Investigation of the structural and functional features of splenic ellipsoids in rainbow trout (Oncorhynchus mykiss)

Molecular characteristics of interleukin (IL)-17A/F3 and its immune response on the pathogen and functional regulation on cytokines in common carp Cyprinus carpio L

Fish interleukin (IL)-17A/F is homologous with mammalian IL-17A and IL-17F, which plays a key role in regulating inflammatory responses and autoimmune diseases. In fish, IL-17A/F1, 2, and 3 have been identified and described. However, IL-17A/F3 has received little attention in fish. In this study, a homolog of IL-17A/F3 was identified in common carp (Cyprinus carpio L.), which was termed as Cc_IL-17A/F3. The deduced amino acid sequence of Cc_IL-17A/F3 has four conserved cysteine residues, which could form two intrachain disulfide bonds. Homology comparison showed that the Cc_IL-17A/F3 was in the range of 31.7-71.9% of sequence similarity with these of other fishes. The Cc_IL-17A/F3 gene was constitutively expressed in various tissues, with higher expression levels in the skin and gills. After common carp were infected by Aeromonas. hydrophila, the mRNA expression levels of Cc_IL-17A/F3 were significantly up-regulated in the spleen, head kidney, gills, and intestine. Based on the indirect immunofluorescence assay, Cc_IL-17A/F3 proteins were found to be obviously increased in the intestine and spleen upon A. hydrophila infection at 24 h post-infection. The recombinant protein rCc_IL-17A/F3 could enhance the gene expression levels of pro-inflammatory cytokines (IL-1β, IL-6, IFN-γ, and TNF-α) as well as chemokines (CXCL8 and CXCL20) in primary head kidney leukocytes. In vivo and in vitro experiments have similar stimulatory effects. When Cc_IL-17A/F3 was overexpressed in common carp, the expressions of pro-inflammatory cytokines and chemokines were significantly up-regulated in head kidney and spleen. In summary, the results derived from the present study suggested that the Cc_IL-17A/F3 plays an important role in defending against bacterial infections, and probably participates in mucosal immunity of the host.

Structural and Functional Aspects of the Spleen in Molly Fish Poecilia sphenops (Valenciennes, 1846): Synergistic Interactions of Stem Cells, Neurons, and Immune Cells

In fish, the spleen is the prime secondary lymphoid organ. It has a role in the induction of adaptive immune responses, in addition to its significance in the elimination of immune complexes. This study was conducted on 18 randomly obtained adult molly fish (Poecilia sphenops) of both sexes using histological, immunohistochemical, and ultrastructural studies to highlight the cellular components of the spleen and their potential role in the immune system. The spleen of molly fish was characterized by the presence of well-distinct melanomacrophage centers, and other basic structures present in higher vertebrates including red and white pulps, blood vessels, and ellipsoids. Some mitotic cells could also be identified in the red pulp. Mast cells with characteristic metachromatic granules could be seen among the splenic cells. Rodlet cells were randomly distributed in the spleen and were also observed around the ellipsoids. The white pulp of the spleen expressed APG5. The expressions were well distinct in the melanomacrophages, leukocytes, and macrophages. Myostatin was expressed in leukocytes and epithelial reticular cells. IL-1β showed immunoreactivity in monocytes and macrophages around the ellipsoids. NF-κB and TGF-β were expressed in macrophages and epithelial reticular cells. Nrf2 expression was detected in stem cells and rodlet cells. Sox-9 had a higher expression in epithelial reticular cells and stem cells. The high frequency of immune cells in the spleen confirmed its role in the regulation of both innate and adaptive immunity, cell proliferation, and apoptosis.

Anatomy of teleost fish immune structures and organs

The function of a tissue is determined by its construction and cellular composition. The action of different genes can thus only be understood properly when seen in the context of the environment in which they are expressed and function. We now experience a renaissance in morphological research in fish, not only because, surprisingly enough, large structures have remained un-described until recently, but also because improved methods for studying morphological characteristics in combination with expression analysis are at hand. In this review, we address anatomical features of teleost immune tissues. There are approximately 30,000 known teleost fish species and only a minor portion of them have been studied. We aim our review at the Atlantic salmon (Salmo salar) and other salmonids, but when applicable, we also present information from other species. Our focus is the anatomy of the kidney, thymus, spleen, the interbranchial lymphoid tissue (ILT), the newly discovered salmonid cloacal bursa and the naso-pharynx associated lymphoid tissue (NALT).

