Cross-immunity and antibody responses to different immobilisation serotypes of Ichthyophthirius multifiliis

Infectivity and genes differentially expressed between young and aging theront cells of the marine fish parasite Cryptocaryon irritans

The ciliated protozoan Cryptocaryon irritans infects a wide range of marine fish and causes the highly lethal white spot disease. This parasite possesses three morphologically and physiologically distinct life stages: an infectious theront, a parasitic trophont, and an asexually reproductive tomont. In the past few years, several attempts have been made to help elucidate how C. irritans transforms from one stage to another using transcriptomic or proteomic approaches. However, there has been no research studying changes in transcription profiles between different time points of a single C. irritans life stage—the development of this parasite. Here we use RNA-seq and compare gene expression profiles of theront cells collected by 1 and 10 hrs after they emerged from tomonts. It has been shown that infectivity of theront cells declines 6–8 hours post-emergence, and we used this characteristic as a physiological marker to confirm the aging of theront cells. We identified a total of 41 upregulated and 90 downregulated genes that were differentially expressed between young and aging theront cells. Using Blast2Go to further analyze functions of these genes, we show that genes related to energy production are downregulated, but quite surprisingly many genes involved in transcription/translation processes are upregulated. We also show that expression of all nine detectable agglutination/immobilization antigen genes, with great sequence divergence, is invariably downregulated. Functions of other differentially expressed genes and indications are also discussed in our study.

Live recombinant Lactococcus lactis vaccine expressing immobilization antigen (i-Ag) for protection against Ichthyophthirius multifiliis in goldfish

The parasite Ichthyophthirius multifiliis (Ich) has been reported in various freshwater fishes worldwide and results in severe losses to both food and aquarium fish production. Lactobacillus strains have a number of properties that make them attractive candidates as delivery vehicles for the presentation to the mucosa of compounds with pharmaceutical interest, in particular vaccines. Here, the present study was conducted to evaluate a live recombinant Lactococcus lactis vaccine expressing immobilization antigen (IAG-52X) in protection against I. multifiliis. A 1266 bp gene fragment containing a potential antigenic epitope of the 48 kDa immobilization antigen of I. multifiliis was assembled from six synthetic ohgonucleotides and cloned into pSIP409 and electrotransformed into Lactobacillus plantarum NC8. The recombinant vaccine candidate was then orally fed into goldfish. The expression of immune-related genes: complement component 3 (C3), MHC I, IgM gene in blood from goldfish at different time points after immunization were evaluated. Immunized fish were than challenged with a lethal dose of infectious I. multifiliis. The cumulative mortality and relative percentage survival (RPS) were also determined. Our results showed that the antibody level in the blood and skin of the immunized fish was statistically significant (P < 0.05) in relation to the control groups. Goldfish orally immunized with NC8-pSIP409- IAG-52X had high serum antibody titers that ranged from 32 to 256 after 28d post immunization, while fish fed with NC8-pSIP409 or PBS had no detectable immobilizing antibody response. Expression of IgM, C3, MHC I genes in the group immunized with IAG-52X were significantly (P < 0.05) up regulated as compared with control group, indicating that different immune cells were actively involved in cellular immune response. The results showed that the average survival rate of fish orally immunized with 10⁸ and 10⁶NC8-pSIP409-IAG-52X was 60% and 50% respectively. Therefore, NC8-pSIP409-IAG-52X could become a promising oral vaccine candidate against I. multifiliis.

Aphanius sophiae (Actinoptrygii, Cyprinodontidae), a new host for Ichthyophthirius multifiliis (Ciliophora) reported from Iran

Aphanius sophiae (Heckel, 1849) or soffia tooth-carp is endemic species to the endorheic Kor River basin, Fars Province, Southwest of Iran. In spite of importance of this species as a biologic control of anopheles larvae, little attention has been paid to its diseases particularly parasitic infections. A total of 24 A. Sophiae (12 males and 12 females) in Safashahr spring, Kor River basin, were collected by using dip net in February 2012. Surface of all fish were gross examined and screened for any swelling and blisters. All of the examined specimens (100 %) were infected with Ichthyophthirius multifiliis. Infected fish typically developed small blister-like raised lesions on the body surface. The current study presents infection of A. sophiae with I. multifiliis for the first time from Iran. Hence, owing to considerable unexplored area of Iran further studies are required to increase our knowledge about parasitic infections of this fish.

