Effect of ranching time on melanomacrophage centres in anterior kidney and spleen of Southern bluefin tuna, Thunnus maccoyii

Histomorphometric evaluation of melanomacrophage centers (MMCs) and CD3+ T cells of two morphs of brown trout (Salmo trutta) fed diets with immunostimulants

The brown trout (Salmo trutta) is a salmonid residing in riverine and coastal waters throughout the Northern Hemisphere, whose various populations evolved into distinct ecological morphs, differing in their migratory tendencies and preferred habitats. Unfortunately, due to progressing degradation of natural environment, the conservation of these populations is of growing importance and is undoubtedly a challenging task. Therefore, various means to refine the preparatory protocols for restocking using hatchery-reared fish are being pursued, some of which involve the administration of immunity-boosting substances. The current study assessed the effects of two dietary immunostimulants: Bioimmuno (4% inosine pranobex and 96% β-glucan) and Focus Plus (commercial preparation by Biomar, Denmark) on two morphs of the brown trout - the river trout (S. trutta morpha fario) and the sea trout (S. trutta morpha trutta). Tissue samples were obtained from ∼75 to 100g fish after 0, 2 and 4 weeks of experimental feeding. Multi-factorial analysis of conducted histological measurements of melanomacrophage centers (MMCs) revealed no changes of their parameters within spleens, but showed a decrease of the occupied tissue area and MMC counts in the livers, progressing with time regardless of the applied diet. Immunohistochemical analysis of CD3+ T cells showed their increased recruitment into mucosal folds of pyloric caeca in the 2-week sampling of trouts fed with the diet with 2% Bioimmuno addition, but this effect was not present in the 4-week sampling. When studying all groups jointly within each morph, there was a significant difference in terms of maintained CD3+ T cells levels, as sea trouts showed significantly higher tissue areas occupied by these cells than river trouts, both in the pyloric caeca and hepatic parenchyma. The study revealed that feeding with a diet enriched with Bioimmuno for 2 weeks may be a favorable enhancement of rearing protocols of brown trout stocks prior to their release, but more studies need to be conducted to test the possibility of an even shorter feeding period.

Chicken Enterococcus faecalis-induced immunoglobulin Y as a prophylactic and therapeutic agent against streptococcosis in red tilapia (Oreochromis hybrid)

Background and Aim

Streptococcosis is a common bacterial disease in red tilapia, in which Enterococcus faecalis infection has not been widely reported. This study aimed to evaluate the efficacy of pellets that contain chicken E. faecalis-induced immunoglobulin Y (IgY) to treat and prevent streptococcosis in red tilapia.

Materials and Methods

We conducted a 28-day study for immunoprophylaxis and immunotherapy, each using four groups with two replications: Healthy control fish (KS), non-IgY pellets (PA and TA), pellets with 25% egg yolk containing E. faecalis-induced IgY (PB and TB), and pellets with 50% egg yolk containing E. faecalis-induced IgY(PC and TC). Indirect enzyme-linked immunosorbent assay was performed on prototype pellets produced with an IgY suspension at 1.63 mg/mL as the standard optical density curve. For the immunoprophylaxis study, pellets of 3% of the average body weight of the experimental fish (0.50 g per fish per day) were given daily until day 14 before the challenge test with E. faecalis (2.1 × 10⁹ Colony-forming unit/mL peroral) on day 15. The data from the observation period on days 15-28 were analyzed. For the immunotherapy study, pellets of 3% of the average body weight (0.50 g per fish per day) were given daily for 21 days (days 8-28) 7 day spost-infection. The data from the immunotherapy study were collected during the observation period on days 8-28. Statistical analysis was performed on non-specific immune variables: Total leukocytes, monocytes, lymphocytes, neutrophils, phagocytic activity, and macrophage capacity; and the semi-quantitative distribution of melanomacrophage centers (MMCs) in the lymphoid organs, such as spleen and liver. Photomacrographic data were analyzed descriptively and qualitatively by comparing the healing process and clinical signs found between experiments in the immunotherapy study.

Results

The pellet with 50% egg yolk with an IgY at 2.43 mg/g pellet, 3% of body weight once daily, was the best formula on experimental fish. The administration of this formulation can also increase non-specific immunity and the distribution of MMCs in the spleen and liver with a survival rate of 55% for 14 days of challenge period in the immunoprophylaxis study and 70% for 21 days of therapy period in the immunotherapy study.

Conclusion

Immunoglobulin Y can be a prophylactic and therapeutic agent against streptococcal infections caused E. faecalis in red tilapia with an optimum dosage of 2.43 mg/g pellet.

