Long-lived effects of administering β-glucans: Indications for trained immunity in fish

Induction of trained immunity using β-glucan and its protective responses in Nile tilapia, Oreochromis niloticus

Emerging and re-emerging diseases in fish cause drastic economic losses in the aquaculture sector. To combat the impact of disease outbreaks and prevent the emergence of infections in culture systems, understanding the advanced strategies for protecting fish against infections is inevitable in fish health research. Therefore, the present study aimed to evaluate the induction of trained immunity and its protective efficacy against Streptococcus agalactiae in tilapia. For this, Nile tilapia and the Tilapia head kidney macrophage primary culture were primed using β-glucan @200 μg/10 g body weight and 10 μg/mL respectively. Expression profiles of the markers of trained immunity and production of metabolites were monitored at different time points, post-priming and training, which depicted enhanced responsiveness. Higher lactate and lactate dehydrogenase (LDH) production in vitro suggests heightened glycolysis induced by priming of the cells using β-glucan. A survival rate of 60% was observed in β-glucan trained fish post challenge with virulent S. agalactiae at an LD50 of 2.6 × 107 cfu/ml, providing valuable insights into promising strategies of trained immunity for combating infections in fish.

Effect of β-glucans on rainbow trout (Oncorhynchus mykiss) IgM+ B cells

β-glucans are carbohydrates present in the cell wall of many fungi, which are often used as immunostimulants in feeds for farmed species. Their capacity to activate innate immune responses directly acting on innate cell populations has been widely documented in fish. However, whether they can affect the functionality of adaptive immune cells has been scarcely explored. In this context, in the current work, we have determined the effects of β-glucans on rainbow trout blood IgM+ B cells in the presence or absence of 2,4,6-trinitrophenyl hapten conjugated to lipopolysaccharide (TNP-LPS), a model antigen. For this, rainbow trout peripheral blood leukocytes were incubated with different doses of β-glucans or media alone in the presence or absence of TNP-LPS for 48 h. The size, levels of expression of surface MHC II, antigen processing and phagocytic capacities and proliferation of IgM+ B cells were then studied by flow cytometry. The number of IgM-secreting cells in the cultures was also estimated by ELISpot. β-glucans significantly decreased the levels of surface MHC II expression and the antigen processing capacities of these cells, especially in the presence of TNP-LPS, while they increased their phagocytic activity. On their own, β-glucans slightly activated the proliferation of IgM+ B cells but reduced that induced by TNP-LPS. In contrast, β-glucans significantly increased the number of cells secreting IgM in the cultures. This effect of β-glucans on the IgM-secreting capacity of B cells was also confirmed through a feeding experiment, in which the IgM-secreting capacity of blood leukocytes obtained from fish fed a β-glucan-supplemented diet for one month was compared to that of leukocytes obtained from fish fed a control diet. Altogether, these findings contribute to increase our knowledge regarding the effects of β-glucans on fish adaptive responses.

Growth, Antioxidant Capacity, and Liver Health in Largemouth Bass (Micropterus salmoides) Fed Multi-Strain Yeast-Based Paraprobiotic: A Lab-to-Pilot Scale Evaluation

A multi-strain yeast-based paraprobiotic (MsYbP) comprising inactive cells and polysaccharides (β-glucan, mannan oligosaccharides, and oligosaccharides) derived from Saccharomyces cerevisiae and Cyberlindnera jadinii could ensure optimal growth and health in farmed fish. This study assessed the impact of an MsYbP on the growth, immune responses, antioxidant capacities, and liver health of largemouth bass (Micropterus salmoides) through lab-scale (65 days) and pilot-scale (15 weeks) experiments. Two groups of fish were monitored: one fed a control diet without the MsYbP and another fed 0.08% and 0.1% MsYbP in the lab-scale and pilot-scale studies, respectively (referred to as YANG). In the lab-scale study, four replicates were conducted, with 20 fish per replicate (average initial body weight = 31.0 ± 0.8 g), while the pilot-scale study involved three replicates with approximately 1500 fish per replicate (average initial body weight = 80.0 ± 2.2 g). The results indicate that the MsYbP-fed fish exhibited a significant increase in growth in both studies (p < 0.05). Additionally, the dietary MsYbP led to a noteworthy reduction in the liver function parameters (p < 0.05), such as alanine aminotransferase (ALT), aspartate aminotransferase (AST) and alkaline phosphatase (AKP), and hepatic nuclear density, indicating improved liver health. Furthermore, the dietary MsYbP elevated the antioxidative capacity of the fish by reducing their malondialdehyde levels and increasing their levels and gene expressions related to antioxidative markers, such as total antioxidant ca-pacity (T-AOC), total superoxide dismutase (T-SOD), glutathione peroxidase (GSH-Px), catalase (CAT), nuclear factor erythroid 2-related factor 2 (nrf2) and kelch-1ike ech-associated protein (keap1) in both studies (p < 0.05). In terms of hepatic immune responses, the lab-scale study showed an increase in inflammation-related gene expressions, such as interleukin-1β (il-1β) and transforming growth factor β1 (tgf-β1), while the pilot-scale study significantly suppressed the expressions of genes related to inflammatory responses, such as tumor necrosis factor α (tnfα) and interleukin-10 (il-10) (p < 0.05). In summary, our findings underscore the role of dietary multi-strain yeast-based paraprobiotics in enhancing the growth and liver health of largemouth bass, potentially through increased antioxidative capacity and the modulation of immune responses, emphasizing the significance of employing yeast-based paraprobiotics in commercial conditions.

Tissue-resident trained immunity in hepatocytes protects against septic liver injury in zebrafish

Trained immunity is classically characterized by long-term functional reprogramming of innate immune cells to combat infectious diseases. Infection-induced organ injury is a common clinical severity phenotype of sepsis. However, whether the induction of trained immunity plays a role in protecting septic organ injury remains largely unknown. Here, through establishing an in vivo β-glucan training and lipopolysaccharide (LPS) challenge model in zebrafish larvae, we observe that induction of trained immunity could inhibit pyroptosis of hepatocytes to alleviate septic liver injury, with an elevated trimethyl-histone H3 lysine 4 (H3K4me3) modification that targets mitophagy-related genes. Moreover, we identify a C-type lectin domain receptor in zebrafish, named DrDectin-1, which is revealed as the orchestrator in gating H3K4me3 rewiring-mediated mitophagy activation and alleviating pyroptosis-engaged septic liver injury in vivo. Taken together, our results uncover tissue-resident trained immunity in maintaining liver homeostasis at the whole-animal level and offer an in vivo model to efficiently integrate trained immunity for immunotherapies.

Long- and short-term dietary β-glucan improves intestinal health and disease resistance in pearl gentian grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀)

β-Glucans are immunostimulants and are widely used in aquaculture industry. The present study was conducted to evaluate the effects of different periods of β-glucan management on growth performance, intestinal health, and disease resistance in pearl gentian grouper (Epinephelus lanceolatus♂ × Epinephelus fuscoguttatus♀). A commercial feed was used as control diet (CD), and the β-glucan diet (βD) was based on CD and further supplemented with 0.1% β-glucan. Grouper in control and long-term β-glucan diet (LGD) groups were fed with CD and βD for 8 weeks, respectively. Groupers in short-term β-glucan diet (SGD) group were fed with CD for the first 4 weeks and βD for the last 4 weeks. We found that LGD and SGD had no effect on growth performance but reduced the mortalities of grouper after challenging with Vibrio harveyi. In addition, both LGD and SGD increased intestinal morphology, enhanced antioxidant capacity, enhanced immunity, inhibited apoptosis, altered the transcriptional profile, and activated mitogen-activated protein kinase (MAPK) and nuclear factor kappa-B (NF-κB) signaling pathway in the intestine of grouper. Furthermore, the effect of LGD on most of the above parameters was comparable to that of SGD. In conclusion, LGD and SGD did not affect growth rate parameters but enhanced the intestinal health and disease resistance of pearl gentian grouper.

Hematological changes in Florida pompano (Trachinotus carolinus) supplemented with β-glucan and Pediococcus acidilactici synbiotic

Florida pompano (Trachinotus carolinus) are a species of growing interest for commercial aquaculture. Effective health monitoring is crucial to the successful growout of the species, and prophylactic and therapeutic use of chemicals and antibiotics has been the traditional strategy for promoting stock health. However, concerns about antimicrobial resistance, chemical residues in seafood products and the environment, and resultant immunosuppression have prompted the industry to identify alternative management strategies, including supplementation with prebiotics, probiotics, and combinations of both (synbiotics). The objectives of this study are to determine and compare hematological, plasma biochemical, and plasma protein electrophoresis data of synbiotic-supplemented (β-glucan and Pediococcus acidilactici) and non-supplemented Florida pompano. Reference intervals for blood analytes are provided for both groups and for subgroups (females, males, large, and small fish) where statistically significant results exist. There are no differences between the hematological and plasma biochemistry analytes between the supplemented and control groups, except for blood urea nitrogen and carbon dioxide, indicating a possible effect of synbiotic supplementation on gill function and osmoregulation. Sex-related and size-related differences are observed within each of the control and supplemented groups; however, biometric measurements do not strongly correlate with blood analytes. These data represent baseline hematological and plasma biochemical data in the Florida pompano and indicate the safety of synbiotic supplementation in this commercially important species. This study serves to further the commercialization of Florida pompano by providing blood analyte reference intervals for health monitoring in the aquaculture setting.

