Generation of primary monocyte-like cultures from rainbow trout head kidney leukocytes

A simple method to evaluate the toxic effects of Prorocentrum lima extracts to fish (sea bass) kidney cells

The diarrhetic shellfish toxins (DSTs) okadaic acid (OA) and its analogues - the dinophysistoxins (DTXs) - are produced by dinoflagellates such as Prorocentrum lima and can bioaccumulate in filter-feeding organisms as they are transferred through the food web. Although there is no assessment of the harmful effects of these toxins on the fish's immune system, this study developed a primary culture protocol for kidney cells from marine fish Centropomus parallelus and evaluated the immunotoxic effects to P. lima extracts containing DSTs. The cells were obtained by mechanical dissociation, segregated with Percoll gradient, and incubated for 24 h at 28 °C in a Leibovitz culture medium supplemented with 2% fetal bovine serum and antibiotics. The exposed cells were evaluated in flow cytometry using the CD54 PE antibody. We obtained >5.0 × 10⁶ viable cells per 1.0 g of tissue that exhibited no cell differentiation. Exposure to 1.2 or 12 ng DST mL^-1 stimulated the immune system activation and increased the proportion of activated macrophages and monocytes in 48 to 52% and in 127 to 146%, respectively. The protocol proved to be an alternative tool to assess the immunotoxic effects of DST exposure on fish's anterior kidney cells.

Transcriptome Profiling of Atlantic Salmon Adherent Head Kidney Leukocytes Reveals That Macrophages Are Selectively Enriched During Culture

The Atlantic salmon (Salmo salar) is an economically important fish, both in aquaculture and in the wild. In vertebrates, macrophages are some of the first cell types to respond to pathogen infection and disease. While macrophage biology has been characterized in mammals, less is known in fish. Our previous work identified changes in the morphology, phagocytic ability, and miRNA profile of Atlantic salmon adherent head kidney leukocytes (HKLs) from predominantly “monocyte-like” at Day 1 of in vitro culture to predominantly “macrophage-like” at Day 5 of culture. Therefore, to further characterize these two cell populations, we examined the mRNA transcriptome profile in Day 1 and Day 5 HKLs using a 44K oligonucleotide microarray. Large changes in the transcriptome were revealed, including changes in the expression of macrophage and immune-related transcripts (e.g. csf1r, arg1, tnfa, mx2), lipid-related transcripts (e.g. fasn, dhcr7, fabp6), and transcription factors involved in macrophage differentiation and function (e.g. klf2, klf9, irf7, irf8, stat1). The in silico target prediction analysis of differentially expressed genes (DEGs) using miRNAs known to change expression in Day 5 HKLs, followed by gene pathway enrichment analysis, supported that these miRNAs may be involved in macrophage maturation by targeting specific DEGs. Elucidating how immune cells, such as macrophages, develop and function is a key step in understanding the Atlantic salmon immune system. Overall, the results indicate that, without the addition of exogenous factors, the adherent HKL cell population differentiates in vitro to become macrophage-like.

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.

Specific function and modulation of teleost monocytes/macrophages: polarization and phagocytosis

Macrophages exist in most tissues and play a variety of functions in vertebrates. Teleost fish species are found in most aquatic environments throughout the world and are quite diverse for a group of vertebrate animals. Due to whole genome duplication and environmental adaptation, teleost monocytes/macrophages possess a variety of different functions and modulations compared with those of mammals. A deeper understanding of teleost monocytes/macrophages in the immune system will not only help develop teleost-specific methods of disease prevention but will also help improve our understanding of the various immune mechanisms in mammals. In this review, we summarize the differences in polarization and phagocytosis of teleost and mammalian macrophages to improve our understanding of the various immune mechanisms in vertebrates.

Teleost-specific TLR25 identified from Schizothorax prenanti may recognize bacterial/viral components and activate NF-κB and type I IFNs signaling pathways

TLR25 is a new member of TLR1 family that is only identified in teleosts, but its function in immune response is still unclear. In current study, the coding sequence (CDS) of TLR25 was cloned from Schizothorax prenanti (named spTLR25), and spTLR25 is 2454 bp in length and coding a protein of 817 aa. The spTLR25 contains a signal peptide, twenty leucine-rich repeat (LRR) domains, a LRR C-terminal (LRRCT) motif, a transmembrane region and a Toll/IL-1 receptor (TIR) domain. Phylogenetic analysis indicates that spTLR25 has the closest relationship with Cyprinus carpio (C. carpio) TLR25-2. The 3D structure of spTLR25 exhibits 5 α-helices and 3 β-sheets in the TIR domain, and 8 α-helices and 6 β-sheets in the LRR domains. The spTLR25 is mainly expressed in immune-related tissues and peripheral blood leukocytes (PBL). Furthermore, the expression levels of spTLR25 were upregulated in spleen, head kidney and liver while S. prenanti was challenged with LPS or Aeromonas hydrophila (Ah), and the upregulation was also detected in head kidney leukocytes (HKL) after LPS and Poly (I:C) stimulation. The luciferase reporter assay demonstrated that NF-κB and type I IFNs signaling pathways can be activated by spTLR25, and this process may involve in the cascade amplification of TLR25-MyD88 signaling. In addition, the co-localization analysis showed that spTLR25 localizes to intracellular region. Taken together, our results reveal that teleost-specific TLR25 may be a multifunctional receptor for recognizing both LPS and Poly (I:C) and may activate NF-κB and type I IFNs signaling pathways.

