Factors influencing in vitro respiratory burst assays with head kidney leucocytes from rainbow trout, Oncorhynchus mykiss (Walbaum)

Modulation of immunity and hepatic antioxidant defense by corticosteroids in pacu (Piaractus mesopotamicus)

We investigated the impact of both the oral administration of hydrocortisone (HC) and an acute stressor on stress, innate immune responses and antioxidant system/oxidative stress responses of juvenile Piaractus mesopotamicus. Fish were either 1) given a commercial feed (C), 2) given a feed supplemented with 400 mg/kg HC, or 3) fed a commercial feed, chased for 2 min and exposed to air for 4 min (S). After initial sampling, fish C and HC were fed and sampled 1, 3, 6, 24 and 72 h post-feeding. Fish S were fed at the same time as the other groups, exposed to a stressor, and sampled 1, 3, 6, 24 and 72 h after. Exposure to the stressor increased circulating glucose and cortisol levels (at 1 and 3 h, respectively), while oral HC increased circulating cortisol at 1 h and glucose at 3 h. The stressor activated respiratory activity of leukocytes (RAL) at 3 h and reduced it at 6 h. HC did not activate RAL, but it did impair it at 6 h. The serum hemolytic activity of the complement system (HAC₅₀) was impaired by the stressor at 1 and 3 h and by HC at 1 h. Regarding the antioxidant system, exposure to the stressor reduced glutathione peroxidase (GPx) and catalase (CAT) activity and decreased concentrations of reduced glutathione (GSH) in the liver up to 6 h. HC only impaired GPx. Additionally, stress induced the accumulation of melano-macrophage (MM) and melano-macrophage centers (MMC), which are biomarkers of oxidative stress, in the spleen. Differences in biomarkers in fish given cortisol and exposed to stress indicate that exogenous hormone was unable to precisely reproduce stress responses.

Phenol-rich fulvic acid as a water additive enhances growth, reduces stress, and stimulates the immune system of fish in aquaculture

Aquaculture has become imperative to cover the demands for dietary animal protein. Simultaneously, it has to overcome prejudices from excessive use of antibiotics and environmental impacts. Natural supplements are traditionally applied orally. In this study, we demonstrated another pathway: the gills. Humic substances are immunostimulants and a natural part of every aquatic ecosystem, making them ideal to be used as bath stimulants. Five and 50 mg C/L of a fulvic acid-rich humic substance was added for 28 days to the water of juvenile rainbow trout (Oncorhynchus mykiss). This fulvic acid is characterized by a high content of phenolic moieties with persistent free radicals and a high electron exchange capacity. The high concentration of the fulvic acid significantly increased growth and reduced the food conversion ratio and the response to a handling-stressor. Phagocytosis and potential killing activity of head kidney leukocytes were increased, as well as the total oxyradical scavenging capacity (TOSC) and lysozyme activity in the gills. In conclusion, immunostimulation via gills is possible with our fulvic acid, and the high phenolic content improved overall health and stress resistance of fish.

β-Glucan enhances respiratory activity of leukocytes suppressed by stress and modulates blood glucose levels in pacu (Piaractus mesopotamicus)

We evaluated the immune response of pacu fed with a β-glucan diet (0.5%) for 10 days. After the feeding period, fish were subjected to handling and 3 h after, inoculated with Aeromonas hydrophila. Fish were sampled before handling (baseline condition), 3, 6, and 24 h and 1 week after inoculation. A higher level of blood glucose was found in fish treated with β-glucan in baseline conditions. Handling and bacterial inoculation increased the circulating levels of cortisol and glucose and promoted the acute inflammatory response (lymphopenia and neutrophilia). β-Glucan prevented the decrease in the respiratory activity of leukocytes observed in the control group at 3 h sampling. β-Glucan did not affect the complement and lysozyme, which were activated 24 h after the bacterial challenge in control fish. A reduction in the number of leukocytes was found in fish treated with β-glucan 1 week after the challenge. We suggest two plausible hypotheses for this event: (1) it could be attributed to a depletion of the immune responses or (2) it could be due to a mobilization of the leukocytes to the spleen for antigen presenting/processing. In general, β-glucan avoided the reduction of the activity of leukocytes after stress and the bacterial challenge and increased the baseline glucose levels. Our findings confirm the immunomodulatory action of glucan and add evidence showing that glucan can have a role in stress response.