Transcriptomic analysis of turbot (Scophthalmus maximus) treated with zymosan a reveals that lncRNAs and inflammation-related genes mediate the protection conferred against Aeromonas salmonicida

Aeromonas salmonicida is one of the most harmful pathogens in finfish aquaculture worldwide. Immunostimulants such as β-glucans are used to enhance the immunity of cultured fish. However, their effects on fish physiology are not completely understood. In the present work, we evaluated the effect of a single intraperitoneal (ip) injection of zymosan A on fish survival against A. salmonicida infection. A single administration of this compound protected fish against A. salmonicida challenge and reduce the bacterial load in the head kidney one week after its administration. Transcriptome analyses of head kidney samples revealed several molecular mechanisms involved in the protection conferred by zymosan A and their regulation by long noncoding RNAs. The transcriptome profile of turbot exposed only to zymosan A was practically unaltered one week after ip injection. However, the administration of this immunostimulant induced significant transcriptomic changes once the fish were in contact with the bacteria and increased the survival of the infected turbot. Our results suggest that the restraint of the infection-induced inflammatory response, the management of apoptotic cell death, cell plasticity and cellular processes involving cytoskeleton dynamics support the protective effects of zymosan A. All this information provides insights on the cellular and molecular mechanisms involved in the protective effects of this widely used immunostimulant.

Effects of dietary β-glucans on the productive performance, blood parameters, and intestinal microbiota of angelfish (Pterophyllum scalare) juveniles

Among the potential feed additives, β-glucans are known to positively affect the growth performance, blood parameters, and intestinal microbiota of fish, even the ornamental species. Therefore, the present study evaluated the effects of the dietary supplementation of different Saccharomyces cerevisiae β-glucans concentrations (0, 0.05, 0.1, and 0.2%) in juvenile angelfish (Pterophyllum scalare) over a 42-day period. Regarding growth performance, no effects were observed on most parameters. However, 0.2% β-glucans supplementation produced higher condition factor values, indicating a better nutritional status. Furthermore, β-glucans supplementation did not affect blood parameters. Regarding intestinal microbiota, β-glucans supplementation increased the abundance of the potentially beneficial bacterial genus Phascolarctobacterium. The high abundance of bacteria from the phylum Bacteroidetes, which can degrade β-glucans, may be attributed to the increased abundance of Phascolarctobacterium spp. In addition, 0.2% β-glucans supplementation produced more operational taxonomic units and higher Sobs (observed species richness), indicating effects on the overall bacterial community structure. These results demonstrate the potential application of β-glucans as a dietary supplement to improve the performance and modulate the intestinal microbiota of angelfish.

A comprehensive review on the dynamic role of toll-like receptors (TLRs) in frontier aquaculture research and as a promising avenue for fish disease management

Toll-like receptors (TLRs) represent a conserved group of germline-encoded pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play a crucial role in inducing the broadly acting innate immune response against pathogens. In recent years, the detection of 21 different TLR types in various fish species has sparked interest in exploring the potential of TLRs as targets for boosting immunity and disease resistance in fish. This comprehensive review offers the latest insights into the diverse facets of fish TLRs, highlighting their history, classification, architectural insights through 3D modelling, ligands recognition, signalling pathways, crosstalk, and expression patterns at various developmental stages. It provides an exhaustive account of the distinct TLRs induced during the invasion of specific pathogens in various fish species and delves into the disparities between fish TLRs and their mammalian counterparts, highlighting the specific contribution of TLRs to the immune response in fish. Although various facets of TLRs in some fish, shellfish, and molluscs have been described, the role of TLRs in several other aquatic organisms still remained as potential gaps. Overall, this article outlines frontier aquaculture research in advancing the knowledge of fish immune systems for the proper management of piscine maladies.

The Combination of β-Glucan and Astragalus Polysaccharide Effectively Resists Nocardia seriolae Infection in Largemouth Bass (Micropterus salmoides)

Effectively treating and preventing outbreaks is crucial for improving the economic benefits of aquaculture. Therefore, utilizing immunostimulants, either alone or in combination, is regarded as a promising strategy. In this study, β-glucan + APS (200 mg/kg + 200 mg/kg), β-glucan (200 mg/kg), APS (200 mg/kg), enrofloxacin (15 mg/kg), and sulfadiazine (15 mg/kg) were added to feed to assess the effects against Nocardia seriolae infection in largemouth bass (Micropterus salmoides) within 14 days. The survival rates did not differ between the enrofloxacin group and the β-glucan + APS group, but both were significantly higher than that of the control group. Additionally, the enrofloxacin group and the β-glucan + APS group exhibited the lowest bacterial loads and tissue damage. Importantly, the β-glucan + APS treatment significantly improved serum enzyme activities (total superoxide dismutase, lysozyme, total protein) and the expression of immune genes (IL-1β, TNF-α, IFN-γ, IgM) compared to the other treatment groups. The enrofloxacin group showed similar efficacy to the β-glucan + APS group in combating N. seriolae infection, but N. seriolae in the enrofloxacin group developed drug resistance. In summary, the combined use of β-glucan and APS is a promising strategy for treating bacterial diseases, thereby contributing to the promotion of sustainable aquaculture development.

Differences between DNA vaccine and single-cycle viral vaccine in the ability of cross-protection against viral hemorrhagic septicemia virus (VHSV) and infectious hematopoietic necrosis virus (IHNV)

Vaccination procedures can be stressful for fish and can bring severe side effects. Therefore, vaccines that can minimize the number of administrations and maximize cross-protection against multiple serotypes, genotypes, or even different species would be highly advantageous. In the present study, we investigated the cross-protective ability of two types of vaccines - viral hemorrhagic septicemia virus (VHSV) G protein-expressing DNA vaccine and G gene-deleted single-cycle VHSV genotype IVa (rVHSV-ΔG) vaccine - against both VHSV genotype Ia and infectious hematopoietic necrosis virus (IHNV) in rainbow trout (Oncorhynchus mykiss). The results showed that rainbow trout immunized with VHSV genotype Ia G gene- or IVa G gene-expressing DNA vaccine were significantly protected against VHSV genotype Ia, but were not protected against IHNV. In contrast to the DNA vaccine, the single-cycle VHSV IVa vaccine induced significant protection against not only VHSV Ia but also IHNV. Considering no significant increase in ELISA titer and serum neutralization activity against IHNV in fish immunized with single-cycle VHSV IVa, the protection might be independent of humoral adaptive immunity. The scarcity of cytotoxic T cell epitopes between VHSV and IHNV suggested that the possibility of involvement of cytotoxic T cell-mediated cellular adaptive immunity would be low. The role of trained immunity (innate immune memory) in cross-protection should be further investigated.

Immunomodulatory Effect and Biological Significance of β-Glucans

β-glucan, one of the homopolysaccharides composed of D-glucose, exists widely in cereals and microorganisms and possesses various biological activities, including anti-inflammatory, antioxidant, and anti-tumor properties. More recently, there has been mounting proof that β-glucan functions as a physiologically active "biological response modulator (BRM)", promoting dendritic cell maturation, cytokine secretion, and regulating adaptive immune responses-all of which are directly connected with β-glucan-regulated glucan receptors. This review focuses on the sources, structures, immune regulation, and receptor recognition mechanisms of β-glucan.

Oral Vaccination of Largemouth Bass (Micropterus salmoides) against Largemouth Bass Ranavirus (LMBV) Using Yeast Surface Display Technology

Largemouth bass ranavirus (LMBV) infects largemouth bass, leading to significant mortality and economic losses. There are no safe and effective drugs against this disease. Oral vaccines that directly target the intestinal mucosal immune system play an important role in resisting pathogens. Herein, the B subunit of Escherichia coli heat-labile enterotoxin (LTB, a mucosal immune adjuvant) and the LMBV main capsid protein (MCP) were expressed using Saccharomyces cerevisiae surface display technology. The yeast-prepared oral vaccines were named EBY100-OMCP and EBY100-LTB-OMCP. The candidate vaccines could resist the acidic intestinal environment. After 7 days of continuous oral immunization, indicators of innate and adaptive immunity were measured on days 1, 7, 14, 21, 28, 35, and 42. High activities of immune enzymes (T-SOD, AKP, ACP, and LZM) in serum and intestinal mucus were detected. IgM in the head kidney was significantly upregulated (EBY100-OMCP group: 3.8-fold; BY100-LTB-OMCP group: 4.3-fold). IgT was upregulated in the intestines (EBY100-OMCP group: 5.6-fold; EBY100-LTB-OMCP group: 6.7-fold). Serum neutralizing antibody titers of the two groups reached 1:85. Oral vaccination protected against LMBV infection. The relative percent survival was 52.1% (EBY100-OMCP) and 66.7% (EBY100-LTB-OMCP). Thus, EBY100-OMCP and EBY100-LTB-OMCP are promising and effective candidate vaccines against LMBV infection.