Inflammatory immune response by lipopolysaccharide-responsive nucleotide binding oligomerization domain (NOD)-like receptors in the Japanese pufferfish (Takifugu rubripes)

Some of NOD-like receptors (NLRs), the cytosolic pattern recognition receptors form a multi-protein complex, inflammasome consisting of one or more NLRs, the adaptor protein ASC and inflammatory caspase to generate mature inflammatory cytokines, interleukin (IL)-1β and IL-18. However, inflammasome-mediated inflammatory cascade involving any NLR member is unknown in a lower vertebrate like fish. Also, inflammatory cytokine induction pathway in response to a specific ligand, namely bacterial lipopolysaccharide (LPS) has not yet been clarified. Therefore, 13 predicted NLR sequences of the Japanese pufferfish, Fugu (Takifugu rubripes) were retrieved in silico and categorized as NLR-C1∼13. Expression analysis of these genes in Fugu head kidney (HK) cells stimulated with a heat-killed Lactobacillus paracasei spp. paracasei (Lpp), LPS, nigericin and a combination of nigericin + LPS showed consistent up-regulations of NLR-C1, 5, 7, 10 and 12 genes in both Lpp and LPS stimulations and NLR-C9 gene in LPS stimulation only. However, nigericin and nigericin + LPS caused an increased expression of NLR-C10 and 12 in HK cells and leukocytes. Fugu treated with Lpp and LPS (in vivo), and infected with Vibrio harveyi had an elevated expression of NLR-C10 and 12. Increased transcription of caspase-1, ASC, IL-1β and IL-18 was recorded in nigericin-stimulated HK cells and leukocytes. Results suggested activation of probable inflammasome-mediated inflammatory cytokine response in Fugu. Moreover, LPS may be a key ligand that induces some of the Fugu NLR-Cs (NLR-C9, 10 and 12). Further characterization and functional analysis of Fugu NLR-C10 and 12 for ligand sensing, and processing of pro-inflammatory cytokine, IL-1β would elucidate the inflammasome evolution in fish.

A Review of Intra- and Extracellular Antigen Delivery Systems for Virus Vaccines of Finfish

Vaccine efficacy in aquaculture has for a long time depended on evaluating relative percent survival and antibody responses after vaccination. However, current advances in vaccine immunology show that the route in which antigens are delivered into cells is deterministic of the type of adaptive immune response evoked by vaccination. Antigens delivered by the intracellular route induce MHC-I restricted CD8+ responses while antigens presented through the extracellular route activate MHC-II restricted CD4+ responses implying that the route of antigen delivery is a conduit to induction of B- or T-cell immune responses. In finfish, different antigen delivery systems have been explored that include live, DNA, inactivated whole virus, fusion protein, virus-like particles, and subunit vaccines although mechanisms linking these delivery systems to protective immunity have not been studied in detail. Hence, in this review we provide a synopsis of different strategies used to administer viral antigens via the intra- or extracellular compartments. Further, we highlight the differences in immune responses induced by antigens processed by the endogenous route compared to exogenously processed antigens. Overall, we anticipate that the synopsis put together in this review will shed insights into limitations and successes of the current vaccination strategies used in finfish vaccinology.

Effects of anti-inflammatory drugs in primary kidney cell culture of a freshwater fish

The non-steroidal anti-inflammatory drugs are emerging contaminants in aquatic ecosystems. This study aimed to evaluate toxic effects of some representative drugs of this pharmaceutical group on primary culture of monocytic lineage of Hoplias malabaricus anterior kidney. The effects of diclofenac, acetaminophen and ibuprofen in cell viability, lipopolysaccharide (LPS)-induced NO production and genotoxicity were evaluated. Cytometry analysis CD11b(+) cells showed 71.5% of stem cells, 19.5% of macrophages and 9% of monocytes. Cell viability was lower in the ficoll compared to percoll separation. LPS-induced NO production by these cells was blocked after treatment with dexamethasone and NG-Methyl-L-Arginine (L-NMMA). Exposure of the cells to diclofenac (0.2-200 ng/mL), acetaminophen (0.025-250 ng/mL) ibuprofen (10-1000 ng/mL) reduced basal NO production and inhibited LPS-induced NO production at all concentrations after 24 h of exposure. Genotoxicity occurred at the highest concentration of diclofenac and at the intermediary concentration of acetaminophen. Genotoxicity was also observed by ibuprofen. In summary, the pharmaceuticals influenced NO production and caused DNA damage in monocytic cells suggesting that these drugs can induce immunosuppression and genotoxicity in fish.

