A Comparative Study on Microstructure, Segregation, and Mechanical Properties of Al-Si-Mg Alloy Parts Processed by GISS-HPDC and SEED-HPDC

There are multiple routes to prepare semi-solid slurries with a globular microstructure for semi-solid forming. The variations in the microstructure of semi-solid slurries prepared using different routes may lead to significant differences in the flow behavior and mechanical properties of rheo-diecasting parts. Therefore, it is crucial to have a comprehensive understanding of the microstructure evolution associated with different slurry preparation routes and their resulting effects. In this study, the gas-induced semi-solid process (GISS) and the swirl enthalpy equilibrium device (SEED) routes were employed to prepare semi-solid Al-Si-Mg slurries for their simplicity and productivity in potential industrial applications. The prepared slurries were then injected into the shoot sleeves of a high-pressure die casting (HPDC) machine to produce tensile test bars. Subsequently, the bars underwent T6 treatment to enhance their mechanical properties. The microstructure, segregation, and mechanical properties of the samples were investigated and compared with those of conventional HPDC. The results indicated that the GISS and SEED can produce semi-solid slurries containing a spherical α-Al primary phase, as opposed to the dendritic structure commonly found in conventional castings. The liquid fraction had a significant effect on the flow behavior, resulting in variations in liquid segregation and mechanical properties. It was observed that a higher solid fraction (>75%) had a suppressing effect on surface liquid segregation. In addition, the tendency for liquid segregation gradually increased along the filling direction due to the special flow behavior of the semi-solid slurry with a low solid fraction. Furthermore, under the same die-casting process parameters, the conventional HPDC samples exhibit higher yield stress (139 ± 3 MPa) compared to SEED-HPDC and GISS-HPDC samples, which may be attributed to the small grain size and the distribution of eutectic phases. After undergoing the T6 treatment, both SEED-HPDC and GISS-HPDC samples showed a significant improvement in yield and tensile strength. These improvements are a result of solution and precipitation strengthening effects as well as the spheroidization of the eutectic Si phase. Moreover, the heat-treated SEED-HPDC samples demonstrate higher ultimate strength (336 ± 5 MPa) and elongation (13.7 ± 0.3%) in comparison to the GISS-HPDC samples (307 ± 4 MPa, 8.8 ± 0.2%) after heat treatment, mainly due to their low porosity density. These findings suggest that both GISS-HPDC and SEED-HPDC processes can be utilized to produce parts with favorable mechanical properties by implementing appropriate heat treatments. However, further investigation is required to control the porosities of GISS-HPDC samples during heat treatment.

PD-L1/BTLA Checkpoint Axis Exploited for Bacterial Immune Escape by Restraining CD8+ T Cell-Initiated Adaptive Immunity in Zebrafish

Programmed death-ligand 1/programmed cell death 1 (PD-L1/PD-1) is one of the most important immune checkpoints in humans and other mammalian species. However, the occurrence of the PD-L1/PD-1 checkpoint in evolutionarily ancient vertebrates remains elusive because of the absence of a PD-1 homolog before its appearance in tetrapods. In this article, we identified, to our knowledge, a novel PD-L1/B and T lymphocyte attenuator (BTLA) checkpoint in zebrafish by using an Edwardsiella tarda-induced bacterial infection model. Results showed that zebrafish (Danio rerio) PD-L1 (DrPD-L1) and BTLA (DrBTLA) were differentially upregulated on MHC class II+ macrophages (Mϕs) and CD8+ T cells in response to E. tarda infection. DrPD-L1 has a strong ability to interact with DrBTLA, as shown by the high affinity (KD = 5.68 nM) between DrPD-L1/DrBTLA proteins. Functionally, the breakdown of DrPD-L1/DrBTLA interaction significantly increased the cytotoxicity of CD8+BTLA+ T cells to E. tarda-infected PD-L1+ Mϕ cells and reduced the immune escape of E. tarda from the target Mϕ cells, thereby enhancing the antibacterial immunity of zebrafish against E. tarda infection. Similarly, the engagement of DrPD-L1 by soluble DrBTLA protein diminished the tolerization of CD8+ T cells to E. tarda infection. By contrast, DrBTLA engagement by a soluble DrPD-L1 protein drives aberrant CD8+ T cell responses. These results were finally corroborated in a DrPD-L1-deficient (PD-L1-/-) zebrafish model. This study highlighted a primordial PD-L1/BTLA coinhibitory axis that regulates CD8+ T cell activation in teleost fish and may act as an alternative to the PD-L1/PD-1 axis in mammals. It also revealed a previously unrecognized strategy for E. tarda immune evasion by inducing CD8+ T cell tolerance to target Mϕ cells through eliciting the PD-L1/BTLA checkpoint pathway.

Single-cell transcriptome profiling reveals diverse immune cell populations and their responses to viral infection in the spleen of zebrafish

Teleost fish are indispensable model organisms for comparative immunology research that should lead to an improved understanding of the general principles of vertebrate immune system design. Although numerous studies on fish immunology have been conducted, knowledge about the cell types that orchestrate piscine immune systems remains limited. Here, we generated a comprehensive atlas of immune cell types in zebrafish spleen on the basis of single-cell transcriptome profiling. We identified 11 major categories from splenic leukocyte preparations, including neutrophils, natural killer cells, macrophages/myeloid cells, T cells, B cells, hematopoietic stem and progenitor cells, mast cells, remnants of endothelial cells, erythroid cells, erythroid progenitors, and a new type of serpin-secreting cells. Notably, we derived 54 potential subsets from these 11 categories. These subsets showed differential responses to spring viremia of carp virus (SVCV) infection, implying that they have diverse roles in antiviral immunity. Additionally, we landscaped the populations with the induced expression of interferons and other virus-responsive genes. We found that trained immunity can be effectively induced in the neutrophil and M1-macrophage subsets by vaccinating zebrafish with inactivated SVCV. Our findings illustrated the complexity and heterogeneity of the fish immune system, which will help establish a new paradigm for the improved understanding of fish immunology.

Essential role of an ERV-derived Env38 protein in adaptive humoral immunity against an exogenous SVCV infection in a zebrafish model

Endogenous retroviruses (ERVs) are the relics of ancient retroviruses occupying a substantial fraction of vertebrate genomes. However, knowledge about the functional association of ERVs with cellular activities remains limited. Recently, we have identified approximately 3,315 ERVs from zebrafish at genome-wide level, among which 421 ERVs were actively expressed in response to the infection of Spring viraemia of carp virus (SVCV). These findings demonstrated the previously unrecognized activity of ERVs in zebrafish immunity, thereby making zebrafish an attractive model organism for deciphering the interplay among ERVs, exogenous invading viruses, and host immunity. In the present study, we investigated the functional role of an envelope protein (Env38) derived from an ERV-E5.1.38-DanRer element in zebrafish adaptive immunity against SVCV in view of its strong responsiveness to SVCV infection. This Env38 is a glycosylated membrane protein mainly distributed on MHC-II+ antigen-presenting cells (APCs). By performing blockade and knockdown/knockout assays, we found that the deficiency of Env38 markedly impaired the activation of SVCV-induced CD4+ T cells and thereby led to the inhibition of IgM+/IgZ+ B cell proliferation, IgM/IgZ Ab production, and zebrafish defense against SVCV challenge. Mechanistically, Env38 activates CD4+ T cells by promoting the formation of pMHC-TCR-CD4 complex via cross-linking MHC-II and CD4 molecules between APCs and CD4+ T cells, wherein the surface subunit (SU) of Env38 associates with the second immunoglobin domain of CD4 (CD4-D2) and the first α1 domain of MHC-IIα (MHC-IIα1). Notably, the expression and functionality of Env38 was strongly induced by zebrafish IFNφ1, indicating that env38 acts as an IFN-stimulating gene (ISG) regulated by IFN signaling. To the best of our knowledge, this study is the first to identify the involvement of an Env protein in host immune defense against an exogenous invading virus by promoting the initial activation of adaptive humoral immunity. It improved the current understanding of the cooperation between ERVs and host adaptive immunity.

Extensive involvement of CD40 and CD154 costimulators in multiple T cell-mediated responses in a perciform fish Larimichthys crocea

CD40 and CD154 are well-characterized costimulatory molecules involved in adaptive humoral immunity in humans and other mammals. These two costimulatory molecules were found to be originated from teleost fish during vertebrate evolution. However, the functionality of fish CD40 and CD154 remains to be explored. In this study, we identified the CD40 and CD154 homologs (LcCD40 and LcCD154) from large yellow croaker (Larimichthys crocea), a marine species of the perciform fish family. The LcCD40 and LcCD154 share conserved structural features to their mammalian counterparts, and are widely expressed in immune-relevant tissues and leukocytes at different transcriptional levels. Immunofluorescence staining and FCM analysis showed that LcCD40 and LcCD154 proteins are distributed on MHC-II⁺ APCs and CD4-2⁺ T cells, and are significantly upregulated in response to antigen stimulation. Co-IP assay exhibited strong association between LcCD40 and LcCD154 proteins. Blockade of LcCD154 with anti-LcCD154 antibody (Ab) or recombinant soluble LcCD40-Ig fusion protein remarkably decreased the MHC-II⁺ APC-initiated CD4⁺ T cell response upon Aeromonas hydrophila stimulation, and alloreactive T cell activation as examined by mixed lymphocyte reaction (MLR). These findings highlight the costimulatory role of LcCD40 and LcCD154 in T cell activities in Larimichthys crocea. Thus, the CD40 and CD154 costimulators may extensively participate in the regulation of multiple T cell-mediated immune responses in teleost fish. It is anticipated that this study would provide a cross-species understanding of the evolutionary history of CD40 and CD154 costimulatory signals from fish to mammals.

