In vitro mitogenesis of peripheral blood lymphocytes from rainbow trout (Salmo gairdneri)

Advances in salmonid fish immunology: A review of methods and techniques for lymphoid tissue and peripheral blood leucocyte isolation and application

Evaluating studies over the past almost 40 years, this review outlines the current knowledge and research gaps in the use of isolated leucocytes in salmonid immunology understanding. This contribution focuses on the techniques used to isolate salmonid immune cells and popular immunological assays. The paper also analyses the use of leucocytes to demonstrate immunomodulation following dietary manipulation, exposure to physical and chemical stressors, effects of pathogens and parasites, vaccine design and application strategies assessment. We also present findings on development of fish immune cell lines and their potential uses in aquaculture immunology. The review recovered 114 studies, where discontinuous density gradient centrifugation (DDGC) with Percoll density gradient was the most popular leucocyte isolation method. Fish head kidney (HK) and peripheral blood (PB) were the main sources of leucocytes, from rainbow trout (Oncorhynchus mykiss) and Atlantic salmon (Salmo salar). Phagocytosis and respiratory burst were the most popular immunological assays. Studies used isolated leucocytes to demonstrate that dietary manipulations enhance fish immunity, while chemical and physical stressors suppress immunity. In addition, parasites, and microbial pathogens depress fish innate immunity and induce pro-inflammatory cytokine gene transcripts production, while vaccines enhance immunity. This review found 10 developed salmonid cell lines, mainly from S. salar and O. mykiss HK tissue, which require fish euthanisation to isolate. In the face of high costs involved with density gradient reagents, the application of hypotonic lysis in conjunction with mico-volume blood methods can potentially reduce research costs, time, and using nonlethal and ethically flexible approaches. Since the targeted literature review for this study retrieved no metabolomics study of leucocytes, indicates that this approach, together with traditional technics and novel flow cytometry could help open new opportunities for in vitro studies in aquaculture immunology and vaccinology.

Procedures for leukocytes isolation from lymphoid tissues and consequences on immune endpoints used to evaluate fish immune status: A case study on roach (Rutilus rutilus)

The effects of two protocols (density gradient versus hypotonic lysis) used for leukocyte isolation from three major lymphoid tissue of fish (head-kidney, spleen and blood) were examined on some cell functional activities (tissue leucocytes distributions, phagocytosis, basal and burst oxidative activities) classically used to estimate the fish immune status. Experiments were conducted on roach (Rutilus rutilus), a cyprinid fish model often studied in different eco-physiological contexts (aquaculture, ecotoxicology …). All of immune endpoints were assessed either immediately after cell isolation or after a 12 h of incubation in order to observe if a post-isolation incubation may influence the leukocytes activities. Compared to the density gradient, hypotonic lysis is associated with granulocytes enrichments of cell suspensions. This is particularly true for leukocyte suspensions isolated from head kidney where granulocytes are naturally abundant. However, important variabilities in leukocyte distributions were observed in head kidney and spleen cells samples obtained by the use of hypotonic lysis for two incubation conditions used (no incubation or 12 h of incubation at 4 °C). The density gradient protocol leads to a transitory increase in basal ROS production in spleen lymphocytes and macrophages The blood leukocytes isolated by this same method exhibit high basal oxidative activities after 12 h of incubation at 4 °C and for the three leukocyte types (lymphocytes, monocytes and granulocytes). The hypotonic lysis is associated with an increase in PMA-induced ROS production especially in head kidney leukocytes. The increases in cell oxidative activities are consistent with increases in granulocyte proportions observed in leukocyte suspensions obtained by hypotonic lysis. Finally, the two protocols have no effect on leukocyte mortality and phagocytic activity. Within limits of our experimental conditions, the spleen is the organ whose leukocyte oxidative activities (stimulated or not) are only slightly influenced by the methods used for leukocyte isolation. This is also the case for the anterior kidney, but for this tissue, it is necessary to incubate the isolated cells for 12 h at 4 °C before functional analyses. Each of the two methodologies used has advantages and disadvantages. The hypotonic lysis allows to isolate a greater variety of leukocytes types whereas the density gradient used ensures a better stability of cells distributions over time. However, for the same fish species and for the same tissue, the method used to isolate leukocytes influences results and must be taken into consideration during acquired data analysis for evaluation of fish immune status.