Evolutionary conservation of divergent pro-inflammatory and homeostatic responses in Lamprey phagocytes

In higher vertebrates, phagocytosis plays a critical role in development and immunity, based on the internalization and removal of apoptotic cells and invading pathogens, respectively. Previous studies describe the effective uptake of these particles by lower vertebrate and invertebrate phagocytes, and identify important molecular players that contribute to this internalization. However, it remains unclear if individual phagocytes mediate internalization processes in these ancient organisms, and how this impacts the balance of pro-inflammatory and homeostatic events within their infection sites. Herein we show that individual phagocytes of the jawless vertebrate Petromyzon marinus (sea lamprey), like those of teleost fish and mice, display the capacity for divergent pro-inflammatory and homeostatic responses following internalization of zymosan and apoptotic cells, respectively. Professional phagocytes (macrophages, monocytes, neutrophils) were the primary contributors to the internalization of pro-inflammatory particles among goldfish (C. auratus) and lamprey (P. marinus) hematopoietic leukocytes. However, goldfish showed a greater ability for zymosan phagocytosis when compared to their jawless counterparts. Coupled to this increase was a significantly lower sensitivity of goldfish phagocytes to homeostatic signals derived from apoptotic cell internalization. Together, this translated into a significantly greater capacity for induction of antimicrobial respiratory burst responses compared to lamprey phagocytes, but also a decreased efficacy in apoptotic cell-driven leukocyte homeostatic mechanisms that attenuate this pro-inflammatory process. Overall, our results show the long-standing evolutionary contribution of intrinsic phagocyte mechanisms for the control of inflammation, and illustrate one effective evolutionary strategy for increased responsiveness against invading pathogens. In addition, they highlight the need for development of complementary regulatory mechanisms of inflammation to ensure continued maintenance of host integrity amidst increasing challenges from invading pathogens.

Fish and mammalian phagocytes differentially regulate pro-inflammatory and homeostatic responses in vivo

Phagocytosis is a cellular mechanism that is important to the early induction of antimicrobial responses and the regulation of adaptive immunity. At an inflammatory site, phagocytes serve as central regulators for both pro-inflammatory and homeostatic anti-inflammatory processes. However, it remains unclear if this is a recent evolutionary development or whether the capacity to balance between these two seemingly contradictory processes is a feature already displayed in lower vertebrates. In this study, we used murine (C57BL/6) and teleost fish (C. auratus) in vitro and in vivo models to assess the evolutionary conservation of this dichotomy at a site of inflammation. At the level of the macrophage, we found that teleost fish already displayed divergent pro-inflammatory and homeostatic responses following internalization of zymosan or apoptotic bodies, respectively, and that these were consistent with those of mice. However, fish and mice displayed significant differences in vivo with regards to the level of responsiveness to zymosan and apoptotic bodies, the identity of infiltrating leukocytes, their rate of infiltration, and the kinetics and strength of resulting antimicrobial responses. Unlike macrophages, significant differences were identified between teleost and murine neutrophilic responses. We report for the first time that activated murine, but not teleost neutrophils, possess the capacity to internalize apoptotic bodies. This internalization translates into reduction of neutrophil ROS production. This may play an important part in the recently identified anti-inflammatory activity that mammalian neutrophils display during the resolution phase of inflammation. Our observations are consistent with continued honing of inflammatory control mechanisms from fish to mammals, and provide added insights into the evolutionary path that has resulted in the integrated, multilayered responses that are characteristic of higher vertebrates.

Regulation of immunity and disease resistance by commensal microbes and chromatin modifications during zebrafish development

How fish larvae are protected from infection before the maturation of adaptive immunity, a process which may take up to several weeks in most species, has long been a matter of speculation. Using a germ-free model, we show that colonization by commensals in newly hatched zebrafish primes neutrophils and induces several genes encoding proinflammatory and antiviral mediators, increasing the resistance of larvae to viral infection. Commensal microbe recognition was found to be mediated mainly through a TLR/MyD88 signaling pathway, and professional phagocytes were identified as the source of these immune mediators. However, the induction of proinflammatory and antiviral genes, but not of antimicrobial effector genes, also required the covalent modification of histone H3 at gene promoters. Interestingly, chromatin modifications were not altered by commensal microbes or hatching. Taken together, our results demonstrate that gene-specific chromatin modifications are associated with the protection of zebrafish larvae against infectious agents before adaptive immunity has developed and prevent pathologies associated with excessive inflammation during development.