Protec™ improves innate immune response and specific antibody response against Lactococcus garvieae in rainbow trout (Oncorhynchus mykiss)

Protec^™ is a commercial aquafeed (Skretting Italia) containing a combination of glucans, vitamin C, vitamin E and zinc (immune support pack). No research information concerning its capability to improve fish immune response is available, so in this study the potential immunomodulatory effects of Protec^™ were investigated in rainbow trout (Oncorhynchus mykiss). Head kidney (HK) leukocytes from adult fish (100 g, n = 6) were in vitro incubated with Protec^™ immune support pack resulting in significantly higher respiratory burst activity and proliferation. Specifically, sonicated Protec^™ immune support pack (160 μg/ml) induced a respiratory burst response similar to that promoted by zymosan and lipopolysaccharide (LPS), while non-sonicated Protec^™ immune support pack induced a response comparable to that of cells stimulated with phorbol myristate acetate (PMA). Moreover, the proliferation of leukocytes exposed to sonicated Protec^™ immune support pack (20 μg/ml) was significantly higher than that of cells stimulated with zymosan, and it was comparable to the proliferation of cells stimulated with phytohaemagglutinin (PHA) and LPS. Afterwards, a feeding trial was performed in a rainbow trout farm. Two groups of juvenile rainbow trout (10 g) were acclimated for 7 weeks before the experiment and fed daily with a commercial control diet (Optiline HE, Skretting Italia) at 2% BW/day. At the end of acclimation, one group of fish was fed with Protec^™ diet (Skretting Italia) at 2% BW/day whereas the other group continued to feed the control diet at the same level for further 4 weeks. Then, fish were sampled (HK leukocytes from n = 6 fish/group, serum from n = 12 fish/group) or intraperitoneally vaccinated against lactococcosis (n = 160/dietary group/time point). Fish fed the same diets for further 4 weeks after vaccination, then feeding returned to the control diet in both groups until the end of the trial. The specific antibody response was recorded at 4 and 8 weeks after vaccination (n = 12 fish/group). The administration of Protec^™ significantly enhanced the respiratory burst activity of leukocytes and the synthesis of specific IgM against Lactococcus garvieae, whereas the serum lysozyme activity was unaffected. The present research suggests that the administration of Protec^™ can improve both innate and adaptive immune response of rainbow trout, proving to be an interesting strategy for enhancing the immune reactivity of fish to vaccines.

Modulation of stress and innate immune response by corticosteroids in pacu (Piaractus mesopotamicus)

Understanding how stress and corticosteroid modulates the innate immune response is one of the keys to improving productivity and reducing losses in intensive aquaculture. Thus, we investigated the effects of dietary corticosteroids (7 days; long-term exposure) and transport (4 h; short-term stress) on stress and innate immune response in pacu. For this end, fish were fed with diets containing dexamethasone (100 mg kg^-1) or hydrocortisone (200 mg kg^-1), followed by transport, and then were intraperitoneally inoculated with heat-killed Aeromonas hydrophila or PBS (sham-inoculation). Fish were sampled after a 7-day feeding period, immediately post-transport and 24 h post-transport and inoculation. The dietary treatment of corticosteroids decreased resting cortisol levels by inhibiting the production of cortisol on the hypothalamus pituitary interrenal-axis. Further, both corticosteroids reduced hematocrit, red blood cells, haemoglobin and hemolytic activity of the complement, while they increased glucose levels and serum lysozyme concentrations. The transport increased cortisol and glucose levels and reduced the humoral immune defenses such as serum lysozyme concentration and hemolytic activity of the complement system. Interestingly, the hemolytic activity of the complement system increased sharply in fish fed with corticosteroids immediately post-transport, when they had their HPI-axis partially suppressed by the corticosteroids. This finding suggests a stimulatory effect of the catecholamines released during the transport on the activity of the complement system. Our results are highly valuable to understanding the stress and innate immune responses to long-term exposure to corticosteroids and short-term stress in fish and may provide insights into how corticosteroids modulate the innate immune system.