Zbtb46 Controls Dendritic Cell Activation by Reprogramming Epigenetic Regulation of cd80/86 and cd40 Costimulatory Signals in a Zebrafish Model

The establishment of an appropriate costimulatory phenotype is crucial for dendritic cells (DCs) to maintain a homeostatic state with optimal immune surveillance and immunogenic activities. The upregulation of CD80/86 and CD40 is a hallmark costimulatory phenotypic switch of DCs from a steady state to an activated one for T cell activation. However, knowledge of the regulatory mechanisms underlying this process remains limited. In this study, we identified a Zbtb46 homolog from a zebrafish model. Zbtb46 deficiency resulted in upregulated cd80/86 and cd40 expression in kidney marrow-derived DCs (KMDCs) of zebrafish, which was accompanied with a remarkable expansion of CD4⁺/CD8⁺ T cells and accumulation of KMDCs in spleen of naive fish. Zbtb46 ^-/- splenic KMDCs exhibited strong stimulatory activity for CD4⁺ T cell activation. Chromatin immunoprecipitation-quantitative PCR and mass spectrometry assays showed that Zbtb46 was associated with promoters of cd80/86 and cd40 genes by binding to a 5'-TGACGT-3' motif in resting KMDCs, wherein it helped establish a repressive histone epigenetic modification pattern (H3K4me0/H3K9me3/H3K27me3) by organizing Mdb3/organizing nucleosome remodeling and deacetylase and Hdac3/nuclear receptor corepressor 1 corepressor complexes through the recruitment of Hdac1/2 and Hdac3. On stimulation with infection signs, Zbtb46 disassociated from the promoters via E3 ubiquitin ligase Cullin1/Fbxw11-mediated degradation, and this reaction can be triggered by the TLR9 signaling pathway. Thereafter, cd80/86 and cd40 promoters underwent epigenetic reprogramming from the repressed histone modification pattern to an activated pattern (H3K4me3/H3K9ac/H3K27ac), leading to cd80/86 and cd40 expression and DC activation. These findings revealed the essential role of Zbtb46 in maintaining DC homeostasis by suppressing cd80/86 and cd40 expression through epigenetic mechanisms.

Essential Role of RIG-I in Hematopoietic Precursor Emergence in Primitive Hematopoiesis during Zebrafish Development

Retinoic acid-inducible gene I (RIG-I) is an important cytosolic pattern recognition receptor crucial for sensing RNA virus infection and initiating innate immune responses. However, the participation of RIG-I in cellular development under physiological conditions remains limited. In this study, the regulatory role of RIG-I in embryonic hematopoiesis was explored in a zebrafish model. Results showed that rig-I was ubiquitously expressed during embryogenesis at 24 h postfertilization (hpf). A defect in RIG-I remarkably disrupted the emergence of primitive hematopoietic precursors and subsequent myeloid and erythroid lineages. In contrast, RIG-I deficiency did not have an influence on the generation of endothelial precursors and angiogenesis and the development of mesoderm and adjacent tissues. The alteration in these phenotypes was confirmed by whole-mount in situ hybridization with lineage-specific markers. In addition, immunostaining and TUNEL assays excluded the abnormal proliferation and apoptosis of hematopoietic precursors in RIG-I-deficient embryos. Mechanistically, RIG-I regulates primitive hematopoiesis through downstream IFN signaling pathways, as shown by the decline in ifnφ2 and ifnφ3 expression, along with rig-I knockdown, and rescue of the defects of hematopoietic precursors in RIG-I-defective embryos after administration with ifnφ2 and ifnφ3 mRNAs. Additionally, the defects of hematopoietic precursors in RIG-I morphants could be efficiently rescued by the wild-type RIG-I but could not be restored by the RNA-binding-defective RIG-I with site mutations at the RNA-binding pocket, which are essential for association with RNAs. This finding suggested that endogenous RNAs may serve as agonists to activate RIG-I-modulated primitive hematopoiesis. This study revealed the functional diversity of RIG-I under physiological conditions far beyond that previously known.

Regulatory role of BTLA and HVEM checkpoint inhibitors in T cell activation in a perciform fish Larimichthys crocea

The BTLA and HVEM are two well-characterized immune checkpoint inhibitors in humans and other mammalian species. However, the occurrence and functionality of these two molecules in non-mammalian species remain poorly understood. In the present study, we identified the BTLA and HVEM homologs from large yellow croaker (Larimichthys crocea), an economically important marine species of the perciform fish family. The Larimichthys crocea BTLA and HVEM (LcBTLA and LcHVEM) share conserved structural features to their mammalian counterparts, and they were expressed in various tissues and cells examined at different transcriptional levels, with particular abundance in immune-relevant tissues and splenic leukocytes. Immunofluorescence staining and flow cytometry analysis showed that LcHVEM and LcBTLA proteins were distributed on MHC-II⁺ APCs and CD4-2⁺ T cells, and a strong interaction between LcBTLA and LcHVEM was detected in splenic leukocytes in the mixed lymphocyte reaction (MLR). By blockade assays using anti-LcBTLA and anti-LcHVEM Abs as well as recombinant soluble LcBTLA and LcHVEM proteins in different combinations, it was found that LcBTLA-LcHVEM interactions play an important inhibitory role in the activation of alloreactive T cells using MLR as a model, and APC-initiated antigen-specific CD4-2⁺ T cells in response to A. hydrophila (A. h) stimulation. These observations highlight the extensive functional roles of LcBTLA and LcHVEM immune-checkpoint inhibitors in allogeneic T cell reactions, and CD4-2⁺ T cell-mediated adaptive immune responses in Larimichthys crocea. Thus, the BTLA-HVEM checkpoint may represent an ancient coinhibitory pathway, which was originated in fish and was conserved from fish to mammals throughout the vertebrate evolution.

Inhibitory Role of an Aeromonas hydrophila TIR Domain Effector in Antibacterial Immunity by Targeting TLR Signaling Complexes in Zebrafish

The Toll/interleukin-1 receptor (TIR) domain is a structural unit responsible for the assembly of signal protein complexes in Toll-like receptor (TLR) and interleukin-1 receptor signaling pathways. TIR domain homologs are found in a considerable number of bacteria and enhance bacterial infection and survival in host organisms. However, whether TIR domain homologs exist in Aeromonas hydrophila, a ubiquitous waterborne bacterium in aquatic environments, remains poorly understood. In this study, a TIR domain protein (TcpAh) was identified from A. hydrophila JBN2301. TIR domain of TcpAh is highly homologous to the counterpart domains in TLRs and myeloid differentiation factor 88 (MyD88). The zebrafish infected with mutant A. hydrophila with tcpAh deletion had a remarkably lower mortality than those infected with the wild-type strain. This result suggests that TcpAh is a crucial virulence factor for A. hydrophila infection. TcpAh exhibited a strong ability to associate with MyD88, tumor necrosis factor receptor-associated factor 3 (TRAF3) and TRAF-associated NF-κB activator-binding kinase 1 (TBK1) in TIR-TIR, TIR-Death domain (DD), and other alternative interactions. This finding suggests that TcpAh extensively interferes with MyD88 and TIR domain-containing adapter inducing interferon (IFN)-β (TRIF) signaling pathways downstream of TLRs. Consequently, CD80/86 expression was suppressed by TcpAh via attenuating TLR-stimulated NF-κB activation, which ultimately led to the impairment of the major costimulatory signal essential for the initiation of adaptive humoral immunity against A. hydrophila infection. We believe that this study is the first to show a previously unrecognized mechanism underlying A. hydrophila evades from host antibacterial defense by intervening CD80/86 signal, which bridges innate and adaptive immunity. The mechanism will benefit the development of therapeutic interventions for A. hydrophila infection and septicemia by targeting TcpAh homologs.

New Insights into IgZ as a Maternal Transfer Ig Contributing to the Early Defense of Fish against Pathogen Infection

IgZ or its equivalent IgT is a newly discovered teleost specific Ig class that is highly specialized in mucosal immunity. However, whether this IgZ/IgT class participates in other biological processes remains unclear. In this study, we unexpectedly discovered that IgZ is highly expressed in zebrafish ovary, accumulates in unfertilized eggs, and is transmitted to offspring from eggs to zygotes. Maternally transferred IgZ in zygotes is found at the outer and inner layers of chorion, perivitelline space, periphery of embryo body, and yolk, providing different lines of defense against pathogen infection. A considerable number of IgZ⁺ B cells are found in ovarian connective tissues distributed between eggs. Moreover, pIgR, the transporter of IgZ, is also expressed in the ovary and colocalizes with IgZ in the zona radiata of eggs. Thus, IgZ is possibly secreted by ovarian IgZ⁺ B cells and transported to eggs through association with pIgR in a paracrine manner. Maternal IgZ in zygotes showed a broad bacteriostatic activity to different microbes examined, and this reactivity can be manipulated by orchestrating desired bacteria in water where parent fish live or immunizing the parent fish through vaccination. These observations suggest that maternal IgZ may represent a group of polyclonal Abs, providing protection against various environmental microbes encountered by a parent fish that were potentially high risk to offspring. To our knowledge, our findings provide novel insights into a previously unrecognized functional role of IgZ/IgT Ig in the maternal transfer of immunity in fish, greatly enriching current knowledge about this ancient Ig class.

