The importance of the Atlantic salmon peritoneal cavity B cell response: Local IgM secreting cells are predominant upon Piscirickettsia salmonis infection.

The intraperitoneal route is favored for administration of inactivated and attenuated vaccines in Atlantic salmon. Nevertheless, the immune responses in the teleost peritoneal cavity (PerC) are still incompletely defined. In this study, we investigated the B cell responses after intraperitoneal Piscirickettsia salmonis (P. salmonis) challenge of Atlantic salmon, focusing on the local PerC response versus responses in the lymphatic organs: spleen and head kidney. We observed a major increase of leukocytes, total IgM antibody secreting cells (ASC), and P. salmonis-specific ASC in the PerC at 3- and 6-weeks post infection (wpi). The increase in ASC frequency was more prominent in the spleen and PerC compared to the head kidney during the observed 6 wpi. The serum antibody response included P. salmonis-specific antibodies and non-specific antibodies recognizing the non-related bacterial pathogen Yersinia ruckeri and the model antigen TNP-KLH. Finally, we present evidence that supports a putative role for the adipose tissue in the PerC immune response.

Primary immunization using low antigen dosages and immunological tolerance in rainbow trout.

Enteric redmouth disease (ERM), caused by the Gram negative enterobacterium Yersinia ruckeri, affects farming of salmonids, but vaccination against ERM confers a certain degree of protection dependent on the administration route. Recent studies on oral vaccination of rainbow trout suggest that immunological tolerance may be induced by primary immunization using a low antigen dosage. We have examined if low dosages of Y. ruckeri antigens, applied in feed or bath exposure over a prolonged period of time, leave rainbow trout more susceptible to infection. Groups of rainbow trout were immunized, either by immersion or feeding using different vaccine dosages, and subsequently challenged by live Y. ruckeri. Survival was recorded and immune reactions in surviving fish were evaluated (ELISA and qPCR). Trout, bath-vaccinated in a highly diluted vaccine or fed the same amount of bacterin in feed over 10 days, were not protected against Y. ruckeri challenge infection and in some cases these sub-optimally immunized fish experienced lower survival compared to non-primed controls. Genes encoding FoxP3 and immune-suppressive cytokines were down-regulated in fish vaccinated with a high antigen dosage when compared to groups exposed to low antigen dosages, suggesting a higher regulatory T cell activity in the latter fish groups. The study suggests that repeated exposure to low antigen concentrations induces some degree of immune tolerance in rainbow trout and we recommend application of high antigen dosages for primary immunization of trout.

Immunogenicity and protective efficacy of Yersinia ruckeri lipopolysaccharide (LPS), encapsulated by alginate-chitosan micro/nanoparticles in rainbow trout (Oncorhyncus mykiss).

Considering the many advantages of oral vaccines in aquaculture, several studies have been conducted in this area recently. In this study, immunization and protective power of the oral vaccine of Yersinia ruckeri encapsulated with Alginate-Chitosan micro/nanoparticles were evaluated in rainbow trout. For this purpose, 720 juvenile rainbow trout (9 ± 1.8 g) were divided into 8 groups in three replications (30 fish each) as follows: Groups A, B and C, were immunized with Yersinia ruckeri lipopolysaccharide (LPS), LPS+Formalin Killed Cells (FKC) and FKC alone, groups D, E, and F were immunized with encapsulated LPS, LPS+FKC and FKC, respectively. The G and H groups considered as encapsulated and non-encapsulated control, respectively. Micro/nanoencapsulation with alginate-chitosan was performed by internal emulsification method and vaccination were conductrd in the first and third weeks via oral route. Sampling was performed on days 0, 30, and 60 of experiment. Anti Y. ruckeri antibody titer in serum, intestine and skin mucus were measured via ELISA method. Non-specific immune response including: serum lysozyme, complement, bactericidal and respiratory burst activity, serum protein and globulin level, as well as white blood cell count were compared among the groups. The expression of IgT gene in the intestine and TCR gene in the anterior kidney were also investigated. At the end of the study, the fish were challenged with Y. ruckeri through immerssion and intraperitoneal routs and the relative survival rate was evaluated. Result showed that the antibody level in serum, skin and intestine was significantly higher in group E and F than control groups (P < 0.05), meanwhile serum, skin and intestine antibody level in all vaccinated groups were significantly (P < 0.01) higher in day 30 and 60 compare to zero day. Non-specific immunity factors including: serum lysozyme, complement, and respiratory burst activity as well as WBC, protein and Globulin level were significantly higher in E and F groups not only in day 30 but also in day 60 of experiment (P < 0.05). Cumulative mortality following injection and bath challenge were significantly (P = 0.004) lower (35%-45%) in groups E and F compare to control group (80%). The IgT and TCR gene expression in groups D, E and F were significantly higher (P < 0.05) than control group. Highest upregulation of IgT and TCR gene expression in vaccinated groups were seen at day 30 and 60 respectively which were significantly (P < 0.001) higher than day zero. Generally, it can be concluded that nano/micronanoencapsulation of Y. ruckeri FKC+LPS with chitosan-alginate, not only increases protective efficacy of oral vaccine, but improves specific and non-specific immune responses in rainbow trout.