Influence of trans-resveratrol on macrophage and lymphocyte activity in rainbow trout (Oncorhynchus mykiss) - in vitro study

The objective of this study was to investigate the effect of trans-resveratrol, a potent antioxidant with anti-inflammatory and chemopreventive properties, naturally occurring in many fruits and plants on lymphocytes proliferation and also on macrophages metabolic and phagocytic activity. The aim of this study was to demonstrate the immunomodulatory effects of the compound on fish immunocompetent cells and determine the type of this interaction (immunosuppression or immunostimulation). Proliferative activity of lymphocytes was studied by MTT assay, and the respiratory burst was evaluated using the respiratory burst activity (RBA) test. Phagocytic killing was tested using the PKA test. The experiment have shown that trans-resveratrol suppressed blood B cells, while there was no significant influence on blood T lymphocytes. However, insignificant stimulatory effect occurred at the lowest concentration. In addition, the compound inhibited proliferation of T and B lymphocytes isolated from the organs. Importantly, trans-resveratrol caused stimulation of blood and organs macrophages phagocytic killing, and also increased the respiratory burst of macrophages isolated from organ. These results suggest a potential use of trans-resveratrol as an immunomodulator of innate immunity in fish. This is particularly important, as this kind of resistance plays leading role in protecting the body against infection. In comparison, adaptive immunity is slower and also much less precise.

Medicinal plant extracts modulate respiratory burst and proliferation activity of rainbow trout (Oncorhynchus mykiss) leukocytes

The present study was designed to investigate the immunomodulatory effects of Aloe vera, Curcuma longa, Echinacea purpurea, Lavandula officinalis, Origanum vulgare, Panax ginseng, and Rheum officinale extracts on leukocytes purified from rainbow trout (Oncorhynchus mykiss) head kidney. The cells were cultured in a medium containing increasing doses of extracts; afterwards, they were tested for reactive oxygen species production after stimulation with phorbol myristate acetate (PMA) and proliferation in the presence or absence of phytohemagglutinin from Phaseolus vulgaris (PHA-P). After a 2-h exposure, the extracts of L. officinalis, O. vulgare, and R. officinale strongly reduced the oxidative burst activity of PMA-stimulated leukocytes, in a dose-dependent manner (P ≤ 0.05). A. vera, C. longa, E. purpurea, and P. ginseng extracts reduced this response with lower efficacy and especially at lower concentrations. On the contrary, the highest concentration of ginseng extract stimulated the respiratory burst of leukocytes compared to untreated control cells. After a 72-h exposure, the extracts of L. officinalis, R. officinale, C. longa, E. purpurea, and P. ginseng had a clear dose-dependent stimulatory effect on leukocyte proliferation (P ≤ 0.05). The results suggest that these medicinal plants can be considered as reliable sources of new antioxidants or immunostimulants to be used in aquaculture.

