Salmonid B lymphocytes demonstrate organ dependent functional heterogeneity

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

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

Clonotypic IgH Response against Systemic Viral infection in Pronephros and Spleen of a Teleost Fish

Upon infection, B lymphocytes develop clonal responses. In teleost fish, which lack lymph nodes, the kinetics and location of B cell responses remain poorly characterized. Fish pronephros is the site of B cell differentiation and the main niche for persistence of plasma cells. In this study, we undertook the analysis of the rainbow trout IgHμ repertoire in this critical tissue for humoral adaptive immunity after primary immunization and boost with a rhabdovirus, the viral hemorrhagic septicemia virus (VHSV). We used a barcoded 5' RACE-cDNA sequencing approach to characterize modifications of the IgHμ repertoire, including VH usage in expressed V(D)J rearrangements, clonal diversity, and clonotype sharing between individual fish and treatments. In the pronephros, our approach quantified the clonotype frequency across the whole IgH repertoire (i.e., with all VH), measuring the frequency of Ag-responding clonotypes. Viral infection led to extensive modifications of the pronephros B cell repertoire, implicating several VH subgroups after primary infection. In contrast, only modest changes in repertoire persisted 5 mo later, including VHSV-specific public expansions. The IgM public response implicating IgHV1-18 and JH5, previously described in spleen, was confirmed in pronephros in all infected fish, strongly correlated to the response. However, the distribution of top clonotypes showed that pronephros and spleen B cells constitute distinct compartments with different IgH repertoires. Unexpectedly, after boost, the frequency of anti-VHSV clonotypes decreased both in pronephros and spleen, raising questions about B cell circulation. A better monitoring of B cell response kinetics in lymphoid tissues will be an essential step to understand B memory and plasmocyte formation mechanisms in fish.

Xbp1-u and Xbp1-s from Nile tilapia (Oreochromis niloticus): Transcriptional profiling upon Streptococcus agalactiae infection and the potential role in B cell activation and differentiation

X-box protein 1 (Xbp1), an essential transcription factor including an unstable form (Xbp1-u) and a stable form (Xbp1-s), plays an vital role in B cell activation and differentiation to plasma cells. In this study, we cloned and identified Xbp1-u gene from Nile tilapia (Oreochromis niloticus), containing 783 bp of nucleotide sequence encoding 260 amino acids. The deduced protein possesses a basic region leucine zipper domain (bZIP) and 26 ribonucleotides of OnXbp1-u transcript. Transcription analysis revealed OnXbp1-u and OnXbp1-s were widely distributed in all examined tissues, with a high expression in immune-related tissues. When stimulated with Streptococcus agalactiae in vivo, the expressions of OnXbp1-u and OnXbp1-s were significantly up-regulated in liver, spleen, head kidney, blood, skin and intestine. After in vitro challenge upon S.agalactiae, the similar up-regulations of OnXbp1-u and OnXbp1-s were also demonstrated in head kidney leukocytes. Moreover, the OnXbp1-u and OnXbp1-s could get involved in LPS-inducible B cell activation and (r)OnIL6-inducible B cell differentiation. Taken together, the results indicated that OnXbp1-u and OnXbp1-s might not only involved in the immune response against S. agalactiae challenge, but also in the B cell activation and differentiation in Nile tilapia.

Long-Lived Plasma Cells Secrete High-Affinity Antibodies Responding to a T-Dependent Immunization in a Teleost Fish

The recent discovery of long-lived plasma cells (LLPCs) in mammals, which provide a constant expression of specific high-affinity antibodies that mediate humoral memory, has caused a dramatic paradigm shift in the study of immunity and vaccine development. In teleost fish, there are few studies regarding the association between LLPCs and antibody production, and the affinity of the antibodies secreted by the LLPCs is poorly understood. In the present study, channel catfish (Ictalurus punctatus) were immunized with trinitrophenylated-keyhole limpet hemocyanin (TNP-KLH) to examine TNP-specific antibody titers, affinities, antibody-secreting cell (ASC) dynamic changes, and especially the affinity of secreted antibodies by LLPCs post-immunization. We demonstrated that TNP-specific LLPCs were generated starting at week 4 post-immunization, achieved a maximal number at week 8, and maintained a comparable level throughout the 18-week post-immunization period, which was correlated with the dynamics of serum antibody titers and affinity maturation in the response. The LLPCs appeared to mostly reside within, or migrate to, the anterior kidney (bone marrow-like tissue in mammals), but a small portion was also located in the spleen and peripheral blood. The antibodies produced by the LLPCs possessed high affinities, indicating that the generation and development of LLPCs were driven by affinity selection in teleosts. Collectively, the results of this study provide insights toward the evolutionary understanding of the affinity-dependent mechanism of LLPC generation and development.