Yersinia ruckeri strain SC09 disrupts proinflammatory activation via Toll/IL-1 receptor-containing protein STIR-3.

Virulent pathogenic microorganisms often enhance their infectivity through immune evasion mechanisms. Our research on the integrative and conjugative element (ICE(r2)) of the virulent fish pathogen Yersinia ruckeri SC09 led to the identification of genes related to immune evasion (designated stir-1, stir-2, stir-3 and stir-4), among which stir-1 and stir-2 were determined as the key contributors to bacterial toxicity and immune evasion. Here, we further examined the ability of stir-3 to mediate immune evasion based on detailed bioinformatic analysis of ICE(r2) from Y. ruckeri SC09. Interactions among the translated STIR-1, STIR-2, STIR-3 and STIR-4 proteins in the secretory process were additionally explored. STIR-3 was positively correlated with bacterial toxicity and inhibited host toll-like receptor (TLR) signaling by interacting with MyD88, thereby facilitating bacterial survival in host cells. Importantly, our data showed co-secretion of STIR-1, STIR-2 and STIR-3 as a complex, with secretion failure occurring in the absence of any one of these proteins. While stir-1, stir-2, stir-3 and stir-4 genes werespecific to Y. ruckeri SC09, the ICE(r2) region where these genes were located is a mobile component widely distributed in bacteria. Therefore, the potential transmission risk of these immune evasion genes requires further research attention.

Expansion of fish CCL20_like chemokines by genome and local gene duplication: Characterisation and expression analysis of 10 CCL20_like chemokines in rainbow trout (Oncorhynchus mykiss).

Mammalian CCL20, or macrophage inflammatory protein-3α, can function as a homeostatic and inflammatory chemokine. In relation to the latter, it is responsible for the chemoattraction of lymphocytes and dendritic cells to mucosal immune sites under inflammatory and pathological conditions. CK1, CK8A and CK8B are rainbow trout (Oncorhynchus mykiss) CC chemokines that were reported previously to be phylogenetically related to mammalian CCL20. In the current study, an additional seven CCL20_L paralogues in rainbow trout are reported, that are divided into three subgroups and have been designated here as: CCL20_L1a (also referred to as CK1), CCL20_L1b1-2, CCL20_L2a (CK8A), CCL20_L2b (CK8B), CCL20_L3a, and CCL20_L3b1-4. Multiple CCL20_L genes were also identified in other salmonids that arose from both whole genome duplication and local gene duplication. Phylogenetic tree, homology and synteny analysis support that CCL20_L1-3 found in salmonids are also present in most teleosts arose from the 3 R whole genome duplication and in some species, local gene duplication. Like mammalian CCL20, rainbow trout CCL20_L molecules possess a high positive net charge with a pI of 9.34-10.16, that is reported to be important for antimicrobial activity. Rainbow trout CCL20_L paralogues are differentially expressed and in general highly expressed in mucosal tissues, such as gills, thymus and intestine. The expression levels of rainbow trout CCL20_L paralogues are increased during development and following PAMP/cytokine stimulation. For example, in RTS-11 cells CCL20_L3b1 and CCL20_L3b2 are highly up-regulated by LPS, Poly I:C, recombinant(r) IFNa and rIL-1ß. Trout CCL20_L paralogues are also increased after Yersinia ruckeri infection or Poly I:C stimulation in vivo, with CCL20_L3b1 and CCL20_L3b2 again highly up-regulated. Overall, this is the first report of the complete CCL20 chemokine subfamily in rainbow trout, and the analysis of their expression and modulation in vitro and in vivo. These results suggest that teleosts possess divergent CCL20_L molecules that may have important roles in anti-viral/anti-bacterial defence and in mucosal immunity.

