as compared to A. annua extract and artesunate. Coagulation of parasite cells was observed with artemisinin treatment, whereas parasites were merely dislodged from their attachment organs and killed some hours later in the same concentration of A. annua. There were positive correlations between the number of parasites dislodged/killed and the concentration of A. annua extract, artemisinin, and artesunate powder, respectively, as well as the duration of exposure of affected fish to the substances. This led to the conclusion that A. annua contains substances that are effective against helminthes parasites of H. longifilis.

MTT2, a copper-inducible metallothionein gene from Tetrahymena thermophila

Metallothioneins (MTs) are ubiquitous, cysteine-rich, metal-binding proteins whose transcriptional activation is induced by a variety of stimuli, in particular heavy metals such as cadmium, copper and zinc. Here we describe the sequence and organization of a novel copper-inducible metallothionein gene (MTT2) from Tetrahymena thermophila. Based on its deduced sequence, the gene encodes a protein 108 amino acids, containing 29 cysteine residues (30%) arranged in motifs characteristic of vertebrate and invertebrate MTs. We demonstrate that the 5'-region of the MTT2 gene can act as an efficient promoter to drive the expression of heterologous genes in the Tetrahymena system. In the latter case, a gene for a candidate vaccine antigen against Ichthyophthirius multifiliis, a ubiquitous parasite of freshwater fish, was expressed at high levels in transformed T. thermophila cell lines. Moreover, the protein was properly folded and targeted to the plasma membrane in its correct three-dimensional conformation. This new copper-inducible MT promoter may be an attractive alternative to the cadmium-inducible MTT1 promoter for driving ectopic gene expression in Tetrahymena and could have a great impact on biotechnological perspectives.

Persistence of host response against glochidia larvae in Micropterus salmoides

Host fish acquire resistance to the parasitic larvae (glochidia) of freshwater mussels (Unionidae). Glochidia metamorphose into juvenile mussels while encysted on host fish. We investigated the duration of acquired resistance of largemouth bass, Micropterus salmoides (Lacepède, 1802) to glochidia of the broken rays mussel, Lampsilis reeveiana (Call, 1887). Fish received three successive priming infections with glochidia to induce an immune response. Primed fish were held at 22-23 degrees C and were challenged (re-infected) at intervals after priming. Metamorphosis success was quantified as the percent of attached glochidia that metamorphosed to the juvenile stage and were recovered alive. Metamorphosis success at 3, 7, and 12 months after priming was significantly lower on primed fish (26%, 40%, and 68% respectively) than on control fish (85%, 93%, and 92% respectively). A second group of largemouth bass was similarly primed and blood was extracted. Immunoblotting was used to detect host serum antibodies to L. reeveiana glochidia proteins. Serum antibodies were evident in primed fish, but not in naive control fish. Acquired resistance of host fish potentially affects natural reproduction and artificial propagation of unionids, many of which are of conservation concern.

Interaction between the blood fluke,Sanguinicola inermisand humoral components of the immune response of carp,Cyprinus carpio

The effect ofSanguinicola inermison serum antibody and complement activity inCyprinus carpiowas assessed using an ELISA and haemolytic assays. Possible immune evasion strategies were assessed using immunodetection of host proteins on the surface of the parasite. Carp acclimatized to 20 or 25 °C were infected by exposure to 500 cercariae or injected intraperitoneally with 150 cercariae, and serum monitored over a 63-day period. In cercariae-injected carp, irrespective of time and temperature, a significant increase occurred in complement activity being greatest at 25 °C. In addition, fish exposed to the cercariae ofS. inermisand maintained at 20 °C the level of complement activity was significantly higher after 5 weeks compared to controls. At 20 °C intraperitoneal injections of parasites increased serum antibody levels which peaked after 7 days. In contrast, at 25 °C, antibody levels were maintained over 63 days. Exposure of fish to infection did not appear to stimulate antibody production. Immunofluorescence studies revealed ‘host-like’ molecules on the surface of the cercarial body exposed to carp serum and adult flukes obtained directly from the fish or cultured for 24 h in L15 medium. The possible role of ‘host-like’ molecules in immune evasion is discussed and the response at different temperatures is related to infection dynamics.