Characterisation and expression analysis of the rainbow trout (Oncorhynchus mykiss) homologue of the human dendritic cell marker CD208/lysosomal associated membrane protein 3

LAMP3/CD208 is a member of the lysosomal-associated membrane protein family and is used as a putative marker for mature dendritic cells (DCs) in humans since it is upregulated upon DC activation and maturation. This paper reports the cloning and sequencing of LAMP3 in rainbow trout. The predicted trout LAMP3 shares the characteristic features of LAMP family members such as a C-terminal lysosomal sorting motif (G-Y-D-R-I) in the short C-terminal cytoplasmic tail, typical for lysosomal targeting, four potential N-linked glycosylation sites (NXS/T), four conserved cysteines in the membrane-proximal domain and the luminal domain divided by a serine/proline-rich region. Expression studies revealed that trout LAMP3, like chicken LAMP3, was constitutively expressed in a wide range of lymphoid tissues, at highest levels in the head kidney, liver and spleen, respectively. LAMP3 was also constitutively expressed in trout head kidney macrophages and RTS11 cells and the expression was shown to be induced in vivo after infection with viral and bacterial pathogens and in cultured macrophages after modulation with microbial mimics (LPS and PolyIC). Thus, it is clear that if LAMP3 is expressed by trout DCs it is not exclusively expressed by them. However, the marker is valuable to further study antigen presentation in fish and to complement already known DC markers.

Long-term stimulation of trout head kidney cells with the cytokines MCSF, IL-2 and IL-6: gene expression dynamics

The production of salmonid leukocyte cell lines from primary cell cultures has been attempted on several occasions, however, to date only monocyte/macrophage like cell lines exist (e.g. RTS-11 and SHK-1 cells). With the increasing number of cytokines discovered in fish in recent years, many of which are growth factors for leukocytes, we now have the possibility of using these molecules to promote leukocyte development and differentiation in culture. We have generated stable cell lines transfected with a variety of plasmids expressing cytokines (Interleukin (IL)-2, IL-6 and Macrophage Colony Stimulating Factor (MCSF)), in order to produce conditioned media rich in these cytokines. The cytokine-conditioned media were used to assess their activity and ability to support the growth of primary head kidney (HK) leukocyte cultures. Here, we describe a series of experiments aimed to determine which cell population(s) of primary HK cultures is supported and will grow in conditioned media containing MCSF, IL-2 or IL-6. For a period of 5 weeks, cells were incubated at 22°C and media were changed every 3-4 days. Samples were taken at different time points, from freshly isolated HK cells (T0), one week post-stimulation (1-WPS), 3-WPS and 5-WPS for RNA extraction. A variety of cell lineage markers (MCSF Receptor 2 (MCSFR2) for macrophages, CD4 and CD8a for T cells and IgM heavy chain for B cells) were then analysed by real-time qPCR to study the cell population dynamics as influenced by the different recombinant cytokines in the cultures. We show here that whilst MCSF appears to drive macrophage differentiation and maintenance, IL-2 and IL-6 seem to preferentially drive lymphocyte differentiation.

Teleost IgSF immunoregulatory receptors

In all animals innate immunity is the first line of immune defense from invading pathogens. The prototypical innate cellular responses such as phagocytosis, degranulation, and cellular cytotoxicity are elicited by leukocytes in a diverse range of animals including fish, amphibians, birds and mammals reinforcing the importance of such primordial defense mechanisms. In mammals, these responses are intricately controlled and coordinated at the cellular level by distinct subsets of immunoregulatory receptors. Many of these surface proteins belong to the immunoglobulin superfamily and in mammals elaborate immunoregulatory receptor networks play a major role in the control of infectious diseases. Recent examination of teleost immunity has begun to further illustrate the complexities of these receptor networks in lower vertebrates. However, little is known about the mechanisms that control how immunoregulatory receptors influence cellular decision making in ectothermic vertebrates. This review focuses on several families of recently discovered immunoglobulin superfamily members in fish that share structural, phylogenetic and in some cases functional relationships with mammalian immunoregulatory receptors. Further characterization of these teleost innate immune receptor families will provide detailed information regarding the conservation and importance of innate immune defense strategies throughout vertebrate evolution.