Differential immune responses of immunoglobulin Z subclass members in antibacterial immunity in a zebrafish model

Immunoglobulin Z (IgZ) or its equivalent immunoglobulin T (IgT) is a newly identified immunoglobulin (Ig) class from teleost fish. This Ig class is characterized by its involvement in mucosa-associated lymphoid tissues (MALTs) for mucosal defence against pathogen infection. Recently, several subclass members of IgZ/IgT, such as IgZ, IgZ2, Igτ1, Igτ2 and Igτ3, have been further identified from zebrafish, common carp and rainbow trout. However, the functional diversity and correlation among these subclasses remain uncertain. Here, we explored the differential immune reactions of the IgZ and IgZ2 subclasses in antibacterial immunity in a zebrafish model. IgZ was extensively distributed in the peripheral serum and skin/gill MALTs and showed a rapid induction upon bacterial infection. IgZ2 was specialized in skin/gill MALTs and showed a strong induction following IgZ production. Correspondingly, the IgZ⁺ B cells had a wider distribution in the systemic primary/secondary lymphoid tissues and MALTs than the IgZ2⁺ B cells, which were predominant in MALTs. IgZ and IgZ2 exhibited a complementary effect in antibacterial immunity by possessing differential abilities. That is, IgZ is preferentially involved in bactericidal reaction that is in part C1q-dependent, and IgZ2 participates in neutralization action through bacteria-coating activity. The production of IgZ largely depended on the αβ T/CD4⁺ T cells, whereas that of IgZ2 did not, suggesting the different dependencies of IgZ and IgZ2 on systemic immunity. Our findings demonstrate that the functional behaviour and mechanism of the IgZ/IgT family are more diverse than previously recognized and thus improve the current knowledge about this ancient Ig class.

CR6-interacting factor-1 contributes to osteoclastogenesis by inducing receptor activator of nuclear factor κB ligand after radiation

BACKGROUND

Radiation induces rapid bone loss and enhances bone resorption and adipogenesis, leading to an increased risk of bone fracture. There is still a lack of effective preventive or therapeutic method for irradiation-induced bone injury. Receptor activator of nuclear factor κB ligand (RANKL) provides the crucial signal to induce osteoclast differentiation and plays an important role in bone resorption. However, the mechanisms of radiation-induced osteoporosis are not fully understood.

AIM

To investigate the role of CR6-interacting factor-1 (Crif1) in osteoclastogenesis after radiation and its possible mechanism.

METHODS

C57BL/6 mice were exposed to Co-60 gamma rays and received 5 Gy of whole-body sublethal irradiation at a rate of 0.69 Gy/min. For in vitro study, mouse bone marrow mesenchymal stem/stromal cells (BM-MSCs) were irradiated with Co-60 at a single dose of 9 Gy. For osteoclast induction, monocyte-macrophage RAW264.7 cells were cocultured with mouse BM-MSCs for 7 d. ClusPro and InterProSurf were used to investigate the interaction interface in Crif1 and protein kinase cyclic adenosine monophosphate (cAMP)-activited catalytic subunit alpha complex. Virtual screening using 462608 compounds from the Life Chemicals database around His120 of Crif1 was carried out using the program Autodock_vina. A tetrazolium salt (WST-8) assay was carried out to study the toxicity of compounds to different cells, including human BM-MSCs, mouse BM-MSCs, and Vero cells.

RESULTS

Crif1 expression increased in bone marrow cells after radiation in mice. Overexpression of Crif1 in mouse BM-MSCs and radiation exposure could increase RANKL secretion and promote osteoclastogenesis in vitro. Deletion of Crif1 in BM-MSCs could reduce both adipogenesis and RANKL expression, resulting in the inhibition of osteoclastogenesis. Deletion of Crif1 in RAW264.7 cells did not affect the receptor activator of nuclear factor κB expression or osteoclast differentiation. Following treatment with protein kinase A (PKA) agonist (forskolin) and inhibitor (H-89) in mouse BM-MSCs, Crif1 induced RANKL secretion via the cAMP/PKA pathway. Moreover, we identified the Crif1-protein kinase cyclic adenosine monophosphate-activited catalytic subunit alpha interaction interface by in silico studies and shortlisted interface inhibitors through virtual screening on Crif1. Five compounds dramatically suppressed RANKL secretion and adipogenesis by inhibiting the cAMP/PKA pathway.

CONCLUSION

Crif1 promotes RANKL expression via the cAMP/PKA pathway, which induces osteoclastogenesis by binding to receptor activator of nuclear factor κB on monocytes-macrophages in the mouse model. These results suggest a role for Crif1 in modulating osteoclastogenesis and provide insights into potential therapeutic strategies targeting the balance between osteogenesis and adipogenesis for radiation-induced bone injury.

The zebrafish NLRP3 inflammasome has functional roles in ASC-dependent interleukin-1β maturation and gasdermin E-mediated pyroptosis

The NLR family pyrin domain containing 3 (NLRP3) inflammasome is one of the best-characterized inflammasomes in humans and other mammals. However, knowledge about the NLRP3 inflammasome in nonmammalian species remains limited. Here, we report the molecular and functional identification of an NLRP3 homolog (DrNLRP3) in a zebrafish (Danio rerio) model. We found that DrNLRP3's overall structural architecture was shared with mammalian NLRP3s. It initiates a classical inflammasome assembly for zebrafish inflammatory caspase (DrCaspase-A/-B) activation and interleukin 1β (DrIL-1β) maturation in an apoptosis-associated speck-like protein containing a caspase-recruitment domain (ASC)-dependent manner, in which DrNLRP3 organizes DrASC into a filament that recruits DrCaspase-A/-B by homotypic pyrin domain (PYD)-PYD interactions. DrCaspase-A/-B activation in the DrNLRP3 inflammasome occurred in two steps, with DrCaspase-A being activated first and DrCaspase-B second. DrNLRP3 also directly activated full-length DrCaspase-B and elicited cell pyroptosis in a gasdermin E (GSDME)-dependent but ASC-independent manner. These two events were tightly coordinated by DrNLRP3 to ensure efficient IL-1β secretion for the initiation of host innate immunity. By knocking down DrNLRP3 in zebrafish embryos and generating a DrASC-knockout (DrASC^-/-) fish clone, we characterized the function of the DrNLRP3 inflammasome in anti-bacterial immunity in vivo The results of our study disclosed the origin of the NLRP3 inflammasome in teleost fish, providing a cross-species understanding of the evolutionary history of inflammasomes. Our findings also indicate that the NLRP3 inflammasome may coordinate inflammatory cytokine processing and secretion through a GSDME-mediated pyroptotic pathway, uncovering a previously unrecognized regulatory function of NLRP3 in both inflammation and cell pyroptosis.

BTLA-HVEM Checkpoint Axis Regulates Hepatic Homeostasis and Inflammation in a ConA-Induced Hepatitis Model in Zebrafish

The BTLA-HVEM checkpoint axis plays extensive roles in immunomodulation and diseases, including cancer and autoimmune disorders. However, the functions of this checkpoint axis in hepatitis remain limited. In this study, we explored the regulatory role of the Btla-Hvem axis in a ConA-induced hepatitis model in zebrafish. Results showed that Btla and Hvem were differentially expressed on intrahepatic Cd8⁺ T cells and hepatocytes. Knockdown of Btla or Hvem significantly promoted hepatic inflammation. Btla was highly expressed in Cd8⁺ T cells in healthy liver but was downregulated in inflamed liver, as evidenced by a disparate proportion of Cd8⁺Btla⁺ and Cd8⁺Btla^- T cells in individuals without or with ConA stimulation. Cd8⁺Btla⁺ T cells showed minimal cytotoxicity to hepatocytes, whereas Cd8⁺Btla^- T cells were strongly reactive. The depletion of Cd8⁺Btla^- T cells reduced hepatitis, whereas their transfer enhanced hepatic inflammation. These observations indicate that Btla endowed Cd8⁺Btla⁺ T cells with self-tolerance, thereby preventing them from attacking hepatocytes. Btla downregulation deprived this tolerization. Mechanistically, Btla-Hvem interaction contributed to Cd8⁺Btla⁺ T cell tolerization, which was impaired by Hvem knockdown but rescued by soluble Hvem protein administration. Notably, Light was markedly upregulated on Cd8⁺Btla^- T cells, accompanied by the transition of Cd8⁺Btla⁺Light^- to Cd8⁺Btla^-Light⁺ T cells during hepatitis, which could be modulated by Cd4⁺ T cells. Light blockade attenuated hepatitis, thereby suggesting the positive role of Light in hepatic inflammation. These findings provide insights into a previously unrecognized Btla-Hvem-Light regulatory network in hepatic homeostasis and inflammation, thus adding a new potential therapeutic intervention for hepatitis.

Characterization of an NLRP1 Inflammasome from Zebrafish Reveals a Unique Sequential Activation Mechanism Underlying Inflammatory Caspases in Ancient Vertebrates

NLRP1 inflammasome is one of the best-characterized inflammasomes in humans and other mammals. However, the existence of this inflammasome in nonmammalian species remains poorly understood. In this study, we report the molecular and functional identification of an NLRP1 homolog, Danio rerio NLRP1 (DrNLRP1) from a zebrafish (D. rerio) model. This DrNLRP1 possesses similar structural architecture to mammalian NLRP1s. It can trigger the formation of a classical inflammasome for the activation of zebrafish inflammatory caspases (D. rerio Caspase [DrCaspase]-A and DrCaspase-B) and maturation of D. rerio IL-1β in a D. rerio ASC (DrASC)-dependent manner. In this process, DrNLRP1 promotes the aggregation of DrASC into a filament with DrASC^CARD core and DrASC^PYD cluster. The assembly of DrNLRP1 inflammasome depends on the CARD-CARD homotypic interaction between DrNLRP1 and DrASC^CARD core, and PYD-PYD interaction between DrCaspase-A/B and DrASC^PYD cluster. The FIIND domain in DrNLRP1 is necessary for inflammasome assembly. To understand the mechanism of how the two DrCaspases are coordinated in DrNLRP1 inflammasome, we propose a two-step sequential activation model. In this model, the recruitment and activation of DrCaspase-A/B in the inflammasome is shown in an alternate manner, with a preference for DrCaspase-A followed by a subsequent selection for DrCaspase-B. By using morpholino oligonucleotide-based knockdown assays, the DrNLRP1 inflammasome was verified to play important functional roles in antibacterial innate immunity in vivo. These observations demonstrate that the NLRP1 inflammasome originated as early as in teleost fish. This finding not only gives insights into the evolutionary history of inflammasomes but also provides a favorable animal model for the study of NLRP1 inflammasome-mediated immunology and diseases.