Omics and imaging combinatorial approach reveals butyrate-induced inflammatory effects in the zebrafish gut

BACKGROUND

Prebiotic feed additives aim to improve gut health by influencing the microbiota and the gut barrier. Most studies on feed additives concentrate on one or two (monodisciplinary) outcome parameters, such as immunity, growth, microbiota or intestinal architecture. A combinatorial and comprehensive approach to disclose the complex and multifaceted effects of feed additives is needed to understand their underlying mechanisms before making health benefit claims. Here, we used juvenile zebrafish as a model species to study effects of feed additives by integrating gut microbiota composition data and host gut transcriptomics with high-throughput quantitative histological analysis. Zebrafish received either control, sodium butyrate or saponin-supplemented feed. Butyrate-derived components such as butyric acid or sodium butyrate have been widely used in animal feeds due to their immunostimulant properties, thereby supporting intestinal health. Soy saponin is an antinutritional factor from soybean meal that promotes inflammation due to its amphipathic nature.

RESULTS

We observed distinct microbial profiles associated with each diet, discovering that butyrate (and saponin to a lesser extent) affected gut microbial composition by reducing the degree of community-structure (co-occurrence network analysis) compared to controls. Analogously, butyrate and saponin supplementation impacted the transcription of numerous canonical pathways compared to control-fed fish. For example, both butyrate and saponin increased the expression of genes associated with immune response and inflammatory response, as well as oxidoreductase activity, compared to controls. Furthermore, butyrate decreased the expression of genes associated with histone modification, mitotic processes and G-coupled receptor activity. High-throughput quantitative histological analysis depicted an increase of eosinophils and rodlet cells in the gut tissue of fish receiving butyrate after one week of feeding and a depletion of mucus-producing cells after 3 weeks of feeding this diet. Combination of all datasets indicated that in juvenile zebrafish, butyrate supplementation increases the immune and the inflammatory response to a greater extent than the established inflammation-inducing anti-nutritional factor saponin. Such comprehensive analysis was supplemented by in vivo imaging of neutrophil and macrophage transgenic reporter zebrafish (mpeg1:mCherry/mpx:eGFPi¹¹⁴) larvae. Upon exposure to butyrate and saponin, these larvae displayed a dose-dependent increase of neutrophils and macrophages in the gut area.

CONCLUSION

The omics and imaging combinatorial approach provided an integrated evaluation of the effect of butyrate on fish gut health and unraveled inflammatory-like features not previously reported that question the usage of butyrate supplementation to enhance fish gut health under basal conditions. The zebrafish model, due to its unique advantages, provides researchers with an invaluable tool to investigate effects of feed components on fish gut health throughout life.

Βeta-glucan stimulation induces trained immunity markers in common carp, Cyprinus carpio

Trained immunity refers to the memory acquired by innate immune cells, leading to cross-protection and non-specific responses to subsequent infection, thereby improving host survival. Trained immunity induction is a combined effect of immune signaling, metabolic changes, and epigenetic modifications. The present study evaluated the induction of markers of the phenomenon of trained immunity in common carp, which is trained using β-glucan. The mammalian target of rapamycin (mtor) and hypoxia-inducible factor (hif1α), the metabolic basis of trained immunity; the histone deacetylase (hdac7), one of the markers of epigenetic modifications, metabolic activity of activated cells and expression profiles of proinflammatory cytokines viz. il6a, tnfαa2, and ifnγ were targeted in the study and analyzed in vivo. Besides in vivo analysis, in vitro analysis of mtorc2, hif1α, hdac7, and ifnγ were analyzed. In vitro analyses were performed on head kidney macrophages isolated and maintained in L-15 media and double trained with β-glucan at 100μg/mL. The culture supernatant was collected at different time intervals and processed for expression studies. Healthy common carp were injected with β-glucan at 20 mg/kg body weight for training followed by a resting phase for 6 days and were restimulated with the same dose. Head kidney was collected from the fish post-induction as well as post-restimulation. The expression profile of mtorc2, hdac7, and hif1α were found elevated post-stimulation of β-glucan. Further, a significantly upregulated expression profile of proinflammatory cytokines (ifnγ, il6a and tnfαa2) was observed. Increased glycolysis in the cells post-β-glucan stimulation was confirmed by the high lactate and LDH production detected in the cell culture supernatant. Overall, the study revealed the expression profile of the trained immunity markers and the increased metabolic activity in cells induced with β-glucan, which further validates that the action of trained immunity is indispensable in fish on encounter with a potential ligand. The study supports the existing reports on trained immunity in teleost fish with evidence at the genomic level. However, further studies are required to understand the responses and actions of trained immune cells during infection in detail.

Modulation of gut microbiota, blood metabolites, and disease resistance by dietary β-glucan in rainbow trout (Oncorhynchus mykiss)

BACKGROUND

Prebiotics are known to have a positive impact on fish health and growth rate, and β-glucans are among the most used prebiotics on the market. In this study, rainbow trout (Oncorhynchus mykiss) were treated with a β-1,3;1,6-glucan dietary supplement (at a dose of 0 g, 1 g, 10 g, and 50 g β-glucan per kg of feed). After 6 weeks, the effect of the β-glucan was evaluated by determining the changes in the microbiota and the blood serum metabolites in the fish. The impact of β-glucan on the immune system was evaluated through a challenge experiment with the bacterial fish pathogen Yersinia ruckeri.

RESULTS

The microbiota showed a significant change in terms of composition following β-glucan treatment, notably an increase in the relative abundance of members of the genus Aurantimicrobium, associated with a decreased abundance of the genera Carnobacterium and Deefgea. Furthermore, analysis of more than 200 metabolites revealed that the relative levels of 53 metabolites, in particular compounds related to phosphatidylcholines, were up- or downregulated in response to the dietary supplementation, this included the amino acid alanine that was significantly upregulated in the fish that had received the highest dose of β-glucan. Meanwhile, no strong effect could be detected on the resistance of the fish to the bacterial infection.

CONCLUSIONS

The present study illustrates the ability of β-glucans to modify the gut microbiota of fish, resulting in alteration of the metabolome and affecting fish health through the lipidome of rainbow trout.

Gut Associated Lymphoid Tissue (GALT) primary cells and stable cell lines as predictive models for intestinal health in rainbow trout (Oncorhynchus mykiss)

The use of functional feeds for farmed fish is now regarded as a key factor in improving fish health and performance against infectious disease. However, the mechanisms by which these nutritional components modulate the immune response are not fully understood. The present study was undertaken to identify the suitability of both primary gut-associated lymphoid tissue (GALT) leucocyte cells and established rainbow trout cell lines as potential alternative methods to test functional feed ingredients prior to full fish feeding trials that can take months to complete. In addition to the primary GALT culture cells, the two rainbow cell lines RTS11 and RTgutGC which are from macrophage and gut epithelial cells, respectively. The cells were stimulated with a variety of pathogen associated molecular patterns (PAMPs) (PHA and Poly I:C) and recombinant rainbow trout IL-1β (rIL-1β), a proinflammatory cytokine, additionally two forms of β-glucan, a prebiotic commonly used aquafeeds were used as stimulants. From this, the suitability of cell models as a health screen for functional feeds was assessed. GALT leucocytes were deemed most effective to act as a health screen over the 4hr time point demonstrating responses to Poly I:C, PHA, and rIL-1β. RTS11 and RTgutGC also responded to the stimulants but did not give a strong T-cell response, most likely reflecting the nature of the cell type as opposed to the mixed cell populations from the primary GALT cell cultures. When stimulated with both forms of β-glucan, GALT leucocytes demonstrated a strong proinflammatory and T-cell response.

Trained Immunity Provides Long-Term Protection against Bacterial Infections in Channel Catfish

Beta glucan exposure induced trained immunity in channel catfish that conferred long-term protection against Edwardsiella ictaluri and Edwardsiella piscicida infections one month post exposure. Flow cytometric analyses demonstrated that isolated macrophages and neutrophils phagocytosed higher amounts of E. ictaluri and E. piscicida. Beta glucan induced changes in the distribution of histone modifications in the monomethylation and trimethylation of H3K4 and modifications in the acetylation and trimethylation of H3K27. KEGG pathway analyses revealed that these modifications affected expressions of genes controlling phagocytosis, phagosome functions and enhanced immune cell signaling. These analyses correlate the histone modifications with gene functions and to the observed enhanced phagocytosis and to the increased survival following bacterial challenge in channel catfish. These data suggest the chromatin reconfiguration that directs trained immunity as demonstrated in mammals also occurs in channel catfish. Understanding the mechanisms underlying trained immunity can help us design prophylactic and non-antibiotic based therapies and develop broad-based vaccines to limit bacterial disease outbreaks in catfish production.

Yeast β-Glucans as Fish Immunomodulators: A Review

Simple Summary

The β-glucan obtained from yeast—a very important molecule for fish production—activates the immune system of fish by different mechanisms and induces protection against pathogens. However, most previous related studies have focused on the use of commercial β-glucan from the yeast Saccharomyces cerevisiae to understand the activation pathways. Experimental β-glucans extracted from other yeasts show other interesting biological activities even at lower doses. This review article analyzes the current information and suggests perspectives on yeast β-glucans.

Abstract

Administration of immunostimulants in fish is a preventive method to combat infections. A wide variety of these biological molecules exist, among which one of the yeast wall compounds stands out for its different biological activities. The β-glucan that forms the structural part of yeast is capable of generating immune activity in fish by cell receptor recognition. The most frequently used β-glucans for the study of mechanisms of action are those of commercial origin, with doses recommended by the manufacturer. Nevertheless, their immune activity is inefficient in some fish species, and increasing the dose may show adverse effects, including immunosuppression. Conversely, experimental β-glucans from other yeast species show different activities, such as antibacterial, antioxidant, healing, and stress tolerance properties. Therefore, this review analyses the most recent scientific reports on the use of yeast β-glucans in freshwater and marine fish.

