Intracellular replication of Edwardsiella tarda in murine macrophage is dependent on the type III secretion system and induces an up-regulation of anti-apoptotic NF-κB target genes protecting the macrophage from staurosporine-induced apoptosis

Functional analysis of OmpA and its contribution to pathogenesis of Edwardsiella tarda

Outer membrane protein A (OmpA), a major component of outer membrane proteins in gram-negative bacteria, is considered to be an important virulence factor in various pathogenic bacteria, but its underlying mechanisms involved in pathogenic process of Edwardsiella tarda has not yet been fully elucidated. E. tarda is an important facultative intracellular pathogen with a broad host range. This bacterium could survive and replicate in macrophages as an escape mechanism from the host defense. To address the functions of OmpA and its potential roles in the pathogenesis of E. tarda, ΔompA mutant strain and ΔompA-C complementary strain were constructed by the allelic exchange method in this study. Here, we demonstrate that the abilities of motility, biofilm formation and adherence to RAW264.7 cells of ΔompA were significantly impaired, although there was no difference in growth between wild-type (WT) strain and ΔompA. Moreover, inactivation of ompA rendered E. tarda more sensitive to oxidative, heat shock and osmotic stress, which simulate the in vivo conditions that E. tarda encounters within the intramacrophage environment. Consist with this observation, ΔompA was also found to be markedly attenuated for growth within macrophages. In addition, compared with the WT strain, ΔompA activated macrophages to release more inflammatory mediators, including tumor necrosis factor alpha (TNF-α), reactive oxygen species (ROS) and nitric oxide (NO). However, flow cytometry analysis revealed that ΔompA induced less apoptosis of RAW264.7 cells as compared with WT strain, characterized by decreased Annexin V binding and the activation of caspase-3. Overall, our findings suggest an importance of OmpA to E. tarda and provide the first comprehensive insight into its functions and potential roles in the pathogenesis of E. tarda, including its effect on interaction with macrophages.

Transcriptional regulation of the Japanese flounder Cu,Zn-SOD (Jfsod1) gene in RAW264.7 cells during oxidative stress caused by causative bacteria of edwardsiellosis

Edwardsiellosis is one of the most important bacterial diseases in fish, sometimes causing extensive economic losses in the aquaculture industry. Our previous studies demonstrated that the Cu,Zn-SOD (sod1) activity has significantly increased in Japanese flounder, Paralichthys olivaceus, hepatopancreas infected by causative bacteria of edwardsiellosis Edwardsiella tarda NUF251. In this study, NUF251 stimulated intracellular superoxide radical production in mouse macrophage RAW264.7 cells, which was reduced by N-acetylcysteine. This result suggests that NUF251 infection causes oxidative stress. To evaluate the regulatory mechanism of Jfsod1 at transcriptional levels under oxidative stress induced by NUF251 infection, we cloned and determined the nucleotide sequence (1124 bp) of the 5'-flanking region of the Jfsod1 gene. The sequence analysis demonstrated that the binding sites for the transcription factors C/EBPα and NF-IL6 involved in the transcriptional regulation of the mammalian sod1 gene existed. We constructed a luciferase reporter system with the 5'-flanking region (-1124/-1) of the Jfsod1 gene, and a highly increased transcriptional activity of the region was observed in NUF251-infected RAW264.7 cells. Further studies using several mutants indicated that deletion of the recognition region of NF-IL6 (-272/-132) resulted in a significant decrease in the transcriptional activity of the Jfsod1 gene in NUF251-infected RAW264.7 cells. In particular, the binding site (-202/-194) for NF-IL6 might play a major role in upregulating the transcriptional activity of the 5'-flanking region of the Jfsod1 gene in response to oxidative stress induced by NUF251 infection. These results could be provided a new insight to understand the pathogenic mechanism of causative bacteria of edwardsiellosis.

CD40/TRAF1 decreases synovial cell apoptosis in patients with rheumatoid arthritis through JNK/NF-κB pathway

INTRODUCTION

A genome-wide association analysis revealed a rheumatoid arthritis (RA)-risk-associated genetic locus on chromosome 9, which contained the tumor necrosis factor receptor-associated factor 1 (TRAF1). However, the detail mechanism by TRAF1 signaled to fibroblast-like synoviocytes (FLSs) apoptosis remains to be fully understood.