Heterogeneity of macrophage activation in fish

In this review, we focus on four different activation states of fish macrophages. In vitro, stimulation with microbial ligands induces the development of innate activated macrophages whereas classically activated macrophages can be induced by stimulation with LPS in combination with (recombinant) IFNγ. Both types of macrophages show elevated phagocytic activity, expression of pro-inflammatory cytokine genes and radical production. Alternatively activated macrophages require the cytokines IL-4/IL-13 for induction of, among others, arginase activity. Until in vitro studies identify the effects of putative IL-4 and IL-13 homologues on fish macrophages, arginase enzyme activity remains the most reliable marker for the presence of alternatively activated macrophages in fish. The best evidence for the existence of regulatory macrophages, associated with the presence of IL-10, comes from in vivo studies, for example during parasitic infections of carp. Altogether, differentially activated macrophages in fish largely resemble the phenotypes of mammalian macrophages. However, the presence of fish-specific ligand recognition by TLRs and of duplicated genes coding for proteins with particular activities, poses additional challenges for the characterization of phenotype-specific gene signatures and cell surface markers.

Flow cytometry assays of respiratory burst in Atlantic salmon (Salmo salar L.) and in Atlantic cod (Gadus morhua L.) leucocytes

The oxidation of dihydrorhodamine 123 (DHR) to the fluorescent rhodamine 123 (RHO) was detected using flow cytometry. This assay for detection of respiratory burst activity was established in peripheral blood leucocytes (PBL) and head kidney leucocytes (HKL) of Atlantic salmon and Atlantic cod. The leucocytes were stimulated by phorbol 12-myristate 13-acetate (PMA). For cod cells 10 times lower concentration of PMA had to be used compared to salmon cells, as higher concentrations were toxic and resulted in considerable cell death. The cells found to be RHO-positive were monocytes/macrophages and neutrophils based on the scatter dot plots, but for salmon also some small cells were found to have high fluorescence intensity both in the flow cytometry analyses and by fluorescence microscopy of cytospin preparations. The nature of these cells is not known. For cod leucocytes, such cells were not obvious. The instrument settings are a bit more demanding for cod, as cod cells die more easily compared to salmon cells. In both assays the limit between negative and positive cells has to be carefully considered. The presented flow cytometry protocols for measurements of respiratory burst in salmon and cod leucocytes can be applied in various studies where respiratory burst functions are involved, such as to verify if it is activated or suppressed in connection with infections and immunostimulation.

Molecular identification and expression analysis of a natural killer cell enhancing factor (NKEF) from rock bream Oplegnathus fasciatus and the biological activity of its recombinant protein

Natural killer cell enhancing factor (NKEF) belongs to the defined peroxiredoxin (Prx) family. Rock bream NKEF cDNA was identified by expressed sequence tag (EST) analysis of rock bream liver that was stimulated with the LPS. The full-length RbNKEF cDNA (1062 bp) contained an open reading frame (ORF) of 594 bp encoding 198 amino acids. RbNKEF was significantly expressed in the gill, liver, and intestine. mRNA expression of NKEF in the head kidney was examined under viral and bacterial challenge via real-time RT-PCR. Experimental challenge of rock bream with Edwardsiella tarda, Streptococcus iniae, and RSIV resulted in significant increases in RbNKEF mRNA in the head kidney. To obtain a recombinant NKEF, the RbNKEF ORF was expressed in Escherichia coli BL21 (DE3), and the purified soluble protein exhibited a single band corresponding to the predicted molecular mass. When kidney leucocytes were treated with a high concentration of rRbNKEF (10 μg/mL), they exhibited significantly enhanced cell proliferation and viability under oxidative stress.

Molecular cloning and expression analysis of a thioredoxin from rock bream, Oplegnathus fasciatus, and biological activity of the recombinant protein

Thioredoxins (TRxs) are a family of small, highly conserved proteins that are essential for the maintenance of cellular homeostasis. TRx1, which contains a conserved redox-active site, Cys-Gly-Pro-Cys, is a proinflammatory cytokine, B cell growth factor, macrophage migration inhibiting factor (MIF), and an immune regulatory cytokine. The TRx1 homologue cDNA was isolated from the rock bream LPS-stimulated liver cDNA library, RbTRx1. RbTRx1 consists of 730 bp full-length cDNA with a 324 bp open reading frame encoding 108 amino acids. When compared with other known TRx1 peptide sequences, the most conserved region of the RbTRx1 peptide was the redox-active site Cys-Gly-Pro-Cys. Phylogenetic analysis grouped the RbTRx1 with other vertebrate TRx1 peptides. Quantitative real-time PCR analysis revealed the presence of RbTRx1 transcripts in liver, gill, kidney, and muscle. The expression of RbTRx1 mRNA in kidney leukocytes was upregulated after bacterial and viral challenge. The kidney leukocytes were treated with a high concentration of rRbTRx1, which significantly enhanced cell proliferation (1 μg/ml and 10 μg/ml) and viability under oxidative stress (10 μg/ml).

