A Lactococcus lactis BFE920 feed vaccine expressing a fusion protein composed of the OmpA and FlgD antigens from Edwardsiella tarda was significantly better at protecting olive flounder (Paralichthys olivaceus) from edwardsiellosis than single antigen vaccines

Lactococcus lactis, a bacterium with probiotic functions and pathogenicity

Lactococcus lactis (L. lactis) is the primary organism for lactic acid bacteria (LAB) and is a globally recognized safe microorganism for the regulation of the intestinal micro-ecological balance of animals and improving the immune performance of the host. L. lactis is known to play a commercially important role in feed fortification, milk fermentation, and vaccine production, but pathogenic L. lactis has been isolated from many clinical cases in recent years, such as the brain of silver carp with Lactococcosis, the liver and spleen of diseased waterfowl, milk samples and padding materials with cow mastitis, and blood and urine from human patients with endocarditis. In dairy farming, where L. lactis has been used as a probiotic in the past, however, some studies have found that L. lactis can cause mastitis in cows, but the lack of understanding of the pathogenesis of mastitis in cows caused by L. lactis has become a new problem. The main objective of this review is to analyze the increasingly serious clinical mastitis caused by L. lactis and combined with the wide application of L. lactis as probiotics, to comprehensively discuss the characteristics and diversity of L. lactis.

Field Efficacy of a Feed-Based Inactivated Vaccine against Vibriosis in Cage-Cultured Asian Seabass, Lates calcarifer, in Malaysia

Vibrio spp. are important aquaculture pathogens that cause vibriosis, affecting large numbers of marine fish species. This study determines the field efficacy of a feed-based inactivated vaccine against vibriosis in cage-cultured Asian seabass. A total of 4800 Asian seabass, kept in a field environment, were separated equally into two groups (vaccinated and non-vaccinated) in duplicate. Fish of Group 1 were orally administered the feed-based vaccine on weeks 0 (prime vaccination), 2 (booster), and 6 (second booster) at 4% body weight, while the non-vaccinated fish of Group 2 were fed with a commercial formulated pellet without the vaccine. Fish gut, mucus, and serum were collected, the length and weight of the fish were noted, while the mortality was recorded at 2-week intervals for a period of 16 weeks. The non-specific lysozyme activities were significantly (p < 0.05) higher in the fish of Group 1 than the non-vaccinated fish of Group 2. Similarly, the specific IgM antibody levels in serum and mucus were significantly (p < 0.05) higher in Group 1 than in Group 2, as seen in the second week, with the highest level 8 weeks after primary immunization. At week 16, the growth performance was significantly (p < 0.05) better in Group 1 and showed lower bacterial isolation in the gut than Group 2. Despite the statistical insignificance (p > 0.05), the survival rate was slightly higher in Group 1 (71.3%) than Group 2 (67.7%). This study revealed that feed-based vaccination improves growth performance, stimulates innate and adaptive immune responses, and increases protection of cultured Asian seabass, L. calcarifer, against vibriosis.

Efficacy of a recombinant M-like protein, SimA as a subunit vaccine candidate against Streptococcus parauberis infection in olive flounder, Paralichthys olivaceus

Streptococcus parauberis, a gram-positive cocci, causes bacterial disease in farmed fish. The recent increase in S. parauberis infection in aquatic farms in South Korea has justified the importance of vaccine development for the prevention of this disease. In this study, we evaluated the effect of subunit vaccines prepared from recombinant M-like protein (SimA) and fibrinogen-binding protein (FBP) candidates with an aluminum hydroxide adjuvant against S. parauberis infection in olive flounder Paralichthys olivaceus. For the in vivo experiment, fish (average length, 7.18 cm; average weight, 3.5 g) were injected intraperitoneally with: phosphate buffer saline (PBS, group 1), PBS/aluminum hydroxide (group 2), FBP/aluminum hydroxide (group 3), SimA/aluminum hydroxide (group 4), and SimA/FBP/aluminum hydroxide (group 5). After 3 weeks, the fish in each group were boosted using PBS (group 1 and 2), FBP (group 3), SimA (group 4), and SimA/FBP (group 5) without adjuvant. We found that the relative percent survival of fish after S. parauberis exposure in group 2, 3, 4, and 5 was 6.25%, 18.75%, 50%, and 12.5%, respectively, whereas the mortality in groups 1 was 80%, respectively. We performed Western blot, ELISA, and quantitative real time RT-PCR (qRT-PCR) after vaccination to investigate the further efficacy of the vaccine. Western blot and ELISA of vaccinated fish serum confirmed the production of specific antibodies against SimA and FBP. Furthermore, results of qRT-PCR showed that recombinant protein SimA induced a remarkably specific-antibody response compared with that in FBP or control and increased the expression of various immune response-related genes including interleukin-8 (IL-8), toll-like receptor 2 (TLR2), tumor necrosis factor-α (TNF-α), CD4-1, and MHC II. Thus, these results indicate that SimA is a potent vaccine candidate for protection against S. parauberis infection.