Reduction of rainbow trout spleen size by splenectomy does not alter resistance against bacterial cold water disease

In lower vertebrates, the contribution of the spleen to anti-bacterial immunity is poorly understood. We have previously reported a phenotypic and genetic correlation between resistance to Flavobacterium psychrophilum, the causative agent of bacterial cold water disease (BCWD) and spleen somatic index (spleen weight normalized to body weight, SI). Fish families with larger pre-challenge SI values were found to have greater BCWD survival (resistance) following intraperitoneal injection of a lethal dose of F. psychrophilum. Since the mammalian spleen is known to be crucial for capture and destruction of encapsulated bacteria, we tested the hypothesis that reduction of spleen size, by surgical splenectomy, should reduce the survival advantage of the larger-spleen, disease-resistant fish. Experiments were performed using two separate lines of fish that had previously been selected either based on BCWD survival (resistant and susceptible), or selected based on spleen size (high and low SI). Following 65 to 81 days post-surgical recovery, fish were challenged with F. psychrophilum and mortality monitored for a minimum of 21 days. No significant difference in the relative survival was detected between splenectomized or sham-operated groups, while SI of splenectomized fish was reduced to an average of 8-12% of control animals. A positive correlation was observed between the SI, measured at the time of splenectomy, and time-to-death post-challenge. In summary, these experiments argue that larger spleen size alone is not sufficient for greater BCWD resistance, but rather it is an indirect indicator of immunological status.

B lymphocyte compartments in the human splenic red pulp: capillary sheaths and periarteriolar regions

The microvasculature of human spleens is still incompletely understood. Two enigmatic types of red pulp microvessels, penicillar arterioles and sheathed capillaries, have already been described in the nineteenth century without gaining much attention afterwards. We performed a detailed analysis of sheathed capillaries to clarify the cellular composition of their sheaths by immunohistological double-staining experiments. Capillary sheaths comprise three different cell types, namely specialized cuboidal CD271(++) inner sheath cells surrounded by CD271(-) macrophages and accumulations of B lymphocytes. The CD271(++) inner sheath cells express the chemokine CXCL13 in a unique single dot pattern. Sheath-associated B lymphocytes consist of IgM(+), IgD(++), and of "switched" cells. T lymphocytes do not accumulate in pericapillary sheaths. The predominant sheath-associated macrophage population is CD163(-)CD68(+) and thus differs from the majority of red pulp macrophages. The sheath-associated macrophages strongly express CD169 only in perifollicular sheaths, but not in sheaths located deeper in the red pulp. IgM(+), IgD(++), and "switched" B cells are also closely associated with red pulp arterioles characterized by the expression of smooth muscle actin in muscle cells and in branched periarteriolar stromal cells. Capillary sheaths are observed in a post-arteriolar position and appear to be of limited length. We suggest to change the term "Vagina periarteriolaris makrophagocytica" of the international histological and embryological terminologies to "Vagina pericapillaris."

Blood cleansing cells in head kidney and spleen in Buenos Aires tetra, Hyphessobrycon anisitsi (Eigenmann), (Characidae: Teleostei)

The general structure and cell types in kidney and spleen in Buenos Aires tetra, Hyphessobrycon anisitsi, family Characidae, are described. The capability and capacity of these organs to clean foreign ferritin from the blood stream are analysed and compared. Head kidney was mainly composed of neutrophils, macrophages, lymphocytes and other white blood cells, whereas unmatured and matured red blood cells were few in number. Spleen often contained much red pulp, that is mainly matured red blood cells between splenic cords, often with some macrophages and neutrophils in the latter. Occasionally, this pulp contained large volumes of unmatured red blood cells, particularly in the periphery of the spleen. The splenic white pulp consisted of ellipsoids composed of an inner endothelial layer covered by a thick sheet of white blood cells, which in the periphery consisted mainly of macrophages. Erythrocytes occupied nearly the entire splenic volume in some specimens, whereas up to half of this volume was filled by ellipsoid macrophages, neutrophils, lymphocytes and other white blood cells in other specimens. The macrophages and sinusoidal endothelial cells in kidney and spleen from ferritin-injected specimens were tightly packed by yellow-brown granules or Prussian blue precipitations, in tissue treated with Mallory stain or acid ferrocyanide, respectively, suggesting a large uptake of foreign ferritin. In the present tetra large amounts of white blood cells are developed in head kidney, where macrophages and sinusoidal endothelial cells play important roles in the cleansing of scavenger and foreign molecules and particles from the blood stream. The spleen seems primarily to be a site for iron recycling and production and storage of red blood cells. Sometimes, however, it was rich in macrophages, neutrophils, lymphocytes and other white blood cells, suggesting functions like blood cleansing and non-specific and specific defence in such specimens.