Antimicrobial and host cell-directed activities of Gly/Ser-rich peptides from salmonid cathelicidins

Cathelicidins, a major family of vertebrate antimicrobial peptides (AMPs), have a recognized role in the first line of defense against infections. They have been identified in several salmonid species, where the putative mature peptides are unusually long and rich in serine and glycine residues, often arranged in short multiple repeats (RLGGGS/RPGGGS) intercalated by hydrophobic motifs. Fragments of 24-40 residues, spanning specific motifs and conserved sequences in grayling or brown, rainbow and brook trout, were chemically synthesized and examined for antimicrobial activity against relevant Gram-positive and Gram-negative salmonid pathogens, as well as laboratory reference strains. They were not active in complete medium, but showed varying potency and activity spectra in diluted media. Bacterial membrane permeabilization also occurred only under these conditions and was indicated by rapid propidium iodide uptake in peptide-treated bacteria. However, circular dichroism analyses indicated that they did not significantly adopt ordered conformations in membrane-like environments. The peptides were not hemolytic or cytotoxic to trout cells, including freshly purified head kidney leukocytes (HKL) and the fibroblastic RTG-2 cell line. Notably, when exposed to them, HKL showed increased metabolic activity, while a growth-promoting effect was observed on RTG-2 cells, suggesting a functional interaction of salmonid cathelicidins with host cells similar to that shown by mammalian ones. The three most active peptides produced a dose-dependent increase in phagocytic uptake by HKL simultaneously stimulated with bacterial particles. The peptide STF(1-37), selected for further analyses, also enhanced phagocytic uptake in the presence of autologous serum, and increased intracellular killing of live E. coli. Furthermore, when tested on HKL in combination with the immunostimulant β-glucan, it synergistically potentiated both phagocytic uptake and the respiratory burst response, activities that play a key role in fish immunity. Collectively, these data point to a role of salmonid cathelicidins as modulators of fish microbicidal mechanisms beyond a salt-sensitive antimicrobial activity, and encourage further studies also in view of potential applications in aquaculture.

Seasonal changes in immune parameters of rainbow trout (Oncorhynchus mykiss), brook trout (Salvelinus fontinalis) and brook trout × Arctic charr hybrids (Salvelinus fontinalis × Salvelinus alpinus alpinus)

Despite the high number of studies concerning seasonality of immune response in fish, information for some fish species is still scarce. Here, we assess seasonal changes in leukocyte counts and several immune parameters in three groups of farmed salmonids, i.e. brook trout (Salvelinus fontinalis), brook trout x Arctic charr hybrids (Salvelinus fontinalis x Salvelinus alpinus alpinus) and rainbow trout (Oncorhynchus mykiss) reared under the same conditions and fed with the same feed. Fish were sampled in five periods of the year (late April, early July, late August, early November and early February) and leukocyte counts, respiratory burst of blood phagocytes, lysozyme concentration in skin mucus and total complement activity were measured. Generalized linear models using fish body length as a continuous predictor and sampling period and fish species as categorical predictors, were significant for each of the parameters analysed. The highest seasonal variations in measured parameters were found in rainbow trout and lowest in hybrids. Our results confirm that measures of innate and adaptive immunity are strongly affected by season in all three groups of salmonids. The results will contribute to the improved assessment of immunocompetence in farmed fishes, essential for future sustainable development in aquaculture.

Purified deoxynivalenol or feed restriction reduces mortality in rainbow trout, Oncorhynchus mykiss (Walbaum), with experimental bacterial coldwater disease but biologically relevant concentrations of deoxynivalenol do not impair the growth of Flavobacterium psychrophilum

Diets containing deoxynivalenol (DON) were fed to rainbow trout Oncorhynchus mykiss (Walbaum) for 4 weeks followed by experimental infection (intraperitoneal) with Flavobacterium psychrophilum (4.1 × 10(6) colony-forming units [CFU] mL(-1) ). Mortality of rainbow trout fed either 6.4 mg kg(-1) DON or trout pair-fed the control diet was significantly reduced (P < 0.05) in comparison with trout fed the control diet to apparent satiation (<0.1 mg kg(-1) DON). In a second experiment, trout were fed one of three experimental diets; a control diet, a diet produced with corn naturally contaminated with DON (3.3 mg kg(-1) DON) or a diet containing purified DON (3.8 mg kg(-1) ); however, these fish were not experimentally infected. The presence of DON resulted in significant reduction (P < 0.0001) in feed intake as well as weight gain after 4 weeks. Respiratory burst of head-kidney leucocytes isolated from rainbow trout fed diets containing purified DON (3.8 mg kg(-1) ) was significantly higher (P < 0.05) at 35 day post-exposure compared with controls. The antimicrobial activity of DON was examined by subjecting F. psychrophilum in vitro to serial dilutions of the chemical. Complete inhibition occurred at a concentration of 75 mg L(-1) DON, but no effect was observed below this concentration (0-30 mg L(-1) ).

