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

An Overview of the Biological Functions and Mechanisms of Action of Medicinal Plants and Seaweeds in the Shrimp Culture

Diseases are major constraints to developing large-scale aquaculture practices in many countries. For decades, synthetic chemotherapeutic agents have been widely applied both as prophylactics and therapeutics to inhibit and control aquatic disease outbreaks. However, their use has become more restricted due to the negative impacts they have on the environment, the host and humans, as well as their limitations in preventing the emergence of antimicrobial-resistant bacteria. Therefore, there is a growing interest in the use of medicinal plants and seaweeds as potential alternatives to antibiotics and other synthetic chemotherapeutics. Medicinal plants and seaweeds can enhance the immune systems of animals, thereby providing protection against numerous diseases while minimizing the adverse effects associated with synthetic chemotherapeutics. Furthermore, the advantages of medicinal plants and seaweeds, such as their effectiveness, easy availability and ability to be applied on a large scale, make them appealing for use in the aquaculture industry. The main goal of this study was to review the existing knowledge of the effects of medicinal plants and seaweeds, as well as their extracts, on shrimp growth, immune response and disease resistance against bacterial and viral agents. Moreover, this paper discusses the application of seaweeds in shrimp culture. We also conducted a literature review to identify gaps in the research and provide recommendations for further advancement in this field of study. Further studies should focus on evaluating other physiological aspects, such as feed and mineral utilization, enzyme activities and histological examination.

Distribution and response strategies of non-specific cytotoxic cell receptor protein 1 in large yellow croaker

The non-specific cytotoxic cell receptor protein 1 (NCCRP1) is considered the universal marker for teleost non-specific cytotoxic cells (NCCs). However, the specific distribution characteristics and response patterns of NCCRP1, and the confirmed existence of NCCs in fish species remain debatable. In this study, we investigated the distribution of NCCRP1 in the croaker and observed the most dominant abundance in the head kidney. While most common markers of cytotoxicity were localized in the trunk kidney lymphocytes (TKLs) and spleen lymphocytes (SPLs), NCCRP1-positive cells were predominantly detected in head kidney lymphocytes (HKLs) with a positive rate of approximately 10 %, where present a huge amount of macrophages (Mϕ) as well. Furthermore, the remarkable induction evidence of NCCRP1 in HKLs was determined. Collectively, these findings contribute significantly to comprehending the immunological function of NCCRP1 in fish species and enhancing our understanding of its evolutionary development.

Nanobubble applications in aquaculture industry for improving harvest yield, wastewater treatment, and disease control

The ever-growing demand for aquaculture has led the industry to seek novel approaches for more sustainable practices. These attempts aim to increase aquaculture yield by increasing energy efficiency and decreasing footprint and chemical demand without compromising animal health. For this, emerging nanobubbles (NBs) aeration technology gained attention. NBs are gas-filled pockets suspended as sphere-like cavities (bulk NBs) or attached to surfaces (surface NBs) with diameters of <1 μm. Compared to macro and microbubbles, NBs have demonstrated unique characteristics such as long residence times in water, higher gas mass transfer efficiency, and hydroxyl radical production. This paper focuses on reviewing NB technology in aquaculture systems by summarizing and discussing uses and implications. Three focus areas were targeted to review the applicability and effects of NBs in aquaculture: (i) NBs aeration to improve the aquaculture harvest yield and subsequent wastewater treatment; (ii) NB application for inactivation of harmful microorganisms; and (iii) NBs for reducing oxidative stress and improving animal health. Thus, this study reviews the research studies published in the last 10 years in which air, oxygen, ozone, and hydrogen NBs were tested to improve gas mass transfer, wastewater treatment, and control of pathogenic microorganisms. The experimental results indicated that air and oxygen NBs yield significantly higher productivity, growth rate, total harvest, survival rate, and less oxygen consumption in fish and shrimp farming. Secondly, the application of air and ozone NBs demonstrated the ability of efficient pollutant degradation. Third, NB application demonstrated effective control of infectious bacteria and viruses, and thus increased fish survival, as well as different gene expression patterns that induce immune responses to infections. Reviewed studies lack robust comparative analyses of the efficacy of macro- and microbubble treatments. Also, potential health and safety implications, as well as economic feasibility through factors such as changes in capital infrastructure, routine maintenance and energy consumption need to be considered and evaluated in parallel to applicability. Therefore, even with a promising future, further studies are needed to confirm the benefits of NB treatment versus conventional aquaculture practices.

