Cellular heterogeneity in red and melanized focal muscle changes in farmed Atlantic salmon (Salmo salar) visualized by spatial transcriptomics

Spatial transcriptomics is a technique that provides insight into gene expression profiles in tissue sections while retaining structural information. We have employed this method to study the pathological conditions related to red and melanized focal changes in farmed Atlantic salmon (Salmo salar). Our findings support a model where similar molecular mechanisms are involved in both red and melanized filet discolorations and genes associated with several relevant pathways show distinct expression patterns in both sample types. Interestingly, there appears to be significant cellular heterogeneity in the foci investigated when looking at gene expression patterns. Some of the genes that show differential spatial expression are involved in cellular processes such as hypoxia and immune responses, providing new insight into the nature of muscle melanization in Atlantic salmon.

The ontogeny of lymphoid organs and IgM+ B-cells in ballan wrasse (Labrus bergylta) reveals a potential site for extrarenal B-cell lymphopoiesis: The pancreas

Vaccination of farmed fish is the most effective prophylactic measure against contagious diseases but requires specific knowledge on when the adaptive immune system is fully developed. The present work describes kidney and spleen morphogenesis as well as B-cell development in the ballan wrasse (Labrus bergylta). The kidney was present at hatching (0 days pot hatching, dph) but was not lymphoid before larvae was 50-60 dph (stage 5), containing abundant Igμ+ cells. The spleen anlage was first observed in larvae at 20-30 dph and was later populated with B-cells. Unexpectedly, we found strong RAG1 signal together with abundant Igμ+ and IgM + cells in the exocrine pancreas of larvae from when the kidney was lymphoid and onwards, suggesting that B-cell lymphopoiesis occurs not only in the head kidney (HK) but also in pancreatic tissue. In this agastric fish, the pancreas is diffused along the intestine and the early presence of IgM+ B-cells in pancreatic tissue might have a role in maintain immune homeostasis in the peritoneal cavity, making a substantial contribution to early protection. IgM-secreting cells in HK indicate the presence of systemic IgM at stage 5, before the first IgM+ cells were identified in mucosal sites. This work together with our previous study on T-cell development in this species indicates that although T- and B-cells start to develop around the same time, B-cells migrate to mucosal tissues ahead of T-cells. This early migration likely involves the production of natural antibodies, contributing significantly to early protection. Moreover, a diet composed of barnacle nauplii did not result in an earlier onset of B-cell lymphopoiesis, as seen in the previous study analysing T-cell development. Nevertheless, components for adaptive immunity indicating putative immunocompetence is likely achieved in early juveniles (>100 dph). Additionally, maternal transfer of IgM to the offspring is also described. These findings provide important insights into the development of the immune system in ballan wrasse and lay the foundation for optimizing prophylactic strategies in the future. Furthermore, this work adds valuable information to broaden the knowledge on the immune system in lower vertebrates.

Identification of a pharyngeal mucosal lymphoid organ in zebrafish and other teleosts: Tonsils in fish?

The constant exposure of the fish branchial cavity to aquatic pathogens causes local mucosal immune responses to be extremely important for their survival. Here, we used a marker for T lymphocytes/natural killer (NK) cells (ZAP70) and advanced imaging techniques to investigate the lymphoid architecture of the zebrafish branchial cavity. We identified a sub-pharyngeal lymphoid organ, which we tentatively named "Nemausean lymphoid organ" (NELO). NELO is enriched in T/NK cells, plasma/B cells, and antigen-presenting cells embedded in a network of reticulated epithelial cells. The presence of activated T cells and lymphocyte proliferation, but not V(D)J recombination or hematopoiesis, suggests that NELO is a secondary lymphoid organ. In response to infection, NELO displays structural changes including the formation of T/NK cell clusters. NELO and gill lymphoid tissues form a cohesive unit within a large mucosal lymphoid network. Collectively, we reveal an unreported mucosal lymphoid organ reminiscent of mammalian tonsils that evolved in multiple teleost fish families.

High-Resolution, 3D Imaging of the Zebrafish Gill-Associated Lymphoid Tissue (GIALT) Reveals a Novel Lymphoid Structure, the Amphibranchial Lymphoid Tissue

The zebrafish is extensively used as an animal model for human and fish diseases. However, our understanding of the structural organization of its immune system remains incomplete, especially the mucosa-associated lymphoid tissues (MALTs). Teleost MALTs are commonly perceived as diffuse and scattered populations of immune cells throughout the mucosa. Yet, structured MALTs have been recently discovered in Atlantic salmon (Salmo salar L.), including the interbranchial lymphoid tissue (ILT) in the gills. The existence of the ILT was only recently identified in zebrafish and other fish species, highlighting the need for in-depth characterizations of the gill-associated lymphoid tissue (GIALT) in teleosts. Here, using 3-D high-resolution microscopy, we analyze the GIALT of adult zebrafish with an immuno-histology approach that reveals the organization of lymphoid tissues via the labeling of T/NK cells with an antibody directed to a highly conserved epitope on the kinase ZAP70. We show that the GIALT in zebrafish is distributed over at least five distinct sub-regions, an organization found in all pairs of gill arches. The GIALT is diffuse in the pharyngeal part of the gill arch, the interbranchial septum and the filaments/lamellae, and structured in two sub-regions: the ILT, and a newly discovered lymphoid structure located along each side of the gill arch, which we named the Amphibranchial Lymphoid Tissue (ALT). Based on RAG2 expression, neither the ILT nor the ALT constitute additional thymi. The ALT shares several features with the ILT such as presence of abundant lymphoid cells and myeloid cells embedded in a network of reticulated epithelial cells. Further, the ILT and the ALT are also a site for T/NK cell proliferation. Both ILT and ALT show structural changes after infection with Spring Viraemia of Carp Virus (SVCV). Together, these data suggest that ALT and ILT play an active role in immune responses. Comparative studies show that whereas the ILT seems absent in most neoteleosts ("Percomorphs"), the ALT is widely present in cyprinids, salmonids and neoteleosts, suggesting that it constitutes a conserved tissue involved in the protection of teleosts via the gills.

