Mucosal immunoglobulins and B cells of teleost fish

Effects of dietary Lactobacillus plantarum (KC426951) in biofloc and stagnant-renewal culture systems on growth performance, mucosal parameters, and serum innate responses of Nile tilapia Oreochromis niloticus

Effects of dietary Lactobacillus plantarum (KC426951) on growth and innate responses of Nile tilapia Oreochromis niloticus were evaluated in biofloc technology system and stagnant-renewal culture system (SRCS). The 90-day-long experiment contained four treatments: SRCS without probiotic (T1), SRCS with probiotic (T2), biofloc without probiotic (T3), and biofloc with probiotic (T4). The administration dose of probiotic was 2 × 10⁸ CFU kg^-1 diet. At the end of experiment, the mean final weights, specific growth rates, feed conversion ratios, and total biomass were significantly (P < 0.05) better in BFT treatments, with no significant effect of probiotic on these parameters in both culture systems. Meanwhile, skin mucosal parameters including total protein (TP), lysozyme (LYZ), alkaline phosphatase (ALP), and protease (PRO) activity were significantly enhanced following probiotic supplementation. T4 treatment displayed a significantly higher LYZ and ALP activity in mucus versus other treatments. Also, serum alternative complement activity was significantly heightened in probiotic-supplemented fish. Superoxide dismutase activity in T4 was detected higher than that of SRCS groups. The results of the current study demonstrated the enhancement of some mucosal and serum innate responses of Nile tilapia in both culture systems upon L. plantarum (KC426951) supplementation.

Recent Advances on Phagocytic B Cells in Teleost Fish

The momentous discovery of phagocytic activity in teleost B cells has caused a dramatic paradigm shift from the belief that phagocytosis is performed mainly by professional phagocytes derived from common myeloid progenitor cells, such as macrophages/monocytes, neutrophils, and dendritic cells. Recent advances on phagocytic B cells and their microbicidal ability in teleost fish position B cells at the crossroads, bridging innate with adaptive immunity. Most importantly, an increasing body of experimental evidence demonstrates that, in both teleosts and mammals, phagocytic B cells can recognize, take up, and destroy particulate antigens and then present those processed antigens to CD4⁺ T cells to elicit adaptive immune responses and that the phagocytosis is mediated by pattern recognition receptors and involves multiple cytokines. Thus, current findings collectively indicate that teleost phagocytic B cells, as well as their counterpart mammalian B1-B cells, can be considered one kind of professional phagocyte. The aim of this review is to summarize recent advances regarding teleost phagocytic B cells, with a particular focus on the recognizing receptors and modulating mechanisms of phagocytic B cells and the process of antigen presentation for T-cell activation. We also attempt to provide new insights into the adaptive evolution of the teleost fish phagocytic B cell on the basis of its innate and adaptive roles.

Proteomic profiling of yellow catfish (Pelteobagrus fulvidraco) skin mucus identifies differentially-expressed proteins in response to Edwardsiella ictaluri infection

Fish mucus acts as a physiological and immunological barrier for maintaining normal fish physiology and conferring defense against pathogens infection. Here we report proteomic profiling of skin mucus of yellow catfish before and after E. ictaluri infection by Label-free LC-MS/MS approach. A total of 918 non-redundant proteins were identified from 54443 spectra referring to yellow catfish genome database. Further annotation via GO and KEGG database revealed complex protein composition of yellow catfish mucus. Besides structural proteins in mucus, a lot of immune-related proteins were retrieved, such as lectins, complement components, antibacterial peptides and immunoglobins. 133 differentially-expressed proteins (DEPs), including 76 up-regulated and 57 down-regulated proteins, were identified, most of which were enriched into 17 pathways centering on "immune system" category with 33 proteins involved. Consistently, significant proliferation of mucus-secreting goblet cells and CYPA-expressing cells were observed along outside of yellow catfish skin after E. ictaluri infection, indicating an enhanced immune response to E. ictaluri infection in yellow catfish skin mucus. The proteomic data provide systematic protein information to comprehensively understand the biological function of yellow catfish skin mucus in response to bacterial infection.

The predominant role of mucosal immunoglobulin IgT in the gills of rainbow trout (Oncorhynchus mykiss) after infection with Flavobacterium columnare

Columnaris disease, induced by Flavobacterium columnare, seriously affects the health of freshwater fish species and damages the mucosal tissues, such as the fins, skin, and gills. Teleosts represent the first bony vertebrate to contain both innate and adaptive immune responses against pathogens. So far, three immunoglobulin isotypes (IgM, IgD, and IgT/IgZ) have been identified in teleost fish, and IgT in mucosal tissues of teleost fish was reported to perform a similar function to IgA in mammals during parasitic infection. However, very limited information is known about the function of IgT in gill mucosal tissues during bacterial infection. In the present study, rainbow trout (Oncorhynchus mykiss) was infected with F. columnare (Fc) via immersion. After Fc infection, the gill structure of rainbow trout showed serious hyperplasia symptoms on the secondary lamellae at 12 h post infection (hpi). Moreover, the mRNA expression levels of NOS2 and cathelicidin-1 were significantly upregulated immediately at 12 hpi and showed high expression throughout the experiment. IgT and IgM showed much higher mRNA expression levels at 28 days post infection (dpi) and 75 dpi, while IgD only showed high mRNA expression levels at 28 dpi. Importantly, the accumulation of IgT⁺ B cells and strong bacteria-specific IgT responses were detected in the gill lamellae of both infected fish (28 dpi) and survivor fish (75 dpi). Overall, our results suggest that IgT and IgT⁺ B cells play a central role in the adaptive immune responses of fish gill mucosa against bacterial infection.

Transcriptome analysis reveals the temporal gene expression patterns in skin of large yellow croaker (Larimichthys crocea) in response to Cryptocaryon irritans infection

Large yellow croaker (Larimichthys crocea) is one of the most important mariculture fish in China. In the past decades, cryptocaryonosis caused by Cryptocryon irritans has led to huge economic losses, posing great threat to the healthy and sustainable development of L. crocea mariculture industry. As the largest immunologically active mucosal organ in fish, skin provides the first defense line against external pathogens. To better understand the gene expression dynamics, the large yellow croakers were artificially infected with C. irritans and their skin tissues were collected at 0 h, 24 h, 48 h, 72 h and 96 h post infection. The total RNA in the skin tissues were extracted and the transcriptome were sequenced. After sequencing, a total of 1,131, 311, 140 million high quality RNA-seq reads were collected. A set of 215, 473, 968, 1055 differentially expressed genes were identified at 24 h, 48 h, 72 h and 96 h post infection respectively. Further analysis clustered these DEGs into six profiles and 75 hub genes for six profiles were identified. Among these hub genes, 18 immune related genes including TLR5, TOPK, NFKBIZ, MAPK14A were identified post C. irritans infection. Cytokine-cytokine receptor interaction was the only pathway that significantly enriched at four timepoints post infection. This study provides an in-depth understanding of skin transcriptome variance of large yellow croaker after C. irritans infection, which would be helpful for further understanding of the molecular mechanism of L. crocea in response to C. irritans infection.

Effects of recombinant Lactobacillus casei on growth performance, immune response and disease resistance in crucian carp, Carassius auratus

In the present study, we constructed two recombinant Lactobacillus casei (L. casei) Lc-pPG-1-AcrV (surface-displayed) and Lc-pPG-2-AcrV (secretory) constitutively expressing AcrV protein of Aeromonas veronii (A. veronii). Expression of recombinant AcrV protein was verified by western blot and immunofluorescence technique. Compared with PBS group, the final weight (FW), weight gain (WG) and specific growth rate (SGR) of fish fed Lc-pPG-1-AcrV, Lc-pPG-2-AcrV and Lc-pPG diets after 56 days observed significantly increase (p < 0.05), while the feed conversion ratio (FCR) showed a significantly decrease (p < 0.05). The recombinant L. casei strains were orally administrated to crucian carp, and significant increased (p < 0.05) the immunoglobulin M (IgM), elevated the acid phosphatase (ACP), alkaline phosphatase (AKP), lysozyme (LZM) and superoxide dismutase (SOD) activity in serum. Moreover, leukocytes phagocytosis percentage and index of the recombinant L. casei were both enhanced. The results demonstrated that the recombinant L. casei could elicit systemic immune responses and increase the serum immunological index. The Interleukin-10 (IL-10), Interleukin-1β (IL-1β), interferon-γ (IFN-γ) and Tumor Necrosis Factor-α (TNF-α) levels in liver, spleen, kidney and intestine have up regulated significantly in tissues (p < 0.05), suggesting that the recombinant L. casei has the ability to induce expression of cytokines and enhance the innate immune response. Higher survival rates were exhibited that crucian carp immunized with Lc-pPG-1-AcrV (67.5%) and Lc-pPG-2-AcrV (52.5%) after challenge with A. veronii. In conclusion, these two recombinant L. casei vaccine were effective in improving crucian carp growth, immunity response and disease resistance. The recombinant L. casei strains may be a promising candidate for the development of an oral vaccine against A. veronii.

