Specific humoral immunity in Osteichthyes

The fish spleen

In the present study, we review the structure and function of fish spleen with special emphasis on its condition in Elasmobranchs, Teleosts and Lungfish. Apart from the amount of splenic lymphoid tissue, the histological organization of the organ ensures the existence of areas involved in antigen trapping, the ellipsoids, and exhibit numerous melano-macrophages which appear isolated or forming the so-called melano-macrophage centres. An extensive discussion on the functional significance of these centres conclude that they are mere accumulations of macrophages consequence of tissue homeostasis rather than primitive germinal centres, as proposed by some authors.

Nanopeptide CI20 remarkably enhances growth performance and disease resistances by improving the mucosal structure, antioxidant capacity, and immunity in mandarin fish (Siniperca chuatsi)

Soybean meal, excessively used in place of fish meal (FM) in aquaculture, has a detrimental impact on fish. In this study, the nanopeptide CI20, which was created by conjugating antimicrobial peptide gcIFN-20H and CMCS, were evaluated the feeding effect in mandarin fish (Siniperca chuatsi). Compared with the control group, 150 mg/kg C-I20-fed fish showed the second highest growth performance with no significant changes in body composition. C-I20-fed fish showed more goblet cells and thicker mucin after feeding. The 150 mg/kg CI20 diet boosted the antioxidant capacity, immunity, and digestive enzymes. After Aeromonas hydrophila and infection spleen and kidney necrosis virus infection, the survival rates in the 150 mg/kg CI20 group were highest. Meanwhile, many tissues in the 150 mg/kg CI20 group had significantly lower pathogen loads than the other groups. Treatment with 150 mg/kg CI20 was effective in increasing antioxidant capacity and immunity. The minimum tissue lesions were observed in the 150 mg/kg CI20 group. The goblet cell number and mucin thickness were significantly increased by CI20 treatment after infection. The study results herein showed that a reasonable dietary concentration of CI20 feed promoted growth performance and disease resistances in fish, suggesting a prospective nano antimicrobial peptide for the aquaculture.

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

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

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).

Immunity elicited by AMP-encoding plasmids fails to increase the protection of European sea bass against nodavirus

Antimicrobial peptides (AMPs) are a potent arm of the innate immune system that can directly kill pathogens and induce immunomodulation. In the marine aquaculture, European sea bass (Dicentrarchus labrax L.) is one of the most prosperous species but is highly susceptible to nodavirus (NNV), which produces high rates of mortality in larvae and juvenile stages. Thus, we aimed to evaluate whether AMPs exert immunomodulatory and/or NNV-preventive actions in sea bass. To do this, plasmids encoding the sea bass AMPs dicentracin (pDIC), beta-defensin (pDB1), hepcidin (pHAMP2) or NK-lysin (pNKL) were generated and intramuscularly injected into sea bass juveniles to evaluate their immunomodulatory and anti-NNV roles. Sea bass muscle transcribes the AMPs and produces an increase in their circulating levels, along with an increase of the antibacterial activity. Immune-related gene analysis revealed a great activation of the inflammatory response and the recruitment of neutrophilic granulocytes at the site of injection. However, AMP-encoding plasmids, namely pHAMP2, negatively affected to NNV disease by increasing fish mortality. In conclusion, plasmids encoding AMPs show immunostimulatory effects on European sea bass but do not improve the resistance to NNV.

RNA-Seq and 16S rRNA Analysis Revealed the Effect of Deltamethrin on Channel Catfish in the Early Stage of Acute Exposure

Deltamethrin (Del) is a widely used pyrethroid insecticide and a dangerous material that has brought serious problems to the healthy breeding of aquatic animals. However, the toxicological mechanisms of Del on channel catfish remain unclear. In the present study, we exposed channel catfish to 0, 0.5, and 5 μg/L Del for 6 h, and analyzed the changes in histopathology, trunk kidney transcriptome, and intestinal microbiota composition. The pathological analyses showed that a high concentration of Del damaged the intestine and trunk kidney of channel catfish in the early stage. The transcriptome analysis detected 32 and 1837 differentially expressed genes (DEGs) in channel catfish trunk kidneys after exposure to 0.5 and 5 μg/L Del, respectively. Moreover, the KEGG pathway and GO enrichment analyses showed that the apoptosis signaling pathway was significantly enriched, and apoptosis-related DEGs, including cathepsin L, p53, Bax, and caspase-3, were also detected. These results suggested that apoptosis occurs in the trunk kidney of channel catfish in the early stage of acute exposure to Del. We also detected some DEGs and signaling pathways related to immunity and drug metabolism, indicating that early exposure to Del can lead to immunotoxicity and metabolic disorder of channel catfish, which increases the risk of pathogenic infections and energy metabolism disorders. Additionally, 16S rRNA gene sequencing showed that the composition of the intestinal microbiome significantly changed in channel catfish treated with Del. At the phylum level, the abundance of Firmicutes, Fusobacteria, and Actinobacteria significantly decreased in the early stage of Del exposure. At the genus level, the abundance of Romboutsia, Lactobacillus, and Cetobacterium decreased after Del exposure. Overall, early exposure to Del can lead to tissue damage, metabolic disorder, immunotoxicity, and apoptosis in channel catfish, and affect the composition of its intestinal microbiota. Herein, we clarified the toxic effects of Del on channel catfish in the early stage of exposure and explored why fish under Del stress are more vulnerable to microbial infections and slow growth.