Ichthyophthirius multifiliis Fouquet and Ichthyophthiriosis in Freshwater Teleosts

The ciliate Ichthyophthirius multifiliis is an important pathogen of freshwater teleosts occurring in both temperate and tropical regions throughout the world. The disease, ichthyophthiriosis, accounts for significant economic losses to the aquaculture industry, including the ornamental fish trade, and epizootics in wild fish populations can result in mass kills. This review attempts to provide a comprehensive overview of the biology of the parasite, covering the free-living and parasitic stages in the life cycle, host-parasite interactions, and the immune response of host and immune evasion strategies by the parasite. Emphasis on the immunological aspects of infection within the fish host, including molecular studies of i-antigens, reflects the current interest in this subject area and the quest to develop a recombinant vaccine against the disease. The current status of methods for the control of ichthyophthiriosis is discussed, together with new approaches in combating this important disease.

Humoral response and susceptibility of five full-sib families of Atlantic salmon, Salmo salar L., to the haemoflagellate, Cryptobia salmositica

Susceptibility and antibody production against pathogenic and vaccine strains of the haemoflagellate, Cryptobia salmositica were investigated in five full-sib families (A-E) of Atlantic salmon, Salmo salar. Humoral response and susceptibility of families were compared within three treatments: infection, vaccination and vaccination followed by challenge. Parasitaemias caused by the vaccine strain of C. salmositica were considerably lower than those caused by the pathogenic strain. All vaccinated families were protected when challenged with the pathogenic strain. Family B had significantly lower parasitaemias (with both strains) than the other families. When naïve fish were infected with the pathogenic strain, this family had a significantly lower and earlier peak parasitaemia (4.3 +/-1.3 x 10(6) parasites mL(-1) blood at 3 weeks post-infection; w.p.i.) than the other families. Family C had the highest peak (11.1 +/- 1.2 x 10(6) parasites mL(-1) blood), which occurred at 4 w.p.i. Antibodies against C. salmositica were detected earlier in Family B (3 w.p.i.) than in Family C (5 w.p.i.). This demonstrates an association of increased susceptibility with a delayed antibody response. Western immunoblot identified antibodies against 112, 181 and 200 kDa antigens earlier in more resistant fish (Family B). Antigenic stimulation leading to a stronger antibody response was shown with the vaccine strain and in the later stages of infection.

Cadmium effects on Ichthyophthirius: evidence for metal-sequestration in fish tissues following administration of recombinant vaccines

We are developing Tetrahymena thermophila as a delivery system for recombinant vaccines against parasitic protozoa, including the common fish parasite, Ichthyophthirius multifiliis. T. thermophila cell lines expressing I. multifiliis genes under the control of a cadmium-inducible metallothionein gene promoter conferred strong protection against a lethal parasite challenge when administered parenterally to naive fish. Nevertheless, given that heavy metals can be toxic to parasites, a question arose as to whether protection resulted from Cd residues carried over with the vaccine, rather than acquired immunity per se. To address this issue, we examined the sensitivity of I. multifiliis to Cd in vitro and determined Cd concentrations in different host tissues following i.p. injection of juvenile channel catfish with the recombinant vaccine. We found that CdCl2 at concentrations > or = 50 ppb were lethal to I. multifiliis theronts in vitro. Furthermore, Cd concentrations were clearly elevated in fish tissues and reached levels equivalent to 74 ng/g wet weight (74 ppb) in the skin within 14 days of injection with recombinant T. thermophila. Nevertheless, fish injected with non-transformed Tetrahymena grown in the presence or absence of CdCl2 showed no significant difference in either relative survival or parasite load following direct challenge with I. multifiliis.

Systemic and cutaneous mucus antibody responses of channel catfish immunized against the protozoan parasite Ichthyophthirius multifiliis

Fish acquire protective immunity against the ciliated protozoan parasite Ichthyophthirius multifiliis following sublethal infection or inoculation with I. multifiliis immobilization antigens (i-antigens). In both cases, parasite-immobilizing antibodies have been identified in sera and mucosal secretions. To investigate the kinetics of this immune response, antibody levels were determined by enzyme-linked immunosorbent assay (ELISA) in the sera and cutaneous mucus of channel catfish (Ictalurus punctatus) that were either infected with parasites or given a single injection of purified i-antigen (5.0 microg/fish) in Freund's incomplete adjuvant. At 5 weeks, infected and inoculated fish had a mean serum (1:80 dilution) antibody absorbance (A405) value of 0.54 +/- 0.17 and 0.35 +/- 0.03, respectively, which were significantly higher (alpha = 0.05) than the pretreatment serum (1:80 dilution) antibody absorbance value of 0.24 +/- 0.05. At 14 weeks, mean serum (1:80 dilution) ELISA absorbance values in the teo groups of fish increased to 0.79 +/- 0.30 and 0.71 +/- 0.24, respectively. In both groups of fish, antibody levels in cutaneous mucus (undiluted) were much lower than those in sera. Infected fish had detectable mucus (undiluted) antibody levels from 3 to 9 weeks, with the highest mean value (0.30 +/- 0.07) occurring at 7 weeks. Although individual inoculated fish produced serum antibody absorbance values comparable to those seen in infected fish, the mean mucus antibody values in this group did not rise above pretreatment levels. I. multifiliis infection induced a transient mucosal antibody response that coincided with the resolution of infection. Whether elicited by infection or intraperitoneal injection of i-antigen, the serum and mucus antibody responses of channel catfish immunized against I. multifiliis did not occur synchronously.