The effect of fishwort (Houttuynia cordata) on skin mucosal, serum immunities, and growth performance of Nile tilapia

The present study addressed the possible effects of fishwort (Houttuynia cordata) powder (FWP) on Nile tilapia's skin mucus parameter, serum immune response, and growth performance. Three hundred twenty tilapia fingerlings (average weight of 39.06 ± 0.16 g) were divided into four treatments and fed four levels of FWP; 0, 5, 10, and 20 g kg^-1 for 72 days. Completed randomised design of the four replications was applied and revealed that fish fed FWP significantly improved skin mucus lysozyme activity (SMLA). The highest value (P < 0.05) was recorded in fish fed 10 g kg^-1 FWP. However, no significant difference in SMLA was observed by feeding the fish 5 and 20 g kg^-1 FWP. Significant (P < 0.05) enhanced skin mucus peroxidase activity (SMPA) was observed in fish fed 10 g kg^-1 FWP, but no significant difference in SMPA was detected between FWP supplemented diets (5 and 20 g kg^-1 FWP) and the control group. Regarding serum immunity, dietary administration of FWP showed significantly (P < 0.05) improved serum lysozyme, peroxidase, alternative complement (ACH50), and phagocytosis vs. the control. The highest values of serum immunity (P < 0.05) were recorded in fish fed 10 g kg^-1 FWP. However, no significance in respiratory burst activity was observed. Similarly, no significant difference in growth performance, feed conversion ratio, and survival rate was observed in fish fed FWP compared to the control. In summary, diets supplemented with FWP (10 g kg^-1) increased the serum and mucosal immunity; however, no FWP supplementations had effects on Nile tilapia growth and survival rate.

Parasitological and histopathological diagnosis of a non-native fish (Oreochromis sp.) with a noticeable presence in a natural Brazilian river environment

Nile tilapia is an exotic species produced in Brazilian aquaculture. The aim of this study was to constitute a pathological diagnosis in Oreochromis sp. with a noticeable presence in a river in southern Brazil, and to verify the origin of these fish in this natural habitat, an environment with high sanitary precariousness due to discharges of industrial effluents. A total of 30 fish were collected during three sampling events and their health status was assessed by parasitological fauna survey, parasitological indices and histopathological examination of the gills, liver, spleen and kidney. Five species of monogenea were identified infesting gills and stomach: Cichlidogyrus sclerosus, C. halli, C. thurstonae, Scutogyrus longicornis and Enterogyrus cichlidarum; and 4 species of trichodinidae infesting the body surface and gills: Trichodina magna, T. compacta, T. centrostrigeata and Paratrichodina africana. The results showed differences according to the seasonality of the collection, with higher parasite count and larger tissue alterations in summer. Histopathological analysis fluctuated from moderate to severe, but necrosis prevailed in all organs and in all fish, indicating that these fish did not enjoy full health.

Anti-foodborne enteritis effect of galantamine potentially via acetylcholine anti-inflammatory pathway in fish

Foodborne enteritis has become a limiting factor in aquaculture. Plant protein sources have already caused enteritic inflammation and inhibition in growth performance. Attempts have been made to find an effective solution to foodborne enteritis. Based on the previously suggested fish cholinergic anti-inflammatory pathway, galantamine, a typical cholinesterase inhibitor, was tested for the repression of pro-inflammatory cytokines for soybean meal induced enteritis by injection into grass carp. Both the phylogenetic analysis of cholinesterase, AchR and bioinformatic prediction, indicated galantamine's potential use as an enteritis drug. The result highlighted galantamine's potential effect for anti-enteritis in fish, especially in carps. Subsequently, a 4-week feeding trail using galantamine as an additive, in a zebrafish soybean meal induced enteritis model, demonstrated the prevention of enteritis. The results demonstrated that galantamine could prevent intestinal pathology, both histologically and molecularly, and also maintain growth performance. Reflected by gene expressional analysis, all mechanical, chemical and immune functions of the intestinal barrier could be protected by galantamine supplementation, which aided molecularly in the control of fish foodborne enteritis, through down-regulating Th17 type proinflammatory factors, meanwhile resuming the level of Treg type anti-inflammatory factors. Therefore, the current results shed light on fish intestinal acetylcholine anti-inflammation, by the dietary addition of galantamine, which could give rise to protection from foodborne enteritis.

TRAF3 enhances TRIF-mediated signaling via NF-κB and IRF3 activation in large yellow croaker Larimichthys crocea

As a member of tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family, TRAF3 is an important regulator of NF-κB and type I interferon (IFN) activation, especially in Toll-like receptors (TLRs)- and retinoic acid inducible gene I (RIG-I)-like receptors (RLRs)-mediated signaling pathway. In the present study, a TRAF3 homologue named Lc-TRAF3 was characterized in large yellow croaker (Larimichthys crocea). The open reading frame (ORF) of Lc-TRAF3 contains 1788 bp encoding a protein of 595 amino acids (aa). Sequence analysis indicated that Lc-TRAF3 is conserved in vertebrates, constituted with a N-terminal RING finger, two TRAF-type zinc fingers, and a C-terminal TRAF-MATH domain. The genome organization of Lc-TRAF3 is conserved in fish, with 13 exons and 12 introns, but different from that in birds or mammals, which contains 10 exons and 9 introns. Lc-TRAF3 was identified as cytosolic protein base on fluorescence microscopy analysis. Expression analysis revealed that Lc-TRAF3 was broadly distributed in examined organs/tissues, with the highest expression level in gill and weakest in brain, and could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation in vivo. Interestingly, overexpression Lc-TRAF3 could induce the activation of NF-κB, and Lc-TRAF3 co-transfected with Lc-TRIF induced a significantly higher level of NF-κB and IRF3 promoter activity, implying that Lc-TRAF3 may function as an enhancer in Lc-TRIF-mediated signaling pathway.

Skin-Associated B Cells in Health and Inflammation

Traditionally, the skin was believed to be devoid of B cells, and studies of the skin immune system have largely focused on other types of leukocytes. Exciting recent data show that B cells localize to the healthy skin of humans and other mammalian species with likely homeostatic functions in host defense, regulation of microbial communities, and wound healing. Distinct skin-associated B cell subsets drive or suppress cutaneous inflammatory responses with important clinical implications. Localized functions of skin-associated B cell subsets during inflammation comprise Ab production, interactions with skin T cells, tertiary lymphoid tissue formation, and production of proinflammatory cytokines but also include immunosuppression by providing IL-10. In this review, we delve into the intriguing new roles of skin-associated B cells in homeostasis and inflammation.

Current use and development of fish vaccines in China

In the past decades, the aquaculture industry made great progress in China, which contributes more than 70% yield of the world's farmed fish. Along with the rapid growth of fish production, increased emergence and outbreak of numbers of diseases pose harm to the aquaculture industry and food safety. From the efficient, safe, environmental and ethical aspects, vaccines is definitely the most appropriate and focused method to control different kinds of fish diseases. In China, researchers have done huge works on the fish vaccines, and so far six domestic aquatic vaccine products along with one imported aquatic vaccine have obtained the national veterinary medicine certificate. More critically, some new vaccines have also entered the field experiment stage and showed broad market prospects. In the present review, authors summarize seven aquatic vaccines, including the live vaccine against grass carp hemorrhagic disease, the inactivated vaccine against Aeromonas hydrophila sepsis in fish, the live vaccine against Edwardsiella tarda in turbot, the anti-idiotypic antibody vaccine against Vibrio alginolyticus, V. parahaemolyticus, and E. tarda in Japanese flounder, the cell-cultured inactivated vaccine against grass carp hemorrhagic disease, the inactivated vaccine against fish infectious spleen and kidney necrosis virus (ISKNV), and the genetically engineered live vaccine against V. anguillarum in turbot. Moreover, different delivery routes of fish vaccines are also compared in this review, along with differential fish immune response after vaccination. All these efforts will ultimately benefit the healthy and sustainable development of aquaculture industry in China.