Pathogen recognition and mechanisms in Atlantic cod (Gadus morhua) head kidney cells: bacteria (LPS) and virus (poly I:C) signals through different pathways and affect distinct genes

Understanding pathogen recognition and mechanisms in Atlantic cod are of significant importance for both basic research on wild populations and health management in aquaculture. A microarray approach was utilized to search for effects of viral (polyinosinic acid:polycytidylic acid), bacterial (lipopolysaccharide) and polyclonal activator (phytohaemoagglutinin) stress in Atlantic cod head kidney cells. LPS cell activation increased mRNA expression of interleukin 8; interleukin-1β; cyclooxygenase 2; leukocyte derived chemotaxin 2; carboxyl-esterase 2 and environmental biomarker cytochrome P450 1A. Mitogen activated protein kinase p38 and cathepsin F were down regulated by LPS. The antiviral responses induced by double stranded RNA clearly increased transcription of Toll like receptor 3 and interferon stimulating gene 15. The phytohaemoagglutinin response seemed to be more non-specific. Special for the phytohaemoagglutinin induction was the increase in major histocompatibility complex class I. CC chemokine type 2 mRNA expression was increased by phytohaemoagglutinin, lipopolysaccharide and polyinosinic acid:polycytidylic acid, while mitogen activated protein kinase p38 and leukocyte derived chemotaxin 2 were down regulated by phytohaemoagglutinin. Oxidative stress related genes like catalase and glutaredoxin and the anti-apoptotic gene Bcl-2 showed no transcriptional changes compared to control in any of the treatments. Eicosanoids like prostaglandin 2, leukotriene B4 and B5 were constitutively produced by cod head kidney cells in vitro. The most remarkable feature of eicosanoid secretion is the higher production of leukotrienes against prostaglandins, indicating that the lipooxygenase pathway is preferred over the cyclooxygenase pathway. Although there were no significant differences in eicosanoid secretion between the groups, polyinosinic acid:polycytidylic acid showed a clear tendency to increase the levels of leukotriene B4 and B5. This study reveals distinct signatures of bacteria and virus transcriptional responses in cod head kidney cells. In addition, the novel finding that cytochrome P450 1A was upregulated during the antibacterial response indicates a connection between immunity and aryl hydrocarbon receptor activation in Atlantic cod.

Fish microsporidia: immune response, immunomodulation and vaccination

Immune response to fish microsporidia is still unknown and there are current research trying to elucidate the events involved in the immune response to this parasite. There is evidence suggesting the role of innate immune response and it is clear that adaptive immunity plays an essential part for eliminating and then mounting a solid resistance against subsequent microsporidian infections. This review article discusses the main mechanisms of resistance to fish microsporidia, which are considered under four main headings. 1) Innate immunity: the inflammatory tissue reaction associated with fish microsporidiosis has been studied at the ultrastructural level, providing identification of many of the inflammatory cells and molecules that are actively participating in the spore elimination, such as macrophages, neutrophils, eosinophilic granular cells, soluble factors and MHC molecules. 2) Adaptive immunity: the study of the humoral response is relatively new and controversial. In some cases, the antibody response is well established and it has a protective role, while in other situations, the immune response is not protective or it is depressed. Study of the cellular response against fish microsporidia is still in its infancy. Although the nature of the microsporidian infection suggests participation of cellular mechanisms, few studies have focused on the cellular immune response of infected fish. 3) Immunomodulation: glucans are compounds that can modulate the immune system and potentiate resistance to microorganisms. These compounds have been proposed that can interact with receptors on the surface of leukocytes that result in the stimulation on non-specific immune responses. 4) Vaccination: little is known about a biological product that could be used as a vaccine for preventing this infection in fish. In the Loma salmonae experience, one of the arguments that favor the production of a vaccine is the development in fish of resistance, associated to a cellular immune response. A recently proved spore-based vaccine to prevent microsporidial gill disease in salmon has recently shown its efficacy by considerably reducing the incidence of infection. This recent discovery would be first anti-microsporidian vaccine that is effective against this elusive parasite.

Cell markers and determinants in fish immunology

Despite the impressive increase in the cloning and expression of genes encoding fish immunoregulatory molecules, the knowledge on "in vivo" and "in vitro" functional immunology of the corresponding peptide products is still at an initial stage. This is partly due to the lacking of specific markers for immunoregulatory peptides, that represent an indispensible tool to dissect immune reactions and to trace the fate of cellular events downstream of the activation. In this review we summarise the available information on functional immune activities of some teleost species and discuss the obtained data in an evolutionary and applied context.

