The effect of peptidoglycan enriched diets on antimicrobial peptide gene expression in rainbow trout (Oncorhynchus mykiss)

Tea polyphenols protect against Flavobacterium columnare-induced gill injury via suppression of oxidative stress, inflammation, and apoptosis in grass carp

Flavobacterium columnare (F. columnare) is one of the deadliest fish pathogens causing bacterial gill rot disease in various freshwater fish species globally. Tea polyphenols (TPs) are an inexpensive product extracted from tea that have received much attention as a feed additive in aquaculture. The current study was designed to investigate the underlying mechanisms and protective effects of dietary TPs against F. columnare-induced gill injury via suppression of oxidative stress, apoptosis, and inflammation in grass carp. TPs were not supplemented to the diet (control) and were supplemented at 40, 80, 120, 160 or 200 mg/kg diet. The feeding experiment was carried out for 60 days, followed by a 3-Day F. columnare challenge test. The results showed that 120 mg/kg TPs in the diet exerted the following five protective effects in fish gill: (1) control gill-rot disease and improved histopathology, (2) inhibit excessive apoptosis, (3) enhance the activity of antioxidant enzymes and upregulate related gene expression via the Nrf2/Keap1 pathway, (4) increase the activity of immune enzymes, And (5) mediate inflammatory cytokine gene expression via the JAK/STAT3 pathway. Taken together, dietary supplementation with TPs is a compelling approach to protect the gill function of fish against F. columnare.

Advancements of fish-derived peptides for mucormycosis: a novel strategy to treat diabetic compilation

Mucormycosis, an extremely fatal fungal infection, is a major hurdle in the treatment of diabetes consequences. The increasing prevalence and restricted treatment choices urge the investigation of novel therapeutic techniques. Because of their effective antimicrobial characteristics and varied modes of action, fish-derived peptides have lately emerged as viable options in the fight against mucormycosis. This review examines the potential further application of fish-derived peptides in diagnosing and managing mucormycosis in relation to diabetic complications. First, we examine the pathophysiology of mucormycosis and the difficulties in treating it in diabetics. We emphasize the critical need for alternative therapeutic methods for tackling the limitations of currently available antifungal medicines. The possibility of fish-derived peptides as an innovative approach to combat mucormycosis is then investigated. These peptides, derived from several fish species, provide wide antimicrobial properties against a variety of diseases. They also have distinct modes of action, such as rupture of cell membranes, suppression of development, and modification of the host immunological response. Furthermore, we investigate the problems and prospects connected with the clinical application of fish-derived peptides. Ultimately, future advances in fish-derived peptides, offer interesting avenues for the management of mucormycosis in the context of diabetic comorbidities. More research and clinical trials are needed to properly investigate these peptide's therapeutic potential and pave the way for their adoption into future antifungal therapies.

Host Defense Peptides in Nutrition and Diseases: A Contributor of Immunology Modulation

Host defense peptides (HDPs) are primary components of the innate immune system with diverse biological functions, such as antibacterial ability and immunomodulatory function. HDPs are produced and released by immune and epithelial cells against microbial invasion, which are widely distributed in humans, animals, plants, and microbes. Notably, there are great differences in endogenous HDP distribution and expression in humans and animals. Moreover, HDP expression could be regulated by exogenous substances, such as nutrients, and different physiological statuses in health and disease. In this review, we systematically assessed the regulation of expression and mechanism of endogenous HDPs from nutrition and disease perspectives, providing a basis to identify the specificity and regularity of HDP expression. Furthermore, the regulation mechanism of HDP expression was summarized systematically, and the differences in the regulation between nutrients and diseases were explored. From this review, we provide novel ideas targeted the immune regulation of HDPs for protecting host health in nutrition and practical and effective new ideas using the immune regulation theory for further research on protecting host health from pathogenic infection and excessive immunity diseases under the global challenge of the antibiotic-abuse-induced series of problems, including food security and microbial resistance.

Growth hormone secretagogue peptide-6 enhances oreochromicins transcription and antimicrobial activity in tilapia (Oreochromis sp.)