Clonal analysis of hematopoietic progenitor cells in the zebrafish

Identification of hematopoietic progenitor cells in the zebrafish (Danio rerio) has been hindered by a lack of functional assays to gauge proliferative potential and differentiation capacity. To investigate the nature of myeloerythroid progenitor cells, we developed clonal methylcellulose assays by using recombinant zebrafish erythropoietin and granulocyte colony-stimulating factor. From adult whole kidney marrow, erythropoietin was required to support erythroid colony formation, and granulocyte colony-stimulating factor was required to support the formation of colonies containing neutrophils, monocytes, and macrophages. Myeloid and erythroid colonies showed distinct morphologies and were easily visualized and scored by their expression of lineage-specific fluorescent transgenes. Analysis of the gene-expression profiles after isolation of colonies marked by gata1:DsRed or mpx:eGFP transgenes confirmed our morphological erythroid and myeloid lineage designations, respectively. The majority of progenitor activity was contained within the precursor light scatter fraction, and more immature precursors were present within the lymphoid fraction. Finally, we performed kinetic analyses of progenitor activity after sublethal irradiation and demonstrated that recovery to preirradiation levels occurred by 14 days after irradiation. Together, these experiments provide the first report of clonal hematopoietic progenitor assays in the zebrafish and establish the number, characteristics, and kinetics of myeloerythroid progenitors during both steady-state and stress hematopoiesis.

Macrophage activation differentially modulates particle binding, phagocytosis and downstream antimicrobial mechanisms

Phagocytosis provides a critical first line of defense against invading pathogens. Engagement of particles through receptor-mediated binding precedes internalization and induction of cellular antimicrobial responses. Phagocytes have the capacity to differentially regulate binding and internalization processes through changes in their receptor profile and modulation of downstream events. This is necessary for the intricate control of phagocytic antimicrobial responses. Several methods are available for evaluation of phagocytosis. Unfortunately, none allow for accurate quantitation of both binding and internalization events. To overcome these limitations, we have developed a novel phagocytosis assay based on a multi-spectral imaging flow cytometry platform. This assay discriminates between internalized and surface-bound particles in a statistically robust manner and allows multi-parametric analysis of phagocytosis and downstream anti-microbial responses. We also devised a novel approach for examination of phagolysosome fusion, which provides an improved capacity for quantitative assessment of phagolysosome fusion in mixed populations of intact cells. Importantly, our approaches are likely amenable to a broad range of comparative model systems based on our examination of murine RAW 264.7 cells and a goldfish primary kidney macrophage (PKM) model system. The latter allowed us to examine the evolutionary conservation of phagocytic antimicrobial responses in a lower vertebrate model. While it has been previously reported that mixed populations of these macrophage cultures are phagocytic, it remained unclear if sub-populations within them contributed differentially to this activity. In accordance with higher vertebrate models, we found that differentiation along the macrophage pathway leads to an increased capacity for phagocytosis in goldfish PKM. Interestingly, cellular activation differentially regulated particle internalization in PKM monocyte and mature macrophage subsets. We also found differential regulation of phagolysosome fusion and downstream production of reactive oxygen intermediates (ROI). The temporal activation of specific phagocytic antimicrobial responses at distinct stages of PKM differentiation suggests specialization within the macrophage compartment early in evolution, geared to meet specific host immunity requirements within specialized niches.

Trypanosomiasis-induced Th17-like immune responses in carp

Background

In mammalian vertebrates, the cytokine interleukin (IL)-12 consists of a heterodimer between p35 and p40 subunits whereas interleukin-23 is formed by a heterodimer between p19 and p40 subunits. During an immune response, the balance between IL-12 and IL-23 can depend on the nature of the pathogen associated molecular pattern (PAMP) recognized by, for example TLR2, leading to a preferential production of IL-23. IL-23 production promotes a Th17-mediated immune response characterized by the production of IL-17A/F and several chemokines, important for neutrophil recruitment and activation. For the cold blooded vertebrate common carp, only the IL-12 subunits have been described so far.

Methodology/Principal Findings

Common carp is the natural host of two protozoan parasites: Trypanoplasma borreli and Trypanosoma carassii. We found that these parasites negatively affect p35 and p40a gene expression in carp. Transfection studies of HEK293 and carp macrophages show that T. carassii-derived PAMPs are agonists of carp TLR2, promoting p19 and p40c gene expression. The two protozoan parasites induce different immune responses as assessed by gene expression and histological studies. During T. carassii infections, in particular, we observed a propensity to induce p19 and p40c gene expression, suggestive of the formation of IL-23. Infections with T. borreli and T. carassii lead to an increase of IFN-γ2 gene expression whereas IL-17A/F2 gene expression was only observed during T. carasssii infections. The moderate increase in the number of splenic macrophages during T. borreli infection contrasts the marked increase in the number of splenic neutrophilic granulocytes during T. carassii infection, along with an increased gene expression of metalloproteinase-9 and chemokines.

