Molecular characterization of a fish-specific toll-like receptor 22 (TLR22) gene from common carp (Cyprinus carpio L.): Evolutionary relationship and induced expression upon immune stimulants

In the host innate immune system, various pattern recognition receptors (PRRs) recognize conserved pathogens-associated molecular patterns (PAMPs), and represent an efficient first line of defense against invading pathogens. TLR22 is one of the fish-specific Toll-like receptors (TLRs), identified in a variety of fish species. In this study, we report the cloning and identification of a TLR22 cDNA from the gills of common carp (Cyprinus carpio L.). The full-length CcTLR22 cDNA was 3301 bp long, including a 32 bp 5'-untranslated region (UTR), an open reading frame (ORF) of 2838 bp and a 432 bp 3'-UTR.The CcTLR22 protein was found to comprise a signal peptide, 16 LRR domains, a LRRCT domain in the extracellular region and a TIR domain in the cytoplasmic region, which fits with the characteristic TLR domain architecture. The genomic organization of CcTLR22 was identified, which was encoded by an uninterrupted exon. Sequence alignment and phylogenetic analysis showed that all known teleost TLR22 members were clustered into an independent clade of the TLR22 family, and showed high amino acid identities with other fish TLRs. Real-time PCR assay showed that CcTLR22 mRNA was expressed in almost all tissues examined, while the levels obviously varied among different tissues. When challenged with poly(I:C) (a viral model) or A. hydrophila bacteria, the expression level of CcTLR22 was up-regulated in a variety of common carp tissues. These results indicate that CcTLR22 plays a significant role in systemic as well as mucosal defence after viral or bacterial stimulation or infection.

Immune-relevant thrombocytes of common carp undergo parasite-induced nitric oxide-mediated apoptosis

Common carp thrombocytes account for 30-40% of peripheral blood leukocytes and are abundant in the healthy animals' spleen, the thrombopoietic organ. We show that, ex vivo, thrombocytes from healthy carp express a large number of immune-relevant genes, among which several cytokines and Toll-like receptors, clearly pointing at immune functions of carp thrombocytes. Few studies have described the role of fish thrombocytes during infection. Carp are natural host to two different but related protozoan parasites, Trypanoplasma borreli and Trypanosoma carassii, which reside in the blood and tissue fluids. We used the two parasites to undertake controlled studies on the role of fish thrombocytes during these infections. In vivo, but only during infection with T. borreli, thrombocytes were massively depleted from the blood and spleen leading to severe thrombocytopenia. Ex vivo, addition of nitric oxide induced a clear and rapid apoptosis of thrombocytes from healthy carp, supporting a role for nitric oxide-mediated control of immune-relevant thrombocytes during infection with T. borreli. The potential advantage for parasites to selectively deplete the host of thrombocytes via nitric oxide-induced apoptosis is discussed.

Adaptive immune responses at mucosal surfaces of teleost fish

This review describes the extant knowledge on the teleostean mucosal adaptive immune mechanisms, which is relevant for the development of oral or mucosal vaccines. In the last decade, a number of studies have shed light on the presence of new key components of mucosal immunity: a distinct immunoglobulin class (IgT or IgZ) and the polymeric Ig receptor (pIgR). In addition, intestinal T cells and their putative functions, antigen uptake mechanisms at mucosal surfaces and new mucosal vaccination strategies have been reported. New information on pIgR of Atlantic cod and common carp and comparison of natural and specific cell-mediated cytotoxicity in the gut of common carp and European seabass, is also included in this review. Based on the known facts about intestinal immunology and mucosal vaccination, suggestions are made for the advancement of fish vaccines.

Protein profiling in the gut of Penaeus monodon gavaged with oral WSSV-vaccines and live white spot syndrome virus

White spot syndrome virus (WSSV) is a pathogen that causes considerable mortality of the farmed shrimp, Penaeus monodon. Candidate 'vaccines', WSSV envelope protein VP28 and formalin-inactivated WSSV, can provide short-lived protection against the virus. In this study, P. monodon was orally intubated with the aforementioned vaccine candidates, and protein expression in the gut of immunised shrimps was profiled. The alterations in protein profiles in shrimps infected orally with live-WSSV were also examined. Seventeen of the identified proteins in the vaccine and WSSV-intubated shrimps varied significantly compared to those in the control shrimps. These proteins, classified under exoskeletal, cytoskeletal, immune-related, intracellular organelle part, intracellular calcium-binding or energy metabolism, are thought to directly or indirectly affect shrimp's immunity. The changes in the expression levels of crustacyanin, serine proteases, myosin light chain, and ER protein 57 observed in orally vaccinated shrimp may probably be linked to immunoprotective responses. On the other hand, altered expression of proteins linked to exoskeleton, calcium regulation and energy metabolism in WSSV-intubated shrimps is likely to symbolise disturbances in calcium homeostasis and energy metabolism.

Molecular characterization of LEAP-2 cDNA in common carp (Cyprinus carpio L.) and the differential expression upon a Vibrio anguillarum stimulus; indications for a significant immune role in skin

