Salmonid T cells assemble in the thymus, spleen and in novel interbranchial lymphoid tissue

Development of a CD3ε mAb that can identify and in vitro activate T cells in large yellow croaker (Larimichthys crocea)

As a hallmark molecule of T cells, CD3ε forms a complex with T-cell receptor (TCR) to transduce antigen signals and drive T cell activation, playing a pivotal role in T cell-mediated immune response. However, the lack of specific monoclonal antibodies (mAbs) targeting CD3ε in fish has substantially impeded the study on adaptive immunity in these species. In this study, we constructed NIH/3T3 cells expressing CD3ε of the large yellow croaker (Larimichthys crocea) via retroviral transduction. Using these cells as immunogen to immune mice, we generated a mAb that specifically recognized a population of spleen leukocytes after cell fusion and screening. This identified population specifically expressed CD3ε, CD4-1 or CD8α, suggesting that it was T cell. Moreover, immunofluorescence demonstrated that the mAb could bind to the surface of some leukocytes, and it was detected as the IgG1 type. These results confirmed the specificity of this CD3ε mAb and its applicability to identify T cells in the large yellow croaker. Subsequently, we revealed the widespread distribution of CD3ε+ T cells in immune-related tissues including spleen, liver, head kidney, gill and peripheral blood by using this mAb. Upon PHA stimulation, the phosphorylation of mTORC1 and MAPK/ERK were enhanced in CD3ε+ T cells. More importantly, this CD3ε mAb could mimic antigenic signaling to induce T-cell activation in vitro, since its treatment activated the mTORC1, MAPK/ERK and Ca2+ pathways which were crucial for T cell activation. Therefore, we generated a CD3ε mAb, which could not only identify but also in vitro activate T cells of the large yellow croaker, providing critical tools for investigating T-cell immune in teleost.

Serotonin Signaling and Macrophage Subsets in Goldfish Gills: Unraveling the Neuroimmune Network for Gill Homeostasis

Goldfish (Carassius auratus) gills function as both respiratory and immune-regulatory organs, integrating neuroendocrine and immune responses to environmental stimuli. This study explores the spatial organization and interaction of neuroendocrine cells (NECs) and immune cells within goldfish gills using confocal immunohistochemistry and transmission electron microscopy. NECs, identified near blood capillaries and nerve fibers, highlight their role in environmental sensing and physiological regulation. These cells express serotonin (5-HT), a neurotransmitter critical for neuroimmune communication. Two distinct macrophage subsets were observed: iNOS-positive macrophages, concentrated in the basal epithelium, suggest a pro-inflammatory role, whereas 5-HT-positive macrophages, dispersed in the subepithelium, likely contribute to immune modulation. The co-localization of MHC-II and CD68 in macrophages further supports an active antigen-processing system in the gills. Ultrastructural analysis revealed diverse immune cells, including rodlet cells, telocytes, and lymphocytes, within the gill epithelium. Telocytes formed intricate networks with immune cells, highlighting their role in immune coordination and tissue homeostasis. These findings provide new insights into the neuroimmune interactions in fish gills, contributing to a broader understanding of aquatic immune systems and environmental adaptability.

Gill Health in Fish Farmed in Recirculating Aquaculture Systems (RAS): A Review

Recirculating Aquaculture Systems (RAS) have been proposed as the future of aquaculture, because they can be used anywhere regardless of access to water, they offer high level of control over farming environment, including biosecurity, and are considered to be sustainable. However, despite of continuous development, there can be still issues with water quality affecting gill health of fish farmed in these systems. This review provides an overview of fish gill structure and gill immune response, and discusses the known impacts of RAS on gill health. Several experimental studies have inadequately reported conditions, particularly water quality, making it difficult to determine if the observed effects were due to water quality issues or RAS system itself. It is crucial for studies investigating the impact of RAS on fish to report water quality during the study. Furthermore, assessments of RAS effects on gill health should include sufficient independent replicates and flow through controls using a common water source. Various methods have been used to assess gill health in RAS, including gill histology, presence of pathogens, gene expression in the gills and gill microbiome analysis. Differences in gill health in fish from RAS and a flow through system have been shown for a number of freshwater and marine fish species. However, these results have been inconsistent across studies, and some results have been challenging to interpret as indicators of gill health. Holistic studies including a number of different methods to assess fish gills would give more conclusive results. More research is needed, in particular, on brackish and marine RAS, to fully understand their impacts on gill health.

B-cell distribution and composition in the gills and the interbranchial lymphoid tissue (ILT) of the Atlantic salmon (Salmo salar L.)

Fish lack organized mucosa-associated lymphoid tissues as found in birds and mammals. However, a distinct aggregation of lymphoid cells, the interbranchial lymphoid tissue (ILT), was identified in the gills of the Atlantic salmon (Salmo salar) in 2008. Since then, additional structures of similar character have been discovered. The function of the ILT remains elusive, and research has focused primarily on its T cell content. Here, we address the B cell composition of the ILT. Gills of Atlantic salmon from two developmental stages (avg 153 g and avg 1,717 g) were examined from both vaccinated and unvaccinated fish for immunoglobulins IgM, IgT, and IgD, as well as for Activation-Induced Cytidine Deaminase (AID) and Paired Box 5 (Pax5) mRNA by morphological methods. There was an abundance of Ig-transcript+ cells throughout the gill epithelium, with IgD mRNA+ cells being the most numerous. Larger fish had a significantly higher number of IgM mRNA+ cells than the smaller fish, whereas vaccination had no significant impact on any of the isotypes. The immunofluorescence results of Igs paired with Pax5 ISH showed B cells in different stages of maturation within the ILT. Additionally, scattered AID+ cells were present within the pILT. In contrast to what was previously assumed, we show that B cells are abundant in the ILT.

Ontogeny of the organized nasopharynx-associated lymphoid tissue in rainbow trout

Understanding the ontogeny of teleost mucosa-associated lymphoid tissues (MALT) is critical for determining the earliest timepoint for effective mucosal vaccination of young fish. Here, we describe the developmental sequence that leads to the formation of an organized MALT structure in rainbow trout, the organized nasopharynx-associated lymphoid tissue (O-NALT). Control rainbow trout were sampled between 340 and 1860 degree days (DD) and routine histology and immunofluorescence staining were used to determine cellular changes in immune cells in the nasal cavity as well as O-NALT formation. We identified that O-NALT is first seeded by CD8α+ T cells at 700 DD followed by IgM+ B cells and cd4-2b+ cells at 1000 DD. Histomorphologically, trout O-NALT is fully formed at 1400 DD. Whole body gene expression analyses uncovered waves of igmh, cd4-2b, and cd8a expression that recapitulate the cellular seeding sequence of O-NALT by specific lymphocyte subsets. Our results indicate that 1) O-NALT formation results from a specific sequence of lymphocyte subset colonization pioneered by CD8α+ T cells and 2) the presence of the full O-NALT structure at 1400 DD may mark this timepoint as the earliest developmental stage at which mucosal vaccines can induce long lasting, specific immune responses.

Comparative analysis of T-cell immunity between Streptococcus agalactiae susceptible and resistant tilapia (Oreochromis niloticus)

Nile tilapia (Oreochromis niloticus) is one of the important economic fish species cultured worldwide. However, Streptococcus agalactiae has emerged as a significant bacterial threat, severely impacting the economy of tilapia industry. The immune response underlying the resistance of tilapia to S. agalactiae are not well understood, hindering the reasonable evaluation of breeding and the formulation of effective strategies. In this study, we investigated the differences in T-cell immunity between S. agalactiae-resistant and -susceptible tilapia. Compared with susceptible tilapia, resistant tilapia exhibited a higher percentage of T cells and BrdU+ T cells during infection, indicating a superior proliferative capacity. Whether infected or not, T cells from resistant fish demonstrated a greater ability to resist apoptosis. Additionally, T cell effector genes, including interleukin (IL)-2, interferon (IFN)-γ, perforin A, and granzyme B were expressed at higher levels in resistant tilapia after infection. Along with these T-cell immune responses, resistant fish showed more effective clearance of infection. Our study elucidates the T-cell immune responses in resistant tilapia, which may contribute to the high resistance of tilapia to S. agalactiae, and provide valuable theoretical references for the selection and evaluation of disease-resistant fish strains in the future.

IL-20 is produced by CD3γδ T cells and induced in the mucosal tissues of grass carp during infection with Aeromonas hydrophila

Interleukin (IL) 20 is a multifunctional cytokine and plays a vital role in regulating autoimmune diseases, inflammation, and immune responses. IL-20 homologs have been described in fish. However, due to the lack of antibodies, cellular sources and immunological functions of fish IL-20 in response to infections have not been fully characterized. In this study, a monoclonal antibody (mAb) was generated against the recombinant grass carp (Ctenopharyngodon idella) IL-20 protein and characterized by immunoblotting, immunofluorescent microscopy and flow cytometry. It was shown that the IL-20 mAb specifically recognized recombinant IL-20 proteins expressed in the E. coli cells and HEK293 cells. Using confocal microscopy, the IL-20+ cells were identified in the head kidney, gills and intestine of grass carp, and induced after infection with Aeromonas hydrophila. Moreover, the IL-20 protein was found to be secreted mainly by CD3γδ T cells which were located predominantly in the gill filaments and intestinal mucosa. Taken together, our results suggest that IL-20 producing T cells are required for the mucosal immunity against bacterial infection in fish.