MATERIALS AND METHODS

Synovial tissue of 10 RA patients and osteoarthritis patients were obtained during joint replacement surgery. We investigated TRAF1 level and FLSs apoptosis percentage in vivo and elucidated the mechanism involved in the regulation of apoptotic process in vitro.

RESULTS

We proved the significant increase of TRAF1 level in FLSs of RA patients and demonstrated that TRAF1 level correlated positively with DAS28 score and negatively with FLSs apoptosis. Treatment with siTRAF1 was able to decrease MMPs levels and the phosphorylated forms of JNK/NF-κB in vitro. Moreover, JNK inhibitor could attenuate expression of MMPs and increase percentage of apoptosis in RA-FLSs, while siTRAF1 could not promote apoptosis when RA-FLSs were pretreated with JNK activator.

CONCLUSIONS

High levels of TRAF1 in RA synovium play an important role in the synovial hyperplasia of RA by suppressing apoptosis through activating JNK/NF-kB-dependent signaling pathways in response to the engagement of CD40.

Orf1B controls secretion of T3SS proteins and contributes to Edwardsiella piscicida adhesion to epithelial cells

Edwardsiella piscicida is a Gram-negative enteric pathogen that causes hemorrhagic septicemia in fish. The type III secretion system (T3SS) is one of its two most important virulence islands. T3SS protein EseJ inhibits E. piscicida adhesion to epithelioma papillosum cyprini (EPC) cells by negatively regulating type 1 fimbria. Type 1 fimbria helps E. piscicida to adhere to fish epithelial cells. In this study, we characterized a functional unknown protein (Orf1B) encoded within the T3SS gene cluster of E. piscicida. This protein consists of 122 amino acids, sharing structural similarity with YscO in Vibrio parahaemolyticus. Orf1B controls secretion of T3SS translocon and effectors in E. piscicida. By immunoprecipitation, Orf1B was shown to interact with T3SS ATPase EsaN. This interaction may contribute to the assembly of the ATPase complex, which energizes the secretion of T3SS proteins. Moreover, disruption of Orf1B dramatically decreased E. piscicida adhesion to EPC cells due to the increased steady-state protein level of EseJ within E. piscicida. Taken together, this study partially unraveled the mechanisms through which Orf1B promotes secretion of T3SS proteins and contributes to E. piscicida adhesion. This study helps to improve our understanding on molecular mechanism of E. piscicida pathogenesis.

Evaluation of UV-B protective properties of leaves and seeds of Achyranthes aspera in Asian catfish Clarias batrachus (Linn.)

The ultraviolet-B (UV-B) radiation is harmful to the aquatic organisms. The UV-B protective properties of leaves and seeds of herb Achyranthes aspera were evaluated in Clarias batrachus. Fish were fed with four diets-EFL1, EFL2 containing 0.25 and 0.5% leaves, EFS containing 0.5% seeds and control, CF. After 83 days of feeding, fish were exposed to UV-B (157 µW/cm²) for 7 days at the rate of 15 min/day. One batch of fish in each treatment was kept unexposed. Significantly higher final weight was found in EFS followed by EFL2 and EFL1 treatments. It was higher in unexposed fish compared to the exposed ones. Among exposed fish, significantly higher lysozyme was found in EFS and myeloperoxidase in EFS and EFL2 compared to others. Nitric oxide synthase and superoxide dismutase levels were significantly higher in liver and head kidney of EFS diet fed fish compared to others. Thiobarbituric acid reactive substances (TBARS) and carbonyl protein levels were minimum in EFS followed by EFL2. The independent sample t-test showed that nitric oxide synthase was significantly higher and myeloperoxidase and TBARS levels were significantly lower in unexposed group compared to the exposed fish in respective treatment. There were up-regulations of TNF-α, iNOS, NF-kB, BAX, Cytochrome c, SOD-c, Caspase 3, Caspase 9, BCL2 in liver and head kidney of leaves and seeds incorporated diets fed fish compared to control. Supplementation of A. aspera seeds and leaves at 0.5% level in diets gave UV-B protection to the fish.