Infectious pancreatic necrosis virus (IPNV) recombinant viral protein 1 (VP1) and VP2-Flagellin fusion protein elicit distinct expression profiles of cytokines involved in type 1, type 2, and regulatory T cell response in rainbow trout (Oncorhynchus mykiss)

In this study, we examined the cytokine immune response against two proteins of infectious pancreatic necrosis virus (IPNV) in rainbow trout (Oncorhynchus mykiss), the virion-associated RNA polymerase VP1 and VP2-Flagellin (VP2-Flg) fusion protein. Since VP1 is not a structural protein, we hypothesize it can induce cellular immunity, an essential mechanism of the antiviral response. At the same time, the fusion construction VP2-Flg could be highly immunogenic due to the presence of the flagellin used as an adjuvant. Fish were immunized with the corresponding antigen in Montanide™, and the gene expression of a set of marker genes of Th1, Th2, and the immune regulatory response was quantified in the head kidney of immunized and control fish. Results indicate that VP1 induced upregulation of ifn-γ, il-12p40c, il-4/13a, il-4/13b2, il-10a, and tgf-β1 in immunized fish. Expression of il-2a did not change in treated fish at the times tested. The antigen-dependent response was analysed by in vitro restimulation of head kidney leukocytes. In this assay, the group of cytokines upregulated after VP1-restimulation was consistent with those upregulated in the head kidney in vivo. Interestingly, VP1 induced il-2a expression after in vitro restimulation. The analysis of sorted lymphocytes showed that the increase of cytokines occurred in CD4-1⁺ T cells suggesting that Th differentiation happens in response to VP1. This is also consistent with the expression of t-bet and gata3, the master regulators for Th1/Th2 differentiation in the kidneys of immunized animals. A different cytokine expression profile was found after VP2-Flg administration, i.e., upregulation occurs for ifn-γ, il-4/13a, il-10a, and tgf-β1, while down-regulation was observed in il-4/13b2 and il-2a. The cytokine response was due to flagellin; only the il-2a effect was dependent upon VP2 in the fusion protein. To the best of our knowledge this study reports for the first-time characteristics of the adaptive immune response induced in response to IPNV VP1 and the fusion protein VP2-Flg in fish. VP1 induces cytokines able to trigger the humoral and cell-mediated immune response in rainbow trout. The analysis of the fish response against VP2-Flg revealed the immunogenic properties of Aeromonas salmonicida flagellin, which can be further tested for adjuvanticity. The novel immunogenic effects of VP1 in rainbow trout open new opportunities for further IPNV vaccine development using this viral protein.

Single and Multi-Strain Probiotics Supplementation in Commercially Prominent Finfish Aquaculture: Review of the Current Knowledge

The Nile tilapia Oreochromis niloticus, Atlantic salmon Salmo salar, rainbow trout Oncorhynchus mykiss, olive flounder Paralichthys olivaceus, common carp Cyprinus carpio, grass carp Ctenopharyngodon idella and rohu carp Labeo rohita are farmed commercially worldwide. Production of these important finfishes is rapidly expanding, and intensive culture practices can lead to stress in fish, often reducing resistance to infectious diseases. Antibiotics and other drugs are routinely used for the treatment of diseases and sometimes applied preventatively to combat microbial pathogens. This strategy is responsible for the emergence and spread of antimicrobial resistance, mass killing of environmental/beneficial bacteria, and residual effects in humans. As an alternative, the administration of probiotics has gained acceptance for disease control in aquaculture. Probiotics have been found to improve growth, feed utilization, immunological status, disease resistance, and to promote transcriptomic profiles and internal microbial balance of host organisms. The present review discusses the effects of single and multi-strain probiotics on growth, immunity, heamato-biochemical parameters, and disease resistance of the above-mentioned finfishes. The application and outcome of probiotics in the field or open pond system, gaps in existing knowledge, and issues worthy of further research are also highlighted.