Single-Cell Transcriptomic Analysis Reveals Neutrophil as Orchestrator during β-Glucan–Induced Trained Immunity in a Teleost Fish

Trained immunity defines long-term memory of innate immunity based on transcriptional, epigenetic, and metabolic modifications of myeloid cells, which are characterized by elevated proinflammatory responses toward homologous or heterologous secondary stimuli in mammals. However, the evidence of trained immunity-associated immune cells and its molecular mechanism in teleost fish remains largely unknown. In this study, we established a trained immunity activation model in turbot (Scophthalmus maximus) and found that administration with β-glucan induces protection against a bacterial infection. Through single-cell RNA sequencing to annotate 14 clusters of innate and adaptive immune cells, as well as two clusters of blood cells, from head kidney and spleen, respectively, we characterized that neutrophil displays cardinal features of trained immunity by analyzing the expression abundance of trained immunity database–related genes at the single-cell level. Subsequently, through establishing an in vivo training and in vitro neutrophil challenge model, we found that the trained neutrophils exhibit a significant elevation of the IL-1R signaling pathway after Edwardsiella piscicida infection. Furthermore, inhibition of neutrophil’s IL-1R signaling pathway through anakinra treatment impaired the heightened production of reactive oxygen, nitrogen species, lactate, as well as the neutrophil extracellular traps formation and bacterial killing ability. Taken together, these findings characterized neutrophil as the orchestrator to express features of trained immunity, and revealed that the IL-1R signaling pathway plays a critical role in induction of trained immunity for bacterial clearance in teleost fish.

Immune responses to prebiotics in farmed salmonid fish: How transcriptomic approaches help interpret responses

Within aquaculture, prebiotics are composed of complex carbohydrate molecules that cannot be digested by the fish directly but are metabolised by the microbial communities within the host gut, with the desire that "healthy" bacterial species are promoted with subsequently improved performance of the fish, there are likely some direct responses of intestinal cells to these dietary components. The sources and processing of prebiotics, which fall under the overarching theme of "functional feeds" are highly varied between species and types of prebiotics administered. How these feeds exert their effect, and the host responses are hard to determine, but new technologies and the development of high-throughput technologies (omics) are enabling the mechanisms and methods of action to be further understood. The recent advances in the availability of 'omics' technologies with the transition from single gene assays to microarray and RNA-seq in fish health have enabled novel functional ingredients to be analysed. This review will focus on recent studies on targeted gene expression and 'omics' technologies to characterize immune responses. Comparisons between the immunomodulatory effect of different prebiotics have been made and specific examples of how transcriptomics techniques have been used to identify immune responses to prebiotics are given.

Heat inactivated mycobacteria, alpha-gal and zebra fish: insights gained from experiences with two promising trained immunity inductors and a validated animal model

Trained immunity (TRAIM) may be defined as a form of memory where innate immune cells such as monocytes, macrophages, dendritic and natural killer (NK) cells undergo an epigenetic reprogramming that enhances their primary defensive capabilities. Cross-pathogen protective TRAIM can be triggered in different hosts by exposure to live microbes or microbe-derived products such as heat-inactivated Mycobacterium bovis or with the glycan α-Gal to elicit protective responses against several pathogens. We review the TRAIM paradigm using two models representing distinct scales of immune sensitization: the whole bacterial cell and one of its building blocks, the polysaccharides or glycans. Observations point out to macrophage lytic capabilities and cytokine regulation as two key components in nonspecific innate immune responses against infections. The study of the TRAIM response deserves attention to better characterize the evolution of host-pathogen cooperation both for identifying the etiology of some diseases and for finding new therapeutic strategies. In this field, the zebrafish provides a convenient and complete biological system that could help to deepen in the knowledge of TRAIM-mediated mechanisms in pathogen-host interactions. This article is protected by copyright. All rights reserved.

β-Glucans from Yeast—Immunomodulators from Novel Waste Resources

β-glucans are a large class of complex polysaccharides with bioactive properties, including immune modulation. Natural sources of these compounds include yeast, oats, barley, mushrooms, and algae. Yeast is abundant in various processes, including fermentation, and they are often discarded as waste products. The production of biomolecules from waste resources is a growing trend worldwide with novel waste resources being constantly identified. Yeast-derived β-glucans may assist the host’s defence against infections by influencing neutrophil and macrophage inflammatory and antibacterial activities. β-glucans were long regarded as an essential anti-cancer therapy and were licensed in Japan as immune-adjuvant therapy for cancer in 1980 and new mechanisms of action of these molecules are constantly emerging. This paper outlines yeast β-glucans’ immune-modulatory and anti-cancer effects, production and extraction, and their availability in waste streams.

β-Glucan-Induced Immuno-Modulation: A Role for the Intestinal Microbiota and Short-Chain Fatty Acids in Common Carp

Dietary supplementation of fish with β-glucans has been commonly associated with immunomodulation and generally accepted as beneficial for fish health. However, to date the exact mechanisms of immunomodulation by β-glucan supplementation in fish have remained elusive. In mammals, a clear relation between high-fibre diets, such as those including β-glucans, and diet-induced immunomodulation via intestinal microbiota and associated metabolites has been observed. In this study, first we describe by 16S rRNA sequencing the active naive microbiota of common carp intestine. Based on the abundance of the genus Bacteroides, well known for their capacity to degrade and ferment carbohydrates, we hypothesize that common carp intestinal microbiota could ferment dietary β-glucans. Indeed, two different β-glucan preparations (curdlan and MacroGard^®) were both fermented in vitro, albeit with distinct fermentation dynamics and distinct production of short-chain fatty acids (SCFA). Second, we describe the potential immunomodulatory effects of the three dominant SCFAs (acetate, butyrate, and propionate) on head kidney leukocytes, showing effects on both nitric oxide production and expression of several cytokines (il-1b, il-6, tnfα, and il-10) in vitro. Interestingly, we also observed a regulation of expression of several gpr40L genes, which were recently described as putative SCFA receptors. Third, we describe how a single in vivo oral gavage of carp with MacroGard^® modulated simultaneously, the expression of several pro-inflammatory genes (il-1b, il-6, tnfα), type I IFN-associated genes (tlr3.1, mx3), and three specific gpr40L genes. The in vivo observations provide indirect support to our in vitro data and the possible role of SCFAs in β-glucan-induced immunomodulation. We discuss how β-glucan-induced immunomodulatory effects can be explained, at least in part, by fermentation of MacroGard^® by specific bacteria, part of the naive microbiota of common carp intestine, and how a subsequent production of SFCAs could possibly explain immunomodulation by β-glucan via SCFA receptors present on leukocytes.

Toxicological damages on copper exposure to IgM+ B cells of Nile tilapia (Oreochromis niloticus) and mitigation of its adverse effects by β-glucan administration

Present investigation was carried out to study toxicological damages of copper exposure and mitigation of its adverse effects with β-glucan administration in IgM⁺ B cells which processes multiple roles similar to macrophages in Nile tilapia (Oreochromis niloticus). IgM⁺ B cells were pretreated with β-glucan (25 μg/mL) for 24 h before exposed to cupric oxide nanoparticles (CuO NPs) or cupric chloride (Cu ions) at the doses of 0, 5, 10, and 20 μg/mL for 24 h, respectively. Our results demonstrated that β-glucan increased reduced glutathione (GSH) to against oxidative damage from CuO NPs and Cu ions exposure in IgM⁺ B cells. The apoptosis process through mitochondrial signaling pathway was depressed in IgM⁺ B cells since the mitochondrial membrane potential (ΔΨm) was protected from copper exposure by β-glucan treatment. Furthermore, the inhibition on phagocytic abilities of IgM⁺ B cells caused by copper exposure could be enhanced with β-glucan treatment via evaluation of microspheres and bioparticles uptake and LPS-induced NO production. Importantly, β-glucan might participate in immunomodulation in IgM⁺ B cells through B cell antigen receptor (BCR) to suppress toxicological effect derived from copper exposure. Taken together, this study provides more information on the toxicological damages in IgM⁺ B cells upon copper exposure and explains the molecular mechanism to reverse adverse effects caused by copper exposure with β-glucan administration.

Beta glucan induced immune priming protects against nervous necrosis virus infection in sevenband grouper

In the present study, we studied the effect of β-glucan on the activation of antiviral immune responses against nervous necrosis virus (NNV) taking into consideration the role of innate immune training. Sevenband grouper primary macrophages showed an attenuated proinflammatory response and elevated antiviral response to NNV infection. In vitro, priming of β-glucan enhanced macrophage viability against NNV infection which is associated with the activation of sustained inflammatory cytokines gene expression. Observations were clear to understand that NLR Family CARD Domain Containing 3 (NLRC3) and caspase-1 activation and subsequent IL-1β production were reduced in β-glucan-primed macrophages. Subsequent markers for training including Lactate and abundance of HIF-1α were elevated in the cells following training. However, the lactate dehydrogenase (LDH) concentrations remained stable among the β-glucan stimulated infected and uninfected groups suggesting similar macrophage health in both groups. In vivo, the NNV-infected fish primed with β-glucan had a higher survival rate (60%) than the control NNV-infected group (40%). Our findings demonstrate that β-glucan induced protective responses against NNV infection and studies are underway to harness its potential applicability for prime and boost vaccination strategies.