Effects of okra (Abelmoschus esculentus L.) leaves, fruits and seeds extracts on European sea bass (Dicentrarchus labrax) leukocytes, and their cytotoxic, bactericidal and antioxidant properties

Okra, Abelmoschus esculentus L., is a popular vegetable crop with many bioactive compounds. The in vitro immunostimulant, cytotoxic, bactericidal and antioxidant activities of ethanolic extracts obtained from different parts of okra (leaves, fruits, and seeds) were studied. Phytochemical screening of hydroalcoholic extracts of okra leaves, fruits and seeds revealed a significant content of total phenols and flavonoids. Significant effects on the activities of leukocytes in the head kidney of European sea bass (Dicentrarchus labrax) (viability, phagocytic ability and capacity, and respiratory burst), as well as on peroxidase leukocyte contents were detected after incubation for 24 h with different concentrations (0.01-1 mg mL^-1) of the extracts. The mean concentrations (0.1 and 0.5 mg mL^-1) of the different extracts increased the phagocytic ability and respiratory activity of head kidney leukocytes. However, the mean concentrations (0.1 mg mL^-1) of leaf and fruit extracts significantly decreased the peroxidase activity of leukocytes. In addition, all ethanolic okra extracts at higher concentrations (1 mg mL^-1) produced a marked reduction in the viability of the DLB-1 cell line compared to the viability recorded in the control samples. In addition, ethanolic extracts used at 0.5 and 1 mg mL^-1 had a significant cytotoxic effect on the viability of PLHC-1 cells. Finally, all doses of seed and leaf extracts at higher concentrations (0.5 and 1 mg mL^-1) showed significant bactericidal activity on two fish pathogenic bacteria, Vibrio anguillarum and V. harveyi strains. Finally, a remarkable antioxidant activity was detected on the ethanolic extracts. All these results point to their possible use as an alternative to chemical compounds in farmed fish.

Adipose Fin as a Natural “Optical Window” for Implantation of Fluorescent Sensors into Salmonid Fish

Simple Summary

Novel optical sensors require implantation into the most transparent organs in order to ensure the most reliable and rapid monitoring of animal health. Widely farmed salmonid fish, such as rainbow trout, have highly translucent adipose fin, which we tested here and showed its high potential as the implantation site for the fluorescent sensors. The filamentous sensors were convenient to inject into the fin, and their optical signal was easily detectable using a simple hand-held device even without immobilization of the fish. Responsiveness of the sensors inside the adipose fin to bodily changes was shown under induced acidosis of fish fluids. The obtained results characterize adipose fin as the favorable site for implantation of optical sensors into salmonids for real-time tracking animal physiological status in basic research and aquaculture.

Abstract

Implantable optical sensors are emerging tools that have the potential to enable constant real-time monitoring of various internal physiological parameters. Such a possibility will open new horizons for health control not only in medicine, but also in animal husbandry, including aquaculture. In this study, we analyze different organs of commonly farmed rainbow trout (Oncorhynchus mykiss) as implantation sites for fluorescent sensors and propose the adipose fin, lacking an endoskeleton, as the optimal choice. The fin is highly translucent due to significantly thinner dermis, which makes the detectable fluorescence of an implanted sensor operating at the visible light range by more than an order of magnitude higher relative to the skin. Compared to the proximal parts of ray fins, the adipose fin provides easy implantation and visualization of the sensor. Finally, we tested fluorescent pH sensors inside the adipose fin and demonstrated the possibility of acquiring their signal with a simple hand-held device and without fish anesthesia. All these features will most likely make the adipose fin the main “window” into the internal physiological processes of salmonid fish with the help of implantable optical sensors.

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.

Assessing Fish Immunotoxicity by Means of In Vitro Assays: Are We There Yet?

There is growing awareness that a range of environmental chemicals target the immune system of fish and may compromise the resistance towards infectious pathogens. Existing concepts to assess chemical hazards to fish, however, do not consider immunotoxicity. Over recent years, the application of in vitro assays for ecotoxicological hazard assessment has gained momentum, what leads to the question whether in vitro assays using piscine immune cells might be suitable to evaluate immunotoxic potentials of environmental chemicals to fish. In vitro systems using primary immune cells or immune cells lines have been established from a wide array of fish species and basically from all immune tissues, and in principal these assays should be able to detect chemical impacts on diverse immune functions. In fact, in vitro assays were found to be a valuable tool in investigating the mechanisms and modes of action through which environmental agents interfere with immune cell functions. However, at the current state of knowledge the usefulness of these assays for immunotoxicity screening in the context of chemical hazard assessment appears questionable. This is mainly due to a lack of assay standardization, and an insufficient knowledge of assay performance with respect to false positive or false negative signals for the different toxicant groups and different immune functions. Also the predictivity of the in vitro immunotoxicity assays for the in vivo immunotoxic response of fishes is uncertain. In conclusion, the currently available database is too limited to support the routine application of piscine in vitro assays as screening tool for assessing immunotoxic potentials of environmental chemicals to fish.