Comparative transcriptome analysis of the transcriptional heterogeneity in different IgM+ cell subsets from peripheral blood of Nile tilapia (Oreochromis niloticus)

In teleost fish, IgM⁺ B cells play important roles in innate and adaptive immunity. Different IgM⁺ B cells are detected in teleost, named IgM^lo and IgM^hi B cell subsets, according to the distinct expression levels of membrane IgM (mIgM). However, the study on the heterogeneity in IgM⁺ B cell subsets remains poorly understood. In this study, the comparative transcriptomic profiles of IgM^-, IgM^lo and IgM^hi from peripheral blood of Nile tilapia (Oreochromis niloticus) were carried out by using RNA-sequencing technique. A total of 6045 and 5470 differentially expressed genes (DEGs) were detected in IgM^lo and IgM^hi cells, respectively, as compared with IgM^- lymphocytes, whereas 3835 genes were differentially expressed when IgM^lo compared to IgM^hi cells. Analysis of the KEGG database indicated that the DEGs were enriched in immune system categories and signaling transduction and interaction in IgM^- vs IgM^hi, IgM^- vs IgM^lo and IgM^lo vs IgM^hi. Comparatively, in IgM^lo vs IgM^hi, GO enrichment analysis indicated that the DEGs enriched in nucleic acid binding transcription factor activity. Analysis of crucial transcription factors for B cell differentiation indicated that IgM^lo and IgM^hi cell clusters belonged to the different B cell subsets. The data generated in this study may provide insights into understanding the heterogeneity of IgM⁺ cells in teleost, and suggest that IgM⁺ B cells play a crucial role in innate immunity.

Molecular characterization and transcriptional expression of a B cell transcription factor Pax5 in Nile tilapia (Oreochromis niloticus)

Pax5 (Paired Box 5), a nuclear transcription factor expressed in B cell specifically, is a key regulator for B cell activation. In this study, we cloned and identified a Pax5 gene (OnPax5) from Nile tilapia (Oreochromis niloticus), which has an open reading frame of 1278 bp, encoding deduced amino acid sequence of 425 residues. OnPax5 contains a conserved DNA-binding domain encoding the paired box, an octapeptide, a homeobox homology region, a transactivation and a repressor domain. OnPax5 is constitutively expressed in various analyzed tissues of tilapia, with a relatively high expression in lymphoid organs, including spleen (SPL), anterior kidney (AK), and thymus. What's more, OnPax5 is highly expressed in leukocytes especially in IgM⁺ lymphocytes sorted from peripheral blood (PBL), SPL and AK. When stimulated with lipopolysaccharide (LPS) in vivo, OnPax5 expression was significantly up-regulated in PBL, SPL and AK. Upon stimulation with LPS, pokeweed mitogen and mouse anti-OnIgM monoclonal antibody in vitro, the expression of OnPax5 was also significantly up-regulated in leukocytes from SPL and AK. Taken together, Pax5, the B cell lineage specific activator factor, might get involved in B cell activation in Nile tilapia.