Biofunctionalization of Endolysins with Oligosacharides: Formulation of Therapeutic Agents to Combat Multi-Resistant Bacteria and Potential Strategies for Their Application

In the aquaculture sector, the biofunctionalization of biomaterials is discussed using materials from algae and analyzed as a possible potential strategy to overcome the challenges that hinder the future development of the application of endolysins in this field. Derived from years of analysis, endolysins have recently been considered as potential alternative therapeutic antibacterial agents, due to their attributes and ability to combat multi-resistant bacterial cells when applied externally. On the other hand, although the aquaculture sector has been characterized by its high production rates, serious infectious diseases have led to significant economic losses that persist to this day. Although there are currently interesting data from studies under in vitro conditions on the application of endolysins in this sector, there is little or no information on in vivo studies. This lack of analysis can be attributed to the relatively low stability of endolysins in marine conditions and to the complex gastrointestinal conditions of the organisms. This review provides updated information regarding the application of endolysins against multi-resistant bacteria of clinical and nutritional interest, previously addressing their important characteristics (structure, properties and stability). In addition, regarding the aquaculture sector, the biofunctionalization of biomaterials is discussed using materials from algae and analyzed as a possible potential strategy to overcome the challenges that hinder the future development of the application of endolysins in this field.

Comparative Reverse Vaccinology of Piscirickettsia salmonis, Aeromonas salmonicida, Yersinia ruckeri, Vibrio anguillarum and Moritella viscosa, Frequent Pathogens of Atlantic Salmon and Lumpfish Aquaculture

Marine finfish aquaculture is affected by diverse infectious diseases, and they commonly occur as co-infection. Some of the most frequent and prevalent Gram-negative bacterial pathogens of the finfish aquaculture include Piscirickettsia salmonis, Aeromonas salmonicida, Yersinia ruckeri, Vibrio anguillarum and Moritella viscosa. To prevent co-infections in aquaculture, polyvalent or universal vaccines would be ideal. Commercial polyvalent vaccines against some of these pathogens are based on whole inactivated microbes and their efficacy is controversial. Identification of common antigens can contribute to the development of effective universal or polyvalent vaccines. In this study, we identified common and unique antigens of P. salmonis, A. salmonicida, Y. ruckeri, V. anguillarum and M. viscosa based on a reverse vaccinology pipeline. We screened the proteome of several strains using complete available genomes and identified a total of 154 potential antigens, 74 of these identified antigens corresponded to secreted proteins, and 80 corresponded to exposed outer membrane proteins (OMPs). Further analysis revealed the outer membrane antigens TonB-dependent siderophore receptor, OMP assembly factor BamA, the LPS assembly protein LptD and secreted antigens flagellar hook assembly protein FlgD and flagellar basal body rod protein FlgG are present in all pathogens used in this study. Sequence and structural alignment of these antigens showed relatively low percentage sequence identity but good structural homology. Common domains harboring several B-cells and T-cell epitopes binding to major histocompatibility (MHC) class I and II were identified. Selected peptides were evaluated for docking with Atlantic salmon (Salmo salar) and Lumpfish MHC class II. Interaction of common peptide-MHC class II showed good in-silico binding affinities and dissociation constants between −10.3 to −6.5 kcal mol−1 and 5.10 × 10−9 to 9.4 × 10−6 M. This study provided the first list of antigens that can be used for the development of polyvalent or universal vaccines against these Gram-negative bacterial pathogens affecting finfish aquaculture.

