Biology of Bony Fish Macrophages

Catechol compounds as dual-targeting agents for fish protection against Ichthyophthirius multifiliis infections

Aquaculture is one of the fastest growing sectors in global food production, recognized as a significant contributor to poverty alleviation, food security, and income generation. However, the frequent occurrence of diseases caused by pathogen infections result in reduced yields and economic losses, posing a substantial constraint to the sustainable development of aquaculture. Here, our study identified that four catechol compounds, quercetin, luteolin, caffeic acid, and chlorogenic acid, exhibited potent antiparasitic effects against Ichthyophthirius multifiliis in both, in vitro and in vivo. The parasite is recognized as one of the most pathogenic to fish worldwide. Using a combination of in silico methods, the dipeptidyl peptidase (DPP) was identified as a critical target for catechol compounds. The two hydroxyl radicals of the catechol group were essential for its binding to and interacting with the DPP protein. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses indicated that catechol compounds disrupt pathways associated with the metabolism and growth of I. multifiliis, thereby exerting antiparasitic effects. Furthermore, these compounds attenuated the expression of proinflammatory cytokines in vivo in fish and promoted macrophage polarization toward M2 phenotype by inhibiting the STAT1 signaling pathway. The dual activity of catechol compounds, acting as both direct antiparasitic and anti-inflammatory agents in fish, offers a promising therapeutic approach for combating I. multifiliis infections in aquaculture.

A novel long non-coding RNA linc-93.2 participates in bisphenol induced oxidative stress and macrophage polarization in red common carp (Cyprinus carpio)

Previous studies show that bisphenol A (BPA) and its analogs induce oxidative stress and promote inflammatory response. However, the key molecules in regulating this process remain unclear. Here, we report significant inductive effects of BPA and bisphenol AF (BPAF) on a newly found long non-coding RNA linc-93.2 accompanied by oxidative stress and activation of pro-inflammatory pathways in treated fish and fish primary macrophages. Silencing linc-93.2 in fish primary macrophages in vitro or fish in vivo significantly promotes the expression of anti-oxidative stress-related genes and anti-inflammatory cytokines. This inhibition of pro-inflammatory cytokine expression, showing cell status disruption towards to M2 polarization. Followed by exposure to BPA or BPAF, silencing linc-93.2 in vitro or in vivo significantly attenuates the increased production of reactive oxygen species and malondialdehyde level aroused by bisphenol treatment, possibly owing to the enhancement of total antioxidant capacity observed in cells and tissue after linc-93.2 knockdown. RNA-sequencing further revealed regulation of nuclear factor-kappa b (NF-κB) in linc-93.2's downstream network, combining with our previous observation on the upstream regulation of linc-93.2 via NF-κB, which together suggest a critical role of linc-93.2 in promoting NF-κB positive feedback loop that may be an important molecular event initiating the immunotoxicity of bisphenols.

The differential impact of iron on ferroptosis, oxidative stress, and inflammatory reaction in head-kidney macrophages of yellow catfish (Pelteobagrus fulvidraco) with and without ammonia stress

Ammonia toxicity in fish is closely related to ferroptosis, oxidative stress, and inflammatory responses. Iron is an essential trace element that plays a key role in many biological processes for cells and organisms, including ferroptosis, oxidative stress response, and inflammation. This study aimed to investigate the effect of iron on indicators of fish exposed to ammonia, specifically on the three aspects mentioned above. The head kidney macrophages of yellow catfish were randomly assigned to one of four groups: CON (normal control), AM (0.046 mg L-1 total ammonia nitrogen), Fe (20 μg mL-1 FeSO4), and Fe + AM (20 μg mL-1 FeSO4, 0.046 mg L-1 total ammonia nitrogen). The cells were pretreated with FeSO4 for 6 h followed by ammonia for 24 h. The study found that iron supplementation led to an excessive accumulation of iron and ROS in macrophages, but it did not strongly induce ferroptosis, oxidative stress, or inflammatory responses. This was supported by a decrease in T-AOC, and the downregulation of SOD, as well as an increase in GSH levels and the upregulation of TFR1, CAT and Nrf2. Furthermore, the mRNA expression of HIF-1, p53 and the anti-inflammatory M2 macrophage marker Arg-1 were upregulated. The results also showed that iron supplementation increased the progression of some macrophages from early apoptosis to late apoptotic cells. However, the combined treatment of iron and ammonia resulted in a stronger intracellular ferroptosis, oxidative stress, and inflammatory reaction compared to either treatment alone. Additionally, there was a noticeable increase in necrotic cells in the Fe + AM and AM groups. These findings indicate that the biological functions of iron in macrophages of fish may vary inconsistently in the presence or absence of ammonia stress.

Mannose modified targeted immersion vaccine delivery system improves protective immunity against Infectious spleen and kidney necrosis virus in mandarin fish (Siniperca chuatsi)

Vaccination is an effective method to prevent viral diseases. However, the biological barrier prevents the immersion vaccine from achieving the best effect without adding adjuvants and carriers. Researches on the targeted presentation technology of vaccines with nanocarriers are helpful to develop immersion vaccines for fish that can break through biological barriers and play an effective role in fish defense. In our study, functionally modified single-walled carbon nanotubes (SWCNTs) were used as carriers to construct a targeted immersion vaccine (SWCNTs-M-MCP) with mannose modified major capsid protein (MCP) to target antigen-presenting cells (APCs), against iridovirus diseases. After bath immunization, our results showed that SWCNTs-M-MCP induced the presentation process and uptake of APCs, triggering a powerful immune response. Moreover, the highest relative percent survival (RPS) was 81.3% in SWCNTs-M-MCP group, which was only 41.5% in SWCNTs-MCP group. Altogether, this study indicates that the SWCNTs-based targeted immersion vaccine induces strong immune response and provided an effective protection against iridovirus diseases.

Extracellular ATP- and adenosine-mediated purinergic signaling modulates inducible nitric oxide synthase (iNOS) gene expression, enzyme activity and nitric oxide production in common carp (Cyprinus carpio) head kidney macrophages

Inducible nitric oxide (NO) synthase (iNOS) is a key immune mediator for production of inflammatory mediator NO from l-arginine. Tight regulation of iNOS expression and enzyme activity is critical for proper NO productions under inflammation and infection conditions. However, the regulatory mechanism for iNOS expression and enzyme activity in fish remains largely unknown. Here, we show that extracellular ATP treatment significantly up-regulates iNOS gene expression and enzyme activity, and consequently leads to enhanced NO production in Cyprinus carpio head kidney macrophages (HKMs). We further show that the extracellular ATP-induced iNOS enzyme activity and NO production can be attenuated by pharmacological inhibition of the ATP-gated P2X4 and P2X7 receptors with their respective specific antagonists, but enhanced by overexpression of P2X4 and P2X7 receptors in grass carp ovary cells. In contrast, adenosine administration significantly reduces iNOS gene expression, enzyme activity and NO production in carp HKMs, and these inhibitory effects can be reversed by pharmacological inhibition of adenosine receptors with the antagonist XAC. Furthermore, LPS- and poly(I:C)-induced iNOS gene expression, enzyme activity, and NO production are significantly attenuated by blockade of P2X4 and P2X7 receptors with their respective specific antagonists in carp HKMs, while overexpression of P2X and P2X7 receptors results in enhanced iNOS gene expression, enzyme activity and NO production in LPS- and poly(I:C)-treated grass carp ovary cells. Taken together, we firstly report an opposite role of extracellular ATP/adenosine-mediated purinergic signaling in modulating iNOS-NO system activity in fish.