Acute acidification stress weakens the head kidney immune function of juvenile Lates calcarifer

Acidized water environment can impact many physiological processes of aquatic animals. The response of the head kidney to acidification, especially the immune response, is of great significance to health. This study analyzed the histological and transcriptional changes under different acidification levels (C group, pH 8.1; P group, pH 7.4; E group, pH 3.5) in the short term (12 h, 36 h and 60 h) in the head kidney of juvenile L. calcarifer. The results showed that the acidification of the water environment caused tissue damage to the head kidney of L. calcarifer, and the damage appeared earlier and was stronger in the extreme pH group. The transcriptional response of L. calcarifer head kidney increased with the increase of acidification level. The two treatments transcriptional responses showed different trends in terms of time. After KEGG function enrichment, with the increase of stimulation time, the proportion of down-regulated pathways was increasing, and the types of pathway enrichment at different acidification levels were quite different at the initial stage. At 12 h, the first category in the P group with the most significant number of pathways was 'Metabolism', and the first category in the E group with the largest number of pathways was 'Human Diseases'. At 60 h, the enrichment pathways of the two groups were highly overlapping in immune-related pathways, which contained 26 common DEGs. They had a dominant expression pattern. In the P group, the expression level decreased with time. In the E group, the down-regulation degree of expression level at 12 h reached the level of the P group at 60 h, and the expression level remained low until 60 h. Through the correlation network, interferon regulatory factor 7 (IRF7), Tripartite motif containing-21 (TRIM21), Signal transducer and activator of transcription 1 (STAT1) and Signal transducer and activator of transcription 3 (STAT3) were found to have the most correlation with other genes. In this study, juvenile L. calcarifer showed different coping strategies to different levels of acute acidification stress, but all of them resulted in the extensive weakening of head kidney immune function.

Changes in the Splenic Melanomacrophage Centre Surface Area in Southern Bluefin Tuna (Thunnus maccoyii) Are Associated with Blood Fluke Infections

Melanomacrophage centres (MMCs) are aggregates of macrophages accumulating various pigments. They have been proposed as an indicator of fish immune response. Blood flukes are common parasites in farmed fish. Two cohorts of wild Southern Bluefin Tuna (Thunnus maccoyi) were examined at transfer, before treatment against blood flukes (pre-treatment) and at harvest. MMCs were assessed in histological sections using image analysis, while Cardicola forsteri and Cardicola orientalis infection severity was determined using qPCR, count of adult flukes in heart flushes and count of eggs in gill filaments. Fish from both cohorts showed the same pattern in the changes in the surface area of MMCs. The surface area of splenic MMCs increased over the ranching duration and was positively correlated to the PCR determined copy numbers of Cardicola forsteri ITS2 rDNA in the gills of those fish. However, the infection with blood fluke was more variable, both between cohorts and individuals within the same cohort. Eggs of blood fluke were detected in renal MMCs using histology. Cardicola forsteri had a higher prevalence than Cardicola orientalis. This study contributes to our understanding of blood fluke infections in Southern Bluefin Tuna and their interactions with MMCs.

Relationship between Southern Bluefin Tuna, Thunnus maccoyii, melanomacrophage centres and Cardicola spp. (Trematoda: Aporocotylidae) infection

Southern Bluefin Tuna (SBT), Thunnus maccoyii, is ranched off Port Lincoln, South Australia and is Australia's second largest economic finfish aquaculture industry. The biggest threats to SBT health identified by the industry are the blood flukes Cardicola forsteri and C. orientalis (Trematoda: Aporocotylidae). Melanomacrophage centres (MMCs) are aggregations of pigmented macrophage like cells present in spleen, kidney and liver of teleost fish. The aim of this study was to quantify MMCs in SBT anterior kidney, liver and spleen to investigate changes in relation to Cardicola spp. Infection. Samples were collected at the end of ranching from pontoons where SBT were treated with PZQ and pontoons with untreated SBT. SBT MMC percentage of surface area cover was highest in SBT spleen and lowest in the liver. Significant positive correlations were identified between SBT MMC area and SBT size in all three organs (p < 0.05). MMC area and parasite infection showed significant positive correlations in the kidney and spleen for Cardicola spp. gill egg counts, and in the kidney for C. forsteri DNA from SBT hearts and gills (p < 0.05). MMCs area increased with increased intensity of Cardicola spp. Infection and MMCs have the potential to be used as an indicator to assess health effects that Cardicola spp. have on SBT.