Comparative evaluation of infection methods and environmental factors on challenge success: Aeromonas salmonicida infection in vaccinated rainbow trout

When testing vaccine-induced protection an effective and reliable challenge method is a basic requirement and we here present a comparative study on different challenge methods used for infection of rainbow trout Oncorhynchus mykiss with Aeromonas salmonicida, a bacterial pathogen eliciting furunculosis. Fish were vaccinated with three different adjuvanted trivalent vaccines containing formalin killed A. salmonicida, Vibrio anguillarum O1 and O2a. These were 1) the commercial vaccine Alpha Ject 3000, 2) an experimental vaccine with water in paraffin oil adjuvant, 3) an experimental vaccine with water in paraffin oil in water adjuvant. Fish were then exposed to A. salmonicida challenge using i.p. injection, cohabitation in freshwater, cohabitation in saltwater (15 ppt) or combined fresh/saltwater cohabitation. Cohabitation reflects a more natural infection mode and was shown to give better differentiation of vaccine types compared to i.p. injection of live bacteria. The latter infection mode is less successful probably due to the intra-abdominal inflammatory reactions (characterized in this study according to the Speilberg scale) induced by i.p. vaccination whereby injected live bacteria more effectively become inactivated at the site of injection. Compared to cohabitation in freshwater, cohabitation in saltwater was less efficient probably due to reduced survivability of A. salmonicida in saltwater, which was also experimentally verified in vitro.

The in vitro effect of kynurenic acid on the rainbow trout (Oncorhynchus mykiss) leukocyte and splenocyte activity

Kynurenic acid (KYNA), an endogenous neuroprotectant formed along the kynurenine pathway of tryptophan degradation, is a selective ligand of the GPR35 receptor, which can be found on the surface of various populations of human immune cells. In infections and inflammations, KYNA produces an anti-inflammatory effect through this receptor, by depressing the synthesis of reactive oxygen species and pro-inflammatory cytokines. However, it is still unrecognized whether receptors for kynurenic acid are also localized on immune cells of poikilothermic animals, or whether KYNA is able to affect these cells. The objective of this study has been to determine the effect of different concentrations of kynurenic acid (12.5 microM to 10 mM) on the viability and mitogenic response of lymphocytes and on the activity of phagocytic cells isolated from blood and the spleen of rainbow trout. The results imply low toxicity of kynurenic acid towards fish immune cells, and the proliferative effect observed at the two lowest concentrations of KYNA (12.5 microM and 25 microM) seems indicative of endogenous kynurenic acid being capable of activating fish lymphocytes. Non-toxic, micromole concentrations of KYNA, however, had no influence on the mitogenic response of lymphocytes nor on the activity of phagocytes in rainbow trout under in vitro conditions. There is some likelihood that such an effect could be observed at lower, nanomole concentrations of KYNA.