Cellular immunity in salmonids infected with the microsporidial parasite Loma salmonae or exposed to non-viable spores

Following a per os challenge of naive rainbow trout with live spores of Loma salmonae, head kidney mononuclear cells (MNC) in culture were able to proliferate in response to crude soluble parasite extract or intact dead spores. A significant response was seen by week 2 post-exposure and a maximum response developed by week 6 or 8, respectively. During this initial challenge, spore filled cysts developed on the gills of challenged fish, and the cysts ruptured by week 12 as is typical for microsporidial gill disease of salmonids (MGDS). Two weeks following this, fish were re-challenged with live spores, and in these fish an enhanced in vitro proliferative response of MNC was immediately apparent, and spore filled cysts did not develop. In contrast, when naive trout were given dead spores by intraperitoneal injection, the most pronounced proliferative responses of MNC developed earlier (week 2 PE) and the response was greater when cells were incubated in vitro with dead spores rather than with crude soluble extract. When these fish were re-challenged per os with live spores, a heightened proliferation in MNC was observed 4 weeks after this exposure and the fish likewise resisted development of xenomas. In fish infected orally or injected intraperitoneally with spores, a marked increase in the response to the mitogen concanavalin A was seen for 22 weeks post-exposure when compared to controls not receiving any spores.

Characterization of a galactose binding serum lectin from the Indian catfish, Clarias batrachus: possible involvement of fish lectins in differential recognition of pathogens

A lectin with molecular mass around 200 kDa was isolated from the serum of the Indian catfish Clarias batrachus. The bioactivity of this serum lectin was Ca2+ and pH dependent. The lectin appeared to be specific for alpha-methyl galactose and sialoglycoproteins like porcine and bovine submaxillary mucin and could agglutinate human, rabbit, mice, rat and chicken erythrocytes. This fish lectin was able to specifically agglutinate different gram negative bacteria. When it was checked against different strains of the fish pathogen Aeromonas sp., it significantly altered the viability and pathogenicity of the bacteria. Binding of the lectin to Aeromonas sp., resulted in a dose dependent increase in the bactericidal activity of fish macrophages. However, when the lectin was checked against different gram positive bacteria it could not agglutinate or affect the viability of those strains and also failed to bring about any significant change in the bactericidal potential of fish macrophages. The lectin was able to induce the proliferation of head kidney lymphocytes of Clarias and helped in the release of 'IL-1' like cytokines from head kidney macrophages.

Rainbow trout (Oncorhynchus mykiss) sIgM- leucocytes secrete an interleukin-2 like growth factor after mitogenic stimulation in vitro

Two secreted proteins were detected in culture supernatants of PHA or PMA stimulated, immunomagnetically separated, sIgM(-) leucocytes of rainbow trout (Oncorhynchus mykiss) with 60kDa and with 12-15kDa (multiple bands). So called conditioned media (CM), containing these proteins, induced significant activation of blood and head kidney leucocytes. Immunomagnetically separated, naive as well as PHA activated sIgM(-) T lymphocytes and LPS prestimulated sIgM(+) B lymphocytes could be identified to be responding to these secreted proteins. Using a monoclonal antibody specific for mouse IL-2 (clone JES6-1A12), one of the multiple 12-15kDa proteins could be stained in Western blots. It was also shown that the induced proliferation was due to this protein in the CM, as the same anti-IL-2 mab was able to block the CM induced proliferation. Furthermore, survival of the IL-2 dependent mouse cell line HT-2 was enhanced after addition of various concentrations of CM. The data presented show, for the first time, that mitogen stimulated trout sIgM(-) leucocytes secrete a cytokine like growth factor sharing functional and structural similarities with mammalian IL-2.

Improved fish lymphocyte culture for chromosome preparation

Cytogenetic methodology is still underdeveloped in fishes compared with mammals. Culture condition for fish lymphocytes was optimized to improve chromosome preparation using the rainbow trout (Oncorhynchus mykiss) as a model after changing the combination of parameters such as mitogens, incubation periods, media, cell components, and freshness of blood. The optimized culture condition included isolation of lymphocytes from fresh blood by a stirring method, their culture in medium 199 supplemented with 10% FBS, 18 microg/ml of phytohemagglutinin (PHA-W) and 100 microg/ml of lipopolysaccharide (LPS) as mitogens, and harvested at 6 days after culture. This condition provided a notably increased mitotic index (MI) of 4.3-10.0% in rainbow trout lymphocytes. In addition, the condition was highly reproducible as shown by the similar level of MI in cultured lymphocytes from 181 individuals without failure. Applicability of this method in a wide range of fish groups was also proven with Ml of 1.1-13.3% in cultured lymphocytes from other 16 freshwater species of Acipenseridae, Anguillidae, Solmonidae, Cyprinidae, and Centrarchidae, and five marine species of Sparidae, Kyphosidae, Paralichthyidae, and Scorpaenidae. Chromosome preparations of improved quality by the present method were successfully applied for the replication R-banding with incorporation of 5-bromo-2'-deoxyuridine and direct R-banding fluorescence in situ hybridization.