CL-K1 Promotes Complement Activation and Regulates Opsonophagocytosis of Macrophages with CD93 Interaction in a Primitive Vertebrate

Collectin is a crucial component of the innate immune system and plays a vital role in the initial line of defense against pathogen infection. In mammals, collectin kidney 1 (CL-K1) is a soluble collectin that has recently been identified to have significant functions in host defense. However, the evolutionary origins of immune defense of CL-K1 and its mechanism in clearance of pathogenic microorganisms remain unclear, especially in early vertebrates. In this study, the Oreochromis niloticus CL-K1 (OnCL-K1) protein was purified and identified, which was capable of binding to two important pathogens of tilapia, Streptococcus agalactiae and Aeromonas hydrophila. Interestingly, OnCL-K1 exhibited direct bactericidal activity by binding to lipoteichoic acid or LPS on cell walls, disrupting the permeability and integrity of the bacterial membrane in vitro. Upon bacterial challenge, OnCL-K1 significantly inhibited the proliferation of pathogenic bacteria, reduced the inflammatory response, and improved the survival of tilapia. Further research revealed that OnCL-K1 could associate with OnMASPs to initiate and regulate the lectin complement pathway. Additionally, OnCD93 reduced the complement-mediated hemolysis by competing with OnMASPs for binding to OnCL-K1. More importantly, OnCL-K1 could facilitate phagocytosis by collaborating with cell surface CD93 in a lectin pathway-independent manner. Moreover, OnCL-K1 also promoted the formation of phagolysosomes, which degraded and killed ingested bacteria. Therefore, this study reveals the antibacterial response mechanism of CL-K1 in primitive vertebrates, including promoting complement activation, enhancing opsonophagocytosis, and killing of macrophages, as well as its internal links, all of which provide (to our knowledge) new insights into the understanding of the evolutionary origins and regulatory roles of the collectins in innate immunity.

Feeding Strategies with Multi-Strain Probiotics Affect Growth, Health Condition, and Disease Resistance in Asian Seabass (Lates calcarifer)

A 16-week feeding trial was done to examine the impacts of continuous feeding (CF) or pulse-feeding (PF) of multi-strain probiotics on Asian seabass (Lates calcarifer, 30.0 ± 0.1 g) juveniles. In this study, three different multi-strain probiotic mixtures were added to a basal diet, including (I) a mixture of different strains of Lactobacillus plantarum, (II) a mixture of the first probiotic (I) + L. delbrueckii sub bulgaricus, L. rhamnosus and L. acidophilus, and (III) a mixture of the second probiotic (II) + two quorum quenching (QQ) bacteria (Bacillus thuringiensis QQ1 and B. cereus QQ2). CF (every day) or PF (every two weeks) strategies were applied for using the abovementioned probiotics to design seven experimental groups including C (control, without probiotics), CF-I (continuous feeding of fish with the probiotic mixture I), CF-II (continuous feeding of fish with the probiotic mixture II), CF-III (continuous feeding of fish with the probiotic mixture III), PF-I (pulse-feeding of fish with the probiotic mixture I), PF-II (pulse-feeding of fish with the probiotic mixture II), and PF-III (pulse-feeding of fish with the probiotic mixture III). Four hundred and twenty fish were stocked into 21 circular polyethylene tanks with 220 L volume (20 fish/tank). Each dietary treatment had three replicates. Tanks were supplied with seawater (temperature = 30.5 °C, salinity = 45 g L-1) in a flow-throw system. Fish in CF-I, CF-II, and CF-III had higher growth rate (ca. 113-145%) and better feed conversion ratio than fish fed C and PF-I (P < 0.05). Fish in the CF-III group had the highest protease activity. Continuous feeding strategy resulted in a higher amount of glutathione and catalase activities in both the liver and plasma as well as higher superoxide dismutase activity in the liver of fish. Pulse-feeding strategy resulted in lower plasma lactate dehydrogenase and aspartate aminotransferase levels than the CF strategy. Regardless of feeding strategy, different probiotic mixtures significantly enhanced blood hemoglobin and hematocrit levels compared to the control. Continuous feeding with the multi-strain probiotics resulted in a higher survival rate against Vibrio harveyi than the PF method. Continuous feeding induced higher mRNA transcription levels of granulocyte-macrophage colony-forming cells and interleukin 10 genes in the gut of fish than PF strategy. In conclusion, continuous feeding with multi-strain probiotics is better than pulse-feeding on growth, feed utilization, antioxidant capacity, and the gut's immune-related genes and led to higher resistance of L. calcarifer in challenge with V. harveyi.

Role of HIF in fish inflammation

The hypoxia-inducing factor (HIF) is a central transcription factor in cellular oxygen sensing and regulation. It is common that the inflammation always appears in many diseases, like infectious diseases in fishes, and the inflammation is often accompanied by hypoxia, as a hallmark of inflammation. Besides coordinating cellular responses to low oxygen, HIF-mediated hypoxia signaling pathway is also crucial for immune responses such as the regulations of innate immune cell phenotype and function, as well as metabolic reprogramming under the inflammation. However, the understanding of the molecular mechanisms by which HIFs regulate the inflammatory response in fish is still very limited. Here, we review the characteristics of HIF as well as its roles in innate immune cells and the infections caused by bacteria and viruses. The regulatory effects of HIF on the metabolic reprogramming of innate immune cells are also discussed and the future research directions are outlooked. This paper will serve as a reference for elucidating the molecular mechanism of HIF regulating inflammation and identifying treatment strategies to target HIF for fish disease.

Ovary of Zebrafish during Spawning Season: Ultrastructure and Immunohistochemical Profiles of Sox9 and Myostatin

This study sought to examine the ovarian cellular and stromal components of the zebrafish (Danio rerio) throughout the spawning season using light and electron microscopic tools. The ovaries of zebrafish showed oocytes in all stages of follicular development and degeneration (atresia). Six stages of oogenesis were demonstrated: oogonia, early oocytes, late oocytes, vacuolated follicles, the yolk globule stage (vitellogenesis), and mature follicles. The SOX9 protein was expressed in the ooplasm of the primary and previtellogenic oocytes and the theca cell layer of the mature follicles. Myostatin was expressed in the granulosa and theca cells. Many stem cells in the ovarian stroma expressed myostatin and SOX9. During the spawning season, the EM results indicated that the zona radiata increased in thickness and was crossed perpendicularly by pore canals that contained processes from both oocytes and zona granulosa. The granulosa cells contained many mitochondria, rER, sER, and vesicles. Meanwhile, the thecal layer consisted of fibroblast-like cells. Atretic follicles could be demonstrated that involved both oocytes and their follicular walls. Several types of cells were distinguished in the ovarian stroma, including mast cells, telocytes, lymphocytes, fibroblasts, endocrine cells, macrophages, adipocytes, dendritic cells, and steroidogenic (stromal) cells. The ovary of the zebrafish serves as a model to investigate follicular development.

Immunonutrition: facilitating mucosal immune response in teleost intestine with amino acids through oxidant-antioxidant balance

Comparative animal models generate fundamental scientific knowledge of immune responses. However, these studies typically are conducted in mammals because of their biochemical and physiological similarity to humans. Presently, there has been an interest in using teleost fish models to study intestinal immunology, particularly intestinal mucosa immune response. Instead of targeting the pathogen itself, a preferred approach for managing fish health is through nutrient supplementation, as it is noninvasive and less labor intensive than vaccine administrations while still modulating immune properties. Amino acids (AAs) regulate metabolic processes, oxidant-antioxidant balance, and physiological requirements to improve immune response. Thus, nutritionists can develop sustainable aquafeeds through AA supplementation to promote specific immune responses, including the intestinal mucosa immune system. We propose the use of dietary supplementation with functional AAs to improve immune response by discussing teleost fish immunology within the intestine and explore how oxidative burst is used as an immune defense mechanism. We evaluate immune components and immune responses in the intestine that use oxidant-antioxidant balance through potential selection of AAs and their metabolites to improve mucosal immune capacity and gut integrity. AAs are effective modulators of teleost gut immunity through oxidant-antioxidant balance. To incorporate nutrition as an immunoregulatory means in teleost, we must obtain more tools including genomic, proteomic, nutrition, immunology, and macrobiotic and metabonomic analyses, so that future studies can provide a more holistic understanding of the mucosal immune system in fish.