Immune protection is dependent on the gut microbiome in a lethal mouse gammaherpesviral infection

Immunopathogenesis in systemic viral infections can induce a septic state with leaky capillary syndrome, disseminated coagulopathy, and high mortality with limited treatment options. Murine gammaherpesvirus-68 (MHV-68) intraperitoneal infection is a gammaherpesvirus model for producing severe vasculitis, colitis and lethal hemorrhagic pneumonia in interferon gamma receptor-deficient (IFNγR-/-) mice. In prior work, treatment with myxomavirus-derived Serp-1 or a derivative peptide S-7 (G305TTASSDTAITLIPR319) induced immune protection, reduced disease severity and improved survival after MHV-68 infection. Here, we investigate the gut bacterial microbiome in MHV-68 infection. Antibiotic suppression markedly accelerated MHV-68 pathology causing pulmonary consolidation and hemorrhage, increased mortality and specific modification of gut microbiota. Serp-1 and S-7 reduced pulmonary pathology and detectable MHV-68 with increased CD3 and CD8 cells. Treatment efficacy was lost after antibiotic treatments with associated specific changes in the gut bacterial microbiota. In summary, transkingdom host-virus-microbiome interactions in gammaherpesvirus infection influences gammaherpesviral infection severity and reduces immune modulating therapeutic efficacy.

A teleost structural analogue to the avian bursa of Fabricius

The bursa of Fabricius is a primary and secondary lymphoid organ considered exclusively present in birds, and studies of this structure have been vital to our current understanding of the adaptive immune system of vertebrates. In this study, we reveal substantial lymphoepithelial tissue in a previously undescribed bursa in Atlantic salmon (Salmo salar), situated caudal to the urogenital papilla of the cloaca and thus analogous to the anatomical placement of the bursa of Fabricius. We investigated three groups of Atlantic salmon at different maturational stages and characterized the structure by applying dissection, radiology, scanning electron microscopy and histological techniques, including immunohistochemistry and in situ hybridization. We found that the epithelial anlage of the salmon cloacal bursa developed into substantial lymphoepithelial tissue and subsequently regressed following sexual maturation. Such a dynamic development is also a key characteristic of the avian bursa. The presence of intraepithelial lymphocytes was concomitant with expression of the leukocyte-attracting chemokine CCL19, indicative of lymphoid organ functions. We did not observe recombination or gene conversion in salmon bursal lymphocytes at any developmental stage, indicating the absence of primary lymphoid organ functions in contrast to the bursa of Fabricius. However, the possibility of the bursa to trap both enteric and environmental antigens, combined with the presence of several antigen-presenting cells residing within the lymphoepithelium, suggest the structure has secondary lymphoid organ functions. We present the discovery of a lymphoid organ in Atlantic salmon with striking topographical similarities to that of the bursa of Fabricius in birds. In addition, the age-dependent dynamics of its lymphoepithelium suggest functions related to the maturation processes of lymphocytes.

Ectopic epithelial cell clusters in salmonid intestine are associated with inflammation

An epizootic incidence of intestinal adenocarcinomas was reported in brood fish of Atlantic salmon (Salmo salar L.) in 2009. The condition was associated with a specific diet inducing enteritis and morphological changes. Here, two field trials of fish up to slaughter size were initiated. In Trial 1, two different feed recipes were used. Feed I was predominantly based on marine ingredients, whereas plant ingredients were limited to soy protein concentrate and wheat. Feed II was lower in fishmeal and without soya protein, which was substituted with plant proteins from other sources. In Trial 2, a commercial feed (Feed III) was included. No macroscopic tumours were observed in 300 fish (Trial 1). At the end of both trials, samples from five different segments of the gastrointestinal tract of a total of 39 fish were investigated with morphological methods. Here, we show the presence of ectopic proliferating epithelial cells only occurring in inflamed intestine and predominantly in the second segment of the mid-intestine. Presence of ectopic epithelial cells in submucosal inflammatory foci may indicate early stages in tumorigenesis, but other possibilities such as proliferative enteric disorders cannot be excluded. Together with inflammation, carcinogenesis should be a focus of investigation in future feed trials.

Time after seawater transfer influences immune cell abundance and responses to SAV3 infection in Atlantic salmon

Pancreas disease (PD) caused by salmonid alphavirus (SAV) severely affects salmonid aquaculture during the seawater phase. To characterize immune cells in target tissues for SAV infection, heart, pancreas and pyloric caeca were analysed from two groups of fish adapted to seawater for 2 and 9 weeks. The sections were scored for the relative abundance of cells expressing MHC class II, IgM, CD3, CD8 or neutrophil/granulocyte markers using immuno-histochemical techniques. In general, necrosis of tissue was more severe in fish infected at 2 weeks post-seawater transfer (wpt) compared with those infected at 9 wpt. At 9 wpt, there were higher numbers of MHC II+ cells in heart, pancreas and pyloric caeca, IgM+ cells in heart and pancreas, and CD3+ cells in pancreas compared to those infected at 2 wpt. The majority of the immune cells infiltrating PD-affected tissues were MHC II+ and CD3+ cells suggesting that antigen-presenting cells and T lymphocytes are the main types of immune cells responding to SAV infection. All the investigated cell types were also observed in pyloric caeca of infected fish, suggesting that this tissue may play a role in the immune response to SAV.

Contrasting expression of immune genes in scaled and scaleless skin of Atlantic salmon infected with young stages of Lepeophtheirus salmonis

Atlantic salmon skin tissues with and without scales were taken from two preferred sites of salmon louse (Lepeophtheirus salmonis) attachment, behind the dorsal fin (scaled) and from the top of the head (scaleless), respectively. Tissues were profiled by qPCR of 32 genes to study responses to copepodids, 4 days post infection (dpi), and during the moult of copepodids to the chalimus stage, at 8 dpi. Basal/constitutive differences were found for many immune-related genes between the two skin sites; e.g., mannose binding protein C was over 100 fold higher expressed in the scaled skin from the back in comparison to the skin without scales from the head. With lice-infection, at 4 dpi most genes in both tissues showed lower values than in the non-infected control. By 8 dpi, the majority of responses increased towards the control levels, including cytokines of Th1, Th17 and Th2 pathways. Immunohistochemistry of three immune factors revealed an even distribution of MHC class II positive cells throughout epidermis, including the top layer of keratinocytes, marked compartmentalization of Mx⁺ and CD8α⁺ cells close to stratum basale, and an increase in numbers of CD8α⁺ cells in response to infection. In conclusion, suppression of immune genes during the copepodid stage likely sets off a beneficial situation for the parasite. At the moult to chalimus stage 8 dpi, only few genes surpassed the non-infected control levels, including CD8α. The gene expression pattern was reflected in the increased number of CD8α expressing cells, thus revealing a relatively minor activation of skin T-cell defenses in Atlantic salmon in response to L. salmonis infection.