Immunisation of channel catfish, Ictalurus punctatus, with Ichthyophthirius multifiliis immobilisation antigens elicits serotype-specific protection

Surface immobilisation antigens (i-antigens) were purified from two strains of Ichthyophthirius multifiliis (NY1 and G5) that represent different i-antigen serotypes, namely A and D, respectively. The efficacy of the purified antigens as subunit vaccines was then tested in challenge studies using parasites of the homologous or heterologous serotype. Three groups of juvenile channel catfish (70 animals per group) were immunised with i-antigens from either the G5 or NY1 isolates, or with bovine serum albumin (BSA) as a control. Proteins were injected intraperitoneally (i.p.) at a dose of 10 microg/fish with complete Freund's adjuvant on day 1, followed by a second injection in incomplete Freund's adjuvant on day 15. Fish immunised with the purified i-antigens developed high titres of serum immobilising antibodies whereas sera from BSA-injected control fish did not. Fish antisera immobilised parasites of the homologous, but not the heterologous strain, and recognised the corresponding i-antigens on Western blots run under non-reducing conditions. On day 36, each group was divided into two subgroups (n=30). One subgroup was challenged with G5 parasites, and the other was challenged with NY1 parasites. When challenged with G5 parasites, 70% of fish immunised with the G5 i-antigens survived. When challenged with NY1 parasites, 33.3% of fish immunised with the NY1 i-antigens survived. All BSA-injected control fish died, as did all fish injected with the purified antigens and challenged with the non-homologous parasite strain. Statistical analyses indicated significant differences among test and control groups with regard to the mean days to death (MDD). While the results of these studies clearly support a role for i-antigens in protection, active immunity in response to natural infection is not serotype-specific. The utility of i-antigens, as well as the existence of other potential vaccine candidates for the prevention of 'white-spot' disease, are discussed.

The use of synthetic genes for the expression of ciliate proteins in heterologous systems

The common fish parasite, Ichthyophthirius multifiliis, expresses abundant glycosylated phosphatidylinositol (GPI)-anchored membrane proteins known as immobilization antigens, or i-antigens. These proteins are targets of the host immune response, and have been identified as potential candidates for recombinant subunit vaccine development. Nevertheless, because Ichthyophthirius utilizes a non-standard genetic code, expression of the corresponding gene products, either as subunit antigens in conventional protein expression systems, or as vector-encoded antigens in the case of DNA vaccines, is far from straightforward. To overcome this problem, we utilized 'assembly polymerase chain reaction' to manufacture synthetic versions of two genes (designated IAG52A[G5/CC] and IAG52B[G5/CC]) encoding approximately 52/55 kDa i-antigens from parasite strain G5. This approach made it possible to eliminate unwanted stop codons and substitute the preferred codon usage of channel catfish for the native sequences of the genes. To determine whether the synthetic alleles could be expressed in cells that use the standard genetic code, we introduced IAG52A[G5/CC] into a variety of heterologous cell types and tested for expression either by immunofluorescence light microscopy or Western blotting. When cloned downstream of appropriate promoters, IAG52A[G5/CC] was expressed in Escherichia coli, mammalian COS-7 cells, and channel catfish where it elicited antigen-specific immune responses. Interestingly, the localization pattern of the corresponding gene product in COS-7 cells indicated that while the protein was correctly folded, it was not present on the cell membrane, suggesting that the signal peptides required for GPI-anchor addition differ in ciliate and mammalian systems. Construction of synthetic alleles should have practical utility in the development of vaccines against Ichthyophthirius, and at the same time, provide a general method for the expression of ciliate genes in heterologous systems.