Transgenerational exposure to ocean acidification impacts the hepatic transcriptome of European sea bass (Dicentrarchus labrax)

Physiological effects of ocean acidification associated with elevated CO2 concentrations in seawater is the subject of numerous studies in teleost fish. While the short time within-generation impact of ocean acidification (OA) on acid-base exchange and energy metabolism is relatively well described, the effects associated with transgenerational exposure to OA are much less known. Yet, the impacts of OA can vary in time with the potential for acclimation or adaptation of a species. Previous studies in our lab demonstrated that transgenerational exposure to OA had extensive effects on the transcriptome of the olfactory epithelium of European sea bass (Dicentrarchus labrax), especially on genes related to ion balance, energy metabolism, immune system, synaptic plasticity, neuron excitability and wiring. In the present study, we complete the previous work by investigating the effect of transgenerational exposure to OA on the hepatic transcriptome of European sea bass. Differential gene expression analysis was performed by RNAseq technology on RNA extracted from the liver of two groups of 18 months F2 juveniles that had been exposed since spawning to the same AO conditions as their parents (F1) to either actual pH or end-of-century predicted pH levels (IPCC RCP8.5), respectively. Here we show that transgenerational exposure to OA significantly impacts the expression of 236 hepatic transcripts including genes mainly involved in inflammatory/immune responses but also in carbohydrate metabolism and cellular homeostasis. Even if this transcriptomic impact is relatively limited compared to what was shown in the olfactory system, this work confirmed that fish transgenerationally exposed to OA exhibit molecular regulation of processes related to metabolism and inflammation. Also, our data expand the up-regulation of a key gene involved in different physiological pathways including calcium homeostasis (i.e. pthr1), which we already observed in the olfactory epithelium, to the liver. Even if our experimental design does not allow to discriminate direct within F2 generation effects from transgenerational plasticity, these results offer the perspective of more functional analyses to determine the potential physiological impact of OA exposure on fish physiology with ecological relevance.

The acute inflammatory response of teleost fish

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

Spirulina promotes macrophages aggregation in zebrafish (Danio rerio) liver

The immune system of teleosts offers many ideas to deepen the immune mechanisms and cells in general. The use of zebrafish as an experimental model is increased in recent years, thanks to its genetic and anatomical characteristics. It is known that several natural compounds exert an action on the immune system, boosting it. Spirulina, a non-toxic blue-green alga, has been declared a superfood for its peculiar biological activities. In this study, we test the immunostimulant effect of spirulina on zebrafish liver macrophages by immunohistochemical analysis using optical and confocal microscopy. Our results have shown an increase in the number of macrophages after feeding with spirulina, furthermore, this natural 'superfood' can induce macrophages aggregation. These data not only provide information on the possible effect of this alga as a complementary feed on the immune cells of teleost, but also improve the knowledge of the immune mechanisms of vertebrates.

Immunity of the intestinal mucosa in teleost fish

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

Anisakidae parasitism activated immune response and induced liver fibrosis in wild anadromous Coilia nasus

Anisakidae nematode larvae is one of the most common parasites in wild anadromous Coilia nasus. This study aims to explore the mechanism of the C. nasus immune response to the parasitism of Anisakid nematode larvae. Results found that Anisakid nematode larvae parasitism caused liver injury as evidenced by histomorphology results as well as high levels of aminotransferase and aspertate aminotransferase. Furthermore, Anisakid nematode larvae parasitism induced an immune response in the host, which was characterized by the elevated populations of macrophages and neutrophils in the liver and head-kidney in the Anisakidae-infected group compared to the noninfected group. The expression of immunoglobulin IgM and IgD in the liver and head-kidney was also increased in the Anisakidae-infected group. The Anisakidae-infected group showed higher activity of antioxidant enzymes catalase and superoxide dismutase, which indicates severe oxidative stress, and increased production of pro-inflammatory cytokines, TNF-α, IL-6 as well as MCP-1 in the liver compared with the noninfected group. As a result of inflammation, livers of hosts in the Anisakidae-infected group showed fibrosis, and elevated expression of associated proteins including α-smooth muscle actin, fibronectin, collagen type I and type III compared with the noninfected group. We demonstrated that Anisakid nematode larvae parasitism results in injury and fibrosis in the liver, and triggers immune cell infiltration and inflammation in the liver and head-kidney of C. nasus. Altogether, the results provide a foundation for building an interaction between parasite and host, and will contribute to C. nasus population and fishery resource protection.

PU.1 Regulates Cathepsin S Expression in Large Yellow Croaker (Larimichthys crocea) Macrophages

Different morphologies have been detected in teleost macrophages. In this study, two macrophage cell lines were sub-cloned from a large yellow croaker head kidney cell line, LYCK. One type of sub-cloned cells was fusiform but the other was round, named LYC-FM and LYC-RM cells respectively, based on their morphologies. Both types showed the characteristics of macrophages, including expression of macrophage-specific marker genes, possession of phagocytic and bactericidal activities, and production of reactive oxygen species (ROS) and nitric oxide (NO). The transcription factor PU.1, crucial for the development of macrophages in mammals, was found to exist in two transcripts, PU.1a and PU.1b, in large yellow croaker, and constitutively expressed in LYC-FM and LYC-RM cells. The expression levels of PU.1a and PU.1b could be upregulated by recombinant large yellow croaker IFN-γ protein (rLcIFN-γ). Further studies showed that both PU.1a and PU.1b increased the expression of cathepsin S (CTSS) by binding to different E26−transformation−specific (Ets) motifs of the CTSS promoter. Additionally, we demonstrated that all three domains of PU.1a and PU.1b were essential for initiating CTSS expression by truncated mutation experiments. Our results therefore provide the first evidence that teleost PU.1 has a role in regulating the expression of CTSS.

Comparative proteomic profiling reveals specific adaption of Vibrio anguillarum to oxidative stress, iron deprivation and humoral components of innate immunity

The gram-negative bacterium Vibrio (Listonella) anguillarum (VA) is the causative agent of vibriosis, a terminal hemorrhagic septicemia affecting the aquacultural industry across the globe. In the current study we used label-free quantitative proteomics to investigate how VA adapts to conditions that mimic defined aspects of vibriosis-related stress such as exposure to oxidative stress (H₂O₂), exposure to humoral factors of innate immunity through incubation with Atlantic salmon serum, and iron deprivation upon supplementation of 2,2'-dipyridyl (DIP) to the growth medium. We also investigated how regulation of virulence factors may be governed by the VA growth phase and availability of nutrients. All experimental conditions explored revealed stress-specific proteomic adaption of VA and only nine proteins were found to be commonly regulated in all conditions. A general observation made for all stress-related conditions was regulation of multiple metabolic pathways. Notably, iron deprivation and exposure to Atlantic salmon serum evoked upregulation of iron acquisition mechanisms. The findings made in the present study represent a source of potential virulence determinants that can be of use in the search for means to understand vibriosis. SIGNIFICANCE: Vibriosis in fish and shellfish caused by V. anguillarum (VA) is responsible for large economic losses in the aquaculture sector across the globe. However, not much is known about the defense mechanism of this pathogen to percept and adapt to the imposed stresses during infection. Analyzing the response of VA to multiple host-related physiochemical stresses, the quantitative proteomic analysis of the present study indicates modulation of several virulence determinants and key defense networks of this pathogen. Our findings provide a theoretical basis to enhance our understanding of VA pathogenesis and can be employed to improve current intervention strategies to control vibriosis in aquaculture.

