Spleen tyrosine kinase from Nile tilapia (Oreochromis niloticus): Molecular characterization, expression pattern upon bacterial infection and the potential role in BCR signaling and inflammatory response

CXCL12/CXCR4 Axis Promotes the Chemotaxis and Phagocytosis of B Cells through the PI3K-AKT Signaling Pathway in an Early Vertebrate

Chemokines play crucial roles in the regulation of immune cell migration and development. The CXCL12/CXCR4 axis has been extensively studied in mammals, but its regulatory mechanism in teleost fish remains unclear. In this study, we used Nile tilapia (Oreochromis niloticus) as a teleost model to investigate the mediation of the CXCL12/CXCR4 axis in IgM+ B cells. Our findings demonstrate that the CXCL12/CXCR4 axis exhibits chemotactic activity on IgM+ B cells and promotes the phagocytosis of IgM+ B cells. Blocking CXCR4 severely impairs the chemotaxis and phagocytosis of IgM+ B cells in vitro and reduces the percentages and numbers of IgM+ B cells that migrate to peripheral blood after pathogen infection in vivo. This reduction in migration leads to a decrease in the inflammatory response, an increase in tissue bacterial load, and a decrease in survival rate. We also discovered that the evolutionarily conserved PI3K-AKT signaling pathway and Girdin are involved in the immune response during Streptococcus agalactiae infection. Inhibitors of the PI3K-AKT signaling pathway prevent the chemotaxis and phagocytosis of IgM+ B cells, impair the expression and phosphorylation levels of related proteins in vitro, and prevent IgM+ B cells chemotaxis into the peripheral blood after pathogen infection in vivo. Furthermore, CXCR4 blocking significantly downregulates the expression of AKT and Girdin. Overall, our study reveals the regulatory mechanism of the CXCL12/CXCR4 axis on IgM+ B cells via the PI3K-AKT signaling pathway in tilapia, suggesting that the functions of the CXCL12/CXCR4 axis in B cells may be conserved between mammals and teleost fish.

Identifying transcriptomic indicators of tertiary treated municipal effluent in longnose dace (Rhinichthys cataractae) caged under semi-controlled conditions in experimental raceways

To evaluate effects of tertiary treated wastewater treatment plant effluent (MWWE) on transcriptomic responses in longnose dace (Rhinichthys cataractae; LND) we conducted a semi-controlled study in experimental raceways (Advancing Canadian Water Assets facility) imbedded in the Pine Creek treatment plant (Calgary, AB). LND collected from a reference site in the Bow River (REF) were caged in raceways containing either 5 % Pine Creek effluent (PC) or Bow River water (BR; control) over 28 d. Liver transcriptomes were analyzed in males and females sampled on days 7, 14 and 28 from BR and PC, and compared to REF fish on day 0. Concurrent with the caging, selected environmental substances of concern were analyzed in the BR and PC. Significantly different unigenes (SDUs) in females (vs males) within both BR and PC raceways increased over time and compared to REF fish. Moreover, SDUs in females and males within the same treatment (i.e., BR, PC) showed a temporal increase as well as compared to REF fish. Time was the dominant factor affecting SDUs, whereas sex and treatment had less of an impact on the transcriptome profiling. Gene Set Enrichment Analysis of BR vs PC over time revealed effects on genes involved in growth, metabolism of carbohydrates and lipids, and immune system on day 7; however, by day 28, 80-100 % of the transcripts localized to enriched biomarkers were associated with tissue immune responses in both sexes. Exposure to 5 % effluent had significant effects on female liver somatic index but no effects were observed on other phenotypic health indices in either sex. BR was used as the source of reference water, but analyses showed trace amounts of ESOCs. Analyses did not point towards definitive response patterns that could be used in field-based ecotoxicogenomic studies on the impacts of well-treated MWWE but suggested compromised adaptive immune responses.