Granulomatous Uveitis Associated with Vaccination in the Atlantic Salmon

This study addressed histologic and immunopathologic changes in ocular tissues and investigated the distribution of major histocompatibility class II (MHC class II)-positive cells in Atlantic salmon ( Salmo salar) suffering from severe postvaccination disease. Twenty-nine fish with generalized inflammation, probably a result of vaccination, were investigated. One individual that had escaped vaccination was included in the study. Material was investigated by cultivation methods for fungi and bacteria. Histology using conventional staining procedures and immunohistochemistry with antisera against MHC class II β chain were performed. No growth was observed from the cultivation investigations. Histology revealed occlusion of the lumen in the larger choroid vessels and in the choriocapillaris, inflammatory infiltrations and loss of structure in the choroid rete, and, in some cases, aggregations of multinucleated giant cells (MGC) and Splendore-Hoeppli material. Immunohistochemistry demonstrated massive MHC class II+ cellular infiltrations in the uveal tract. Such infiltrations were also seen in the ventral ciliary cleft, a condition that is associated with glaucoma. Immunoreactive cells included dendritelike cells, epithelioid cells, and MGCs. The endothelia of smaller vessels were frequently MHC class II+, and immunoreactive infiltrations were seen in the optic nerve in several individuals. No pathologic changes were detected in the unvaccinated individual. In conclusion, generalized inflammatory reactions in fish may lead to severe ocular inflammation, occlusion of uveal vessels, and perivascular changes with MHC class II+ upregulation in cells in the uveal tract and optic nerve.

Coronary changes in the Atlantic salmon Salmo salar L: characterization and impact of dietary fatty acid compositions

Consumption of fatty acids from fishes is widely regarded as beneficial for preventing cardiovascular disorders. Nevertheless, salmonids themselves are victims of vascular diseases. As the pathogenesis and nature of these changes are elusive, they are here addressed using novel morphological and transcriptional approaches. Coronary arteries of wild Atlantic salmon Salmo salar L., (n = 12) were investigated using histological and immunohistochemical techniques, and RT-qPCR was employed to investigate expression of stretch-induced genes. In an experimental trial, fish were fed diets with different fatty acids composition, and histological features of the coronary arteries (n = 36) were investigated. In addition, the heart fatty acid profile (n = 60) was analysed. There were no differences in morphological or immunological features between wild fish and groups of experimental fish. Arteriosclerotic lesions consisted of smooth muscle cells in dissimilar differential stages embedded in considerable amounts of extracellular matrix in a similar fashion to what is seen in early stages of human atherosclerosis. No fat accumulations were observed, and very few inflammatory cells were present. In affected arteries, there was an induction of stretch-related genes, pointing to a stress-related response. We suggest that salmon may have a natural resistance to developing atherosclerosis, which corresponds well with their high investment in lipid metabolism.

Vaccination and triploidy increase relative heart weight in farmed Atlantic salmon, Salmo salar L

Heart morphology is particularly plastic in teleosts and differs between farmed and wild Atlantic salmon. However, little is known about how different culture practices and sex affect heart morphology. This study investigated how vaccination, triploidy and sex affected heart size and heart morphology (ventricle shape, angle of the bulbus arteriosus) in farmed Atlantic salmon for 18 months following vaccination (from c. 50-3000 g body weight). In addition, hearts were examined histologically after 7 months in sea water. All fish sampled were sexually immature. Vaccinated fish had significantly heavier hearts relative to body weight and a more triangular ventricle than unvaccinated fish, suggesting a greater cardiac workload. Irrespective of time, triploids had significantly heavier hearts relative to body weight, a more acute angle of the bulbus arteriosus and less fat deposition in the epicardium than diploids. The ventricle was also more triangular in triploids than diploids at seawater transfer. Sex had transient effects on the angle of the bulbus arteriosus, but no effect on relative heart weight or ventricle shape. From a morphological perspective, the results indicate that vaccination and triploidy increase cardiac workload in farmed Atlantic salmon.

Vaccination with outer membrane vesicles from Francisella noatunensis reduces development of francisellosis in a zebrafish model

Infection of fish with the facultative intracellular bacterium Francisella noatunensis remains an unresolved problem for aquaculture industry worldwide as it is difficult to vaccinate against without using live attenuated vaccines. Outer membrane vesicles (OMVs) are biological structures shed by Gram-negative bacteria in response to various environmental stimuli. OMVs have successfully been used to vaccinate against both intracellular and extracellular pathogens, due to an ability to stimulate innate, cell-mediated and humoral immune responses. We show by using atomic force and electron microscopy that the fish pathogenic bacterium F. noatunensis subspecies noatunensis (F.n.n.) shed OMVs both in vitro into culture medium and in vivo in a zebrafish infection model. The main protein constituents of the OMV are IglC, PdpD and PdpA, all known Francisella virulence factors, in addition to the outer membrane protein FopA and the chaperonin GroEL, as analyzed by mass spectrometry. The vesicles, when used as a vaccine, reduced proliferation of the bacterium and protected zebrafish when subsequently challenged with a high dose of F.n.n. without causing adverse effects for the host. Also granulomatous responses were reduced in F.n.n.-challenged zebrafish after OMV vaccination. Taken together, the data support the possible use of OMVs as vaccines against francisellosis in fish.

Transcriptional characterization of the T cell population within the salmonid interbranchial lymphoid tissue

Previously, our group has shown that the interbranchial lymphoid tissue (ILT) is a distinct structure largely consisting of T cells embedded in a meshwork of epithelial cells, with no direct resemblance to previously described lymphoid tissues. In this study, we aim to focus on the T cell population and the possibility of the ILT being a thymus analog. By characterizing structural responsiveness to Ag challenge, the presence of recombination activating genes, and different T cell-related transcripts, we attempt to further approach the immunological function of the ILT in salmonid gills. In addition to eight healthy individuals, a group of eight infectious salmon anemia virus-challenged fish were included to observe T cell responses related to infection. The results showed reduced size of ILT in the infected group, no expression of RAG-1 and -2, and a high degree of T cell diversity within the ILT. Taking into account that the ILT can be regarded as a strategically located T cell reservoir and possibly an evolutionary forerunner of mammalian MALTs right at the border to the external environment, the alteration in transcription observed may likely represent a shift in the T cell population to optimize local gill defense mechanisms.