Increased melanomacrophage centres in the liver of reproductively dysfunctional female greater amberjack Seriola dumerili (Risso, 1810)

The greater amberjack Seriola dumerili is a new aquaculture fish that may display reproductive dysfunctions. During extensive follicular atresia, which is a common reproductive dysfunction in females during vitellogenesis, part of the reabsorbed yolk returns to the liver to be metabolized and recycled. Melanomacrophage centres (MMCs) are aggregates of macrophage-like cells that play a role in the destruction, detoxification and recycling of endogenous and exogenous materials, and have been associated with systemic stress. Wild and captive-reared greater amberjack were sampled in the Mediterranean Sea during two different phases of the reproductive cycle. The liver of reproductively dysfunctional captive-reared females sampled during the spawning season showed a high density of both MMCs and apoptotic cells. A weak liver anti-cytochrome P450 monooxygenase 1A immunoreactivity was observed, suggesting that the examined fish were not exposed to environmental pollutants. We propose that the observed increase in MMCs and apoptosis in captive-reared fish was related to the hepatic overload associated to the metabolism of yolk proteins reabsorbed during extensive follicular atresia. Since follicular atresia is a frequent physiological and pathological event in teleosts, we suggest that the reproductive state should be always assessed when MMCs are used as markers of exposure to stress or pollutants.

Melanomacrophages and melanomacrophage centres in Osteichthyes

Melanomacrophages (MMs) are phagocytizing cells with high amounts of pigments including melanin which can be found in a number of cold-blooded species. In Osteichthyes, these cells cluster to form so-called melanomacrophage centres (MMCs), which are predominantly present in the stroma of hematopoietic and lymphoid tissues, that is, in the kidney and spleen. The functionality of MMs and MMCs results from their involvement and role in the defence reactions, related to both the innate and the adaptive immune mechanisms, and in processes unrelated to defence functions as well. There is evidence that MMCs are structurally and functionally similar to mammals' germinal centres (GCs). It appears that mature IgM+ B cells in Osteichthyes can be the equivalent of mIgM+ centrocytes in mammals, whereas MMs can be, in terms of the function, the equivalent of follicular dendritic cells (FDCs), and MMCs can be, in terms of clustered specific cells, the equivalent of GCs. This paper presents selected facts about the structural and functional similarity between GCs and MMCs and about the involvement and role of MMCs and MMs in the immune response. The facts help get a proper picture of the location of MMs and MMCs within the structure of the fish immune system, also in the context of their evolutionary relationship with GCs and of the possibility of pointing out the evolutionary closeness between MMCs in Osteichthyes and GCs in mammals.

Characterisation and 3D structure of melanomacrophage centers in shorthorn sculpins (Myoxocephalus scorpius)

Melanomacrophage centres (MMCs) are distinct aggregations of pigment-containing cells in internal organs of fish, amphibians and reptiles. Although MMCs are commonly used as biomarkers for anthropogenic exposure in many environmental monitoring programs, a substantial knowledge on characteristics of MMCs is required prior to the assessment of MMC responses. The present study was the first to determine the 3D structure of splenic MMCs of a fish from a number of consecutive histology sections by use of the Fiji and AutoCad software. Most splenic MMCs of shorthorn sculpins (Myoxocephalus scorpius) had spherical shape and limited variation in size (maximum diameter). We confirmed the close relationship between MMCs and blood vessels in spleen of shorthorn sculpins as 97% of investigated MMCs (60 whole MMCs over 510 μm thickness of the samples) were closely associated with splenic blood capillaries (mainly ellipsoids) at least once in a set of consecutive sections. In this paper, we describe variations in morphology, density, size, area, distribution, pigments and response to pathogens of MMC populations from different organs (spleen, kidney, liver, pancreas and gills). Additionally, we provide evidence suggesting the presence and dominance of pheomelanin in MMCs of shorthorn sculpins.