Dynamics of Gill Responses to a Natural Infection with Neoparamoeba perurans in Farmed Tasmanian Atlantic Salmon

Gill health has become a significant global challenge for Atlantic salmon (Salmo salar) aquaculture, particularly during the marine phase of farming. The increasing prevalence of gill pathologies has been linked to rising seawater temperatures, underscoring the need to evaluate existing tools for monitoring gill health and to develop novel approaches for early detection. In this study, we investigated the gill responses of commercially farmed Atlantic salmon to natural infection with Neoparamoeba perurans during an outbreak of amoebic gill disease (AGD) in Tasmania. Our focus spanned the low AGD prevalence, high AGD prevalence, and post-freshwater treatment stages of the outbreak. Evaluations of gill tissue included assessments of the gross AGD score, histopathological score, abundance of N. perurans (measured by 18S rRNA gene expression), and expression levels of inflammation-related transcripts. We demonstrated a strong correlation between different measures of AGD-related gill pathology and significant differences between distinct stages of the N. perurans outbreak. Post-treatment, fish exhibited considerable variability in their responses to the freshwater bath, highlighting the necessity for personalized management strategies that consider genetic, environmental, and health status factors. The expression patterns of angiogenin-1 (ANG1) and complement C1q tumour necrosis factor-related protein 3-like (C1QTNF3) emphasize their potential as biomarkers for early detection of gill damage in salmon aquaculture worldwide.

Organisation of the Atlantic salmon (Salmo salar) thymus and its content of Ig-expressing cells

The thymus of fishes is located as a dual organ in a rostrodorsal projection within the gill chamber and is covered by the operculum. The histological organization of the teleost fish thymus displays considerable diversity, particularly in salmonids where a clear distinction between the thymus cortex and medulla is yet to be defined. Recent interest has focused on the role of B cells in thymic function, but the presence of these cells within the salmon thymus remains poorly understood. In this morphological study, we applied in situ hybridization to investigate developing Atlantic salmon thymi for the expression of recombination activating (Rag) genes 1 and 2. We identified the location of the cortex, aligning with the previously described inner zone. Expression of IgM and IgD transcripts was predominantly observed in cells within the outer and subcapsular zones, with lesser expression in the cortex and inner zone. IgT expression was confined to a limited number of cells in the inner zone and capsule. The location of the thymus medulla could not be established. Our results are discussed in the context of the recently identified lymphoid organs, namely the intrabranchial lymphoid tissue (ILT) and the salmon bursa.

Resistance of Nile tilapia fed with Padina boergesenii extract to Pseudomonas putida infection

The aim of this research was to estimate the immunopotentiation effect of brown algae Padina boergesenii water extract on Nile tilapia, Oreochromis niloticus through resistance to Pseudomonas putida infection. Gas Chromatography Mass Spectrometry was utilized to characterize the seaweed phytoconstituents. One hundred and twenty-six fish were divided in triplicates into two equal groups corresponding to two diet variants that used to feed Nile tilapia for 20 successive days: a basal (control), and P. boergesenii water extract supplemented group. Fish samples were collected at 10-days intervals throughout the experiment. Serum biochemical constituents, total antioxidant capacity (TAC), and some immune related genes expression of the spleen and intestinal tissues of experimental fish were studied, as well as histological examination of fish immune tissues. Moreover, following 20 days of feeding, the susceptibility of Nile tilapia to P. putida infection was evaluated to assess the protective effect of the used extract. The findings indicated that the studied parameters were significantly increased, and the best immune response profiles were observed in fish fed P. boergesenii water extract for 20 successive days. A bacterial challenge experiment using P. putida resulted in higher survival within the supplemented fish group than the control. Thus, the lowered post-challenge mortality of the fish may be related to the protection provided by the stimulation of the innate immune system, reduced oxidative stress by higher activity of TAC, and elevated levels of expression of iterleukin-1beta (IL-1β), beta-defensin (β-defensin), and natural killer-lysin (NKl). Moreover, the constituents of the extract used showed potential protective activity for histological features of the supplemented fish group when compared to the control. Collectively, this study presents a great insight on the protective role of P. boergesenii water extract as an additive in Nile tilapia feed which suggests its potential for improving the immune response against P. putida infection.

Functional role of TrIL-1β in Takifugu rubripes defense against Cryptocaryon irritans infection

The Japanese puffer, Takifugu rubripes, is a commercially important fish species in China that is under serious threat from white spot disease (cyptocaryoniasis), which leads to heavy economic losses. We previously found that interleukin-1β (IL-1β), an important cytokine with a potential role in resistance against pathogens, was one of the most significantly differentially up-regulated proteins in the gills and spleen of T. rubripes infected by the protozoan parasite Cryptocaryon irritans. In this study, we assessed the potential function of T. rubripes IL-1β (TrIL-1β) in fish infected with C. irritans. Phylogenetic analysis indicated that the TrIL-1β protein sequence was most closely related to that of Atlantic salmon (Salmo salar) (67.2 %). The incubation experiments revealed that TrIL-1β may reduce trophont activity by destroying membranes. Immunofluorescence experiments showed that recombinant TrIL-1β promoted the expression of endogenous IL-1β, which penetrated and disrupted the cell membranes of trophonts. Transmission electron microscopy showed that the IL-1β group had less tissue damage compared with control groups of fish. IL-1β-small interfering RNA and IL-1β overexpression experiments were performed in head kidney primary cells, and challenge experiments were performed in vitro. Quantitative RT-PCR results showed that TrIL-1β regulated and activated MyD88/NF-κB and MyD88/MAPK/p38 signaling pathways during C. irritans infection. TrIL-1β also promoted the differential expression of IgM, showing that it was involved in humoral immunity of T. rubripes. The cumulative mortality experiment show that TrIL-1β could protect fish against C. irritans infection. These results enrich current knowledge about the molecular structure of TrIL-1β. They also suggested that recombinant TrIL-1β could be used as an adjuvant in a subunit vaccine against C. irritans infection, which is of profound importance for the prevention and control of parasitic diseases in T. rubripes.

Aggregation and proliferation of B cells and T cells in MALTs upon Cryptocaryon irritans infection in large yellow croaker Larimichthys crocea

Mucosal immunity in mucosa-associated lymphoid tissues (MALTs) plays crucial roles in resisting infection by pathogens, including parasites, bacteria and viruses. However, the mucosal immune response in the MALTs of large yellow croaker (Larimichthys crocea) upon parasitic infection remains largely unknown. In this study, we investigated the role of B cells and T cells in the MALTs of large yellow croaker following Cryptocaryon irritans infection. Upon C. irritans infection, the total IgM and IgT antibody levels were significantly increased in the skin mucus and gill mucus. Notably, parasite-specific IgM antibody level was increased in the serum, skin and gill mucus following parasitic infection, while the level of parasite-specific IgT antibody was exclusively increased in MALTs. Moreover, parasitic infection induced both local and systemic aggregation and proliferation of IgM+ B cells, suggesting that the increased levels of IgM in mucus may be derived from both systemic and mucosal immune tissues. In addition, we observed significant aggregation and proliferation of T cells in the gill, head kidney and spleen, suggesting that T cells may also be involved in the systemic and mucosal immune responses upon parasitic infection. Overall, our findings provided further insights into the role of immunoglobulins against pathogenic infection, and the simultaneous aggregation and proliferation of both B cells and T cells at mucosal surfaces suggested potential interactions between these two major lymphocyte populations during parasitic infection.

Slc43a2+ T cell metastasis from spleen to brain in RGNNV infected teleost

The origin of T cells in the teleost's brain is unclear. While viewing the central nervous system (CNS) as immune privileged has been widely accepted, previous studies suggest that T cells residing in the thymus but not in the spleen of the teleost play an essential role in communicating with the peripheral organs. Here, we identified nine T cell subpopulations in the thymus and spleen of orange-spotted grouper (Epinephelus coioices) through single-cell RNA-sequencing analysis. After viral CNS infection with red-spotted grouper nervous necrosis virus (RGNNV), the number of slc43a2+ T cells synchronously increased in the spleen and brain. During the infection tests in asplenic zebrafish (tlx1▲ zebrafish model), no increase in the number of slc43a2+ T cells was observed in the brain. Single-cell transcriptomic analysis indicated that slc43a2+ T cells mature and functionally differentiate within the spleen and then migrate into the brain to trigger an immune response. This study suggests a novel route for T cell migration from the spleen to the brain during viral infection in fish.