LEAP-2 is a cysteine-rich cationic antimicrobial peptide (AMP) playing an important role in host innate immune system. LEAP-2 genes have been identified from higher vertebrates and several fish species. Here we report the cloning and identification of two LEAP-2 cDNA sequences from the liver of common carp (Cyprinus carpio L.). The LEAP-2A cDNA was 1325 bp long and contained an ORF of 279 bp encoding a protein of 92 amino acids. The LEAP-2B cDNA was 608 bp long and contained an ORF of 276 bp encoding a protein of 91 amino acids. Both LEAP-2 proteins consisted of 41 amino acid residues and shared four cysteines at the conserved positions in the predicted mature peptides, highly similar to LEAP-2 of other species. Sequence alignment showed that LEAP-2 amino acid sequences were well conserved in different species, and the phylogenetic relation of LEAP-2 was coincident with evolution of biological species. Expression analysis data revealed that LEAP-2A and LEAP-2B mRNAs were expressed in a wide range of common carp tissues including liver, spleen, head kidney, skin, gills, hindgut and foregut. When injected intraperitoneally with Vibrio anguillarum, the expression level of common carp LEAP-2A was quickly up-regulated in liver, spleen, head kidney, skin, gills, foregut and hindgut, however, the expression level of LEAP-2B was similarly up-regulated in spleen, skin, gills and hindgut but not in liver, head kidney and foregut. Our results showed that the LEAP-2A had a markedly high constitutive expression in skin, and the LEAP-2A and the LEAP-2B had a significantly high up-regulated expression after stimulus in skin. This differential expression of LEAP-2 in common carp suggests that it may play a key role in immune responses against invading pathogens and both LEAP-2 molecules may be involved in mucosal immunity.

Truncated VP28 as oral vaccine candidate against WSSV infection in shrimp: an uptake and processing study in the midgut of Penaeus monodon

Several oral vaccination studies have been undertaken to evoke a better protection against white spot syndrome virus (WSSV), a major shrimp pathogen. Formalin-inactivated virus and WSSV envelope protein VP28 were suggested as candidate vaccine components, but their uptake mechanism upon oral delivery was not elucidated. In this study the fate of these components and of live WSSV, orally intubated to black tiger shrimp (Penaeus monodon) was investigated by immunohistochemistry, employing antibodies specific for VP28 and haemocytes. The midgut has been identified as the most prominent site of WSSV uptake and processing. The truncated recombinant VP28 (rec-VP28), formalin-inactivated virus (IVP) and live WSSV follow an identical uptake route suggested as receptor-mediated endocytosis that starts with adherence of luminal antigens at the apical layers of gut epithelium. Processing of internalized antigens is performed in endo-lysosomal compartments leading to formation of supra-nuclear vacuoles. However, the majority of WSSV-antigens escape these compartments and are transported to the inter-cellular space via transcytosis. Accumulation of the transcytosed antigens in the connective tissue initiates aggregation and degranulation of haemocytes. Finally the antigens exiting the midgut seem to reach the haemolymph. The nearly identical uptake pattern of the different WSSV-antigens suggests that receptors on the apical membrane of shrimp enterocytes recognize rec-VP28 efficiently. Hence the truncated VP28 can be considered suitable for oral vaccination, when the digestion in the foregut can be bypassed.

Ubiquitous presence of piscidin-1 in Atlantic cod as evidenced by immunolocalisation

Background

Antimicrobial peptides (AMPs), the natural antibiotics bestowed upon all forms of life, consist of small molecular weight proteins with a broad spectrum antimicrobial activity against a variety of pathogenic microorganisms. Piscidins are one of the AMP families that are imperative for the innate defence mechanisms of teleosts. Atlantic cod, a basal fish belonging to the superorder Paracanthopterygii also possesses multiple piscidin peptides. Two piscidin paralogues (pis1 and pis2) and a novel alternative splice variant of pis2 of this fish were previously described by us. To shed light on other potent roles of these molecules, now we have mapped the distribution of piscidin 1 (Pis1), in different tissues and organs of cod through immunohistochemistry (IHC) employing an affinity purified polyclonal antibody specific to Pis1.

Results

Various cell types and tissues of Atlantic cod including those from the immune organs of naïve fish are armed with Pis1 peptide. Different types of the blood leucocytes and phagocytic cells among the leucocytes examined gave a relatively strong indication of Pis1 immunopositivity. In addition, other cell types such as hematopoietic cells, epithelial cells and multi-granular cells located in the mucosal and hematopoietic tissues were also Pis1-immunoreactive. More interestingly, chondrocytes appear to produce Pis1 and this is the first report on the presence of an AMP in cartilage tissue of fish. Furthermore, Pis1 immunopositivity was detected in other tissues and organs of naïve fish including neural tissues, exocrine and endocrine glands, compound gland cells, excretory kidney, intestinal and respiratory epithelial cells, swim bladder, skin and hypodermis layer, myosepta, liver, heart, eye and oocytes.

Conclusions

Pis1 peptide is produced by various cell types located in different tissues and organs of Atlantic cod. It is present in all immune-related organs of naïve fish and the elevated peptide expression following phagocytosis strongly suggest their involvement in innate defence. Further, its widespread occurrence in non-immune tissues and organs of apparently healthy fish implies that piscidin may have other functions in addition to its role as an immune effector molecule.

Molecular cloning and expression of two β-defensin and two mucin genes in common carp (Cyprinus carpio L.) and their up-regulation after β-glucan feeding

In this study, we described the partial structure, mRNA tissue distribution and regulation of two carp mucin and two β-defensin genes. This is the first description of these genes in fish. The genes might provide relevant tools to monitor feed-related improvements of fish health under aquaculture conditions. Carp mucin 2 and mucin 5B genes show a high similarity to their mammalian and avian counterparts. The carp β-defensin 1 and β-defensin 2 genes cluster together well with their piscine family members. The influence of a β-glucan immunomodulant on the expression of these genes in mucosal tissues could be confirmed for the first time. Muc5B expression was significantly increased in the skin. For Muc2 no significant up- or down-regulation could be observed. Significantly higher expression levels of β-defensin 2 in gills and both β-defensin genes in skin were found. Thus, the mucosal system can be influenced by the addition of β-glucans to the food.