Blood cell characterization and transcriptome analysis reveal distinct immune response and host resistance of different ploidy cyprinid fish following Aeromonas hydrophila infection

Aeromonas hydrophila can pose a great threat to survival of freshwater fish. In this study, A. hydrophila infection could decrease blood cell numbers, promote blood cell damage as well as alter the levels of alkaline phosphatase (ALP), lysozyme (LZM), aspartate aminotransferase (AST), total antioxidant capacity (T-AOC), total superoxide dismutase (SOD), catalase (CAT) and malondialdehyde (MDA) in immune-related tissues of red crucian carp (RCC, 2 N = 100) and triploid cyprinid fish (3 N fish, 3 N = 150). In addition, the significant alternation of antioxidant status was observed in PBMCs isolated from RCC and 3 N following LPS stimulation. The core differential expression genes (DEGs) involved in apoptosis, immunity, inflammation and cellular signals were co-expressed differentially in RCC and 3 N following A. hydrophila challenge. NOD-like receptor (NLR) signals appeared to play a critical role in A. hydrophila-infected fish. DEGs of NLR signals in RCCah vs RCCctl were enriched in caspase-1-dependent Interleukin-1β (IL-1β) secretion, interferon (IFN) signals as well as cytokine activation, while DEGs of NLR signals in 3Nah vs 3Nctl were enriched in caspase-1-dependent IL-1β secretion and antibacterial autophagy. These results highlighted the differential signal regulation of different ploidy cyprinid fish to cope with bacterial infection.

To React or Not to React: The Dilemma of Fish Immune Systems Facing Myxozoan Infections

Myxozoans are microscopic, metazoan, obligate parasites, belonging to the phylum Cnidaria. In contrast to the free-living lifestyle of most members of this taxon, myxozoans have complex life cycles alternating between vertebrate and invertebrate hosts. Vertebrate hosts are primarily fish, although they are also reported from amphibians, reptiles, trematodes, mollusks, birds and mammals. Invertebrate hosts include annelids and bryozoans. Most myxozoans are not overtly pathogenic to fish hosts, but some are responsible for severe economic losses in fisheries and aquaculture. In both scenarios, the interaction between the parasite and the host immune system is key to explain such different outcomes of this relationship. Innate immune responses contribute to the resistance of certain fish strains and species, and the absence or low levels of some innate and regulatory factors explain the high pathogenicity of some infections. In many cases, immune evasion explains the absence of a host response and allows the parasite to proliferate covertly during the first stages of the infection. In some infections, the lack of an appropriate regulatory response results in an excessive inflammatory response, causing immunopathological consequences that are worse than inflicted by the parasite itself. This review will update the available information about the immune responses against Myxozoa, with special focus on T and B lymphocyte and immunoglobulin responses, how these immune effectors are modulated by different biotic and abiotic factors, and on the mechanisms of immune evasion targeting specific immune effectors. The current and future design of control strategies for myxozoan diseases is based on understanding this myxozoan-fish interaction, and immune-based strategies such as improvement of innate and specific factors through diets and additives, host genetic selection, passive immunization and vaccination, are starting to be considered.

Haematological and metabolic profiles associated with age and sex in giant kokopu (Galaxias argenteus) (Gmelin 1789) broodstock

This study characterized selected peripheral blood (PB) haematological parameters, liver, serum and muscle metabolic features in 3- and 5-year-old male and female giant kokopu (Galaxias argenteus) broodstock reared indoor at 16°C. Sex and age did not affect PB total cell count and haematocrit values. Nonetheless, higher erythrocytes in 5-year-old fish, elevated thrombocyte and lymphocyte counts in 3-year-old fish indicate age-specific cellular regulation. Higher thrombocyte counts in female fish suggest sex-specific regulation. At a metabolic level, liver abundance for long chain saturated fatty acids (FAs) was higher in males, whereas females had elevated levels of polyunsaturated FAs. Essential and non-essential amino acids (AAs) in liver and serum were also elevated in females compared to males. These findings suggest differential allocation of FAs and AAs to reflect requirements for gonadal, development and provisioning. Similarly, age significantly resulted in higher liver and serum abundances of some non-essential AAs in 3-year-olds compared to 5-year-old fish, suggesting higher metabolism in younger fish. Overall, results enhance our understanding of sex- and age-based differences in fish haematology, muscle, liver, and serum metabolite profiles in healthy G. argenteus. Future studies should carefully consider potential age- and sex-specific differences in metabolic responses.