Transcriptional response of Atlantic salmon (Salmo salar) after primary versus secondary exposure to infectious salmon anemia virus (ISAV)

Following an infection with a specific pathogen, the acquired immune system of many teleostean fish, including salmonids, is known to retain a specific memory of the infectious agent, which protects the host against subsequent infections. For example, Atlantic salmon (Salmo salar) that have survived an infection with a low-virulence infectious salmon anemia virus (ISAV) isolate are less susceptible to subsequent ISAV infections. A greater understanding of the mechanisms and immunological components involved in this acquired protection against ISAV is fundamental for the development of efficacious vaccines and treatments against this pathogen. To better understand the immunity components involved in this observed resistance, we have used an Atlantic salmon DNA microarray to study the global gene expression responses of preexposed Atlantic salmon (fish having survived an infection with a low-virulence ISAV isolate) during the course of a secondary infection, 18 months later, with a high-virulence ISAV isolate. We present global gene expression patterns in both preexposed and naïve fish, following exposure by either cohabitation with infected fish or by direct intra-peritoneal injection of a high-virulence ISAV isolate. Our results show a clear reduction of ISAV viral loads in head-kidney of secondary infected fish compared to primary infected fish. Further, we note a lower-expression of many antiviral innate immunity genes in the secondary infected fish, such as the interferon induced GTP-binding protein Mx, CC-chemokine 19 and signal transducer and activator of transcription 1 (STAT 1), as well as MHC class I antigen presentation involved genes. Potential acquired immunity genes such as GILT, leukocyte antigen transcript CD37 and Ig mu chain C region membrane-bound form were observed to be over-expressed in secondary infected fish. The observed differential gene expression profile in secondary and primary infected fish head-kidney provides great insight into immunity components involved during primary and secondary ISAV infection.

Dissecting teleost B cell differentiation using transcription factors

B cell developmental pathways in teleost fishes are poorly understood. In the absence of serological reagents, an alternative approach to dissecting teleost B cell development is to use transcription factors that are differentially expressed during B cell development. This review discusses the structure and function of six transcription factors that play essential roles during teleost B cell development: Ikaros, E2A, EBF, Pax5, Blimp1, and XbpI. Research on alternative splicing of both the Ikaros and Pax5 genes in rainbow trout is presented, including their functional significance. An application is discussed that should aid in elucidating teleost B cell development and activation, by using transcription factors as developmental markers in flow cytometric analysis. Possible future studies in teleost B cell development are suggested in the context of gene regulation. Lastly, broader impacts and practical applications are discussed.

Comparative analyses of B cell populations in trout kidney and mouse bone marrow: establishing "B cell signatures"

This study aimed to identify the frequency and distribution of developing B cell populations in the kidney of the rainbow trout, using four molecular B cell markers that are highly conserved between species, including two transcription factors, Pax5 and EBF1, recombination-activating gene RAG1, and the immunoglobulin heavy chain mu. Three distinct B cell stages were defined: early developing B cells (CLP, pro-B, and early pre-B cells), late developing B cell (late pre-B, immature B, and mature B cells), and IgM-secreting cells. Developmental stage-specific, combinatorial expression of Pax5, EBF1, RAG1 and immunoglobulin mu was determined in trout anterior kidney cells by flow cytometry. Trout staining patterns were compared to a well-defined primary immune tissue, mouse bone marrow, and using mouse surface markers B220 and CD43. A remarkable level of similarity was uncovered between the primary immune tissues of both species. Subsequent analysis of the entire trout kidney, divided into five contiguous segments K1-K5, revealed a complex pattern of early developing, late developing, and IgM-secreting B cells. Patterns in anterior kidney segment K1 were most similar to those of mouse bone marrow, while the most posterior part of the kidney, K5, had many IgM-secreting cells, but lacked early developing B cells. A potential second B lymphopoiesis site was uncovered in segment K4 of the kidney. The B cell patterns, or "B cell signatures" described here provide information on the relative abundance of distinct developing B cell populations in the trout kidney, and can be used in future studies on B cell development in other vertebrate species.

Molecular and cellular analysis of B-cell populations in the rainbow trout using Pax5 and immunoglobulin markers

To date, the trout B-cell is poorly defined, as many essential molecular markers are not yet available for this species. In mammalian systems, the transcription factor Pax5, expressed from pre-B through plasmablast stages, provides an important marker for B-cell differentiation. In a previous study we showed that Pax5 is expressed in the trout. Here we identify trout B-cell populations that vary in expression of Pax5, membrane and secreted Ig. Immune tissues were separated based on concentration of surface IgM, and analyzed by qPCR and flow cytometry. Results suggest that spleen and PBL contain mostly resting B cells, which lack secreted Ig. While the great majority of splenic B cells become strongly activated upon LPS stimulation, PBLs do not. Additionally, anterior kidney contains both developing B and Ig-secreting B-cell populations, but few resting, mature B cells. Lastly, posterior kidney contains multiple B-cell populations in various states of activation. We conclude that trout immune tissues contain multiple, developmentally diverse and tissue-specific B-cell populations as defined by their relative expression of Pax5, surface IgM, and secreted IgM.