Teleost intestinal immunology

Teleosts clearly have a more diffuse gut associated lymphoid system, which is morphological and functional clearly different from the mammalian GALT. All immune cells necessary for a local immune response are abundantly present in the gut mucosa of the species studied and local immune responses can be monitored after intestinal immunization. Fish do not produce IgA, but a special mucosal IgM isotype seems to be secreted and may (partly) be the recently described IgZ/IgT. Fish produce a pIgR in their mucosal tissues but it is smaller (2 ILD) than the 4-5 ILD pIgR of higher vertebrates. Whether teleost pIgR is transcytosed and cleaved off in the same way needs further investigation, especially because a secretory component (SC) is only reported in one species. Teleosts also have high numbers of IEL, most of them are CD3-ɛ+/CD8-α+ and have cytotoxic and/or regulatory function. Possibly many of these cells are TCRγδ cells and they may be involved in the oral tolerance induction observed in fish. Innate immune cells can be observed in the teleost gut from first feeding onwards, but B cells appear much later in mucosal compartments compared to systemic sites. Conspicuous is the very early presence of putative T cells or their precursors in the fish gut, which together with the rag-1 expression of intestinal lymphoid cells may be an indication for an extra-thymic development of certain T cells. Teleosts can develop enteritis in their antigen transporting second gut segment and epithelial cells, IEL and eosinophils/basophils seem to play a crucial role in this intestinal inflammation model. Teleost intestine can be exploited for oral vaccination strategies and probiotic immune stimulation. A variety of encapsulation methods, to protect vaccines against degradation in the foregut, are reported with promising results but in most cases they appear not to be cost effective yet. Microbiota in fish are clearly different from terrestrial animals. In the past decade a fast increasing number of papers is dedicated to the oral administration of a variety of probiotics that can have a strong health beneficial effect, but much more attention has to be paid to the immune mechanisms behind these effects. The recent development of gnotobiotic fish models may be very helpful to study the immune effects of microbiota and probiotics in teleosts.

Proteome reference map of the skin mucus of Atlantic cod (Gadus morhua) revealing immune competent molecules

The skin mucosal proteome of Atlantic cod (Gadus morhua) was mapped using a 2D PAGE, LC-MS/MS coupled approach. Mucosal proteins from naive fish were identified primarily by similarity searches across various cod EST databases. The identified proteins were clustered into 8 groups based on gene ontology classification for biological process. Most of the proteins identified from the gel are hitherto unreported for cod. Galectin-1, mannan binding lectin (MBL), serpins, cystatin B, cyclophilin A, FK-506 binding protein, proteasome subunits (alpha-3 and -7), ubiquitin, and g-type lysozyme are considered immune competent molecules. Five of the aforementioned proteins were cloned and their tissue distribution was analysed by RT-PCR.

Common carp have two subclasses of bonyfish specific antibody IgZ showing differential expression in response to infection

Immunoglobulin heavy chains identified in bony fish are broadly classified into three classes namely IgM, IgD and IgZ. The most recently described isotype is IgZ, a teleosts-fish specific isotype that shows variations in gene structure across teleosts. In this study we have identified two IgZ subclasses in common carp. IgZ1 is a four constant heavy chain domains containing antibody isolated across teleosts and IgZ2 is a two constant domains containing heavy chain chimera with a μ1 and ζ4 domain. Sequence analyses suggest that these subtypes are expressed from two separate genomic loci. Expression analyses show that IgZ1 is more abundant in systemic organs and IgZ2 chimera is preferentially expressed at mucosal sites. The basal expression level of IgM in fish is much higher than of the other isotypes. We show that IgZ1 expression in systemic and mucosal organs is responsive to blood parasites, while mucosal parasite infection induces IgM and IgZ2 gene expression. This report is the first to show differential expression of the IgZ variants in response to pathogens and suggests that the IgZ subtypes in carps may have mutually exclusive humoral functions.

Time-related changes of the intestinal morphology of Atlantic salmon, Salmo salar L., at two different soybean meal inclusion levels

Abstract Soybean meal (SBM) induces enteritis in the distal intestine of Atlantic salmon. The present study assesses the effects of SBM concentrations on the kinetics of the enteritis process. Fish of 300 g, kept at 12 degrees C, were fed diets with different SBM inclusions: 0%, 10% and 20% SBM for 57 days. Samples of the distal intestine of five fish per treatment were taken for histological and electron microscopical analysis. A semi-quantitative scoring system was used to assess the degree of the morphological changes induced by SBM feeding in the distal intestinal epithelium. The first signs of enteritis appeared earlier in the salmon fed the 20SBM diet than in those fed the 10SBM diet. Thereafter, the condition increased steadily, displaying no signs of recovery. Furthermore, at the lower concentration, the process marking the onset of enteritis began more gradually than at the higher concentration and it displayed a tendency to stabilize after 13-20 days of continuous feeding. Electron microscopy indicated that the endocytosis process was hampered at day 3 of 20SBM and at 7 days of 10SBM. Furthermore, a strong reduction of microvilli was already evident after 7 days of 20SBM feeding, thus indicating a decreased uptake capacity of the distal enterocytes. In addition, transformation and migration of eosinophilic granulocytes was observed, which, in combination with the lysozyme C immunoreactivity supports their protective role during the inflammatory process in the distal gut of Atlantic salmon. It can be concluded that the severity of enteritis and its kinetics are concentration-dependent, showing no signs of recovery during feeding with diets containing SBM.

Immunological differences in intestine and rectum of Atlantic cod (Gadus morhua L.)