Characterization of an IgM-like immunoglobulin from silver catfish (Rhamdia quelen) serum and its use for the production of polyclonal antibodies and development of immunoassays

Abstract: Knowledge on fish immunoglobulin (Ig) characteristics and the availability of monoclonal or polyclonal antibodies to fish Igs are essential to evaluate the humoral immune response and the Ig distribution on leukocyte cells. We demonstrated that silver catfish serum Ig is composed of one immunodominant H chain with approximately 75k Da and one L chain with approximately 28 kDa, similar to human IgM. Rabbit polyclonal antibodies to the catfish IgM-like Ig recognized both the H and L chain and were useful in developing an indirect ELISA to measure the production of antibodies in fish immunized with bovine serum albumin. Dot blot and western blot cross-reactivity studies indicated a wide degree of epitope sharing amongst Ig from several Siluriformes and Characiformes fish indigenous to Brazilian rivers. In these fish species, polyclonal antibodies reacted mostly with the H chain. The results presented here are central to the development of tools and strategies to investigate the antibody production to inoculated antigens and tissue distribution of Ig molecules in native fish species. Furthermore, because of the wide range of cross-reactivity, polyclonal antibodies to silver catfish IgM-like Ig might be used to develop immunoassays to measure the humoral immune response in other fish species.

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.

Grass carp (Ctenopharyngodon idella) TRAF6 and TAK1: molecular cloning and expression analysis after Ichthyophthirius multifiliis infection

Ichthyophthirius multifiliis, a pathogenic ciliate parasite, infects almost all freshwater fish species and causes significant economic losses. Tumor necrosis factor receptor-associated factor 6 (TRAF6) and transforming growth factor-β-activated kinase 1 (TAK1) are two important signaling molecules involved in toll-like receptor (TLR) signal transduction. To date, the roles of TRAF6 and TAK1 in host defense against fish parasites are still poorly understood. In the present study, TRAF6 (CiTRAF6) and TAK1 (CiTAK1) were identified from grass carp (Ctenopharyngodon idella). The full-length cDNA sequence of CiTRAF6 (2250 bp) includes an open reading frame (ORF) of 1629 bp, which shows a high similarity to that of Cyprinus carpio TRAF6 and encodes a putative protein of 542 amino acids containing one RING domain, two zinc fingers, one coiled-coil region, and one MATH domain. The full-length CiTAK1 cDNA sequence is 2768 bp and includes an ORF of 1626 bp that encodes a putative protein of 541 amino acids containing a conserved serine/threonine protein kinase catalytic domain and a coiled-coil region. Phylogenetic analysis showed that CiTRAF6 and CiTAK1 were clustered with TRAF6 and TAK1 of other teleosts, respectively. CiTRAF6 and CiTAK1 were both constitutively expressed in all examined tissues but with varied expression levels. The highest expressions of CiTRAF6 and CiTAK1 were in the head kidney and spleen, respectively. The expression profiles of CiTRAF6 and CiTAK1 were detected in grass carp after I. multifiliis infection. Expressions of both genes were significantly up-regulated in the skin, gill, head kidney, and spleen at most time points after infection, indicating that CiTRAF6 and CiTAK1 may play essential roles in grass carp defense against I. multifiliis.

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.

Use of vaccination against enteric septicemia of catfish and columnaris disease by the U.S. catfish industry

Vaccination is an effective strategy used for the protection of food animals against infectious diseases. A 2010 U.S. Department of Agriculture questionnaire examined U.S. catfish industry use (in 2009) of two commercial vaccines that provide protection against enteric septicemia of catfish (ESC) and columnaris disease, catfish producers' opinions regarding the percentage of vaccinated fish they expect to be protected, and producers' general expectations regarding survival of vaccinated fish compared with unvaccinated fish. During 2009, 9.7% of the total fingerling operations used one or both vaccines; 12.3% of the total industry fry production was vaccinated against ESC, and 17.0% was vaccinated against columnaris disease. Of the producers who grew food-sized catfish to harvest, 6.7% used vaccinated catfish. The farms that did not use vaccinated fish for grow out had a mean size of 63.4 water surface hectares (156.6 water surface acres). The operations that used vaccinated fish were larger (mean size = 206.6 water surface hectares, or 510.6 water surface acres). The producers that stocked ESC-vaccinated fish for grow out represented 19.0% of the total water surface area of food fish production; producers that stocked columnaris-vaccinated fish represented 16.6% of the total area. Of the producers that stocked ESC-vaccinated catfish, 41.9% thought that survival was better in vaccinated fish than in unvaccinated fish; of the producers that stocked columnaris-vaccinated catfish, 46.2% thought that vaccinated fish displayed better survival. However, 37.5% of producers that used the ESC vaccine and 39.7% of producers that used the columnaris vaccine did not know whether vaccination improved survival rates. When all producers were asked about their expectations regarding the percentage of vaccinated fish that would be protected from disease, 52.4% responded that they expected 100% of their fish to be protected. More producer information about reasonable expectations regarding vaccine efficacy, the conditions under which immunosuppression and vaccine failure can occur, and assessment of vaccine performance may result in increased use of vaccination as a tool for the catfish industry.