Molecular and functional characterization of MST2 in grass carp during bacterial infection

The regulation of host redox homeostasis is critically important in the immune response to pathogens. The "mammalian sterile 20-like" kinase 2 (MST2) has been shown to play a role in apoptosis, cell proliferation, and cancer; however, few studies have examined its ability to modulate redox homeostasis during innate immunity, especially in teleost fish. In this study, we cloned the MST2 gene of Ctenopharyngodon idella (CiMST2) and analyzed its tissue distribution. CiMST2 was present in most of the studied tissues, and it was most highly expressed in brain tissue. Expression patterns analysis revealed that MST2 mRNA and protein were significantly up-regulated under bacterial infection, suggesting that it is involved in the immune response. Bacterial stimulation significantly increased the level of antioxidases. To explore the interplay between CiMST2 and antioxidant regulation, we examined the effects of CiMST2 overexpression and conducted RNA interference assays in vitro. CiMST2 overexpression significantly increased the expression levels of nuclear factor E2-related factor 2 (Nrf2) and other antioxidases and vice versa, revealing that CiMST2 regulated host redox homeostasis via Nrf2-antioxidant responsive element (ARE) signaling. Overall, our findings provide a new perspective on the role of MST2 in innate immunity in teleosts as well as insights that will aid the prevention and control of disease in the grass carp farming industry.

Integrated analysis of mRNA and long non-coding RNA expression profiles reveals the potential roles of lncRNA-mRNA network in carp macrophage immune regulation

Long non-coding RNAs (lncRNAs) have emerged as a hot topic in research as mounting evidence has indicated their transcriptional or post-transcriptional regulatory potential in multiple biological processes. Previous studies have revealed the involvement of lncRNAs in the immunoregulation of mammalian macrophages by changing mRNA expression; however, studies on the lncRNAs in fish macrophages and their potential roles in the immune system remain unknown. Primary macrophages were isolated from the head kidney (HK) of red common carp (Cyprinus carpio) and high-throughput lncRNA-mRNA sequencing was performed using the Illumina HiSeq platform. The results revealed that the most highly expressed mRNAs in primary HK macrophages were mainly involved in immune-related signal pathways. Furthermore, the most enriched immune-related GO term and KEGG pathway of the mRNAs were "immune system development" and "chemokine signaling pathway," respectively. A total of 20,333 lncRNAs, composed of 10,512 known and 9821 novel lncRNAs, were identified, and functional enrichment analysis of the lncRNA-mRNA network indicated that the expressed lncRNAs in primary HK macrophages could be associated with the regulation of multiple immune-related signaling pathways. In addition, the expressions of several selected lncRNAs and their related mRNAs were determined in carp macrophages following a 6-h exposure to lipopolysaccharide (LPS) and Poly(I: C), the results of which confirmed the co-expression regulation of lncRNAs and target mRNAs in the immune response of carp macrophages. These results suggest the correlative of the lncRNA-mRNA network in fish macrophage immune response, which may further affect the cross-talk of various signaling pathways by interaction with other network genes. Here, we provided fundamental data about the transcriptome profiles of primary HK macrophages from red common carp by analysis of the lncRNA-mRNA network, and ultimately suggest the potential roles of lncRNA-mRNA networks in immune regulation in teleost fish.

Transcriptome Profiling of Atlantic Salmon Adherent Head Kidney Leukocytes Reveals That Macrophages Are Selectively Enriched During Culture

The Atlantic salmon (Salmo salar) is an economically important fish, both in aquaculture and in the wild. In vertebrates, macrophages are some of the first cell types to respond to pathogen infection and disease. While macrophage biology has been characterized in mammals, less is known in fish. Our previous work identified changes in the morphology, phagocytic ability, and miRNA profile of Atlantic salmon adherent head kidney leukocytes (HKLs) from predominantly “monocyte-like” at Day 1 of in vitro culture to predominantly “macrophage-like” at Day 5 of culture. Therefore, to further characterize these two cell populations, we examined the mRNA transcriptome profile in Day 1 and Day 5 HKLs using a 44K oligonucleotide microarray. Large changes in the transcriptome were revealed, including changes in the expression of macrophage and immune-related transcripts (e.g. csf1r, arg1, tnfa, mx2), lipid-related transcripts (e.g. fasn, dhcr7, fabp6), and transcription factors involved in macrophage differentiation and function (e.g. klf2, klf9, irf7, irf8, stat1). The in silico target prediction analysis of differentially expressed genes (DEGs) using miRNAs known to change expression in Day 5 HKLs, followed by gene pathway enrichment analysis, supported that these miRNAs may be involved in macrophage maturation by targeting specific DEGs. Elucidating how immune cells, such as macrophages, develop and function is a key step in understanding the Atlantic salmon immune system. Overall, the results indicate that, without the addition of exogenous factors, the adherent HKL cell population differentiates in vitro to become macrophage-like.

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.

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.

Salmonid Antibacterial Immunity: An Aquaculture Perspective

The aquaculture industry is continuously threatened by infectious diseases, including those of bacterial origin. Regardless of the disease burden, aquaculture is already the main method for producing fish protein, having displaced capture fisheries. One attractive sector within this industry is the culture of salmonids, which are (a) uniquely under pressure due to overfishing and (b) the most valuable finfish per unit of weight. There are still knowledge gaps in the understanding of fish immunity, leading to vaccines that are not as effective as in terrestrial species, thus a common method to combat bacterial disease outbreaks is the use of antibiotics. Though effective, this method increases both the prevalence and risk of generating antibiotic-resistant bacteria. To facilitate vaccine design and/or alternative treatment efforts, a deeper understanding of the teleost immune system is essential. This review highlights the current state of teleost antibacterial immunity in the context of salmonid aquaculture. Additionally, the success of current techniques/methods used to combat bacterial diseases in salmonid aquaculture will be addressed. Filling the immunology knowledge gaps highlighted here will assist in reducing aquaculture losses in the future.

Changes in the immune function of rainbow trout (Oncorhynchus mykiss) provide insights into strategies against BDE-47 stress

Polybrominated diphenyl ethers (PBDEs) are ubiquitous in marine ecosystems and have been suggested to bioaccumulate in aquatic food webs, with potentially negative impacts on marine organism. In this study, a 21-day experiment was performed under controlled laboratory conditions, in which 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), the most biotoxic PBDE in the marine environment, was fed to rainbow trout (Oncorhynchus mykiss) at concentrations of 50 and 500 ng g^-1 in the diet. BDE-47 significantly decreased the specific growth rate of O. mykiss and was highly concentrated in the liver and head kidney, as evidenced by increased bioaccumulation factor (BAF) values. Tissue observation revealed impairment of the microstructure of the head kidney. Important immune factors in the skin, blood and head kidney were significantly inhibited by BDE-47 treatment (p < 0.05), whereas the respiratory burst activity of macrophages was enhanced. Additionally, immune-related genes were strongly downregulated following BDE-47 exposure (p < 0.05). In a bacterial challenge, the treatment groups had much higher mortality than did the control group (p < 0.05). BDE-47 accumulated and impaired immune organs, and the hierarchy of immune responses was impaired, consequently reducing O. mykiss resistance to pathogen invasion.

Characterization of miRNAs in Cultured Atlantic Salmon Head Kidney Monocyte-Like and Macrophage-Like Cells

Macrophages are among the first cells to respond to infection and disease. While microRNAs (miRNAs) are involved in the process of monocyte-to-macrophage differentiation in mammals, less is known in teleost fish. Here, Atlantic salmon head kidney leukocytes (HKLs) were used to study the expression of miRNAs in response to in vitro culture. The morphological analysis of cultures showed predominantly monocyte-like cells on Day 1 and macrophage-like cells on Day 5, suggesting that the HKLs had differentiated from monocytes to macrophages. Day 5 HKLs also contained a higher percentage of phagocytic cells. Small RNA sequencing and qPCR analysis were applied to examine the miRNA diversity and expression. There were 370 known mature Atlantic salmon miRNAs in HKLs. Twenty-two miRNAs (15 families) were downregulated while 44 miRNAs (25 families) were upregulated on Day 5 vs. Day 1. Mammalian orthologs of many of the differentially expressed (DE) miRNAs are known to regulate macrophage activation and differentiation, while the teleost-specific miR-2188, miR-462 and miR-731 were also DE and are associated with immune responses in fish. In silico predictions identified several putative target genes of qPCR-validated miRNAs associated with vertebrate macrophage differentiation. This study identified Atlantic salmon miRNAs likely to influence macrophage differentiation, providing important knowledge for future functional studies.