Effects of mercuric chloride and sodium selenite on some immune responses of blue gourami, Trichogaster trichopterus (Pallus)

The immunotoxicological effects of mercuric chloride and sodium selenite on blue gourami were studied. Some immune responses ranging from non-specific to specific were investigated. These include tissue lysozyme activity, kidney lymphocyte proliferation and plasma agglutinating antibody titre against bacteria. After 2 weeks of chronic exposure, 0.09 mg/l of Hg2+ alone induced a significant increase of kidney lysozyme activity of 4196.3 +/- 1171.0 U/g, but it decreased to 1577.4 +/- 902.4 U/g when exposed simultaneously to equiconcentration of selenium. Plasma lysozyme activity was also increased by co-administration of Hg2+ and SeO3(2-). The level of plasma agglutinating antibody against Aeromonas hydrophila L37 was lowered in the chemical-treated fish. This indicates that the fish immunity was impaired by action of mercury and selenium. However, the in vitro lymphocyte proliferation test shows that mercury concentration lower than 0.045 mg/l Hg2+ enhanced the mitotic rate of kidney lymphocytes by approximately 30%. A high concentration of mercury caused irreversible damaging effects on con A-induced lymphoblastogenesis. In contrast, the inhibitory effect of low concentrations of mercury could be removed by washing. On the other hand, selenium showed a suppressive effect on the lymphocyte proliferation even at 0.5 mg/l.

Effect of the microsporidian Enterocytozoon salmonis on the immune response of chinook salmon

The protozoan parasite Enterocytozoon salmonis is an intranuclear microsporidian of salmonid mononuclear leukocytes. Experimental infections were initiated in chinook salmon to determine the effects of parasitism on selected host immune functions. The humoral antibody response to dinitrophenylated-keyhole limpet hemocyanin and the in vitro blastogenic responses of isolated mononuclear leukocytes to mitogens (concanavalin A, lipopolysaccharide and phytohemagglutinin-P) were evaluated. The humoral response as detected by enzyme-linked immunosorbent assays was suppressed following infection. The degree of suppression increased as the severity of the infection progressed. Additionally, the response to mitogen-induced lymphoproliferation was impaired. These results suggest that infection with E. salmonis may cause suppression of host cell immune functions, thus increasing the susceptibly of infected fish to other pathogens.

The effect of eicosanoids on rainbow trout, Oncorhynchus mykiss, leucocyte proliferation

Proliferation of rainbow trout head kidney leucocytes in response to the mitogen phytohaemagglutinin-P (PHA-P) was modulated in the presence of inhibitors of eicosanoid synthesis and by exogenous eicosanoids. The presence of indomethacin, a cyclooxygenase inhibitor, resulted in a stimulatory effect, whereas the presence of nordihydroguiaretic acid, a lipoxygenase inhibitor, resulted in an inhibitory effect on mitogenicity. The addition of prostaglandins and lipoxins was also found to be inhibitory, whilst the addition of leukotrienes was stimulatory. Some class/series effects of the eicosanoids were also apparent. Prostaglandin E2 was a more potent inhibitor than prostaglandin E3, and proliferation was more sensitive to the effects of leukotriene B4 than to leukotriene B5. Whilst PHA-P was able to directly induce the release of prostaglandins from head kidney leucocytes, it did not induce the release of lipoxygenase products.

Mitogenesis of rainbow trout peripheral blood lymphocytes requires homologous plasma for optimal responsiveness

An improved method for obtaining optimal mitogenic responses in peripheral blood lymphocytes has been devised by utilizing autologous or homologous rainbow trout plasma. The use of 10% plasma in culture results in up to a 60-fold increase in the proliferative potential of the peripheral blood lymphocyte response to lipopolysaccharide when compared to the more routinely used fetal bovine serum. Furthermore, it has been observed that lymphocytes which were unresponsive to in vitro mitogenic challenge when cultured in fetal bovine serum, responded well when cultured in the presence of trout plasma. Also in contrast to previous mitogen studies where maximal stimulation was reported to occur on Day 4-5 of culture, the stimulatory effects of lipopolysaccharide were greatest on Day 10 when plasma was employed. Together these data suggest that former conditions of lymphocyte cell culture, employing only fetal bovine serum, not only fail to provide the optimal conditions for cell growth, but in many cases the essential conditions. However, these requirements are met by supplementation with trout plasma, which seems to contain heat-stable factors responsible for the enhanced mitogenic responsiveness.