Mucosal Vaccine Development for Veterinary and Aquatic Diseases

Because most pathogens and food antigens enter the host via the mucosal surfaces, effective mucosal immunity is critical for maintaining homeostasis through immune regulation, tolerance, and induction of effective immune responses when needed. Thus the mucosa-associated lymphoid tissues represent an important target for vaccination. Indeed, more than 20 years of research have clearly demonstrated the benefits of mucosal vaccination versus systemic vaccination. Such benefits include local induction of secretory immunoglobulin A (SIgA) as well as activation and maturation of mucosal dendritic cells, homing of effector cells to the mucosal surfaces, expression of specific host defense peptides, and other innate effector molecules. In addition, mucosal vaccination offers the opportunity to induce colostral and lactogenic immunity during pregnancy and the possibility of avoiding neutralization of early life vaccines by maternal antibodies, both of which are critical for protecting the most susceptible from infectious diseases. Moreover, mucosal administration offers the advantage of inducing both effective systemic immunity and mucosal immunity, enhancing vaccine efficacy and providing improved protection. A number of animal vaccines are already administered via the mucosal surfaces, with many more to come over the next few years. It is gratifying to see that veterinary vaccine development has yet again taken a leadership role in exploring innovative approaches and technologies to mucosal vaccination. For the veterinary field, considerations for mucosal vaccine development and use necessarily include costs (often pennies per dose), mass delivery that preferably avoids animal restraint, and economic and trade considerations. In this chapter, we provide an overview of some of the existing vaccine technologies and discuss their advantages and disadvantages.

Viral-Infected Change of the Digestive Tract Microbiota Associated With Mucosal Immunity in Teleost Fish

The digestive tract is a unique series of organs that is inhabited by a range of commensal microbes while also exposed to an overwhelming load of dietary antigens. It is widely known that mammals have evolved complex and efficient immune strategies to protect the mucosa of the digestive tract. However, in the early vertebrates, the roles of mucosal immune defense and microbial communities in the different segments of the digestive tract are not well-understood. Here, we constructed a bath infection model with infectious hematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss). Importantly, following viral infection, we found that the IHNV distribution and the reactions of immune-related genes had similar trends that decreased across the digestive tract. Hematoxylin and eosin (H & E) and alcian blue (A & B) staining of the trout digestive tract showed that the pathological changes only occurred in the buccal and pharyngeal mucosal tissues. Moreover, the increased diversity of the microbial community was only detected in the buccal mucosa through 16S rRNA gene sequencing, suggesting that the magnitude of the immune response and microbial community changes are related to the IHNV load and the original microbial diversity. In addition, the loss of digestive tract dominant species and increased colonization of opportunistic bacteria were discovered in the buccal mucosal surface indicating that a secondary bacterial infection occurred in this mucosal tissue.

Immune response of teleost fish to helminth parasite infection

Abstract Fish immune systems have become the subject of several studies due to the growing development of aquaculture and fisheries, and the demand for healthy produce for human consumption. Among the parasites responsible for diseases in fish farming, helminths stand out because they cause infections in farmed fish and decrease food conversion, zootechnical performance and meat quality. In the present review, the components that participate in the innate and adaptive immune responses of teleost fish that have so far been described are presented in order to summarize the defenses that these hosts have recourse to, in combating different groups of helminth parasites.

Advances in salmonid fish immunology: A review of methods and techniques for lymphoid tissue and peripheral blood leucocyte isolation and application

Evaluating studies over the past almost 40 years, this review outlines the current knowledge and research gaps in the use of isolated leucocytes in salmonid immunology understanding. This contribution focuses on the techniques used to isolate salmonid immune cells and popular immunological assays. The paper also analyses the use of leucocytes to demonstrate immunomodulation following dietary manipulation, exposure to physical and chemical stressors, effects of pathogens and parasites, vaccine design and application strategies assessment. We also present findings on development of fish immune cell lines and their potential uses in aquaculture immunology. The review recovered 114 studies, where discontinuous density gradient centrifugation (DDGC) with Percoll density gradient was the most popular leucocyte isolation method. Fish head kidney (HK) and peripheral blood (PB) were the main sources of leucocytes, from rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Phagocytosis and respiratory burst were the most popular immunological assays. Studies used isolated leucocytes to demonstrate that dietary manipulations enhance fish immunity, while chemical and physical stressors suppress immunity. In addition, parasites, and microbial pathogens depress fish innate immunity and induce pro-inflammatory cytokine gene transcripts production, while vaccines enhance immunity. This review found 10 developed salmonid cell lines, mainly from S. salar and O. mykiss HK tissue, which require fish euthanisation to isolate. In the face of high costs involved with density gradient reagents, the application of hypotonic lysis in conjunction with mico-volume blood methods can potentially reduce research costs, time, and using nonlethal and ethically flexible approaches. Since the targeted literature review for this study retrieved no metabolomics study of leucocytes, indicates that this approach, together with traditional technics and novel flow cytometry could help open new opportunities for in vitro studies in aquaculture immunology and vaccinology.

Review on Immersion Vaccines for Fish: An Update 2019

Immersion vaccines are used for a variety of aquacultured fish to protect against infectious diseases caused by bacteria and viruses. During immersion vaccination the antigens are taken up by the skin, gills or gut and processed by the immune system, where the resulting response may lead to protection. The lack of classical secondary responses following repeated immersion vaccination may partly be explained by the limited uptake of antigens by immersion compared to injection. Administration of vaccines depends on the size of the fish. In most cases, immersion vaccination is inferior to injection vaccination with regard to achieved protection. However, injection is problematic in small fish, and fry as small as 0.5 gram may be immersion vaccinated when they are considered adaptively immunocompetent. Inactivated vaccines are, in many cases, weakly immunogenic, resulting in low protection after immersion vaccination. Therefore, during recent years, several studies have focused on different ways to augment the efficacy of these vaccines. Examples are booster vaccination, administration of immunostimulants/adjuvants, pretreatment with low frequency ultrasound, use of live attenuated and DNA vaccines, preincubation in hyperosmotic solutions, percutaneous application of a multiple puncture instrument and application of more suitable inactivation chemicals. Electrostatic coating with positively charged chitosan to obtain mucoadhesive vaccines and a more efficient delivery of inactivated vaccines has also been successful.

Effect of Bacillus velezensis on Aeromonas veronii-Induced Intestinal Mucosal Barrier Function Damage and Inflammation in Crucian Carp (Carassius auratus)

Aeromonas veronii is an emerging aquatic pathogen causing hemorrhagic septicemia in humans and animals. Probiotic is an effective strategy for controlling enteric infections through reducing intestinal colonization by pathogens. Here we report that the consumption of Bacillus velezensis regulated the intestinal innate immune response and decreased the degree of intestinal inflammation damage caused by the A. veronii in Crucian carp. In this study, we isolated four strains of B. velezensis, named C-11, S-22, L-17 and S-14 from apparently healthy Crucian carp, which exerted a broad-spectrum antimicrobial activity inhibiting both Gram-positive and Gram-negative bacteria especially the fish pathogens. B. velezensis isolates showed typical Bacillus characteristics by endospore staining, physiological and biochemical test, enzyme activity analysis (amylase, protease, and lipase), and molecular identification. Here, Bacillus-containing dietary was orally administrated to Crucian carp for 8 weeks before A. veronii challenge. Immunological parameters and the expression of immune-related genes were measured at 2, 4, 6, 8, and 10 weeks post-administration. The results showed that B. velezensis was found to promote the increase in the phagocytic activities of peripheral blood leukocytes (PBLs) and head kidney leukocytes (HKLs), as well as the increase in interleukin 1β (IL-1β), IL-10 and tumor necrosis factor α (TNF-α) concentration of serum. Lysozyme levels (113.76 U/mL), ACP activity (25.32 U/mL), AKP activity (130.08 U/mL), and SOD activity (240.63 U/mL) were maximum (P < 0.05) in the B. velezensis C-11 treated group at 8 week. Our results showed that Crucian carp fed with the diet containing B. velezensis C-11 and S-22 developed a strong immune response with significantly higher (P < 0.05) levels of IgM in samples of serum, mucus of skin and intestine compared to B. velezensis L-17 and S-14 groups. Moreover, B. velezensis spores appeared to show no toxicity and damage in fish, which could inhabit the gut of Crucian carp. B. velezensis restrained up-regulation of pro-inflammation cytokines (IL-1β, IFN-γ, and TNF-α) mRNA levels in the intestine and head kidney at final stage of administration, and the expression of IL-10 was increased throughout the 10-week trial. A. veronii infection increased the population of inflammatory cells in the intestinal villi in the controls. In contrast, numerous goblet cells and few inflammatory cells infiltrated the mucosa in the B. velezensis groups after challenge with A. veronii. Compared with A. veronii group, B. velezensis could safeguard the integrity of intestinal villi. The highest post-challenge survival rate (75.0%) was recorded in B. velezensis C-11 group. The present data suggest that probiotic B. velezensis act as a potential gut-targeted therapy regimens to protecting fish from pathogenic bacteria infection.