Chronic effects of mercury on Bufo gargarizans larvae: Thyroid disruption, liver damage, oxidative stress and lipid metabolism disorder

Mercury is severely detrimental to organisms and is ubiquitous in both terrestrial and aquatic ecosystems. In the present study, we examined the effects of chronic mercury (Hg) exposure on metamorphosis, body size, thyroid microstructures, liver microstructural and ultrastructural features, and transcript levels of genes associated with lipid metabolism, oxidative stress and thyroid hormones signaling pathways of Chinese toad (Bufo gargarizans) tadpoles. Tadpoles were exposed to mercury concentrations at 0, 6, 12, 18, 24 and 30 µg/L from Gosner stage 26-42 of metamorphic climax. The present results showed that high dose mercury (24 and 30 µg/L) decelerated metamorphosis rate and inhibited body size of B. gargarizans larvae. Histological examinations have clearly exhibited that high mercury concentrations caused thyroid gland and liver damages. Moreover, degeneration and disintegration of hepatocytes, mitochondrial vacuolation, and endoplasmic reticulum breakdown were visible in the ultrastructure of liver after high dose mercury treatment. Furthermore, the larvae exposed to high dose mercury demonstrated a significant decrease in type II iodothyronine deiodinase (Dio2) and thyroid hormone receptor α and β (TRα and TRβ) mRNA levels. Transcript level of superoxide dismutase (SOD) and heat shock protein (HSP) were significantly up regulated in larvae exposed to high dose mercury, while transcript level of phospholipid hydroperoxide glutathione peroxidase (PHGPx) was significantly down regulated. Moreover, exposure to high dose mercury significantly down regulated mRNA expression of carnitine palmitoyltransferase (CPT), sterol carrier protein (SCP), acyl-CoA oxidase (ACOX) and peroxisome proliferator-activated receptor α (PPAPα), but significantly up regulated mRNA expression of fatty acid elongase (FAE), fatty acid synthetase (FAS) and Acetyl CoA Carboxylase (ACC). Therefore, we conclude that high dose mercury induced thyroid function disruption, liver oxidative stress and lipid metabolism disorder by damaging thyroid and liver cell structures and altering the expression levels of relevant genes.

Melanomacrophage Centers As a Histological Indicator of Immune Function in Fish and Other Poikilotherms

Melanomacrophage centers (MMCs) are aggregates of highly pigmented phagocytes found primarily in the head kidney and spleen, and occasionally the liver of many vertebrates. Preliminary histological analyses suggested that MMCs are structurally similar to the mammalian germinal center (GC), leading to the hypothesis that the MMC plays a role in the humoral adaptive immune response. For this reason, MMCs are frequently described in the literature as “primitive GCs” or the “evolutionary precursors” to the mammalian GC. However, we argue that this designation may be premature, having been pieced together from mainly descriptive studies in numerous distinct species. This review provides a comprehensive overview of the MMC literature, including a phylogenetic analysis of MMC distribution across vertebrate species. Here, we discuss the current understanding of the MMCs function in immunity and lingering questions. We suggest additional experiments needed to confirm that MMCs serve a GC-like role in fish immunity. Finally, we address the utility of the MMC as a broadly applicable histological indicator of the fish (as well as amphibian and reptilian) immune response in both laboratory and wild populations of both model and non-model vertebrates. We highlight the factors (sex, pollution exposure, stress, stocking density, etc.) that should be considered when using MMCs to study immunity in non-model vertebrates in wild populations.

Histological changes, lipid metabolism and oxidative stress in the liver of Bufo gargarizans exposed to cadmium concentrations

Chinese toad (Bufo gargarizans) were exposed to different concentrations of cadmium (5, 50, 100, 200 and 500 μg Cd L^-1) from Gosner stage 3-42. Metamorphosis rate, body weight, total length and body length were measured. Histological alterations in thyroid gland and liver were examined. Changes in hepatocyte were also examined using Transmission electron microscopic. In addition, the mRNA expression of several genes involved in lipid metabolism, oxidative stress and thyroid hormones signaling pathways were also measured. Our results showed that 200 and 500 μg Cd L^-1 decreased the metamorphosis rate and inhibited the body size of B. gargarizans larvae at G42. Moreover, histological examinations have clearly exhibited that cadmium caused liver damage. Ultrastructural examination revealed lipid accumulation and abnormal mitochondria. Exposure to 200 and 500 μg Cd L^-1 significantly up-regulated mRNA expression of D2, SOD, GPx, ACC and FAE, but down-regulated mRNA expression of TRα, TRβ, PPARα, ACOX, CPT and SCP. However, low Cd concentration (5, 50 and 100) exposure did not cause any effect in genes expression. Thus, we conclude that high Cd concentrations could affect the normal processes of lipid metabolism though increasing lipid synthesis and reducing the ability of fatty acid β-oxidation, and disturb thyroid hormone pathways in liver, and induced oxidative stress. In addition, lipid metabolism might be regulated by THs. To our knowledge, the present study is the first to report the influence of cadmium on hepatic lipid metabolism in B. gargarizans and will greatly provide new insights into cadmium hepatotoxicity in amphibian.