β-glucan mitigation on toxicological effects in monocytes/macrophages of Nile tilapia (Oreochromis niloticus) following copper exposure

The protective effect of β-glucan against toxicological effects caused by copper oxide nanoparticles (Cu NPs) and copper ions (Cu ions) were studied in monocytes/macrophages (MO/MФ) of Nile tilapia (Oreochromis niloticus). Our results demonstrated that CuO NPs and Cu ions exposure aroused strong oxidative lesion in MO/MФ by detection of cellular reactive oxygen species (ROS) and reduced glutathione (GSH), as well as identification of several antioxidant-related cytokines. Meanwhile, the serious pro-inflammatory responses were accompanied during the processes of oxidative lesion by TNFα, IL-1β, and IL-6 genes validation. Copper induced MO/MФ underwent apoptosis through mitochondrial signaling pathway by mitochondrial membrane potential (ΔΨm) detection and Bax, Bcl-2, Cyt-c, Apaf-1, Caspase 9, Caspase 3 genes validation. Furthermore, the phagocytic abilities were inhibition in MO/MФ by evaluation of microspheres (0.5 and 1.0 μm beads) and bioparticles (S. agalactiae and A. hydrophila) uptake and LPS-induced NO production. However, β-glucan might participate in immunomodulation through C-type lectin receptor (CLR) and complement receptor 3 (CR3) to suppress pro-inflammatory responses, thereby revered all the copper induced aforementioned adverse effects in MO/MΦ. Taken together, our results provide insights on the mechanisms through β-glucan administration to mitigate toxicological effects of CuO NPs and Cu ions exposure to the MO/MΦ, which will benefit aspects related to fish farming and aquaculture production.

Dietary Agaricus blazei Spent Substrate Improves Disease Resistance of Nile Tilapia (Oreochromis niloticus) against Streptococcus agalactiae In Vivo

This study evaluated the effects of the feeding of spent mushroom substrate from Agaricus blazei on Nile tilapia (Oreochromis niloticus). The safety of 0–1000 μg/mL A. blazei spent substrate water extract (ABSSE) was demonstrated in the primary hepatic and splenic macrophages and the THK cell line (a cell line with characteristics of melanomacrophages) using a cytotoxicity assay. Here, 10 μg/mL of crude ABSSE promoted the phagocytic activity of macrophages and THK cells. Stimulating ABSSE-primed THK cells with lipopolysaccharides or peptidoglycan resulted in higher expression levels of four cytokine genes (e.g., interleukinz (IL)-1β, IL-12b, IL-8 and tumor necrosis factor α (TNFα)) and one cytokine gene (TNFα), respectively. An in vitro bacterial growth inhibition assay demonstrated that ABSSE could inhibit the growth of Streptococcus agalactiae. In the first feeding trial, Nile tilapia were fed with experimental feed containing 0, 1, or 5% of A. blazei spent substrate (ABSS) for seven and fourteen days followed by bacterial challenge assay. The best result was obtained when Nile tilapia were continuously fed for seven days on a diet containing 1% ABSS, with the survival rate being higher than in groups with 0% and 5% ABSS after challenge with S. agalactiae. In the second trial, fish were fed diets supplemented with 0% or 1% ABSS for seven days, and then all the groups were given the control feed for several days prior to bacterial challenge in order to investigate the duration of the protective effect provided by ABSS. The results showed that the protective effects were sustained at day 7 after the feed was switched. Overall, spent mushroom substrate from A. blazei is a cost-effective feed additive for Nile tilapia that protects fish from S. agalactiae infection.

Conservation of members of the free fatty acid receptor gene family in common carp

Accumulating evidence supports the crucial role intestinal microbiota and their metabolites play in the homeostasis of organisms. An important class of metabolites that have been shown to affect the immune system are short chain fatty acids (SCFAs). These SCFAs can affect the host cells via passive diffusion or via ligation to receptors, among others G-protein coupled receptor (GPR) 41 and 43. GPR41 and GPR43 are both part of a family of GPR40-related receptors. Mammalian studies have shown an important role for GPR41 and GPR43 in the modulation of immune responses by SCFAs. However, up till date, no validated coding sequences for orthologues of these SCFA receptors have been published for teleost fish. We used genomic resources and cDNA cloning, to identify and validate ten coding sequences for gpr40L genes in common carp. Phylogenetic analysis showed a division into three subclasses, putatively named class a, b and c, and showed the common carp genes had a closer phylogenetic relationship to mammalian GPR43 than to mammalian GPR41. Synteny analysis revealed a clear conservation of syntenic relationships between gpr40L in the genomes of spotted gar and common carp with the relevant region in the human genome. This conservation of synteny validates the genes identified, as gpr40L. Finally, presence of gpr40L genes was investigated in silico for genomes of 25 different, mostly teleost, fish species largely confirming the observations for gpr40L of common carp with regards to both, subdivision in three subclasses a-c and conservation of synteny. Our data provide an important first step towards an understanding of the role and function of receptors for SCFAs and immunomodulation in fish.

Secretome Profiling of Atlantic Salmon Head Kidney Leukocytes Highlights the Role of Phagocytes in the Immune Response to Soluble β-Glucan

β‐Glucans (BG) are glucose polymers which are produced in bacteria and fungi but not in vertebrate organisms. Being recognized by phagocytic leukocytes including macrophages and neutrophils through receptors such as dectin-1 and Complement receptor 3 (CR3), the BG are perceived by the innate immune system of vertebrates as foreign substances known as Pathogen Associated Molecular Patterns (PAMPs). The yeast-derived BG has been recognized for its potent biological activity and it is used as an immunomodulator in human and veterinary medicine. The goal of the current study was to characterize the immunostimulatory activity of soluble yeast BG in primary cultures of Atlantic salmon (Salmo salar) head kidney leukocytes (HKLs) in which phagocytic cell types including neutrophils and mononuclear phagocytes predominate. The effect of BG on the secretome of HKL cultures, including secretion of extracellular vesicles (EVs) and soluble protein55s was characterized through western blotting and mass spectrometry. The results demonstrate that, along with upregulation of proinflammatory genes, BG induces secretion of ubiquitinated proteins (UbP), MHCII-containing EVs from professional antigen presenting cells as well as proteins derived from granules of polymorphonuclear granulocytes (PMN). Among the most abundant proteins identified in BG-induced EVs were beta-2 integrin subunits, including CD18 and CD11 homologs, which highlights the role of salmon granulocytes and mononuclear phagocytes in the response to soluble BG. Overall, the current work advances the knowledge about the immunostimulatory activity of yeast BG on the salmon immune system by shedding light on the effect of this PAMP on the secretome of salmon leukocytes.

Transgenerational effects of β-glucan on thermal tolerance, growth performance, and immune gene expression of endangered cyprinid Tor putitora progeny

Nutritional programming signifies a process in which broodstock feeding approaches have long-term effects on the subsequent progeny. The present study aimed to elucidate whether supplementing golden mahseer, Tor putitora broodstock diets with β-glucan affects progeny growth performance, survival, thermal tolerance, and non-specific immunity. Initially, the growth performance of progeny produced from brooders fed with different levels of β-glucan was non-significant. However, on the 15th and 35th DPH, the maximum weight was observed in fry obtained from the brooders fed with 0.5% followed by 1.0% β-glucan. Furthermore, on 50th DPH, significantly higher weight was registered in the fry from the 0.5% β-glucan fed group while 1.0% β-glucan group had no transgenerational effect on growth. The condition factor of fry obtained from golden mahseer brooders fed with a 0.5% β-glucan diet was greater than the control and 1.0% β-glucan fed group. On the other hand, we did not find any significant transgenerational influence of β-glucan on the survival of the progeny. The thermal tolerance of fry produced from brooders fed with β-glucan was significantly modulated at both end-points (CT_max and CT_min). Expression of interleukin-1β was significantly up-regulated in fry obtained from β-glucan fed brooders. In contrast, the expression level of tumor necrosis factor-α was significantly higher only in fry produced from 1.0% β-glucan fed brooders. The expression of immunoglobulin light chain and serum amyloid A gene was significantly higher in fry produced from 0.5% β-glucan fed brooders. Overall results suggest that the dietary provisioning of β-glucan in golden mahseer brooders can be a strategy to produce healthy and robust fry in captivity for stock enhancement and conservation programs.

Immune Status and Hepatic Antioxidant Capacity of Gilthead Seabream Sparus aurata Juveniles Fed Yeast and Microalga Derived β-glucans

This work aimed to evaluate the effects of dietary supplementation with β-glucans extracted from yeast (Saccharomyces cerevisiae) and microalga (Phaeodactylum tricornutum) on gene expression, oxidative stress biomarkers and plasma immune parameters in gilthead seabream (Sparus aurata) juveniles. A practical commercial diet was used as the control (CTRL), and three others based on CTRL were further supplemented with different β-glucan extracts. One was derived from S. cerevisiae (diet MG) and two different extracts of 21% and 37% P. tricornutum-derived β-glucans (defined as Phaeo21 and Phaeo37), to give a final 0.06% β-glucan dietary concentration. Quadruplicate groups of 95 gilthead seabream (initial body weight: 4.1 ± 0.1 g) were fed to satiation three times a day for 8 weeks in a pulse-feeding regimen, with experimental diets intercalated with the CTRL dietary treatment every 2 weeks. After 8 weeks of feeding, all groups showed equal growth performance and no changes were found in plasma innate immune status. Nonetheless, fish groups fed β-glucans supplemented diets showed an improved anti-oxidant status compared to those fed CTRL at both sampling points (i.e., 2 and 8 weeks). The intestinal gene expression analysis highlighted the immunomodulatory role of Phaeo37 diet after 8 weeks, inducing an immune tolerance effect in gilthead seabream intestine, and a general down-regulation of immune-related gene expression. In conclusion, the results suggest that the dietary pulse administration of a P. tricornutum 37% enriched-β-glucans extract might be used as a counter-measure in a context of gut inflammation, due to its immune-tolerant and anti-oxidative effects.