Diploid and triploid Chinook salmon (Oncorhynchus tshawytscha) have altered microRNA responses in immune tissues after infection with Vibrio anguillarum

Production of sterile fishes through artificial retention of a third set of chromosomes (triploidy) is a sustainable alternative for aquaculture since it reduces escapee pressure on wild populations. However, these fishes have reduced survival in stressful conditions and in response to infection. In this study, the impact of Vibrio anguillarum infection on diploid and triploid Chinook salmon (Oncorhynchus tshawytscha) was investigated to identify if there was any significant immune regulation by microRNAs (miRNA). Small RNAs from hindgut, head kidney, and spleen were sequenced to determine if miRNA transcript abundance was altered due to ploidy and infection in nine-month old full-sibling diploids and triploids. All three tissues had differentially expressed miRNA prior to infection, indicating subtle changes in epigenetic regulation due to increased ploidy. Additionally, miRNA were altered by infection, but there was only a difference in spleen miRNA expression between diploids and triploids at three days of infection. Furthermore, one miRNA (ssa-miR-2188-3p) was confirmed as having an altered response to infection in triploids compared to diploids, implicating potential immune dysregulation due to increased ploidy. The miRNAs identified in this study are predicted to target immune pathways, providing evidence for their importance in regulating responses to pathogens. This study is the first to investigate how increased ploidy alters miRNA expression in response to infection. Additionally, it provides evidence for epigenetic dysregulation in triploid fishes, which may contribute to their poor performance in response to stress.

Histomorphometric evaluation of melanomacrophage centers (MMCs) and CD3+ T cells of two morphs of brown trout (Salmo trutta) fed diets with immunostimulants

The brown trout (Salmo trutta) is a salmonid residing in riverine and coastal waters throughout the Northern Hemisphere, whose various populations evolved into distinct ecological morphs, differing in their migratory tendencies and preferred habitats. Unfortunately, due to progressing degradation of natural environment, the conservation of these populations is of growing importance and is undoubtedly a challenging task. Therefore, various means to refine the preparatory protocols for restocking using hatchery-reared fish are being pursued, some of which involve the administration of immunity-boosting substances. The current study assessed the effects of two dietary immunostimulants: Bioimmuno (4% inosine pranobex and 96% β-glucan) and Focus Plus (commercial preparation by Biomar, Denmark) on two morphs of the brown trout - the river trout (S. trutta morpha fario) and the sea trout (S. trutta morpha trutta). Tissue samples were obtained from ∼75 to 100g fish after 0, 2 and 4 weeks of experimental feeding. Multi-factorial analysis of conducted histological measurements of melanomacrophage centers (MMCs) revealed no changes of their parameters within spleens, but showed a decrease of the occupied tissue area and MMC counts in the livers, progressing with time regardless of the applied diet. Immunohistochemical analysis of CD3+ T cells showed their increased recruitment into mucosal folds of pyloric caeca in the 2-week sampling of trouts fed with the diet with 2% Bioimmuno addition, but this effect was not present in the 4-week sampling. When studying all groups jointly within each morph, there was a significant difference in terms of maintained CD3+ T cells levels, as sea trouts showed significantly higher tissue areas occupied by these cells than river trouts, both in the pyloric caeca and hepatic parenchyma. The study revealed that feeding with a diet enriched with Bioimmuno for 2 weeks may be a favorable enhancement of rearing protocols of brown trout stocks prior to their release, but more studies need to be conducted to test the possibility of an even shorter feeding period.

The acute inflammatory response of teleost fish

Acute inflammation is crucial to the immune responses of fish. The process protects the host from infection and is central to induction of subsequent tissue repair programs. Activation of proinflammatory signals reshapes the microenvironment within an injury/infection site, initiates leukocyte recruitment, promotes antimicrobial mechanisms and contributes to the resolution of inflammation. Inflammatory cytokines and lipid mediators are primary contributors to these processes. Uncontrolled or persistent induction results in delayed tissue healing. The kinetics by which inducers and regulators of acute inflammation exert their actions is essential for understanding the pathogenesis of fish diseases and identifying potential treatments. Although, a number of these are well-conserved across, others are not, reflecting the unique physiologies and life histories of members of this unique animal group.

Investigation of the functional role of UNC93B1 in Nile tilapia (Oreochromis niloticus): mRNA expression, subcellular localization, and physical interaction with fish-specific TLRs

Nile tilapia (Oreochromis niloticus) is one of the major food fish worldwide. The farming business, on the other hand, has faced considerable obstacles, such as disease infestations. Toll-like receptors (TLRs) play an important function in the activation of the innate immune system in response to infections. Unc-93 homolog B1 (UNC93B1) is a key regulator of nucleic acid (NA)-sensing TLRs. Here the UNC93B1 gene, which was cloned from Nile tilapia tissue for this investigation, had the same genetic structure as a homologous gene in humans and mice. Phylogenetic analysis revealed that Nile tilapia UNC93B1 clustered with UNC93B1 from other species and separately from the UNC93A clade. The gene structure of the Nile tilapia UNC93B1 was found to be identical to that of human UNC93B1. Our gene expression studies revealed that Nile tilapia UNC93B1 was highly expressed in the spleen, followed by other immune-related tissues such as the head kidney, gills, and intestine. Moreover, Nile tilapia UNC93B1 mRNA transcripts were up-regulated in vivo in the head kidney and spleen tissues from poly I:C and Streptococcus agalactiae injected Nile tilapia, as well as in vitro in LPS stimulated Tilapia head kidney (THK) cells. The Nile tilapia UNC93B1-GFP protein signal was detected in the cytosol of THK cells and was co-localized with endoplasmic reticulum and lysosome but not with mitochondria. Moreover, the results of a co-immunoprecipitation and immunostaining analysis showed that Nile tilapia UNC93B1 can be pulled down with fish-specific TLRs such as TLR18 and TLR25 from Nile tilapia, and was found to be co-localized with these fish-specific TLRs in the THK cells. Overall, our findings highlight the potential role of UNC93B1 as an accessory protein in fish-specific TLR signaling.

Effects of juvenile thermal preconditioning on the heat-shock, immune, and stress responses of rainbow trout upon a secondary thermal challenge

Higher water temperatures and pathogens are both significant factors that negatively affect the welfare of teleost fish. In aquaculture, compared to natural populations, these problems are especially exacerbated, as the animals have relatively limited mobility, and the higher density promotes faster spread of infectious diseases. Because of the potential harm these stressors can inflict, methods that can limit the damage of these stressors are particularly valuable. As a method of interest, early-life thermal preconditioning of animals demonstrated some potential for effective improvements in thermotolerance. However, the potential effects of the method on the immune system via the heat-stress model have not been explored. In this experiment, juvenile-stage thermal preconditioned rainbow trout (Oncorhynchus mykiss) were subjected to a secondary thermal challenge, animals were collected and sampled at the time of lost equilibrium. The effects of preconditioning on the general stress response was assessed by measuring the plasma cortisol levels. In addition, we also examined hsp70 and hsc70 mRNA levels in the spleen and gill tissues, as well as IL-1β, IL-6, TNF-α, IFN-1, β2m, and MH class I transcripts via qRT-PCR. No changes in CTmax were observed between the preconditioned and control cohorts upon the second challenge. IL-1β and IL-6 transcripts were generally upregulated with increased temperature of the secondary thermal challenge, whereas IFN-1 transcripts were upregulated in the spleen, but downregulated in the gills, along with MH class I. The juvenile thermal preconditioning produced a series of changes in transcript levels for IL-1β, TNF-α, IFN-1, and hsp70 but the dynamics of these differences were inconsistent. Finally, analysis of plasma cortisol levels presented significantly lower cortisol levels in the pre-conditioned animals compared to the non-pre-conditioned control cohort.

IL-4/13 expressing CD3γ/δ+ T cells regulate mucosal immunity in response to Flavobacterium columnare infection in grass carp

Interleukin (IL) 4 and 13 are signature cytokines orchestrating Th2 immune response. Teleost fish have two homologs, termed IL-4/13A and IL-4/13B, and have been functionally characterized. However, what cells express IL-4/13A and IL-4/13B has not been investigated in fish. In this work, the recombinant IL-4/13A and IL-4/13B proteins of grass carp (Ctenopharyngodon idella) were produced in the Escherichia coli (E. coli) cells and purified. Monoclonal antibodies (mAbs) against the recombinant CiIL-4/13A and CiIL-4/13B proteins were prepared and characterized. Western blotting analysis showed that the CiIL-4/13A and CiIL-4/13B mAbs could specifically recognize the recombinant proteins expressed in the E. coli cells and HEK293T cells and did not cross-react with each other. Confocal microscopy revealed that the CiIL-4/13A⁺ and CiIL-4/13B⁺ cells were present in the gills, intestine and spleen and could be upregulated in fish infected with Flavobacterium columnare (F. columnare). Interestingly, the cells expressing CiIL-4/13A and CiIL-4/13B were mostly CD3γ/δ⁺ cells. The CD3γ/δ⁺/IL-4/13A⁺ and CD3γ/δ⁺/IL-4/13B⁺ cells were significantly upregulated in the gill filaments and the intestinal mucosa after F. columnare infection. Our results imply that the CD3γ/δ⁺/IL-4/13A⁺ and CD3γ/δ⁺/IL-4/13B⁺ cells are important for homeostasis and the regulation of mucosal immunity.

