Anaphylatoxin-like molecules generated during complement activation induce a dramatic enhancement of particle uptake in rainbow trout phagocytes

Proteomics analysis of differentially abundant proteins in the rohu kidney infected with Edwardsiella tarda

Edwardsiella tarda (Et) is a zoonotic gram-negative pathogen with a diverse host range, including fish. However, the in-depth molecular mechanisms underlying the response of Labeo rohita (rohu) kidney to Et are poorly understood. A proteomic and histopathological analysis was performed for the rohu kidney after Et infection. The histopathology of the infected rohu kidney showed vacuolation and necrosis. After LC-MS/MS analysis, ~1240 proteins were identified with ≥2 unique peptides. A total of 96 differentially abundant proteins (DAPs) were observed between the control and Et infected group (ET). Metascape and STRING analysis were used for the gene ontology (GO), and protein-protein interaction network (PPI) for the significant pathways of DAPs. In PPI, low-abundant proteins were mapped to metabolic pathways and oxidative phosphorylation (cox5ab, uqcrfs1). High-abundance proteins were mapped to ribosomes (rplp2), protein process in the ER (hspa8), and immune system (ptgdsb.1, muc2). Our label-free proteomic approach in the rohu kidney revealed abundant enriched proteins involved in vesicle coat (ehd4), complement activation (c3a.1, c9, c7a), phagosome (thbs4, mapk1), metabolic reprogramming (hao1, glud1a), wound healing (vim, alox5), and the immune system (psap) after Et infection. A targeted proteomics approach of multiple reaction monitoring (MRM) validated the DAPs (nprl3, ambp, vmo1a, hspg2, muc2, hao1 and glud1a) between control and ET. Overall, the current analysis of histology and proteome in the rohu kidney provides comprehensive data on pathogenicity and the potential immune proteins against Et.

Δfur mutant as a potential live attenuated vaccine (LAV) candidate protects American eels (Anguilla rostrata) from Vibrio harveyi infection

The eel farming industry is highly susceptible to Vibriosis. Although various types of vaccines against Vibriosis have been investigated, there is limited research on decreasing the virulence of Vibrions through gene knockout and utilizing it as live attenuated vaccines (LAV). In this study, we aim to develop a LAV candidate against Vibrio harveyi infection in American eels (Anguilla rostrata) using a ferric uptake regulator (fur) gene mutant strain of V. harveyi (Δfur mutant). After the eels were administrated with the Δfur mutant at the dose of 4 × 102 cfu/g body weight, the phagocytic activity of the leucocytes, plasma IgM antibody titers, activity of lysozyme and Superoxide Dismutase (SOD) enzyme, and gene expression levels of 18 immune related proteins were detected to evaluate the protection effect of the LAV. Preliminary findings suggest that the LAV achieved over 60% relative percent survival (RPS) after the American eels were challenged by a wild-type strain of V. harveyi infection on 28 and 42 days post the immunization (dpi). The protection was mainly attributed to increased plasma IgM antibody titers, higher levels of lysozyme, enhanced activity of SOD and some regulated genes encoded immune related proteins. Together, the Δfur mutant strain of V. harveyi, as a novel LAV vaccine, demonstrates promising protective effects against V. harveyi infection in American eels, thus presenting a potential candidate vaccine for fish farming.

Insights into the Antibacterial Properties of Complement Peptides C3a, C4a, and C5a across Vertebrates

Complement peptides C3a, C4a, and C5a are important components of innate immunity in vertebrates. Although they diverged from a common ancestor, only C3a and C4a can act as antibacterial peptides in Homo sapiens, suggesting that C5a has evolved into a purely chemotactic molecule; however, the antibacterial properties of C3a, C4a, and C5a across vertebrates still require elucidation. In this article, we show that, unlike those in H. sapiens, Mus musculus C3a, C4a, and C5a all possess antibacterial activities, implying that the antibacterial properties of C3a, C4a, and C5a have evolved divergently in vertebrates. The extremely different net charge, a key factor determining the antibacterial activities of cationic antimicrobial peptides, of vertebrate C3a, C4a, and C5a supports this speculation. Moreover, the antibacterial activity of overlapping peptides covering vertebrate C3a, C4a, and C5a further strongly supports the speculation, because their activity is positively correlated with the net charge of source molecules. Notably, the structures of C3a, C4a, and C5a are conserved in vertebrates, and the inactive overlapping peptides can become antibacterial peptides if mutated to possess enough net positive charges, indicating that net charge is the only factor determining the antibacterial properties of vertebrate C3a, C4a, and C5a. More importantly, many vertebrate C3a-, C4a-, and C5a-derived peptides possess high antibacterial activities yet exhibit no hemolytic activities, suggesting the application potential in anti-infective therapy. Taken together, our findings reveal that vertebrate C3a, C4a, and C5a are all sources of antibacterial peptides that will facilitate the design of excellent peptide antibiotics.