Nervous Necrosis Virus-like Particle (VLP) Vaccine Stimulates European Sea Bass Innate and Adaptive Immune Responses and Induces Long-Term Protection against Disease

The rapidly increasing Mediterranean aquaculture production of European sea bass is compromised by outbreaks of viral nervous necrosis, which can be recurrent and detrimental. In this study, we evaluated the duration of protection and immune response in sea bass given a single dose of a virus-like particle (VLP)-based vaccine. Examinations included experimental challenge with nervous necrosis virus (NNV), serological assays for NNV-specific antibody reactivity, and immune gene expression analysis. VLP-vaccinated fish showed high and superior survival in challenge both 3 and 7.5 months (1800 and 4500 dd) post-vaccination (RPS 87 and 88, OR (surviving) = 16.5 and 31.5, respectively, p < 0.01). Although not providing sterile immunity, VLP vaccination seemed to control the viral infection, as indicated by low prevalence of virus in the VLP-vaccinated survivors. High titers of neutralizing and specific antibodies were produced in VLP-vaccinated fish and persisted for at least ~9 months post-vaccination as well as after challenge. However, failure of immune sera to protect recipient fish in a passive immunization trial suggested that other immune mechanisms were important for protection. Accordingly, gene expression analysis revealed that VLP-vaccination induced a mechanistically broad immune response including upregulation of both innate and adaptive humoral and cellular components (mx, isg12, mhc I, mhc II, igm, and igt). No clinical side effects of the VLP vaccination at either tissue or performance levels were observed. The results altogether suggested the VLP-based vaccine to be suitable for clinical testing under farming conditions.

Analysis of the white sturgeon (Acipenser transmontanus) immune response during immunostimulation and Veronaea botryosa infection

Systemic phaeohyphomycosis caused by Veronaea botryosa is regarded as an important emerging mycotic disease of sturgeon aquaculture. However, no vaccines or treatments are currently available. The effects of dietary β-glucan supplementation on resistance to V. botryosa infection was examined in controlled challenges by exposing immunostimulated and control fish to ~7.25 × 10⁵ fungal spores/fish via intra-muscular injection. Six weeks post-challenge, cumulative mortality was determined and antibodies to acute phase-proteins (APP) were used to quantify the conserved APP peptides in the serum of challenged and control fish using Western blot. Transcript levels for all tested pro-inflammatory cytokines, APP, and regulatory cytokines in the spleen were similar amongst treatments at the end of the three-week feeding period. However, significantly higher survival occurred in fingerlings fed 0.3% β-glucans compared to non-immunostimulated fish groups (p < 0.05) six weeks post-challenge. A strong proinflammatory response was detected in exposed treatment groups, and greater survival at 6 weeks was associated with higher transcript abundance of Il-17 in fish fed β-glucans. Findings support the important role of this cytokine in response to fungal infection.

A mixture of β-Glucan and Mannanoligosaccharide Ameliorated the Growth Rate, Digestive Enzyme Activity, Intestinal Morphometry, and Immunity of Common Carp (Cyprinus carpio)

The feasible strategies to keep up the growth and wellbeing of aquatic organisms by using beneficial dietary additives are highly recommended. Among these additives, β-glucan (BG) and mannanoligosaccharide (MOS) were examined on common carp performances in the current experiment. For 80 days, carps were fed four diets with 0%, 1%, 2%, and 3% BG/MOS (1:1) and distributed in four groups (triplicates). Higher final weight and weight gain (P<0.05) were detected in carps fed 1%, 2%, and 3% of BG/MOS than the control group. The feed conversion ratio showed lower values (P<0.05) in carps fed BG/MOS mixture at 2%, and 3% than 0% and 1% levels. The activities of amylase, lipase, and protease were higher in fish fed BG/MOS mixture than in the control group. Interestingly, fish fed BG/MOS mixture had markedly increased villi length and width with branched villi integrity. Further, the count of goblet cells was increased (P<0.05) in groups fed BG/MOS mixture than in carps fed the basal diet. The hemoglobin and hematocrit levels and the counts of red and white blood cells were higher in carps fed BG/MOS mixture than the control with the highest counts in 3% level. Additionally, the blood total protein, globulin, and phagocytic activity were higher (P<0.05) in groups that received BG/MOS mixture at 3% than the control. The lysozyme had a higher value in 1% group than the control (P<0.05). Therefore, using the mixture of BG/MOS at 2–3% is suggested for improving the growth rate, intestinal health, blood health, and immunity of common carp.

Immunological memory in teleost fish

Immunological memory can be regarded as the key aspect of adaptive immunity, i.e. a specific response to first contact with an antigen, which in mammals is determined by the properties of T, B and NK cells. Re-exposure to the same antigen results in a more rapid response of the activated specific cells, which have a unique property that is the immunological memory acquired upon first contact with the antigen. Such a state of immune activity is also to be understood as related to "altered behavior of the immune system" due to genetic alterations, presumably maintained independently of the antigen. It also indicates a possible alternative mechanism of maintaining the immune state at a low level of the immune response, "directed" by an antigen or dependent on an antigen, associated with repeated exposure to the same antigen from time to time, as well as the concept of innate immune memory, associated with epigenetic reprogramming of myeloid cells, i.e. macrophages and NK cells. Studies on Teleostei have provided evidence for the presence of immunological memory determined by T and B cells and a secondary response stronger than the primary response. Research has also demonstrated that in these animals macrophages and NK-like cells (similar to mammalian NK cells) are able to respond when re-exposed to the same antigen. Regardless of previous reports on immunological memory in teleost fish, many reactions and mechanisms related to this ability require further investigation. The very nature of immunological memory and the activity of cells involved in this process, in particular macrophages and NK-like cells, need to be explained. This paper presents problems associated with adaptive and innate immune memory in teleost fish and characteristics of cells associated with this ability.

Sustainable plant-based diets promote rainbow trout gut microbiota richness and do not alter resistance to bacterial infection

Background

Farmed fish food with reduced fish-derived products are gaining growing interest due to the ecological impact of fish-derived protein utilization and the necessity to increase aquaculture sustainability. Although different terrestrial plant proteins could replace fishmeal proteins, their use is associated with adverse effects. Here, we investigated how diets composed of terrestrial vegetal sources supplemented with proteins originating from insect, yeast or terrestrial animal by-products affect rainbow trout (Onchorynchus mykiss) gut microbiota composition, growth performance and resistance to bacterial infection by the fish pathogen Flavobacterium psychrophilum responsible for frequent outbreaks in aquaculture settings.

Results

We showed that the tested regimes significantly increased gut bacterial richness compared to full vegetal or commercial-like diets, and that vegetal diet supplemented with insect and yeast proteins improves growth performance compared to full vegetal diet without altering rainbow trout susceptibility to F. psychrophilum infection.

Conclusion

Our results demonstrate that the use of insect and yeast protein complements to vegetal fish feeds maintain microbiota functions, growth performance and fish health, therefore identifying promising alternative diets to improve aquaculture’s sustainability.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-021-00107-2.

Dietary Synbiotics Can Help Relieve the Impacts of Deltamethrin Toxicity of Nile Tilapia Reared at Low Temperatures

The optimal water temperature for the normal growth of Nile tilapia is between 26 and 28 °C, and the toxicity of pesticides is strongly related to water temperature. An alternate approach to augmenting the resistance of fish to ambient water toxicity and low water temperature via synbiotic feeding was proposed. In this study, fish were allocated into four groups with 10 fish in each replicate, where they were fed a basal diet or synbiotics (550 mg/kg) and kept at a suboptimal water temperature (21 ± 2 °C). The prepared diets were fed to Nile tilapia for 30 days with or without deltamethrin (DMT) ambient exposure (15 μg/L). The groups were named control (basal diet without DMT toxicity), DMT (basal diet with DMT toxicity), synbiotic (synbiotics without DMT toxicity), and DMT + synbiotic (synbiotics with DMT toxicity). The results displayed upregulated transcription of catalase, glutathione peroxidase, and interferon (IFN-γ) genes caused by DMT exposure and synbiotic feeding when compared with the controls. Moreover, HSP70 and CASP3 genes displayed increased transcription caused by DMT exposure without synbiotic feeding. However, fish fed with synbiotics showed downregulated HSP70 and CASP3 gene expressions. The transcription of IL-1β and IL-8 genes were also decreased by DMT exposure, while fish fed synbiotics showed upregulated levels. DMT exposure resulted in irregular histopathological features in gills, intestine, spleen, and liver tissues, while fish fed synbiotics showed regular, normal, and protected histopathological images. Our results indicated that dietary synbiotics ameliorated histopathological damages in DMT-exposed tilapia through alleviation of oxidative stress and inflammation as well as enhancing the immunity.