Immunity of the intestinal mucosa in teleost fish

The paper presents the problem of intestinal mucosa immunity in teleost fish. The immunity of the intestinal mucosa in teleost fish depends on the elements and mechanisms with different organizational/structural and functional properties than in mammals. The organization of the elements of intestinal mucosal immunitya in these animals is associated with the presence of immune cells that fulfil the functions assigned to the induction and effector sites of mucosal immunity in mammals; they are located at various histological sites of the mucosa - in the lamina propria (LP) and in the surface epithelium. The presence of mucosa-associated lymphoid tissue (MALT) has not been demonstrated in teleost fish, and the terminology used in relation to the structure and function of the mucosa immunity components in teleost fish is inadequate. In this article, we review the knowledge of intestinal mucosal immunity in teleost fish, with great potential for knowledge and practical applications especially in the field of epidemiological safety. We discuss the organization and functional properties of the elements that determine this immunity, according to current data and taking into account the tissue definition and terminology adopted by the Society for Mucosal Immunology General Assembly (13th ICMI in Tokyo, 2007).

Immunohistochemistry of the Gut-Associated Lymphoid Tissue (GALT) in African Bonytongue (Heterotis niloticus, Cuvier 1829)

Heterotis niloticus is a basal teleost, belonging to the Osteoglossidae family, which is widespread in many parts of Africa. The digestive tract of H. niloticus presents similar characteristics to those of higher vertebrates, exhibiting a gizzard-like stomach and lymphoid aggregates in the intestinal lamina propria. The adaptive immune system of teleost fish is linked with each of their mucosal body surfaces. In fish, the gut-associated lymphoid tissue (GALT) is generally a diffuse immune system that represents an important line of defense against those pathogens inhabiting the external environment that can enter through food. The GALT comprises intraepithelial lymphocytes, which reside in the epithelial layer, and lamina propria leukocytes, which consist of lymphocytes, macrophages, granulocytes, and dendritic-like cells. This study aims to characterize, for the first time, the leukocytes present in the GALT of H. niloticus, by confocal immuno- fluorescence techniques, using specific antibodies: toll-like receptor 2, major histocompatibility complex class II, S100 protein, serotonin, CD4, langerin, and inducible nitric oxide synthetase. Our results show massive aggregates of immune cells in the thickness of the submucosa, arranged in circumscribed oval-shaped structures that are morphologically similar to the isolated lymphoid follicles present in birds and mammals, thus expanding our knowledge about the intestinal immunity shown by this fish.

The thymus and T-cell ontogeny in ballan wrasse (Labrus bergylta) is nutritionally modelled

Marine fish larvae often experience high mortality unrelated to predation during early life stages, and farmed ballan wrasse (Labrus bergylta) is no exception. Knowing when the adaptive immune system is developed and fully functional, and how nutrition may modulate these processes is therefore of importance to establish effective prophylactic measures and will also extend the relatively limited knowledge on the immune system in lower vertebrates. The thymus anlage of ballan wrasse was found to be histologically visible for the first time at larval stage 3 (20-30 days post hatch, dph) and becomes lymphoid at stage 5 (50-60 dph) correlating with an increase of T-cell marker transcripts. At this stage, a clear zonation into a RAG1⁺ cortex and a RAG1^- CD3ϵ⁺ medulla was distinguished, indicating that T-cell maturation processes in ballan wrasse are similar to other teleosts. The higher abundance of CD4-1⁺ compared to CD8β⁺ cells in the thymus together with the apparent lack of CD8β⁺ cells in gill, gut, and pharynx, where CD4-1⁺ cells were identified, indicates that helper T-cells have a more prominent role during larval development compared to cytotoxic T-cells. As ballan wrasse lacks a stomach but has an exceptionally high IgM expression in the hindgut, we hypothesize that helper T-cells are crucial for activation and recruitment of IgM⁺ B-cells and possibly other leukocytes to the gut during early development. Nutritional factors such as DHA/EPA, Zn and Se may lead to an earlier expression of certain T-cell markers as well as a larger size of the thymus, indicating an earlier onset of adaptive immunity. Including live feeds that supplies the larva with higher amounts of these nutrients can therefore be beneficial for ballan wrasse farming.

Induction of attenuated Nocardia seriolae and their use as live vaccine trials against fish nocardiosis

Nocardia seriolae, a Gram-positive facultative intercellular pathogen, has been identified as the causative agent of fish nocardiosis, causing substantial mortality and morbidity of a wide range of fish species. Looking into that fact, the effective vaccine against this pathogen is urgently needed to control the significant losses in aquaculture practices. In order to induct attenuated strains for developing the potential live vaccines, the mutagenic N. seriolae strain S-250 and U-20 were obtained from wild-type strain ZJ0503 through continuous passaging and ultraviolet (UV) irradiation, respectively. Additionally, the biological characteristic, virulence, stability, mediating immune response and supplying protective efficacy to hybrid snakehead of the S-250 and U-20 strains were determined in the present study. The results showed that U-20 strain displayed dramatic changes in morphological characteristic and significant decreased in the virulence to hybrid snakehead, while that of S-250 strain had no obvious different in comparison to ZJ0503 strain. When hybrid snakehead were intraperitoneally injected with ZJ0503, S-250 and U-20 strains at their respective sub-clinical dosage, the non-specific immunity parameters (serum LYZ, POD, ACP, AKP and SOD activities), specific antibody (IgM) titers production and immune-related genes (CC1, CC2, IL-1β, IL-8, TNFα, IFNγ, MHCIα, MHCIIα, CD4, CD8α, TCRα and TCRβ) expression were up-regulated, indicating that they were able to trigger humoral and cell-mediated immune responses. Furthermore, the protective efficacy in hybrid snakehead after vaccination with ZJ0503, S-250 and U-20 strains, in terms of relative percentage survival (RPS), were 28.85%, 56.89% and 89.65% respectively. Taken together, two attenuated N. seriolae strains S-250 and U-20 were obtained successfully and they could elicit strong immune response and supply protective efficacy to hybrid snakehead against N. seriolae, which suggested that these two attenuated strains were the potential candidates for live vaccine development to control fish nocardiosis in aquaculture.

Multi-Organ Transcriptome Response of Lumpfish (Cyclopterus lumpus) to Aeromonas salmonicida Subspecies salmonicida Systemic Infection

Lumpfish is utilized as a cleaner fish to biocontrol sealice infestations in Atlantic salmon farms. Aeromonas salmonicida, a Gram-negative facultative intracellular pathogen, is the causative agent of furunculosis in several fish species, including lumpfish. In this study, lumpfish were intraperitoneally injected with different doses of A. salmonicida to calculate the LD50. Samples of blood, head-kidney, spleen, and liver were collected at different time points to determine the infection kinetics. We determined that A. salmonicida LD50 is 102 CFU per dose. We found that the lumpfish head-kidney is the primary target organ of A. salmonicida. Triplicate biological samples were collected from head-kidney, spleen, and liver pre-infection and at 3- and 10-days post-infection for RNA-sequencing. The reference genome-guided transcriptome assembly resulted in 6246 differentially expressed genes. The de novo assembly resulted in 403,204 transcripts, which added 1307 novel genes not identified by the reference genome-guided transcriptome. Differential gene expression and gene ontology enrichment analyses suggested that A. salmonicida induces lethal infection in lumpfish by uncontrolled and detrimental blood coagulation, complement activation, inflammation, DNA damage, suppression of the adaptive immune system, and prevention of cytoskeleton formation.