Heat and immunity: an experimental heat wave alters immune functions in three-spined sticklebacks (Gasterosteus aculeatus)

Global climate change is predicted to lead to increased temperatures and more extreme climatic events. This may influence host-parasite interactions, immunity and therefore the impact of infectious diseases on ecosystems. However, little is known about the effects of rising temperatures on immune defence, in particular in ectothermic animals, where the immune system is directly exposed to external temperature change. Fish are ideal models for studying the effect of temperature on immunity, because they are poikilothermic, but possess a complete vertebrate immune system with both innate and adaptive immunity. We used three-spined sticklebacks ( Gasterosteus aculeatus) originating from a stream and a pond, whereby the latter supposedly were adapted to higher temperature variation. We studied the effect of increasing and decreasing temperatures and a simulated heat wave with subsequent recovery on body condition and immune parameters. We hypothesized that the immune system might be less active at low temperatures, but will be even more suppressed at temperatures towards the upper tolerable temperature range. Contrary to our expectation, we found innate and adaptive immune activity to be highest at a temperature as low as 13 °C. Exposure to a simulated heat wave induced long-lasting immune disorders, in particular in a stickleback population that might be less adapted to temperature variation in its natural environment. The results show that the activity of the immune system of an ectothermic animal species is temperature dependent and suggest that heat waves associated with global warming may immunocompromise host species, thereby potentially facilitating the spread of infectious diseases.

β-glucan enriched bath directly stimulates the wound healing process in common carp (Cyprinus carpio L.)

Wound healing is a complex and well-organized process in which physiological factors and immune mechanisms are involved. A number of different immune modulators have been found to enhance the non-specific defence system in vertebrates, among which β-glucans are the most powerful and extensively investigated. The aim of the present study was to investigate the biological impact of two different commercially available β glucan containing products on the wound healing process in carp. Throughout a two week experiment fish were kept either untreated (control), or in water supplemented with the two different types of β-glucans. The wound healing process was monitored using a multispectral visualisation system. The correlation between wound closure and immune response was investigated by measuring the gene expression patterns of IL-1β, IL-6 family member M17, IL-8 and Muc5b, and measurement of production of radical oxygen species. PAMPs/DAMPs stimulation caused by the wounding and or β-glucans resulted in an inflammatory response by activating IL-1β, IL-6 family member M17 and IL-8 and differences in the expression pattern were seen depending on stimuli. IL-1β, IL-6 family member M17 and IL-8 were activated in all wounds regardless of treatment. Expression of all three interleukins was highly up regulated in control wounded muscle already at day 1 post-wounding and decreased at subsequent time-points. The reverse was the case with control wounded skin, where expression increased from day 1 through day 14. The results for the β-glucan treated wounds were more complex. The images showed significantly faster wound contraction in both treated groups compared to the control. The obtained results clearly demonstrated that a β glucan enriched bath promotes the closure of wounds in common carp and induce a local change in cytokine expression.

Effects of excretory/secretory products from Anisakis simplex (Nematoda) on immune gene expression in rainbow trout (Oncorhynchus mykiss)

Excretory/secretory (ES) products are molecules produced by parasitic nematodes, including larval Anisakis simplex, a parasite occurring in numerous marine fish hosts. The effects of these substances on host physiology have not been fully described. The present work elucidates the influence of ES substances on the fish immune system by measuring immune gene expression in spleen and liver of rainbow trout (Oncorhynchus mykiss) injected intraperitoneally with ES products isolated from A. simplex third stage larvae. The overall gene expression profile of exposed fish showed a generalized down-regulation of the immune genes tested, suggesting a role of ES proteins in immunomodulation. We also tested the enzymatic activity of the ES proteins and found that lipase, esterase/lipase, valine and cysteine arylamidases, naphthol-AS-BI-phosphohydrolase and α-galactosidase activities were present in the ES solution. This type of hydrolytic enzyme activity may play a role in nematode penetration of host tissue. In addition, based on the notion that A. simplex ES products may have an immune-depressive effect (by minimizing immune gene expression) it could also be suggested that worm enzymes directly target host immune molecules which would add to a decreased host immune response and increased worm survival.