An assessment of the use of drug and non-drug interventions in the treatment ofIchthyophthirius multifiliisFouquet, 1876, a protozoan parasite of freshwater fish

Infection by the ciliate protozoanIchthyophthirius multifiliisFouquet, 1876 causes significant economic losses in freshwater aquaculture worldwide. Following the ban on the use of malachite green for treating food fish, there has been extensive research aimed at identifying suitable replacements. In this paper we critically assess drug and non-drug interventions, which have been tested for use or have been employed against this parasite and evaluate possibilities for their application in farm systems. Current treatments include the administration of formaldehyde, sodium chloride (salt), copper sulphate and potassium permanganate. However, purportedly more environmentally friendly drugs such as humic acid, potassium ferrate (VI), bronopol and the peracetic acid-based products have recently been tested and represent promising alternatives. Further investigation, is required to optimize the treatments and to establish precise protocols in order to minimize the quantity of drug employed whilst ensuring the most efficacious performance. At the same time, there needs to be a greater emphasis placed on the non-drug aspects of management strategies, including the use of non-chemical interventions focusing on the removal of free-swimming stages and tomocysts ofI. multifiliisfrom farm culture systems. Use of such strategies provides the hope of more environmentally friendly alternatives for the control ofI. multifiliisinfections.

Gene expression profiling of a fish parasite Ichthyophthirius multifiliis: Insights into development and senescence-associated avirulence

The ciliate parasite Ichthyophthirius multifiliis (Ich) infects many freshwater fish, causing white spot disease that leads to heavy economic losses to aquaculture and ornamental industries. Despite its economic importance, molecular studies examining fundamental processes such as life stage regulation and infectivity have been scarce. In this study, we developed an oligo microarray platform using all available I. multifiliis expressed sequence tag (EST) information as well as probes designed through comparative genomics to other protozoa. Gene expression profiling for developmental and virulence factors was conducted using this platform. For the developmental study, the microarray was used to examine gene expression profiles between the three major life stages of Ich: infective theront, parasitic trophont, and reproductive tomont. A total of 135 putative I. multifiliis genes were found to be differentially expressed among all three life-stages. Examples of differentially expressed transcripts among life stages include immobilization antigens and epiplasmin, as well as various other transcripts involved in developmental regulation and host-parasite interactions. I. multifiliis has been shown to lose infectivity at later cell divisions potentially due to cellular senescence. Therefore, the microarray was also used to explore expression of senescence-associated genes as related to the passage number of the parasite. In this regard, comparison between tomont early and late passages yielded 493 differently expressed genes; 1478 differentially expressed genes were identified between trophont early and late passages. The EST-derived oligo microarray represents a first generation array of this ciliate and provided reproducible expression data as validated by quantitative RT-PCR.

Elicited cross-protection and specific antibodies in Mozambique tilapia (Oreochromis mossambicus) against two different immobilization serotypes of Cryptocaryon irritans isolated in Hawaii

The objective of this study was to determine whether immunization of Mozambique tilapia with different Cryptocaryon irritans i-antigen serotypes elicited cross-protection against challenge infection by both serotypes. Fish were directly exposed to live theronts of isolate W1 or isolate K1, that express different surface i-antigens. There was no significant difference in the number of trophonts infecting the fish between the two isolates, W1 and K1, following primary exposure. Serum from immunized fish exposed to live theronts showed higher immobilization titres and ELISA values against homologous isolates than to heterologous isolates after the primary exposure. However, mucus antibody did not immobilize theronts although the ELISA results clearly indicated that mucus antibodies recognizing C. irritans were generated. In a study with Western blot analyses, serum antibodies recognized only an antigen of the corresponding serotype and no proteins common to both serotypes were identified. Sequence analyses of 754 bases of rDNA nucleotide sequence including complete nuclear ribosomal ITS-1-5.8S rDNA-ITS-2 region were conducted and found to be identical for W1- and K1-isolates. These findings confirmed that both isolates were members of the species, C. irritans, and that rDNA analysis would not distinguish the two isolates. In conclusion, despite the fact that the immobilization assays and ELISA detected two serotypes in vitro, challenge assays provided evidence for only one type of C. irritans.