Growth Hormone-Releasing Peptide 6 (GHRP-6) (His-(D-Trp)-Ala-Trp-(D-Phe)-Lys-NH2) is an agonist of the growth hormone secretagogue receptor. GHRP-6 mimics the effect of ghrelin. The present study focuses on the immunomodulatory effects of GHRP-6 in tilapia with and without the presence of Pseudomonas aeruginosa infection. GHRP-6 up-regulated the transcription levels of three piscidin-like antimicrobial peptides (Oreochromicins I, II, and III) and granzyme in a tissue-dependent manner. Antimicrobial activity stimulation in serum (lysozyme and anti-protease activity) was also confirmed. Besides, GHRP-6 enhanced the in vitro antimicrobial activity against P. aeruginosa in tilapia gills mucus and serum samples and decreased the bacterial load in vivo after infection with this Gram-negative bacterium. Our results evidenced, for the first time, a direct link between a growth hormone secretagogue ghrelin mimetic in fish and the enhancement of antimicrobial peptides transcription, which suggests that this secretagogue is capable to lead the activation of microbicidal activity in tilapia. Thus, these results open new possibilities for GHRP-6 application in aquaculture to stimulate the teleost immune system as an alternative treatment against opportunistic bacteria.

Development of species-specific IgM antibodies and elevation of mucosal immune response in Labeo rohita using recombinant bicistronic nano DNA vaccine priming

Immunoglobulin (IgM) is the primary immunoglobulin essential for defense mechanisms in fish. It is difficult to reliably quantify IgM because a lack of standardization in methodology and limited availability of commercially reagents. In the present study, a polyclonal antibody was developed for the specific detection and quantification of IgM in Labeo rohita. Recombinant bicistronic NanoDNA plasmid (RBND Vac) encoding the glyceraldehyde-3-phosphate dehydrogenase gene of Edwarsiella tarda conjugated with poly (lactic-co-glycolic acid) - Chitosan (PLGA-Chit) was developed and its potential as a DNA vaccine, to prevent the infection of E. tarda in L. rohita was investigated. Two treatment groups [T1 - (PLGA-Chit-NPs-pDNA), T2 - (PLGA-NPs-pDNA) and one control group (T0 - 1 × PBS)] were utilized. Polyclonal antibody was developed to estimate IgM titers in the serum and mucosal associated tissues (MAT) using Enzyme-linked Immunosorbent Assay (ELISA) technique. Additionally, immune gene expression was studied using qRT-PCR. Vaccinated groups also exhibited a significant increase in the total serum protein, globulin concentration and relatively less mortality was observed in T1 group. IgM level in serum and mucosal tissues (skin, gill and gut) increased significantly days post vaccination compared to control group, also non-specific immune parameters (myeloperoxidase and lysozyme levels) showed significant improvement in vaccinated fish. Furthermore, histopathological examination confirmed minor damage in physiological structure of kidney and liver tissues in vaccinated fish. Knowledge of the immunoglobulin in L. rohita primed with RBND Vac complex provides the better protection against E. tarda. The normal physiology findings of this study will aid in monitoring changes in the health status of fish, when the animals undergo vaccination by immersion method.

Changes in rainbow trout (Oncorhynchus mykiss) growth and mucosal immune parameters after dietary administration of grape (Vitis vinifera) seed extract

The effect of dietary grape (Vitis vinifera) seed extract (GSE) on growth performance and mucosal immune parameters in rainbow trout (Oncorhynchus mykiss) fry was studied. Fish (1.3 g mean weight) were randomly distributed in nine tanks (15 fish per tank) and fed diets containing GSE at 0 (control), 100, and 200 mg kg^-1for 60 days. The results showed that growth parameters were enhanced in both treatment groups compared to the control group. Histological examination of fish skin showed higher epidermis thickness, goblet cell density, and volume density in the GSE groups compared to the values of the control group. Furthermore, the villus height, goblet cell density, and intraepithelial lymphocytes were increased in the fish intestine in those fish fed GSE, with respect to control fish. Feeding fish with low dose of GSE (100 mg kg^-1) up-regulated the expression of some immune-relevant genes, including complement component 3 (C3), lysozyme (Lys), omDB-3, interferon gamma (IFN-γ), and tumor necrosis factor-α (TNF-α) in different mucosal tissues. However, feeding fish the high dose of GSE (200 mg kg^-1) mostly enhanced expression of these genes in the skin. Besides, skin mucus of fish fed GSE showed bactericidal activity against Yersinia ruckeri. It was concluded that GSE, especially at 100 mg kg^-1, modulates the growth performance and mucosal immunity of rainbow trout.