Investigating the impacts of H2O2 treatment on gills of healthy Atlantic salmon reveals potential changes to mucus production with implications on immune activity

Current treatment strategies for relevant infectious diseases in Atlantic salmon (Salmo salar L.) include the use of low salinity or freshwater bathing. However, often availability is restricted, and hydrogen peroxide (H₂O₂) is used as an alternative. The potential impacts of H₂O₂ on fish mucosal tissues, especially the gills therefore need to be considered. In this study the mucosal and immunological effects of H₂O₂ treatment on the gills of healthy Atlantic salmon were examined by gene expression (qPCR) and immunohistochemistry (IHC) investigating T-cell, B-cell, and mucin activity. Healthy fish were treated with H₂O₂ and sampled at different times: 4 h, 24 h and 14 days post-H₂O₂ treatment (dpt) (total n = 18) to investigate the effect of holding time and H₂O₂ treatment. Treatment with H₂O₂ resulted in up-regulation of markers for T-cell activity and anti-inflammatory response and down-regulation of mucin expression in the gills at 14 dpt compared to fish sampled prior to treatment (0h; n = 5 fish). These findings were supported by IHC analysis, which despite being highly variable between samples, showed an increase in the number of CD3⁺ T cells at 14 dpt in 50% of treated fish compared to pre-treatment fish. The results from this study suggest that H₂O₂ treatment does not immune compromise healthy Atlantic salmon after 14 dpt (i.e., post-recovery) but modulates gill immune activity and disrupts the mucus covering of the gills. However, further studies are required to determine whether the effects observed are related to H₂O₂ treatment in isolation or other variables such as holding time or environmental factors.

LC-qTOF-MS analysis of fish immune organs reveals the distribution of amino acids in response to metabolic adaptation of the survival phenotype in grouper against Vibrio infection

Epinephelus fuscoguttatus is economically crucial to various Southeast Asia countries where they are reared in fish farms to meet the demand for supply. However, a systemic infectious disease known as vibriosis has steadily and extensively affected the fish farming industry. The disease is caused by Vibrio spp., which are pathogenic gram-negative bacteria. This study focused on understanding the host's metabolic adaptation against Vibrio vulnificus infection, which features a survival phenotype, by profiling the metabolites in grouper fingerlings that survived the experimental infection. Mapping of the pathways is crucial to explain the roles of metabolites in fish immunity. A solvent extraction method was used on the grouper's immune organs (gills, liver and spleen) prior to Liquid Chromatography-Quadrupole Time-of-Flight Mass Spectrometry (LC-qTOF-MS) analysis. The metabolites identified in fingerlings that survived experimental infections were mostly amino acids (primary metabolites). Glutamine (0.44%), alanine (0.68%), phenylalanine (2.63%) and tyrosine (2.60%) were highly abundant in survived-infected gills. Aspartic acid (13.57%) and leucine (4.01%) were highly abundant in the livers of the survived-infected fish and lysine was highly abundant in both gills (2.94%) and liver (3.64%) of the survived-infected fish. Subsequent bioinformatics analysis revealed the involvement of the identified functional amino acids in various immune-related pathways. The current findings facilitate the comprehension of the metabolic adaptation of grouper fingerlings that exhibited a survival phenotype against Vibrio infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-022-03269-1.

Cluster of differentiation antigens: essential roles in the identification of teleost fish T lymphocytes

Cluster of differentiation (CD) antigens are cell surface molecules expressed on leukocytes and other cells associated with the immune system. Antibodies that react with CD antigens are known to be one of the most essential tools for identifying leukocyte subpopulations. T lymphocytes, as an important population of leukocytes, play essential roles in the adaptive immune system. Many of the CD antigens expressed on T lymphocytes are used as surface markers for T lymphocyte classification, including CD3, CD4 and CD8 molecules. In this review, we summarize the recent advances in the identification of CD molecules on T lymphocytes in teleosts, with emphasis on the functions of CD markers in the classification of T lymphocyte subsets. We notice that genes encoding CD3, co-receptors CD4 and CD8 have been cloned in several fish species and antibodies have been developed to study protein expression in morphological and functional contexts. T lymphocytes can be divided into CD4⁺ and CD8⁺ cells discriminated by the expression of CD4 and CD8 molecules in teleost, which are functionally similar to mammalian helper T cells (Th) and cytotoxic T cells (Tc), respectively. Further studies are still needed on the particular characteristics of teleost T cell repertoires and adaptive responses, and results will facilitate the health management and development of vaccines for fish.

Lympho-Hematopoietic Microenvironments and Fish Immune System

In the last 50 years information on the fish immune system has increased importantly, particularly that on species of marked commercial interest (i.e., salmonids, cods, catfish, sea breams), that occupy a key position in the vertebrate phylogenetical tree (i.e., Agnatha, Chondrichtyes, lungfish) or represent consolidated experimental models, such as zebrafish or medaka. However, most obtained information was based on genetic sequence analysis with little or no information on the cellular basis of the immune responses. Although jawed fish contain a thymus and lympho-hematopoietic organs equivalents to mammalian bone marrow, few studies have accounted for the presumptive relationships between the organization of these cell microenvironments and the known immune capabilities of the fish immune system. In the current review, we analyze this topic providing information on: (1) The origins of T and B lymphopoiesis in Agnatha and jawed fish; (2) the remarkable organization of the thymus of teleost fish; (3) the occurrence of numerous, apparently unrelated organs housing lympho-hematopoietic progenitors and, presumably, B lymphopoiesis; (4) the existence of fish immunological memory in the absence of germinal centers.

Structural characteristics and mucosal immune response of the interbranchial lymphoid tissue in the gills of flounder (Paralichthys olivaceus)

A specialized lymphoepithelial tissue termed the interbranchial lymphoid tissue (ILT) is recently identified in several fish species. However, the structural variation and mucosal immune functions of the ILT remain largely unknown. In this study, the anti-Zap-70 MAb was firstly determined to specifically recognize ZAP-70 protein, and CD4-1⁺, CD4-2⁺ and CD8β⁺ T-cells, but not IgM⁺ B cells, in peripheral blood leucocytes of flounder (Paralichthys olivaceus). Then we found that aggregates of Zap-70⁺ cells were located in the epithelium covering the bottom of the interbranchial cleft and along the afferent and efferent edges of the filaments in a cross view, where a meshwork of epithelial cells containing diffused lymphoid cells was exhibited, confirming these structures as the ILT; In a sagittal view, Zap-70⁺ cells were situated at the base of the filaments (here named as proximal ILT, pILT) and in the interlamellar epithelium (named as distal ILT, dILT). Also, a few IgM⁺ B cells were distributed at these sites. The lymphoepithelium within pILT and dILT was very thin with a low number of Zap-70⁺ cells in premetamorphosis and postclimax larvae of flounder, and got thicker containing much more Zap-70⁺ cells in juvenile and adult individuals. The aggregates of CD4-1⁺/Zap-70⁺, CD4-2⁺/Zap-70⁺, and CD8β⁺/Zap-70⁺ T-cell subsets were identified in the ILT. Post bath vaccination with inactivated Edwardsiella tarda and then intraperitoneal injection of EdU, the amounts of EdU⁺ and Zap-70⁺ cells obviously increased at 3 d and 7 d, and co-localization of EdU⁺/Zap-70⁺ cells identified the presence of proliferative T cells; meanwhile, MHC class II-expressing cells were increased. These findings indicated that the ILT in gills of flounder was an important site for the induction of local T cell-mediated immunity, which would lead to a better understanding of mucosal immunity and defense mechanisms of teleost fish.

Antigen-Presenting Cells and T Cells Interact in a Specific Area of the Intestinal Mucosa Defined by the Ccl25-Ccr9 Axis in Medaka

Organized intestinal mucosal immune response appears to be restricted to tetrapods. In teleost fish, there is no evidence for the existence of a particular intestinal region that facilitates the interaction of antigen-presenting cells (APCs) and T cells, such as secondary lymphoid organs. Indeed, despite their importance in the defense against pathogens, the location and manner of APC-T cell interaction within the fish gut is unknown. Here, using non-invasive live imaging of newly developed transgenic reporter lines, we addressed the spatial organization and behavior of APCs and T cells in the intestine of medaka fish both during homeostasis and inflammation. We report that Ccr9a⁺ T cells are recruited to a band in the lamina propria next to the muscularis mucosa in which Ccl25-expressing cells are present. Ccr9a⁺ T cells contact APCs for several minutes, in a process mediated by connexin 43. This type of interaction was observed in homeostasis and inflammation, with the interaction being longer and more frequent during inflammation. Thus, our results demonstrate that the mucosal immune response in the intestine of medaka is organized and endowed with a specific region with specialized microenvironment and function.