The effect of vaccination, ploidy and smolt production regime on pathological melanin depositions in muscle tissue of Atlantic salmon, Salmo salar L

The presence of melanin in muscle fillets of farmed salmon represents a considerable quality problem for the salmon industry with major economic concerns. In this study, we have examined the presence of abnormal pigmentation in vaccinated versus unvaccinated Atlantic salmon, Salmo salar L., and evaluated possible differences between diploid and triploid fish. Furthermore, the impact of the smolt production regime at ambient (4.5 °C) versus elevated temperature (16 °C) was investigated. Pigmented muscle spots were analysed for the expression of genes involved in melanization (tyrosinase gene family) and immune-related response in addition to morphological investigations. The proportion of fish with intramuscular melanin deposits was not significantly different between vaccinated and unvaccinated fish, regardless of ploidy. However, an interaction between vaccination and smolt regime was shown, where smoltification at elevated temperature after vaccination increased the number of affected individuals compared with vaccination followed by simulated natural smoltification. Furthermore, there were overall more fish with melanin spots amongst the triploids compared with their diploid counterparts. Transcription of the tyrosinase gene family confirmed an onsite melanogenesis in all pigment spots. The histological examination and the expression of the immune-related genes revealed a chronic polyphasic myopathy that was not affected by vaccination, ploidy or smolt production regime.

Pathological pigmentation in cardiac tissues of Atlantic salmon (Salmo salar L.) with cardiomyopathy syndrome

It is widely accepted that melanin formation may play an immunologic role in invertebrates and ectothermic vertebrates. In farmed Atlantic salmon, cardiomyopathy syndrome (CMS) is a common viral disease associated with severe cardiac inflammation that may be accompanied by heavy melanisation of the heart. By the use of histology, laser capture microdissection and transcription analysis of tyrosinase genes, we here show that this melanisation is linked to de novo melanogenesis by melanomacrophages, suggesting an active part in the inflammatory reaction. No general systemic activation of the extracutaneous pigmentary system in response to viral infections with affinity to the heart was observed.

Innate and adaptive immune responses of Arctic charr (Salvelinus alpinus, L.) during infection with Aeromonas salmonicida subsp. achromogenes and the effect of the AsaP1 toxin

Aeromonas salmonicida subsp. achromogenes, the causative agent of atypical furunculosis in many fish species, secretes the toxic metalloendopeptidase AsaP1. This study aimed to analyze innate and adaptive immune parameters induced in Arctic charr (Salvelinus alpinus, L.) infected with wild type (wt) A. salmonicida subsp. achromogenes and its isogenic asaP1 deletion mutant (AsaP1-deficient). Head-kidney, liver and spleen were obtained from i.p. infected charr (wt, AsaP1-deficient), during a time schedule of 7 d post infection. Reverse transcription quantitative real-time PCR (RT-qPCR) was applied to study the expression of immune parameters: pro-inflammatory cytokines IL-1β and TNF-α; anti-inflammatory cytokine IL-10; chemokines CXCL-8 (IL-8) and CC-chemokine; the cytokines IFN-γ and IL-4/13A as tracers for Th1 and Th2 immune responses, respectively; and the cell markers CD8α and CD83. In addition, lymphoid organs were histopathologically examined at days 3 and 7 post infection, including B (IgM) and T (CD3ε) cell staining. The detected immune responses were initially driven by innate mechanisms represented by the up-regulation of pro-inflammatory cytokines and chemokines and later on by adaptive Th2 related responses cumulating in B-cell recruitment as shown by regulation of immune parameters in spleen and head-kidney, with significant differences between mutant and wt infected fish. Histological sections revealed IgM-positive cells around ellipsoid arterioles in spleen, while CD3ε positive cells were found in clusters scattered all over the section. However, histopathological differences were only detected between infected and non-infected fish, but not between AsaP1-deficient mutant and wt infected fish. This work represents the first study on innate and adaptive immune responses of Arctic charr induced by a bacterial infection.

Transcription of the tyrosinase gene family in an Atlantic salmon leukocyte cell line (SHK-1) is influenced by temperature, but not by virus infection or bacterin stimulation

The present study was performed to address putative links between the immune and pigmentary systems. A pigment-producing leukocyte-like cell-line (SHK-1 cells) of Atlantic salmon (Salmo salar L.) was exposed to different temperatures, poly I:C, bacterin or infected with virus (infectious pancreatic necrosis virus or infectious salmon anaemia virus). The effect of this stimulation regarding the transcription-pattern of the tyrosinase gene family (melanin genes) and the immune-related genes MHC class II and IFN-1 was analysed using real-time RT-qPCR. At 10°C cultivation, tyrosinase and dopachrome tautomerase remained unregulated. At 15°C, a moderate up-regulation was induced, while at 20°C, these genes were up-regulated in an exponential manner over time. Temperature did not affect the transcription of the immune-related genes. Virus infections, poly I:C or bacterin had no influence on the transcription of the melanogenesis-related genes, but triggered the immune-related genes. Our findings revealed no connections between the pigmentary and immune systems, but demonstrated a hereto undiscovered temperature-effect on the tyrosinase gene family.

Characterisation of a monoclonal antibody detecting Atlantic salmon endothelial and red blood cells, and its association with the infectious salmon anaemia virus cell receptor

Endothelial cells (ECs) line the luminal surfaces of the cardiovascular system and play an important role in cardiovascular functions such as regulation of haemostasis and vasomotor tone. A number of fish and mammalian viruses target these cells in the course of their infection. Infectious salmon anaemia virus (ISAV) attacks ECs and red blood cells (RBCs) of farmed Atlantic salmon (Salmo salar L.), producing the severe disease of infectious salmon anaemia (ISA). The investigation of ISA has up to now been hampered by the lack of a functional marker for ECs in Atlantic salmon in situ. In this study, we report the characterisation and use of a novel monoclonal antibody (MAb) detecting Atlantic salmon ECs (e.g. vessel endothelium, endocardial cells and scavenger ECs) and RBCs. The antibody can be used with immunohistochemistry, IFAT and on Western blots. It appears that the epitope recognised by the antibody is associated with the ISAV cellular receptor. Besides being a tool to identify ECs in situ, it could be useful in further studies of the pathogenicity of ISA. Finally, the detection of an epitope shared by ECs and RBCs agrees with recent findings that these cells share a common origin, thus the MAb can potentially be used to study the ontogeny of these cells in Atlantic salmon.