A review of soluble factors and receptors involved in fish skin immunity: The tip of the iceberg

The immune system of fish possesses soluble factors, receptors, pathways and cells very similar to those of the other vertebrates' immune system. Throughout evolutionary history, the exocrine secretions of organisms have accumulated a large reservoir of soluble factors that serve to protect organisms from microbial pathogens that could disrupt mucosal barrier homeostasis. In parallel, a diverse set of recognition molecules have been discovered that alert the organism to the presence of pathogens. The known functions of both the soluble factors and receptors mentioned above encompass critical aspects of host defense, such as pathogen binding and neutralization, opsonization, or modulation of inflammation if present. The molecules and receptors cooperate and are able to initiate the most appropriate immune response in an attempt to eliminate pathogens before host infection can begin. Furthermore, these recognition molecules, working in coordination with soluble defence factors, collaboratively erect a robust and perfectly coordinated defence system with complementary specificity, activity and tissue distribution. This intricate network constitutes an immensely effective defence mechanism for fish. In this context, the present review focuses on some of the main soluble factors and recognition molecules studied in the last decade in the skin mucosa of teleost fish. However, knowledge of these molecules is still very limited in all teleosts. Therefore, further studies are suggested throughout the review that would help to better understand the functions in which the proteins studied are involved.

Syk regulates the haemocyte autophagy through inducing the mRNA expressions of autophagy-related genes and the cleavage of CgLC3 in oyster antibacterial immunity

Spleen tyrosine kinase (Syk) is reported to be involved in activating the autophagy. Recently, a homologue of Syk was identified from Pacific oyster Crassostrea gigas (defined as CgSyk). In the present study, the molecular characteristics of CgSyk and its regulation mechanism in autophagy were investigated in oyster C. gigas. The full-length cDNA of CgSyk was of 4566 bp with an open reading frame (ORF) of 1989 bp. CgSyk encoded a polypeptide of 662 amino acids, containing two Src homology 2 (SH2) domains and one tyrosine kinase catalytic (TyrKc) domain. The deduced amino acid sequence of CgSyk shared low similarity with the previously identified Syks from other species. In the phylogenetic tree, CgSyk was first clustered with Crassostrea virginica CvSyk, and then classified into a branch of invertebrate Syks. In CgSyk-RNAi oysters, the mRNA expressions of CgLC3, CgP62, CgBeclin-1 and CgATG5 in haemocytes decreased significantly at 12 h after Vibrio splendidus stimulation. At the same time, the abundance of CgLC3Ⅱ in haemocytes, and the autophagy rate of haemocytes in CgSyk-RNAi oysters decreased significantly at 12 h after V. splendidus stimulation. All the results collectively suggested that CgSyk regulated the autophagy through inducing the mRNA expressions of autophagy-related genes and the cleavage of CgLC3 to defend against bacterial invasion in oysters.

Affinity-Driven Site-Specific High Mannose Modification Determines the Structural Polymerization and Function of Tetrameric IgM in a Primitive Vertebrate

Teleost tetramer IgM is the predominant Ig in the immune system and plays essential roles in host defense against microbial infection. Due to variable disulfide polymerization of the monomeric subunits, tetrameric IgM possesses considerable structural diversity. Previous work indicated that the teleost IgM H chain was fully occupied with complex-type N-glycans. However, after challenge with trinitrophenyl (TNP) Ag, the complex N-glycans in the Asn-509 site of Oreochromis niloticus IgM H chain transformed into high mannose. This study, therefore, was conducted to examine the functional roles of the affinity-related high-mannose modification in tilapia IgM. The TNP-specific IgM Ab affinity maturation was revealed in tilapia over the response. A positive correlation between TNP-specific IgM affinity and its disulfide polymerization level of isomeric structure was demonstrated. Mass spectrometric analysis indicated that the relationship between IgM affinity and disulfide polymerization was associated with the Asn-509 site-specific high-mannose modification. Furthermore, the increase of high mannose content promoted the combination of IgM and mannose receptor (MR) on the surface of phagocytes. Moreover, the increased interaction of IgM and MR amplified the phagocytic ability of phagocytes to Streptococcus agalactiae. To our knowledge, this study demonstrates that site-specific high-mannose modification associates with IgM Ab affinity and its structural disulfide polymerization and amplifies the phagocytosis of phagocytes by the combination of IgM and MR. The present study provides evidence for understanding the association of IgM structure and function during the evolution of the immune system.