A Yersinia ruckeri TIR Domain-Containing Protein (STIR-2) Mediates Immune Evasion by Targeting the MyD88 Adaptor.

TIR domain-containing proteins are essential for bacterial pathogens to subvert host defenses. This study describes a fish pathogen, Yersinia ruckeri SC09 strain, with a novel TIR domain-containing protein (STIR-2) that affects Toll-like receptor (TLR) function. STIR-2 was identified in Y. ruckeri by bioinformatics analysis. The toxic effects of this gene on fish were determined by in vivo challenge experiments in knockout mutants and complement mutants of the stir-2 gene. In vitro, STIR-2 downregulated the expression and secretion of IL-6, IL-1ß, and TNF-α. Furthermore, the results of NF-κB-dependent luciferase reporter system, co-immunoprecipitation, GST pull-down assays, and yeast two-hybrid assay indicated that STIR-2 inhibited the TLR signaling pathway by interacting with myeloid differentiation factor 88 (MyD88). In addition, STIR-2 promoted the intracellular survival of pathogenic Yersinia ruckeri SC09 strain by binding to the TIR adaptor protein MyD88 and inhibiting the pre-inflammatory signal of immune cells. These results showed that STIR-2 increased virulence in Y. ruckeri and suppressed the innate immune response by inhibiting TLR and MyD88-mediated signaling, serving as a novel strategy for innate immune evasion.

Distinct response of immune gene expression in peripheral blood leucocytes modulated by bacterin vaccine candidates in rainbow trout Oncorhynchus mykiss: A potential in vitro screening and batch testing system for vaccine development in aquaculture.

Fish aquaculture is the world's fastest growing food production industry and infectious diseases are a major limiting factor. Vaccination is the most appropriate method for controlling infectious diseases and a key reason for the success of salmonid cultivation and has reduced the use of antibiotics. The development of fish vaccines requires the use of a great number of experimental animals that are challenged with virulent pathogens. In vitro cell culture systems have the potential to replace in vivo pathogen exposure for initial screening and testing of novel vaccine candidates/preparations, and for batch potency and safety tests. PBL contain major immune cells that enable the detection of both innate and adaptive immune responses in vitro. Fish PBL can be easily prepared using a hypotonic method and is the only way to obtain large numbers of immune cells non-lethally. Distinct gene expression profiles of innate and adaptive immunity have been observed between bacterins prepared from different bacterial species, as well as from different strains or culturing conditions of the same bacterial species. Distinct immune pathways are activated by pathogens or vaccines in vivo that can be detected in PBL in vitro. Immune gene expression in PBL after stimulation with vaccine candidates may shed light on the immune pathways involved that lead to vaccine-mediated protection. This study suggests that PBL are a suitable platform for initial screening of vaccine candidates, for evaluation of vaccine-induced immune responses, and a cheap alternative for potency testing to reduce animal use in aquaculture vaccine development.

Dietary Application of the Probiotic Lactobacillus plantarum 426951 Enhances Immune Status and Growth of Rainbow Trout (Oncorhynchus mykiss) Vaccinated Against Yersinia ruckeri.