IFN-γ Manipulates NOD1-Mediated Interaction of Autophagy and Edwardsiella piscicida to Augment Intracellular Clearance in Fish

Edwardsiella piscicida is an intracellular pathogenic bacterium accounting for significant losses in farmed fish. Currently, cellular and molecular mechanisms underlying E. piscicida-host cross-talk remain obscure. In this study, we revealed that E. piscicida could increase microtubule-associated protein L chain 3 (LC3) puncta in grass carp (Ctenopharyngodon idella) monocytes/macrophages and a carp cell line, Epithelioma papulosum cyprini The autophagic response was confirmed by detecting the colocalization of E. piscicida with LC3-positive autophagosomes and LysoTracker-probed lysosomes in the cells. Moreover, we unveiled the autophagic machinery targeting E. piscicida by which the nucleotide-binding oligomerization domain receptor 1 (NOD1) functioned as an intracellular sensor to interact and recruit autophagy-related gene (ATG) 16L1 to the bacteria. Meanwhile, E. piscicida decreased the mRNA and protein levels of NOD1 and ATG16L1 in an estrogen-related receptor-α-dependent manner, suggesting a possible mechanism for this bacterium escaping autophagy. Subsequently, we examined the effects of various E. piscicida virulence factors on NOD1 expression and found that two of them, EVPC and ESCB, could reduce NOD1 protein expression via ubiquitin-dependent proteasomal degradation. Furthermore, an intrinsic regulator IFN-γ was found to enhance the colocalization of E. piscicida with NOD1 or autophagosomes, suggesting its involvement in the interaction between autophagy and E. piscicida Along this line, a short-time treatment of IFN-γ caused intracellular E. piscicida clearance through an autophagy-dependent mechanism. Collectively, our works demonstrated NOD1-mediated autophagy-E. piscicida dialogues and uncovered the molecular mechanism involving autophagy against intracellular bacteria in fish.

Edwardsiella piscicida Interferes with Classical Endocytic Trafficking and Replicates in a Specialized Replication-Permissive Niche in Nonphagocytic Cells

Edwardsiella piscicida is an intracellular pathogen within a broad spectrum of hosts. Essential to E. piscicida's virulence is its ability to invade and replicate inside host cells, yet the survival mechanisms and the nature of the replicative compartment remain unknown. Here, we characterized its intracellular lifestyle in nonphagocytic cells and showed that the intracellular replication of E. piscicida in nonphagocytic cells is dependent on its type III secretion system (T3SS) but not its type VI secretion system. Following internalization, E. piscicida is contained in vacuoles that transiently mature into early endosomes but subsequently bypasses the classical endosome pathway and fusion with lysosomes, which depend on its T3SS. Following rapid escape from the degradative pathway, E. piscicida was found to create a specialized replication-permissive niche characterized by endoplasmic reticulum (ER) markers. Furthermore, we found that a T3SS effector, EseJ, is responsible for the intracellular replication of E. piscicida by preventing endosome/lysosome fusion. In vivo experiments also confirmed that EseJ is necessary for bacterial colonization by E. piscicida in the epithelial layer, followed by systemic dissemination in both zebrafish and mice. Thus, this work elucidates the tactics used by E. piscicida to survive and proliferate within host nonphagocytic cells. IMPORTANCE E. piscicida is a facultative intracellular bacterium associated with septicemia and fatal infections in many animals, including fish and humans. However, little is known about its intracellular life, which is important for successful invasion of the host. The present study is the first comprehensive characterization of E. piscicida's intracellular lifestyle in host cells. Upon internalization, E. piscicida is transiently contained in Rab5-positive vacuoles, but the pathogen prevents further endosome maturation and fusion with lysosomes by utilizing a T3SS effector, EseJ. In addition, the bacterium creates a specialized replication niche for rapid growth via an interaction with the ER. Our study provides new insights into the strategies used by E. piscicida to successfully establish an intracellular lifestyle that contributes to its survival and dissemination during infection.