Gut Immune System and the Implications of Oral-Administered Immunoprophylaxis in Finfish Aquaculture

The gastrointestinal immune system plays an important role in immune homeostasis regulation. It regulates the symbiotic host-microbiome interactions by training and developing the host's innate and adaptive immunity. This interaction plays a vital role in host defence mechanisms and at the same time, balancing the endogenous perturbations of the host immune homeostasis. The fish gastrointestinal immune system is armed with intricate diffused gut-associated lymphoid tissues (GALTs) that establish tolerance toward the enormous commensal gut microbiome while preserving immune responses against the intrusion of enteric pathogens. A comprehensive understanding of the intestinal immune system is a prerequisite for developing an oral vaccine and immunostimulants in aquaculture, particularly in cultured fish species. In this review, we outline the remarkable features of gut immunity and the essential components of gut-associated lymphoid tissue. The mechanistic principles underlying the antigen absorption and uptake through the intestinal epithelial, and the subsequent immune activation through a series of molecular events are reviewed. The emphasis is on the significance of gut immunity in oral administration of immunoprophylactics, and the different potential adjuvants that circumvent intestinal immune tolerance. Comprehension of the intestinal immune system is pivotal for developing effective fish vaccines that can be delivered orally, which is less labour-intensive and could improve fish health and facilitate disease management in the aquaculture industry.

Feeding olive flounder (Paralichthys olivaceus) with Lactococcus lactis BFE920 expressing the fusion antigen of Vibrio OmpK and FlaB provides protection against multiple Vibrio pathogens: A universal vaccine effect

Vibriosis, an illness caused by the Vibrio bacteria species, results in significant economic loss in olive flounder farms. Here we present a novel anti-Vibrio feed vaccine protecting multiple strains of Vibrio pathogens, a universal vaccine effect. The vaccine was generated by engineering Lactococcus lactis BFE920 to express the fusion antigens of Vibrio outer membrane protein K (OmpK) and flagellin B subunit (FlaB). These antigen genes are highly conserved among Vibrio species. Olive flounder (7.1 ± 0.8 g and 140 ± 10 g) were fed the vaccine adsorbed to a regular feed (1 × 10⁷ CFU/g) for one week with a 1-week interval, repeating three times (a triple boost). The vaccinated fish increased the significant levels of antigen-specific antibodies, T cell numbers (CD4-1, CD4-2, and CD8α), cytokine production (T-bet and IFN-γ), and innate immune responses (TLR5M, IL-1β, and IL-12p40). Also, the survival rates of adult and juvenile fish fed the vaccine were significantly elevated when challenged with V. anguillarum, V. alginolyticus, and V. harveyi. In addition, weight gain rate and feed conversion ratio were improved in vaccinated fish. The feed vaccine protected multiple Vibrio pathogens, a universal vaccine effect, by activating innate and adaptive immune responses. This oral vaccine may be developed as an anti-Vibrio vaccine to protect against a broad spectrum of Vibrio pathogens.

Mechanisms Used by Probiotics to Confer Pathogen Resistance to Teleost Fish

Probiotics have been defined as live microorganisms that when administered in adequate amounts confer health benefits to the host. The use of probiotics in aquaculture is an attractive bio-friendly method to decrease the impact of infectious diseases, but is still not an extended practice. Although many studies have investigated the systemic and mucosal immunological effects of probiotics, not all of them have established whether they were actually capable of increasing resistance to different types of pathogens, being this the outmost desired goal. In this sense, in the current paper, we have summarized those experiments in which probiotics were shown to provide increased resistance against bacterial, viral or parasitic pathogens. Additionally, we have reviewed what is known for fish probiotics regarding the mechanisms through which they exert positive effects on pathogen resistance, including direct actions on the pathogen, as well as positive effects on the host.

Vaccine Efficacy of a Newly Developed Feed-Based Whole-Cell Polyvalent Vaccine against Vibriosis, Streptococcosis and Motile Aeromonad Septicemia in Asian Seabass, Lates calcarifer

