HMG20A from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

A novel C-type lectin, perlucin, from the small abalone, Haliotis diversicolor involved in the innate immune defense against Vibrio harveyi infection

C-type lectins (CTLs), a member of pattern recognition receptors, play an important role in the innate immunity by recognizing invading microorganisms. In this study, a novel perlucin gene (designated as HdPer 3), a typical CTLs was cloned and characterized from the small abalone Haliotis diversicolor. The open reading frame of HdPer 3 was 471 bp, encoding a protein of 156 amino acids that included a single carbohydrate-recognition domain. HdPer 3 was widely expressed in all tested tissues and developmental stage. HdPer 3 expression was significantly up-regulated after Vibrio harveyi infection, suggesting that HdPer 3 was activated in response to bacterial infection. The encapsulation ability of hemocytes could be significantly enhanced by the recombinant protein HdPer 3 (rHdPer 3). To understand the regulation mechanism of the HdPer 3, HdPer 3 was silenced in vivo by RNAi. Knocking down HdPer 3 decreased the hemocytes phagocytosis. Meanwhile, knocking down HdPer 3 can reduce the expression of 2 phagocytosis-related genes (Rab and Dynamin), TNF-α, and 2 MAPK pathway-related genes (MAPK-X1 and Ras) after V. harveyi infection. Moreover, HdPer 3 interference could increase the bacterial load in the hemolymph and the mortality of abalones after V. harveyi infection. All these results suggested that HdPer 3 played a crucial role in the defense against V. harveyi infection by recognizing bacterial pathogens and activating the expression of immune-related genes.

Siglec7 functions as an inhibitory receptor of non-specific cytotoxic cells and can regulate the innate immune responses in a primitive vertebrate (Oreochromis niloticus)

In mammals, siglec7, an integral component of the siglecs, is principally found on the surface of natural killer (NK) cells, macrophages, and monocytes, where it interacts with various pathogens to perform immunological regulatory activities. Nonetheless, the immune defense and mechanism of siglec7 in early vertebrates remain unknown. In this study, we identified siglec7 from Oreochromis niloticus (OnSiglec7) and revealed its immune functions. Specifically, OnSiglec7 was abundantly expressed in immune-related tissues of healthy tilapia and its transcription level was strongly activated after being challenged with A. hydrophila, S. agalactiae, and Poly: IC. Meanwhile, OnSiglec7 protein was purified and analyzed, which could recognize multiple pathogens through binding and agglutinating activity. Moreover, OnSiglec7-positive cells were mainly distributed in non-specific cytotoxic cells (NCC) of tilapia HKLs and showed cell membrane localization. Furthermore, OnSiglec7 blockage affected multiple innate immune responses (inflammation, apoptosis, and pyroptosis process) by regulating the activation of MAPK, NF-κB, TLR, and JAK-STAT pathways. Finally, OnSiglec7 blockage also greatly enhanced the cytotoxic effect of tilapia NCC. Summarily, this study uncovers immune functions and mechanisms of siglec7 in primitive vertebrates, thereby enhancing our understanding of the systemic evolution and ancient functions of other siglecs within the host's innate immune system (to our knowledge).

The role of HMGB2 in the immune response of Nile tilapia (Oreochromis niloticus) to streptococcal infection

High mobility group protein B2 (HMGB2) is an abundant chromatin-associated protein with pivotal roles in transcription, cell proliferation, differentiation, inflammation, and tumorigenesis. However, its immune function in Nile tilapia (Oreochromis niloticus) remains unclear. In this study, we identified a homologue of HMGB2 from Nile tilapia (On-HMGB2) and investigated its functions in the immune response against streptococcus infection. The open reading frame (ORF) of On-HMGB2 spans 642 bp, encoding 213 amino acids, and contains two conserved HMG domains. On-HMGB2 shares over 80 % homology with other fish species and 74%-76 % homology with mammals. On-HMGB2 was widely distributed in various tissues, with its highest transcript levels in the liver and the lowest in the intestine. Knockdown of On-HMGB2 promoted the inflammatory response in Nile tilapia, increased the bacterial load in the tissues, and led to elevated mortality in Nile tilapia following Streptococcus agalactiae infection. Taken together, On-HMGB2 significantly influences the immune system of Nile tilapia in response to streptococcus infection.