Comparative Transcriptomic Immune Responses of Mullet (Mugil cephalus) Infected by Planktonic and Biofilm Lactococcus Garvieae

Lactococcus garvieae is an important pathogen of fish, associated with high rates of mortality and infection recurrence in summer or stressful conditions. Chronic infection and disease recurrence have also been reported to be associated with biofilms. However, the impact of biofilm and planktonic bacterial infection on fish immune responses remains unclear. In this study, de novo sequencing was used to compare differences of the spleen transcriptome in planktonic- and biofilm-infected mullets. Among the 181,024 unigenes obtained, 3,392 unigenes were associated with immune response genes. Comparative analysis of the gene expression between infection with the L. garvieae planktonic type and biofilm type identified a total of 3,120 and 3,489 differentially expressed genes in response to planktonic and biofilm infection, respectively, of which 1,366 and 1,458 genes were upregulated, and 1,754 and 1,458 genes were downregulated, respectively. Gene ontology enrichment analysis of immune genes identified genes involved in the complement system, toll-like receptor signaling, and antigen processing, which were further verified by qPCR. Additionally, genes encoding TLR2, IL-1β, TNF-α, C7, and MHC class II peptides were downregulated in response to biofilm infection. Importantly, the results show that biofilm infection induces a different immune pathway response compared with planktonic bacterial infection and, furthermore, illustrates that the prevention of biofilm formation may be a necessary and new strategy for controlling bacterial infection in aquaculture.

Complement C3a Enhances the Phagocytic Activity of B Cells Through C3aR in a Fish

The complement system is an important part of the immune system of teleost fish. Besides, teleost B cells possess both phagocytic activity and adaptive humoral immune function, unlike mammalian B1 cells with phagocytic activity and B2 cells specific to adaptive humoral immunity. However, the cross talk between complement system and phagocytic B cells in teleost fish still requires elucidation. Here, we show that, unlike tetrapods with a single C3 gene, nine C3 genes were identified from the grass carp (Ctenopharyngodon idella) genome, named C3.1-C3.9. Expression analysis revealed that C3.1 is the dominant C3 molecule in grass carp, for its expression was significantly higher than that of the other C3 molecules both at the mRNA and protein levels. The C3a fragment of C3.1 (C3a.1) was determined after the conserved C3 convertase cleavage site. Structural analysis revealed that C3a.1 consists of four α-helixes, with the C-terminal region forming a long α-helix, which is the potential functional region. Interestingly, we found that the recombinant GST-C3a.1 protein and the C-terminal α-helix peptide of C3a.1 both could significantly enhance the phagocytic activity of IgM⁺ B cells. Further study revealed that the C3a receptor (C3aR) was highly expressed in grass carp IgM⁺ B cells, and the phagocytosis-stimulating activity of C3a.1 could be dramatically inhibited by the anti-C3aR antibodies, indicating that C3a.1 performed the stimulating function through C3aR on IgM⁺ B cells. Taken together, our study not only uncovered the novel phagocytosis-stimulating activity of C3a, but also increased our knowledge of the cross talk between complement system and phagocytic B cells in teleost fish.

Complement C3 and Activated Fragment C3a Are Involved in Complement Activation and Anti-Bacterial Immunity

In the complement system, C3 is a central component in complement activation, immune defense and immune regulation. In all pathways of complement activation, the pivotal step is conversion of the component C3 to C3b and C3a, which is responsible to eliminate the pathogen and opsonization. In this study, we examined the immunological properties of C3 and its activated fragment C3a from Japanese flounder (Paralichthys olivaceus) (PoC3 and PoC3a), a teleost species with important economic value. PoC3 is composed of 1655 amino acid residues, contains the six domains and highly conserved GCGEQ sequence of the C3 family. We found that PoC3 expression occurred in nine different tissues and was upregulated by bacterial challenge. In serum, PoC3 was able to bind to a broad-spectrum of bacteria, and purified native PoC3 could directly kill specific pathogen. When PoC3 expression in Japanese flounder was knocked down by siRNA, serum complement activity was significantly decreased, and bacterial replication in fish tissues was significantly increased. Recombinant PoC3a (rPoC3a) exhibited apparent binding capacities to bacteria and Japanese flounder peripheral blood leukocytes (PBL) and induce chemotaxis of PBL. Japanese flounder administered rPoC3a exhibited enhanced resistance against bacterial infection. Taken together, these results indicate that PoC3 is likely a key factor of complement activation, and PoC3 and PoC3a are required for optimal defense against bacterial infection in teleost.

Potential Applications of Fluorescence-Activated Cell Sorting (FACS) and Droplet-Based Microfluidics in Promoting the Discovery of Specific Antibodies for Characterizations of Fish Immune Cells