IMPORTANCE

In this work, four Bacillus velezensis strains isolated from apparently healthy Crucian carp, which exhibited a broad-spectrum antibacterial activity especially the fish pathogens. Administration of B. velezensis induced the enhancement of the intestinal innate immune response through reducing intestinal colonization by pathogens. The isolation and characterization would help better understand probiotic can be recognized as an alternative of antimicrobial drugs protecting human and animal health.

Vaccines and immune protection of principal Mediterranean marine fish species

This review describes and summarizes the knowledge on established and experimental vaccines developed against viral and bacterial pathologies affecting the most important farmed marine finfish species present in the Mediterranean area, namely European seabass Dicentrarchus labrax, sea bream Sparus aurata, turbot Psetta maxima and meagre Argyrosomus regius. The diseases that have been recorded in seabass, sea bream and meagre are caused by bacteria Vibrio anguillarum, Photobacterium damselae, Tenacibaculum maritimum as well as by viruses such as Viral Encephalopathy and Retinopathy/Viral Nervous Necrosis and Lymphocystic disease. The main pathologies of turbot are instead bacteriosis provoked by Tenacibaculum maritimum, Aeromonas sp. and Vibrio anguillarum, and virosis by viral hemorrhagic septicaemia virus. Some vaccines have been optimized and are now regularly available for the majority of the above-mentioned pathogens. A measurable immune protection has been conferred principally against Vibrio anguillarum, Photobacterium damselae sub. piscicida and VER/VNN.

Transcriptomic characterization of adult zebrafish infected with Streptococcus agalactiae

Streptococcus agalactiae is a major aquaculture pathogen infecting various saltwater and freshwater fish. To better understand the mechanism of the immune responses to S. agalactiae in wildtype zebrafish, the transcriptomic profiles of two organs containing mucosal-associated lymphoid tissues from S. agalactiae-infected and non-infected groups were obtained using RNA-seq techniques. In the intestines, 6735 and 12908 differently expressed genes (DEGs) were identified at 24 hpi and 48 hpi, respectively. Among 66 and 116 significantly enriched pathways, 15 and 21 pathways were involved in immune system or signal transduction at 24 hpi and 48 hpi, respectively. A number of genes involved in Toll-like receptor signaling pathway, RIG-I-like receptor signaling pathway, NOD-like receptor signaling pathway, T cell receptor signaling pathway, B cell receptor signaling pathway, Antigen processing and presentation, NF-kappa B signaling pathway and PI3K-Akt signaling pathway were significantly downregulated. In the skins, 3113 and 4467 DEGs were identified at 24 hpi and 48 hpi, respectively. Among 24 and 56 significantly enriched pathways, 4 and 13 pathways were involved in immune system or signal transduction at 24 hpi and 48 hpi, respectively. More immune-related signaling pathways including Leukocyte transendothelial migration, Cytokine-cytokine receptor interaction, PI3K-Akt signaling pathway, IL-17 signaling pathway, MAPK signaling pathway, TNF signaling pathway, Complement and coagulation cascades, Hematopoietic cell lineage and Jak-STAT signaling pathway were differently enriched for upregulated DEGs at 48 hpi, which were completely different from that in the intestines. Furthmore, comparative transcriptome analysis revealed that the downregulated 1618 genes and upregulated 1622 genes existed both at 24 hpi and 48 hpi for the intestine samples. In the skins, the downregulated 672 genes and upregulated 428 genes existed both at 24 hpi and 48 hpi. Three pathways related to immune processes were significantly enriched for downregulated DEGs both in the intestines and skins collected at 24 hpi and 48 hpi, which included Antigen processing and presentation, Intestinal immune network for IgA production and Hematopoietic cell lineage. Interaction network analysis of DEGs identified the main DEGs in the sub-network of complement and coagulation cascades both in the intestines and skins. Twenty of DEGs involved in complement and coagulation cascades were further validated by Real-time quantitative PCR. Altogether, the results obtained in this study will provide insight into the immune response of zebrafish against S. agalactiae XQ-1 infection in fatal conditions, and reveal the discrepant expression pattern of complement and coagulation cascades in the intestines and skins.

Pharyngeal Immunity in Early Vertebrates Provides Functional and Evolutionary Insight into Mucosal Homeostasis

The pharyngeal organ is located at the crossroad of the respiratory and digestive tracts in vertebrate, and it is continuously challenged by varying Ags during breathing and feeding. In mammals, the pharyngeal mucosa (PM) is a critical first line of defense. However, the evolutionary origins and ancient roles of immune defense and microbiota homeostasis of PM are still unknown. In this study, to our knowledge, we are the first to find that diffuse MALT is present in PM of rainbow trout, an early vertebrate. Importantly, following parasitic infection, we detect that strong parasite-specific mucosal IgT and dominant proliferation of IgT⁺ B cell immune responses occurs in trout PM, providing, to our knowledge, the first demonstration of local mucosal Ig responses against pathogens in pharyngeal organ of a nonmammal species. Moreover, we show that the trout PM microbiota is prevalently coated with secretory IgT and, to a much lesser degree, by IgM and IgD, suggesting the key role of mucosal Igs in the immune exclusion of teleost pharyngeal bacteria. Overall, to our knowledge, our findings provide the first evidence that pharyngeal mucosal immunity appear earlier than tetrapods.

A Comparison of the Innate and Adaptive Immune Systems in Cartilaginous Fish, Ray-Finned Fish, and Lobe-Finned Fish

The immune system is composed of two subsystems-the innate immune system and the adaptive immune system. The innate immune system is the first to respond to pathogens and does not retain memory of previous responses. Innate immune responses are evolutionarily older than adaptive responses and elements of innate immunity can be found in all multicellular organisms. If a pathogen persists, the adaptive immune system will engage the pathogen with specificity and memory. Several components of the adaptive system including immunoglobulins (Igs), T cell receptors (TCR), and major histocompatibility complex (MHC), are assumed to have arisen in the first jawed vertebrates-the Gnathostomata. This review will discuss and compare components of both the innate and adaptive immune systems in Gnathostomes, particularly in Chondrichthyes (cartilaginous fish) and in Osteichthyes [bony fish: the Actinopterygii (ray-finned fish) and the Sarcopterygii (lobe-finned fish)]. While many elements of both the innate and adaptive immune systems are conserved within these species and with higher level vertebrates, some elements have marked differences. Components of the innate immune system covered here include physical barriers, such as the skin and gastrointestinal tract, cellular components, such as pattern recognition receptors and immune cells including macrophages and neutrophils, and humoral components, such as the complement system. Components of the adaptive system covered include the fundamental cells and molecules of adaptive immunity: B lymphocytes (B cells), T lymphocytes (T cells), immunoglobulins (Igs), and major histocompatibility complex (MHC). Comparative studies in fish such as those discussed here are essential for developing a comprehensive understanding of the evolution of the immune system.

Effects of Assam tea extract on growth, skin mucus, serum immunity and disease resistance of Nile tilapia (Oreochromis niloticus) against Streptococcus agalactiae

The present study aimed to assess the possible effects of Assam tea (Camellia sinensis) extract (ATE) on growth performances, immune responses, and disease resistance of Nile tilapia, Oreochromis niloticus against Streptococcus agalactiae. Five levels of ATE were supplemented into the based diet at 0, 1, 2, 4, and 8 g kg^-1 feed of Nile tilapia fingerlings (10.9 ± 0.04 g initial weight) in triplicate. After four and eight weeks of feeding, fish were sampled to determine the effects of the tea supplements upon their growth performance, as well as serum and mucosal immune responses. A disease challenge using S. agalactiae was conducted at the end of the feeding trial. Fish fed ATE revealed significantly improved serum lysozyme, peroxidase, alternative complement (ACH50), phagocytosis, and respiratory burst activities compared to the basal control fed fish (P < 0.05). The mucus lysozyme and peroxidase activities were ameliorated through ATE supplementation in the tilapia diets. Supplementation of ATE significantly (P < 0.05) enhanced final body weight, weight gain, and specific growth rate; while a decreased feed conversion ratio was revealed at 2 g kg^-1 inclusion level, after four and eight weeks. Challenge test showed that the relative percent survival (RSP) of fish in each treatment was 33.33%, 60.00%, 83.33%, 76.68%, and 66.68% in groups fed 0, 1, 2, 4, and 8 g kg^-1, respectively. In summary, diets supplemented with ATE especially at 2 g kg^-1 increased the humoral and mucosal immunity, enhanced growth performance, and offered higher resistance against S. agalactiae infection in Nile tilapia.