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.

Effect of immunization of channel catfish with inactivated trophonts on serum and cutaneous antibody titers and survival against Ichthyophthirius multifiliis

Two trials were conducted to determine the effect of immunization of channel catfish with inactivated trophonts on serum and cutaneous antibody titers and survival against Ichthyophthirius multifiliis Fouquet (Ich). In trial I, catfish were immunized intraperitoneally (IP) with: 1) 1% formalin-inactivated trophonts, 2) 3% formalin-inactivated trophonts and 3) freeze-thawed trophonts. Positive and negative control catfish were immunized with live theronts and 5% bovine serum albumin (BSA), respectively. At day 14, 28 and 50 post-immunizations, no statistical difference was noted in serum or cutaneous anti-Ich antibody titers to formalin-inactivated trophonts or freeze-thawed trophonts. The survival of catfish challenged with live theronts ranged from 33.3% to 43.3% for the formalin-inactivated or freeze-thawed trophonts at 50 d post-immunization. The survival of catfish immunized with live theront and BSA was 93.3 and 0%, respectively. In trial II, catfish were IP immunized with sonicated trophonts at doses of 1) 5 trophonts or 10.2 microg protein g(-1) fish, 2) 10 trophonts or 20.4 microg protein g(-1) fish, 3) 20 trophonts or 40.8 microg protein g(-1) fish, and 4) 5% BSA as the control. Fish immunized with 10 or 20 trophonts g(-1) fish showed highest serum (1/210 to 1/480) and cutaneous antibody titers (1/48 to 1/52), respectively, at 22 d post-immunization and survival (63.3-60.0%). The fish immunized with 5 trophonts g(-1) fish had titers of 1/52 and 1/12 for serum and cutaneous antibody and survival of 23.3%. BSA immunized catfish had background titers and a survival of 6.7%. There was a significant correlation between doses of sonicated trophonts used to immunize and catfish survival (correlation coefficient = 0.859, p < 0.01). These results indicate that doses of sonicated trophonts, but not formalin-inactivated or freeze-thawed trophonts provided both serum and cutaneous antibody responses and survival to live trophont challenge.

Acute toxicity of peracetic acid (PAA) formulations to Ichthyophthirius multifiliis theronts

Peracetic acid (PAA) is an antimicrobial disinfectant used in agriculture, food processing, and medical facilities. It has recently been suggested as a means to control infestations of Ichthyophthirius multifiliis. The purpose of this study was to determine the acute toxicity of two products containing 4.5% and 40% PAA to I. multifiliis theronts from two geographically separate isolates. Theronts were exposed to concentrations of PAA in 96-well plates containing groundwater at 23 degrees C. Acute toxicity was observed over a 4-h period. No significant difference in the median lethal concentration (LC(50)) estimates was evident between the two isolates at 4 h with the 4.5% PAA product (0.146 versus 0.108 mg/l PAA), while there was a statistical difference between the 4 h LC(50) with the 40% PAA product (0.274 versus 0.158 mg/l PAA). These results suggest that PAA is toxic to I. multifiliis theronts at low concentrations and that one of the isolates was more resistant to this compound.

Antigenic and cross-protection studies on two turbot scuticociliate isolates

The protection induced in turbot by inactivated vaccines containing either of two isolates (I(1) and C(1)) of the scuticociliate parasite Philasterides dicentrarchi, which causes important mortalities in turbot cultures, was evaluated in the present study. The results obtained after challenging the fish with the two isolates show that vaccination protected fish only against the homologous isolate, but did not confer cross-protection. The two isolates constitute two serotypes, as shown in the immobilization tests with mouse and turbot anti-I(1) and anti-C(1) antisera, in which only the homologous antisera immobilized the ciliates. ELISA assays, using total antigen free of proteases (TAWP), cytosolic antigens (CYA), ciliar antigens (CA) or membrane protein fraction (MPF), were also carried out. Differences in the levels of antibodies produced in mouse against the homologous and heterologous antigens were observed; these differences were significantly different when the antigen preparations used in the ELISA were TAWP, CYA or CA. Nevertheless, ELISA assays using turbot sera against TAWP did not show significant differences in the levels of antibodies against the homologous and heterologous antigens. Antigenic cross-reactivity was also detected in the Western blot assays, as well as significant differences in the patterns of antigenic recognition in the two isolates - in both reduced and non-reduced TAWP antigens, but which was noteworthy when mouse antisera were used. The results obtained in the present study demonstrate for the first time the existence of serotypes of the ciliate parasite of turbot Philasterides dicentrarchi that display clear antigenic differences, which must be taken into consideration in the future development of a vaccine against scuticociliatosis.