This study was aimed to assess the effect of oral application of Lactobacillus plantarum (2 × 107 CFU g-1 feed) as a probiotic on growth performance and immune status of vaccinated rainbow trout (29.5 ± 2 g) to yersiniosis at 16 ± 2 °C for 72 days. Fish were randomly allocated into 12 fiber glass tanks (4100 L) at a density of 80 fish per tank (240 fish per treatment). The results revealed that the activity of lysozyme and alkaline phosphatase was significantly higher in immunized fish fed with diet supplemented with probiotic (vaccine +probiotic) than that in the immunized group fed with basal diet (vaccine group) while no significant differences in levels of hematological parameters, complements, total IgM, proteins, and the intestine lactic acid bacteria (LAB) were detected. Also, significantly a better growth performance in terms of feed conversion ratio, weight gain, and thermal growth coefficient was seen in the vaccine + probiotic group than that in the vaccine group. These results indicate that feeding probiotic after vaccination can enhance the efficacy of immersion vaccination to Yersinia ruckeri.

Dissecting the immune pathways stimulated following injection vaccination of rainbow trout (Oncorhynchus mykiss) against enteric redmouth disease (ERM).

Enteric redmouth disease (ERM or yersiniosis) is one of the most important diseases of salmonids and leads to significant economic losses. It is caused by the Gram-negative bacterium Yersinia ruckeri but can be controlled by bacterin vaccination. The first commercial ERM vaccine was licenced in 1976 and is one of the most significant and successful health practices within the aquaculture industry. Although ERM vaccination provides complete protection, knowledge of the host immune response to the vaccine and the molecular mechanisms that underpin the protection elicited is limited. In this report, we analysed the expression in spleen and gills of a large set of genes encoding for cytokines, acute phase proteins (APPs) and antimicrobial peptides (AMPs) in response to ERM vaccination in rainbow trout, Oncorhynchus mykiss. Many immune genes in teleost fish are known to have multiple paralogues that can show differential responses to ERM vaccination, highlighting the necessity to determine whether all of the genes present react in a similar manner. ERM vaccination immediately activated a balanced inflammatory response with correlated expression of both pro- and anti-inflammatory cytokines (eg IL-1ß1-2, TNF-α1-3, IL-6, IL-8 and IL-10A etc.) in the spleen. The increase of pro-inflammatory cytokines may explain the systemic upregulation of APPs (eg serum amyloid A protein and serum amyloid protein P) and AMPs (eg cathelicidins and hepcidin) seen in both spleen and gills. We also observed an upregulation of all the α-chains but only one ß-chain (p40B2) of the IL-12 family cytokines, that suggests specific IL-12 and IL-23 isoforms with distinct functions might be produced in the spleen of vaccinated fish. Notably the expression of Th1 cytokines (IFN-γ1-2) and a Th17 cytokine (IL-17A/F1a) was also up-regulated and correlated with enhanced expression of the IL-12 family α-chains, and the majority of pro- and anti-inflammatory cytokines, APPs and AMPs. These expression profiles may suggest that ERM vaccination activates host innate immunity and expression of specific IL-12 and IL-23 isoforms leading to a Th1 and Th17 biased immune response. A late induction of Th2 cytokines (IL-4/13B1-2) was also observed, that may have a homeostatic role and/or involvement in antibody production. This study has increased our understanding of the host immune response to ERM vaccination and the adaptive pathways involved. The early responses of a set of genes established in this study may provide essential information and function as biomarkers in future vaccine development in aquaculture.

Effect of oral booster vaccination of rainbow trout against Yersinia ruckeri depends on type of primary immunization.

Vaccination of rainbow trout against Enteric Redmouth Disease (ERM) caused by Yersinia ruckeri can be successfully performed by administering vaccine (a bacterin consisting of formalin killed bacteria) by immersion, bath or injection. Booster immunization is known to increase the protection of fish already primed by one of these vaccination methods. Oral vaccination of trout (administering vaccine in feed) is an even more convenient way of presenting antigen to the fish but the effect of an oral booster has not previously been described in detail. The present work describes to what extent protection may be enhanced by oral boostering following priming with different administration methods. The study confirms that vaccination by 30 s dip into a bacterin (diluted 1:10) may confer a significant protection compared to non-vaccinated fish. The immunity may be optimized by booster immunization either provided as dip (most effective), bath (less effective) or orally (least effective). Oral immunization may be used as booster after dip but applied as a single oral application it induced merely a slight and statistically non-significant response. It is noteworthy that primary oral immunization followed by an oral booster vaccination showed a trend for an even weaker response. It should be investigated if continued exposure to a low antigen concentration - as performed by two oral immunizations - may induce tolerance to the pathogen and thereby leave the fish more vulnerable.