The defence system of the distal gut (hindgut and rectum) of Atlantic cod, (Gadus morhua L.) was studied using (immuno)histochemical, electron microscopical and real-time quantitative PCR techniques. The uptake and transport of macromolecules in the intestinal epithelium was also investigated. In this study we observed that cod has many and large goblet cells in its intestinal epithelium and that IgM(+) cells are present in the lamina propria and their number is considerably higher in the rectum than in the intestine. Myeloperoxidase staining revealed low numbers of granulocytes in and under the epithelium of the distal intestine, whereas high numbers were found clustered in the submucosa of the rectum. Electron microscopy not only confirmed these observations, but also revealed the presence of lymphoid cells and macrophages within the intestinal epithelium. Acid phosphatase staining demonstrated more positive macrophage-like cells in the rectum than in the distal intestine. Antigen uptake studies showed a diffused absorption of horse radish peroxidase (HRP) and LTB-GFP, whereas ferritin uptake could not be detected. Basal gene expression of cytokines (IL-1beta, IL-8 and IL-10) and immune relevant molecules (hepcidin and BPI/LPB) were compared in both the intestine and rectum and revealed approximately 2-9 times higher expression in the rectum, of which IL-1beta expression showed the most prominent difference. The present results clearly indicate that intestinal immunity is very prominent in the rectum of cod.

Soybean meal induces intestinal inflammation in common carp (Cyprinus carpio L.)

The development of soybean meal (SBM) induced enteritis in the hindgut of the omnivorous common carp (Cyprinus carpio L.). The developed condition was assessed when carp, continuously fed on animal protein, were transferred to a diet in which 20% of the protein was replaced by SBM. After week 1, most of the inflammation parameters were already present, but at week 3, a strong aggravation of the condition was observed which included a shortening of the mucosal folds, the disappearance of the supranuclear vacuoles, an increased number of goblet cells, a thickened lamina propria and sub-epithelial mucosa with increased numbers of basophilic granulocytes as well as a decreased uptake capacity of enterocytes (impaired endocytosis and microvilli). Contrary to previous observations made with respect to Atlantic salmon, common carp start to recover from the fourth to the fifth week after switching to SBM feeding. At this stage, the supranuclear vacuoles refill and most of the parameters revert to basal levels. During the enteritis process, a real-time quantitative PCR analysis was conducted to measure the expression of inflammatory and anti-inflammatory cytokine genes in the isolated intraepithelial lymphocytes (IEL). The pro-inflammatory interleukin 1 beta (IL-1 beta) and tumour necrosis factor alpha1 (TNF-alpha1) genes were up-regulated during the inflammation process while the anti-inflammatory interleukin 10 (IL-10) was down-regulated after an initial up-regulation at week 1. Transforming growth factor beta (TGF-beta) expression showed an up-regulation from week 3 onwards despite the high Ct value and the low primer efficiency shown. This study confirms the contribution of IEL (mainly T-like cells) and basophils in the enteritis process. In addition, the results show a clear involvement of up- and down-regulated cytokine genes in both the onset and recovery of the SBM-induced enteritis in the hindgut of carp.

Expression of the polymeric Immunoglobulin Receptor (pIgR) in mucosal tissues of common carp (Cyprinus carpio L.)

The mucosal immune system seems to be an important defence mechanism for fish but the binding of IgM in mucosal organs is poorly described in fish. In this study the gene encoding the polymeric Immunoglobulin Receptor (pIgR) in carp has been isolated and sequenced from a liver cDNA-library and aligned with other species. The pIgR of carp consists of 2 Ig domains, a transmembrane and an intracellular region, together 327 amino acids. In situ hybridisations with sense and anti-sense DIG-labelled pIgR RNA probes were performed on liver, gut and skin of common carp (Cyprinus carpio L.) and in these organs only anti-sense probes were found to hybridise. In liver the majority of hepatocytes was stained around the nucleus. In gut and skin, staining could be detected around the nucleus of the epithelial cells, but in gut also a subpopulation of lymphoid cells was stained in epithelium and lamina propria. The specific in situ hybridisation of the epithelia and hepatocytes coincides with the in situ binding of FITC-labelled carp IgM to the same cells. RT-PCR results indicate the expression of the pIgR gene in all lymphoid organs of carp, but not in muscle. Macrophages/neutrophils enriched by adherence or sorted B cells (MACS) did not show expression of the pIgR gene and are excluded as the pIgR expressing lymphoid cells in the intestine. The relevance of pIgR staining and gene expression in mucosal organs is discussed.

Differential contribution of neutrophilic granulocytes and macrophages to nitrosative stress in a host-parasite animal model

Tyrosine nitration is a hallmark for nitrosative stress caused by the release of reactive oxygen and nitrogen species by activated macrophages and neutrophilic granulocytes at sites of inflammation and infection. In the first part of the study, we used an informative host-parasite animal model to describe the differential contribution of macrophages and neutrophilic granulocytes to in vivo tissue nitration. To this purpose common carp (Cyprinus carpio) were infected with the extracellular blood parasite Trypanoplasma borreli (Kinetoplastida). After infection, serum nitrite levels significantly increased concurrently to the upregulation of inducible nitric oxide synthase (iNOS) gene expression. Tyrosine nitration, as measured by immunohistochemistry using an anti-nitrotyrosine antibody, dramatically increased in tissues from parasite-infected fish, demonstrating that elevated NO production during T. borreli infection coincides with nitrosative stress in immunologically active tissues. The combined use of an anti-nitrotyrosine antibody with a panel of monoclonal antibodies specific for several carp leukocytes, revealed that fish neutrophilic granulocytes strongly contribute to in vivo tissue nitration most likely through both, a peroxynitrite- and an MPO-mediated mechanism. Conversely, fish macrophages, by restricting the presence of radicals and enzymes to their intraphagosomal compartment, contribute to a much lesser extent to in vivo tissue nitration. In the second part of the study, we examined the effects of nitrosative stress on the parasite itself. Peroxynitrite, but not NO donor substances, exerted strong cytotoxicity on the parasite in vitro. In vivo, however, nitration of T. borreli was limited if not absent despite the presence of parasites in highly nitrated tissue areas. Further, we investigated parasite susceptibility to the human anti-trypanosome drug Melarsoprol (Arsobal), which directly interferes with the parasite-specific trypanothione anti-oxidant system. Arsobal treatment strongly decreased T. borreli viability both, in vitro and in vivo. All together, our data suggest an evolutionary conservation in modern bony fish of the function of neutrophilic granulocytes and macrophages in the nitration process and support the common carp as a suitable animal model for investigations on nitrosative stress in host-parasite interactions. The potential of T. borreli to serve as an alternative tool for pharmacological studies on human anti-trypanosome drugs is discussed.