Dietary Chitosan Nanoparticles: Potential Role in Modulation of Rainbow Trout (Oncorhynchus mykiss) Antibacterial Defense and Intestinal Immunity against Enteric Redmouth Disease

Bio-nanotechnology employing bio-sourced nanomaterial is an emerging avenue serving the field of fish medicine. Marine-sourced chitosan nanoparticles (CSNPs) is a well-known antimicrobial and immunomodulatory reagent with low or no harm side effects on fish or their human consumers. In this study, in vitro skin mucus and serum antibacterial activity assays along with intestinal histology, histochemical, and gene expression analyses were performed to evaluate the impact of dietary CSNPs (5 g kg⁻¹ dry feed) on rainbow trout resistance against ‘enteric redmouth’ disease. Two treatment conditions were included; short-term prophylactic-regimen for 21 days before the bacterial challenge, and long-term therapeutic-regimen for 21 days before the challenge and extended for 28 days after the challenge. Our results revealed higher antibacterial defense ability and positive intestinal histochemical and molecular traits of rainbow trout after dietary CSNPs. The prophylactic-regimen improved trout health while the therapeutic regimen improved their disease resistance and lowered their morbidity. Therefore, it is anticipated that CSNPs is an effective antibacterial and immunomodulatory fish feed supplement against the infectious threats. However, the CSNPs seem to be more effective in the therapeutic application rather than being used for short-term prophylactic applications.

Antimicrobial Peptides of Salmonid Fish: From Form to Function

Antimicrobial peptides (AMPs) are small, usually cationic, and amphiphilic molecules that play a crucial role in molecular and cellular host defense against pathogens, tissue damage, and infection. AMPs are present in all metazoans and several have been discovered in teleosts. Some teleosts, such as salmonids, have undergone whole genome duplication events and retained a diverse AMP repertoire. Salmonid AMPs have also been shown to possess diverse and potent antibacterial, antiviral, and antiparasitic activity and are induced by a variety of factors, including dietary components and specific molecules also known as pathogen-associated molecular patterns (PAMPs), which may activate downstream signals to initiate transcription of AMP genes. Moreover, a multitude of cell lines have been established from various salmonid species, making it possible to study host-pathogen interactions in vitro, and several of these cell lines have been shown to express various AMPs. In this review, the structure, function, transcriptional regulation, and immunomodulatory role of salmonid AMPs are highlighted in health and disease. It is important to characterize and understand how salmonid AMPs function as this may lead to a better understanding of host-pathogen interactions with implications for aquaculture and medicine.

The Application of Single-Cell Ingredients in Aquaculture Feeds—A Review

Single-cell ingredients (SCI) are a relatively broad class of materials that encompasses bacterial, fungal (yeast), microalgal-derived products or the combination of all three microbial groups into microbial bioflocs and aggregates. In this review we focus on those dried and processed single-cell organisms used as potential ingredients for aqua-feeds where the microorganisms are considered non-viable and are used primarily to provide protein, lipids or specific nutritional components. Among the SCI, there is a generalised dichotomy in terms of their use as either single-cell protein (SCP) resources or single-cell oil (SCO) resources, with SCO products being those oleaginous products containing 200 g/kg or more of lipids, whereas those products considered as SCP resources tend to contain more than 300 g/kg of protein (on a dry basis). Both SCP and SCO are now widely being used as protein/amino acid sources, omega-3 sources and sources of bioactive molecules in the diets of several species, with the current range of both these ingredient groups being considerable and growing. However, the different array of products becoming available in the market, how they are produced and processed has also resulted in different nutritional qualities in those products. In assessing this variation among the products and the application of the various types of SCI, we have taken the approach of evaluating their use against a set of standardised evaluation criteria based around key nutritional response parameters and how these criteria have been applied against salmonids, shrimp, tilapia and marine fish species.

Defensins: Transcriptional regulation and function beyond antimicrobial activity

Defensins are one the largest group of antimicrobial peptides and are part of the innate defence. Defensins are produced by animals, plants and fungi. In animals and plants, defensins can be constitutively or differentially expressed both locally or systemically which confer defence before and a stronger response after infection. Immune signalling pathways regulate the gene expression of defensins. These pathways include cellular receptors, which recognise pathogen-associated molecular patterns and are found both in plants and animals. After recognition, signalling pathways and, subsequently, transcriptional factors are activated. There is an increasing number of novel functions in defensins, such as immunomodulators and immune cell attractors. Identification of defensin triggers could help us to elucidate other new functions. The present article reviews the different elicitors of defensins with a main focus on human, fish and marine invertebrate defensins.