Response of Ig-positive cells to Enteromyxum scophthalmi (Myxozoa) experimental infection in turbot, Scophthalmus maximus (L.): A histopathological and immunohistochemical study

In recent years, a new parasite that causes severe losses has been detected in farmed turbot, Scophthalmus maximus (L.), in Northwestern Spain. Dead fish showed emaciation and cachexia caused by severe necrotizing enteritis that affected all portions of the digestive tract. The parasite was classified as a myxosporean and named Enteromyxum scophthalmi. This study was set up to gain insights into the immune response of fish against this parasitic infection. The kinetics of immunoglobulin positive (Ig+) cells in spleen, kidney and intestine from turbot experimentally infected with E. scophthalmi was studied. For evaluating both the progress of infection and the lesions induced by the parasite, we performed histopathological studies and for detecting Ig+ cells in situ we used an indirect immunohistochemical method. In fish exposed to the parasite, the number of Ig+ cells significantly increased in the intestine, the target organ of the parasite, whereas in spleen and kidney, haematopoietic organs where the parasite was not detected, the number of Ig+ cells decreased. Furthermore, the pattern of distribution of Ig+ cells changed in all three organs examined in recipient/infected fish compared with control fish (not exposed to the parasite). The results obtained in this study indicate that the infection by E. scophthalmi in turbot induced an immune response that involved changes in the number and distribution of Ig+ cells.

Immunodetection of lymphocyte subpopulations involved in allograft rejection in a teleost, Dicentrarchus labrax (L.)

Monoclonal antibodies which recognize antigenic determinants expressed by T-cells and Ig-bearing cells, respectively, allowed lymphocyte subpopulations involved in allograft rejection of muscle transplants to be identified in the teleost fish Dicentrarchus labrax (L.). The monoclonal antibody DLT15 first allowed recognizing T-cells involved in an in vivo antigen-driven cellular response in teleosts. Immunohistochemical studies showed a high density of lymphocytes in allografts and provided evidence of predominance of T-cells. The heterogeneity of the cell populations recognized by the antibodies was evidenced by the different size, cytology, and staining patterns of T-cells and Ig-bearing cells.

Lymphocyte development in fish and amphibians

Recently, molecular markers such as recombination activating genes (RAG), terminal deoxynucleotidyl transferase (TdT), stem cell leukemia hematopoietic transcription factor (SCL), Ikaros and gata-binding protein (Gata)-family members have been isolated and characterized from key lower vertebrates, adding to our growing knowledge of lymphopoiesis in ectotherms. In all gnathostomes there appear to be two main embryonic locations derived from the early mesoderm, both intra- and extraembryonic, which contribute to primitive and definitive hematopoiesis based upon their differential expression of SCL, Gata-1, Gata-2 and myeloblastosis oncogene (c-myb). In teleosts, a unique intraembryonic location for hematopoietic stem cells termed the intermediate cell mass (ICM) of Oellacher appears to be responsible for primitive or definitive hematopoiesis depending upon the species being investigated. In Xenopus, elegant grafting studies in combination with specific molecular markers has led to a better definition of the roles that ventral blood islands and dorsal lateral plate play in amphibian hematopoiesis, that of primitive and definitive lymphopoiesis. After the early embryonic contribution to hematopoiesis, specialized tissues must assume the role of providing the proper microenvironment for T and B-lymphocyte development from progenitor stem cells. In all gnathostomes, the thymus is the major site for T-cell maturation as evidenced by strong expression of developmental markers such as Ikaros, Rag and TdT plus expression of T-cell specific markers such as T-cell receptor beta and lck. In this respect, several zebrafish mutants have provided new insights on the development of the thymopoietic environment. On the other hand, the sites for B-cell lymphopoiesis are less clear among the lower vertebrates. In elasmobranchs, the spleen, Leydig's organ and the spiral valve may all contribute to B-cell development, although pre-B cells have yet to be fully addressed in fish. In teleosts, the kidney is undeniably the major source of B-cell development based upon functional, cellular and molecular indices. Amphibians appear to use several different sites (spleen, bone marrow and/or kidney) depending upon the species in question.