Multiple infections of several bacterial species are often observed under natural farm conditions. The infections would cause a much more significant loss compared to a single infectious agent. Vaccination is an essential strategy to prevent diseases in aquaculture, and oral vaccination has been proposed as a promising technique since it requires no handling of the fish and is easy to perform. This research attempts to develop and evaluate a potential feed-based polyvalent vaccine that can be used to treat multiple infections by Vibrios spp., Streptococcus agalactiae, and Aeromonas hydrophila, simultaneously. The oral polyvalent vaccine was prepared by mixing formalin-killed vaccine of V. harveyi, S. agalactiae, and A. hydrophila strains with commercial feed pellet, and palm oil as an adjuvant was added to improve their antigenicity. Thereafter, a vaccinated feed pellet was tested for feed quality analysis in terms of feed stability in water, proximate nutrient analysis, and palatability, safety, and growth performance using Asian seabass, Lates calcarifer as a fish host model. For immune response analysis, a total of 300 Asian seabass juveniles (15.8 ± 2.6 g) were divided into two groups in triplicate. Fish of group 1 were not vaccinated, while group 2 was vaccinated with the feed-based polyvalent vaccine. Vaccinations were carried out on days 0 and 14 with oral administration of the feed containing the bacterin at 5% body weight. Samples of serum for antibody and lysozyme study and the spleen and gut for gene expression analysis were collected at 7-day intervals for 6 weeks. Its efficacy in protecting fish was evaluated in aquarium challenge. Following vaccination by the polyvalent feed-based vaccine, IgM antibody levels showed a significant (p < 0.05) increase in serum against Vibrio harveyi, Aeromonas hydrophila, and Streptococcus agalactiae and reached the peak at week 3, 5, and 6, respectively. The high-stimulated antibody in the serum remained significantly higher than the control (p < 0.05) at the end of the 6 weeks vaccination trial. Not only that, but the serum lysozyme level was also increased significantly at week 4 (p < 0.05) as compared to the control treatment. The immune-related gene, dendritic cells, C3, Chemokine ligand 4 (CCL4), and major histocompatibility complex class I (MHC I) showed significantly higher expression (p < 0.05) after the fish were vaccinated with the oral vaccine. In the aquarium challenge, the vaccine provided a relative percentage survival of 75 ± 7.1%, 80 ± 0.0%, and 80 ± 0.0% after challenge with V. harveyi, A. hydrophila, and S. agalactiae, respectively. Combining our results demonstrate that the feed-based polyvalent vaccine could elicit significant innate and adaptive immunological responses, and this offers an opportunity for a comprehensive immunization against vibriosis, streptococcosis, and motile aeromonad septicemia in Asian seabass, Lates calcarifer. Nevertheless, this newly developed feed-based polyvalent vaccination can be a promising technique for effective and large-scale fish immunization in the aquaculture industry shortly.

Plasmid Replicons for the Production of Pharmaceutical-Grade pDNA, Proteins and Antigens by Lactococcus lactis Cell Factories

The Lactococcus lactis bacterium found in different natural environments is traditionally associated with the fermented food industry. But recently, its applications have been spreading to the pharmaceutical industry, which has exploited its probiotic characteristics and is moving towards its use as cell factories for the production of added-value recombinant proteins and plasmid DNA (pDNA) for DNA vaccination, as a safer and industrially profitable alternative to the traditional Escherichia coli host. Additionally, due to its food-grade and generally recognized safe status, there have been an increasing number of studies about its use in live mucosal vaccination. In this review, we critically systematize the plasmid replicons available for the production of pharmaceutical-grade pDNA and recombinant proteins by L. lactis. A plasmid vector is an easily customized component when the goal is to engineer bacteria in order to produce a heterologous compound in industrially significant amounts, as an alternative to genomic DNA modifications. The additional burden to the cell depends on plasmid copy number and on the expression level, targeting location and type of protein expressed. For live mucosal vaccination applications, besides the presence of the necessary regulatory sequences, it is imperative that cells produce the antigen of interest in sufficient yields. The cell wall anchored antigens had shown more promising results in live mucosal vaccination studies, when compared with intracellular or secreted antigens. On the other side, engineering L. lactis to express membrane proteins, especially if they have a eukaryotic background, increases the overall cellular burden. The different alternative replicons for live mucosal vaccination, using L. lactis as the DNA vaccine carrier or the antigen producer, are critically reviewed, as a starting platform to choose or engineer the best vector for each application.

Modulation of transcriptomic profile in aquatic animals: Probiotics, prebiotics and synbiotics scenarios