Increased parasite resistance of greater amberjack (Seriola dumerili Risso 1810) juveniles fed a cMOS supplemented diet is associated with upregulation of a discrete set of immune genes in mucosal tissues

The main objective of this study was to determine the effect of two forms of mannan oligosaccharides (MOS: Bio-Mos^® and cMOS: Actigen^®, Alltech Inc, USA) and their combination on greater amberjack (Seriola dumerili) growth performance and feed efficiency, immune parameters and resistance against ectoparasite (Neobenedenia girellae) infection. Fish were fed for 90 days with 5 g kg^-1 MOS, 2 g kg^-1 cMOS or a combination of both prebiotics, in a Seriola commercial base diet (Skretting, Norway). At the end of the feeding period, no differences were found in growth performance or feed efficiency. Inclusion of MOS also had no effect on lysozyme activity in skin mucus and serum, but the supplementation of diets with cMOS induced a significant increase of serum bactericidal activity. Dietary cMOS also reduced significantly greater amberjack skin parasite levels, parasite total length and the number of parasites detected per unit of fish surface following a cohabitation challenge with N. girellae, whereas no effect of MOS was detected on these parameters. Of 17 immune genes studied cMOS dietary inclusion up-regulated hepcidin, defensin, Mx protein, interferon-γ (IFNγ), mucin-2 (MUC-2), interleukin-1β (IL-1B), IL-10 and immunoglobulin-T (IgT) gene expression in gills and/or skin. MOS supplementation had a larger impact on spleen and head kidney gene expression, where piscidin, defensin, iNOS, Mx protein, interferons, IL-1β, IL-10, IL-17 and IL-22 were all upregulated. In posterior gut dietary MOS and cMOS both induced IL-10, IgM and IgT, but with MOS also increasing piscidin, MUC-2, and IL-1β whilst cMOS induced hepcidin, defensin and IFNγ. In general, the combination of MOS and cMOS resulted in fewer or lower increases in all tissues, possibly due to an overstimulation effect. The utilization of cMOS at the dose used here has clear benefits on parasite resistance in greater amberjack, linked to upregulation of a discrete set of immune genes in mucosal tissues.

Effects of cortisol and lipopolysaccharide on expression of select growth-, stress- and immune-related genes in rainbow trout liver

Many studies have shown that stress-induced cortisol levels negatively influence growth and immunity in finfish. Despite this knowledge, few studies have assessed the direct effects of cortisol on liver immune function. Using real-time PCR, the expression of three cortisol-responsive genes (GR: glucocorticoid receptor, IGF-1: insulin-like growth factor-I and SOCS-1: suppressor of cytokine signaling-I), genes involved with innate and adaptive immunity (IL-1β: interleukin-1 beta, IgM: immunoglobin-M and Lyz: lysozyme), and liver-specific antimicrobial peptides (hepcidin and LEAP-2A: liver-expressed antimicrobial peptide-2A) was studied in vitro using rainbow trout liver slices. The abundances of GR, SOCS-1 and IGF-1 mRNAs were suppressed by cortisol treatment. Abundance of IL-1β mRNA was upregulated by LPS and suppressed by cortisol treatment in a time-dependent manner. While abundance of IgM mRNA was suppressed by cortisol treatment and stimulated by LPS, there were no effects of cortisol or LPS on abundance of Lyz mRNA. Abundance of hepcidin and LEAP-2A mRNA levels were suppressed by cortisol treatment and stimulated by LPS. These results demonstrate that cortisol directly suppresses abundance of GR, IGF-1, IL-1β, IgM, hepcidin, LEAP-2A and SOCS-1 mRNA transcripts in the rainbow trout liver. We report for the first time, a suppressive effect of cortisol (within 8 h of treatment) on hepcidin and LEAP-2A mRNAs in rainbow trout liver, which suggests that acute stress may negatively affect liver immune function in rainbow trout.