Establishment and characterization of two head kidney macrophage cell lines from large yellow croaker (Larimichthys crocea)

Two continuous macrophage cell lines (LCM07 and LCM10) were established for the first time from the head kidney of the marine fish large yellow croaker (Larimichthys crocea). To date, both cell lines have been subcultured for more than 100 passages in 12 months. Notably, the LCM07 and LCM10 cells have distinct morphology and immune function. LCM07 cells showed strong contact inhibition in crowded conditions, while this was not observed in the LCM10 cells because they could grow in an overlapping manner. Correspondingly, LCM10 cells were slenderer than LCM07 cells. LCM07 cells had stronger phagocytic ability than LCM10 cells, while LCM10 cells had stronger respiratory burst activity after incubation with lipopolysaccharide (LPS) and phorbol ester (PMA). LCM07 cells had stronger Escherichia coli killing ability than LCM10 cells. The mRNA of macrophage markers, namely that of CD11b, CD114, CD68, CD86, CD209, and CD163, were all expressed in primary macrophages as well as the two cell lines. The mRNA expression levels of selected inflammatory cytokines, namely interleukin (IL)-1β, IL-8, and tumor necrosis factor (TNF)α, were all upregulated after incubation with LPS. LPS also regulated key components of the mitogen-activated protein kinase (MAPK) signaling pathway, i.e., p38, ERK (extracellular signal-regulated kinase), and JNK (Jun N-terminal kinase) and their phosphorylated forms. Arachidonic acid (ARA) downregulated the LPS-induced upregulation of IL-1β, IL-8, and TNFα, revealing that LCM07 and LCM10 cells are useful for studying nutritional immunity. In conclusion, two distinct macrophage cell lines have been established for the first time from the head kidney of marine fish, which could be useful for studying immunity and nutritional immunity.

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

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

Molecular and functional characterization of a mannose-binding lectin/ficolin-associated protein (MAp44) from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

The lectin pathway of the complement system has a pivotal role in the defense against infectious organisms. Mannose-binding lectin/ficolin-associated protein (MAp44), a multifunctional complement regulator, regulates the complement activation by competing with MASP-1, MASP-2 and MASP-3 for MBL and ficolin binding sites. In this study, we described the identification and functional characterization of a MAp44 homologue (OnMAp44) from Nile tilapia (Oreochromis niloticus) at molecular, cellular and protein levels. The open reading frame (ORF) of OnMAp44 is 1140 bp of nucleotide sequence encoding a polypeptide of 379 amino acids. The deduced amino acids sequence has four characteristic structures, including two C1r/C1s-Uegf-BMP domains (CUB), one epidermal growth factor domain (EGF) and one complement control protein domains (CCP). Expression analysis revealed that the OnMAp44 was highly expressed in liver, and widely existed in other examined tissues. In addition, the OnMAp44 expression was significantly up-regulated in spleen and head kidney following challenges with Streptococcus agalactiae and Aeromonas hydrophila. The up-regulations of OnMAp44 mRNA and protein expression were also observed in hepatocytes and monocytes/macrophages in vitro stimulation with S. agalactiae and A. hydrophila. Recombinant OnMAp44 protein was able to participate in the regulation of inflammation and migration reaction. Taken together, the results indicated that OnMAp44 was likely to involve in the immune response to bacterial infection in Nile tilapia.

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

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

The interleukin-6 regulates the function of monocytes/macrophages (MO/MФ) via the interleukin-6 receptor β in ayu (Plecoglossus altivelis)

Interleukin-6 (IL-6) is one of the most pleiotropic cytokines because of its wide range of effects on cells of the immune and non-immune systems in the body. However, the role of IL-6 in fish monocytes/macrophages (MO/MФ) is poorly understood. In this study, we cloned the cDNA sequence of the IL-6 gene from ayu (Plecoglossus altivelis) and demonstrated using a tissue distribution assay that ayu interleukin-6 (PaIL-6) mRNA is expressed in all tested tissues. Changes in expression were observed in immune tissues as well as in MO/MФ after a Vibrio anguillarum infection; subsequently, PaIL-6 was expressed and purified to prepare anti-PaIL-6 antibodies. Recombinant PaIL-6 protein (rPaIL-6) treatment enhanced pro-inflammatory cytokine expression. Ayu interleukin-6 receptor β (PaIL-6Rβ) knockdown resulted in decreased pro-inflammatory cytokine expression in MO/MФ treated with rPaIL-6, whereas no significant changes were observed after ayu interleukin-6 receptor α (PaIL-6Rα) knockdown in MO/MФ. PaIL-6 and PaIL-6Rβ knockdown in MO/MФ inhibited the phosphorylation of signal transducer and activator of transcription 1. Moreover, PaIL-6Rβ knockdown inhibited the phagocytic and bactericidal ability of ayu MO/MФ treated with rPaIL-6. These data indicate that PaIL-6 may be able to regulate the function of ayu MO/MФ.

Characterization of a new il-4/13 homologue in grass carp (Ctenopharyngodon idella) and its cooperation with M-CSF to promote macrophage proliferation

In this study, a new il-4/13 cDNA was isolated from grass carp (Ctenopharyngodon idella) using homologous cloning. The phylogenetic tree and sequence alignment of the deduced amino acid (aa) sequence showed that it was closer to grass carp il-4/13b (gcil-4/13b) than other homologues and therefore named gcil-4/13b-like (gcil-4/13bl). It has 399-nt coding sequence (CDS) which is less than gcil-4/13b (408 nt). In addition, the cloned gcil-4/13bl gene is approximately 1600 bp in length and has a conserved genetic structure consisting of four exons and three introns. Compared to gcil-4/13b gene, it has a variety of nucleotides variation across the CDS and contains a longer intron 3, suggesting that it is a new gcil-4/13 gene. The gcil-4/13bl transcripts were ubiquitously expressed in almost all selected tissues, and there was almost only gcil-4/13bl detected in brain and head kidney (HK). Recombinant grass carp (rgc) Il-4/13bl was prepared by using Escherichia coli (E. coli) Rosetta-gami 2 (DE3). The functional study demonstrated that rgcIl-4/13bl significantly upregulated arginase-2 gene expression and arginase activity, whilst downregulated nitric oxide (NO) production as well as the transcript levels of inducible nitric oxide synthesase (inos) and ifn-γ in freshly isolated grass carp HK monocytes/macrophages (M0/Mϕ). These data suggested that the newly cloned il-4/13bl had the conserved functions to activate M2-type but antagonize M1-type macrophages. Furthermore, rgcIl-4/13bl was able to drive the proliferation of M0/Mϕ which were pre-treated by rgcM-csf, indicating the involvement of gcIl-4/13bl in the proliferation of macrophages. Here we not only identified a new il-4/13-encoding gene in grass carp, but also for the first time revealed a novel function of fish Il-4/13 combined with M-csf engaging in M0/Mϕ proliferation.

Erythroid Progenitor Cells in Atlantic Salmon (Salmo salar) May Be Persistently and Productively Infected with Piscine Orthoreovirus (PRV)

Piscine orthoreovirus (PRV-1) can cause heart and skeletal muscle inflammation (HSMI) in farmed Atlantic salmon (Salmo salar). The virus targets erythrocytes in the acute peak phase, followed by cardiomyocytes, before the infection subsides into persistence. The persistent phase is characterized by high level of viral RNA, but low level of viral protein. The origin and nature of persistent PRV-1 are not clear. Here, we analyzed for viral persistence and activity in various tissues and cell types in experimentally infected Atlantic salmon. Plasma contained PRV-1 genomic dsRNA throughout an 18-week long infection trial, indicating that viral particles are continuously produced and released. The highest level of PRV-1 RNA in the persistent phase was found in kidney. The level of PRV-1 ssRNA transcripts in kidney was significantly higher than that of blood cells in the persistent phase. In-situ hybridization assays confirmed that PRV-1 RNA was present in erythroid progenitor cells, erythrocytes, macrophages, melano-macrophages and in some additional un-characterized cells in kidney. These results show that PRV-1 establishes a productive, persistent infection in Atlantic salmon and that erythrocyte progenitor cells are PRV target cells.