Optimisation of phagocytosis assay in rainbow trout (Oncorhynchus mykiss)

A phagocytosis assay is one of the most commonly used functional immunological methods. There are many possible ways of assessing leucocytes and their ability to ingest different particles. The aim of this study was to optimise the phagocytosis assay in rainbow trout (Oncorhynchus mykiss) using labelled zymosan particles (Alexa Fluor 488 and Texas Red conjugate). Whole blood was incubated with the particles under different conditions and leucocytes were subsequently isolated by haemolysis in a hypotonic environment. The effect of the different incubation time, temperature, blood volume and dilution on the phagocytic activity was evaluated by flow cytometry. Our experiments showed that the incubation for at least 2 h at 15 °C provided optimal results, while the blood volume and dilution had no significant effect. The optimised assay will be used for the examination of fish health and in further experimental studies.

Blood Will Tell: What Hematological Analyses Can Reveal About Fish Welfare

Blood analyses provide substantial information about the physiological aspects of animal welfare assessment, including the activation status of the neuroendocrine and immune system, acute and long-term impacts due to adverse husbandry conditions, potential diseases, and genetic predispositions. However, fish blood is still not routinely analyzed in research or aquaculture for the assessment of health and/or welfare. Over the years, the investigative techniques have evolved from antibody-based or PCR-based single-parameter analyses to now include transcriptomic, metabolomic, and proteomic approaches and from hematological observations to fluorescence-activated blood cell sorting in high-throughput modes. The range of testing techniques established for blood is now broader than for any other biogenic test material. Evaluation of the particular characteristics of fish blood, such as its cell composition, the nucleation of distinct blood cells, or the multiple isoforms of certain immune factors, requires adapted protocols and careful attention to the experimental designs and interpretation of the data. Analyses of fish blood can provide an integrated picture of the endocrine, immunological, reproductive, and genetic functions under defined environmental conditions and treatments. Therefore, the scarcity of high-throughput approaches using fish blood as a test material for fish physiology studies is surprising. This review summarizes the wide range of techniques that allow monitoring of informative fish blood parameters that are modulated by different stressors, conditions, and/or treatments. We provide a compact overview of several simple plasma tests and of multiparametric analyses of fish blood, and we discuss their potential use in the assessment of fish welfare and pathologies.

Novel insights into host-pathogen interactions of large yellow croakers ( Larimichthys crocea) and pathogenic bacterium Pseudomonas plecoglossicida using time-resolved dual RNA-seq of infected spleens

Host-pathogen interactions are highly complex, involving large dynamic changes in gene expression during infection. These interactions are fundamental to understanding anti-infection immunity of hosts, as well as the pathogenesis of pathogens. For bacterial pathogens interacting with animal hosts, time-resolved dual RNA-seq of infected tissue is difficult to perform due to low pathogen load in infected tissue. In this study, an acute infection model of Larimichthys crocea infected by Pseudomonas plecoglossicida was established. The spleens of infected fish exhibited typical symptoms, with a maximum bacterial load at two days post-injection (dpi). Time-resolved dual RNA-seq of infected spleens was successfully applied to study host-pathogen interactions between L. crocea and P. plecoglossicida. The spleens of infected L. crocea were subjected to dual RNA-seq, and transcriptome data were compared with those of noninfected spleens or in vitro cultured bacteria. Results showed that pathogen-host interactions were highly dynamically regulated, with corresponding fluctuations in host and pathogen transcriptomes during infection. The expression levels of many immunogenes involved in cytokine-cytokine receptor, Toll-like receptor signaling, and other immune-related pathways were significantly up-regulated during the infection period. Furthermore, metabolic processes and the use of oxygen in L. crocea were strongly affected by P. plecoglossicida infection. The WGCNA results showed that the metabolic process was strongly related to the entire immune process. For P. plecoglossicida, the expression levels of motility-related genes and flagellum assembly-related genes were significantly up-regulated. The results of this study may help to elucidate the interactions between L. crocea and P. plecoglossicida.