Optimal dietary alcoholic extract of lotus leaf improved growth performance and health status of grass carp (Ctenopharyngodon idellus)

Grass carp (Ctenopharyngodon idellus) is one of the most important aquaculture fish in China. This study tried to explore the effects of dietary alcoholic extract of lotus leaf (AELL) addition on the growth performance and health status of grass carp by feeding juvenile fish (average weight: 34 ± 1 g) with four different experimental diets: control, AELL7, AELL14 and AELL21 for 8 weeks. At the end of the growth trial, the highest values of final body weight (FBW), weight gain rate (WGR), specific growth rate (SGR) and feed intake (FI) all occurred in group AELL14 (P < 0.05). Compared to control, the crude lipid content of whole-body and the serum malondialdehyde (MDA) in the three experimental groups decreased, while the serum superoxide dismutase (SOD), glutathione peroxidase (GSH) and total antioxidant capacity (T-AOC) values almost all increased in the three experimental groups. The highest serum immunoglobulin M (IgM) concentration occurred in AELL14 group (P < 0.05). In AELL14 and AELL21 groups, both the serum complement 3 (C3) concentration and lysozyme (LYS) activity were significantly higher, whereas the final cumulative mortality in challenge test was significantly lower, when compared to those in control group (P < 0.05). The AELL exerted dose-dependent beneficial effects on grass carp health through up-regulating related gene expressions and enzyme activity. In conclusion, the optimal dietary AELL level is 0.14% for juvenile grass carp.

Screening of immuno-modulatory potential of different herbal plant extracts using striped catfish (Pangasianodon hypophthalmus) leukocyte-based in vitro tests

Many medicinal plants have been shown to possess biological effects, including immuno-modulatory activities on human and other mammals. However, studies about the potential mechanisms of plant extracts on the humoral and tissular immunities in fish have received less attention. This study aimed to screen the immunestimulating properties of 20 ethanol plant extracts on striped catfish Pangasianodon hypophthalmus leukocytes. The peripheral blood mononuclear cells (PBMCs) and head kidney leukocytes (HKLs) of striped catfish (50 ± 5 g per fish) were stimulated at 10 and 100 μg of each plant extract per mL of cell culture medium. Several humoral immune parameters (lysozyme, complement and total immunoglobulin) were examined at 24-h post stimulation (hps). Furthermore, the responses of four cytokine genes, namely il1β, ifrγ 2a and b, and mhc class II were assessed by quantitative real-time PCR at 6, 12, 24, and 48 hps. The results showed that lysozyme, complement as well as total immunoglobulin levels in both PBMCs and HKLs were regulated by some of the plant extracts tested in a concentration-dependent manner; some plant extracts induced the highest immune responses at the low dose (10 μg mL^-1) while others were more efficient at high dose (100 μg mL^-1). Among the extracts, five extracts including garlic Allium sativum L. (As), neem Azadirachta indica A. Juss (Ai), asthma-plant Euphorbia hirta L. (Eh), bhumi amla Phyllanthus amarus Schum. et Thonn (Pa), and ginger Zingiber officinale Rosc (Zo) induced significant changes in the expression of pro-inflammatory cytokine (il1β), antiviral cytokines (ifrγ 2a and b) and adaptive immune cytokine (mhc class II) in striped catfish cells. Pa always modulated the strongest expression of the four cytokines in PBMCs and HKLs over the whole experimental period (p < 0.05), whereas Zo did not stimulate the mhc class II expression in striped catfish leukocytes throughout experimental periods. These in vitro results demonstrated that some plant extracts could differently modulate great potential immune response in fish, supporting their applications in further in vivo experiments.

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

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

Glutamine protects against LPS-induced inflammation via adjusted NODs signaling and enhanced immunoglobulins secretion in rainbow trout leukocytes

To evaluate effects of glutamine (GLN) on fish immune responses, leukocytes were isolated from head kidney of rainbow trout and cultured in GLN-free DMEM media supplemented with different combinations of lipopolysaccharide (LPS) and GLN. LPS significantly increased expression of pro-inflammatory cytokines, while GLN supplementation alleviated LPS-induced inflammation. Leukocytes in +GLN + LPS group showed more active GLN anabolism and catabolism, which signals could be sensed by O-GlcNAcylation, and then affected LPS binding to cell surface (LBP) and adjusted NODs signaling. The mRNA expression of immunoglobulins (Igs) and their receptor (pIgR) was also significantly increased after GLN supplementation. Further analysis showed that GLN increased the percentage of IgM⁺ B cells and IgT⁺ B cells, accompanied with the increased IgM and IgT secretion in culture media, which further increased complement C3 expression to perform effector functions. All these results illustrated the regulating mechanism of GLN against LPS-induced inflammation both via adjusted NODs signaling and increased Igs⁺ B cells to secrete Igs.

Nasal Vaccination Drives Modifications of Nasal and Systemic Antibody Repertoires in Rainbow Trout

Bony fish represent the most basal vertebrate branch with a dedicated mucosal immune system, which comprises immunologically heterogeneous microenvironments armed with innate and adaptive components. In rainbow trout (Oncorhynchus mykiss), a nasopharynx-associated lymphoid tissue (NALT) was recently described as a diffuse network of myeloid and lymphoid cells located in the olfactory organ of fish. Several studies have demonstrated high levels of protection conferred by nasal vaccines against viral and bacterial pathogens; however, the mechanisms underlying the observed protection are not well understood. We applied 5'RACE and a deep sequencing-based approach to investigate the clonal structure of the systemic and mucosal rainbow trout B cell repertoire. The analysis of Ig repertoire in control trout suggests different structures of IgM and IgT spleen and NALT repertoires, with restricted repertoire diversity in NALT. Nasal and injection vaccination with a bacterial vaccine revealed unique dynamics of IgM and IgT repertoires at systemic and mucosal sites and the remarkable ability of nasal vaccines to induce spleen Ig responses. Our findings provide an important immunological basis for the effectiveness of nasal vaccination in fish and other vertebrate animals and will help the design of future nasal vaccination strategies.

Molecular cloning and functional characterization of TRIF in large yellow croaker Larimichthys crocea

As an adaptor in Toll-like receptor (TLR) signaling pathway, Toll/interleukin-1 receptor (TIR) domain containing adaptor inducing interferon-β (TRIF) mediates downstream signaling cascades and plays important roles in host innate immune responses. In the present study, a TRIF ortholog named Lc-TRIF was identified in large yellow croaker (Larimichthys crocea). Sequence comparison analysis revealed that Lc-TRIF has a conserved TIR domain but without TRAF6 binding motif. The genome structure of Lc-TRIF is conserved, with two exons and one intron. Syntenic comparison showed that the loci of fish TRIF was different from that in mammals or birds, and TRAM was absent in the genomes of fish, amphibians, and birds, but present in mammals and reptiles. Expression analysis revealed that Lc-TRIF was broadly expressed in examined organs/tissues, with the highest expression level in gill and weakest in brain, and could be up-regulated under poly I:C, LPS, PGN, and Pseudomonas plecoglossicida stimulation. Fluorescence microscopy results showed that Lc-TRIF exhibited a global localization throughout the entire cell including the nucleus in HEK 293T cells. Additionally, luciferase assays demonstrated that Lc-TRIF expression could significantly induce NF-κB, type I IFN, IRF3 as well as IRF7 promoter activation. These results collectively indicated that Lc-TRIF was function in host antiviral and antibacterial responses via NF-κB and IRF3/7 related signaling pathway.

Nanoencapsulation of inactivated-viral vaccine using chitosan nanoparticles: Evaluation of its protective efficacy and immune modulatory effects in olive flounder (Paralichthys olivaceus) against viral haemorrhagic septicaemia virus (VHSV) infection