Aquaculture and fisheries have provided protein sources for human consumption for a long time, but diseases have induced declines in product benefits and raised concerns, resulting in great losses to these industries in many countries. The overuse of antibiotics for the treatment of diseases has increased the chemical concentrations in culture systems and weakened the natural immunity of aquatic organisms. Concerns regarding the detrimental effects of antibiotics on the environment and human health due to residual antibiotic-related issues encourage the development of reliable, environmental and health safety methods, such as vaccines, probiotics, prebiotics, synbiotics and phytobiotics, for protection against disease and for reducing and possibly eliminating disease occurrence. Immunity has been effectively enhanced by pro-, pre-, and synbiotics, which confer strong protection and reduce the risks associated with stressors and disease outbreaks in culture systems. These agents confer several benefits, including enhancing both host growth and immune responses against pathogens, while sustaining health and environmental stability, and their use is thus widely accepted. Alterations in gene expression in individual cells could serve as an indicator of the immunity and growth rate of aquatic animals after pro-, pre- and synbiotic feeding. This review addresses the potential use of pro, pre- and synbiotics as immunostimulants for improved aquaculture management and environmental health and chronicles the recent insights regarding the application of pro-, pre- and synbiotics with special emphasis on their immunomodulatory and antioxidative responses based on gene expression changes. Furthermore, the current review describes the research gaps and other issues that merit further investigation.

Surface display of hirame novirhabdovirus (HIRRV) G protein in Lactococcus lactis and its immune protection in flounder (Paralichthys olivaceus)

Background

Hirame novirhabdovirus (HIRRV) can infect a wide range of marine and freshwater fish, causing huge economic losses to aquaculture industry. Vaccine development, especially oral vaccine, has become an effective and convenient way to control aquatic infectious diseases. HIRRV glycoprotein (G), an immunogenic viral protein is a potential vaccine candidate for prevention of the disease. Here, we aimed to construct a recombinant Lactococcus lactis strain expressing HIRRV-G on the cell surface as an oral vaccine to prevent HIRRV.

Results

Glycoprotein gene of HIRRV was successfully cloned and expressed in L. lactis NZ9000 in a surface-displayed form, yielding Ll:pSLC-G. An approximately 81 kDa recombinant G protein (containing LysM anchoring motif) was confirmed by SDS-PAGE, western blotting and mass spectrometry analysis. The surface-displayed G protein was also verified by immunofluorescence and flow cytometry assays. Furthermore, to evaluate the potential of Ll:pSLC-G as oral vaccine candidate, flounders were continuously fed with commercial diet pellets coated with 1.0 × 10⁹ cfu/g of induced Ll:pSLC-G for 1 week. Four weeks later, booster vaccination was performed with the same procedure. Compared with the controls, Ll:pSLC-G elicited significantly higher levels of specific IgM against HIRRV in flounder gut mucus at the second week and in serum at the fourth week (p < 0.05). Meanwhile, oral immunization with Ll:pSLC-G could provide 60.7% protection against HIRRV infection and a significantly lower virus load was detected than the controls on the third day post-challenge (p < 0.01). Moreover, on the first day post 1-week feeding, approximately 10⁴–10⁵ recombinant L. lactis cells were detected in every gram of foregut, midgut and hindgut of flounder, which were mainly localized at the bottom of gut mucus layer; and on day 21, 10²–10³L. lactis cells could still be recovered.

Conclusions

HIRRV-G protein was successfully expressed on the surface of L. lactis cells, which could trigger mucosal and humoral immune response of flounder and provide considerable immune protection against HIRRV. It suggests that genetically engineered L. lactis expressing G protein can be employed as a promising oral vaccine against HIRRV infection.

Studies on the Use of Flagellin as an Immunostimulant and Vaccine Adjuvant in Fish Aquaculture

Immunostimulants and vaccines are important for controlling infectious diseases in fish aquaculture. In this study we assess the potential of flagellin to be used for such purposes in rainbow trout (Oncorhynchus mykiss). A recombinant flagellin from the salmonid pathogen Yersinia ruckeri (YRF) has been produced previously by us and shown to be a potent activator of inflammatory cytokines, acute phase proteins and antimicrobial peptides in vitro. Here we show that YRF is the most potent inflammatory activator of three bacterial PAMPs (LPS, peptidoglycan and flagellin) tested. The host response to flagellin was next studied in vivo. The YRF modulated gene expression was examined in two systemic (spleen and liver) and two mucosa-associated (gills and skin) tissues. YRF injection initiated a transient systemic inflammatory response with key pro-inflammatory cytokines (IL-1β, TNFα, IL-6, and IL-11 etc.) and chemokines (CXCL_F4 and CXCL-8) induced rapidly (by 6 h) but subsiding quickly (by 24 h) in multiple tissues. Consequently, a variety of anti-microbial pathways were activated systemically with heightened expression of acute phase proteins, antimicrobial peptides and complement genes in multiple tissues, which was sustained to 24 h in the liver and mucosal tissues. The Th17 cytokine IL-17A/F1 was also induced in the spleen and liver, and Th2 cytokine IL-4/13 was induced in the liver. However, the anti-inflammatory IL-10 and the Th1 cytokine IFNγ were refractory. A secreted form of TLR5 (TLR5s) was induced by flagellin in all tissues examined whilst the membrane form was refractory, suggesting that TLR5s may function as a negative feedback regulator. Trout liver appeared to be an important organ responding to flagellin stimulation, with marked induction of IL-11, IL-23P19, IL-17C1, SAA, and cathelicidin-2. YRF induced a strong antibody response. These antibodies reacted against the middle domain of YRF and were able to decrease YRF bioactivity. Intact YRF was necessary for its bioactivity, as deletion of the N-terminal, C terminal or middle domain of YRF led to functional loss. This study suggests that flagellin could be a potent immunostimulant and vaccine adjuvant for fish aquaculture.