Haemocyte reactions in WSSV immersion infected Penaeus monodon

White spot syndrome virus (WSSV) has been a major cause of shrimp mortality in aquaculture worldwide in the past decades. In this study, WSSV infection (by immersion) and behaviour recruitment of haemocytes is investigated in gills and midgut, using an antiserum against the viral protein VP28 and a monoclonal antibody recognising haemocytes (WSH8) in a double immunohistochemical staining and in addition transmission electron microscopy was applied. More WSH 8(+) haemocytes were detected at 48 and 72 h post-infection in the gills of infected shrimp compared to uninfected animals. Haemocytes in the gills and midgut were not associated with VP28-immunoreactivity. In the gills many other cells showed virus replication in their nuclei, while infected nuclei in the gut cells were rare. Nevertheless, the epithelial cells in the midgut showed a clear uptake of VP28 and accumulation in supranuclear vacuoles (SNV) at 8h post-infection. However, epithelial nuclei were never VP28-immunoreactive and electron microscopy study suggests degradation of viral-like particles in the SNV. In contrast to the gills, the midgut connective tissue shows a clear increase in degranulation of haemocytes, resulting in the appearance of WSH8-immunoreactive thread-like material at 48 and 72 h post-infection. These results indicate recruitment of haemocytes upon immersion infection in the gills and degranulation of haemocytes in less infected organs, like the midgut.

Improved uptake of plant-derived LTB-linked proteins in carp gut and induction of specific humoral immune responses upon infeed delivery

Oral vaccination of fish is an effortless and stress free immunisation method which can be used for almost any age. However, vaccination via the mucosal route does have disadvantages. For example, the vaccine may induce tolerance and has to be protected to escape digestion. Also the vaccine should be efficiently delivered to immune-competent cells in the gut or other lymphoid organs. In addition, it should be cost effective. Here we present a novel fish vaccination model using potato tubers as vaccine production and delivery system. The model vaccines discussed here include fusion proteins consisting of a gut adhesion molecule (LTB) and a viral peptide or green fluorescent protein (GFP) expressed in potato tubers. The adhesion molecule mediates binding to and uptake from the gut, whereas the viral peptide or GFP functions as model vaccine antigen provoking the induction of an immune response. We demonstrate that fusion to LTB facilitates an elevated uptake of the model vaccines in carp gut mucosa. The plant-derived fusion proteins also elicit a specific systemic humoral immune response upon oral application of crude tuber material incorporated into a standard dietary feed pellet. The data presented here show the promising potentials of the plant as a production system for oral vaccines in aquaculture and feed mediated immunisation of fish.

The effect of oral immuno-stimulation in juvenile carp (Cyprinus carpio L.)

The effect of a 2-week period of oral immuno-stimulation from the age of 2 or 6 weeks post-fertilisation (wpf; before and after reaching the ability to produce antibodies) onwards was investigated on various immune functions of the common carp, Cyprinus carpio. The immuno-stimulants Aeromonas salmonicida lipopolysaccharide, Yeast DNA (containing unmethylated CpG motifs) or high-M alginate (an extract of algae containing poly-mannuronic acid) were used. The effect of this treatment was studied on the kinetics of B cells in head kidney and peripheral blood leucocytes using flow cytometry, on the total plasma IgM level using ELISA, on cytokine and inducible nitric oxide synthase (iNOS) expression in the intestine, and acute phase protein expression in the liver, using real time quantitative PCR, and on exposure to Vibrio anguillarum. Oral administration of immuno-stimulants from 6 wpf resulted in decreased WCI12(+) (B) cell percentages in PBL (only after administration of LPS) and head kidney (all test groups), and a decreased total IgM level in plasma, suggesting that suppressive effects are strongly indicative of oral or juvenile tolerance. After administration from 2 wpf, the effects on WCI12(+) (B) cell percentages were less pronounced: the group fed with Yeast DNA showed higher percentages compared to the control group at 6 wpf, but lower percentages at 8 wpf. No changes were observed in the cytokine or iNOS expression levels in the intestine or acute phase protein expression in the liver. A challenge with V. anguillarum resulted in an initially higher cumulative mortality in the group fed with LPS, but lower mortality in the groups fed with Yeast DNA or high-M alginate compared to the control group, providing a provisional warning especially for the use of pathogen-derived immuno-stimulants, such as A. salmonicida LPS, in larval and juvenile fish.

Carp (Cyprinus carpio L.) innate immune factors are present before hatching

Expression of the innate immune factors, complement factor 3 (C3), alpha2-macroglobulin (alpha2M), serum amyloid A (SAA) and a complement factor 1 r/s--mannose binding lectin associated serine protease-like molecule (C1/MASP2), was determined with Real Time Quantitative-PCR in carp (Cyprinus carpio L.) ontogeny around hatching. Furthermore, the expression of C3 mRNA and the presence of C3 protein were studied in carp embryos and larvae using In Situ Hybridisation, Western Blotting and Immunohistochemistry. C3, alpha2M, SAA and C1/MASP2 mRNA were produced by embryos from 12 h post-fertilisation, which is relatively long before hatching (2 days post-fertilisation (dpf)), indicating either involvement of these factors in development itself or more probably a preparation of the immune system for the post-hatching period. In addition, maternal mRNA of the aforementioned innate immune factors and maternal C3- and immunoglobulin protein was present in unfertilised eggs. Furthermore, C3 mRNA production was situated in the yolk syncytial layer in embryos from 24 h post-fertilisation to 5 dpf, followed by the liver in larvae, providing a new sequence of C3 production in teleost development.