Transcriptome analysis and protein-protein interaction in resistant and susceptible families of Japanese flounder (Paralichthys olivaceus) to understand the mechanism against Edwardsiella tarda

Edwardsiella tarda is one of the most harmful bacterial pathogens for aquaculture flatfish. After artificial infection of 47 Japanese flounder (Paralichthys olivaceus) families, resistant and susceptible families were identified in this study. High-throughput sequencing was performed on the liver transcriptome of uninfected groups (PoRU and PoSU) and infected groups (PoRC and PoSC). Through assembly and annotation, a total of 3012 and 1386 differentially expressed genes (DEGs) were identified in PoRU vs. PoSU and PoRC vs. PoSC. The significant enrichment pathways between PoRU and PoSU were mainly in metabolic and biosynthesis pathways. A total of thirty dominant enrichment pathways between PoRC and PoSC mainly focused on some immune-related pathways, including the hematopoietic cell lineage, cytokine-cytokine receptor interaction, complement and coagulation cascades, antigen processing and presentation, the intestinal immune network for immunoglobulin A (IgA) production and T/B cell receptor signaling pathway. Under the protein-protein interaction (PPI) analysis, hub genes, including CD molecules, complement component factors and chemokines, were identified in the network, and 16 core genes were differentially expressed in resistant and sustainable families in quantitative polymerase chain reaction (qPCR) validation. This study represents the first transcriptome analysis based on resistant and susceptible families and provides resistant genes to understand the potential molecular mechanisms of antibacterial function in marine fish. The results obtained in this study provide crucial information on gene markers for resistant breeding of Japanese flounder.

Molecular characterization and expression analysis of chemokine (CXCL12) from Nile tilapia (Oreochromis niloticus)

Chemokines are a class of small molecular weight cytokines of 6-14 kDa, exerting important roles in the regulation of various inflammatory diseases and immune cell migration. In this study, we have identified the CXCL12 gene from Nile tilapia (Oreochromis niloticus), including CXCL12a (OnCXCL12a) and CXCL12b (OnCXCL12b). The open reading frames of OnCXCL12a and OnCXCL12b are 309 and 297 bp, encoding 102 and 98 amino acids, respectively. Multiple alignment showed that OnCXCL12a and OnCXCL12b have characteristics of CXC chemokines and share high identity with CXCL12 amino acid sequences from the known species. Tissue distribution in the healthy fish indicated that OnCXCL12a and OnCXCL12b expressed in all examined tissues, with the highest expression in muscle and anterior kidney, respectively. After challenged by Streptococcus agalactiae, Poly(I:C) and LPS in vivo and in vitro, OnCXCL12 is transcriptionally up-regulated in immune tissues and cells significantly. The recombinant OnCXCL12 proteins, (r)OnCXCL12a and (r)OnCXCL12b, enhance the release of nitric oxide and increase the expression of inflammatory cytokines (TNF-α, IL-6, and IL-10) in anterior kidney leukocytes, as well as exhibit chemotactic activity for leukocytes from anterior kidney. Summarizing, these results indicate that OnCXCL12 is involved in the immune response of Nile tilapia against pathogen infection and may play an important role in mediating inflammatory response.

Relative content detection of oligomannose modification of IgM heavy chain induced by TNP-antigen in an early vertebrate through nanoLC-MS/MS

N-glycan modification is reported to be important in regulating the structure and function of immunoglobulins in mammals. While, the study on teleost immunoglobulin glycosylation is still limitted. In this study, we constructed a TNP-antigen driven model, and detected the site-specific N-glycans of PBS-immunized and TNP-specific Oreochromis niloticus serum IgM through ¹⁸O-labeling and nanoLC-MS/MS. These methods are widely used for peptide enrichment and protein modification identification, but rarely used in detecting the level of N-glycosylation in teleost Igs that driven by specific antigen. The results revealed that there are four N-glycosylation sites in O.niloticus IgM heavy chain, namely, the Asn-315 site in the CH2 domain, the Asn-338 site in the CH3 domain, and the Asn-509 and Asn-551 sites in the CH4 domain, All of the four residues were efficiently N-glycosylated. After immunized with TNP-antigen, the signal strength of oligomannose in the TNP-specific IgM in primary mass spectrometry was significantly higher than that in the PBS-immunized IgM. Notably, the TNP-specific IgM had an Asn-509 site fully occupied with oligomannose, while only a small amount of oligomannose was found in the PBS-immunized IgM of this site. N-glycans in other sites were mainly complex-type with a low content of fucosylation and sialylated. The oligomannose in TNP-specific IgM was further verified to be essential for the binding of IgM and MBL. These results demonstrated that the TNP-antigen induced the site-specific oligomannose modification of O.niloticus IgM heavy chain, and played an important role in the interaction of IgM and MBL, which provided insights into the evolutionary understanding of the IgM oligomannose modification and function.