Immune-modulation of two BATF3 paralogues in rainbow trout Oncorhynchus mykiss.

Basic leucine zipper transcription factor ATF-like (BATF) -3 is a member of the activator protein 1 (AP­1) family of transcription factors and is known to play a vital role in regulating differentiation of antigen-presenting cells in mammals. In this study, two BATF3 homologues (termed BATF3a and BATF3b) have been identified in rainbow trout (Oncorhynchus mykiss). Both genes were constitutively expressed in tissues, with particularly high levels of BATF3a in spleen, liver, pyloric caecae and head kidney. BATF3a was also more highly induced by PAMPs and cytokines in cultured cells, with type II IFN a particularly potent inducer. In rIL-4/13 pre-stimulated cells, the viral PAMPS polyI:C and R848 had the most pronounced effect on BATF3 expression. BATF3 expression could also be modulated in vivo, following infection with Yersinia ruckeri, a bacterial pathogen causing redmouth disease in salmonids, or with the rhabdovirus IHNV. The results suggest that BATF3 may be functionally conserved in regulating the differentiation and activation of immune cells in lower vertebrates and could be explored as a potential marker for comparative investigation of leucocyte lineage commitment across the vertebrate phyla.

Induced expression of cathelicidins in trout (Oncorhynchus mykiss) challenged with four different bacterial pathogens.

Cathelicidins are an important family of antimicrobial peptide effectors of innate immunity in vertebrates. Two members of this group, CATH-1 and CATH-2, have been identified and characterized in teleosts (ray-finned fish). In this study, we investigated the expression of these genes in different tissues of rainbow trout challenged with 4 different inactivated pathogens. By using qPCR, we detected a strong induction of both cath-1 and cath-2 genes within 24 hours after intraperitoneal inoculation with Lactococcus garvieae, Yersinia ruckeri, Aeromonas salmonicida, or Flavobacterium psychrophilum cells. Up to 700-fold induction of cath-2 was observed in the spleen of animals challenged with Y. ruckeri. Moreover, we found differences in the intensity and timing of gene up-regulation in the analyzed tissues. The overall results highlight the importance of cathelicidins in the immune response mechanisms of salmonids.

Secondary immune response of rainbow trout following repeated immersion vaccination.

Teleosts are able to raise a protective immune response, comprising both innate and adaptive elements, against various pathogens. This is the basis for a widespread use of vaccines, administered as injection or immersion, in the aquaculture industry. It has been described that repeated injection vaccination of fish raises a secondary immune response, consisting of rapid, accelerated and increased antibody reaction. This study reports how rainbow trout responds to repeated immersion vaccination against yersiniosis (ERM) caused by the bacterial pathogen Yersinia ruckeri. It was found that rainbow trout does not raise a classical secondary response following repeated immersion vaccination. Serum antibody titres were merely slightly increased even after three immunizations, using 30-s immersion into a bacterin consisting of formalin-inactivated Y. ruckeri (serotype O1, biotypes 1 and 2), performed over a 3-month period. The densities of IgM-positive lymphocytes in spleen of fish immunized three times were increased compared to control fish, but no general trend for an increase with the number of immunizations was noted. The lack of a classical secondary response following repeated immersion vaccination may partly be explained by limited uptake of antigen by immersion compared to injection.

Characterisation of ZBTB46 and DC-SCRIPT/ZNF366 in rainbow trout, transcription factors potentially involved in dendritic cell maturation and activation in fish.