Akin to their mammalian counterparts, teleost fish possess a complex assortment of highly specialized immune cells that are capable of unleashing potent innate immune responses to eradicate or mitigate incoming pathogens, and also differentiate into memory lymphocytes to provide long-term protection. Investigations into specific roles and functions of fish immune cells depend on the precise separation of each cell type. Commonly used techniques, for example, density gradient centrifugation, rely on immune cells to have differing sizes or densities and thus fail to separate between similar cell types (e.g. T and B lymphocytes). Furthermore, a continuously growing database of teleost genomic information has revealed an inventory of cellular markers, indicating the possible presence of immune cell subsets in teleost fish. This further complicates the interpretation of results if subsets of immune cells are not properly separated. Consequently, monoclonal antibodies (mAbs) against specific cellular markers are required to precisely identify and separate novel subsets of immune cells in fish. In the field of fish immunology, mAbs are largely generated using the hybridoma technology, resulting in the development of mAbs against specific cellular markers in different fish species. Nevertheless, this technology suffers from being labour-intensive, time-consuming and most importantly, the inevitable loss of diversities of antibodies during the fusion of antibody-expressing B lymphocytes and myeloma cells. In light of this, the focus of this review is to discuss the potential applications of fluorescence-activated cell sorting and droplet-based microfluidics, two emerging technologies capable of screening and identifying antigen-specific B lymphocytes in a high-throughput manner, in promoting the development of valuable reagents for fish immunology studies. Our main goal is to encourage the incorporation of alternative technologies into the field of fish immunology to promote the production of specific antibodies in a high-throughput and cost-effective way, which could better allow for the precise separation of fish immune cells and also facilitate the identification of novel immune cell subsets in teleost fish.

A Siglec-1-like lectin from Nile tilapia (Oreochromis niloticus) possesses functions of agglutination and mediation of macrophage phagocytic activity

Siglec-1, one of the sialic acid-binding immunoglobulin-type lectins, is closely related to the recognition of host-pathogen and cell-cell interactions in the adaptive and innate immune systems. In this communication, a Siglec-1-like gene (OnSiglec-1-like) from Nile tilapia (Oreochromis niloticus) was analyzed. Relative expression revealed that the OnSiglec-1-like was expressed in all tested tissues, and the highest expression was found in the anterior kidney. Upon Streptococcus agalactiae (S. agalactiae) infection, the expression of OnSiglec-1-like was up-regulated in anterior kidney and spleen significantly in vivo. Additionally, the same phenomenon was observed in anterior kidney leukocytes upon LPS and S. agalactiae challenges as well in vitro. Western-blotting and ELISA analyses revealed that recombinant OnSiglec-1-like protein possessed high binding activity to LTA, LPS and S. agalactiae. Further, the recombinant OnSiglec-1-like was able to agglutinate S. agalactiae. Moreover, with the digestion of specific sialidase, the phagocytic ability of macrophages to S. agalactiae was greatly enhanced. Taken together, these results indicated that the Siglec-1-like possesses conserved functions of agglutination and promotion of macrophage phagocytic activity in Nile tilapia.

A novel hepatic lectin of zebrafish Danio rerio is involved in innate immune defense

ASGPR (asialoglycoprotein receptor, also known as hepatic lectin) was the first identified animal lectin, which participated in a variety of physiological processes. Yet its detailed immune functions are not well studied in lower vertebrates. After reporting a zebrafish hepatic lectin (Zhl), we identified a novel hepatic lectin (zebrafish hepatic lectin-like, Zhl-l) in zebrafish. The zhl-l was mainly expressed in liver in a tissue specific manner. And challenge with LPS/LTA induced a significant change of zhl-l expression. What's more, recombinant C-type lectin domain (rCTLD) of Zhl-l had the activity of agglutinating and binding to both Gram-negative and Gram-positive bacteria. It promoted the phagocytosis of bacteria by carp macrophages. Moreover, rCTLD could bind to insoluble lipopolysaccharide (LPS), lipoteichoic acid (LTA) and peptidoglycan (PGN) independent of Ca²⁺, which was inhibited by galactose. Interestingly, Zhl-l was located in the membrane, and its overexpression could upregulate the production of pre-inflammatory cytokines. Taken together, these results indicated that Zhl-l played a role in immune defense, and would provide further information to understand functions of C-type lectin family and the innate immunity in vertebrates.

Complement factor C5 in Atlantic salmon (Salmo salar): Characterization of cDNA, protein and glycosylation

Complement component 5 (C5) is an essential factor of the defensive complement system in all vertebrates. We report the characterization of C5 cDNA and protein from Atlantic salmon (Salmo salar), a teleost fish species of high importance in aquaculture. The C5 cDNA cloned from liver is 5079 nucleotides long, whose translation product has a molecular weight of 190 kDa, with the classical β-α orientation and motifs/sites for β-α cleavage (⁶⁷⁸RPKR⁶⁸¹) and cleavage by C5 convertases (R⁷⁵⁸). Mass spectrometric analysis show a single N-linked, biantennary, complex glycan at N¹¹²⁵. Moreover, the N-linked glycan displays an unusual modification in the form of acetylated sialic acid residues. Three anti-C5 antisera produced in mice using purified C5 worked in immunohistochemical analyses of formalin fixed liver tissue. The purification method, whereby inactive and activated (C5b) forms were isolated, opens for interesting studies on the complement function in fish, including possible connection to stress, disease and glycosylation.

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.