Conclusion/Significance

This is the first study that provides evidence for a Th17-like immune response in fish in response to infection with a protozoan parasite.

Characterization and functional analysis of goldfish (Carassius auratus L.) tumor necrosis factor-alpha

We identified and characterized two isoforms of tumor necrosis factor-alpha (TNFalpha) from the goldfish, TNFalpha-1 and TNFalpha-2. At the protein level, goldfish TNFalpha-1 and TNFalpha-2 were most homologous to carp TNFalpha-1 and TNFalpha-2, respectively. Phylogenetically, the two goldfish isoforms grouped most closely with the carp TNFalpha isoforms and TNF species of other cyprinids. Real-time PCR analysis revealed constitutive expression of goldfish TNFalpha-1 and TNFalpha-2 in all tissues with TNFalpha-2 mRNA levels higher than TNFalpha-1 in all tissues examined. A modest up-regulation in expressions of goldfish TNFalpha-1 and TNFalpha-2 in kidney-derived monocytes and significant increase in expression of both isoforms in mature macrophages were observed in response to activation with macrophage-activating factors. TNFalpha-2 was subsequently expressed using a prokaryotic expression system and the recombinant molecule (rTNFalpha-2) was functionally characterized. The rTNFalpha-2 induced a dose-dependent chemotactic response and enhanced phagocytosis of primary goldfish macrophages. Furthermore, rTNFalpha-2 primed the respiratory burst in monocytes and induced nitric oxide production of primary goldfish macrophages. Our results indicate that goldfish TNFalpha is a central regulatory and effector cytokine of inflammatory and antimicrobial responses of the goldfish.

Head Kidney-Derived Macrophages of Common Carp (Cyprinus carpio L.) Show Plasticity and Functional Polarization upon Differential Stimulation

Cells from the myeloid lineage are pluripotent. To investigate the potential of myeloid cell polarization in a primitive vertebrate species, we phenotypically and functionally characterized myeloid cells of common carp (Cyprinus carpio L.) during culture. Flow cytometric analysis, Ab labeling of cell surface markers, and light microscopy showed the presence of a major population of heterogeneous macrophages after culture. These head kidney-derived macrophages can be considered the fish equivalent of bone marrow-derived macrophages and show the ability to phagocytose, produce radicals, and polarize into innate activated or alternatively activated macrophages. Macrophage polarization was based on differential activity of inducible NO synthase and arginase for innate and alternative activation, respectively. Correspondingly, gene expression profiling after stimulation with LPS or cAMP showed differential expression for most of the immune genes presently described for carp. The recently described novel Ig-like transcript 1 (NILT1) and the CXCR1 and CXCR2 chemokine receptors were up-regulated after stimulation with cAMP, an inducer of alternative activation in carp macrophages. Up-regulation of NILT1 was also seen during the later phase of a Trypanosoma carassii infection, where macrophages are primarily alternatively activated. However, NILT1 could not be up-regulated during a Trypanoplasma borreli infection, a model for innate activation. Our data suggest that NILT1, CXCR1, and CXCR2 could be considered markers for alternatively activated macrophages in fish.

Development of goldfish macrophages in vitro

Over 100 years after the first description of macrophages by Metchnikoff, there are still questions as to the mechanisms leading to the heterogeneity of their lineage. Current views are based on the mononuclear phagocyte system (MPS) theory, where all mammalian macrophages are derived from circulating blood monocytes and ultimately from hematopoietic stem cells in the bone marrow. Our studies on the regulation of fish macrophage development, suggested that teleosts have alternate pathways of monopoiesis, which undoubtedly contribute to macrophage heterogeneity in the goldfish. Macrophage heterogeneity has been attributed to a network of positive and negative regulators of macrophage development, including soluble mediators known as colony-stimulating factors of which two (M-CSF and GM-CSF) promote formation and growth of mature macrophages. In contrast to our knowledge of CSFs and their receptors in mammals, there is no published information about fish macrophage CSFs. Since fish macrophages generate their own growth factors, it is reasonable to assume that pathways of fish macrophage development and hematopoiesis may be distinct from those of mammalian macrophages. More importantly, the presence of fish progenitor/stem cells and developing macrophages in long-term cultures, allowed us to address pathways of macrophage differentiation, which could not be addressed in mammalian macrophage culture systems. Characterization of primary kidney macrophage (PKM) cultures from goldfish hematopoietic tissues (kidney) indicated that three distinct subpopulations developed in response to endogenous macrophage growth factors. These macrophage subpopulations expressed several differentiation markers, including the hematopoietic stem cell antigen AC133, c-kit, granulin, CD63, macrosialin, c/EBPbeta, legumain, and the colony-stimulating factor receptor-1 (CSF-1R). In the goldfish, there appeared to be a stringent control between those early progenitors that self-renewed, and those that were recruited into the maturation pathways. We report that upon commitment, goldfish macrophages developed through two distinct differentiation pathways: one consistent with the "classical" pathway (MPS) of macrophage development (progenitors-->monocytes-->mature macrophages), and an "alternate" pathway (AP-macrophages) where mature macrophages appeared to rapidly develop from early progenitors in the absence of an intermediate monocyte stage. AP-macrophages represent a unique subset of spontaneously growing cells. Their self-renewal was promoted by endogenous macrophage growth factors (MGF), and effectively controlled by a novel soluble form of the CSF-1R (sCSF-1R). The discovery of sCSF-1R in the goldfish highlights the inherent complexity in the hematopoietic regulatory machinery of teleosts.