ZBTB46 and DC-SCRIPT/ZNF366 are two zinc finger transcription factors that play important roles in regulating differentiation of dendritic cells in mammals. In this study, the ZBTB46 and DC-SCRIPT/ZNF366 homologues were identified in rainbow trout Oncorhynchus mykiss and their expression analysed in vivo and in vitro. As transcription factors, they are well conserved in sequence, genomic organisation and gene synteny. Their expression was differentially modulated by bacterial and viral PAMPs in the monocyte/macrophage-like cell line RTS-11, in primary head kidney (HK) macrophages, and in HK macrophages cultured with IL-4/13A. In the RTS-11 cells and primary HK macrophages, all the ZBTB46 and DC-SCRIPT/ZNF366 homologues were down-regulated by interferon gamma (type II IFN) but unaffected by IFN2 (type I IFN), administered as recombinant proteins to cell cultures. In fish gills, infection with amoebae (Paramoebae perurans) resulted in reduction of ZBTB46 and DC-SCRIPT/ZNF366 expression in Atlantic salmon Salmo salar, whilst infection with Yersinia ruckeri induced gene expression in rainbow trout.

Antigen Uptake during Different Life Stages of Zebrafish (Danio rerio) Using a GFP-Tagged Yersinia ruckeri.

Immersion-vaccines (bacterins) are routinely used for aquacultured rainbow trout to protect against Yersinia ruckeri (Yr). During immersion vaccination, rainbow trout take up and process the antigens, which induce protection. The zebrafish was used as a model organism to study uptake mechanisms and subsequent antigen transport in fish. A genetically modified Yr was developed to constitutively express green fluorescent protein (GFP) and was used for bacterin production. Larval, juvenile and adult transparent zebrafish (tra:nac mutant) received a bath in the bacterin for up to 30 minutes. Samples were taken after 1 min, 15 min, 30 min, 2 h, 12 h and 24 h. At each sampling point fish were used for live imaging of the uptake using a fluorescence stereomicroscope and for immunohistochemistry (IHC). In adult fish, the bacterin could be traced within 30 min in scale pockets, skin, oesophagus, intestine and fins. Within two hours post bath (pb) Yr-antigens were visible in the spleen and at 24 h in liver and kidney. Bacteria were associated with the gills, but uptake at this location was limited. Antigens were rarely detected in the blood and never in the nares. In juvenile fish uptake of the bacterin was seen in the intestine 30 min pb and in the nares 2 hpb but never in scale pockets. Antigens were detected in the spleen 12 hpb. Zebrafish larvae exhibited major Yr uptake only in the mid-intestine enterocytes 24 hpb. The different life stages of zebrafish varied with regard to uptake locations, however the gut was consistently a major uptake site. Zebrafish and rainbow trout tend to have similar uptake mechanisms following immersion or bath vaccination, which points towards zebrafish as a suitable model organism for this aquacultured species.

Expression and antimicrobial activity of c-type lysozyme in taimen (Hucho taimen, Pallas).

Lysozymes are important defense proteins of the innate immune system and possess high antibacterial activities. In the present study, a full-length c-type lysozyme cDNA (HtLysC) was cloned and characterized from taimen (Hucho taimen, Pallas). The cDNA contains an open reading frame (ORF) of 432 bp encoding 143 amino acid (aa), with 97% identity to LysC of Rainbow trout (Oncorhynchus mykiss). The amino acid sequence possessed a LYZ1 domain (16-140 aa) which contained two conserved residues (Glu 50 and Asp 67), eight conserved cysteine residues and a calcium binding site. RT-PCR analysis showed that HtLysC transcripts were most abundant in liver and less in muscle. The expression of HtLysC was up-regulated in the liver when challenged with Yersinia ruckeri. The recombinant HtLysC (rHtLysC) had lytic activities against Micrococcus lysodeikticus, Aeromonas salmonicida and Y. ruckeri. Enzyme assay showed that the optimal temperature and pH of rHtLysC were 55 °C and 6.0, respectively. Taken together, these results indicated that HtLysC might play an important role in innate immune defense against bacterial pathogens as a functional lysozyme.

Protection of rainbow trout against yersiniosis by lpxD mutant Yersinia ruckeri.