Characterization of myocardial lesions associated with cardiomyopathy syndrome in Atlantic salmon, Salmo salar L., using laser capture microdissection

Cardiomyopathy syndrome (CMS) in Atlantic salmon, Salmo salar L., is characterized by focal infiltration in the spongy myocardium and endocardium of the heart. The origin of the mononuclear infiltrate is unknown. Using experimentally infected fish, we investigated localization of the causative agent, piscine myocarditis virus (PMCV), within the heart and characterized the cell population associated with myocardial lesions. Cellular and transcriptional characteristics in the lesions were compared with adjacent non-infiltrated tissues using laser capture microdissection, RT-qPCR and immunohistochemistry. Our results reveal that PMCV is almost exclusively present in myocardial lesions. The inflammatory infiltrate comprises a variety of leucocyte populations, including T cells, B cells, MHC class II(+) and CD83(+) cells, most likely of the macrophage line. Correlation analyses demonstrated co-ordinated leucocyte activity at the site of the virus infection. Cellular proliferation and/or DNA repair was demonstrated within the myocardial lesions. Different cell populations, mainly myocytes, stained positive for proliferating cell nuclear antigen (PCNA). Densities of endothelial cells and fibroblasts were not significantly increased. The simultaneous presence of PMCV and various inflammatory cells in all myocardial lesions analysed may indicate that both viral lytic and immunopathological effects may contribute to the pathogenesis of CMS.

Localization of natriuretic peptides in the cardiac pacemaker of Atlantic salmon (Salmo salar L.)

This study describes the location of the primary pacemaker at the sino-atrial (SA) junction and the localization of salmon cardiac peptide (sCP) and ventricular natriuretic peptide (VNP) in Atlantic salmon (Salmo salar L.). The pacemaker tissue appeared lightly stained and composed of: (1) wavy nerve bundles with oval elongated wavy appearing nuclei with pointed ends, (2) ganglion cells (12-22 μm) with granular cytoplasm and (3) wide muscle fibers with large nuclei (modified cardiomyocytes) clearly distinguishing them from the other myocardial cells. Pacemaker tissue was further evaluated using immunohistochemical staining. Immunoreactivity of natriuretic peptides (sCP and VNP) antisera showed specific staining in pacemaker ganglion cells in addition to the cardiomyocytes. Positive staining with anti-CD3ɛ antisera in the pacemaker ganglion cells is a novel finding in teleosts and is consistent with observations in mammals. In conclusion, the Atlantic salmon pacemaker was shown to be located at the SA node and to harbor sCP and VNP peptides, suggesting a possible neuromodulatory and/or neurotransmitter role for these cardiac hormones within the teleost heart.

Melanogenesis in visceral tissues ofSalmo salar. A link between immunity and pigment production?

Melanogenesis is mostly studied in melanocytes and melanoma cells, but much less is known about other pigment cell systems. Liver, spleen, kidney, and other organs of lower vertebrates harbour a visceral pigment cell system with an embryonic origin that differs from that of melanocytes. In teleosts, melanin-containing cells occur in the reticulo-endothelial system and are mainly in the kidney and spleen. The Atlantic salmon ( Salmo salar L.) is an ichthyic breeding species of considerable economic importance. The accumulation of pigments in salmon visceral organs and musculature adversely affects the quality of fish products and is a problem for the aquaculture industry. With the aim to reveal novel functions and behaviour of the salmonid extracutaneous pigment system, we investigated aspects of the melanogenic systems in the tissues of Atlantic salmon, as well as in SHK-1 cells, which is a long-term cell line derived from macrophages of the Atlantic salmon head-kidney. We demonstrate that a melanogenic system is present in SHK-1 cells, head-kidney, and spleen tissues. As teleosts lack lymph nodes and Peyer’s patches, the head-kidney and spleen are regarded as the most important secondary lymphoid organs. The detection of tyrosinase activity in lymphoid organs indicates that a link exists between the extracutaneous pigmentary system and the immune system in salmon.

Pigment-producing granulomatous myopathy in Atlantic salmon: a novel inflammatory response

Melanin comprises a complex group of pigmented polymers whose primary function is ascribed to dermal solar protection, but may also have an interesting role in innate immunity. In ectothermic vertebrates, melanogenesis is reported in leukocyte populations, but it is not known if this occurs in connection with inflammatory reactions. Melanin accumulations in ectopic locations, in particular muscle, represent a serious quality problem in salmon production. Here, we investigated such changes for the expression of dopachrome tautomerase and tyrosinase as well as some important immune genes and pathogens. Furthermore, the nature of the pathological changes was addressed by morphological methods. Gene transcripts encoding key enzymes in melanogenesis, suggesting a de novo melanin synthesis in pigmented muscle, were found. MHC class II transcripts were up-regulated and there was no indication of bacterial or viral infection. The histological examination revealed granulomatous inflammation with distribution of MHC class II positive cells and T cells, analogous to the pattern found in mammals. Importantly, in contrast to mammals pigmented cells were contributing in the inflammation. We demonstrate that melanin production occurs in granulomatous inflammation in salmon, revealing a close and hitherto unreported link between the pigmentary and immune systems.

Presence and interaction of inflammatory cells in the spleen of Atlantic cod, Gadus morhua L., infected with Francisella noatunensis

Serious infectious diseases, accompanied by macrophage-dominated chronic inflammation, are common in farmed Atlantic cod. To increase knowledge relating to morphological aspects of such inflammatory responses, cod were challenged with Francisella noatunensis, an important bacterial pathogen of this fish species. Tissue and cell dynamics in the spleen were examined sequentially over 60 days. Small clusters of mainly macrophage-like cells (MLCs) staining for non-specific esterase and acid phosphatase developed with time. These foci were transiently infiltrated by pleomorphic proliferating cells of unknown nature and by granulocyte-like cells (GCLCs) staining for peroxidase and lysozyme. The latter cell type, which appeared to be resident in the red pulp of control fish, migrated into the inflammatory foci of infected fish. Cells expressing genes encoding IFN-γ and IL-8 increased in number during the study period. Bacteria were detected only in the MLCs and their number increased despite the extensive inflammation. Our results demonstrate an intimate spatial relationship in inflammatory foci between at least three cell types. The presence of GCLCs, together with MLCs, suggests pyogranulomatous inflammation as a more appropriate descriptive term than granulomatous inflammation.