Lympho-Hematopoietic Microenvironments and Fish Immune System

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

Effects of acute deltamethrin exposure on kidney transcriptome and intestinal microbiota in goldfish (Carassius auratus)

As a widely used synthetic pyrethroid insecticide, deltamethrin (DM) causes serious health problems to aquatic organisms. However, the comprehensive understanding of the adverse effect of DM on aquatic organisms has received limited attention. In this study, goldfish (Carassius auratus) were exposed to 0 (control group), 0.2 and 2 µg/L DM for 96 h. The kidney transcriptome and intestinal microbiota were investigated. Comparative transcriptome analysis identified 270 and 711 differentially expressed genes (DEGs) in goldfish kidneys after exposure to 0.2 and 2 µg/L DM, respectively. KEGG pathway analysis revealed that the apoptosis pathway was markedly regulated and the regulation of programmed cell death was significantly enriched by the GO analysis. Several apoptosis-related genes including cathepsin L and cytochrome c were also detected. These results indicated that apoptosis occurred in the goldfish kidney after acute exposure to sublethal concentration of DM. Besides, some immune and drug metabolism-related DEGs were identified, indicating that exposure to DM caused immunotoxicity and metabolic disruption in goldfish. Additionally, 16 S rRNA gene sequencing analysis revealed a remarkable alteration in the composition of the intestinal microbial community of DM-treated goldfish. At the phylum level, the abundance of Proteobacteria, Firmicutes and Fusobacteria was increased, whereas the abundance of Bacteroidetes was reduced significantly after DM exposure. At the genus level, the abundance of Aeromonas, Cetobacterium, Dielma and Pseudorhodobacter was reduced, whereas Akkermansia was increased after DM exposure. In summary, exposure to DM could induce apoptosis and immunotoxicity in goldfish kidneys and affect the composition of the intestinal microbiota in goldfish. This study provides a comprehensive analysis of the adverse effect of DM exposure on the goldfish and will be helpful for understanding the toxicological mechanisms of DM in fish.

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.

Identification of an IgD/IgT chimera in the European sea bass (Dicentrarchus labrax L.)

Three classes of immunoglobulins have been identified in Teleosts: IgM, IgT/Z and IgD. They are fundamental for fish immune responses and, therefore, their functional activities are heavily investigated. In this paper, we describe the identification of a new IgD/IgT chimera in sea bass (Dicentrarchus labrax) from a gills transcriptome. This transcript joined the first six constant domains of the IgD chain with the two terminal constant domains of IgT, generating a long in-frame coding sequence with a junction between the canonical δ6 exon splicing donor site and the τ3 exon splicing acceptor site. Studies performed on genomic DNA confirmed the presence of the sequence and identifies and intronic region of 656 bp within this joining region. The basal expression of the IgD/IgT chimera was investigated both in silico and in vivo: high level of expression was found in gills, gut and head kidney. Moreover, IgD/IgT transcripts were up-regulated after in vitro stimulation of sea bass HK leukocytes with LPS. The IgD/IgT chimera was found also in two congener species, Morone saxatilis and Morone chrysops. It is not possible to have a precise idea on the evolutionary scenario that lead to the appearance of this sequence due to the lack of genomic information, but we could speculate that an ancestral duplication of the entire IgH locus was followed by the chimerization of Cδ/Cτ in one of the two loci. Finally, the IgD/IgT high basal expression in tissues and organs fundamental for sea bass immune response and its modulation after LPS stimulation provide a very preliminary indication that this unusual Ig variant could have a functional activity.