Epitheliocystis in Greater Amberjack: Evidence of a Novel Causative Agent, Pathology, Immune Response and Epidemiological Findings

Epitheliocystis is a fish gill disease caused by a broad range of intracellular bacteria infecting freshwater and marine fish worldwide. Here we report the occurrence and progression of epitheliocystis in greater amberjack reared in Crete (Greece). The disease appears to be caused mainly by a novel Betaproteobacteria belonging to the Candidatus Ichthyocystis genus with a second agent genetically similar to Ca. Parilichlamydia carangidicola coinfecting the gills in some cases. After a first detection of the disease in 2017, we investigated epitheliocystis in the following year's cohort of greater amberjack juveniles (cohort 2018) transferred from inland tanks to the same cage farm in the open sea where the first outbreak was detected. This cohort was monitored for over a year together with stocks of gilthead seabream and meagre co-farmed in the same area. Our observations showed that epitheliocystis could be detected in greater amberjack gills as early as a month following the transfer to sea cages, with ionocytes at the base of the gill lamellae being initially infected. Cyst formation appears to trigger a proliferative response, leading to the fusion of lamellae, impairment of gill functions and subsequently to mortality. Lesions are characterized by infiltration of immune cells, indicating activation of the innate immune response. At later stages of the outbreak, cysts were no longer found in ionocytes but were observed in mucocytes at the trailing edge of the filament. Whole cysts appeared finally to be expelled from infected mucocytes directly into the water, which might constitute a novel means of dispersion of the infectious agents. Molecular screening indicates that meagre is not affected by this disease and confirms the presence of previously described epitheliocystis agents, Ca. Ichthyocystis sparus, Ca. Ichthyocystis hellenicum and Ca. Similichlamydia spp., in gilthead seabream. Prevalence data show that the bacteria persist in both gilthead seabream and greater amberjack cohorts after first infection.

RNA-Seq of Single Fish Cells - Seeking Out the Leukocytes Mediating Immunity in Teleost Fishes

The immune system is a complex and sophisticated biological system, spanning multiple levels of complexity, from the molecular level to that of tissue. Our current understanding of its function and complexity, of the heterogeneity of leukocytes, is a result of decades of concentrated efforts to delineate cellular markers using conventional methods of antibody screening and antigen identification. In mammalian models, this led to in-depth understanding of individual leukocyte subsets, their phenotypes, and their roles in health and disease. The field was further propelled forward by the development of single-cell (sc) RNA-seq technologies, offering an even broader and more integrated view of how cells work together to generate a particular response. Consequently, the adoption of scRNA-seq revealed the unexpected plasticity and heterogeneity of leukocyte populations and shifted several long-standing paradigms of immunology. This review article highlights the unprecedented opportunities offered by scRNA-seq technology to unveil the individual contributions of leukocyte subsets and their crosstalk in generating the overall immune responses in bony fishes. Single-cell transcriptomics allow identifying unseen relationships, and formulating novel hypotheses tailored for teleost species, without the need to rely on the limited number of fish-specific antibodies and pre-selected markers. Several recent studies on single-cell transcriptomes of fish have already identified previously unnoticed expression signatures and provided astonishing insights into the diversity of teleost leukocytes and the evolution of vertebrate immunity. Without a doubt, scRNA-seq in tandem with bioinformatics tools and state-of-the-art methods, will facilitate studying the teleost immune system by not only defining key markers, but also teaching us about lymphoid tissue organization, development/differentiation, cell-cell interactions, antigen receptor repertoires, states of health and disease, all across time and space in fishes. These advances will invite more researchers to develop the tools necessary to explore the immunology of fishes, which remain non-conventional animal models from which we have much to learn.

RNA form baker's yeast cultured with and without lipopolysaccharide (LPS) modulates gene transcription in an intestinal epithelial cell model, RTgutGC from rainbow trout (Oncorhynchus mykiss)

The objective of this study was to evaluate if the intestinal RTgutGC cell line could be suitable for research on dietary ingredients and their function as modulators of inflammation during lipopolysaccharide (LPS) induced stress. The RTgutGC cells cultured together with RNA from baker's yeast, reached confluency after 72 h. The cells were grown in either compete L-15 (CM) or nutrient deprived L-15 (DM). Then, the RTgutGC cells were exposed to LPS or RNA from baker's yeast, either alone, or in combination, in CM or DM. All cultures were harvested following LPS challenge for 48 h and 72 h. LPS induced transcription of Interleukin 1β (IL-1β), Interleukin -8 (IL-8), Toll like receptor 3 (TLR3), interferon regulating factor 3 (irf3), Nuclear factor ĸβ (NFĸβ), one of the multidrug transporters, ABCC2, and glutamine synthase 1 (GLS01) in RTgutGC cells at one or both sampling points (48 h and/or 72 h post LPS challenge). RNA from baker's yeast in culture alone, (cultured 120 h and 144 h with RTgutGC cells and harvested at the respective LPS sampling points) induced transcription of INF1, TNFα and ticam/trif, not induced by LPS. In addition, RNA from baker's yeast affected IL-1β, TLR3, irf3 and NFĸβ, comparable to the responses triggered by LPS. RNA from baker's yeast alone did not affect ABCC2 or GLS01 transcriptions in this set up. So, LPS and RNA from baker's yeast affects distinct but also common gene transcripts in this intestinal cell line. Culturing RTgutGC cells in DM, adding a combination of LPS and RNA from baker's yeast, reduced IL-1β transcription compared to cells grown in CM, 48 h and 72 h post LPS challenge. Also, in RTgutGC cells, grown in DM, the LPS induced transcription of ABCC2 declined, measured 48 h post LPS challenge. Possibly indicating that optimal transcription of IL-1β and ABBC2 in RTgutGC cells, cultured over time, requires access of adequate nutrients under stressful condition. RNA from baker's yeast induced INF1 transcription in the RTgutGC cells, regardless if the medium was complete or deprived of nutrients. However, culturing RTgutGC cells in DM enriched with RNA from baker's yeast for a longer period of time (120 h, 144 h), seemed beneficial for INF1 transcription.

High-Resolution, 3D Imaging of the Zebrafish Gill-Associated Lymphoid Tissue (GIALT) Reveals a Novel Lymphoid Structure, the Amphibranchial Lymphoid Tissue

The zebrafish is extensively used as an animal model for human and fish diseases. However, our understanding of the structural organization of its immune system remains incomplete, especially the mucosa-associated lymphoid tissues (MALTs). Teleost MALTs are commonly perceived as diffuse and scattered populations of immune cells throughout the mucosa. Yet, structured MALTs have been recently discovered in Atlantic salmon (Salmo salar L.), including the interbranchial lymphoid tissue (ILT) in the gills. The existence of the ILT was only recently identified in zebrafish and other fish species, highlighting the need for in-depth characterizations of the gill-associated lymphoid tissue (GIALT) in teleosts. Here, using 3-D high-resolution microscopy, we analyze the GIALT of adult zebrafish with an immuno-histology approach that reveals the organization of lymphoid tissues via the labeling of T/NK cells with an antibody directed to a highly conserved epitope on the kinase ZAP70. We show that the GIALT in zebrafish is distributed over at least five distinct sub-regions, an organization found in all pairs of gill arches. The GIALT is diffuse in the pharyngeal part of the gill arch, the interbranchial septum and the filaments/lamellae, and structured in two sub-regions: the ILT, and a newly discovered lymphoid structure located along each side of the gill arch, which we named the Amphibranchial Lymphoid Tissue (ALT). Based on RAG2 expression, neither the ILT nor the ALT constitute additional thymi. The ALT shares several features with the ILT such as presence of abundant lymphoid cells and myeloid cells embedded in a network of reticulated epithelial cells. Further, the ILT and the ALT are also a site for T/NK cell proliferation. Both ILT and ALT show structural changes after infection with Spring Viraemia of Carp Virus (SVCV). Together, these data suggest that ALT and ILT play an active role in immune responses. Comparative studies show that whereas the ILT seems absent in most neoteleosts ("Percomorphs"), the ALT is widely present in cyprinids, salmonids and neoteleosts, suggesting that it constitutes a conserved tissue involved in the protection of teleosts via the gills.

Kinetics of CD4-1+ lymphocytes in brown trout after exposure to viral haemorrhagic septicaemia virus

T-helper cells express CD4 as a co-receptor that binds to major histocompatibility complex class II to synchronize the immune response against upcoming threats via mediating several cytokines. We have previously reported the presence of CD4 homologues in brown trout. The study of cellular immune responses in brown trout is limited by the availability of specific antibodies. We here describe the generation of a polyclonal antibody against CD4-1 that allows for the investigation of CD4⁺ cells. We used this novel tool to study CD4⁺ cells in different tissues during viral haemorrhagic septicaemia infection (VHSV) using flow cytometric technique. Flow cytometric analyses revealed an enhanced level of surface CD4-1 expression in the infected group in major lymphoid organs and in the intestine. These results suggest an important role for the T-helper cells within the immune response against viruses, comparable to the immune response in higher vertebrates.

Mucosal immunoglobulins of teleost fish: A decade of advances

Immunoglobulins (Igs) are complex glycoproteins that play critical functions in innate and adaptive immunity of all jawed vertebrates. Given the unique characteristics of mucosal barriers, secretory Igs (sIgs) have specialized to maintain homeostasis and keep pathogens at bay at mucosal tissues from fish to mammals. In teleost fish, the three main IgH isotypes, IgM, IgD and IgT/Z can be found in different proportions at the mucosal secretions of the skin, gills, gut, nasal, buccal, and pharyngeal mucosae. Similar to the role of mammalian IgA, IgT plays a predominant role in fish mucosal immunity. Recent studies in IgT have illuminated the primordial role of sIgs in both microbiota homeostasis and pathogen control at mucosal sites. Ten years ago, IgT was discovered to be an immunoglobulin class specialized in mucosal immunity. Aiming at this 10-year anniversary, the goal of this review is to summarize the current status of the field of fish Igs since that discovery, while identifying knowledge gaps and future avenues that will move the field forward in both basic and applied science areas.