Influence of vaccination on the nitric oxide response of gilthead seabream following infection with Photobacterium damselae subsp. piscicida

The nitric oxide (NO) response of vaccinated and non-vaccinated juvenile gilthead seabream was studied in vivo and the NO response of isolated kidney macrophages of fish was studied in vitro. Fish were vaccinated with formalin-killed Photobacterium damselae subsp. piscicida (Pdp) with or without Freund's incomplete adjuvant (FIA) and control fish received phosphate buffered saline (PBS). Thirty days later, fish were injected with a sublethal dose of Pdp and 3 fish/group were bled at time periods thereafter and serum nitrite and citrulline levels were determined as a measure of the NO response. All infected groups showed an increase in NO metabolites from 6h to 27 days, with peak levels at 24 h. However, the response in bacterin-vaccinated fish was significantly higher than in the non-vaccinated group and the bacterin plus FIA resulted in a further significant enhancement. Similarly enhanced NO responses were produced in vitro by isolated macrophages obtained from vaccinated compared with non-vaccinated fish 30 days after vaccination following infection, with the response in macrophages from fish vaccinated with the bacterin plus FIA being significantly higher than those from fish vaccinated with the bacterin alone. Thus, vaccination resulted in an enhanced NO response to infection with Pdp in vivo and in vitro. Furthermore, the level of protection of fish to experimental challenge with virulent Pdp correlated with the level of the NO responses in the different groups.

Anaphylatoxin-like molecules generated during complement activation induce a dramatic enhancement of particle uptake in rainbow trout phagocytes

Here we have identified a serum fraction containing approximately 8-kDa molecules with an unexpected capacity to greatly enhance particle uptake in trout head kidney leukocytes (HKLs). This approximately 8-kDa particle-uptake enhancing fraction (PUEF-8) was purified from complement-activated serum by gel filtration chromatography. Mass spectrometric analysis and reactivity of anti-trout C3-1 and C4 antibodies, indicated the presence of C3a, C4a and C5a molecules in PUEF-8. Using a newly developed flow cytometric assay that measures the capacity of cells to ingest fluorescent beads, we showed that PUEF-8 induced a striking enhancement (344+/-50% higher than the PBS control value) in the number of HKLs ingesting three or more beads. In contrast, the effect of PUEF-8 on peripheral blood leukocytes (PBLs) was almost negligible. Interestingly, PUEF-8 acted as a strong chemoattractant for both HKLs and PBLs. These findings suggest a novel role for the anaphylatoxins generated during complement activation in teleost fish.

Modulation of the activity of sea bass (Dicentrarchus labrax) head-kidney macrophages by macrophage activating factor(s) and lipopolysaccharide

The aim of this study was to establish the requirements for macrophage activating factor (MAF) production by sea bass head-kidney leucocytes and the kinetics of macrophage activation when exposed to MAF-containing supernatants and/or lipopolysaccharide (LPS), a known macrophage stimulant. MAF activity was found in culture supernatants of total head-kidney leucocytes pulsed with 5 microg ml(-1)Con A, 5 or 10 ng ml(-1)PMA and 100 ng ml(-1)calcium ionophore, or 10 microg ml(-1)Con A alone, as assessed by the capacity to prime macrophages for enhanced production of reactive oxygen intermediates (ROI). Mixed leucocyte cultures from two or eight fish showed higher MAF activity after stimulation, indicating that a mixed leucocyte reaction was also important for MAF production. MAF-induced activation of macrophage cultures was highest at 18 h of exposure and was lost by 72 h except for MAF induced by Con A-stimulation alone. LPS primed macrophages for increased ROI production at early incubation times and down-regulated ROI production after 24 h. LPS had no effect in further stimulating the MAF-induced priming effect on production of ROI and down-regulated the MAF-priming by 48 h. Sea bass head-kidney macrophages did not show increased nitrite production when exposed to MAF and/or LPS, which may be related to their differentiation status.