Immunological response of the Sub-Antarctic Notothenioid fish Eleginops maclovinus injected with two strains of Piscirickettsia salmonis

Eleginops maclovinus is an endemic fish to Chile that lives in proximity to salmonid culture centers, feeding off of uneaten pellet and salmonid feces. Occurring in the natural environment, this interaction between native and farmed fish could result in the horizontal transmission of pathogens affecting the aquaculture industry. The aim of this study was to evaluate the innate and adaptive immune responses of E. maclovinus challenged with P. salmonis. Treatment injections (in duplicate) were as follows: control (100 μL of culture medium), wild type LF-89 strain (100 μL, 1 × 10⁸ live bacteria), and antibiotic resistant strain Austral-005 (100 μL, 1 × 10⁸ live bacteria). The fish were sampled at various time-points during the 35-day experimental period. The gene expression of TLRs (1, 5, and 8), NLRCs (3 and 5), C3, IL-1β, MHCII, and IgMs were significantly modulated during the experimental period in both the spleen and gut (excepting TLR1 and TLR8 spleen expressions), with tissue-specific expression profiles and punctual differences between the injected strains. Anti-P. salmonis antibodies increased in E. maclovinus serum from day 14-28 for the LF-89 strain and from day 14-35 for the Austral-005 strain. These results suggest temporal activation of the innate and adaptive immune responses in E. maclovinus tissues when injected by distinct P. salmonis strains. The Austral-005 strain did not always cause the greatest increases/decreases in the number of transcripts, so the magnitude of the observed immune response (mRNA) may not be related to antibiotic resistance. This is the first immunological study to relate a pathogen widely studied in salmonids with a native fish.

Antimicrobial and host cell-directed activities of Gly/Ser-rich peptides from salmonid cathelicidins

Cathelicidins, a major family of vertebrate antimicrobial peptides (AMPs), have a recognized role in the first line of defense against infections. They have been identified in several salmonid species, where the putative mature peptides are unusually long and rich in serine and glycine residues, often arranged in short multiple repeats (RLGGGS/RPGGGS) intercalated by hydrophobic motifs. Fragments of 24-40 residues, spanning specific motifs and conserved sequences in grayling or brown, rainbow and brook trout, were chemically synthesized and examined for antimicrobial activity against relevant Gram-positive and Gram-negative salmonid pathogens, as well as laboratory reference strains. They were not active in complete medium, but showed varying potency and activity spectra in diluted media. Bacterial membrane permeabilization also occurred only under these conditions and was indicated by rapid propidium iodide uptake in peptide-treated bacteria. However, circular dichroism analyses indicated that they did not significantly adopt ordered conformations in membrane-like environments. The peptides were not hemolytic or cytotoxic to trout cells, including freshly purified head kidney leukocytes (HKL) and the fibroblastic RTG-2 cell line. Notably, when exposed to them, HKL showed increased metabolic activity, while a growth-promoting effect was observed on RTG-2 cells, suggesting a functional interaction of salmonid cathelicidins with host cells similar to that shown by mammalian ones. The three most active peptides produced a dose-dependent increase in phagocytic uptake by HKL simultaneously stimulated with bacterial particles. The peptide STF(1-37), selected for further analyses, also enhanced phagocytic uptake in the presence of autologous serum, and increased intracellular killing of live E. coli. Furthermore, when tested on HKL in combination with the immunostimulant β-glucan, it synergistically potentiated both phagocytic uptake and the respiratory burst response, activities that play a key role in fish immunity. Collectively, these data point to a role of salmonid cathelicidins as modulators of fish microbicidal mechanisms beyond a salt-sensitive antimicrobial activity, and encourage further studies also in view of potential applications in aquaculture.

Edwardsiella tarda evades serum killing by preventing complement activation via the alternative pathway

Edwardsiella tarda is a Gram-negative bacterium with a broad host range that includes a wide variety of farmed fish as well as humans. E. tarda has long been known to be able to survive in host serum, but the relevant mechanism is unclear. In this study, we investigated the fundamental question, i.e. whether E. tarda activated serum complement or not. We found that (i) when incubated with flounder serum, E. tarda exhibited a high survival rate (87.6%), which was slightly but significantly reduced in the presence of Mg(2+); (ii) E. tarda-incubated serum possessed strong hemolytic activity and bactericidal activity, (iii) compared to the serum incubated with a complement-sensitive laboratory Escherichia coli strain, E. tarda-incubated serum exhibited much less chemotactic activity, (iv) in contrast to the serum incubated with live E. tarda, the serum incubated with heat-inactivated E. tarda exhibited no apparent hemolytic capacity. Taken together, these results indicate for the first time that E. tarda circumvents serum attack by preventing, to a large extent, complement activation via the alternative pathway, and that heat-labile surface structures likely play an essential role in the complement evasion of E. tarda.

Functional characterization of mannose-binding lectin in zebrafish: implication for a lectin-dependent complement system in early embryos

The lectin pathway involves recognition of pathogen-associated molecular patterns by mannose-binding lectin (MBL), and the subsequent activation of associated enzymes, termed MBL-associated serine proteases (MASPs). In this study, we demonstrate that the transcript of MBL gene is present in the early embryo of zebrafish, and MBL protein is also present in the embryo. In addition, we show that recombinant zebrafish MBL was able to bind the Gram-negative bacterium Escherichia coli and the Gram-positive bacterium Staphylococcus aureus, and rMBL was able to promote the phagocytosis of E. coli and S. aureus by macrophages, indicating that like mammalian MBL, zebrafish MBL performs a dual function in both pattern recognition and opsonization. Importantly, we show that microinjection of anti-MBL antibody into the early developing embryos resulted in a significantly increased mortality in the embryos challenged with Aeromonas hydrophila (pathogenic to zebrafish); and injection of rMBL into the embryos (resulting in increase in MBL in the embryo) markedly promoted their resistance to A. hydrophila; and this promoted bacterial resistance was significantly reduced by the co-injection of anti-MBL antibody with rMBL but not by the injection of anti-actin antibody with rMBL. These suggest that the lectin pathway may be already functional in the early embryos in zebrafish before their immune system is fully matured, protecting the developing embryos from microbial infection. This work provides a new angle to understand the immune role of the lectin pathway in early development of animals.