Rapid induction of experimental AA amyloidosis in mink by intravenous injection of amyloid enhancing factor

Studies of amyloid enhancing factor (AEF)-induced amyloidosis are commonly performed in mice. In mink, earlier studies of amyloid A (AA) amyloidosis showed that the predeposition phase was highly variable. Thus, the aim of the study was to establish an AEF-induced AA amyloidosis model in mink to facilitate studies of early amyloid deposition in a species with prominent ellipsoids, anatomical structures lacking in mice but present in most other mammals. AEF was extracted from mink spleens containing AA. Mink received one intravenous injection of AEF and repeated subcutaneous injections of lipopolysaccharide (LPS) as an inflammatory stimulus. On day 4, small amounts of amyloid were detected in the marginal zone in the spleen. On day 7, considerable amyloid deposition was detected in the ellipsoids and marginal zones in the spleen and in the space of Disse in the liver. By immunohistochemistry, the deposits were identified as AA amyloid. Immunolabeling was also detected in lymphoid follicles and the red pulp of some animals. Control animals receiving only AEF were negative. Control animals receiving only LPS were negative except for one of three animals which had small amounts of amyloid in the spleen. The mink AEF model is a suitable tool to study the development of AA amyloidosis in a species with a spleen containing both well-developed ellipsoids and marginal zone.

Distribution of the professional phagocytic granulocytes of the bony fish gilthead seabream (Sparus aurata L.) during the ontogeny of lymphomyeloid organs and pathogen entry sites

Although it is believed that fish fry depend fundamentally on their innate defence mechanisms, the ontogeny of fish innate immune cells is poorly understood. In the present study, we have used a specific monoclonal antibody against acidophilic granulocytes (AGs), the main professional phagocytic cell type of the bony fish gilthead seabream, to study their localization during the development of the main lymphomyeloid organs, namely the head kidney, spleen and thymus, and of the two major portals for pathogen entry, namely the gills and intestine. AGs were observed in the posterior intestine and in the blood earlier than in the haematopoietic kidney (21 vs. 27 days post-hatching, dph). AGs were observed scattered between other cells of the haematopoietic lineage in the head kidney of larvae, but were grouped around the blood vessels of this organ in juveniles and adults, where they were also much more numerous. In the spleen and in the thymus, AGs were observed much later (62 dph) and appeared scattered. AGs were also observed in the gill lamella and the posterior intestine near the anus throughout development.

Filter Function and Immune Complex Trapping in Splenic Ellipsoids

The role of splenic ellipsoids in the trapping of particulate material and immune complexes was investigated in mink (Mustela vison). The ellipsoids were prominent, with typical features such as a permeable endothelium and a discontinuous basement membrane surrounded by a sheath of macrophages and reticular cells. Ellipsoidal trapping of circulating particles was demonstrated 10 min after intracardiac injection of colloidal carbon and fluorescent microspheres. Preformed peroxidase-antiperoxidase immune complexes were detected in ellipsoids 10 min and also 1 h after intracardiac injection. Erythrocytes were frequently observed in the ellipsoidal sheath, and many phagocytized fragments of erythrocytes were found in the ellipsoidal macrophages. It was concluded that mink ellipsoids are effective blood filters with a role in retention of circulating particulate material, and that mammalian splenic ellipsoids also have the ability to trap immune complexes.