Viral haemorrhagic septicaemia virus (VHSV), a (-) ssRNA virus belonging to the genus Novirhabdovirus of rhabdoviridae family, is the aetiological agent of viral haemorrhagic septicaemia (VHS) disease which causes huge economic losses in farmed olive flounder (Paralichthys olivaceus) and significant mortalities among several other marine fish species in Korea, Japan, and China. Previously, we developed an inactivated vaccine viz., formalin-inactivated VHSV mixed with squalene as adjuvant which was effective in conferring protective immunity (58-76% relative percentage survival) against VHSV but the mode of administration was intraperitoneal injection which is not feasible for small sized fingerling fish. To overcome this limitation, we presently focused on replacing the injection route of vaccine delivery by oral and immersion routes. In this context, we encapsulated the inactivated VHSV vaccine with chitosan nanoparticles (CNPs-IV) by water-in-oil (W/O) emulsification method. After encapsulation, two sets of in vivo vaccination trials were conducted viz., preliminary trial-I and final trial-II. In preliminary trial-I, olive flounder fingerlings (10.5 ± 1.7 g) were vaccinated with CNPs-IV by different delivery strategies involving oral and immersion routes (single/booster dose) followed by challenge with VHSV (1 × 10⁶ TCID₅₀ virus/fish) to evaluate an effective method amongst different applied delivery strategies. Subsequently, a final trial-II was conducted to better understand the immune mechanism behind the efficacy of the employed delivery strategy and also to further improvise the delivery mechanism with prime-boost (primary immersion and oral boosting) combination in order to improve the transient anti-VHSV response in the host. Evaluation of RPS analysis in trial-I revealed higher RPS of 46.7% and 53.3% in the CNPs-IV (immersion) and CNPs-IV (immersion/immersion) groups, respectively compared to 0% RPS in the CNPs-IV (oral) group and 20% RPS in the CNPs-IV (oral/oral) group when calculated against 100% cumulative mortality percentage in the NVC (non-vaccinated challenged) control group, whereas, in the trial-II, RPS of 60% and 66.6% were obtained for CNPs-IV (immersion/immersion) and CNPs-IV (immersion/oral) groups, respectively. In addition, specific (anti-VHSV) antibody titre in the fish sera, skin mucus and intestinal mucus of the immunized groups were significantly (p < 0.05) enhanced following vaccination. Furthermore, CNPs-IV immunized fish showed significant (p < 0.05) upregulation of different immune gene transcripts (IgM, IgT, pIgR, MHC-I, MHC-II, IFN-γ, and Caspase3) compared to control, in both the systemic (kidney) and mucosal (skin and intestine) immune compartments of the host post immunization as well as post challenge. To conclude, mucosal immunization with CNPs-IV vaccine can orchestrate an effective immunization strategy in organizing a coordinative immune response against VHSV in olive flounder thereby exhibiting higher protective efficacy to the host with minimum stress.

The key differentially expressed genes and proteins related to immune response in the spleen of pufferfish (Takifugu obscurus) infected by Aeromonas hydrophila

The immune mechanism elicited in pufferfish (Takifugu obscurus) against the invasion of Aeromonas hydrophila is still poorly understood. We examined the spleen of pufferfish at the transcriptome and proteome levels by using Illumina-seq and TMT coupled mass spectrometry after 12 h infection by A. hydrophila, respectively. A total of 2,339 genes (1,512 up-regulated and 827 down-regulated) and 537 (237 up-regulated and 300 down-regulated) proteins were identified. GO and KEGG analyses revealed that the responses to stimulus were the main biological processes, intestinal immune network for IgT production and calcium signaling pathway. Fourteen genes (8 up-regulated and 6 down-regulated) and proteins (5 up-regulated and 9 down-regulated) involved immune responses or signal transduction were validated by qRT-PCR and parallel reaction monitoring to confirm the reliability of the transcriptomic and proteomic analyses, respectively. Moreover, qRT-PCR and flow cytometry were used to detect dynamics of the genes in calcium signaling pathway and changes of concentration of cytoplasm Ca²⁺ in spleen cells within a 72 h challenge. This study provides the findings regarding immune response, especially intestinal immune network for IgT production pathway and calcium signaling pathway at the molecular, protein and cellular in pufferfish after infection by A. hydrophila. These results would provide a new insight and molecular targets into the response to pathogenic infection in pufferfish.

Development of a live vector vaccine against infectious hematopoietic necrosis virus in rainbow trout

Infectious hematopoietic necrosis virus (IHNV) leads to serious disease and economic losses in the salmonid aquaculture industry. The present study aimed to develop an effective and efficient vaccine to protect rainbow trout (Oncorhynchus mykiss) against IHNV infection. Administered via the immersion route, a live vector vaccine containing the regions of the IHNV glycoprotein (G) induced immune responses in rainbow trout. Use of the immersion route induced more-efficient mucosal immunity than intramuscular injection vaccination. IHNV G gene expression was detected in the spleens of rainbow trout at 3, 7 and 15 days post-vaccination (dpv). The G gene expression continuously decreased between 3 and 15 dpv. In addition, the expression of TLR-3, TLR-7 and TLR-8 was upregulated after vaccination, and the highest expression levels of IFN-1, Mx-1, Mx-3, Vig-1 and Vig-2 were observed at 3 dpv. Four markers of the adaptive immune response (CD4, CD8, IgM and IgT) gradually increased. When experimental fish were challenged with IHNV by immersion, significant differences in cumulative percentage mortality were observed in the vaccinated fish and the unvaccinated (empty-plasmid-vaccinated) fish. The relative survival rate was 92% and 6% in the vaccinated group and empty-plasmid group, respectively. Serum antibody levels gradually increased in the vaccinated fish, unlike in the unvaccinated fish, after 7 dpv. Our results suggest there was a significant increase in fish immune responses and resistance to infection with IHNV following administration of the live vector vaccine. Therefore, this live vector vaccine is a promising vaccine that may be utilized to protect rainbow trout against IHNV.

A recombinant adenovirus targeting typical Aeromonas salmonicida induces an antibody-mediated adaptive immune response after immunization of rainbow trout

Aeromonas salmonicida, the oldest known fish pathogen and currently endemic throughout most of the world in both fresh and marine waters, causes severe economic losses to the salmon farming industry. Although there have been many studies on the prevention of furunculosis over the past few decades, it is still prevalent in many fisheries. In this study, a recombinant adenovirus vaccine candidate harboring the highly immunogenic Vapa gene (pAd-easy-cmv-Vapa) was successfully constructed and tested. The immune protection rate and specific antibody levels in the peripheral blood were then determined after immunizing rainbow trout. In addition, relative levels of IgM and IgT in the head kidney and hindgut before and after immunization were measured by quantitative reverse transcription PCR. Western blotting results indicated that the recombinant adenovirus could infect HEK-293 cells and express the A layer protein (encoded by Vapa). Further, survival analysis of fish 28 days after challenge showed that immunization significantly lowered the mortality rate (40%) compared to that in the control group (76.6%) and empty vector group (73.6%). This also led to an increase in specific antibodies in peripheral serum. In addition, levels of IgM and IgT in the head kidney and hindgut were increased to varying degrees. In conclusion, our research provides a candidate vaccine for the prevention of Aeromonas salmonicida A450 infection in rainbow trout and lays the foundation for future research on adaptive immune mechanisms associated with rainbow trout antibodies.

Molecular characterization and expression analysis of secretory immunoglobulin M (IgM) heavy chain gene in rohu, Labeo rohita

Immunoglobulin M (IgM) is the major isotype among teleost immunoglobulins. The present study was aimed to explore IgM heavy chain gene and its expression profile in rohu. Full-length IgM heavy chain cDNA of rohu consisted of 1994 bp encoding a polypeptide of 576 amino acid residues including a leader peptide, variable (VH) and constant (CH1-CH2-CH3-CH4) domains confirming the secretory form of IgM. The sequence carries conserved residues such as cysteine, tryptophan and amino acid motifs like 'YYCAR' and 'FDYWGKGT-VTV-S'. The predicted 3 D model confirmed various domains of rohu IgM heavy chain. Phylogenetic tree analysis revealed that IgM heavy chain gene of rohu shared the same cluster with that of other cyprinid fishes. Tissue distribution analysis showed the predominant level of IgM heavy chain gene expression in kidney, spleen and intestine. IgM heavy chain gene expression in rohu kidney was found to be up-regulated and reached a maximum at 7 days post-challenge with Aeromonas hydrophila. These findings demonstrate the first report of full-length secretory IgM heavy chain gene in rohu. Besides, IgM heavy chain gene was highly expressed in major lymphoid tissues and bacterial challenge influenced its expression which further confirmed its role in the adaptive humoral immune response.

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

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

Molecular cloning of polymeric immunoglobulin receptor-like (pIgRL) in flounder (Paralichthys olivaceus) and its expression in response to immunization with inactivated Vibrio anguillarum

In the present work, the polymeric immunoglobulin receptor-like (pIgRL) from flounder (Paralichthys olivaceus) was firstly cloned and identified. The full length cDNA of flounder pIgRL was of 1393 bp including an open reading frame of 1053 bp, and the deduced pIgRL sequence encoded 350 amino acids, with a predicted molecular mass of 39 kDa. There were two immunoglobulin-like domains in flounder pIgRL. In healthy flounder, the transcriptional level of pIgRL was detected in different tissues by real-time PCR, showing the highest level in the skin and gills, and higher levels in the spleen and hindgut. After flounders were vaccinated with inactivated Vibrio anguillarum via intraperitoneal injection and immersion, the pIgRL mRNA level increased firstly and then declined in all tested tissues during 48 h, and the maximum expression levels in the gills, skin, spleen and hindgut in immersion group, or in the spleen, head kidney, skin and gills in injection group, were higher than in other tested tissues. In addition, recombinant protein of the extracellular region of flounder pIgRL was expressed in Escherichia coli BL21 (DE3), and rabbit anti-pIgRL polyclonal antibodies were prepared, which specifically reacted with the recombinant pIgRL, and a 39 kDa protein confirmed as natural pIgRL by liquid chromatography-mass spectrometry in skin mucus of flounder. Co-immunoprecipitation assay and western-blotting demonstrated that the pIgRL, together with IgM, could be immunoprecipitated by anti-pIgRL antibody in gut, skin and gill mucus of flounder, suggesting the existence of pIgRL-IgM complexes. These results indicated that the flounder pIgRL was probably involved in the mucosal IgM transportation and played important roles in mucosal immunity.