Polyclonal hypergammaglobulinemia and autoantibody production induced by vaccination in farmed Atlantic salmon

The introduction of oil-adjuvanted vaccines in salmon aquaculture made large-scale production feasible by reducing the impact of infections. Vaccines given intraperitoneally (ip) contain oil adjuvant such as mineral oil. However, in rodents, a single ip injection of adjuvant hydrocarbon oil induces lupus-like systemic autoimmune syndrome. We have recently reported that autoimmune disease in farmed salmon, characterized by production of various autoantibodies, immune complex glomerulonephritis, liver thrombosis, and spinal deformity, are previously unrecognized side effects of vaccination. In the present study, we examined whether vaccination-induced autoantibody production in farmed Atlantic salmon is a mere result of polyclonal B-cell activation. Sera were collected from 205 vaccinated and unvaccinated Atlantic salmon (experimental, 7 farms) and wild salmon. Total IgM levels and autoantibodies to salmon blood cell (SBC) extract in sera were measured by ELISA and the relationship between hypergammaglobulinemia and autoantibody production was analyzed. Comparison of endpoint titers vs levels/units using a single dilution of sera in detection of autoantibodies to SBC showed near perfect correlation, justifying the use of the latter for screening. Both total IgM and anti-SBC antibodies are increased in vaccinated salmon compared with unvaccinated controls, however, they do not always correlate well when compared between groups or between individuals, suggesting the involvement of antigen-specific mechanisms in the production of anti-SBC autoantibodies. The primary considerations of successful vaccine for aquaculture are cost-effectiveness and safety. Vaccination-induced autoimmunity in farmed Atlantic salmon may have consequences on future vaccine development and salmon farming strategy. Evaluation for polyclonal hypergamamglobulinemia and autoimmunity should be included as an important trait when vaccine efficacy and safety are evaluated in future.

Salmonid T cells assemble in the thymus, spleen and in novel interbranchial lymphoid tissue

In modern bony fishes, or teleost fish, the general lack of leucocyte markers has greatly hampered investigations of the anatomy of the immune system and its reactions involved in inflammatory responses. We have previously reported the cloning and sequencing of the salmon CD3 complex, molecules that are specifically expressed in T cells. Here, we generate and validate sera recognizing a peptide sequence of the CD3ε chain. Flow cytometry analysis revealed high numbers of CD3ε(+) or T cells in the thymus, gill and intestine, whereas lower numbers were detected in the head kidney, spleen and peripheral blood leucocytes. Subsequent morphological analysis showed accumulations of T cells in the thymus and spleen and in the newly discovered gill-located interbranchial lymphoid tissue. In the latter, the T cells are embedded in a meshwork of epithelial cells and in the spleen, they cluster in the white pulp surrounding ellipsoids. The anatomical organization of the salmonid thymic cortex and medulla seems to be composed of three layers consisting of a sub-epithelial medulla-like zone, an intermediate cortex-like zone and finally another cortex-like basal zone. Our study in the salmonid thymus reports a previously non-described tissue organization. In the intestinal tract, abundant T cells were found embedded in the epithelium. In non-lymphoid organs, the presence of T cells was limited. The results show that the interbranchial lymphoid tissue is quantitatively a very important site of T cell aggregation, strategically located to facilitate antigen encounter. The interbranchial lymphoid tissue has no resemblance to previously described lymphoid tissues.

Manifestations of systemic autoimmunity in vaccinated salmon

The development of systemic autoimmunity may result as an undesired side-effect following vaccination, and this condition was recently shown to occur in farmed salmon (Salmo salar). Several of previously reported side-effects following vaccination of fish should therefore be reviewed in the light of this condition. Here, organs and pathological changes in three separate groups of fish severely affected by vaccination were investigated by different morphological methods (n=84). Granulomas or microgranulomas were observed at the injection site and in several organs. Mott cells were observed in all tissues examined. Pannus-like changes with lymphocyte infiltrates were observed in spines. In conclusion, the reactions following vaccination were of a systemic nature that may be explained by a pathogenetic mechanism caused by systemic autoimmunity.

Identification and characterization of a novel intraepithelial lymphoid tissue in the gills of Atlantic salmon

In addition to being the respiratory organ in fish, the gills form a barrier against the external milieu. Innate and adaptive immune system components have been detected in the gills, but lymphoid cell accumulations similar to that seen in the mammalian mucosa have not been described. The present investigations revealed cell accumulations on the caudal edge of interbranchial septum at the base of the gill filaments in the Atlantic salmon. Cytokeratin immunohistochemical staining and identification of a basal membrane and desmosome cell junctions by electron microscopy showed that the cell accumulation was located intraepithelially. Major histocompatibility complex (MHC) class II+ cells were detected by immunohistochemistry, and laser capture micro-dissection and subsequent RT-PCR analysis revealed expression of T-cell receptor transcripts in the investigated tissue, suggesting the presence of T cells. The intraepithelial tissue reported here may be a suitable location for immune surveillance of gill infections, as well as a target site for new vaccine approaches and investigations of epithelial immunity. This is the first description of a lymphocyte cell aggregation within a teleostian gill epithelium network, illustrating a phylogenetically early form of leukocyte accumulations in a respiratory organ.