Endocytosis of horse-spleen ferritin in the black tetra, Gymnocorymbus ternetzi (Characidae: Teleostei)

The capability and capacity of various tissues in black tetra Gymnocorymbus ternetzi (Boulenger), family Characidae, to take up horse-spleen ferritin from the blood stream are described. In the head kidney from specimens injected intraperitoneally with horse-spleen ferritin, numerous large cells were tightly packed by yellow-brown granules when the period of time between injection and sacrifice was over 2h. These cellular granules were tightly packed with Prussian blue precipitation in tissues treated with an acid ferrocyanide solution, i.e., they displayed a deep blue colour. Similarly, smaller cells were also observed in the trunk kidney, gills and peripheral connective tissue of the pancreas and spleen at this stage after ferritin injection, but they were always few in number. Yellow-brown granules or Prussian blue granules were not found in endothelial cell layers of heart and liver. We suggest that practically all clearance of the blood from foreign and scavenger macromolecules is performed by macrophages in the head kidney of the black tetra, whereas the spleen seems to function mainly as a reservoir for red blood cells in this species.

Splenic ellipsoids: an early target for deposition of AA amyloid induced in mink

The spleen is the primary target for spontaneous as well as experimental AA amyloidosis in animals such as mice and mink, and is therefore a valuable organ for study of the initial phases of amyloid fibrillogenesis and deposition. We have investigated splenic amyloid AA deposits induced in the mink, and we demonstrate a novel target for AA, namely the splenic ellipsoids. We show presence of amyloid P component (AP), glycosaminoglycans (GAGs) and apolipoprotein E (apoE), all well-known common elements of amyloid, co-localizing with AA. In addition, apolipoprotein AI (apoAI) was seen co-localized to the AA deposits in the ellipsoids. We hypothesize that the ellipsoids may be important splenic structures for initial AA formation. The apoAI in the ellipsoids could displace SAA from acute phase HDL at this site, thereby making SAA available for amyloid formation and deposition.

Distribution and retention of antigens of Aeromonas salmonicida in Atlantic salmon (Salmo salar L.) vaccinated with a DeltaaroA mutant or formalin-inactivated bacteria in oil-adjuvant

In this study we report the differences in distribution and retention of Aeromonas salmonicida antigens after vaccination with two different vaccines. Parr of Atlantic salmon (Salmo salar) were given intraperitoneal injections of either a commercial, monovalent furunculosis vaccine (Apoject) or live, attenuated A. salmonicida (DeltaaroA). Fish were sampled at weeks 2, 4 and 12 post-vaccination and head kidney and spleen were collected. Presence of LPS and 16S rDNA in isolated leukocytes were investigated by immunocytochemistry and polymerase chain reaction (PCR).16S rDNA was detected in head kidney and spleen of all DeltaaroA vaccinated and most Apoject-vaccinated fish at weeks 2 and 4. At week 12, 16S rDNA was detected in none of the DeltaaroA vaccinated fish, but it was detected in head kidney of 75% of Apoject-vaccinated fish. LPS was detected in both vaccination groups at all sampling times, but most frequently in the DeltaaroA vaccinated fish (in head kidney 75-83% vs. 50%, in spleen 58-67% vs. 17-25%).

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.

Immune-complex trapping in the splenic ellipsoids of rainbow trout (Oncorhynchus mykiss)

Rainbow trout (Oncorhynchus mykiss), immunised with horseradish peroxidase, were given horseradish peroxidase intravenously, and the trapping of antigen in the spleen was followed 1, 24, and 48 h after injection. After 1 h, the localisation of horseradish peroxidase indicated that the antigen had been extensively trapped in the walls of the splenic ellipsoids. The colocalization of horseradish peroxidase with rainbow trout immunoglobulin M and complement factor 3 was shown with a double immunofluorescence technique and suggested that horseradish peroxidase was trapped in the form of immune complexes. After 24 and 48 h, very little horseradish peroxidase was detected in the ellipsoids, and horseradish peroxidase was mainly found in association with large cells with prominent cytoplasmic extensions. In non-immunized fish given horseradish peroxidase intravenously, antigen was not detected in ellipsoids. Thus, the observed difference between immunised and non-immunized trout suggests a specific role for the splenic ellipsoids in rapid immune-complex trapping and invites speculation on its significance in a secondary immune response.