Genetic Engineering of Lactococcus lactis Co-producing Antigen and the Mucosal Adjuvant 3' 5'- cyclic di Adenosine Monophosphate (c-di-AMP) as a Design Strategy to Develop a Mucosal Vaccine Prototype

Lactococcus lactis is a promising candidate for the development of mucosal vaccines. More than 20 years of experimental research supports this immunization approach. In addition, 3′ 5′- cyclic di-adenosine monophosphate (c-di-AMP) is a bacterial second messenger that plays a key role in the regulation of diverse physiological functions (potassium and cellular wall homeostasis, among others). Moreover, recent studies showed that c-di-AMP has a strong mucosal adjuvant activity that promotes both humoral and cellular immune responses. In this study, we report the development of a novel mucosal vaccine prototype based on a genetically engineered L. lactis strain. First, we demonstrate that homologous expression of cdaA gen in L. lactis is able to increase c-di-AMP levels. Thus, we hypothesized that in vivo synthesis of the adjuvant can be combined with production of an antigen of interest in a separate form or jointly in the same strain. Therefore, a specifically designed fragment of the trans-sialidase (TScf) enzyme from the Trypanosoma cruzi parasite, the etiological agent of Chagas disease, was selected to evaluate as proof of concept the immune response triggered by our vaccine prototypes. Consequently, we found that oral administration of a L. lactis strain expressing antigenic TScf combined with another L. lactis strain producing the adjuvant c-di-AMP could elicit a TS-specific immune response. Also, an additional L. lactis strain containing a single plasmid with both cdaA and tscf genes under the Pcit and Pnis promoters, respectively, was also able to elicit a specific immune response. Thus, the current report is the first one to describe an engineered L. lactis strain that simultaneously synthesizes the adjuvant c-di-AMP as well as a heterologous antigen in order to develop a simple and economical system for the formulation of vaccine prototypes using a food grade lactic acid bacterium.

Gene expression analysis of isolated salmonid GALT leucocytes in response to PAMPs and recombinant cytokines

Increased knowledge of the immune response of the intestine, a physiologically critical organ involved in absorption, secretion and homeostasis in a non-sterile environment, is needed to better understand the mechanisms involved in the induction of long-lasting immunity and, subsequently, the development of efficacious gastrointestinal immunization approaches. To this end, analysis of isolated gut cells will give an insight into the cell types present and their immune capability. Hence, in this study we first optimised a method for salmonid gut leucocyte isolation and characterised the cells on the basis of their expression of a range of selected cell markers associated with T & B cells and dendritic cells. The GALT leucocytes were then stimulated with a variety of PAMPs, recombinant cytokines and PHA, as a means to help characterise the diversity of the immune repertoire present in such cells. The stimulants tested were designed to examine the nature of the antibacterial, antiviral and T cell type responses in the cells (at the transcript level) using a panel of genes relevant to innate and adaptive immunity. The results showed distinct responses to the stimulants, with a clear delineation seen between the stimulant used (eg viral or bacterial PAMP) and the pathway elicited. The changes in the expression patterns of the immune genes in these cells indicates that the salmonid intestine contains a good repertoire of competent immune cells able to respond to different pathogen types. Such information may aid the development of efficient priming by oral vaccination in salmonids.