CD4 homologues in Atlantic salmon

In mammals CD4 is a membrane glycoprotein on Th cells with four extracellular immunoglobulin-like (Ig-like) domains (D1-D4). It functions as a co-receptor during immune recognition between the TCR and the MHC II/peptide complex. The cytoplasmic domain binds p56lck, a protein kinase responsible for phosphorylating CD3 which is the first interaction in a cascade leading to T cell activation. We have previously reported a CD4-2 gene in rainbow trout (Oncorhynchus mykiss) which was found adjacent to the CD4-1 gene by synteny analysis. There are two subtypes (a and b) of CD4-2 in rainbow trout, with two Ig-like extracellular domains. Here we present the homologues of mammalian CD4 in Atlantic salmon (Salmo salar): CD4-1 with four extracellular domains and CD4-2a and CD4-2b with two extracellular domains. A Southern blot analysis shows two copies of the CD4-1 gene in the genomic DNA of the closely related rainbow trout. The genes for CD4-1 and CD4-2 have been sequenced and show typical traits for CD4 genes, such as the code for the first domain (D1) being divided between two exons and the other domains being largely coded for by single exons. The corresponding translated cDNAs show little (13-17%) identity to higher vertebrates and are approximately 37% similar to other translated, teleost sequences but are 89% identical to the closely related rainbow trout. However they exhibit conserved features such as the Lck binding motif in their cytoplasmic domains and the order of variable and constant type Ig-like domains. qRT-PCR data are presented describing the differential tissue expression of these genes together with other T cell markers (TCR and CD3) in several individuals.

Vaccination-induced systemic autoimmunity in farmed Atlantic salmon

Over half of the salmon consumed globally are farm-raised. The introduction of oil-adjuvanted vaccines into salmon aquaculture made large-scale production feasible by preventing infections. The vaccines that are given i.p. contain oil adjuvant such as mineral oil. However, in rodents, a single i.p. injection of adjuvant hydrocarbon oil induces lupus-like systemic autoimmune syndrome, characterized by autoantibodies, immune complex glomerulonephritis, and arthritis. In the present study, whether the farmed salmon that received oil-adjuvanted vaccine have autoimmune syndrome similar to adjuvant oil-injected rodents was examined. Sera and tissues were collected from vaccinated or unvaccinated Atlantic salmon (experimental, seven farms) and wild salmon. Autoantibodies (immunofluorescence, ELISA, and immunoprecipitation) and IgM levels (ELISA) in sera were measured. Kidneys and livers were examined for pathology. Autoantibodies were common in vaccinated fish vs unvaccinated controls and they reacted with salmon cells/Ags in addition to their reactivity with mammalian Ags. Diffuse nuclear/cytoplasmic staining was common in immunofluorescence but some had more specific patterns. Serum total IgM levels were also increased in vaccinated fish; however, the fold increase of autoantibodies was much more than that of total IgM. Sera from vaccinated fish immunoprecipitated ferritin and approximately 50% also reacted with other unique proteins. Thrombosis and granulomatous inflammation in liver, and immune-complex glomerulonephritis were common in vaccinated fish. Autoimmunity similar to the mouse model of adjuvant oil-induced lupus is common in vaccinated farmed Atlantic salmon. This may have a significant impact on production loss, disease of previously unknown etiology, and future strategies of vaccines and salmon farming.

Histological, digestive, metabolic, hormonal and some immune factor responses in Atlantic salmon, Salmo salar L., fed genetically modified soybeans

The paper reports the second and final part of an experiment aiming to study physiological and health-related effects of genetically modified (GM) soybean meal (SBM) type Roundup Ready soybean (RRS) in diets for post-smolt Atlantic salmon. For 3 months salmon were fed diets containing 172 g kg(-1) full-fat SBM from RRS (GM-soy) or an unmodified, non-isogenic line (nGM-soy), or a reference diet with fishmeal as the sole protein source (FM). Slight differences in anti-nutrient levels were observed between the GM and nGM-soy. Histological changes were observed only in the distal intestine of the soy-fed fish. The incidence of moderate inflammation was higher in the GM-soy group (9 of 10 sampled fish) compared with the nGM-soy group (7 of 10). However, no differences in the concomitant decreases in activities of digestive enzymes located in the brush border (leucine aminopeptidase and maltase) and apical cytoplasm (acid phosphatase) of enterocytes or in the number of major histocompatibility complex class II+ cells, lysozyme activity, or total IgM of the distal intestine were observed. GM compared with nGM-soy fed fish had higher head kidney lysozyme (11,856 vs. 10,456 units g(-1) tissue) and a tendency towards higher acid phosphatase (0.45 vs. 0.39 micromol h(-1) kg(-1) body mass in whole tissue) activities, respectively. Plasma insulin and thyroxin levels, and hepatic fructose-1,6-bisphosphatase and ethoxyresorufin-O-deethylase activities were not significantly affected. It is not possible, however, to conclude whether the differences in responses to GM-soy were due to the genetic modification or to differences in soy cultivars in the soy-containing diets. Results from studies using non-modified, parental line soybeans as the control group are necessary to evaluate whether genetic modification of soybeans in diets poses any risk to farmed Atlantic salmon.

Isolation of the Atlantic salmon tyrosinase gene family reveals heterogenous transcripts in a leukocyte cell line

In ectothermic vertebrates, visceral organs harbor melanin-containing cells. Their ability as pigment producers is nevertheless disputed. To address expression of the key genes for melanogenesis in Atlantic salmon (Salmo salar), a tyrosinase-positive leukocyte cell line (SHK-1) and skin were used to obtain full-length tyrosinase (Tyr), tyrosinase-like protein-1 (Tyrp1), and dopachrome tautomerase (Dct) mRNA transcripts. In the SHK-1 cells, two different Tyrp1 transcripts were identified, one lacking exon 1. However, only the full-length version of Tyrp1 was identified in the skin. Sequencing of Tyrp1 genomic region revealed that the two Tyrp1 transcripts might originate from two different loci, possibly a result of pseudo-tetraploidity of the Atlantic salmon genome. Expression of Tyr, Tyrp1 and Dct was investigated by quantitative real-time reverse transcriptase polymerase cain reaction showing highest expression in the SHK-1 cell line and skin, intermediate in pronephros, and negligible or absent in liver and muscle. Histological approaches were used to demonstrate melanin and revealed presence of melanized cells in skin, kidney and liver, and absence of such cells in muscle. In addition to verify melanin synthesis abilities of visceral-located cells, our results indicate loci-specific transcription differences between populations of melanin-producing cells in Atlantic salmon.