Ontogeny of the common carp (Cyprinus carpio L.) innate immune system

The ontogeny of the teleost innate immune system was studied in carp using cellular, histological and quantitative molecular techniques. Carp myeloid cells first appeared ventro-lateral of the aorta at 2 days post fertilization (the start of hatching), and subsequently around the sinuses of the vena cardinalis (or posterior blood islet), head kidney and trunk kidney. In addition, the hematopoietic tissue around the sinuses of the vena cardinalis transformed into that of the trunk kidney, which is the first description of the ontogeny of the trunk kidney hematopoietic tissue in teleosts. The mAb's used in this study reacted with carp myeloid surface molecules that are already transcribed and processed from the first appearance of myeloid cells, and thus serve a significant role in unraveling ontogenetic processes of teleost immunology. Finally, this study associated the first appearance of myeloid cells with an immune response on the molecular level: 2 days post fertilization embryos responded to LPS injection with upregulation of interleukin 1-beta, inducible nitric oxide synthase and serum amyloid A, and down-regulation of complement factor 3 and alpha2-macroglobulin, implying a functional embryonic innate defense system.

The ontogeny of mucosal immune cells in common carp (Cyprinus carpio L.)

The ontogeny of carp (Cyprinus carpio L.) immune cells was studied in mucosal organs (intestine, gills and skin) using the monoclonal antibodies WCL38 (intraepithelial lymphocytes), WCL15 (monocytes/macrophages) and WCI12 (B cells). In addition, recombination activating gene 1 expression was examined in the intestine with real time quantitative PCR and in situ hybridization to investigate extrathymic generation of lymphocytes. WCL38(+) intraepithelial lymphocytes (putative T cells) appeared in the intestine at 3 days post-fertilization (dpf), which is shortly after hatching but before feeding, implying an important function at early age. These lymphoid cells appear in the intestine before the observation of the first thymocytes at 3-4 dpf, and together with the expression of recombination activating gene 1 in the intestine, suggests that similar to mammals at least part of these cells are generated in the intestine. WCL15(+)monocytes/macrophages appeared in the lamina propria of the intestine at 7 dpf, but considerably later in the epithelium, while WCI12(+) (B) cells appeared in intestine and gills at 6-7 weeks. From these results it can be concluded that putative T cells occur much earlier than B cells, and that B cells appear much later in the mucosae than in other internal lymphoid organs (2 wpf).

Phylogeny and ontogeny of fish leucocytes

In contrast to higher vertebrates, most fish species hatch at the embryonic stage of life. Consequently, they have to defend against a variety of micro-organisms living in their aquatic environment. This paper is focussed on the development of leucocytes functioning within this early innate system and later on in the acquired immune system (B and T cells). Most of the data are derived from cyprinid fish (zebrafish, carp), which are excellent models to study early ontogeny. Attention is also paid to the phylogeny of leucocytes, with special attention to early chordates. It is clear that young fish use innate mechanisms during the first weeks/months of their development. In zebrafish, a variety of hematopoietic genes have been sequenced which allow a detailed picture of the development of the distinct leucocytes and their precursors. In cyprinids and sea bass, the thymus is the first lymphoid organ and T cells appear to be selected there much earlier than the first detection of T cell-dependent antibody responses. The first B cells are most probably generated in head kidney. Although T cells are selected earlier than B cells, T cell independent responses occur earlier than the T cell-dependent responses. The very early (pre-thymic) appearance of T-like cells in gut of sea bass and carp suggests an extra-thymic origin of these cells. However, B cells populate the GALT much later than spleen or kidney, indicating a rather late appearance of mucosal immunity. The first plasma cells are found long after the intake of food in cyprinids, but in many marine fish they appear around the first food intake. In general, acquired immunity is not correlated to food intake.

Development and function of the thymus in teleosts

The thymus plays a pivotal role in the development of the adaptive immune system, an important factor that separates higher vertebrates from the rest of the animal phyla. The development of functional T-cells from thymocytes is a crucial step in the development of a functional vertebrate immune system and whilst recent advances in molecular and developmental biology have advanced our understanding of T-cell development, they have also provided potential model species across the vertebrate phyla including the zebrafish (Danio rerio). However, this species is one of more than 20,000 species of fish that could assist in elucidating the development of the vertebrate thymus and, consequently, the evolution of the vertebrate immune response. In this paper we review the knowledge of the teleost thymus through the organogenesis and development studies in teleosts together with advances in molecular and functional approaches. Where necessary we will combine this knowledge with that obtained in higher vertebrates.

Rag expression identifies B and T cell lymphopoietic tissues during the development of common carp (Cyprinus carpio)

The generation of lymphoid cells during carp development was studied by analyzing expression of the recombination activating genes (rag) using in situ hybridization and real time quantitative PCR. These data were combined with immunohistochemistry using the mAb's WCL9 (cortical thymocytes) and WCI12 (B cells). Carp rag-1 and rag-2 showed 90 and 89% amino acid identity, respectively, to the corresponding zebrafish sequences. Rag-1 was first expressed in the thymus at 4 days post-fertilization (dpf), while both rag-1+/WCL9+ and rag-1-/WCL9- areas were distinguished from 1 week post-fertilization (wpf), suggesting early cortex/medulla differentiation. From 6 dpf, rag-1+ cells were also present cranio-lateral of the head kidney. From 1 wpf, rag-1/rag-2 was expressed in kidney (together with immunoglobulin heavy chain expression) but not in spleen, while WCI12+ cells appeared 1 week later in both organs, suggesting B cell recombination in kidney but not in spleen. Rag-1 expression exceeded rag-2 levels in thymus and in head- and trunk-kidney of juveniles, but this ratio was reversed in head- and trunk-kidney from approximately 16 wpf onwards. Rag-1/rag-2 expression was detected in thymi of animals over 1-year-old, but in kidney only at low levels, indicating life-long new formation of putative T cells but severely reduced formation of B cells in older fish.