Host responses against the fish parasitizing ciliate Ichthyophthirius multifiliis

Recent studies have shown that fish are able to mount protective immune responses against various parasites. One of the best characterized parasite-host system in this context is the ciliate Ichthyophthirius multifiliis (Ich) parasitizing a range of freshwater fishes. Both specific and non-specific host defence mechanisms are responsible for the protection of fish against challenge infections with this ciliate. The specific humoral components comprise at least specific antibodies. The non-specific humoral elements included are the alternative complement pathway and probably lectins. Cellular factors involved in the specific response are B-cells and putative T-cells. The non-specific effector cells recognized are various leukocytes. In addition, goblet-cells and mast cells (EGC-cells) may have a function. The NCC-cell (suggested analogue to NK-cells in mammals) seems to play a role in the non-specific response. This well documented protective response in freshwater fishes against Ich has urged the development of anti-parasitic vaccines. Indeed, such products based on formalin killed parasites have been developed and found to offer the vaccinated host a satisfactory protection. However, the collection of parasites for vaccine production is extremely laborious. It involves keeping infected fish due to the fact that in vitro propagation of the parasite is still insufficiently developed. Gaining knowledge of amino acid sequences and its encoding DNA-sequences for the protective antigens (i-antigens) in the parasite was a major breakthrough. That achievement made it possible to produce a recombinant protein in E. coli and preliminary results indicated a certain protection of fish vaccinated with this product. Recent work has shown that the free-living and easily cultivated ciliate Tetrahymena can be transformed and express the i-antigen. This path seems to be promising for future development of vaccines against Ich. A novel approach in fish is the development of DNA-vaccines. Successful DNA-vaccination trials have been conducted in fish against viral infections and the technology also makes it possible to develop a DNA-vaccine against Ich. Other approaches to immuno-protection against Ich have been the use of heterologous vaccines. Thus, both bath and injection vaccination using live or killed (un-transformed) Tetrahymena has been reported to offer treated fish a certain level of protection. Such protection could be explained by non-specific reactions and the efficacy and duration of this vaccination type should be further evaluated.

The I-antigens of Ichthyophthirius multifiliis are GPI-anchored proteins

The parasitic ciliate Ichthyophthirius multifiliis has abundant surface membrane proteins (i-antigens) that when clustered, trigger rapid, premature exit from the host. Similar antigens are present in free-living ciliates and are GPI-anchored in both Paramecium and Tetrahymena. Although transmembrane signalling through GPI-anchored proteins has been well-documented in metazoan cells, comparable phenomena have yet to be described in protists. Since premature exit of Ichthyophthirius is likely to involve a transmembrane signalling event, we sought to determine whether i-antigens are GPI-anchored in these cells as well. Based on their solubility properties in Triton X-114, the i-antigens of Ichthyophthirius are amphiphilic in nature and partition with the detergent phase. Nevertheless, following treatment of detergent lysates with phospholipase C, the same proteins become hydrophilic. Concomitantly, they are recognized by antibodies against a cross-reacting determinant exposed on virtually all GPI-anchored proteins following cleavage with phospholipase C. Finally, when expressed in recombinant form in Tetrahymena thermophila, full-length i-antigens are restricted to the membrane, while those lacking hydrophobic C-termini are secreted from the cell. Taken together, these observations argue strongly that the i-antigens of Ichthyophthirius multifiliis are, in fact, GPI-anchored proteins.

Chemoattraction of Ichthyophthirius multifiliis (Ciliophora) theronts to host molecules

Mechanisms in the host-finding process of Ichthyophthirius multifiliis were studied in vitro by a novel bioassay using 24-well multidishes supplied with bottom layers of agar with chemoattractants. It was shown that low molecular weight molecules (carbohydrates, amino acids, fatty acids, urea) did not attract theronts. In contrast, sera and mucus from a range of teleosts (including marine fish) were effective attractants. Fractionation by gel filtration of fish serum allowed determination of the molecular size of the attracting proteins. Further biochemical studies suggested the chemoattractants to be present in fractions with host immunoglobulin and some still undetermined proteins. No clear association between enzyme activity and chemotactic potential was seen. The high chemoattractive effect of serum from various unrelated teleosts corresponds to the low host specificity of I. multifiliis and suggests that serum factors in mucus could be involved in host finding of the parasite. Society for Parasitology Inc.

Partial cross protection against Ichthyophthirius multifiliis in Gyrodactylus derjavini immunized rainbow trout

Partial cross protection against a skin-parasitic ciliate has been recorded in rainbow trout previously immunized with an ectoparasitic platyhelminth. The susceptibility to infection by Ichthyophthirius multifiliis differed significantly between naive and Gyrodactylus derjavini immunized rainbow trout. Fish partly immune to the ectoparasitic monogenean G. derjavini became less infected and experienced lower mortality than naive fish when exposed to I. multifiliis infections. In vitro studies on immobilization of theronts using decomplemented (heat-inactivated) serum from G. derjavini immune or non-immune hosts showed no immobilization. However, untreated serum from both immune and non-immune fish containing intact complement immobilized theronts (titre 128-256). In addition, non-specific priming of the host response with interleukin (IL-1), bacterial lipopolysaccharide (LPS), concanavalin A (Con A) or mannan did confer a partial resistance to I. multifiliis infection. This will suggest that non-specific factors including complement could be partly responsible for the host response against infections with this ciliate.