Identification of DEAD-Box RNA Helicase DDX41 as a Trafficking Protein That Involves in Multiple Innate Immune Signaling Pathways in a Zebrafish Model

DDX41 is an important sensor for host recognition of DNA viruses and initiation of nuclear factor-κB (NF-κB) and IFN signaling pathways in mammals. However, its occurrence and functions in other vertebrates remain poorly defined. Here, a DDX41 ortholog [Danio rerio DDX41 (DrDDX41)] with various conserved structural features to its mammalian counterparts was identified from a zebrafish model. This DrDDX41 was found to be a trafficking protein distributed in the nucleus of resting cells but transported into the cytoplasm under DNA stimulation. Two nuclear localization signal motifs were localized beside the coiled-coil domain, whereas one nuclear export signal motif existed in the DEADc domain. DrDDX41 acts as an initiator for the activation of NF-κB and IFN signaling pathways in a Danio rerio STING (DrSTING)-dependent manner through its DEADc domain, which is a typical performance of mammalian DDX41. These observations suggested the conservation of DDX41 proteins throughout the vertebrate evolution, making zebrafish an alternative model in understanding DDX41-mediated immunology. With this model system, we found that DrDDX41 contributes to DrSTING–Danio rerio STAT6 (DrSTAT6)-mediated chemokine (Danio rerio CCL20) production through its DEADc domain. To the best of our knowledge, this work is the first report showing that DDX41 is an upstream initiator in this newly identified signaling pathway. The DrDDX41-mediated signaling pathways play important roles in innate antibacterial immunity because knockdown of either DrDDX41 or DrSTING/DrSTAT6 significantly reduced the survival of zebrafish under Aeromonas hydrophilia or Edwardsiella tarda infection. Our findings would enrich the current knowledge of DDX41-mediated immunology and the evolutionary history of the DDX41 family.

Costimulatory Function of Cd58/Cd2 Interaction in Adaptive Humoral Immunity in a Zebrafish Model

CD58 and CD2 have long been known as a pair of reciprocal adhesion molecules involved in the immune modulations of CD8⁺ T and NK-mediated cellular immunity in humans and several other mammals. However, the functional roles of CD58 and CD2 in CD4⁺ T-mediated adaptive humoral immunity remain poorly defined. Moreover, the current functional observations of CD58 and CD2 were mainly acquired from in vitro assays, and in vivo investigation is greatly limited due to the absence of a Cd58 homology in murine models. In this study, we identified cd58 and cd2 homologs from the model species zebrafish (Danio rerio). These two molecules share conserved structural features to their mammalian counterparts. Functionally, cd58 and cd2 were significantly upregulated on antigen-presenting cells and Cd4⁺ T cells upon antigen stimulation. Blockade or knockdown of Cd58 and Cd2 dramatically impaired the activation of antigen-specific Cd4⁺ T and mIgM⁺ B cells, followed by the inhibition of antibody production and host defense against bacterial infections. These results indicate that CD58/CD2 interaction was required for the full activation of CD4⁺ T-mediated adaptive humoral immunity. The interaction of Cd58 with Cd2 was confirmed by co-immunoprecipitation and functional competitive assays by introducing a soluble Cd2 protein. This study highlights a new costimulatory mechanism underlying the regulatory network of adaptive immunity and makes zebrafish an attractive model organism for the investigation of CD58/CD2-mediated immunology and disorders. It also provides a cross-species understanding of the evolutionary history of costimulatory signals from fish to mammals as a whole.

Coordination of Bactericidal and Iron Regulatory Functions of Hepcidin in Innate Antimicrobial Immunity in a Zebrafish Model

Hepcidin acts as both an antimicrobial peptide and a hormonal regulator of iron homeostasis; however, the biological significance of this dual-function in immune reactions remains elusive. In this study, we provide experimental evidence regarding the coordination of this dual-function in the innate antimicrobial immunity using a zebrafish model. The transcription of hepcidin gene was significantly upregulated in liver by Aeromonas hydrophila (A.h) DNA stimulation, which was accompanied by an increase of hepcidin protein and a decrease of iron concentration in serum. Thus, an enhanced bactericidal activity against A.h and Escherichia coli and inhibitory effects on A.h growth and OmpA expression were observed in A.h cells, the latter of which made the bacterium more susceptible to complement attack. The enhanced bacteriostatic activities in serum following the stimulation were dramatically impaired by neutralizing hepcidin or restoring iron to the samples. Immuno-protection assay showed that zebrafish administrated with A.h DNA or designed CpG-ODNs had a significantly enhanced defence against A.h and Vibrio alginolyticus infections, which was also eliminated by the neutralization of hepcidin. Results indicate that the induction of hepcidin leads to the decrease of iron in circulation, which eventually limits iron availability to invading microorganisms, thus contributing to host defence.

RIG-I: a multifunctional protein beyond a pattern recognition receptor

It was widely known that retinoic acid inducible gene I (RIG-I) functions as a cytosolic pattern recognition receptor that initiates innate antiviral immunity by detecting exogenous viral RNAs. However, recent studies showed that RIG-I participates in other various cellular activities by sensing endogenous RNAs under different circumstances. For example, RIG-I facilitates the therapy resistance and expansion of breast cancer cells and promotes T cell-independent B cell activation through interferon signaling activation by recognizing non-coding RNAs and endogenous retroviruses in certain situations. While in hepatocellular carcinoma and acute myeloid leukemia, RIG-I acts as a tumor suppressor through either augmenting STAT1 activation by competitively binding STAT1 against its negative regulator SHP1 or inhibiting AKT-mTOR signaling pathway by directly interacting with Src respectively. These new findings suggest that RIG-I plays more diverse roles in various cellular life activities, such as cell proliferation and differentiation, than previously known. Taken together, the function of RIG-I exceeds far beyond that of a pattern recognition receptor.

Mutual Regulation of NOD2 and RIG-I in Zebrafish Provides Insights into the Coordination between Innate Antibacterial and Antiviral Signaling Pathways

Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) and retinoic acid-inducible gene I (RIG-I) are two important cytosolic pattern recognition receptors (PRRs) in the recognition of pathogen-associated molecular patterns (PAMPs), initiating innate antibacterial and antiviral signaling pathways. However, the relationship between these PRRs, especially in teleost fish models, is rarely reported. In this article, we describe the mutual regulation of zebrafish NOD2 (DrNOD2) and RIG-I (DrRIG-I) in innate immune responses. Luciferase assays were conducted to determine the activation of NF-κB and interferon signaling. Morpholino-mediated knockdown and mRNA-mediated rescue were performed to further confirm the regulatory roles between DrNOD2 and DrRIG-I. Results showed that DrNOD2 and DrRIG-I shared conserved structural hallmarks with their mammalian counterparts, and activated DrRIG-I signaling can induce DrNOD2 production. Surprisingly, DrNOD2-initiated signaling can also induce DrRIG-I expression, indicating that a mutual regulatory mechanism may exist between them. Studies conducted using HEK293T cells and zebrafish embryos showed that DrRIG-I could negatively regulate DrNOD2-activated NF-κB signaling, and DrNOD2 could inhibit DrRIG-I-induced IFN signaling. Moreover, knocking down DrRIG-I expression by morpholino could enhance DrNOD2-initiated NF-κB activation, and vice versa, which could be rescued by their corresponding mRNAs. Results revealed a mutual feedback regulatory mechanism underlying NOD2 and RIG-I signaling pathways in teleosts. This mechanism reflects the coordination between cytosolic antibacterial and antiviral PRRs in the complex network of innate immunity.

Characterization of γδ T Cells from Zebrafish Provides Insights into Their Important Role in Adaptive Humoral Immunity

γδ T cells represent an evolutionarily primitive T cell subset characterized by distinct T cell receptors (TCRs) and innate and adaptive immune functions. However, the presence of this T cell subset in ancient vertebrates remains unclear. In this study, γδ T cells from a zebrafish (Danio rerio) model were subjected to molecular and cellular characterizations. The constant regions of zebrafish TCR-γ (DrTRGC) and δ (DrTRDC) were initially identified. Zebrafish γδ T cells accounted for 7.7–20.5% of the total lymphocytes in spleen, head kidney, peripheral blood, skin, gill, and intestine tissues. They possess typical morphological features of lymphocytes with a surface phenotype of γ⁺δ⁺CD4⁻CD8⁺. Zebrafish γδ T cells functionally showed a potent phagocytic ability to both soluble and particulate antigens. They can also act as an antigen-presenting cell to initiate antigen (KLH)-specific CD4⁺ T_KLH cell activation and to induce B cell proliferation and IgM production. Particularly, zebrafish γδ T cells also play a critical role in antigen-specific IgZ production in intestinal mucus. These findings demonstrated that γδ T cells had been originated as early as teleost fish, which providing valuable insights into the evolutionary history of T cell subset. It is anticipated that this study would be used as a guide to develop a zebrafish model for the cross-species investigation of γδ T cell biology.