Development of a cost-effective oral vaccination method against viral disease in fish

Different vaccination methods have been applied to protect fish against the detrimental effects of various pathogens. Several studies have shown the potentials of oral vaccination. In theory oral vaccination is an effortless and stress-free method which can be applied at almost any age. In general, however, the vaccine has to be protected to avoid digestion, which results in high costs for application in aquaculture. In this paper we introduce a cost-effective oral vaccination strategy for viral diseases of fish. The vaccines discussed here include fusion proteins consisting of a gut adhesion molecule and a viral peptide expressed in plants. The adhesion molecule mediates binding to and uptake from the gut, whereas the viral peptide functions as vaccine antigen mediating the induction of a humoral immune response. The first pilot studies using a fusion of the gut adhesion molecule and well-characterised heterologous linear B- and T-cell viral epitopes, produced in potato tubers, showed a promising binding and subsequent uptake in the end gut of carp. The results further indicated that a specific humoral immune response was evoked.

Potential involvement of rainbow trout thrombocytes in immune functions: a study using a panel of monoclonal antibodies and RT-PCR

The functional relationship between fish and mammalian thrombocytes is relatively unknown. In this study, a panel of monoclonal antibodies (mAbs) was used to investigate the functional properties of rainbow trout thrombocytes. The mAbs recognize cell-surface molecules on thrombocytes with molecular weights ranging from 17 to 160 kDa. Flow cytometric and immuno-electron microscopic analyses demonstrate that these molecules are expressed at different levels and that surface expression increased upon activation with bovine collagen. Two of these cell-surface molecules (17 and 21 kDa) were directly involved in collagen-induced aggregation of thrombocytes since aggregation was blocked upon pre-treatment with mAbs that recognize the two surface markers. Interestingly, the percentage of thrombocytes in blood increased after stimulation using different antigens. The transcriptional profile of trout thrombocytes was then examined after immuno-magnetic enrichment using the described mAbs to assess potential roles of trout thrombocytes in immune functions. Trout thrombocytes express components of the MHC class Ia pathway, IL1beta, TNFalpha, TGFbeta, the interleukin receptor common gamma chain as well as CXC and CC chemokines. MHC class IIB and TNFalpha were expressed at low levels in resting thrombocytes. No evidence was found for the expression of TCRalphabeta, Ig heavy chain, CD8alpha or CK1 mRNA. Taken together, these results suggest that rainbow trout thrombocytes express molecules involved in activation, aggregation and genes encoding proteins, that are involved in antigen presentation and immune regulation.

Multiple acute temperature stress affects leucocyte populations and antibody responses in common carp, Cyprinus carpio L

Stress is a potential factor causing increased susceptibility of fish to pathogens. In this study, stress-induced immunological changes that may contribute to a decreased immune status were investigated. A 3 h drop in ambient water temperature of 9 degrees C was used as a relative mild and acute stress model for carp. Effects of this stressor on the dynamics of leucocyte populations were determined with specific monoclonal antibodies. The relative number of circulating B-lymphocytes in the total leucocyte population decreased significantly within 4 h after the onset of single or multiple cold shocks. This decrease was reversible, as B-lymphocyte numbers were restored within 24 h. Most probably, a redistribution of B-lymphocytes contributed to this phenomenon. In head kidney, an increase was measured in the relative number of B-lymphocytes. Granulocyte numbers showed opposite reactions: the percentage of granulocytes in the total leucocyte population nearly doubled in circulation and decreased significantly in the head kidney. This demonstrates that in vivo, a mild stressor differentially alters the distribution of leucocytes. In stressed carp, the percentage of apoptotic lymphocytes in blood is significantly higher compared with the unstressed animals. B-lymphocytes as well as Ig- lymphoid cells contributed to this increased apoptosis. Labelling of blood lymphocytes with a polyclonal antiserum against the glucocorticoid receptor also showed, besides B-lymphocytes, part of the Ig- lymphoid cell population to be glucocorticoid receptor positive. As the distribution of B-lymphocytes was substantially affected, the effect of temperature stress on T-lymphocyte-independent (trinitrophenyl-lipopolysaccharide) and T-lymphocyte-dependent (dinitrophenyl-keyhole limpet hemocyanin) humoral antibody responses was determined. Kinetics of the primary antibody response to the T-lymphocyte-independent antigen showed lower antibody titres in stressed carp during the onset of the immune response, implying a slower development of the antibody response against the T-lymphocyte-independent antigen.

Increased efficacy of immersion vaccination in fish with hyperosmotic pretreatment

Immersion vaccination is common practice in aquaculture, because of its convenience for mass vaccination with sufficient protection. However, the mechanisms of antigen uptake and presentation, resulting in a protective immune response and the role of the innate immune system therein are largely unknown. The impact of immersion vaccination on fish physiology and on the ensuing innate and specific immune response was characterized with fluorescently labeled particulate and soluble model antigens. Vaccination of common carp by direct immersion (DI) or hyperosmotic immersion (HI; direct immersion, preceded by a brief immersion in a hypertonic solution) greatly enhanced the uptake of soluble, but not particulate antigen through temporary disruption of the integrity of the epithelia of gills and skin. Damage induced is mild and does not impose additional stress over the handling associated with immersion vaccination. Especially HI briefly but strongly activates the innate immune system. We conclude that HI more effectively increased the uptake of vaccine and enhanced the efficacy by which vaccine components are processed and presented by the innate immune system, dually enhancing the mucosal immune response. Understanding the mechanisms involved in uptake and processing of vaccine in the early phase of the immune response will greatly benefit the design of immersion vaccination.