Antiviral defense in salmonids - Mission made possible?

Viral diseases represent one of the major threats for salmonid aquaculture. Survival from viral infections are highly dependent on host innate antiviral immune defense, where interferons are of crucial importance. Neutralizing antibodies and T cell effector mechanisms mediate long-term antiviral protection. Despite an immune cell repertoire comparable to higher vertebrates, farmed fish often fail to mount optimal antiviral protection. In the quest to multiply and spread, viruses utilize a variety of strategies to evade or escape the host immune system. Understanding the specific interplay between viruses and host immunity at depth is crucial for developing successful vaccination and treatment strategies in mammals. However, this knowledge base is still limited for pathogenic fish viruses. Here, we have focused on five RNA viruses with major impact on salmonid aquaculture: Salmonid alphavirus, Infectious salmon anemia virus, Infectious pancreatic necrosis virus, Piscine orthoreovirus and Piscine myocarditis virus. This review explore the protective immune responses that salmonids mount to these viruses and the existing knowledge on how the viruses counteract and/or bypass the immune response, including their IFN antagonizing effects and their mechanisms to establish persisting infections.

Intestinal immunity of dogfish Scyliorhinus canicula spiral valve: A histochemical, immunohistochemical and confocal study

The present study describes histochemical and immunohistochemical characteristics of the spiral valve and its associated lymphoid tissue (GALT) in the dogfish Scyliorhinus canicula. The mucosal surface of the spiral valve represents the first line of defense against pathogens coming from the external environment through food. Epithelial, mucus and immune cells play a key role in controlling the inflammatory response. Valve intestine of S. canicula had many folds lined by simple columnar cells and goblet cells, which later reacted positive to PAS, AB and AB-PAS, histochemical stains differentiated the different types of mucins; lectin histochemistry (PNA and WGA), detected neutral and acid mucins secreted that plays an important role in protection against invading pathogens. Integrin α5β1 was expressed in enterocytes that line the valve's folds with greater marking in the apical part of the cells. Laminin was found on the apical side of the epithelium, in fibrillar and cellular elements of the lamina propria and in the muscularis mucosa. In the spiral valve gut-associated lymphoid tissue (GALT) has been studied. For the first time, massive leucocytes aggregates were identified by confocal immunofluorescence techniques, using the following antibodies: TLR2, S100, Langerin/CD207. Our results expand knowledge about Dogfish valve intestine giving important news in understanding comparative immunology.

Mucosal and systemic immune responses in Senegalese sole (Solea senegalensis Kaup) bath challenged with Tenacibaculum maritimum: A time-course study

Tenacibaculosis, caused by Tenacibaculum maritimum, continues to inflict substantial losses among cultured marine species, particularly in the Senegalese sole. However, the immune mechanisms in fish involved in fighting against this disease are still poorly understood. Thus, the present study aimed to investigate the skin mucus's terminal carbohydrate composition, several immune-related enzymes (i.e. lysozyme, peroxidase, proteases and antiproteases), the haemolytic activity of complement and the bactericidal activity in the skin mucus and plasma of the Senegalese sole in a time-course study following a bath challenge with T. maritimum. The haematological profile and the kinetics of cell migration post-infection were also considered. The bath challenge induced slight variations in the terminal carbohydrate composition of Senegalese sole skin mucus. In general, results from this study showed a delay in the mucosal immune response compared to that found at the systemic level (i.e. blood and plasma). For instance, a significant increase in the skin mucus's lysozyme, complement, protease and antiprotease activities were observed at the end of the experiment (14 d post-challenge). Interestingly, the higher activity of these enzymes could be related to the skin mucus's bactericidal capacity and haemolytic complement activity, suggesting that these enzymes play an important role in the defence against Gram-negative bacteria. The haematological profile revealed a significant increase in circulating neutrophils in challenged fish after 48 and 72 h, which was positively correlated to the increments observed in peroxidase and lysozyme activities, respectively, in the plasma of challenged fish at the same time. Although the route of entry and the survival strategy of T. maritimum are still not fully elucidated, results from the present study will contribute to this endeavour through the study of the mucosal immune responses of fish against this particular pathogen.

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.

Role of teleost B cells in viral immunity

Teleost fish possess all the necessary elements to mount an adaptive immune response. Despite this, the important physiological and structural differences between the mammalian and the teleost fish immune system, anticipate significant changes regarding how this response is coordinated and executed. B cells are key players in adaptive immune responses through the production of antibodies. However, recent studies performed in mammals and other species including fish point to many additional functions of B cells within both the adaptive and the innate immune system, in many occasions taking part in the crosstalk between these two arms of the immune response. Furthermore, it should be taken into account that fish B cells share many functional and phenotypical features with innate B cell populations from mammals, which will surely condition their response to antigens. Concerning viral infections, although most studies undertaken to date in fish have been focused on characterizing antibody production, some recent studies have demonstrated that fish B cells are able to interact with viruses at different levels. In this sense, in the current review, we have tried to provide an overview of what is currently known regarding the role of teleost B cells in antiviral immunity.

Pichia pastoris yeast as a vehicle for oral vaccination of larval and adult teleosts

Oral vaccination is of major interest because it can be used for mass vaccination of fish of various size and age. Given that their administration is relatively easy and stress-free, oral vaccines have both economic and animal welfare benefits. Yet, mostly due to their limited efficacy, only very few oral vaccines are available to aquaculture industry. Here we present a method for oral vaccine delivery based on the yeast Pichia pastoris. We could express a model antigen, green fluorescent protein (GFP), in this yeast and subsequently show delivery of the GFP protein to the intestine of juvenile flounder or adult carp and trout. We tested this approach in several commercially-relevant fish species, from juvenile to adult stage. To test the oral delivery of antigen to larval fish, the GFP-expressing Pichia pastoris was first fed to planktonic crustacean Daphnia or rotifers that served as 'bioencapsulation vehicles' and afterwards, fed to flounder larvae. Again, we could show delivery of intact GFP protein to the intestine. In rainbow trout, the orally-administered GFP-expressing yeast elicited a rapid local innate immune response in the intestine and a subsequent systemic response in the spleen. Our results show that Pichia pastoris is a good vehicle for oral antigen delivery and that it can be used in non-encapsulated form for older fish or in bioencapsulated form for larval fish. We discuss the immunomodulatory properties of the yeast itself, and its potential to enhance local immune responses and act as an adjuvant.

PLGA encapsulated inactivated-viral vaccine: Formulation and evaluation of its protective efficacy against viral haemorrhagic septicaemia virus (VHSV) infection in olive flounder (Paralichthys olivaceus) vaccinated by mucosal delivery routes

Viral haemorrhagic septicaemia virus (VHSV), an OIE listed viral pathogen, is the etiological agent of a contagious disease, causing huge economic losses in farmed olive flounder (Paralichthys olivaceus) and significant mortalities among several other marine fish species in Korea, Japan, and China. In continuation with our previous work, where injection vaccination with inactivated VHSV mixed with squalene (as adjuvant) conferred higher protective immunity to olive flounder, the present study focused on replacing the injection route of vaccine delivery by immersion/oral route to overcome the limitations of the parenteral immunization method. Here, we encapsulated the inactivated VHSV vaccine with PLGA (poly lactic-co-glycolic acid) nanoparticles (PNPs-IV) and evaluated its ability to induce protective immunity in olive flounder (12.5 ± 1.5 g) by initially immunizing the fishes by immersion route followed by a booster with the same dose two weeks later with half of the fish through immersion route and other half through oral route (incorporated into fish feed). Cumulative mortalities post-challenge (1 × 10⁶ TCID₅₀ virus/fish) with virulent VHSV-isolate, were lower in vaccinated fish and RPS of 60% and 73.3% were obtained for PNPs-IV (immersion/immersion) and PNPs-IV (immersion/oral) groups, respectively. In addition, specific (anti-VHSV) antibody titre in the fish sera, skin mucus and intestinal mucus of the immunized groups were significantly (p < 0.05) enhanced following vaccination. Furthermore, PNPs-IV immunized fish showed significant (p < 0.05) upregulation of different immune gene transcripts (IgM, IgT, pIgR, MHC-I, MHC-II, IFN-γ, and Caspase3) compared to controls, in both the systemic (kidney) and mucosal (skin and intestine) immune compartment of the host post immunization as well as post challenge. Thus it can be inferred that the adopted immunization strategy efficiently protected and transported the inactivated viral antigen to target immune organs and positively stimulated the protective immune response against VHSV in olive flounder.