A Siglec-1-like lectin from Nile tilapia (Oreochromis niloticus) possesses functions of agglutination and mediation of macrophage phagocytic activity

Siglec-1, one of the sialic acid-binding immunoglobulin-type lectins, is closely related to the recognition of host-pathogen and cell-cell interactions in the adaptive and innate immune systems. In this communication, a Siglec-1-like gene (OnSiglec-1-like) from Nile tilapia (Oreochromis niloticus) was analyzed. Relative expression revealed that the OnSiglec-1-like was expressed in all tested tissues, and the highest expression was found in the anterior kidney. Upon Streptococcus agalactiae (S. agalactiae) infection, the expression of OnSiglec-1-like was up-regulated in anterior kidney and spleen significantly in vivo. Additionally, the same phenomenon was observed in anterior kidney leukocytes upon LPS and S. agalactiae challenges as well in vitro. Western-blotting and ELISA analyses revealed that recombinant OnSiglec-1-like protein possessed high binding activity to LTA, LPS and S. agalactiae. Further, the recombinant OnSiglec-1-like was able to agglutinate S. agalactiae. Moreover, with the digestion of specific sialidase, the phagocytic ability of macrophages to S. agalactiae was greatly enhanced. Taken together, these results indicated that the Siglec-1-like possesses conserved functions of agglutination and promotion of macrophage phagocytic activity in Nile tilapia.

SHP1 tyrosine phosphatase gets involved in host defense against Streptococcus agalactiae infection and BCR signaling pathway in Nile tilapia (Oreochromis niloticus)

Src homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1), a kind of protein tyrosine phosphatases (PTPs), is a critical regulator of antigen receptor signal transduction. Signal transduction of BCR is regulated by phosphatases in teleost as in mammals. In this study, SHP1 from Nile tilapia (Oreochromis niloticus) (OnSHP1) was identified and characterized, including the expression pattern against bacterial infection and regulation function in BCR signaling pathway. The open reading frame of OnSHP1 contains 1749 bp of nucleotide sequence, encoding a protein of 582 amino acids. The OnSHP1 protein was highly conversed compared to that of other species, including two amino-terminal SH2 domains at the N terminus and a PTP catalytic domain. Transcriptional expression analysis revealed that OnSHP1 was detected in all examined tissues and highly expressed in spleen. The up-regulated OnSHP1 expression was observed in peripheral blood, spleen and anterior kidney following challenge with Streptococcus agalactiae or lipopolysaccharide (LPS) in vivo, as well as that displayed in leukocytes stimulated with S. agalactiae or LPS in vitro. Further, after induction with mouse anti-tilapia IgM monoclonal antibody in vitro, OnSHP1 was significantly up-regulated in leukocytes. When spleen leukocytes treated with PTP Inhibitor II in vitro, the phosphorylation level of OnSHP1 at the phosphorylation sites (Y⁵³⁵ and Y⁵⁵⁷) and the cytoplasmic free Ca²⁺ concentration were up-regulated significantly. Overall, the findings of this study indicate that SHP1 gets involved in host defense against bacterial infection and BCR signaling pathway in Nile tilapia.

CD38 play roles in T cell-dependent response and B cell differentiation in nile tilapia (Oreochromis niloticus)

CD38 is a multifunctional cell surface molecule that plays a crucial role in B cell activation, differentiation, and maturation in mammals with an increased expression in B cell maturation. In this study, a CD38-like molecule (OnCD38) was cloned and identified from Nile tilapia (Oreochromis niloticus), and its functional characterization was investigated. The open reading frame of OnCD38 is 828 bp of the nucleotide sequence, encoding a polypeptide of 275 amino acids. The deduced amino acid sequence of OnCD38 is highly homologous to other teleost fish and similar to mammals, containing extracellular, intracellular and transmembrane regions. Subcellular localization studies revealed that OnCD38 molecules were presented on the surface of B cells. Three healthy tilapia were used in each experimental group and control group. Following keyhole limpet hemocyanin (KLH) challenge in vivo, the mRNA expression of OnCD38 was significantly up-regulated in peripheral blood, spleen, and head kidney, with an earlier up-regulation in the second challenge than the first one. The up-regulation of OnCD38 expression was also detected in head kidney leukocytes after stimulation with LPS, recombinant HomoIL-10 ((r)HomoIL-10), (r)OnIL-10, and LPS plus (r)OnIL-10 in vitro. Furthermore, the OnCD38 expression increased with the differentiation of B cells, reaching a high level (10.1 fold higher than resting mature B cells) at the plasma-like B cells. Taken together, in this study, these results indicate that the OnCD38 is likely involved in the T cell-dependent response and plays roles in B cell differentiation in Nile tilapia.