Specific function and modulation of teleost monocytes/macrophages: polarization and phagocytosis

Macrophages exist in most tissues and play a variety of functions in vertebrates. Teleost fish species are found in most aquatic environments throughout the world and are quite diverse for a group of vertebrate animals. Due to whole genome duplication and environmental adaptation, teleost monocytes/macrophages possess a variety of different functions and modulations compared with those of mammals. A deeper understanding of teleost monocytes/macrophages in the immune system will not only help develop teleost-specific methods of disease prevention but will also help improve our understanding of the various immune mechanisms in mammals. In this review, we summarize the differences in polarization and phagocytosis of teleost and mammalian macrophages to improve our understanding of the various immune mechanisms in vertebrates.

CpGs Induce Differentiation of Atlantic Salmon Mononuclear Phagocytes Into Cells With Dendritic Morphology and a Proinflammatory Transcriptional Profile but an Exhausted Allostimulatory Activity

Due to their ability to present foreign antigens and prime naïve T cells, macrophages, and dendritic cells (DCs) are referred to as professional antigen-presenting cells (APCs). Although activated macrophages may function as APCs, these cells are particularly effective at directly engaging pathogens through phagocytosis, and production of antimicrobial compounds. On the other hand, DCs possess superb antigen-presenting and costimulatory capacity and they are essential for commencement and regulation of adaptive immune responses. In in vitro models, development of mature mammalian DCs from monocytes requires sequential exposure to growth factors (including GM-CSF and IL-4) and proinflammatory stimuli such as toll-like receptor (TLR) ligands. Currently, except for IL-4/13, neither orthologs nor functional analogs of the growth factors which are essential for the differentiation of mammalian DCs (including GM-CSF and FLT3) have been identified in teleosts and data about differentiation of piscine APCs is scant. In the present study, primary salmon mononuclear phagocytes (MPs) stimulated in vitro for 5-7 days with a B-class CpG oligodeoxynucleotides (ODN 2006PS) underwent morphological differentiation and developed "dendritic" morphology, characterized by long, branching pseudopodia. Transcriptional profiling showed that these cells expressed high levels of proinflammatory mediators characteristic for M1 polarized MPs. However, the cells treated with CpGs for 7 days downregulated their surface MHCII molecules as well as their capacity to endocytose ovalbumin and exhibited attenuated allostimulatory activity. This concurred with transcriptional downregulation of costimulatory CD80/86 and upregulation of inhibitory CD274 (B7-H1) genes. Despite their exhausted allostimulatory activity, these cells were still responsive to re-stimulation with gardiquimod (a TLR7/8 ligand) and further upregulated a wide array of immune genes including proinflammatory mediators such as intereukin-1 beta and tumor necrosis factor. Overall, the presented data highlight the disparate effects TLR ligands may have on the proinflammatory status of APCs, on one side, and their antigen-presenting/costimulatory functions, on the other. These findings also indicate that despite the poor phylogenetic conservation of the growth factors involved in the differentiation of DCs, some of the processes that orchestrate the development and the differentiation of professional APCs are conserved between teleosts in mammals.

Glucocorticoid receptor in ayu (Plecoglossus altivelis): Genomic and non-genomic effects on monocytes/macrophages function

The glucocorticoid receptor (GR) is an important feedback regulator of the hypothalamic-pituitary-interrenal (HPI) axis. However, there are a limited number of studies focused on host-pathogen interactions in which an association between GR and immune response has been evaluated in monocytes/macrophages (MO/MФ) after being challenged with highly pathogenic bacteria. Here, we cloned the cDNA sequence of the glucocorticoid receptor (PaGR) gene from ayu fish. The PaGR transcript was expressed in all tissues, and changes in expression were observed in immune tissues and MO/MФ after live Vibrio anguillarum infection. Subsequently, PaGR was expressed and purified to prepare anti-PaGR antibodies. We analyzed the subcellular localization of PaGR. PaGR was expressed not only in the intracellular space but also in the plasma membrane. PaGR activation decreased the expression of pro-inflammatory cytokines and increased the expression of anti-inflammatory cytokines. However, PaGR activation suppressed the phagocytosis activity of V. anguillarum-infected ayu MO/MФ via a non-genomic pathway. Interestingly, PaGR activation could enhance MO/MФ bacterial killing capability and apoptosis. Therefore, PaGR may modulate the immune response in ayu MO/MФ by genomic and non-genomic pathways.

Innate Immunity Provides Biomarkers of Health for Teleosts Exposed to Nanoparticles

In recent years, the unique properties of nanoparticles have fostered novel applications in various fields such as biology, pharmaceuticals, agriculture, and others. Unfortunately, their rapid integration into daily life has also led to environmental concerns due to uncontrolled release of nanoparticles into the aquatic environment. Despite increasing awareness of nanoparticle bioaccumulation in the aquatic environment, much remains to be learned about their impact on aquatic organisms and how to best monitor these effects. Herein, we provide the first review of innate immunity as an emerging tool to assess the health of fish following nanoparticle exposure. Fish are widely used as sentinels for aquatic ecosystem pollution and innate immune parameters offer sensitive and reliable tools that can be harnessed for evaluation of contamination events. The most frequent biomarkers highlighted in literature to date include, but are not limited to, parameters associated with leukocyte dynamics, oxidative stress, and cytokine production. Taken together, innate immunity offers finite and sensitive biomarkers for assessment of the impact of nanoparticles on fish health.

Insulin favors acute inflammatory reaction in alloxan-diabetic tilapia during infectious aerocystitis

ABSTRACT: In vertebrates, the inflammatory reaction is responsible for modulating the initial nonspecific defense until specific immunity is acquired. In this context, numerous studies in mammals have demonstrated the participation of insulin in the inflammatory response, favoring cell proliferation and the migratory capacity of endothelial cells, vascular smooth muscle cells and monocytes, as well as mediating the expression of pro-thrombotic and pro-fibrotic factors. However, little is known about the effect of this peptidic hormone on the inflammatory reaction in teleostean fish. In order to evaluate the participation of insulin in the acute inflammatory response of Nile tilapia, Oreochromis niloticus, during aerocystitis induced by Aeromonas hydrophila, and 48 aloxane-diabetic tilapia were used, constituting two groups: diabetics treated with insulin and diabetics without treatment. After six, 24, and 48 hours of inflammatory stimulation, tilapia were submitted to deep anesthesia for euthanasia and necropsy, and thus, obtaining exudate and harvesting of the swim bladder for analysis of the inflammatory reaction. Based on this premise, the present study demonstrated the participation of insulin in the acute inflammatory reaction of alloxan-diabetic tilapia by favors the cellular accumulation in the exudate, the proliferative effect of fibrous tissue and neovascularization in the inflamed site. Such findings reinforce the old hypothesis that insulin plays an important role in the innate immune response during acute inflammatory reaction, being an important pro-inflammatory hormone. However, Nile tilapia proved to be a promising experimental model for studies and advances in research involving diabetes mellitus.

Amphibian (Xenopus laevis) Interleukin-8 (CXCL8): A Perspective on the Evolutionary Divergence of Granulocyte Chemotaxis

The glutamic acid-leucine-arginine (ELR) motif is a hallmark feature shared by mammalian inflammatory CXC chemokines such the granulocyte chemo-attractant CXCL8 (interleukin-8, IL-8). By contrast, most teleost fish inflammatory chemokines lack this motif. Interestingly, the amphibian Xenopus laevis encodes multiple isoforms of CXCL8, one of which (CXCL8a) possesses an ELR motif, while another (CXCL8b) does not. These CXCL8 isoforms exhibit distinct expression patterns during frog development and following immune challenge of animals and primary myeloid cultures. To define potential functional differences between these X. laevis CXCL8 chemokines, we produced them in recombinant form (rCXCL8a and rCXCL8b) and performed dose-response chemotaxis assays. Our results indicate that compared to rCXCL8b, rCXCL8a is a significantly more potent chemo-attractant of in vivo-derived tadpole granulocytes and of in vitro-differentiated frog bone marrow granulocytes. The mammalian CXCL8 mediates its effects through two distinct chemokine receptors, CXCR1 and CXCR2 and our pharmacological inhibition of these receptors in frog granulocytes indicates that the X. laevis CXCL8a and CXCL8b both chemoattract tadpole and adult frog granulocytes by engaging CXCR1 and CXCR2. To delineate which frog cells are recruited by CXCL8a and CXCL8b in vivo, we injected tadpoles and adult frogs intraperitoneally with rCXCL8a or rCXCL8b and recovered the accumulated cells by lavage. Our transcriptional and cytological analyses of these tadpole and adult frog peritoneal exudates indicate that they are comprised predominantly of granulocytes. Interestingly, the granulocytes recruited into the tadpole, but not adult frog peritonea by rCXCL8b, express significantly greater levels of several pan immunosuppressive genes.