Characterization of SIGIRR/IL-1R8 Homolog from Zebrafish Provides New Insights into Its Inhibitory Role in Hepatic Inflammation

Single Ig IL-1R-related molecule (SIGIRR, also called IL-1R8 or Toll/IL-1R [TIR]8), a negative regulator for Toll/IL-1R signaling, plays critical roles in innate immunity and various diseases in mammals. However, the occurrence of this molecule in ancient vertebrates and its function in liver homeostasis and disorders remain poorly understood. In this study, we identified a SIGIRR homology from zebrafish (Danio rerio [DrSIGIRR]) by using a number of conserved structural and functional hallmarks to its mammalian counterparts. DrSIGIRR was highly expressed in the liver. Ablation of DrSIGIRR by lentivirus-delivered small interfering RNA in the liver significantly enhanced hepatic inflammation in response to polyinosinic-polycytidylic acid [poly(I:C)] stimulation, as shown by the upregulation of inflammatory cytokines and increased histological disorders. In contrast, depletion of TIR domain-containing adaptor inducing IFN-β (TRIF) or administration of TRIF signaling inhibitor extremely abrogated the poly(I:C)-induced hepatic inflammation. Aided by the zebrafish embryo model, overexpression of DrSIGIRR in vivo significantly inhibited the poly(I:C)- and TRIF-induced NF-κB activations; however, knockdown of DrSIGIRR promoted such activations. Furthermore, pull-down and Duolink in situ proximity ligation assay assays showed that DrSIGIRR can interact with the TRIF protein. Results suggest that DrSIGIRR plays an inhibitory role in TRIF-mediated inflammatory reactions by competitive recruitment of the TRIF adaptor protein from its TLR3/TLR22 receptor. To our knowledge, this study is the first to report a functional SIGIRR homolog that existed in a lower vertebrate. This molecule is essential to establish liver homeostasis under inflammatory stimuli. Overall, the results will enrich the current knowledge about SIGIRR-mediated immunity and disorders in the liver.

Essential Roles of TIM-1 and TIM-4 Homologs in Adaptive Humoral Immunity in a Zebrafish Model

TIM-1 and TIM-4 proteins have become increasingly attractive for their critical functions in immune modulation, particularly in CD4(+) Th2 cell activation. Thus, these proteins were hypothesized to regulate adaptive humoral immunity. However, further evidence is needed to validate this hypothesis. This study describes the molecular and functional characteristics of TIM-1 and TIM-4 homologs from a zebrafish (Danio rerio) model (D. rerio TIM [DrTIM]-1 and DrTIM-4). DrTIM-1 and DrTIM-4 were predominantly expressed in CD4(+) T cells and MHC class II(+) APCs under the induction of Ag stimulation. Blockade or knockdown of both DrTIM-1 and DrTIM-4 significantly decreased Ag-specific CD4(+) T cell activation, B cell proliferation, Ab production, and vaccinated immunoprotection against bacterial infection. This result suggests that DrTIM-1 and DrTIM-4 serve as costimulatory molecules required for the full activation of adaptive humoral immunity. DrTIM-1 was detected to be a trafficking protein located in the cytoplasm of CD4(+) T cells. It can translocate onto the cell surface under stimulation by TIM-4-expressing APCs, which might be a precise regulatory strategy for CD4(+) T cells to avoid self-activation before APCs stimulation. Furthermore, a unique alternatively spliced soluble DrTIM-4 variant was identified to exert a negative regulatory effect on the proliferation of CD4(+) T cells. The above findings highlight a novel costimulatory mechanism underlying adaptive immunity. This study enriches the current knowledge on TIM-mediated immunity and provides a cross-species understanding of the evolutionary history of costimulatory systems throughout vertebrate evolution.

Cysteine-independent Catalase-like Activity of Vertebrate Peroxiredoxin 1 (Prx1)

Peroxiredoxins (Prxs) are a ubiquitous family of antioxidant proteins that are known as thioredoxin peroxidases. Here we report that Prx1 proteins from Tetraodon nigroviridis and humans also possess a previously unknown catalase-like activity that is independent of Cys residues and reductants but dependent on iron. We identified that the GVL motif was essential to the catalase (CAT)-like activity of Prx1 but not to the Cys-dependent thioredoxin peroxidase (POX) activity, and we generated mutants lacking POX and/or CAT activities for individually delineating their functional features. We discovered that the TnPrx1 POX and CAT activities possessed different kinetic features in reducing H2O2. The overexpression of wild-type TnPrx1 and mutants differentially regulated the intracellular levels of reactive oxygen species and p38 phosphorylation in HEK-293T cells treated with H2O2. These observations suggest that the dual antioxidant activities of Prx1 may be crucial for organisms to mediate intracellular redox homeostasis.

Low-molecular-weight fibroblast growth factor 2 attenuates hepatic fibrosis by epigenetic down-regulation of Delta-like1

Liver fibrosis, a major cause of end-stage liver diseases, is closely regulated by multiple growth factors and cytokines. The correlation of fibroblast growth factor 2 (FGF2) with chronic liver injury has been reported, but the exact functions of different FGF2 isoforms in liver fibrogenesis remain unclear. Here, we report on the differential expression patterns and functions of low- and high-molecular-weight FGF2 (namely, FGF2(lmw) and FGF2(hmw) , respectively) in hepatic fibrogenesis using a CCl4 -induced mouse liver fibrosis model. FGF2(hmw) displayed a robust increase in CCl4 -induced hepatic fibrosis and promoted fibrogenesis. In contrast, endogenous FGF2(lmw) exhibited a slight increase in hepatic fibrosis and suppressed this pathological progression. Moreover, exogenous administration of recombinant FGF2(lmw) potently ameliorated CCl4 -induced liver fibrosis. Mechanistically, we showed that FGF2(lmw) treatment attenuated hepatic stellate cell activation and fibrosis by epigenetic down-regulation of Delta-like 1 expression through the p38 mitogen-activated protein kinase pathway.

CONCLUSION

FGF2(lmw) and FGF2(hmw) have distinct roles in liver fibrogenesis. These findings demonstrate a potent antifibrotic effect of FGF2(lmw) administration, which may provide a novel approach to treat chronic liver diseases.

Characterization of surface phenotypic molecules of teleost dendritic cells

Dendritic cells (DCs) are among the most important professional antigen-presenting cells (APCs) that participate in various biological activities in mammals. However, evidence of the existence of DCs in teleost fish and other lower vertebrates remains limited. In this study, phenotypic and functional characteristics of teleost DCs were described in a zebrafish model. An improved method to efficiently enrich DCs was established. Immunofluorescence staining revealed that the surface phenotypic hallmarks of mammalian DCs, including MHC-II, CD80/86, CD83, and CD209, were distributed on the surfaces of zebrafish DCs (DrDCs). Functional analysis results showed that DrDCs could initiate antigen-specific CD4(+) T cell activation, in which MHC-II, CD80/86, CD83, and CD209 are implicated. Hence, teleost DCs exhibit conserved immunophenotypes and functions similar to those of their mammalian counterparts. Our findings contributed to the current understanding of the evolutionary history of DCs and the DC-regulatory mechanisms of adaptive immunity.

A novel C1q-domain-containing (C1qDC) protein from Mytilus coruscus with the transcriptional analysis against marine pathogens and heavy metals

The C1q-domain-containing (C1qDC) proteins, which are involved in various processes of vertebrates, are important pattern recognition receptors in innate immunity of invertebrates. In present study, a novel C1qDC was identified from Mytilus coruscus (designated as McC1qDC), which was 917 bp in length encoding 236 amino acids with a typical signal peptide of 19 amino acid residues in N-terminus. Based on its conserved C1q domain and molecular architecture of 10 β-strand jelly-roll folding topology structure, McC1qDC might be classified as a member of the C1q family. The mRNA transcript of McC1qDC was predominantly detectable in the hemocytes, and a less degree in gill, gonad and mantle, but trace in foot, adductor and digestive gland. Upon induction by Vibrio harveyi and Vibrio alginolyticus, McC1qDC expression was significantly up-regulated. Time-dependent mRNA expression of McC1qDC was found during copper and cadmium exposure for its heavy metal-binding domain. These results indicated that McC1qDC was a novel member of the C1qDC protein family as a pattern recognition receptor against pathogens, and might be developed as a potential indicator for monitoring heavy metals pollution.

Conserved inhibitory role of teleost SOCS-1s in IFN signaling pathways

The suppressor of cytokine signaling 1 (SOCS-1) protein is a critical regulator in the immune systems of humans and mammals, which functions classically as an inhibitor of the IFN signaling pathways. However, data on functional characterisation of SOCS-1 in ancient vertebrates are limited. In this study, we report the function of teleost SOCS-1s in IFN signaling in fish models (zebrafish and Tetraodon) and human cells. Structurally, teleost SOCS-1s share conserved functional domains with their mammalian counterparts. Functionally, teleost SOCS-1s could be significantly induced upon stimulation with IFN stimulants and zebrafish IFNφ1. Overexpression of teleost SOCS-1s could dramatically suppress IFNφ1-induced Mx, Viperin and PKZ activation in zebrafish, and IFN-induced ISG15 activation in HeLa cells. Furthermore, a SOCS-1 variant that lacks the KIR domain was also characterised. This study demonstrates the conserved negative regulatory role of teleost SOCS-1s in IFN signaling pathways, providing perspective into the functional conservation of SOCS-1 proteins during evolution.