Complement C5a acts as molecular adjuvant in fish by enhancing antibody response to soluble antigen

C5a, the most potent anaphylatoxin generated during complement activation, has important pro-inflammatory actions and has also been shown to enhance antigen-specific antibody response in mammals, thereby acting as a molecular adjuvant. In rainbow trout, C5a has been shown to have a chemoattractant ability and its receptor has also been found on potential APCs. In this study, we tested the possible role of trout C5a as a molecular adjuvant. We demonstrated the presence of native C5a in trout serum using the antibody generated by recombinant trout C5a, and then we generated recombinant infectious hematopoietic necrosis virus glycoprotein (G), and a G-C5a fusion protein to test the adjuvant activity of trout C5a. Recombinant G-C5a displayed a potent chemoattractant activity in contrast to G alone, indicating that the C5a portion of the fusion protein was functional. Thereafter, G-C5a, partially emulsified in a small quantity of IFA, was injected into one group of trout, while the other group of trout was inoculated with the same dose of recombinant G. At four to sixteen weeks post-injection, the serum IgM antibody levels of the fish injected with recombinant G-C5a were obviously higher than those injected with G protein alone. Thus, these results suggest, for the first time, that C5a acts as molecular adjuvant in teleost fish by enhancing antibody response to a soluble antigen.

Antibiotic uptake by cultured Atlantic cod leucocytes and effect on intracellular Francisella noatunensis subsp. noatunensis replication

The granuloma disease caused by Francisella noatunensis subsp. noatunensis in farmed Atlantic cod has not been successfully treated by use of antibacterials, even when antibacterial resistance testing indicates a sufficient effect. The reason for this treatment failure may be the intracellular existence of the bacteria within immune cells, mainly macrophages. To investigate the effect of antibacterials on intracellular Francisella replication, we established a protocol for the detection of drugs within Atlantic cod immune cells using high-performance liquid chromatography (HPLC). When the uptake and intracellular concentrations of oxolinic acid and flumequine were analysed in isolated adherent head kidney leucocytes (HKLs) by HPLC, we found that uptake was rapid and the intracellular concentrations reflected the extracellular exposure concentrations. To investigate the effect of the antibacterial compounds on intracellular bacterial replication, adherent HKLs experimentally infected with the bacteria were analysed using flow cytometry and intracellular labelling of bacteria by specific antibodies. We found that flumequine did not inhibit intracellular bacterial replication. Unexpectedly, the results indicated that the intracellularly effiacy of the drug was reduced. The HPLC method used proved to be highly applicable for accurate determination of intracellular drug concentrations. When combined with sensitive and specific flow cytometry analyses for identification and measurement of intracellular bacterial replication, we suggest that this approach can be very valuable for the design of antibacterial treatments of intracellular pathogens.

Functional analysis of domain of unknown function (DUF) 1943, DUF1944 and von Willebrand factor type D domain (VWD) in vitellogenin2 in zebrafish

Vitellogenin (Vg), the precursor of egg-yolk proteins in all oviparous organisms, shares a similar domain structure combination. In most cases, Vg contains the Vitellogenin_N domain, the domain of unknown function (DUF) 1943, and the von Willebrand factor type D domain (VWD), which are present in different forms of Vg from both vertebrates and invertebrates. Occasionally, a DUF1944 domain is also present in between DUF1943 and VWD in some Vg proteins of vertebrates. Recent studies have shown that Vg participates in immune defense of host with multiple functions. However, whether all Vg proteins encoded by different vg genes play an immune role is unknown. In addition, the correlation of different domains in Vg with the multiple immune functions remains completely unclear. Here we demonstrated clearly that recombinant proteins, rDUF1943, rDUF1944 and rVWD from zebrafish Vg2 interacted with both the Gram-positive bacteria Staphylococcus aureus, Bacillus subtilis and Micrococcus luteus and the Gram-negative bacteria Escherichia coli and Vibrio anguillarum, as well as their signature components LTA and LPS. Moreover, both rDUF1943 and rDUF1944 promoted the phagocytosis of E. coli and S. aureus by carp macrophages. These suggest that both DUF1943 and DUF1944 as well as VWD may contribute to the function of Vg as a pattern recognition receptor, and DUF1943 and DUF1944 also contribute to the function of Vg as an opsonin. This study also opens a new angle for identification of function of genes of unknown function, which have the domains DUF1943 and DUF1944.