The in vitro effect of commercially available noble metal nanocolloids on the rainbow trout (Oncorhynchus mykiss) leukocyte and splenocyte activity

The growing popularity of nanotechnology in the past decade has increased nanomaterial concentrations in the environment and the risk of their toxicity for aquatic organisms. Metal nanoparticles, which are easily absorbed and accumulated by fish, are probably able to interact directly with their immunocompetent cells. The objective of this study was to evaluate the in vitro effect of commercially available silver, gold and copper nanocolloids on the rainbow trout leukocyte and splenocyte activity. At high concentrations, all of the nanocolloids studied had adverse effects on the proliferative response of trout lymphocytes, and the most toxic of them, silver, decreased also the respiratory burst activity of splenocytes. Low concentrations of silver nanocolloid, however, had a stimulating effect on the lymhocyte proliferation.

Comparative study of β-glucan induced respiratory burst measured by nitroblue tetrazolium assay and real-time luminol-enhanced chemiluminescence assay in common carp (Cyprinus carpio L.)

The respiratory burst is an important feature of the immune system. The increase in cellular oxygen uptake that marks the initiation of the respiratory burst is followed by the production of reactive oxygen species (ROS) such as superoxide anion and hydrogen peroxide which plays a role in the clearance of pathogens and tissue regeneration processes. Therefore, the respiratory burst and associated ROS constitute important indicators of fish health status. This paper compares two methods for quantitation of ROS produced during the respiratory burst in common carp: the widely used, single-point measurement based on the intracellular reduction of nitroblue tetrazolium (NBT) and a real-time luminol-enhanced assay based on the detection of native chemiluminescence. Both assays allowed for detection of dose-dependent changes in magnitude of the respiratory burst response induced by β-glucans in head kidney cells of carp. However, whereas the NBT assay was shown to detect the production of only superoxide anions, the real-time luminol-enhanced assay could detect the production of both superoxide anions and hydrogen peroxide. Only the chemiluminescence assay could reliably record the production of ROS on a real-time scale at frequent and continual time intervals for time course experiments, providing more detailed information on the respiratory burst response. The real-time chemiluminescence assay was used to measure respiratory burst activity in macrophage and neutrophilic granulocyte-enriched head kidney cell fractions and total head kidney cell suspensions and proved to be a fast, reliable, automated multiwell microplate assay to quantitate fish health status modulated by β-glucans.

PAMP induced expression of immune relevant genes in head kidney leukocytes of rainbow trout (Oncorhynchus mykiss)

Host immune responses elicited by invading pathogens depend on recognition of the pathogen by specific receptors present on phagocytic cells. However, the reactions to viral, bacterial, parasitic and fungal pathogens vary according to the pathogen-associated molecular patterns (PAMPs) on the surface of the invader. Phagocytic cells are known to initiate a respiratory burst following an exposure to the pathogen, but the underlying and associated specific elements are poorly elucidated in fish. The present study describes the differential response of head kidney leukocytes from rainbow trout (Oncorhynchus mykiss) to different PAMPs mimicking viral (poly I:C), bacterial (flagellin and LPS) and fungal infections (zymosan and β-glucan). Transcript of cytokines related to inflammation (IL-1β, IL-6, IL-10 and TNF-α) was highly up-regulated following LPS exposure whereas flagellin or poly I:C induced merely moderate reactions. In contrast, IFN-γ expression was significantly higher in the poly I:C stimulated group compared to the LPS group. When head kidney cells were exposed to zymosan or β-glucan, genes encoding IL-1β, TNF-α, IL-6 and IL-10 became up-regulated. Their level of up-regulation was comparable to LPS but the kinetics differed. In particular, TNF-α induction was considerably slower when stimulated with zymosan or β-glucan. The gene encoding the COX-2 enzyme, a central element during initiation of inflammatory reactions, was significantly higher in stimulated cells although a depressing effect of high concentrations of LPS and zymosan became evident after 4h exposure. This study suggests that rainbow trout leukocytes respond differently to viral, bacterial and fungal PAMPs, which may reflect activation of specific signaling cascades eventually leading to activation of different immune effector molecules.