Influence of dietary lipid composition on cardiac pathology in farmed Atlantic salmon, Salmo salar L

The present study investigated the short-term (5 months) effect of replacing dietary marine oils with vegetable oils on the development of arteriosclerotic changes in the heart of Atlantic salmon, Salmo salar. The experiment was performed as a randomized observer-blinded and controlled trial. Farmed Atlantic salmon were randomly sampled from a study population containing 900 individuals. The salmon were divided into three groups and given diets with either 100% fish oil (Diet 1), a 50/50% mixture of fish oil and rapeseed oil (Diet 2) or 100% rapeseed oil (Diet 3). Ten sexually immature salmon from each dietary group were sampled in March and August 2002. Additionally, 47 sexually mature wild salmon were randomly collected in mid-September 2001. Serial histological sections were taken from the bulbus arteriosus and ventricle wall for histopathological evaluation of the coronary arteries and myocardium. No significant differences in mean coronary changes recorded by the main variable 'mean range lesion' (MRL) were detected between the groups in March or August. MRL increased significantly between March and August with Diet 2 (P < 0.01), was nearly significant with Diet 3 (P = 0.06) and was unchanged with Diet 1. This pattern coincided with the Diet 2 group having the highest increase in heart weight. MHC class II immunoreactive cells in the coronary changes were detected in sections from one individual in each group. Heart weight was the most dominant variable in the data set and explained linearly 15.5% of the variation in MRL. Body weight, fish length and heart weight were all significantly, positively and linearly correlated to MRL. The Diet 2 group had the highest growth rate and also exhibited a significant increase in MRL. The possible influence of diet composition on weight gain and MRL needs to be further elucidated. Increase in heart weight seems to be the dominating predictor of the appearance of MRL in Atlantic salmon. However, the present results cannot exclude the possibility that differences in fatty acid composition of fish feed can influence the development of arteriosclerotic changes in Atlantic salmon.

MHC class II+ cells in the gills of Atlantic salmon (Salmo salar L.) affected by amoebic gill disease

Amoebic gill disease (AGD) is characterised by the association of Neoparamoeba sp. with hyperplastic gill tissue of affected fishes, however, the identity and role of host cells associated with AGD lesions are not known. Here, we investigated cells with an immunological role that were associated with AGD lesions by locating cellular MHC class II beta chain. A tank housing Atlantic salmon (Salmo salar) was inoculated with Neoparamoeba sp., and MHC class II beta chain expression in the gills was qualitatively assessed by immunohistochemistry. In AGD-naïve control fish, MHC class II+ cells were detected basolateral to the interlamellar epithelium as well as upon the interlamellar and secondary epithelium. In the gills of AGD affected fish MHC class II+ cells were observed in both affected and unaffected tissue. Within AGD lesions, numerous MHC class II+ cells were present and these cells exhibited variable levels of expression suggesting that like mammals, MHC class II expression is highly regulated. The presence of MHC class II+ cells within gill lesions is indicative of immune cell trafficking and these cells could contribute in an antigen presentation capacity to the development of an antibody response in fish chronically affected by AGD.

Vaccine-associated granulomatous inflammation and melanin accumulation in Atlantic salmon, Salmo salar L., white muscle

The purpose of this study was to investigate the nature of variably sized pigmented foci encountered in fillets of farmed Atlantic salmon, Salmo salar L. The material was sampled on the fillet production line and on salmon farms from fish with an average size of 3 kg from various producers. The fish had been routinely vaccinated by injection. Gross pathology, histology, immunohistochemistry using antisera against major histocompatibility complex (MHC) class II beta chain and transmission electron microscopy (TEM) were used to characterize the changes. Macroscopically, melanized foci were seen penetrating from the peritoneum deep into the abdominal wall, sometimes right through to the skin, and also embedded in the caudal musculature. Histological investigation revealed muscle degeneration and necrosis, fibrosis and granulomatous inflammation containing varying numbers of melano-macrophages. Vacuoles, either empty or containing heterogeneous material, were frequently seen. The presence of abundant MHC class II+ cells indicated an active inflammatory condition. TEM showed large extracellular vacuoles and leucocytes containing homogeneous material of lipid-like appearance. The results showed that the melanized foci in Atlantic salmon fillet resulted from an inflammatory condition probably induced by vaccination. The described condition is not known in wild salmon and in farmed salmon where injection vaccination is not applied.

Vaccination induces major histocompatibility complex class II expression in the Atlantic salmon eye

The aim of the study was to investigate the presence, distribution and density of major histocompatibility complex (MHC) class II+ cells in the ocular tissues of the Atlantic salmon, Salmo salar, prior to and following vaccination. Eyes were collected 14 days prior to and at 4, 11, 25 and 39 days and 4 months subsequent to vaccination with a commercial fish vaccine. A quantitative analysis was performed in sections on the number of immunopositive cells in the retinal layers. In all groups, MHC class II+ cells were detected in the area of the limbus but not in the central parts of the cornea. In the uvea, immunopositive cells were present in unvaccinated and vaccinated fish. Abundant immunopositive cells were identified in the choroid rete (or choroid gland) in all groups as well as in the ventral ciliary cleft, where macrophage-like MHC class II+ cells were seen. Quantitative histology of the retina revealed a significant increase in MHC class II+ cells in the outer plexiform layer (OPL) and the inner nuclear layer (INL) 4 days following vaccination. Positive cells were detected in all layers of the retina with the exception of the photoreceptor layer.

Production of rabbit antisera against recombinant MHC class II beta chain and identification of immunoreactive cells in Atlantic salmon (Salmo salar)

In the present work, rabbit antisera recognising the Atlantic salmon (Salmo salar) MHC class II beta chain polypeptide were produced and used in immunoblotting, immunohistochemistry and immunogold electron microscopy. A construct encoding the beta1 and beta2 domains fused to the E. coli protein thioredoxin was used to express the recombinant MHC class II beta chain. Immunoblotting revealed a band of approximately 30kDa in total protein samples from head kidney, spleen, gills, thymus and blood leukocytes, while being absent in muscle. The distribution of MHC class II positive cells was immunohistochemically demonstrated in Atlantic salmon epithelial and haemopoietic tissues. Ultrastructural demonstration of immunoreactive organelles in mid-kidney cells was performed by immunogold electron microscopy. The results indicate expression in lymphocytes, macrophages, epithelial cells and endritic-like cells. This is the first study to address morphological MHC class II expression in a fish species.

Subclinical herpesvirus infection in farmed turbot Scophthalmus maximus

Subclinical infections with a herpesvirus were detected by light and electron microscopic examination of juvenile turbot collected during a national surveillance programme. Virions detected in the epidermis of skin and in the epithelium of gills had a morphology consistent with those of Herpesvirus scophthalmi described from turbot in the United Kingdom and Denmark. This is the first report of herpesvirus infection in turbot in Norway.