Interplay between invertebrate C3a with vertebrate macrophages: functional characterization of immune activities of amphioxus C3a

Our current knowledge of the structure and function of C3a comes from the study of vertebrate C3a anaphylatoxins, virtually nothing is known about the structure and function of C3a molecules in invertebrates. Here we demonstrated that C3a from the invertebrate chordate Branchiostoma japonicum, BjC3a, was similar to vertebrate C3a possessing potential antibacterial activity, as revealed by sequence analysis and computational modeling. The antibacterial activity of BjC3a was definitely confirmed by both antibacterial assay and TEM observation showing that recombinant BjC3a was directly bactericidal. Additionally, recombinant BjC3a, like vertebrate C3a, was capable of inducing sea bass macrophage migration and enhancing macrophage phagocytosis and respiratory burst response. Moreover, recombinant BjC3a-desArg (generated by removal of the C-terminal arginine), like mammalian C3a-desArg, retained the immunological activities of BjC3a such as antibacterial and respiratory burst-stimulating activities, indicating that the immunological functions of C3a-desArg were conserved throughout chordate evolution. Altogether, our findings show that invertebrate (amphioxus) BjC3a is able to interact with vertebrate (sea bass) macrophages and mediate immune activities, suggesting the emergence of the inflammatory pathway of the complement system similar to that of vertebrates in the basal chordate amphioxus.

Francisella noatunensis subsp. noatunensis replicates within Atlantic cod (Gadus morhua L.) leucocytes and inhibits respiratory burst activity

Francisella noatunensis subsp. noatunensis, causing granulomatosis in cod, has been shown to reside within cod immune cells, mainly within monocytes and macrophages. In the present study, we analysed the ability of the bacterium to replicate within adherent cells isolated from head kidney by in vitro infection of leucocytes. Two different technical approaches for flow cytometry analyses were performed for detection of intracellular bacteria. The presence of the wild type was assessed after identification by intracellular binding of specific antibodies to the pathogen. The other way was to use green fluorescent protein (GFP) transformed bacterium for infection studies allowing direct measurements of fluorescence from infected cells. By both methods we found an increase in fluorescence in infected cells, verifying bacterial replication, both after 4 and 28 h post infection in leucocytes isolated from head kidney (HKL). The GFP transformed bacterium was similar to the wild type in growth and infectivity pattern, showing that it can be a valuable tool for further studies of infection routes and pathology. Further, F. noatunensis subsp. noatunensis was found to inhibit respiratory burst activity, a potent pathogen killing mechanism, in cod leucocytes, but not in such cells from salmon. Our findings may indicate that inhibition of respiratory burst during Francisella infection is a key to its intracellular existence. This strategy seems to be conserved through evolution as it is also observed during infections in higher vertebrates caused by bacteria within the Francisella genus. The results presented here, showing the intracellular existence of Francisella, its replication within leucocytes and the inhibitory effect on respiratory burst, strongly support that these factors contribute to disease and pathology in infected cod. The intracellular replication shown in the present study might contribute to explain the problems of obtaining protective vaccines against Francisella and effective antibiotic treatment of infected fish.

Calreticulin is a microbial-binding molecule with phagocytosis-enhancing capacity

Calreticulin (CRT) is a highly conserved calcium-binding protein mainly involved in directing proper conformation of proteins and controlling calcium level. Accumulating data also show that CRT is emerging as an immune-relevant molecule. In this study, we demonstrated that the CRT gene from the amphioxus Branchiostoma japonicum, named Bjcrt, consisted of a signal peptide, three domains (N-, P-, C-domains) and an ER retrieval signal sequence (KDEL), which appears to be the ancient form of vertebrate CRTs, and Bjcrt was expressed in a tissue-specific manner, with the most abundant expression in the notochord. We also demonstrated for the first time that the recombinant BjCRT (rBjCRT) was able to bind the Gram-negative bacterium Escherichia coli and the Gram-positive bacterium Staphylococcus aureus. Moreover, both BjCRT as well as human recombinant calreticulin were able to promote the phagocytosis of E. coli and S. aureus by sea bass macrophages. These results indicate that CRT is a microbial-binding molecule and possesses an ability to enhance phagocytosis, a novel function assigned to CRT, reenforcing the notion that CRT is an immune-relevant molecule associated with host immune responses.

Cytokine Gene Expression in CD4 Positive Cells of the Japanese Pufferfish, Takifugu rubripes

CD4⁺ T (Th) cells are a central component of the adaptive immune response and are divided into distinct sets based on their specific cytokine production pattern. Several reports have suggested that fish possess Th subset activity similar to that of mammals. The aim of the present study was to isolate CD4⁺ T cells from the blood of Japanese pufferfish, Fugu rubripes, and to characterize their cytokine expression profile. We produced a specific antibody against Fugu CD4 and performed cell sorting with the magnetic activated cell sorting system. Sorted Fugu CD4⁺ cells were characterized by morphology and expression analysis of cell marker genes. Fugu CD4⁺ cells expressed T-cell marker genes but not macrophage or B-cell marker genes. In addition, peripheral blood lymphocytes were stimulated with lipopolysaccharide (LPS), polycytidylic acid (polyI:C), concanavalin A (ConA) prior to sorting, and then Multiplex RT-PCR was used to examine the expression of Th cytokines by the stimulated Fugu CD4⁺ cells. LPS and polyI:C stimulation upregulated the expression of Th1, Th17 and Treg cytokines and downregulated the expression of Th2 cytokines. ConA stimulation upregulated the expression of all Th cytokines. These results suggest that fish exhibit the same upregulation of Th-specific cytokine expression as in mammals.