Fish antimicrobial peptides (AMP's) as essential and promising molecular therapeutic agents: A review

Antimicrobial peptides (AMPs) are generally considered as an essential component of innate immunity, thereby providing the first line of defense against wide range of pathogens. In addition, they can also kill the pathogens which are generally resistant to number of antibiotics, thereby providing the avenues for the development of future therapeutic agents. Fishes are constantly challenged by variety of pathogens which not only shows detrimental effect on their health but also increases risk of becoming resistant to conventional antibiotics. As fishes rely more on innate immunity, AMPs can serve as a potential defensive weapons in fishes for combating emerging devastating diseases. Generally, AMPs show multidimensional properties like rapid diffusion to the site of infection, recruitment of other immune cells to infected tissues and vigorous potential to rapidly neutralize broad range of pathogens (bacterial, fungal and viral). AMPs also exhibit diverse biological effect like endotoxin neutralization, immunomodulation and induction of angiogenesis in mammals. Due to these properties AMPs have become one of the most promising therapeutic agents to be studied. Till date, many AMPs have been isolated from the fishes but not fully characterized at molecular level. This review provides an overview of the structures, functions, and putative mechanisms of major families of fish AMPs. Further, we also highlighted how fish AMPs can be used as a novel therapeutic tool which is the theme of future research in drug development.

Host–parasite transcriptomics during immunostimulant-enhanced rejection of salmon lice (Lepeophtheirus salmonis) by Atlantic salmon (Salmo salar)

Salmon lice ( Lepeophtheirus salmonis) are important ectoparasites of wild and farmed salmonids and cause major losses to the salmon farming industry throughout the Northern Hemisphere. With the emergence of resistance to several commonly used parasiticides, novel control strategies and integration of multiple treatment options are needed, including host immunostimulation. Here, we investigate the effects of a functional feed containing a peptidoglycan and nucleotide formulation on L. salmonis infection of Atlantic salmon ( Salmo salar) by characterizing lice infection levels, the expression of several host immune genes, and the parasite transcriptomic response to the immunostimulated host. Although initial infection intensities were low, the low dose (LD) immunostimulant diet reduced the total lice burden by 50% relative to controls. Immunostimulant fed hosts up-regulated interleukin-1β in the skin and spleen. This gene has been implicated in successful responses of several salmonid species to salmon lice but is typically not observed in Atlantic salmon, suggesting a favorable influence on the immune response. Lice infecting LD immunostimulated salmon overexpressed genes putatively involved in parasite immunity, including carboxylesterases, and underexpressed genes putatively involved in feeding (e.g., proteases). These lice response genes further improve the characterization of the transcriptome of the non-model parasite by identifying genes potentially involved in evading host immunity.

The response of fish to immunostimulant diets

In order to maintain fish health and to improve performance immunostimulants have been used as dietary additives to improve weight gain, feed efficiency, and/or disease resistance in cultured fish. In aquaculture, non-specific immunostimulants have been widely used probably due to the limited knowledge of the immune response in fish and the ease of their application. Many studies have been carried out to assess the effect of dietary immunostimulants in fish including algal derivatives, herb and plant extract containing diets using a wide range of downstream analytical techniques. Many immunostimulants are based upon tradition and folklore transferred through generations and specific to certain geographical regions rather than known biological properties. However, there are studies in which it is possible to observe a clear and direct dose-dependent stimulatory effect upon the immune system. Other dietary supplements used contain PAMPs (Pathogen Associated Molecular Patterns) as immunostimulants whose recognition depends upon PRR (pathogen recognition receptor) interactions including the TLRs (Toll-like receptor). Despite the growing interest in the use of immunostimulants across the aquaculture industry the underlying mechanisms of ligand recognition, extract composition and activation of the fish immune response remains fragmented. In this review we focus upon the last 15 years of studies addressing the assessment of: (1) plant, herb and algae extracts; and (2) PAMPs, upon non-specific immune parameters of activation and immunostimulant diet efficacy.