New function for Escherichia coli xanthosine phophorylase (xapA): genetic and biochemical evidences on its participation in NAD(+) salvage from nicotinamide

BACKGROUND

In an effort to reconstitute the NAD(+) synthetic pathway in Escherichia coli (E. coli), we produced a set of gene knockout mutants with deficiencies in previously well-defined NAD(+)de novo and salvage pathways. Unexpectedly, the mutant deficient in NAD(+) de novo and salvage pathway I could grow in M9/nicotinamide medium, which was contradictory to the proposed classic NAD(+) metabolism of E. coli. Such E. coli mutagenesis assay suggested the presence of an undefined machinery to feed nicotinamide into the NAD(+) biosynthesis. We wanted to verify whether xanthosine phophorylase (xapA) contributed to a new NAD(+) salvage pathway from nicotinamide.

RESULTS

Additional knockout of xapA further slowed down the bacterial growth in M9/nicotinamide medium, whereas the complementation of xapA restored the growth phenotype. To further validate the new function of xapA, we cloned and expressed E. coli xapA as a recombinant soluble protein. Biochemical assay confirmed that xapA was capable of using nicotinamide as a substrate for nicotinamide riboside formation.

CONCLUSIONS

Both the genetic and biochemical evidences indicated that xapA could convert nicotinamide to nicotinamide riboside in E. coli, albeit with relatively weak activity, indicating that xapA may contribute to a second NAD(+) salvage pathway from nicotinamide. We speculate that this xapA-mediated NAD(+) salvage pathway might be significant in some bacteria lacking NAD(+) de novo and NAD(+) salvage pathway I or II, to not only use nicotinamide riboside, but also nicotinamide as precursors to synthesize NAD(+). However, this speculation needs to be experimentally tested.

Discovery of an unusual alternative splicing pathway of the immunoglobulin heavy chain in a teleost fish, Danio rerio

In present study, we identified a novel membrane immunoglobulin M isotype from zebrafish (Danio rerio), which was designated as mIgM-2, adding a new member to the Immunoglobulin family in teleost fish. The full length of cloned mIgM-2 cDNA was 611 bp, encoding 150 amino acids. The putative mIgM-2 protein sequence consists of one constant region and a trans-membrane region. Phylogenetic analysis showed that mIgM-2 grouped with the known zebrafish IgM sequences. The mIgM-2 mRNA was widely expressed in immune-related tissues including intestine, kidney and skin. In vivo stimulation with LPS significantly up-regulates the expression of mIgM-2. Our results will add new insight into the immunoglobulin class diversity of teleost fish, and to better understand the evolutionary history of adaptive immunity from fish to mammals as a whole.

Delta-like 1 serves as a new target and contributor to liver fibrosis down-regulated by mesenchymal stem cell transplantation

Chronic liver injury always progresses to fibrosis and eventually to cirrhosis, a massive health care burden worldwide. Delta-like 1 (Dlk1) is well known as an inhibitor of adipocyte differentiation. However, whether it is involved in liver fibrosis remains unclear. Here, we provide the first evidence that Dlk1 is a critical contributor to liver fibrosis through promoting activation of hepatic stellate cells (HSCs) during chronic liver injury. We found that upon liver injury, Dlk1 was dramatically induced and initially expressed in hepatocytes and then into the HSCs by a paracrine manner. It leads to the activation of HSCs, which is considered to be a pivotal event in liver fibrogenesis. Two forms (∼50 and ∼25 kDa) of the Dlk1 protein were detected by Western blot analysis. In vitro administration of Dlk1 significantly promoted HSC activation, whereas in vivo knockdown of Dlk1 dramatically inhibited HSC activation and the subsequent fibrosis. The large soluble form (∼50 kDa) of Dlk1 was shown to contribute to HSC activation. We were encouraged to find the Dlk1-promoted HSC activation and liver fibrosis can be depressed by transplantation of bone marrow-mesenchymal stem cells (BM-MSCs). Furthermore, we demonstrated that FGF2 was up-regulated in BM-MSCs under injury stimulation, and it probably participated in the inhibition of Dlk1 by BM-MSCs. Our findings provide a novel role of Dlk1 in liver fibrosis leading to a better understanding of the molecular basis in fibrosis and cirrhosis and also give insights into the cellular and molecular mechanisms of MSC biology in liver repair.

Identification of Treg-like cells in Tetraodon: insight into the origin of regulatory T subsets during early vertebrate evolution

CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg cells) are critical for the maintenance of peripheral tolerance, and the suppression of autoimmune diseases and even tumors. Although Treg cells are well characterized in humans, little is known regarding their existence or occurrence in ancient vertebrates. In the present study, we report on the molecular and functional characterization of a Treg-like subset with the phenotype CD4-2(+)CD25-like(+)Foxp3-like(+) from a pufferfish (Tetraodon nigroviridis) model. Functional studies showed that depletion of this subset produced an enhanced mixed lymphocyte reaction (MLR) and nonspecific cytotoxic cell (NCC) activity in vitro, as well as inflammation of the intestine in vivo. The data presented here will not only enrich the knowledge of fish immunology but will also be beneficial for a better cross-species understanding of the evolutionary history of the Treg family and Treg-mediated regulatory networks in cellular immunity.

Characterization of C1q in teleosts: insight into the molecular and functional evolution of C1q family and classical pathway

C1qs are key components of the classical complement pathway. They have been well documented in human and mammals, but little is known about their molecular and functional characteristics in fish. In the present study, full-length cDNAs of c1qA, c1qB, and c1qC from zebrafish (Danio rerio) were cloned, revealing the conservation of their chromosomal synteny and organization between zebrafish and other species. For functional analysis, the globular heads of C1qA (ghA), C1qB (ghB), and C1qC (ghC) were expressed in Escherichia coli as soluble proteins. Hemolytic inhibitory assays showed that hemolytic activity in carp serum can be inhibited significantly by anti-C1qA, -C1qB, and -C1qC of zebrafish, respectively, indicating that C1qA, C1qB, and C1qC are involved in the classical pathway and are conserved functionally from fish to human. Zebrafish C1qs also could specifically bind to heat-aggregated zebrafish IgM, human IgG, and IgM. The involvement of globular head modules in the C1q-dependent classical pathway demonstrates the structural and functional conservation of these molecules in the classical pathway and their IgM or IgG binding sites during evolution. Phylogenetic analysis revealed that c1qA, c1qB, and c1qC may be formed by duplications of a single copy of c1qB and that the C1q family is, evolutionarily, closely related to the Emu family. This study improves current understanding of the evolutionary history of the C1q family and C1q-mediated immunity.

Induction of hepatic differentiation of mouse bone marrow stromal stem cells by the histone deacetylase inhibitor VPA

Bone marrow stromal stem cells (BMSSCs) may have potential to differentiate in vitro and in vivo into hepatocytes. Here, we investigated the effects of valproic acid (VPA) involved in epigenetic modification, a direct inhibitor of histone deacetylase, on hepatic differentiation of mouse BMSSCs. Following the treatment of 2.5 mM VPA for 72 hrs, the in vitro expanded, highly purified and functionally active mouse BMSSCs from bone marrow were either exposed to some well-defined cytokines and growth factors in a sequential way (fibroblast growth factor-4 [FGF-4], followed by HGF, and HGF + OSM + ITS + dexamethasone, resembling the order of secretion during liver embryogenesis) or transplanted (caudal vein) in mice submitted to a protocol of chronic injury (chronic i.p. injection of CCl4). Additional exposure of the cells to VPA considerably improved the in vitro differentiation, as demonstrated by a more homogeneous cell population exhibited epithelial morphology, increasing expression of hepatic special genes and enhanced hepatic functions. Further more, in vivo results indicate that the pre-treatment of VPA significantly increased the homing efficiency of BMSSCs to the site of liver injury and, additionally, for supporting hepatic differentiation as well as in vitro. We have demonstrated the usefulness of VPA in the transdifferentiation of BMSSCs into hepatocytes both in vitro and in vivo, and regulation of fibroblast growth factor receptors (FGFRs) and c-Met gene expression through post-translational modification of core histones might be the primary initiating event for these effects. This mode could be helpful for liver engineering and clinical therapy.

Identification of cytokines involved in hepatic differentiation of mBM-MSCs under liver-injury conditions

AIM: To identify the key cytokines involved in hepatic differentiation of mouse bone marrow mesenchymal stem cells (mBM-MSCs) under liver-injury conditions.

METHODS: Abdominal injection of CCl₄ was adopted to duplicate a mouse acute liver injury model. Global gene expression analysis was performed to evaluate the potential genes involved in hepatic commitment under liver-injury conditions. The cytokines involved in hepatic differentiation of mBM-MSCs was functionally examined by depletion experiment using specific antibodies, followed by rescue experiment and direct inducing assay. The hepatic differentiation was characterized by the expression of hepatic lineage genes and proteins, as well as functional features.

RESULTS: Cytokines potentially participating in hepatic fate commitment under liver-injury conditions were initially measured by microarray. Among the up-regulated genes determined, 18 cytokines known to closely relate to liver growth, repair and development, were selected for further identification. The fibroblast growth factor-4 (FGF-4), hepatocyte growth factor (HGF) and oncostatin M (OSM) were finally found to be involved in hepatic differentiation of mBM-MSCs under liver-injury conditions. Hepatic differentiation could be dramatically decreased after removing FGF-4, HGF and OSM from the liver-injury conditioned medium, and could be rescued by supplementing these cytokines. The FGF-4, HGF and OSM play different roles in the hepatic differentiation of mBM-MSCs, in which FGF-4 and HGF are essential for the initiation of hepatic differentiation, while OSM is critical for the maturation of hepatocytes.