A highly phagocytic cell line TO from Atlantic salmon is CD83 positive and M-CSFR negative, indicating a dendritic-like cell type

Leucocyte cell lines are valuable tools for immunological studies. In this study the TO cell line, originating from Atlantic salmon head kidney leucocytes, is described with respect to enzyme cytochemistry, functional studies, reactivity with leucocyte specific antibodies and immune gene expression. Pronounced characteristics of the TO cell line are the rapid adherence to the plastic growth surface, high phagocytic capacity and bactericidal functions. No respiratory burst activity, and little or no NO production were detected under the experimental conditions tested, and thus the TO cells appear to have other effective killing mechanisms. The cells are reactive with a leucocyte specific monoclonal antibody (MAb), but does not bind a neutrophil specific MAb or stain for myeloperoxidase. Real-time RT-PCR showed the expression in TO cells of several immune genes, some of which were significantly regulated following LPS stimulation. The expression of CD83 might indicate a dendritic cell (DC) origin of the TO cells, as this marker is considered a hallmark for DC. Expression of TCR-alpha or the macrophage marker M-CSFR was not detected. Based on the present analyses the TO cells display a mixture of known characteristics for macrophages and DCs. At the same time the TO cells lack some central functions of phagocytic/myeloid cells. As the TO cells are developed to a long-term culture one cannot exclude that some functions might have been lost in this process. Nevertheless, the features of the TO cells indicate their potential as a model system for immunological studies of salmon phagocytic cells.

The innate immune response of finfish--a review of current knowledge

The decline in the fisheries of traditional marine species has been an incentive for the diversification of today's aquaculture sector into the intensive rearing of many finfish species. The increasing interest in commercial farming of different finfish species is expected to result in similar environmental and husbandry-related problems as have been experienced in the development of the salmonid farming industry. An understanding of the biology of the fish species being cultured, in particular the immune response is important for improved husbandry and health management of the species. The innate immune system of fish has generated increasing interest in recent years and is now thought to be of key importance in primary defence and in driving adaptive immunity. This review focuses on key components (cellular and humoral) of the innate immune responses of different fish species of commercial importance.

Characterization and functional analysis of goldfish (Carassius auratus L.) tumor necrosis factor-alpha

We identified and characterized two isoforms of tumor necrosis factor-alpha (TNFalpha) from the goldfish, TNFalpha-1 and TNFalpha-2. At the protein level, goldfish TNFalpha-1 and TNFalpha-2 were most homologous to carp TNFalpha-1 and TNFalpha-2, respectively. Phylogenetically, the two goldfish isoforms grouped most closely with the carp TNFalpha isoforms and TNF species of other cyprinids. Real-time PCR analysis revealed constitutive expression of goldfish TNFalpha-1 and TNFalpha-2 in all tissues with TNFalpha-2 mRNA levels higher than TNFalpha-1 in all tissues examined. A modest up-regulation in expressions of goldfish TNFalpha-1 and TNFalpha-2 in kidney-derived monocytes and significant increase in expression of both isoforms in mature macrophages were observed in response to activation with macrophage-activating factors. TNFalpha-2 was subsequently expressed using a prokaryotic expression system and the recombinant molecule (rTNFalpha-2) was functionally characterized. The rTNFalpha-2 induced a dose-dependent chemotactic response and enhanced phagocytosis of primary goldfish macrophages. Furthermore, rTNFalpha-2 primed the respiratory burst in monocytes and induced nitric oxide production of primary goldfish macrophages. Our results indicate that goldfish TNFalpha is a central regulatory and effector cytokine of inflammatory and antimicrobial responses of the goldfish.

Modulation of carp (Cyprinus carpio) neutrophil functions during an infection with the haemoparasite Trypanoplasma borreli

Trypanoplasma borreli is an extracellular blood parasite of common carp (Cyprinus carpio) transmitted by fish-biting leeches. The infestation with this parasite in juvenile carp may range between 75% and 100%, especially in fish recovering from the first hibernation period. T. borreli is perfectly adapted to its prolonged survival in a cyprinid host. Elevated numbers of activated neutrophils in peripheral blood and tissues are reported during T. borreli infection, but in context of the disease, the direct reason for elevated neutrophil numbers and their role during the infection remain unclear. In this study, a quantitative transmigration system, permitting the harvest of highly pure (> or = 97%) neutrophil populations was applied to investigate the modulation of carp neutrophil functions during T. borreli infection. We demonstrate time-dependent kinetics of a serum-induced down-regulation of neutrophil chemotaxis and an up-regulation of ROS production during the course of infection. With highly pure neutrophil populations, we could show that this divergent alteration of neutrophil functions was neither caused by T. borreli metabolites nor by the parasite itself. Moreover, when added to highly purified neutrophils, parasite metabolites did not alter the leukotriene B4-induced neutrophil chemotaxis nor the Staphylococcus aureus-induced ROS production. We conclude that the haemoparasite T. borreli does not interact with neutrophils directly, but indirectly modulates their functions via serum factors induced by parasite interaction with other components of the immune system.