Yersinia ruckeri is a Gram negative bacteria causing yersiniosis in freshwater and marine fish. Lipid A, important for pathogenesis of Gram negative bacteria, biosynthesis pathway requires nine enzyme catalyzed steps. Although there are nine genes encoding lipid A biosynthesis in bacteria, biosynthesis of lipopolysaccharides relies on lpxD gene that encodes the third pathway enzyme. The roles of LpxD in Y. ruckeri virulence have not been studied. In the present study, in-frameshift deletion of lpxD gene and their role in Y. ruckeri virulence in rainbow trout were determined. For this purpose, 92% of the Y. ruckeri lpxD genes were deleted by homologous recombination. After running in SDS-PAGE and staining with silver stain, no LPS was detectable in the Y. ruckeri ΔlpxD mutant. Virulence and immunogenicity of the Y. ruckeri ΔlpxD mutant (YrΔlpxD) were determined in rainbow trout. Rainbow trout immunized with YrΔlpxD with immersion, or intraperitoneal injection method displayed superior protection (relative percentage survival ≥ 84%) after exposure to wild type Y. ruckeri. In conclusion, our results indicated that deletion of the lpxD gene causes significant attenuation of Y. ruckeri in rainbow trout, and LPS deficient YrΔlpxD could be used as a live attenuated vaccine against Y. ruckeri in rainbow trout. This vaccine can protect fish and it can be applied to fish with different methods such as immersion or injection.

Yersinia ruckeri lipopolysaccharide is necessary and sufficient for eliciting a protective immune response in rainbow trout (Oncorhynchus mykiss, Walbaum).

Enteric redmouth disease (ERM), caused by Yersinia ruckeri, has been controlled successfully using immersion-applied bacterin vaccines for several decades. While the host response to vaccination and the mechanism of protection of this vaccine have been elucidated, the bacterial components eliciting protection have remained unclear. Here we show that highly purified serotype O1 Y. ruckeri lipopolysaccharide (LPS) is sufficient to induce a protective response to experimental challenge in rainbow trout (Oncorhynchus mykiss). Dose response experiments demonstrated that Y. ruckeri LPS at doses of 1 ng/fish and above resulted in essentially complete protection and doses as low as 0.01 ng/fish (1.38 ng/kg) resulted in significant protection, thus demonstrating the extremely high potency of this immunogen. Analysis of the Y. ruckeri genome identified a cluster of putative O-antigen biosynthetic genes specific to serotype O1 strains. This cluster primarily consisted of genes encoding proteins predicted to function in the biosynthesis of legionamic acid, a nonulosonic acid known to be part of the O-polysaccharide repeat of O1 Y. ruckeri. Mutation of the nab2 gene, a nonulosonic acid biosynthesis gene (nab gene), resulted in production of severely truncated forms of LPS. Vaccination with bacterin vaccines derived from the nab2 mutant and its wild type parent strain demonstrated that LPS is a required component of the whole-cell bacterin vaccine and suggests that LPS is the only cellular component contributing to the protective response elicited by this vaccine. We speculate that the exceptionally high potency of Y. ruckeri LPS accounts for the unusual success of this vaccine when delivered by immersion.

Re-examination of the rainbow trout (Oncorhynchus mykiss) immune response to flagellin: Yersinia ruckeri flagellin is a potent activator of acute phase proteins, anti-microbial peptides and pro-inflammatory cytokines in vitro.

Flagellin is the principal component of bacterial flagellum and a major target of the host immune system. To provide new insights into the role of flagellin in fish immune responses to flagellated microorganisms, a recombinant flagellin from Yersinia ruckeri (rYRF) was produced and its bioactivity investigated in the trout macrophage cell line RTS-11 and head kidney cells. rYRF is a potent activator of pro-inflammatory cytokines, acute phase proteins, antimicrobial peptides and subunits of the IL-12 cytokine family. This and the synergy seen with IFN-γ to enhance further expression of specific IL-12 and TNF-α isoforms may suggest that flagellin could be a useful immune stimulant or adjuvant for use in aquaculture. Gene paralogues were often differentially modulated, highlighting the need to study all of the paralogues of immune genes in fish to gain a full understanding of the effects of PAMPs or other stimulants, and the potential immune responses elicited.