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.

Effects of Cell Differentiation on the Phagocytic Activities of IgM+ B Cells in a Teleost Fish

Teleost B cells have phagocytic activities for ingesting particulate antigens, such as bacteria, in addition to the functional secretion of immunoglobulins (Igs). In the present study, the phagocytic activities of IgM⁺ B cells under various differentiational conditions residing in peripheral blood leukocytes were investigated in a teleost fish Nile tilapia (Oreochromis niloticus). The IgM⁺ B cells were recognized as IgM^lo or IgM^hi subsets based on their membrane IgM (mIgM) levels. The mIgM, secreted IgM (sIgM), major histocompatibility complex class II and reactive oxygen species were detected. Expressions of transcription factors (Pax5 and Blimp-1) and B cell signaling molecules (CD79a, CD79b, BLNK, and LYN) suggested that IgM^lo B cells were resembling as plasma-like cells and IgM^hi resembling as naïve/mature B cells, respectively. Analysis of phagocytic activities demonstrated that both IgM^lo and IgM^hi B cells have a similar phagocytic ability (phagocytosis percentage); however, the phagocytic capacity [phagocytic index and the mean fluorescence intensity (MFI)] of IgM^hi B cells was significantly higher than that of IgM^lo B cells. Taken together, the results indicated that B cell differentiation may cause the decrease of phagocytic capacity but not phagocytic ability of phagocytic IgM⁺ B cells in teleost. The finding may provide an evolutionary evidence for understanding the greater specialization of the B cell in more sophisticated adaptive humoral immunity, by decreasing phagocytic activity in order to contribute its function more specifically into antibody-secreting.

Blimp-1 is involved in B cell activation and maturation in Nile tilapia (Oreochromis niloticus)

B lymphocyte-induced maturation protein 1 (Blimp-1), a transcription factor containing zinc finger, is required and sufficient to trigger terminal differentiation of B cells in mammals. The Blimp-1 (OnBlimp-1) from Nile tilapia (Oreochromis niloticus) was identified and characterized its expression pattern during B cell activation and maturation. The cDNA of OnBlimp-1 open reading frame is 2547 bp encoding a protein of 848 amino acids and the predicted molecular weight is 93.36 kDa. OnBlimp-1 contains a SET domain and five ZnF_C2H2 domains, which shares high homology with that of other species. OnBlimp-1 transcription was detected in all examined tissues with high expression in the spleen (SPL). Analysis of sorted lymphocyte populations, including IgM⁺ and IgM^- cells from peripheral blood (PBL), SPL and anterior kidney (AK), indicated that the OnBlimp-1 transcription was highly expressed in the IgM⁺ B cells. Upon LPS stimulation, OnBlimp-1 expression was up-regulated in tissues of PBL, SPL and AK significantly. The expression of OnBlimp-1, as well as the secreted IgM, was significantly up-regulated in the SPL and AK leukocytes stimulated with anti-OnIgM monoclonal antibody and LPS in vitro, respectively. Above results suggest that OnBlimp-1, a cytokine regulating the terminal differentiation of activated B cells to antibody-secreting cells, is likely to play important roles in B cell activation and maturation in Nile tilapia.

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.