B-Lymphoma cells with epigenetic silencing of Pax5 trans-differentiate into macrophages, but not other hematopoietic lineages

In mice, zygotic or pro-B-cell-specific knock-out of the Pax5 gene allows differentiation of pro-B-cells into all hematopoietic lineages. We previously generated and characterized a murine B-cell lymphoma, dubbed Myc5, whose cells spontaneously lose Pax5 expression when cultured in vitro, but regain it when re-injected into syngeneic mice. In cultured Myc5 cells, the loss of Pax5 correlates with the acquisition of myeloid markers, such as CD11b and F4/80. Here, we sought to determine whether these cells are truly B-macrophage-restricted or, like Pax5-null progenitors, can give rise to additional hematopoietic lineages. In vitro differentiation assays with various cytokines showed that Myc5 cells do not differentiate into NK cells, dendritic cells, neutrophils, or osteoclasts. At the same time, in the presence of macrophage colony-stimulating factor (M-CSF), they readily phagocytose latex beads and provide T-cell help. Both phenomena are indicative of the bona fide macrophage phenotype. Conversely, enforced Pax5 re-expression in macrophage-like Myc5 cells led to down-regulation of the M-CSF receptor and re-acquisition of some B-cell surface markers (e.g., CD79a) and lineage-specific transcription factors (e.g., IRF4 and Blimp). Retrovirally encoded Pax5 also restored expression of several master B-cell differentiation proteins, such as the IL-7 receptor and transcription factor E2A. In contrast, levels of EBF were unaffected by Pax5 suggesting that EBF acts exclusively upstream of Pax5 and might contribute to Pax5 expression. Indeed, transduction with an EBF-encoding retrovirus partly reactivated endogenous Pax5. Our data reveal the complex relationship between B-cell-specific transcription factors and suggest the existence of numerous feedback mechanisms.

B lymphocytes from early vertebrates have potent phagocytic and microbicidal abilities

The present paradigm dictates that phagocytosis is accomplished mainly by 'professional' phagocytes (such as macrophages and monocytes), whereas B cells lack phagocytic capabilities. Here we demonstrate that B cells from teleost fish have potent in vitro and in vivo phagocytic activities. Particle uptake by B cells induced activation of 'downstream' degradative pathways, leading to 'phagolysosome' formation and intracellular killing of ingested microbes. Those results indicate a previously unknown function for B cells in the innate immunity of these primitive animals. A considerable proportion of Xenopus laevis B cells were also phagocytic. Our findings support the idea that B cells evolved from an ancestral phagocytic cell type and provide an evolutionary framework for understanding the close relationship between mammalian B lymphocytes and macrophages.

Recent advances on the complement system of teleost fish

The complement system plays an essential role in alerting the host of the presence of potential pathogens, as well as in their clearing. In addition, activation of the complement system contributes significantly in the orchestration and development of an acquired immune response. Although the complement system has been studied extensively in mammals, considerably less is known about complement in lower vertebrates, in particular teleost fish. Here we review our current understanding of the role of fish complement in phagocytosis, respiratory burst, chemotaxis and cell lysis. We also thoroughly review the specific complement components characterized thus far in various teleost fish species. In addition, we provide a comprehensive compilation on complement host-pathogen interactions, in which we analyze the role of fish complement in host defense against bacteria, viruses, fungi and parasites. From a more physiological perspective, we evaluate the knowledge accumulated on the influence of stress, nutrition and environmental factors on levels of complement activity and components, and how the use of this knowledge can benefit the aquaculture industry. Finally, we propose future directions that are likely to advance our understanding of the molecular evolution, structure and function of complement proteins in teleosts. Such studies will be pivotal in providing new insights into complement-related mechanisms of recognition and defense that are essential to maintaining fish homeostasis.

Evolution of anaphylatoxins, their diversity and novel roles in innate immunity: Insights from the study of fish complement

Anaphylatoxins are small molecules ( approximately 9 kDa) that are generated as a result of the activation of the complement system. These molecules play an important role in inflammation, and they are responsible for the activation of various innate and adaptive immune processes. The study of these important inflammatory molecules has been restricted to mammalian species so far. Recent studies have shown that teleost fish, unlike any other known animal species, contain multiple forms of the C3a anaphylatoxin, all of which are functionally active and play a prominent role in inducing superoxide production in fish leukocytes. The C5a anaphylatoxin has also been characterized in these animals, and like in mammals, it plays an important role in leukocyte chemotaxis and in triggering the respiratory burst of leukocytes. Interestingly, it has been shown that rainbow trout anaphylatoxins play an unexpected role in enhancing phagocytosis of particles. C5a and C3a receptors have recently been cloned and characterized in rainbow trout, suggesting that the duplication of these receptors from a common ancestor occurred before the emergence of teleosts. The studies derived from these molecules in teleost fish indicate that the basic structure and function of anaphylatoxins and their receptors, have been conserved for more than 300 million years.