Apolipoprotein Eb (On-ApoEb) protects Oreochromis niloticus against Streptococcus agalactiae infection

Apolipoprotein E (ApoE), a critical targeting protein, has been found to play an essential role in the protection against infection and inflammation. However, the immune functions of ApoE against bacterial infection in fish have not been investigated. In this study, a full-length cDNA for ApoE, named On-ApoEb was cloned from Oreochromis niloticus. The predicted cDNA sequence was 831bp in length and coded for a protein of 276 amino acid residues, which shared 63.87%-98.55% identity with ApoEb from other fishes, and about 22% identity with ApoEb from mammals. On-ApoEb from O. niloticus was highly expressed in the liver and could be activated in the tissues (liver, spleen, brain, and intestine) after infection with Streptococcus agalactiae. Moreover, the results revealed that On-ApoEb could decrease the expression levels of pro-inflammatory factors, immune-related pathways, and apoptosis, while increasing the expression levels of anti-inflammatory factors. Furthermore, On-ApoEb was noted to improve the survival rate and reduce the bacterial load in the liver and spleen. These results suggested that On-ApoEb was connected with immune response and had anti-inflammation and anti-apoptosis activities.

A novel perlucin-like protein (PLP) protects Litopenaeus vannamei against Vibrio harveyi infection

C-type lectins (CTLs), as pattern recognition receptors (PRRs), play an important role in the innate immunity of Litopenaeus vannamei. In this study, a novel CTL, named perlucin-like protein (PLP), was identified from L. vannamei, which shared homology sequences of PLP from Penaeus monodon. PLP from L. vannamei was expressed in the hepatopancreas, eyestalk, muscle and brain and could be activated in the tissues (hepatopancreas, muscle, gill and intestine) after infection with the pathogen Vibrio harveyi. Bacteria (Vibrio alginolyticus, V. parahaemolyticus, V. harveyi, Streptococcus agalactiae and Bacillus subtilis) could be bound and agglutinated by the PLP recombinant protein in a Ca2+-dependent manner. Moreover, PLP could stabilise the expression of the immune-related genes (ALF, SOD, HSP70, Toll4 and IMD) and apoptosis gene (Caspase2). The RNAi of PLP could remarkably affect the expression of antioxidant gene, antimicrobial peptide genes, other CTLs, apoptosis genes, Toll signaling pathways, and IMD signaling pathways. Moreover, PLP reduced the bacterial load in the hepatopancreas. These results suggested that PLP was involved in the innate immune response against V. harveyi infection by recognising bacterial pathogens and activating the expression of immune-related and apoptosis genes.

Molecular characterization and functional analysis of CD209E from Nile Tilapia (Oreochromis Niloticus) involved in immune response to bacterial infection

CD209 plays significant roles in pathogen recognition, innate and adaptive immunity, and cell-cell interactions. In the present study, a CD209 antigen-like protein E from Nile tilapia (Oreochromis niloticus) (designated as OnCD209E) was identified and characterized. OnCD209E contains an open reading frame (ORF) of 771 bp encoding a 257 amino acid protein, as well as the carbohydrate recognition domain (CRD). Multiple sequence analysis exhibits that the amino acid sequence of OnCD209E was relatively high homologous to that of partial fish, especially the highly conserved CRD, in which four conserved disulfide-bonded cysteine residues, WIGL conserved motif and two Ca²⁺/carbohydrate-binding sites (EPD and WFD motifs) were founded. Quantitative real-time PCR and Western Blot revealed that OnCD209E mRNA/protein is generally expressed in all tissues examined, but with wealth in head kidney and spleen tissues. The mRNA expression of OnCD209E was significantly increased in brain, head kidney, intestine, liver, and spleen tissues in response to the stimulation with polyinosinic-polycytidylic acid, Streptococcus agalactiae and Aeromonas hydrophila in vitro. Recombinant OnCD209E protein exhibited detectable bacterial binding and agglutination activity against different bacteria as well as inhibited the proliferation of tested bacteria. Subcellular localization analysis revealed that OnCD209E was mostly localized in the cell membrane. Moreover, overexpression of OnCD209E could activate nuclear factor-kappa B reporter genes in HEK-293T cells. Collectively, these results demonstrated that CD209E may potentially involve in immune response of Nile tilapia against bacterial infection.