Mechanisms of Fish Macrophage Antimicrobial Immunity

Overcrowding conditions and temperatures shifts regularly manifest in large-scale infections of farmed fish, resulting in economic losses for the global aquaculture industries. Increased understanding of the functional mechanisms of fish antimicrobial host defenses is an important step forward in prevention of pathogen-induced morbidity and mortality in aquaculture setting. Like other vertebrates, macrophage-lineage cells are integral to fish immune responses and for this reason, much of the recent fish immunology research has focused on fish macrophage biology. These studies have revealed notable similarities as well as striking differences in the molecular strategies by which fish and higher vertebrates control their respective macrophage polarization and functionality. In this review, we address the current understanding of the biological mechanisms of teleost macrophage functional heterogeneity and immunity, focusing on the key cytokine regulators that control fish macrophage development and their antimicrobial armamentarium.

Intestinal granular cells of a cartilaginous fish, thornback ray Raja clavata: Morphological characterization and expression of different molecules

This investigation aims to fill gaps in our understanding of the intestinal immune cells of elasmobranchs. Whole digestive tracts of fifteen thornback ray Raja clavata were provided by a trawl fleet from the Gulf of Asinara (Sardinia, western Mediterranean Sea). Histochemical, immunohistochemical and ultrastructural observations were conducted on the spiral intestine. Three types of granular cells were identified; type I in epithelium, types II and III in lamina propria-submucosa, with each of them containing cytoplasmic granules with different ultrastructural characteristics. Data on size and density of each granular cell type are provided. Immunostaining of intestinal sections showed the reactivity of the granular cells: type I cells were positive for lysozyme, mast cell tryptase and tumor necrosis factor-ɑ based on antibody staining; type III cells were immune-reactive to anti-interleukin 6 antibody, whilst type II cells were negative to all the antibodies used. Comparison of each granular cell type with immune cells of teleosts or mammals and an hypothesis on their nature and function are reported. A potential role for granular cells in intestinal cellular immunity is also discussed with respect to type I and type III cells having similarities to Paneth cells and neutrophils, respectively.

Does dietary insect meal affect the fish immune system? The case of mealworm, Tenebrio molitor on European sea bass, Dicentrarchus labrax

Feeding small European sea bass, Dicentrarchus labrax, for 6 weeks with Tenebrio molitor larval meal showed significant anti-inflammatory responses (ceruloplasmin, myeloperoxidase and nitric oxide). Serum bacteriolytic activity against a Gram negative bacterium was not significantly affected by dietary Tenebrio, while both lysozyme antibacterial activity and serum trypsin inhibition usually linked to the anti-parasite activity of the fish, were significantly enhanced. The latter may be due to the similarities in the composition of the exoskeleton of parasites and insects that may therefore act as an immunostimulant potentially increasing the anti-parasitic activity. The addition of exogenous proteases significantly decreased both trypsin-inhibition and serum bacteriolytic activity probably through direct inhibition of the proteins responsible for these immune functions. Further investigation involving bacterial or parasitic challenges will be necessary to assess if the effects of dietary mealworm meal on the immune system observed in the present study are translated into an improved resistance to diseases.

Histopathological and ultrastructural assessment of two mugilid species infected with myxozoans and helminths

The histopathology and ultrastructure of the intestine of mullets, Liza ramada and Liza saliens, from Comacchio lagoons (northern Italy) naturally infected with myxozoans and helminths were investigated and described. Sixty-two (80.5%) of 77 mullets harboured one or more of the following parasites species: Myxobolus mugchelo (Myxozoa), Neoechinorhynchus agilis (Acanthocephala), Haplosplanchnus pachysomus and Dicrogaster contractus (Digenea). Co-occurrence of helminths with myxozoans was common. The main damage caused by digeneans was destruction of the mucosal epithelium of the villi, necrosis and degeneration of intestinal epithelial cells. More severe intestinal damage was caused by acanthocephalans which reach the submucosa layer with their proboscis. At the site of helminths infection, several mast cells (MCs), rodlet cells (RCs), mucous cells and few neutrophils and macrophages were observed in the epithelium. RCs and mucous cells exhibited discharge activity in close vicinity to the worm's tegument. M. mugchelo conspicuous plasmodia were encysted mainly in muscle and submucosa layers of the intestine. Indeed, spores of M. mugchelo were documented within the epithelial cells of host intestine and in proximity to MCs. Degranulation of the MCs near the myxozoans was very frequent.

Liver of the fish Gymnotus inaequilabiatus and nematode larvae infection: Histochemical features and expression of proliferative cell nuclear antigen

Histopathological lesions due to third-larval stage of nematode Brevimulticaecum sp. within the liver of a subpopulation of 31 Gymnotus inaequilabiatus from the Pantanal Region (Brazil) were studied with histochemical and immunohistochemical methods. In 93.5% of fish, livers harboured nematode larvae and the intensity of infection ranged from 8 to 293. In livers with highest number of larvae, the hepatic tissue was occupied primarily by the nematodes. Each larva was encircled by focal inflammatory granulomatous reaction. Within the thickness of the granuloma, three concentric layers were recognized: an inner layer of densely packed epithelioid cells, a middle layer of mast cells (MCs) entrapped in a thin fibroblast-connective mesh and an outer layer of fibrous connective tissue with fibroblasts. Epithelioid cells and fibroblasts within the thickness of the granuloma wall were positive for proliferative cell nuclear antigen (PCNA). Moreover, several hepatocytes in infected liver were immunoreactive to PCNA. Occurrence of rodlet cells and MCs in parenchyma, in close proximity to the encysted nematode larvae and near the blood vessel of infected liver, was observed. Macrophage aggregates (MAs) were numerous within the granulomas and scattered in parenchyma of the infected liver. High quantity of haemosiderin was encountered in MAs and hepatocytes of infected liver.

Impacts of Low Temperature on the Teleost Immune System

As poikilothermic vertebrates, fish can experience changes in water temperature, and hence body temperature, as a result of seasonal changes, migration, or efflux of large quantities of effluent into a body of water. Temperature shifts outside of the optimal temperature range for an individual fish species can have negative impacts on the physiology of the animal, including the immune system. As a result, acute or chronic exposure to suboptimal temperatures can impair an organisms' ability to defend against pathogens and thus compromise the overall health of the animal. This review focuses on the advances made towards understanding the impacts of suboptimal temperature on the soluble and cellular mediators of the innate and adaptive immune systems of fishes. Although cold stress can result in varying effects in different fish species, acute and chronic suboptimal temperature exposure generally yield suppressive effects, particularly on adaptive immunity. Knowledge of the effects of environmental temperature on fish species is critical for both the optimal management of wild species and the best management practices for aquaculture species.

LncRNA and mRNA profiling during activation of tilapia macrophages by HSP70 and Streptococcus agalactiae antigen

Objectives

To investigate the lncRNA profiling during tilapia peritoneal macrophages (TPMs) activation and discuss the relationship between lncRNA and mRNA.