Smell of Infection: A Novel, Noninvasive Method for Detection of Fish Excretory-Secretory Proteins

Chemical signals are produced by aquatic organisms following predatory attacks or perturbations such as parasitic infection. Ectoparasites feeding on fish hosts are likely to cause release of similar alarm cues into the environment due to the stress, wounding, and immune response stimulated upon infection. Alarm cues are often released in the form of proteins, antimicrobial peptides, and immunoglobulins that provide important insights into bodily function and infection status. Here we outline a noninvasive method to identify potential chemical cues associated with infection in fish by extracting, purifying, and characterizing proteins from water samples from cultured fish. Gel free proteomic methods were deemed the most suitable for protein detection in saline water samples. It was confirmed that teleost proteins can be characterized from water and that variation in protein profiles could be detected between infected and uninfected individuals and fish and parasite only water samples. Our novel assay provides a noninvasive method for assessing the health condition of both wild and farmed aquatic organisms. Similar to environmental DNA monitoring methods, these proteomic techniques could provide an important tool in applied ecology and aquatic biology.

Molecular identification and expression analysis of TAB1 from orange-spotted grouper (Epinephelus coioides)

Transforming growth factor-β activated kinase 1 (TAK1) is a crucial signal transducer in multiple signaling pathways. TAK1 binds TAB1, TAB2, and TAB3, which act as activators and adaptors that specifically regulate the activation of TAK1. To date, the role of TABs is largely unknown in fish. In the present study, a TAB1 cDNA sequence was identified in grouper (Epinephelus coioides), and designated EcTAB1. The full-length open reading frame of EcTAB1 is 1, 521 bp; it encodes 506 amino acids that contains an N-terminal PP2C domain. Many important functional sites in mammalian TAB1 were conserved in TAB1 from grouper and from other fish. Multiple sequence alignment showed that EcTAB1 protein shared high sequence identity with TAB1 of other fish, especially with Stegastes partitus (95% identity). TAB1 was clustered into the same subgroup with other fish TAB1 in the phylogenetic tree. Tissue expression analysis indicated that TAB1 was widely distributed in different tissues. After infection with Cryptocaryon irritans, EcTAB1 expression was up-regulated in the infection site (gills). Besides, EcTAB1 was expressed throughout the grouper spleen (GS) cells and significantly enhanced the activation of NF-κB.

The proliferation and clonal migration of B cells in the systemic and mucosal tissues of channel catfish suggests there is an interconnected mucosal immune system

IgM transcripts from different mucosal and systemic tissues from a single adult channel catfish have been evaluated. Arrayed heavy chain cDNA libraries from each of these different mucosal and systemic tissues were separately constructed, hybridized with VH family specific probes and a variety of approaches were used to define their structural relationships. Baseline hybridization studies indicated that the tissue libraries had different VH expression patterns, and sequencing studies indicated this was not simply due to varying proportions of the same B cell population. In the systemic tissues of PBL, spleen, and anterior kidney >95% of the sequenced clones in the arrayed libraries represented different heavy chain rearrangements. Diversity was also found in the mucosal libraries of skin, gill lamellae, and two non-adjoining regions of the intestine, but additional populations were identified which indicated localized clonal expansion. Various clonal sets were characterized in detail, and their genealogies indicated somatic mutation accompanied localized clonal expansion with some members undergoing additional mutations and expansion after migration to different mucosal sites. PCR analyses indicated these mucosal clonal sets were more abundant within different mucosal tissues rather than in the systemic tissues. These studies indicate that the mucosal immune system in fish can express B cell transcripts differently from those found systemically. These studies further indicate that the mucosal immune system is interconnected with clonal B cells migrating between different mucosal tissues, results which yield new insight into immune diversity in early vertebrate phylogeny.

Oral delivery of Bacillus subtilis spores expressing grass carp reovirus VP4 protein produces protection against grass carp reovirus infection

Grass carp (Ctenopharyngodon idellus) hemorrhagic disease (GCHD), caused by grass carp reovirus (GCRV), has given rise to an enormous loss in grass carp industry during the past years. Up to date, vaccination remained to be the most effective way to protect grass carp from GCHD. Oral vaccination is of major interest due to its advantages of noninvasive, time-saving, and easily-operated. The introduction of oral vaccination has profound impact on aquaculture industry because of its feasibility of extensive application for fish in various size and age. However, the main challenge in developing oral vaccine is that antigens are easily degraded and are easy to induce tolerance. Bacillus subtilis (B. subtilis) spores would be an ideal oral vaccine delivery system for their robust specialty, gene operability, safety and adjuvant property. VP4 protein is the major outer capsid protein encoded by GCRV segment 6 (S6), which plays an important role in viral invasion and replication. In this study, we used B. subtilis spores as the oral delivery system and successfully constructed the B. subtilis CotC-VP4 recombinant spores (CotC-VP4 spores) to evaluate its protective efficacy in grass carp. Grass carp orally immunized with CotC-VP4 spores showed a survival rate of 57% and the relative percent survival (RPS) of 47% after the viral challenge. Further, the specific IgM levels in serum and the specific IgZ levels in intestinal mucus were significantly higher in the CotC-VP4 group than those in the Naive group. The immune-related genes including three innate immune-related genes (IL-4/13A, IL-4/13B, CSF1R), four adaptive immune-related genes (BAFF, CD4L, MHC-II, CD8), three inflammation-related genes (IL-1β, TNF-α, TGF-β) and interferon type I (IFN-I) related signaling pathway genes were significantly up-regulated in the CotC-VP4 group. The study demonstrated that the CotC-VP4 spores produced protection in grass carp against GCRV infection, and triggered both innate and adaptive immunity post oral immunization. This work highlighted that Bacillus subtilis spores were powerful platforms for oral vaccine delivery, and the combination of Bacillus subtilis spores with GCRV VP4 protein was a promising oral vaccine.

Antibody responses to chimeric peptides derived from parasite antigens in mice and other animal species

Peptide vaccines constitute an interesting alternative to classical vaccines due to the possibility of selecting specific epitopes, easy of production and safety. However, an inadequate design may render these peptides poorly immunogenic or lead to undesirable outcomes (e.g., formation of B neoepitopes). As an approach to vaccine development, we evaluated the antibody response to chimeras composed of two or three known B epitopes from Trichinella and Fasciola, and several linkers (GSGSG, GPGPG and KK) in species as different as mice, sheep and turbot. All these species could mount an effective immune response to the short chimeric peptides. Nevertheless, this response depended on several factors including a favorable orientation of B-cell epitopes, adequateness of linkers and/or probability of formation of T neoepitopes. We also observed that, at least in mice, the inclusion of a decoy epitope may have favorable consequences on the antibody response to other epitopes in the chimera.

The Change of Teleost Skin Commensal Microbiota Is Associated With Skin Mucosal Transcriptomic Responses During Parasitic Infection by Ichthyophthirius multifillis

Teleost skin serves as the first line of defense against invading pathogens, and contain a skin-associated lymphoid tissue (SALT) that elicit gut-like immune responses against antigen stimulation. Moreover, exposed to the water environment and the pathogens therein, teleost skin is also known to be colonized by diverse microbial communities. However, little is known about the interactions between microbiota and the teleost skin mucosal immune system, especially dynamic changes about the interactions under pathogen infection. We hypothesized that dramatic changes of microbial communities and strong mucosal immune response would be present in the skin of aquatic vertebrate under parasite infection. To confirm this hypothesis, we construct an infected model with rainbow trout (Oncorhynchus mykiss), which was experimentally challenged by Ichthyophthirius multifiliis (Ich). H & E staining of trout skin indicates the successful invasion of Ich and shows the morphological changes caused by Ich infection. Critically, increased mRNA expression levels of immune-related genes were detected in trout skin from experimental groups using qRT-PCR, which were further studied by RNA-Seq analysis. Here, through transcriptomics, we detected that complement factors, pro-inflammatory cytokines, and antimicrobial genes were strikingly induced in the skin of infected fish. Moreover, high alpha diversity values of microbiota in trout skin from the experimental groups were discovered. Interestingly, we found that Ich infection led to a decreased abundance of skin commensals and increased colonization of opportunistic bacteria through 16S rRNA pyrosequencing, which were mainly characterized by lose of Proteobacteria and increased intensity of Flavobacteriaceae. To our knowledge, our results suggest for the first time that parasitic infection could inhibit symbionts and offer opportunities for other pathogens' secondary infection in teleost skin.

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

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

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