Oral vaccination using Artemia coated with recombinant Saccharomyces cerevisiae expressing cyprinid herpesvirus-3 envelope antigen induces protective immunity in common carp (Cyprinus carpio var. Jian) larvae

Cyprinid herpesvirus 3 (CyHV-3) is the etiological agent of koi herpersvirus disease (KHVD), which causes serious economic losses in global common carp and ornamental koi carp production of larvae as well as adult type fish. To control KHVD, vaccines against CyHV-3 utilizing different immunization routes have been developed, among them, oral vaccination is the most desirable method to prevent fish diseases occurring at the early larval stage. Here, we developed an oral subunit vaccine through the Saccharomyces cerevisiae cell surface display of CyHV-3 envelope protein pORF65, then, the recombinant yeast fed to Artemia which served as bio-encapsulation vector by subsequently feeding the common carp (Cyprinus carpio var. Jian) larvae. The fluorescent observation showed that the Artemia and S. cerevisiae could deliver intact antigen to the hindgut of carp larvae suggesting the possibility of the vector for oral immunization. On this basis, after three immunizations at a week interval, the oral vaccine induced high level of specific anti-pORF65 antibody. Meanwhile, a significant difference of immune-related genes expression occurred including cxca, IL-1β, IFN-a1, lysozyme, IgM and CD8α between vaccined group and blank control group. In addition, 30% of relative percent survival of carp larvae after immunization was obtained post the animal infection assay, offered an certain immune protection. Our results indicated that the oral pORF65 subunit vaccine bioencapsulated in Artemia induced the activation of immune response and high level of antibodies, which could be served as an oral vaccine candidate for the prevention of CyHV-3 infection.

Protective immunity against CyHV-3 infection via different prime-boost vaccination regimens using CyHV-3 ORF131-based DNA/protein subunit vaccines in carp Cyprinus carpio var. Jian

Cyprinid Herpesvirus 3 (CyHV-3), also known as Koi Herpesvirus (KHV), causes Koi Herpesvirus Disease (KHVD) which leads to serious economic losses worldwide. To exploit DNA/subunit vaccine candidates, CyHV-3 ORF131 gene and cDNA was cloned and analyzed in the present study. Major B cell epitopes of deduced CyHV-3 pORF131 was also predicted. Then the complete CDS of CyHV-3 ORF131 was inserted into pEGFP-N1 vector and a modified pYD1/EBY100 system to construct the DNA and subunit vaccine, respectively. Subsequently, carp were immunized with homologous and heterologous prime-boost regimens relying on the constructed DNA and oral subunit vaccines. Then the protective immunity generated from different vaccines and regimens as well as the capacity of yeast (Saccharomyces cerevisiae) as an oral vaccine vehicle was evaluated. Our study confirmed that CyHV-3 ORF131 gene consisted of 2 introns and 3 exons encoding a 428 amino acids peptide. Further analysis indicated that four fragments of CyHV-3 pORF131 contained the major B cell epitopes (Cys20~Val140, Ser169~Tyr245, Thr258~Pro390, Phe414~Gln428), which could be linked and expressed in E. coli (BL21) as a truncated pORF131. The expression of full-length CyHV-3 pORF131 by pEGFP-N1 and yeast surface display was verified by In vitro assays before vaccination. Immunization of carp with CyHV-3 ORF131 DNA and subunit vaccines could evoke the activation of immune-related genes such as CXCa, CXCR1, IL-1β, TNF-α, INF-a1, Mx-1, IgM, IgT1 and production of specific serum IgM measured by ELISA. RPS (relative percent of survival) ranging from 53.33% to 66.67% was acquired post challenge test. Moreover, flow cytometry analysis illustrated the delivery of surface-displayed CyHV-3 pORF131 to midgut after oral gavage. Thus, our findings suggest that CyHV-3 ORF131 can serve as DNA/subunit vaccines candidate and the yeast as an ideal oral vaccine vehicle.

Chemokine CXCb1 stimulates formation of NETs in trunk kidney neutrophils of common carp

Both in mammals and in fish, CXC chemokines activate leukocytes and regulate their migration both under normal physiological and inflammatory conditions. Moreover, in mammalian neutrophils CXC chemokines also stimulate the formation of neutrophil extracellular traps (NETs). Here, we investigated the effects of recombinant carp CXCL8s and CXCb1 on NET formation in neutrophils from the head (HK) and trunk (TK) kidney of carp. We found that neither recombinant CXCL8s nor CXCb1 stimulated DNA release in HK-derived neutrophils, while in TK-derived cells rcCXCb1 stimulated the release of NETs, composed of extracellular DNA co-localized with citrulline H3 histone and neutrophil elastase. Furthermore, CXCb1-induced NET release required NADPH oxidase activity, while it did not change upon treatment with CXCR inhibitors. In conclusion, we demonstrated, for the first time in fish, that CXCb1 chemokine induces formation of NETs in TK-derived neutrophils and this process is ROS-dependent. The difference between HK and TK-derived neutrophils is probably related to differences in the maturation state of these cells.