A novel C-type lectin (CLEC12B) from Nile tilapia (Oreochromis niloticus) is involved in host defense against bacterial infection

C-type lectin (CLEC) is a family of carbohydrate-binding protein that has high affinity for calcium and mediates multiple biological events including adhesion between cells, the turnover of serum glycoproteins, and the innate immune system's reaction to prospective invaders. However, it's ill-defined for how CLEC effects bony fish's innate immunity to bacterial infection. Therefore, CLEC12B, a member of the C-type lectin domain family, was found in Nile tilapia (Oreochromis niloticus) and its functions in bacterial infection were examined. The OnCLEC12B consist of a C-type lectin domain, a transmembrane domain, and a hypothetical protein of 308 amino acids that encoded by 927 bp basic group. Besides, the OnCLEC12B protein have a series of highly conserved amino acid sites with other CLEC12B proteins. Subcellular localization showed that OnCLEC12B located in cell membrane. Transcriptional levels investigation showed that OnCLEC12B was extensively expressed in all selected organs and has high expression in the liver. The transcriptional levels of OnCLEC12B were induced by Streptococcus agalactiae and Aeromonas hydrophila in the liver, spleen, head kidney, brain, and intestine. Afterward, invitro study revealed that several kinds of pathogens could be bound and agglutinated by recombinant protein of OnCLEC12B (rOnCLEC12B). Moreover, rOnCLEC12B could not only promote the proliferation of monocytes/macrophages but also encourage its phagocytosis on S.agalactiae and A.hydrophila, and its over-expression could significantly suppress the activation of the NF-κB pathway. Summarily, our results indicated that OnCLEC12B gets involved in fish immunization activities to pathogens infection.

Vasoactive Intestinal Peptide (VIP) Protects Nile Tilapia (Oreochromis niloticus) against Streptococcus agalatiae Infection

Vasoactive intestinal peptide (VIP), a member of secretin/glucagon family, is involved in a variety of biological activities such as gut motility, immune responses, and carcinogenesis. In this study, the VIP precursor gene (On-VIP) and its receptor gene VIPR1 (On-VIPR1) were identified from Nile tilapia (Oreochromis niloticus), and the functions of On-VIP in the immunomodulation of Nile tilapia against bacterial infection were investigated and characterized. On-VIP and On-VIPR1 contain a 450 bp and a 1326 bp open reading frame encoding deduced protein of 149 and 441 amino acids, respectively. Simultaneously, the transcript of both On-VIP and On-VIPR1 were highly expressed in the intestine and sharply induced by Streptococcus agalatiae. Moreover, the positive signals of On-VIP and On-VIPR1 were detected in the longitudinal muscle layer and mucosal epithelium of intestine, respectively. Furthermore, both in vitro and in vivo experiments indicated several immune functions of On-VIP, including reduction of P65, P38, MyD88, STAT3, and AP1, upregulation of CREB and CBP, and suppression of inflammation. Additionally, in vivo experiments proved that On-VIP could protect Nile tilapia from bacterial infection and promote apoptosis and pyroptosis. These data lay a theoretical basis for further understanding of the mechanism of VIP guarding bony fish against bacterial infection.

Japanese Flounder HMGB1: A DAMP Molecule That Promotes Antimicrobial Immunity by Interacting with Immune Cells and Bacterial Pathogen

High mobility group box (HMGB) proteins are DNA-associated proteins that bind and modulate chromosome structures. In mammals, HMGB proteins can be released from the cell nucleus and serve as a damage-associated molecular pattern (DAMP) under stress conditions. In fish, the DAMP function of HMGB proteins in association with bacterial infection remains to be investigated. In this study, we examined the immunological functions of two HMGB members, HMGB1 and HMG20A, of Japanese flounder. HMGB1 and HMG20A were expressed in multiple tissues of the flounder. HMGB1 was released from peripheral blood leukocytes (PBLs) upon bacterial challenge in a temporal manner similar to that of lactate dehydrogenase release. Recombinant HMGB1 bound to PBLs and induced ROS production and the expression of inflammatory genes. HMGB1 as well as HMG20A also bound to various bacterial pathogens and caused bacterial agglutination. The bacteria-binding patterns of HMGB1 and HMG20A were similar, and the binding of HMGB1 competed with the binding of HMG20A but not vice versa. During bacterial infection, HMGB1 enhanced the immune response of PBLs and repressed bacterial invasion. Collectively, our results indicate that flounder HMGB1 plays an important role in antimicrobial immunity by acting both as a modulator of immune cells and as a pathogen-interacting DAMP.