Materials and Methods

RNA sequencing was used to investigate the lncRNA and mRNA profiles of TPMs activation following stimulation with Streptococcus agalactiae (Sa) antigen, heat shock protein 70 (HSP70) and HSP70+Sa. The expressions of lncRNA and mRNA were confirmed by qPCR. 356 lncRNA, 10173 mRNA and 1782 transcripts of uncertain coding potential (TUCP) were differentially expressed by pairwise comparison. These lncRNAs were shorter in length, fewer in exon number and higher in expression levels as compared with mRNAs. 683 lncRNAs and 4320 mRNAs were co-located, while 316 lncRNAs and 9997 mRNAs were in co-expression networks. Seven mRNAs (ANKRD34A, FMODA, GJA3, CNTN5, BMP10, BAI2 and HS3ST6) were involved in both networks of LNC_00035 and LNC_000466. Differentially expressed genes were involved in signaling pathways, such as "phosphorylation", "cytokine-cytokine receptor interaction", "endocytosis" and "MHC protein complex". LNC_000792, LNC_000215, LNC_000035 and LNC_000310, with cis and/or trans relationships with mRNAs, were also involved in ceRNA network.

Conclusions

These results might represent the first identified expression profile of lncRNAs and mRNAs in tilapia macrophages activated by HSP70 and Sa.

Transcriptome profiling of antiviral immune and dietary fatty acid dependent responses of Atlantic salmon macrophage-like cells

BACKGROUND

Due to the limited availability and high cost of fish oil in the face of increasing aquaculture production, there is a need to reduce usage of fish oil in aquafeeds without compromising farm fish health. Therefore, the present study was conducted to determine if different levels of vegetable and fish oils can alter antiviral responses of salmon macrophage-like cells (MLCs). Atlantic salmon (Salmo salar) were fed diets containing 7.4% (FO7) or 5.1% (FO5) fish oil. These diets were designed to be relatively low in EPA + DHA (i.e. FO7: 1.41% and FO5: 1%), but near the requirement level, and resulting in comparable growth. Vegetable oil (i.e. rapeseed oil) was used to balance fish oil in experimental diets. After a 16-week feeding trial, MLCs isolated from fish in these dietary groups were stimulated by a viral mimic (dsRNA: pIC) for 6 h (qPCR assay) and 24 h (microarray and qPCR assays).

RESULTS

The fatty acid composition of head kidney leukocytes varied between the two dietary groups (e.g. higher 20:5n-3 in the FO7 group). Following microarray assays using a 44K salmonid platform, Rank Products (RP) analysis showed 14 and 54 differentially expressed probes (DEP) (PFP < 0.05) between the two diets in control and pIC groups (FO5 vs. FO7), respectively. Nonetheless, Significance Analysis of Microarrays (SAM, FDR < 0.05) identified only one DEP between pIC groups of the two diets. Moreover, we identified a large number (i.e. 890 DEP in FO7 and 1128 DEP in FO5 overlapping between SAM and RP) of pIC-responsive transcripts, and several of them were involved in TLR-/RLR-dependent and cytokine-mediated pathways. The microarray results were validated as significantly differentially expressed by qPCR assays for 2 out of 9 diet-responsive transcripts and for all of the 35 selected pIC-responsive transcripts.

CONCLUSION

Fatty acid-binding protein adipocyte (fabp4) and proteasome subunit beta type-8 (psmb8) were significantly up- and down-regulated, respectively, in the MLCs of fish fed the diet with a lower level of fish oil, suggesting that they are important diet-responsive, immune-related biomarkers for future studies. Although the different levels of dietary fish and vegetable oils involved in this study affected the expression of some transcripts, the immune-related pathways and functions activated by the antiviral response of salmon MLCs in both groups were comparable overall. Moreover, the qPCR revealed transcripts responding early to pIC (e.g. lgp2, map3k8, socs1, dusp5 and cflar) and time-responsive transcripts (e.g. scarb1-a, csf1r, traf5a, cd80 and ctsf) in salmon MLCs. The present study provides a comprehensive picture of the putative molecular pathways (e.g. RLR-, TLR-, MAPK- and IFN-associated pathways) activated by the antiviral response of salmon MLCs.

Isolation and characterization of monocyte/macrophage from peripheral blood of half smooth tongue sole (Cynoglossus semilaevis)

In the present study, the peripheral blood cells of half smooth tongue sole (Cynoglossus semilaevis) were examined by blood smear under the light microscopy. The proportion of main types of blood cells are as following: erythrocyte occupied the majority (92.3%), followed by thrombocyte (4.15%), granulocyte (1.7%), lymphocyte (1.5%) and monocyte (0.3%), respectively. Meanwhile, the isolation method of monocytes was established, by density gradient centrifugation to isolate mononuclear leukocytes of peripheral blood. In primary culture, the monocytes were adhered to the bottom of the flask without feeder cells and separated easily with suspended leukocytes in the medium in 3 h. After suspended leukocytes were removed, the monocytes multiplied rapidly with the two doubly during the 24 h, then the cells proliferated and kept stable until 48 h. When co-cultured with suspended leukocytes after three days, the monocytes could derive to typical macrophages, of which the size enlarged significantly and showed various forms such as like fried eggs, and giant irregular shape with pseudopod because cells fusion or deformation occurred until macrophages died in about two weeks. Monocytes showed strong respiratory burst activity after treated with Phorbol ester PMA and challenged by bacteria respectively. In addition, macrophage of half smooth tongue sole had typical macrophage features such as phagocytic capability, positive esterase activity, and the considerable expression of M-CSFR, MHC-II, IL-6, IL-10, TNF and arginase genes. That arginase expression in macrophages (3d and 5d after differentiation) was upregulated fluctuant suggest that the cultivation was mixture of alternatively activated type macrophage (M2) in the majority while the classically activated type (M1) win the minority. Furthermore, MHC-Ⅱ, M-CSFR and IL-6 were significantly induced following LPS challenge. Collectively, the present study will be useful for the study on half smooth tongue sole immune systems and immune function.

Monocyte/Macrophage: NK Cell Cooperation-Old Tools for New Functions

Monocyte/macrophage and natural killer (NK) cells are partners from a phylogenetic standpoint of innate immune system development and its evolutionary progressive interaction with adaptive immunity. The equally conservative ways of development and differentiation of both invertebrate hemocytes and vertebrate macrophages are reviewed. Evolutionary conserved molecules occurring in macrophage receptors and effectors have been inherited by vertebrates after their common ancestor with invertebrates. Cytolytic functions of mammalian NK cells, which are rooted in immune cells of invertebrates, although certain NK cell receptors (NKRs) are mammalian new events, are characterized. Broad heterogeneity of macrophage and NK cell phenotypes that depends on surrounding microenvironment conditions and expression profiles of specific receptors and activation mechanisms of both cell types are discussed. The particular tissue specificity of macrophages and NK cells, as well as their plasticity and mechanisms of their polarization to different functional subtypes have been underlined. The chapter summarized studies revealing the specific molecular mechanisms and regulation of NK cells and macrophages that enable their highly specific cross-cooperation. Attention is given to the evolving role of human monocyte/macrophage and NK cell interaction in pathogenesis of hypersensitivity reaction-based disorders, including autoimmunity, as well as in cancer surveillance and progression.

Evolutionary Aspects of Macrophages Polarization

Macrophages constitute a heterogeneous population of myeloid cells that are essential for maintaining homeostasis and as a first line of innate responders controlling and organizing host defenses against pathogens. Monocyte-macrophage lineage cells are among the most functionally diverse and plastic cells of the immune system. They undergo specific activation into functionally distinct phenotypes in response to immune signals and microbial products. In mammals, macrophage functional heterogeneity is defined by two activation states, M1 and M2, which represent two polar ends of a continuum exhibiting pro-inflammatory and tissue repair activities, respectively. While the ancient evolutionary origin of macrophages as phagocytic defenders is well established, the evolutionary roots of the specialized division of macrophages into subsets with polarized activation phenotypes is less well defined. Accordingly, this chapter focuses on recent advances in the understanding of the evolution of macrophage polarization and functional heterogeneity with a focus on ectothermic vertebrates.