CONCLUSION: FGF-4, HGF and OSM are the key cytokines involved in the liver-injury conditioned medium for the hepatic differentiation of mBM-MSCs.

Identification and characterization of a novel immunoglobulin Z isotype in zebrafish: implications for a distinct B cell receptor in lower vertebrates

It was generally believed that teleost fish possess only few immunoglobulin isotypes such as IgM and IgD. The newly discovered IgZ/IgZ-like molecules in zebrafish and several other fish species greatly enriched our knowledge of immunoglobulin family in lower vertebrates. In the present study, we report a second IgZ-like isotype in zebrafish, which was designated as IgZ-2, adding a novel member to the immunoglobulin family in teleost fish. The IgZ-2 heavy chain encoding gene exhibited 76.5% nucleotide sequence identity to the previously reported IgZ. The putative IgZ-2 protein consists of four constant regions in its extracellular region, a transmembrane domain, and a short cytoplasmic tail. A number of conserved domains or residues such as the four Ig domains, cysteines required for Ig fold, hydrophobic and hydrophilic residues consistent with the CART domain, and Thr, Ser, Tyr residues known to be essential for association with the B cell co-receptors CD79A/B, were identified. Phylogenetic analysis showed that IgZ-2 grouped with IgZ and other known teleost IgZ-like sequences. The IgZ-2 transcripts were widely expressed in immune-related tissues, and could be significantly up-regulated by in vivo stimulation with LPS in various tissues including head kidney, spleen, intestine, skin, and gill. However, hardly IgZ-2 transcripts were detected during embryonic development until 2 weeks after fertilization. Double immunofluorescence staining showed that IgZ-2 and IgM were co-localized on B cell surfaces. Flow cytometric analysis showed the percentage of IgZ-2-positive cells could be dramatically up-regulated by interleukin-4 (IL-4), a cytokine known to activate the proliferation and differentiation of B cells. These observations indicated that the newly cloned IgZ-2 could be a novel B cell receptor. Our results will add new insights into the immunoglobulin class diversity of teleost fish, and also help us understand better the evolutionary history of adaptive immunity from fish to mammals as a whole.

Direct hepatic differentiation of mouse embryonic stem cells induced by valproic acid and cytokines

AIM: To develop a protocol for direct hepatic lineage differentiation from early developmental progenitors to a population of mature hepatocytes.

METHODS: Hepatic progenitor cells and then mature hepatocytes from mouse embryonic stem (ES) cells were obtained in a sequential manner, induced by valproic acid (VPA) and cytokines (hepatocyte growth factor, epidermal growth factor and insulin). Morphological changes of the differentiated cells were examined by phase-contrast microscopy and electron microscopy. Reverse transcription polymerase chain reaction and immunocytochemical analyses were used to evaluate the gene expression profiles of the VPA-induced hepatic progenitors and the hepatic progenitor-derived hepatocytes. Glycogen storage, cytochrome P450 activity, transplantation assay, differentiation of bile duct-like structures and tumorigenic analyses were performed for the functional identification of the differentiated cells. Furthermore, FACS and electron microscopy were used for the analyses of cell cycle profile and apoptosis in VPA-induced hepatic differentiated cells.

RESULTS: Based on the combination of VPA and cytokines, mouse ES cells differentiated into a uniform and homogeneous cell population of hepatic progenitor cells and then matured into functional hepatocytes. The progenitor population shared several characteristics with ES cells and hepatic stem/progenitor cells, and represented a novel progenitor cell between ES and hepatic oval cells in embryonic development. The differentiated hepatocytes from progenitor cells shared typical characteristics with mature hepatocytes, including the patterns of gene expression, immunological markers, in vitro hepatocyte functions and in vivo capacity to restore acute-damaged liver function. In addition, the differentiation of hepatic progenitor cells from ES cells was accompanied by significant cell cycle arrest and selective survival of differentiating cells towards hepatic lineages.

CONCLUSION: Hepatic cells of different developmental stages from early progenitors to matured hepatocytes can be acquired in the appropriate order based on sequential induction with VPA and cytokines.

The DC-SIGN of zebrafish: insights into the existence of a CD209 homologue in a lower vertebrate and its involvement in adaptive immunity

Dendritic cell-specific ICAM-3-grabbing nonintegrin (DC-SIGN/CD209) has become hot topic in recent studies because of its important roles in immune responses and immune escape. CD209 has been well characterized in humans and several other mammals, but little documentation exists about it in lower vertebrates. This is the first report on the identification and functional characterization of a fish DC-SIGN/CD209 molecule. The zebrafish DC-SIGN/CD209 cDNA translates into 343 aa organized into three domains structurally conserved among vertebrates. An EPN motif essential for interacting with Ca(2+) and for recognizing mannose-containing motifs has been identified. Several conserved motifs crucial for internalization and signal transduction are also present within the cytoplasmic tail. Phylogenetic analysis supports the hypothesis that CD209 family members diverged from a common ancestor. The expression of DC-SIGN/CD209 in immune-related tissues can be significantly up-regulated by exogenous Ags and IL-4. This molecule associates with various APCs, including macrophages, B lymphocytes, and a possible dendritic cell-like (CD83(+)/CD80(+)CD209(+)) population. Functionally, T cell activation, Ab (IgM) production, and bacterial vaccination-elicited immunoprotection can be dramatically inhibited by a CD209 blockade after stimulation with keyhole limpet hemocyanin (KLH) in vivo or challenged with Aeromonas hydrophila, suggesting that DC-SIGN/CD209 in zebrafish is crucial for the initiation and development of adaptive immunity. Phagocytosis analysis showed that DC-SIGN/CD209 does not participate in the uptake of KLH Ag, suggesting that other mechanisms might exist that underlie DC-SIGN/CD209 involvement. We hope that the present study will contribute to a better cross-species understanding of the evolutionary history of the DC-SIGN/CD209 family.

Recruitment of endogenous bone marrow mesenchymal stem cells towards injured liver

Recent studies suggest that mesenchymal stem cells (MSCs) possess a greater differentiation potential than once thought and that they have the capacity to regenerate damaged tissues/organs. However, the evidence is insufficient, and the mechanism governing the recruitment and homing of MSCs to these injured sites is not well understood. We first examined the MSCs circulating in peripheral blood and then performed chemotaxis, wound healing and tubule-formation assays to investigate the migration capability of mouse bone marrow MSCs (mBM-MSCs) in response to liver-injury signals. In addition, BM-MSCs from donor enhanced green fluorescent protein transgenic male mice were transplanted into liver-injured co-isogenic female recipients, either by intra-bone marrow injection or through the caudal vein, to allow in vivo tracking analysis of the cell fate after transplantation. Donor-derived cells were analysed by in vivo imaging analysis, PCR, flow cytometry and frozen sections. Microarray and real-time PCR were used for chemokine/cytokine and receptor analyses. We successfully isolated circulating MSCs in peripheral blood of liver-injured mice and provided direct evidence that mBM-MSCs could be mobilized into the circulation and recruited into the liver after stimulation of liver injury. CCR9, CXCR4 and c-MET were essential for directing cellular migration towards the injured liver. The recruited mBM-MSCs may play different roles, including hepatic fate specification and down-regulation of the activity of hepatic stellate cells which inhibits over-accumulation of collagen and development of liver fibrosis. Our results provide new insights into liver repair involving endogenous BM-MSCs and add new information for consideration when developing clinical protocols involving the MSCs.

CD154-CD40 interactions are essential for thymus-dependent antibody production in zebrafish: insights into the origin of costimulatory pathway in helper T cell-regulated adaptive immunity in early vertebrates

The CD154-CD40-mediated costimulatory pathway is critical for T-B cell cooperation in thymus-dependent (TD) immune response in mammals. However, little is known about its existence and occurrence in lower vertebrates. Here, we report on the identification and functional characterization of CD154 and CD40 homologs from the zebrafish (Danio rerio) model. Zebrafish CD154 is a type II membrane-bound protein with a TNF homology domain in its extracellular C-terminal region, whose tertiary structure is a sandwich containing two stacked sheets with "jelly roll" topology, just as the human TNF members do. The zebrafish CD40 is a type I membrane-bound protein with a sequence pattern of four cysteine-rich domains in its extracellular N-terminal region. The consensus TNFR-associated factor (TRAF)2- and TRAF6-binding motifs in mammalian CD40 are found in the cytoplasmic tail of zebrafish CD40, which indicates similar signal transduction mechanisms to higher vertebrates. Zebrafish CD154 and CD40 are widely distributed and can be up-regulated by thymus-dependent Ag. The production of IgM was dramatically decreased by anti-CD154 or soluble CD40, and it was enhanced by soluble CD154 or CD154-encoding plasmid in vivo. Thymus-dependent Ag-induced CD154 expression was inhibited by cyclosporin A, suggesting that CD154 functionally associates with T cells. Double immunofluorescence staining showed that CD40 and membrane IgM colocalized in B cells. CD154-CD40 binding assays showed that CD154 specifically binds to CD40 at homodimeric form. Our results provide the first evidence for the existence of the functional CD154-CD40-mediated costimulatory pathway and helper T cell regulatory mechanism underlying adaptive immunity in a fish species.

Midkine accumulated in nucleolus of HepG2 cells involved in rRNA transcription

AIM: To investigate the ultrastructural location of midkine (MK) in nucleolus and function corresponding to its location.