Gill Transcriptome Analysis Revealed the Difference in Gene Expression Between Freshwater and Seawater Acclimated Guppy (Poecilia reticulata)

Guppy (Poecilia reticulata) can adapt to a wide range of salinity changes. To investigate the gene expression changes in the guppy exposed to seawater, we characterized its gill transcriptome using RNA sequencing. Experimental fish were exposed to salinity increase from 0 to 30‰ within 4 days, while control fish were cultured in freshwater (0‰ salinity). Seven days after salinity exposure, the gills were sampled and the mortality within 2 weeks was recorded. No significant difference in the cumulative mortality at the second week was found between the two groups. Transcriptomic analysis identified 3477 differentially expressed genes (DEGs), including 1067 upregulated and 2410 downregulated genes. These DEGs were enriched in several biological processes, including ion transport, ion homeostasis, ATP biosynthetic process, metabolic process, and immune system process. Oxidative phosphorylation was the most activated pathway. DEGs involved in the pathway "endoplasmic reticulum (ER)-mediated phagocytosis," "starch and sucrose metabolism," and "steroid biosynthesis" were mainly downregulated; chemokines and interleukins involved in "cytokine-cytokine receptor interaction" were differentially expressed. The present results suggested that oxidative phosphorylation had essential roles in osmoregulation in the gills of seawater acclimated guppy, during which the decline in the expression of genes encoding V-ATPases and calreticulin had a negative effect on the phagocytosis and immune response. Besides, several metabolic processes including "starch and sucrose metabolism" and "steroid biosynthesis" were affected. This study elucidates transcriptomic changes in osmotic regulation, metabolism, and immunity in seawater acclimated guppy.

Modulation of the mucosal immune response of red tilapia (Oreochromis sp.) against columnaris disease using a biomimetic-mucoadhesive nanovaccine

Columnaris, a highly contagious bacterial disease caused by Flavobacterium columnare, is recognized as one of the most important infectious diseases in farmed tilapia, especially during the fry and fingerling stages of production. The disease is associated with characteristic lesions in the mucosa of affected fish, particularly their skin and gills. Vaccines delivered via the mucosa are therefore of great interest to scientists developing vaccines for this disease. In the present study, we characterized field isolates of F. columnare obtained from clinical columnaris outbreaks in red tilapia to select an isolate to use as a candidate for our vaccine study. This included characterizing its colony morphology, genotype and virulence status. The isolate was incorporated into a mucoadhesive polymer chitosan-complexed nanovaccine (CS-NE), the efficacy of which was determined by experimentally infecting red tilapia that had been vaccinated with the nanoparticles by immersion. The experimental infection was performed 30-days post-vaccination (dpv), which resulted in 89% of the unvaccinated control fish dying, while the relative percentage survival (RPS) of the CS-NE vaccinated group was 78%. Histology of the mucosal associated lymphoid tissue (MALT) showed a significantly higher presence of leucocytes and a greater antigen uptake by the mucosal epithelium in CS-NE vaccinated fish compared to control fish and whole cell vaccinated fish, respectively, and there was statistically significant up-regulation of IgT, IgM, TNF α, IL1-β and MHC-1 genes in the gill of the CS-NE vaccinated group. Overall, the results of our study confirmed that the CS-NE particles achieved better adsorption onto the mucosal surfaces of the fish, elicited great vaccine efficacy and modulated the MALT immune response better than the conventional whole cell-killed vaccine, demonstrating the feasibility of the mucoadhesive nano-immersion vaccine as an effective delivery system for the induction of a mucosal immune response against columnaris disease in tilapia.

Adaptive immune responses in channel catfish exposed to Edwardsiella ictaluri live attenuated vaccine and wild type strains through the specific gene expression profiles

We extend the previous findings on the differential activity of immune-related genes in the lymphoid organs of channel catfish in the 7 days post-challenge (dpc) with E. ictaluri live attenuated vaccines (LAVs) and wild type (WT) strains by assessing the expression of these genes in the 21 dpc. The expression of T and B cell-specific genes were significantly elevated in the spleen at 14 dpc and in the AK at 21 dpc in catfish treated with E. ictaluri WT and LAV strains compared to a non-treated control group. The gene expression of IFN-γ correlated with adaptive immunity genes in the lymphoid tissues of catfish. These data indicate that two novel LAVs were able to trigger the activation of T helper1 polarization cytokine IFN-γ gene and specific lymphocyte genes in the spleen followed by their activation in the AK of catfish without causing inflammation, thus providing protective immunity in E. ictaluri infection.

Immunoglobulins in teleosts

Immunoglobulins are glycoproteins which are produced as membrane-bound receptors on B-cells or in a secreted form, known as antibodies. In teleosts, three immunoglobulin isotypes, IgM, IgT, and IgD, are present, each comprising two identical heavy and two identical light polypeptide chains. The basic mechanisms for generation of immunoglobulin diversity are similar in teleosts and higher vertebrates. The B-cell pre-immune repertoire is diversified by VDJ recombination, junctional flexibility, addition of nucleotides, and combinatorial association of light and heavy chains, while the post-immune repertoire undergoes somatic hypermutation during clonal expansion. Typically, the teleost immunoglobulin heavy chain gene complex has a modified translocon arrangement where the Dτ-Jτ-Cτ cluster of IgT is generally located between the variable heavy chain (VH) region and the Dμ/δ-Jμ/δ-Cμ-Cδ gene segments, or within the set of VH gene segments. However, multiple genome duplication and deletion events and loss of some individual genes through evolution has complicated the IgH gene organization. The IgH gene arrangement allows the expression of either IgT or IgM/IgD. Alternative splicing is responsible for the regulation of IgM/IgD expression and the secreted versus transmembrane forms of IgT, IgD, and IgM. The overall structure of IgM and IgT is usually conserved across species, whereas IgD has a large variety of structures. IgM is the main effector molecule in both systemic and mucosal immunity and shows a broad range of concentrations in different teleost species. Although IgM is usually present in higher concentrations under normal conditions, IgT is considered the main mucosal Ig.

Molecular and cellular characterization of European sea bass CD3ε+ T lymphocytes and their modulation by microalgal feed supplementation

The CD3 coreceptor is a master T cell surface marker, and genes encoding CD3ζ, γδ, and ε chains have been reported in several teleost fish. Here, a complete cDNA sequence of CD3ɛ chain was identified from a sea bass (Dicentrarchus labrax L.) gill transcriptome. Its basal expression was quantified in both lymphoid and non-lymphoid organs of sea bass juveniles with real-time qPCR analysis. After either in vitro stimulation of head kidney leukocytes with the T-cell mitogen phytohaemagglutinin or in vivo stimulation with an orally administered Vibrio anguillarum vaccine, CD3ε expression levels increased in head kidney leukocytes, confirming that CD3ε T cells may play important roles in fish systemic protection against pathogens. Further, three peptides were designed on the CD3ɛ cytoplasmic tail region and employed as immunogens for antibody production in rabbit. One antiserum so obtained, named RACD3/1, immunostained a band of the expected size in a western blot of a sea bass thymocyte lysate. The distribution of CD3ε⁺ lymphocyte population in the lymphoid organs and mucosal tissues was addressed in healthy fish by IHC. In decreasing percentage order, CD3ε⁺ lymphocytes were detected by flow cytometry in thymus, peripheral blood leukocytes, gills, head kidney, gut, and spleen. Finally, a significant in vivo enhancement of CD3ε⁺ T intestinal lymphocytes was found in fish fed on diets in which 100% fish meal was replaced by the microalgae Nannochloropsis sp. biomass. These results indicate that CD3ε⁺ T cells are involved in nutritional immune responses.

Anatomy of teleost fish immune structures and organs

The function of a tissue is determined by its construction and cellular composition. The action of different genes can thus only be understood properly when seen in the context of the environment in which they are expressed and function. We now experience a renaissance in morphological research in fish, not only because, surprisingly enough, large structures have remained un-described until recently, but also because improved methods for studying morphological characteristics in combination with expression analysis are at hand. In this review, we address anatomical features of teleost immune tissues. There are approximately 30,000 known teleost fish species and only a minor portion of them have been studied. We aim our review at the Atlantic salmon (Salmo salar) and other salmonids, but when applicable, we also present information from other species. Our focus is the anatomy of the kidney, thymus, spleen, the interbranchial lymphoid tissue (ILT), the newly discovered salmonid cloacal bursa and the naso-pharynx associated lymphoid tissue (NALT).