Dietary β-glucan (MacroGard®) improves innate immune responses and disease resistance in Nile tilapia regardless of the administration period

The effects of dietary β-glucan on innate immune responses have been shown in a number of different vertebrate species. However, there is conflicting information about the period of administration (shorter vs. longer), and it is also unclear to what extent β-glucan's effects can be observed post-treatment in fish. Thus, we fed Nile tilapia for 0 (control group; 45 days of control diet), 15 (30 days of control followed by 15 days of β-glucan), 30 (15 days of control followed by 30 days of β-glucan) or 45 days with a diet containing 0.1% of β-glucan (MacroGard®). We evaluated the growth performance at the end of the β-glucan feeding trial and the innate immune function immediately after the feeding trial and 7 and 14 days post-feeding trial. In addition, at day 10 post-feeding trial, we assessed the tilapia's resistance against a bacterial infection. No significant differences were observed in growth performance between the groups; however, fish fed with β-glucan for 30 and 45 days had higher (approx. 8%) relative weight gain compared to the control. Regardless of the administration period, fish fed with β-glucan had higher innate immune responses immediately after the feeding trial such as lysozyme activity in plasma, liver and intestine and respiratory burst compared to the control, and in general these differences were gradually reduced over the withdrawal period (up to 14 days). No differences were observed in the plasma hemolytic activity of the complement or myeloperoxidase activity in plasma or intestine. Moreover, fish from the control group had early mortalities (2 vs. 4-5 days post-infection, respectively) and a lower survival rate (60 vs. 80%, respectively) compared to fish fed with β-glucan for 15 or 30 days, and, interestingly, fish fed for 45 days with β-glucan had no mortality. This study indicates that regardless of the administration period (i.e., 15 up to 45 days), the β-glucan improved the innate immune responses and the tilapia's resistance to disease, and this protection could be observed up to 10 days post-feeding trial, adding in vivo evidence that β-glucan may contribute to a trained innate immunity. Additionally, we showed that a longer period of administration did not cause immunosuppression as previously hypothesized but promoted further growth and immune performance. These findings are relevant to the aquaculture industry and demonstrate that a longer β-glucan feeding protocol may be considered to achieve better results.

Trained immunity contributes to the prevention of Mycobacterium tuberculosis infection, a novel role of autophagy

Mycobacterium tuberculosis (M. tuberculosis) is the pathogen which causes tuberculosis (TB), a significant human public health threat. Co-infection of M. tuberculosis and the human immunodeficiency virus (HIV), emergence of drug resistant M. tuberculosis, and failure to develop highly effective TB vaccines have limited control of the TB epidemic. Trained immunity is an enhanced innate immune response which functions independently of the adaptive/acquired immune system and responds non-specifically to reinfection with invading agents. Recently, several studies have found trained immunity has the capability to control and eliminate M. tuberculosis infection. Over the past decades, however, the consensus was adaptive immunity is the only protective mechanism by which hosts inhibit M. tuberculosis growth. Furthermore, autophagy plays an essential role in the development of trained immunity. Further investigation of trained immunity, M. tuberculosis infection, and the role of autophagy in this process provide new possibilities for vaccine development. In this review, we present the general characteristics of trained immunity and autophagy. We additionally summarize several examples where initiation of trained immunity contributes to the prevention of M. tuberculosis infection and propose future directions for research in this area.

β-glucans: wide-spectrum immune-balancing food-supplement-based enteric (β-WIFE) vaccine adjuvant approach to COVID-19

Conventional vaccines to combat COVID-19 through different approaches are at various stages of development. The complexity of COVID-19 such as the potential mutations of the virus leading to antigenic drift and the uncertainty on the duration of the immunity induced by the vaccine have hampered the efforts to control the COVID-19 pandemic. Thus, we suggest an alternative interim treatment strategy based on biological response modifier glucans such as the Aureobasidium pullulans AFO-202-derived β-glucan, which has been reported to induce trained immunity, akin to that induced by the Bacille Calmette-Guérin vaccine, by epigenetic modifications at the central level in the bone marrow. These β-glucans act as pathogen-associated molecular patterns, activating mucosal immunity by binding with specific pathogen recognition receptors such as dectin-1 and inducing both the adaptive and innate immunity by reaching distant lymphoid organs. β-Glucans have also been used as immune adjuvants for vaccines such as the influenza vaccine. Therefore, until a conventional vaccine is widely available, an orally consumable vaccine adjuvant that acts like biosimilars, termed as the wide-spectrum immune-balancing food-supplement-based enteric (β-WIFE) vaccine adjuvant approach, with well-reported safety is worth in-depth investigation and can be considered for a clinical trial.

Systemic and Mucosal B and T Cell Responses Upon Mucosal Vaccination of Teleost Fish

The development of mucosal vaccines against pathogens is currently a highly explored area of research in both humans and animals. This is due to the fact that mucosal vaccines have the potential to best elicit protective responses at these mucosal surfaces, which represent the frontline of host defense, thus blocking the pathogen at its initial replication sites. However, in order to provide an efficient long-lasting protection, these mucosal vaccines have to be capable of eliciting an adequate systemic immune response in addition to local responses. In aquaculture, the need for mucosal vaccines has further practical implications, as these vaccines would avoid the individual manipulation of fish out of the water, being beneficial from both an economic and animal welfare point of view. However, how B and T cells are organized in teleost fish within these mucosal sites and how they respond to mucosally delivered antigens varies greatly when compared to mammals. For this reason, it is important to establish which mucosally delivered antigens have the capacity to induce strong and long-lasting B and T cell responses. Hence, in this review, we have summarized what is currently known regarding the adaptive immune mechanisms that are induced both locally and systemically in fish after mucosal immunization through different routes of administration including oral and nasal vaccination, anal intubation and immersion vaccination. Finally, based on the data presented, we discuss how mucosal vaccination strategies could be improved to reach significant protection levels in these species.

Epidermal Club Cells in Fishes: A Case for Ecoimmunological Analysis

Epidermal club cells (ECCs), along with mucus cells, are present in the skin of many fishes, particularly in the well-studied Ostariophysan family Cyprinidae. Most ECC-associated literature has focused on the potential role of ECCs as a component of chemical alarm cues released passively when a predator damages the skin of its prey, alerting nearby prey to the presence of an active predator. Because this warning system is maintained by receiver-side selection (senders are eaten), there is want of a mechanism to confer fitness benefits to the individual that invests in ECCs to explain their evolutionary origin and maintenance in this speciose group of fishes. In an attempt to understand the fitness benefits that accrue from investment in ECCs, we reviewed the phylogenetic distribution of ECCs and their histochemical properties. ECCs are found in various forms in all teleost superorders and in the chondrostei inferring either early or multiple independent origins over evolutionary time. We noted that ECCs respond to several environmental stressors/immunomodulators including parasites and pathogens, are suppressed by immunomodulators such as testosterone and cortisol, and their density covaries with food ration, demonstrating a dynamic metabolic cost to maintaining these cells. ECC density varies widely among and within fish populations, suggesting that ECCs may be a convenient tool with which to assay ecoimmunological tradeoffs between immune stress and foraging activity, reproductive state, and predator-prey interactions. Here, we review the case for ECC immune function, immune functions in fishes generally, and encourage future work describing the precise role of ECCs in the immune system and life history evolution in fishes.

Trained innate immunity

The innate immune system acts rapidly in an identical and nonspecific way every time the body is exposed to pathogens. As such, it cannot build and maintain immunological memory to help prevent reinfection. Researchers contend that trained immunity is influenced by intracellular metabolic pathways and epigenetic remodeling. The purpose of this review was to explore the topic of trained innate immunity based on the results of relevant previous studies. This systematic review entailed identifying articles related to trained innate immunity. The sources were obtained from PubMed using different search terms that included "trained innate immunity," "trained immunity," "trained," "innate," "immunity," and "immune system." Boolean operators were used to combine terms and phrases. A review of previous study results revealed that little is currently known about the molecular and cellular processes that mediate or induce a trained immune response in animals. However, it is believed that alterations in the phenotypes of cell populations and the numbers of specific cells may play a critical role in mediating the trained immune response. Increasing evidence shows that the protective processes and actions that occur during a secondary infection are not entirely linked to the adaptive immune system. Instead, these events also involve heightened activation of innate immune cells. While trained innate immune cells may have a shorter memory, they assist in the fight against pathogens and provide cross-protection. Identification of the mechanisms and molecules that underlie trained innate immunity has highlighted important features of the human immune response. Such advances continue to open doors for future research on how the body responds to disease-causing pathogens.