METHODS: To investigate the ultrastructural location of MK in nucleolus with immunoelectronic microscopy. To study the role that MK plays in ribosomal biogenesis by real-time PCR. The effect of MK on anti-apoptotic activity of HepG2 cells was studied with FITC-conjugated annexin V and propidium iodide PI double staining through FACS assay.

RESULTS: MK mainly localized in the granular component (GC), dense fibrillar component (DFC) and the border between the DFC and fibrillar center (FC). The production of 45S precursor rRNA level was decreased significantly in the presence of MK antisense oligonucleotide in the HepG2 cells. Furthermore, it was found that exogenous MK could protect HepG2 from apoptosis significantly.

CONCLUSION: MK was constitutively translocated to the nucleolus of HepG2 cells, where it accumulated and mostly distributed at DFC, GC components and at the region between FC and DFC, MK played an important role in rRNA transcription, ribosome biogenesis, and cell proliferation in HepG2 cells. MK might serve as a molecular target for therapeutic intervention of human carcinomas.

Pre-B cell colony-enhancing factor in lower vertebrates: first evidence of this cytokine being involved in antioxidant activity by reconstruction of a novel NAD salvage pathway in E. coli

The pre-B cell colony-enhancing factor identified in mammals is an important cytokine involved in multiple functions, such as immunoregulation, cellular proliferation and differentiation. However, little is known about its existence and function in lower vertebrates; therefore, we investigated the characterization, expression and especially the biofunction of this factor in Tetraodon nigroviridis, a model organism of lower vertebrates. We focus on the question of whether the pre-B cell colony-enhancing factor of lower vertebrates contributes to the NAD mediated antioxidant activity by its involvement in the biosynthesis of NAD through pyridine nucleotide cycles. Experimental data demonstrated that by transforming fish pre-B cell colony-enhancing factor into Escherichia coli cells, the amounts of NAD and NADP significantly increased, and cellular antioxidant activity greatly improved. This is the first report about vertebrate pre-B cell colony-enhancing factor acting as a nicotinamide phosphoribosyltransferase to reconstruct a pyridine nucleotide cycle III pathway in E. coli and playing an important role in antioxidant stress by up-regulation of NAD biosynthesis. We hope that our observations may enrich the study of pre-B cell colony-enhancing factor, and contribute to a better understanding of the molecular and functional evolution of the pre-B cell colony-enhancing factor family in both lower vertebrates and mammals as a whole.

Lipopolysaccharide induces apoptosis in Carassius auratus lymphocytes, a possible role in pathogenesis of bacterial infection in fish

Lipopolysaccharide (LPS), the endotoxin of Gram-negative bacteria, is capable of eliciting a wide variety of pathophysiological effects, including endotoxin shock, tissue injury and lethality in both humans and animals. It is also a potent stimulant to initiate the proliferation, differentiation and activation of B lymphocytes and macrophages, resulting in changes of inflammatory cytokines, such as TNF-alpha, IL1-beta, IL6, IL-8 and IL-12, and enhancement of immune responses. However, little is known about its effect on the induction of apoptosis in lymphocytes. In the present study, the lymphocytes from Carassius auratus were employed for this purpose. The cells were exposed to LPS at various doses for different time periods. By careful apoptotic characteristic analysis, such as condensation of nuclear chromatin, fragmentation of genomic DNA and formation of apoptotic bodies, it provided the first evidence that LPS had apoptotic-inducing effect on fish lymphocytes in a time- and dose-dependent manner. LPS exposure induced significant increase of intracellular reactive oxygen species (ROS), loss of mitochondrial transmembrane potential (DeltaPsi), depletion of ATP production, down-regulation of Bcl-2 expression, up-regulation of Bax and mitochondrial NO-synthase (mNOS) expression, and selective activation of caspase-9 rather than caspase-8. Each of these observations suggests that the LPS-induced apoptosis in C. auratus lymphocytes occurs largely via the mitochondrial apoptotic pathway. This observation was different from the mechanism behind the LPS-induced apoptosis in mammalian macrophages/thymocytes that occurs via the TNF-alpha-mediated death-receptor pathway. Our study suggested the existence of a possible novel role in the pathogenesis of Gram-negative bacterial infection in fish and even in mammals, which may contribute to the therapy of bacterial diseases. Also, it will help to gain more insights into the mechanisms of septic shock and of LPS-induced immunosuppression and autoimmunity.

Vacuolar H+-ATPase

The vacuolar H(+)-ATPase (V-ATPase) is a universal component of eukaryotic organisms, which is present in both intracellular compartments and the plasma membrane. In the latter, its proton-pumping action creates the low intravacuolar pH, benefiting many processes such as, membrane trafficking, protein degradation, renal acidification, bone resorption, and tumor metastasis. In this article, we briefly summarize recent studies on the essential and diverse roles of mammalian V-ATPase and their medical applications, with a special emphasis on identification and use of V-ATPase inhibitors.

In vitro differentiation of mouse bone marrow stromal stem cells into hepatocytes induced by conditioned culture medium of hepatocytes

The differentiation potential of adult stem cells has long been believed to be limited to the tissue or germ layer of their origin. However, recent studies have demonstrated that adult stem cells may encompass a greater potential than once thought. In the present study, we examined whether murine bone marrow derived stromal stem cells (BMSSCs) are able to differentiate into functional hepatocytes in vitro. BMSSCs were isolated from murine femora and tibiae, and the mesodermal multilineage differentiation potentials of these cells were functionally characterized. To effectively induce hepatic differentiation, we designed a novel protocol by using hepatocyte-conditioned medium. Hepatic differentiation from mouse BMSSCs was examined by a variety of assays at morphological and molecular levels. Morphologically, mouse BMSSCs became round and epithelioid, binucleated after induction. Differentiated cells were harvested on Days 0, 10, and 20 and subjected to examination of hepatocyte characteristics by reverse transcription-polymerase chain reaction (RT-PCR) and immunocytochemistry. We detected AFP, HNF-3beta, CK19, CK18, ALB, TAT, and G-6-Pase at the mRNA and/or protein levels, hepatocyte-like cells by culture in conditioned medium further demonstrated in vitro functions characteristic of liver cells, including glycogen storage, and urea secretion. Moreover, transplantation of the differentiated cells into liver-injured mice partially restored serum albumin level and significantly suppressed transaminase activity. Our findings indicated the transdifferentiation potential of mouse BMSSCs developing into the functional hepatocyte-like cells by conditioned culture medium and, hence, may serve as a model system for the study of mechanisms involved in the transdifferentiation, and a cell source for cell therapy of hepatic diseases.

Construction of a recombinant plasmid containing multi-copy CpG motifs and its effects on the innate immune responses of aquatic animals

Bacterial DNA and synthetic oligodeoxynucleotides (ODNs) containing unmethylated CpG dinucleotides (CpG motifs) have been shown to induce potential immune responses. In this study, we designed a recombinant plasmid containing multi-copy CpG motifs, and observed its effects on innate immune responses of fish and prawn. The results showed that such plasmid DNA, compared to the vacant vector, can highly induce the activation of head kidney macrophages and the proliferation of peripheral blood leukocytes in Carassius auratus and Lateolabrax japonicus in vitro, as well as the activity of humoral defense proteins and the antibacterial activity of haemolymph in Litopenaeus vannamei in vivo. It implies that the multi-copy CpG motifs harboured in plasmid could contribute to these innate immunostimulatory effects. Therefore, the study suggested that the plasmid containing multi-copy CpG motifs might have its potential application in improving host resistance to pathogen insults in aquaculture, and have its notable advantages of high efficacy, economical cost and application to a broad range of aquatic species.

Bone morphogenetic protein

Bone morphogenetic proteins (BMPs) are multi-functional growth factors belonging to the transforming growth factor-beta superfamily. It has been demonstrated that BMPs had been involved in the regulation of cell proliferation, survival, differentiation and apoptosis. However, their hallmark ability is that play a pivotal role in inducing bone, cartilage, ligament, and tendon formation at both heterotopic and orthotopic sites. In this review, we mainly concentrate on BMP structure, function, molecular signaling and potential medical application.

Identification and characterization of suppressor of cytokine signaling 1 (SOCS-1) homologues in teleost fish

The suppressor of cytokine signaling 1 (SOCS-1), as the name implies, is a member of proteins that regulate cytokine signaling pathways by inhibiting the events of key tyrosine phosphorylation on cytokine receptors and signaling molecules such as Janus kinase family members. Although mammalian SOCS-1 homologues were characterized in several species, no similar research work has been reported in fish yet. In this paper, we initially cloned the SOCS-1 genes from Tetraodon nigroviridis and Danio rerio, and identified other two SOCS-1 genes from Fugu rubripes and Gasterosteus aculeatus. The results showed that the fish SOCS-1-encoding genes consist of two exons and a single intron, a typical characteristic of SOCS family in gene organization. Moreover, two alternatively spliced transcripts that encoded 220 and 196 amino acids were obtained in T. nigroviridis, proving the distinct existence of alternative splicing in fish SOCS-1 different from higher vertebrates. By reverse transcriptase polymerase chain reaction (PCR) and real-time quantitative PCR, gene expression studies indicated that both two alternatively spliced transcripts of Tetraodon SOCS-1 were expressed extensively in major tissues as we examined and their corresponding expression levels could be strikingly raised at 3 h postinjection with lipopolysaccharide, which strongly suggested that SOCS-1 proteins in fish might be involved in inflammatory responses. This is the first report of cloning and characterization of SOCS-1 complementary deoxyribonucleic acids and genes in fish.