Spleen and kidney of Atlantic salmon (Salmo salar L.) show histochemical changes early in the course of experimentally induced infectious salmon anaemia (ISA)

Infectious salmon anaemia (ISA) is a disease of farmed Atlantic salmon (Salmo salar L.) in Norway that affects both erythrocytic and leucocytic cells. Both cell types are possible target cells for the aetiological ISA agent, which is probably a virus. In the present study the distribution and phenotype of leucocyte populations in the spleen and head kidney of Atlantic salmon that were developing ISA have been examined. Frozen tissues were collected from fish at various times after inoculation with ISA-infective material. Immune and enzyme histochemical techniques were used to characterise the response of leucocyte populations. Acid phosphatase positive macrophages predominantly in the red pulp of the spleen appeared to have engulfed erythrocytes at day 4 after infection. Evidence of degradation products of phagocytosed erythrocytes was present in macrophages in red pulp of the spleen at day 7 after infection, in addition to the usual site of erythrophagocytosis in melanomacrophage accumulations. Signs of erythrophagocytosis were not found in the head or body portions of the kidney. The activation of macrophages in the spleen at day 7 was suggested by decreased reactivity for the enzyme 5' nucleotidase. From day 7, clusters of immunoglobulin positive (Ig +) cells were present in the head kidney, while from day 11, the ellipsoids of the spleen showed reactivity for Ig and complement factor C3. These observations are discussed in relation to early immunoglobulin production and possible immune complex trapping. The present results suggest that the leucocyte populations in Atlantic salmon respond to ISA infection through macrophage activation and the initiation of an immune response.

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

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

Ontogeny of IgM and IgM-bearing cells in rainbow trout

We have studied the ontogenic development of immunoglobulin M (IgM) and of IgM-bearing cells in the rainbow trout, Oncorhynchus mykiss. Lymphocytes showing cytoplasmic IgM were first observed in embryos at 12 days before hatch (14 degrees C). At this stage, no cells positive for surface IgM were present. Lymphocytes bearing surface IgM were observed at 8 days before hatch (14 degrees C). Unfertilized trout eggs contained detectable amounts of IgM (11.2 +/- 2.6 micrograms/g of egg weight), indicating that transfer of IgM from mother to embryo can occur in salmonids. The levels of IgM from whole fish increase slowly after the appearance of intraembryonic cells that express surface IgM. The amount of IgM/g of tissue peaks around hatch, but this parameter shows lower values up to 2 months after hatch.

Phylogeny of immune recognition: antigen processing/presentation in channel catfish immune responses to hemocyanins

Studies were conducted to address the role(s) of antigen (Ag) processing/presentation in channel catfish immune responses. Vigorous and specific secondary in vitro proliferative and antibody (Ab) responses were obtained to keyhole limpet and Limulus polyphemus hemocyanins with peripheral blood leukocytes (PBL) from catfish previously primed in vivo with Ag. In addition, such antigen-specific in vitro proliferative and Ab responses were efficiently elicited by antigen-pulsed and subsequently paraformaldehyde-fixed autologous PBL used as putative antigen-presenting cells (APC) but not by APC fixed prior to Ag pulsing. Treatment of these putative APC with lysosomotropic agents, protease inhibitors, or the ionophore monensin prior to or during pulsing with Ag significantly inhibited both in vitro responses. Furthermore, the use of radiolabeled protein indicated that both untreated and inhibitor-treated PBL but not erythrocytes take up Ag; however, only untreated PBL were able to degrade Ag. Immune restriction was indicated by the use of allogeneic PBL as APC in that only strong MLRs were generated with no detectable antibodies produced in vitro. Finally, the employment of isolated leukocyte subpopulations demonstrated that both catfish B (sIg+) lymphocytes and monocytes were efficient Ag presentors.