Lyn is involved in host defense against S. agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

Lyn, a member of Src protein kinase family, plays a crucial role in immune reactions against pathogenic infection. In this study, Lyn from Nile tilapia (Oreochromis niloticus) (OnLyn) was identified and characterized at expression pattern against bacterial infection, and regulation function in BCR signaling. The open reading frame of OnLyn contained 1536 bp of nucleotide sequence encoded a protein of 511 amino acids. The OnLyn protein was highly conversed to other species Lyn, including SH3, SH2 and a catalytic Tyr kinase (TyrKc) domain. Transcriptional expression analysis revealed that OnLyn was detected in all examined tissues and was highly expressed in the head kidney. The up-regulation OnLyn expression was observed in the head kidney and spleen following challenge with Streptococcus agalactiae (S. agalactiae) in vivo, and was also displayed in head kidney leukocytes challenge with S. agalactiae and LPS in vitro. In addition, after induction with mouse anti-OnIgM mAb in vitro, the OnLyn expression and phosphorylation of OnLyn (Y⁵⁰⁷) were significantly up-regulated in the head kidney leukocytes. Moreover, after treatment with AZD0530 and mouse anti-OnIgM monoclonal antibody, the down-regulation of cytoplasmic free-Ca²⁺ concentration was detected in the head kidney leukocytes in vitro. Taken together, the findings of this study revealed that OnLyn might play potential roles in BCR signaling and get involved in host defense against bacterial infection in Nile tilapia.

B cell receptor accessory molecule CD79 gets involved in response against Streptococcus agalactiae infection and BCR signaling in Nile tilapia (Oreochromis niloticus)

CD79, composed of two distinct chains called CD79a and CD79b, is a transmembrane protein that forms a B cell antigen receptor with membrane immunoglobulin, and generates a signal following antigen recognition by the B cell receptor. In this study, the CD79a (OnCD79a) and CD79b (OnCD79b) were cloned and identified from Nile tilapia (Oreochromis niloticus). The cDNA of ORF for OnCD79a and OnCD79b are 669 and 627 bp, coding 222 and 208 amino acids, respectively. The deduced protein analysis showed that both CD79a andCD79b contain an immunoreceptor tyrosine-based activation motif in their intracellular tails that used to propagate a signal in a B cell. Expression analysis revealed that both CD79a and CD79b expressed at high levels in immune tissues, such as anterior kidney and spleen, and in IgM⁺ B cells. Upon Streptococcus agalactiae (S. agalactiae) infection, the expressions of OnCD79a and OnCD79b were significantly up-regulated in anterior kidney and spleen. The significant up-regulations of OnCD79a and OnCD79b were also detected in leukocytes after in vitro challenge with S. agalactiae. Further, stimulations of LPS and anti-OnIgM monoclonal antibody induced significant up-regulations of OnCD79a and OnCD79b in leukocytes. Taken together, the results of this study indicated that CD79 molecule, playing roles in BCR signaling, was likely to get involved in host defense against bacterial infection in Nile tilapia.

Interleukin-6 gets involved in response to bacterial infection and promotes antibody production in Nile tilapia (Oreochromis niloticus)

Interleukin 6 (IL-6), a pleiotropic cytokine, plays an important role in humoral immune response, not only inducing the differentiation of B cells into plasma cells, but also promoting antibody-secreting cells (ASCs) to produce antibodies. In this study, Nile tilapia (Oreochromis niloticus) IL-6 (OnIL-6) was identified and characterized at expression level in response to bacterial infection and promotion of antibody production. The open reading frame of OnIL-6 ORF is consisted of 663 bp encoding a polypeptide of 220 amino acids. The deduced OnIL-6 protein contained an IL-6/G-CSF family signature, two conserved cysteine, and four α-helix bundles, which was highly homologous to other species. Spatial mRNA expression analysis revealed that the highest expression of OnIL-6 was observed in the thymus. After in vivo challenges of lipopolysaccharide (LPS) and Streptococcus agalactia (S. agalactiae), OnIL-6 expressions were significantly up-regulated in head kidney and spleen. The similar up-regulation of OnIL-6 was observed in the head kidney and spleen leukocytes in vitro stimulation with LPS and S. agalactiae. In addition, inducement with the recombinant OnIL-6 ((r)OnIL-6) in vitro caused significant increases in expressions of both sIgM and mIgM. Moreover, the (r)OnIL-6 stimulation enhanced the secretion of sIgM (more especially in P50 plasma-like B cells) and the production of mIgM in P60 and P70 B cell subsets (resting B cells, activated B cells and plasmablast-like B cells) in vitro. Taken together, this study indicated that OnIL-6 might be involved in host defense against bacterial infection and promote the production of antibody in Nile tilapia.