Molecular cloning of a putative agglutination/immobilization antigen located on the surface of a novel agglutination/immobilization serotype ofCryptocaryon irritans

A surface agglutination/immobilization antigen was purified from the novel agglutination/immobilization serotype (serotype G37) of the ciliated protozoanCryptocaryon irritans, a parasite of seawater fishes. Serum from fish immunized withC. irritanstheronts had agglutination/immobilization activity against therontsin vitro. However, fish and rabbit antisera raised against serotype G32 (reported previously) caused little agglutination/immobilization of serotype G37 theronts. Immunological analysis indicated that the 37 kDa theront surface membrane protein may be the agglutination/immobilization antigen of this serotype. The full-length 37 kDa antigen cDNA contained 1171 base pairs, encoding a 331–amino acid protein with hydrophobic N- and C-termini, which are characteristically found in proteins containing a C-terminal glycosylphosphatidylinositol anchor. In addition, the genetically characterized nucleotide sequences of the first internal transcribed spacer region of ribosomal DNA of these 2 serotypes were compared. The internal transcribed spacer rDNA sequence of serotype G32 was identical to that of isolates from Pingtung, Taiwan, and from the USA. On the other hand, the sequences of serotype G37 were not identical to those of anyC. irritansisolate.

Can the surface immobilization antigens of Philasterides dicentrarchi (Ciliophora: Scuticociliatida) be used as target antigens to develop vaccines in cultured fish?

In order to test whether immobilization antigens (i-antigens) of Philasterides dicentrarchi could be suitable antigenic targets against scuticociliatosis, polyclonal olive flounder (Paralichthys olivaceus) sera were raised against P. dicentrarchi by immunization with lysates of ciliates grown using chinook salmon epithelial (CHSE) cells, and the ability of the immune sera to kill the ciliates via classical complement pathway was analyzed in relation to agglutination activity. The immune sera showed clear agglutination activity against the CHSE-cultured ciliates. However, the agglutinated ciliates were not killed but escaped from the agglutinated mass within a few hours. Ciliates isolated from fish artificially infected with the same population of CHSE-cultured ciliates were not agglutinated by the immune sera even at the lowest dilution. In antibody-dependent complement-mediated killing (ADCK), the immune sera completely killed the CHSE-cultured ciliates at relatively higher serum dilutions (showing low or no agglutination activity). However, CHSE-cultured ciliates were not killed completely at lower immune serum dilutions (showing high agglutination activity). In contrast to CHSE-cultured ciliates, the ciliates isolated from infected fish were killed at lower dilutions of the immune sera in spite of no agglutination response. Considering the presence of various i-antigen types, ability to change i-antigen type in response to corresponding antibody, and relatively low ADCK activity at high agglutination titer, i-antigens of P. dicentrarchi may not be good targets for subunit vaccine development. To develop subunit vaccines against scuticociliatosis, other surface antigens expressed constitutively or expressed specifically under the infection state for survival of the ciliates in the host fish might be more favorable to elicit protective antibodies than the surface i-antigens.

Cutaneous antibody-secreting cells and B cells in a teleost fish

Antibodies in cutaneous mucus and skin of teleosts play a critical role in the protective immune response against infection. We demonstrate by ELISPOT that antibody-secreting cells (ASC), which include LPS-inducible B cells (plasmablasts) and non-replicating plasma cells, reside in low numbers in the skin of channel catfish. Following immunization against the protozoan parasite Ichthyophthirius multifiliis, which infects skin and gills, the number of ASC in skin increased 20-fold, indicating that the number of ASC in skin is dynamic and increases in response to parasite infection. The number of ASC in skin remained elevated for at least 17 weeks after the last parasite exposure. Cutaneous ASC included I. multifiliis-specific ASC, which undoubtedly serve as the primary source of cutaneous antibodies that confer long-term humoral immunity against reinfection. Our demonstration that skin contains B cells and plasma cells suggests that it is an integral component of the teleost immune system.