Gut immunity in European sea bass (Dicentrarchus labrax): a review

In this review, we summarize and discuss the trends and supporting findings in scientific literature on the gut mucosa immune role in European sea bass (Dicentrarchus labrax L.). Overall, the purpose is to provide an updated overview of the gastrointestinal tract functional regionalization and defence barriers. A description of the available information regarding immune cells found in two immunologically-relevant intestinal compartments, namely epithelium and lamina propria, is provided. Attention has been also paid to mucosal immunoglobulins and to the latest research investigating gut microbiota and dietary manipulation impacts. Finally, we review oral vaccination strategies, as a safe method for sea bass vaccine delivery.

Characterization of CD3γ/δ+ cells in grass carp (Ctenopharyngodon idella)

CD3 is an essential component of the TCR-CD3 complex which plays a key role in adaptive immunity. Non-mammalian CD3 complex consists of CD3γ/δ, CD3ε and CD3ζ subunits. In this study, homologues of CD3γ/δ and CD3ε (termed CiCD3γ/δ and CiCD3ε) have been identified in grass carp (Ctenopharyngodon idella). Like their counterparts from other vertebrates, the CiCD3γ/δ and CiCD3ε are clustered in the same locus in the genome and encode proteins which are structurally conserved, comprising a signal peptide, an extracellular domain, a transmembrane domain and a cytoplasmic tail containing two ITAM motifs. Sequence analyses identified two novel conserved motifs in the cytoplasmic tail of CiCD3γ/δ and CiCD3ε, one is composed of an arginine and lysine motif (RK or RR) at the C terminus of CiCD3γ/δ and a proline rich domain (PxxPxP/Q) located at the N terminus of ITAM motifs of CiCD3ε. Both genes were highly expressed at the mRNA level in the spleen and gills of healthy fish and could be modulated by infection of Flavobacterium columnare and grass carp reovirus. A monoclonal antibody against the CiCD3γ/δ (GC38T) was produced and showed good reactivity with the native molecule in Western blotting analysis and flow cytometry. The CiCD3γ/δ⁺ cells were analysed in the primary leucocytes, accounting for 5.5% of lymphocytes isolated from spleen, 4.5% from head kidney and 2.8% from peripheral blood. The CiCD3γ/δ⁺ cells were localized in the gills and head kidney by fluorescent confocal microscopy.

A Major QTL for Resistance to Vibrio anguillarum in Rainbow Trout

Genetic selection of disease resistant fish is a major strategy to improve health, welfare and sustainability in aquaculture. Mapping of single nucleotide polymorphisms (SNP) in the fish genome may be a fruitful tool to define relevant quantitative trait loci (QTL) and we here show its use for characterization of Vibrio anguillarum resistant rainbow trout (Oncorhynchus mykiss). Fingerlings were exposed to the pathogen V. anguillarum serotype O1 in a solution of 1.5 × 10⁷ cfu/ml and observed for 14 days. Disease signs appeared 3 days post exposure (dpe) whereafter mortality progressed exponentially until 6 dpe reaching a total mortality of 55% within 11 days. DNA was sampled from all fish – including survivors – and analyzed on a 57 k Affymetrix SNP platform whereby it was shown that disease resistance was associated with a major QTL on chromosome 21 (Omy 21). Gene expression analyses showed that diseased fish activated genes associated with innate and adaptive immune responses. The possible genes associated with resistance are discussed.

Revisiting the Teleost Thymus: Current Knowledge and Future Perspectives

Simple Summary

The thymus is the immune organ producing T lymphocytes that are essential to create immunity after encountering pathogens or vaccination. This review summarizes the thymus localization and histological studies, cell composition, and function in teleost fishes. We also describe how seasonal changes, photoperiod, water temperature fluctuations, and hormones can affect thymus development in fish species. Overall, the information helps identify future studies needed to understand thymus function in fish species and the immune system’s evolutionary origins. Since fish are exposed to pathogens, especially under aquaculture conditions, knowledge about the fish thymus and T lymphocyte can also help improve fish farming protocols, considering intrinsic and environmental conditions that can contribute to achieving the best vaccine responsiveness for disease resistance.

Abstract

The thymus in vertebrates plays a critical role in producing functionally competent T-lymphocytes. Phylogenetically, the thymus emerges early during evolution in jawed cartilaginous fish, and it is usually a bilateral organ placed subcutaneously at the dorsal commissure of the operculum. In this review, we summarize the current understanding of the thymus localization, histology studies, cell composition, and function in teleost fishes. Furthermore, we consider environmental factors that affect thymus development, such as seasonal changes, photoperiod, water temperature fluctuations and hormones. Further analysis of the thymus cell distribution and function will help us understand how key stages for developing functional T cells occur in fish, and how thymus dynamics can be modulated by external factors like photoperiod. Overall, the information presented here helps identify the knowledge gaps and future steps needed for a better understanding of the immunobiology of fish thymus.

A practical guide to unbiased quantitative morphological analyses of the gills of rainbow trout (Oncorhynchus mykiss) in ecotoxicological studies

Rainbow trout (Oncorhynchus mykiss) are frequently used as experimental animals in ecotoxicological studies, in which they are experimentally exposed to defined concentrations of test substances, such as heavy metals, pesticides, or pharmaceuticals. Following exposure to a broad variety of aquatic pollutants, early morphologically detectable toxic effects often manifest in alterations of the gills. Suitable methods for an accurate and unbiased quantitative characterization of the type and the extent of morphological gill alterations are therefore essential prerequisites for recognition, objective evaluation and comparison of the severity of gill lesions. The aim of the present guidelines is to provide practicable, standardized and detailed protocols for the application of unbiased quantitative stereological analyses of relevant morphological parameters of the gills of rainbow trout. These gill parameters inter alia include the total volume of the primary and secondary gill lamellae, the surface area of the secondary gill lamellae epithelium (i.e., the respiratory surface) and the thickness of the diffusion barrier. The featured protocols are adapted to fish of frequently used body size classes (300-2000 g). They include well-established, conventional sampling methods, probes and test systems for unbiased quantitative stereological analyses of light- and electron microscopic 2-D gill sections, as well as the application of modern 3-D light sheet fluorescence microscopy (LSFM) of optically cleared gill samples as an innovative, fast and efficient quantitative morphological analysis approach. The methods shown here provide a basis for standardized and representative state-of-the-art quantitative morphological analyses of trout gills, ensuring the unbiasedness and reproducibility, as well as the intra- and inter-study comparability of analyses results. Their broad implementation will therefore significantly contribute to the reliable identification of no observed effect concentration (NOEC) limits in ecotoxicological studies and, moreover, to limit the number of experimental animals by reduction of unnecessary repetition of experiments.

Comparative study of cartilaginous fish divulges insights into the early evolution of primary, secondary and mucosal lymphoid tissue architecture

Cartilaginous fish are located at a pivotal point in phylogeny where the adaptive immune system begins to resemble that of other, more-derived jawed vertebrates, including mammals. For this reason, sharks and other cartilaginous fish are ideal models for studying the natural history of immunity. Insights from such studies may include distinguishing the (evolutionarily conserved) fundamental aspects of adaptive immunity from the (more recent) accessory. Some lymphoid tissues of sharks, including the thymus and spleen, resemble those of mammals in both appearance and function. The cartilaginous skeleton of sharks has no bone marrow, which is also absent in bony fish despite calcified bone, but cartilaginous fish have other Leydig's and epigonal organs that function to provide hematopoiesis analogous to mammalian bone marrow. Conserved across all vertebrate phylogeny in some form is gut-associated lymphoid tissues, or GALT, which is seen from agnathans to mammals. Though it takes many forms, from typhlosole in lamprey to Peyer's patches in mammals, the GALT serves as a site of antigen concentration and exposure to lymphocytes in the digestive tract. Though more complex lymphoid organs are not present in agnathans, they have several primitive tissues, such as the thymoid and supraneural body, that appear to serve their variable lymphocyte receptor-based adaptive immune system. There are several similarities between the adaptive immune structures in cartilaginous and bony fish, such as the thymus and spleen, but there are mechanisms employed in bony fish that in some instances bridge their adaptive immune systems to that of tetrapods. This review summarizes what we know of lymphoid tissues in cartilaginous fishes and uses these data to compare primary and secondary tissues in jawless, cartilaginous, and bony fishes to contextualize the early natural history of vertebrate mucosal immune tissues.

Anatomy, immunology, digestive physiology and microbiota of the salmonid intestine: Knowns and unknowns under the impact of an expanding industrialized production

Increased industrialized production of salmonids challenges aspects concerning available feed resources and animal welfare. The immune system plays a key component in this respect. Novel feed ingredients may trigger unwarranted immune responses again affecting the well-being of the fish. Here we review our current knowledge concerning salmon intestinal anatomy, immunity, digestive physiology and microbiota in the context of industrialized feeding regimes. We point out knowledge gaps and indicate promising novel technologies to improve salmonid intestinal health.