The roles of haemocytes and the lymphoid organ in the clearance of injected Vibrio bacteria in Penaeus monodon shrimp

In order to study the reaction of Penaeus monodon haemocytes, live Vibrio anguillarum bacteria were injected and the shrimp were periodically sampled. Immuno-double staining analysis with specific antisera against the haemocyte granules and bacteria showed that large numbers of haemocytes encapsulated the bacteria at the site of injection. A rapid decrease of live circulating bacteria was detected in the haemolymph. Bacterial clearance in the haemolymph was induced by humoral factors, as observed by agglutinated bacteria, and followed by uptake in different places in the body. Bacteria mainly accumulated in the lymphoid organ (LO), where they, or their degradation products, could be detected for at least 7 days after injection. The LO consists of folded tubules with a central haemal lumen and a wall, layered with cells. The haemolymph, including the antigens, seemed to migrate from the central tubular lumen through the wall, where the bacteria are arrested and their degradation is started. Electron microscopy of the LO revealed the presence of many phagocytic cells that morphologically resemble small-granular haemocytes. It is proposed that haemocytes settle in the tubule walls before they phagocytose. Immunostaining suggests that many of the haemocytes degranulate in the LO, producing a layer of fibrous material in the outer tubule wall. These findings might contribute to the reduced haemocyte concentration in the haemolymph of diseased animals or following injection of foreign material. It is proposed that the LO is a filter for virtually all foreign material encountered in the haemolymph. Observations from the present study are similar to clearance mechanisms in the hepatic haemolymph vessel in most decapod crustaceans that do not possess a LO. The experimental shrimp appeared to contain many LO spheroids, where bacterial antigens were finally observed as well. It is proposed that the spheroids have a degradation function for both bacterial and viral material, and that their presence is primarily related to the history of the infectious burden of the shrimp.

Preliminary study on haemocyte response to white spot syndrome virus infection in black tiger shrimp Penaeus monodon

White spot syndrome virus (WSSV) has been a major cause of shrimp mortality in aquaculture in the past decade. In contrast to extensive studies on the morphology and genome structure of the virus, little work has been done on the defence reaction of the host after WSSV infection. Therefore, we examined the haemocyte response to experimental WSSV infection in the black tiger shrimp Penaeus monodon. Haemolymph sampling and histology showed a significant decline in free, circulating haemocytes after WSSV infection. A combination of in situ hybridisation with a specific DNA probe for WSSV and immuno-histochemistry with a specific antibody against haemocyte granules in tissue sections indicated that haemocytes left the circulation and migrated to tissues where many virus-infected cells were present. However, no subsequent haemocyte response to the virus-infected cells was detected. The number of granular cells decreased in the haematopoietic tissue of infected shrimp. In addition, a fibrous-like immuno-reactive layer appears in the outer stromal matrix of tubule walls in the lymphoid organ of infected shrimp. The role of haemocytes in shrimp defence after viral infection is discussed.

The role of the haematopoietic tissue in haemocyte production and maturation in the black tiger shrimp (Penaeus monodon)

The haematopoietic tissue (HPT) of the black tiger shrimp (Penaeus monodon) is located in different areas in the cephalothorax, mainly at the dorsal side of the stomach and in the onset of the maxillipeds and, to a lesser extent, towards the antennal gland. In young and in experimentally stimulated animals, the HPT is expanded in relatively larger and more numerous lobules throughout the cephalothorax. Four cell types could be identified in the HPT by electron microscopy. The type 1 cells are the presumed precursor cells that give rise to a large- and a small-granular young haemocyte, denominated as the type 2 and type 3 cells, respectively. A gradient of maturation from the type 1 towards the type 2 or 3 cells could frequently be observed. The presumed precursor cells are located towards the exterior of the lobules and maturing young haemocytes towards the inner part, where they can be released into the haemal lacunae. The type 4 cells show typical features of interstitial cells. Different stimulation experiments were carried out and various techniques were used to study the HPT in relation to the (circulating) haemocytes. The majority of the cells in the HPT are able to proliferate and proliferation can be increased significantly after the injection of saline and, to a much higher extent, after LPS injection. The circulating haemocytes of crustaceans are generally divided into hyaline (H), semigranular (SG) or granular (G) cells, of which large- and small-granular variants of each of these were suggested in the present study. Even after stimulation in this study, the circulating haemocytes scarcely divide. The high variations that were found in the total haemocyte count in the stimulation experiments were not accompanied by significant differences in differential haemocyte count and, therefore, appeared to be a less useful indicator of stress or health in P. monodon. Light and electron microscopical observations support the regulation of the populations of the different haemocyte types in the circulation by (stored) haemocytes from the connective tissue. In conclusion, according to morphological and immuno-chemical criteria, it is proposed in the present study to divide the haemocytes into a large-and a small-granular developmental series. After extensive morphological observations, it is suggested that the hyaline cells are the young and immature haemocytes of both the large- and small-granular cell line that are produced in the HPT, and can be released into the haemolymph. Indications were found that the granular cells, of at least the large-granular cell line, mature and accumulate in the connective tissue and are easily released into the haemolymph. Combining the results of the present study with literature, this proposed model for haemocyte proliferation, maturation and reaction will be discussed.