A novel dietary multi-strain yeast fraction modulates intestinal toll-like-receptor signalling and mucosal responses of rainbow trout (Oncorhynchus mykiss)

This study was conducted to evaluate the mucosal immune responses of rainbow trout when supplementing an experimental formulated feed with multi-strain yeast fraction product (Saccharomyces cerevisiae and Cyberlindnera jardinii). In total, 360 fish (initial BW 23.1 ± 0.2 g) were randomly allotted into three dietary treatments in an 8-week feeding trial. The dietary treatments included basal diet (control) and control + 1.5 g/kg multi-strain yeast fraction product (MsYF) fed continuously and pulsed every two weeks between control and MsYF diet. No negative effects on growth performance of feeding the MsYF supplemented diet were observed. SGR and FCR averaged 2.30 ± 0.03%/day and 1.03 ± 0.03, respectively, across experimental groups. Muscularis thickness in the anterior intestine after 8 weeks of feeding was significantly elevated by 44.3% in fish fed the MsYF continuously, and by 14.4% in fish fed the MsYF pulsed (P < 0.02). Significant elevations in goblet cell density in the anterior and posterior (>50% increase) intestine were observed after 8 weeks of feeding the MsYF supplemented diet (P< 0.03). In contrast, lamina propria width was significantly lower in fish fed the experimental diets (>10% reduction). The gene expression analysis of the intestine revealed significant elevations in expression of tlr2, il1r1, irak4, and tollip2 after 4 weeks of feeding the MsYF. Significant elevations in effector cytokines tnfα, il10 and tgfβ were observed after 4 weeks of feeding the MsYF regime. After 8 weeks significant elevations in the gene expression levels of il1β, ifnγ, and il12 were observed in fish fed the MsYF. Likewise, the expression of the transcription factor gata3 was significantly elevated (P<0.01). Supplementation of the multi-strain yeast fraction product positively modulates the intestinal mucosal response of rainbow trout through interaction with toll-like receptor two signalling pathway and potential for increased capacity of delivery of antigens to the underlying mucosal associated lymphoid tissue.

Stimulation of Mytilus galloprovincialis Hemocytes With Different Immune Challenges Induces Differential Transcriptomic, miRNomic, and Functional Responses

Mediterranean mussels (Mytilus galloprovincialis) are marine bivalve molluscs with high resilience to biotic and abiotic stress. This resilience is one of the reasons why this species is such an interesting model for studying processes such as the immune response. In this work, we stimulated mussel hemocytes with poly I:C, β-glucans, and LPS and then sequenced hemocyte mRNAs (transcriptome) and microRNAs (miRNome) to investigate the molecular basis of the innate immune responses against these pathogen-associated molecular patterns (PAMPs). An immune transcriptome comprising 219,765 transcripts and an overview of the mussel miRNome based on 5,175,567 non-redundant miRNA reads were obtained. The expression analyses showed opposite results in the transcriptome and miRNome; LPS was the stimulus that triggered the highest transcriptomic response, with 648 differentially expressed genes (DEGs), while poly I:C was the stimulus that triggered the highest miRNA response, with 240 DE miRNAs. Our results reveal a powerful immune response to LPS as well as activation of certain immunometabolism- and ageing/senescence-related processes in response to all the immune challenges. Poly I:C exhibited powerful stimulating properties in mussels, since it triggered the highest miRNomic response and modulated important genes related to energy demand; these effects could be related to the stronger activation of these hemocytes (increased phagocytosis, increased NO synthesis, and increased velocity and accumulated distance). The transcriptome results suggest that after LPS stimulation, pathogen recognition, homeostasis and cell survival processes were activated, and phagocytosis was induced by LPS. β-glucans elicited a response related to cholesterol metabolism, which is important during the immune response, and it was the only stimulus that induced the synthesis of ROS. These results suggest a specific and distinct response of hemocytes to each stimulus from a transcriptomic, miRNomic, and functional point of view.

Natural Feed Additives Modulate Immunity and Mitigate Infection with Sphaerospora molnari (Myxozoa:Cnidaria) in Common Carp: A Pilot Study

Myxozoans are a diverse group of cnidarian parasites, including important pathogens in different aquaculture species, without effective legalized treatments for fish destined for human consumption. We tested the effect of natural feed additives on immune parameters of common carp and in the course of a controlled laboratory infection with the myxozoan Sphaerospora molnari. Carp were fed a base diet enriched with 0.5% curcumin or 0.12% of a multi-strain yeast fraction, before intraperitoneal injection with blood stages of S. molnari. We demonstrate the impact of these treatments on respiratory burst, phagocytosis, nitric oxide production, adaptive IgM+ B cell responses, S. molnari-specific antibody titers, and on parasite numbers. Both experimental diets enriched B cell populations prior to infection and postponed initial parasite proliferation in the blood. Curcumin-fed fish showed a decrease in reactive oxygen species, nitric oxide production and B cell density at late-stage infection, likely due to its anti-inflammatory properties, favoring parasite propagation. In contrast, multi-strain yeast fraction (MsYF)-fed fish harbored the highest S. molnari-specific antibody titer, in combination with the overall lowest parasite numbers. The results demonstrate that yeast products can be highly beneficial for the outcome of myxozoan infections and could be used as effective feed additives in aquaculture.

Development of Fish Immunity and the Role of β-Glucan in Immune Responses

Administration of β-glucans through various routes, including immersion, dietary inclusion, or injection, have been found to stimulate various facets of immune responses, such as resistance to infections and resistance to environmental stress. β-Glucans used as an immunomodulatory food supplement have been found beneficial in eliciting immunity in commercial aquaculture. Despite extensive research involving more than 3000 published studies, knowledge of the receptors involved in recognition of β-glucans, their downstream signaling, and overall mechanisms of action is still lacking. The aim of this review is to summarize and discuss what is currently known about of the use of β-glucans in fish.

Effects of dietary yeast cell wall on biochemical indices, serum and skin mucus immune responses, oxidative status and resistance against Aeromonas hydrophila in juvenile Persian sturgeon (Acipenser persicus)

The present study aims to shed light on the effects of yeast cell wall (ImmunoWall®) supplementation on biochemical indices, oxidative status, serum and mucus immune responses as well as disease resistance of juvenile Persian sturgeon (Acipenser persicus). For this purpose, one hundred fifty three juvenile Persian sturgeons (47.78 ‏± 0.39 g) were distributed into nine tanks (500 L) and fed with basal diets containing two levels of yeast cell wall (YCW) 0.5% (T1) and 1% (T2) and a diet without YCW as control (0%). As shown by the results obtained at the end of 56-day feeding trial, YCW had no significant effect on glucose, cortisol, SGOT, lysozyme and IgM in serum (P > 0.05) albeit an enhancement of cholesterol, LDH, ALP and SOD and ACH50 was observed in fish fed YCW supplemented diets. However, plasma triglyceride levels were lower in fish fed YCW compared with the control group. Also, total protein content, lysozyme and protease activities in skin mucus were unaffected by the supplemented diets (P > 0.05) and only total immunoglobulin and ALP enzyme activity were significantly increased in T₁ and T₂ groups (P > 0.05). The cumulative mortality of the fish fed supplemented diets at the end of disease challenge was 100% where cumulative mortality of those fed the control diet was 75% (P < 0.05). The present study shows that increasing immune parameters in serum and mucus of juvenile Persian sturgeon by YCW dietary supplementation did not improve resistance against Aeromonas hydrophila. According to the obtained results, the YCW supplementation at 0.5 and 1% in the juvenile Persian sturgeon diet is not recommended.

Microalgal extracts induce larval programming and modify growth and the immune response to bioactive treatments and LCDV in Senegalese sole post-larvae

Immunostimulants are key molecules in aquaculture since they heighten defensive responses and protection against pathogens. The present study investigated the treatment of Senegalese sole larvae with a whole-cell crude extract of the microalgae Nannochloropsis gaditana (Nanno) and programming of growth and the immune system. Larvae at hatch were treated with the Nanno extracts for 2 h and thereafter were cultivated for 32 days post-hatch (dph) in parallel with an untreated control group (CN). Dry weight and length at 21 days post-hatch (dph) were higher in post-larvae of the Nanno than CN group. These differences in weight were later confirmed at 32 dph. To evaluate changes in the immune response associated with Nanno-programming treatments, the Nanno and CN post-larvae were supplied with two bioactive compounds yeast β-glucan (Y) and a microalga extract from the diatom Phaeodactylum tricornutum (MAe). The bioactive treatments were administrated to the treatment groups through the live prey (artemia metanauplii, 200 artemia mL^-1) enriched for 30 min with MAe or Y (at 2 mg mL^-1 SW) or untreated prey in the case of the negative control (SW). The effect of the treatments was assessed by monitoring gene expression, enzyme activity and mortality over 48 h. The post-larvae sole supplied with the bioactive compounds Y and MAe had increased mortality at 48 h compared to the SW group. Moreover, mortality was higher in Nanno-programmed than CN post-larvae. Lysozyme and total anti-protease enzymatic activities at 6 and 24 h after the start of the trial were significantly higher in the Nanno and MAe supplied post-larvae compared to their corresponding control (CN and SW, respectively). Immune gene transcripts revealed that il1b, cxc10 and mx mRNAs were significantly different between Nanno and CN post-larvae at 6 and 24 h. Moreover, the expression of il1b, tnfa, cxc10, irf3, irf7 and mx was modified by bioactive treatments but with temporal differences. At 48 h after bioactive treatments, Y and SW post-larvae were challenged with the lymphocystis disease virus (LCDV). No difference existed in viral copy number between programming or bioactive treatment groups at 3, 6 and 24 h after LCDV challenge although the total number of copies reduced with time. Gene expression profiles in the LCDV-challenged group indicated that post-larvae triggered a wide defensive response compared to SWC 24 h after challenge, which was modulated by programming and bioactive compound treatments. Cluster analysis of expressed genes separated the SW and Y groups indicating long-lasting effects of yeast β-glucan treatment in larvae. A noteworthy interaction between Nanno-programming and Y-treatment on the regulation of antiviral genes was observed. Overall, the data demonstrate the capacity of microalgal crude extracts to modify sole larval plasticity with long-term effects on larval growth and the immune responses.