The gene for an abundant parasite coat protein predicts tandemly repetitive metal binding domains

Immobilization antigens are highly abundant surface membrane proteins that coat the surface of hymenostomatid ciliates. While their function is unknown, recent studies with the common fish parasite, Ichthyophthirius multifiliis, suggest their involvement in a novel mechanism of humoral immunity involving an effect of antibody on parasite behavior. To gain further insight into the nature of these proteins, we have cloned a gene encoding the 48kDa i-antigen of I. multifiliis. Analysis of the gene (designated IAG48[G1]) reveals a single, uninterrupted reading frame that predicts a protein of 442 amino acids. Based on its deduced amino acid sequence, the protein contains hydrophobic amino acid domains at its N- and C-terminus that are characteristic of signal peptide and GPI-anchor addition sites, respectively. The most striking feature of the predicted protein, however, is a series of tandem repeats that spans most of its length. The repeats themselves are characterized by periodic cysteine residues that fall into register when the homologous segments are aligned. Interestingly, the spacing of cysteines (C-X2,3-C) within a framework of larger (C-X2-C-X20-C-X3-C-X20-C-X2-C) motifs is entirely consistent with the structure of known zinc-binding proteins. Finally, comparison of the coding sequence of the 48kDa i-antigen gene with a partial cDNA previously thought to encode this protein reveals nearly complete identity except at their 3' ends, suggesting that alternative forms of the antigen exist.

Immunolabeling analysis of biosynthetic and degradative pathways of cell surface components (glycocalyx) in Paramecium cells

Biosynthetic and degradative pathways of glycocalyx components are largely unknown in Paramecium and in some related parasitic protozoa. We isolated cell surface (glyco-)proteins, i.e., surface antigens (SAg) and used them in the native (nSAg) or denatured (dSAg) state to produce antibodies (AB) for immunolocalization by confocal imaging and by quantitative immunogold EM-labeling of ultrathin sections or of freeze-fracture replicas. Antibodies against nSAg or dSAg, respectively, yield different labeling densities over individual structures, thus indicating biosynthetic or degradative pathways, respectively. We derive the following biosynthetic way: ER --> Golgi apparatus --> non-regulated/non-dense core vesicle transport --> diffusional spread over non-ciliary (somatic) and ciliary cell membrane. For degradation we show the following pathways: Concentration of nSAg in the cytostome --> nascent digestive vacuole --> mature vacuoles --> release of dSAg at cytoproct, with partial retrieval by "discoidal vesicles". A second internalization pathway proceeds via coated pits ("parasomal sacs") --> early endosomes ("terminal cisternae") --> digestive vacuoles. Dense packing of SAg in the glycocalyx may drive them into the endo-/phagocytic pathway. Still more intriguing is the site of nSAg integration into the cell membrane by unstimulated exocytosis. We consider unconspicuous clear vesicles relevant for nSAg export, probably via sites which most of the time are occupied by coated pits. This could compensate for membrane retrieval by coated pits, while scarcity of smooth profiles at these sites may be explained by the much longer time period required for coated pit formation as compared to exocytosis.

Ichthyophthirius multifiliis: a model of cutaneous infection and immunity in fishes

The parasitic ciliate Ichthyophthirius multifiliis offers a useful system for the study of cutaneous immunity against an infectious microorganism. Naive fish usually die following infection, but animals surviving sublethal parasite exposure become resistant to subsequent challenge. This resistance correlates with the presence of humoral antibodies in the sera and cutaneous mucus of immune fish. A mechanism of immunity has recently been elucidated that involves antibody binding to surface proteins (referred to as immobilization antigens or i-antigens) located on the parasite cell and ciliary membranes. Antibody-mediated cross-linking of i-antigens triggers a response by the parasite resulting in its exit from the host. These effects can be observed directly on the surface of live fish. In addition to allowing the observation of effector responses in vivo, Ichthyophthirius also provides a means to study the ontogeny of the mucosal immune response. The sites of antigen capture and presentation, and the sites of antibody production, are unknown with regard to cutaneous immunity. Because the external epithelial surfaces of fish are often the points of pathogen entry, a basic understanding of the inductive immune mechanisms and immune cell interactions in the skin and gills is extremely important with regard to vaccine development. The development of Ichthyophthirius as an experimental system and how it might be used to address these issues are discussed in this review.