Immunomodulatory effect of cathelicidins in response to a β-glucan in intestinal epithelial cells from rainbow trout

The aim of the present study was to characterize intestinal immune mechanisms involved in the response to β-glucans in rainbow trout. Among the immune effectors regulated in response to immunostimulants, host defense peptides (HDPs) are abundantly expressed in epithelial linings, suggesting their important role in the mucosal immune response. Therefore, the immunomodulatory properties of expressed HDPs in the epithelial intestinal cells of rainbow trout in response to the β-glucan, zymosan, were assessed. The results showed that zymosan increased the production of the HDP, cathelicidin, and the cytokine, IL-1β, in the intestinal epithelial RTgutGC cell line at the transcript and protein levels. Thus, cathelicidin-2 variants were produced and were shown to (i) induce the production of IL-1β in RTgutGC cells and (ii) display a synergic effect with zymosan in IL-1β upregulation. Importantly, the colocalization of both rtCATH-2 and IL-1β was detected in the intestinal epithelial cells of rainbow trout fed with a 0.3% zymosan-supplemented diet. We propose that trout cathelicidins are expressed by intestinal epithelial cells and exert immunomodulatory effects to improve the local intestinal immune response triggered by immunostimulants.

The longevity of the antimicrobial response in rainbow trout (Oncorhynchus mykiss) fed a peptidoglycan (PG) supplemented diet

This study builds upon previous work studying antimicrobial peptide (AMP) gene expression in rainbow trout (Oncorhynchus mykiss) fed a peptidoglycan (PG) enriched diet. The aims here were 1) to evaluate how long AMP expression is elevated in skin with continuous feeding of fish with the PG enriched diet for 21 or 28 days, and 2) to assess the impact of stopping PG feeding at day 14 when sampled at day 21 or 28. The rainbow trout were divided into 6 groups, with two fed a control commercial diet for the duration of the experiment and the other four given the same diet enriched with 10 mg PG/Kg for 14 days (PG 1-14) or continuously (PG continuous), the former reverting back to the commercial diet at day 14. No mortalities occurred during the study and there were no significant differences in growth among the fish in the different diet groups. The expression of six AMP genes was studied by real-time PCR in the skin, since these genes were shown to be induced in response to the PG enriched diets in a previous experiment. We show that continuous PG treatment for 21 or 28 days maintained high levels of AMP expression, although in general the levels decreased with time on the diets. Withdrawal of the PG diets at day 14 resulted in a fall in expression level especially apparent with omCATH-1, omCATH-2 and omLEAP-2a, but with omDB-3 and omDB-4 remaining at elevated levels (x10) in comparison to fish given a control diet. These results confirm that orally administered PG clearly enhances the trout innate immune system and could be used as a means to boost fish defences. Future studies should be conducted to verify the impact on survival after pathogen challenge in trout fed PG enriched diets under these regimes.

Antimicrobial peptides from fish

Antimicrobial peptides (AMPs) are found widely distributed through Nature, and participate in the innate host defense of each species. Fish are a great source of these peptides, as they express all of the major classes of AMPs, including defensins, cathelicidins, hepcidins, histone-derived peptides, and a fish-specific class of the cecropin family, called piscidins. As with other species, the fish peptides exhibit broad-spectrum antimicrobial activity, killing both fish and human pathogens. They are also immunomodulatory, and their genes are highly responsive to microbes and innate immuno-stimulatory molecules. Recent research has demonstrated that some of the unique properties of fish peptides, including their ability to act even in very high salt concentrations, make them good potential targets for development as therapeutic antimicrobials. Further, the stimulation of their gene expression by exogenous factors could be useful in preventing pathogenic microbes in aquaculture.

The mucosal immune system of fish: the evolution of tolerating commensals while fighting pathogens

The field of mucosal immunology research has grown fast over the past few years, and our understanding on how mucosal surfaces respond to complex antigenic cocktails is expanding tremendously. With the advent of new molecular sequencing techniques, it is easier to understand how the immune system of vertebrates is, to a great extent, orchestrated by the complex microbial communities that live in symbiosis with their hosts. The commensal microbiota is now seen as the "extended self" by many scientists. Similarly, fish immunologist are devoting important research efforts to the field of mucosal immunity and commensals. Recent breakthroughs on our understanding of mucosal immune responses in teleost fish open up the potential of teleosts as animal research models for the study of human mucosal diseases. Additionally, this new knowledge places immunologists in a better position to specifically target the fish mucosal immune system while rationally designing mucosal vaccines and other immunotherapies. In this review, an updated view on how teleost skin, gills and gut immune cells and molecules, function in response to pathogens and commensals is provided. Finally, some of the future avenues that the field of fish mucosal immunity may follow in the next years are highlighted.