Identification and Characterization of EvpQ, a Novel T6SS Effector Encoded on a Mobile Genetic Element in Edwardsiella piscicida

In this study, a hypothetical protein (ORF02740) secreted by Edwardsiella piscicida was identified. We renamed the ORF02740 protein as EvpQ, which is encoded by a mobile genetic element (MGE) in E. piscicida genome. The evpQ gene is spaced by 513 genes from type VI secretion system (T6SS) gene cluster. Low GC content, three tRNA, and three transposase genes nearby evpQ define this MGE that evpQ localizes as a genomic island. Sequence analysis reveals that EvpQ shares a conserved domain of C70 family cysteine protease and shares 23.91% identity with T3SS effector AvrRpt2 of phytopathogenic Erwinia amylovora. Instead, EvpQ of E. piscicida is proved to be secreted at a T6SS-dependent manner, and it can be translocated into host cells. EvpQ is thereof a novel T6SS effector. Significantly decreased competitive index of ΔevpQ strain in blue gourami fish (0.53 ± 0.27 in head kidney and 0.44 ± 0.19 in spleen) indicates that EvpQ contributes to the pathogenesis of E. piscicida. At 8-, 18-, and 24-h post-subculture into DMEM, the transcription of evpQ was found to be negatively regulated by Fur and positively regulated by EsrC, and the steady-state protein levels of EvpQ are negatively controlled by RpoS. Our study lays a foundation for further understanding the pathogenic role of T6SS in edwardsiellosis.

Marine Peptide-N6NH2 and Its Derivative-GUON6NH2 Have Potent Antimicrobial Activity Against Intracellular Edwardsiella tarda in vitro and in vivo

Edwardsiella tarda is a facultative intracellular pathogen in humans and animals. There is no effective way except vaccine candidates to eradicate intracellular E. tarda. In this study, four derivatives of marine peptide-N6NH2 were designed by an introduction of unnatural residues or substitution of natural ones, and their intracellular activities against E. tarda were evaluated in macrophages and in mice, respectively. The minimum inhibitory concentration (MIC) value of N6NH2 and GUON6NH2 against E. tarda was 8 μg/mL. GUON6NH2 showed higher stability to trypsin, lower toxicity (<1%) and longer post-antibiotic effect (PAE) than N6NH2 and other derivatives. Antibacterial mechanism results showed that GUON6NH2 could bind to LPS and destroyed outer/inner cell membranes of E. tarda, superior to N6NH2 and norfloxacin. Both N6NH2 and GUON6NH2 were internalized into macrophages mainly via lipid rafts, micropinocytosis, and microtubule polymerization, respectively, and distributed in the cytoplasm. The intracellular inhibition rate of GUON6NH2 against E. tarda was 97.05–100%, higher than that in case of N6NH2 (96.82–100%). In the E. tarda-induced peritonitis mouse model, after treatment with of 1 μmol/kg N6NH2 and GUON6NH2, intracellular bacterial numbers were reduced by 1.54- and 1.97-Log₁₀ CFU, respectively, higher than norfloxacin (0.35-Log₁₀ CFU). These results suggest that GUON6NH2 may be an excellent candidate for novel antimicrobial agents to treat infectious diseases caused by intracellular E. tarda.

RGD-binding integrins mediated phagocytosis involved in the entry of Edwardsiella tarda into mudskipper MO/MФ

The versatile fish pathogen Edwardsiella tarda is an intracellular pathogen with the ability to invade and replicate in host phagocytes. However, the mechanism mediating the uptake of E. tarda in fish monocytes/macrophages (MO/MΦ) is not yet understood. Generating mudskipper kidney-derived MO/MФ transcriptomic resources from mudskipper challenged by E. tarda is crucial for understanding the molecular mechanisms underlying the mudskipper invasion process. In the present study, a total of 1185 up-regulated and 885 down-regulated differentially expressed genes (DEGs) were identified using RNA-seq. Enrichment and pathway analysis of DEGs revealed the centrality of the phagosome and regulation of actin cytoskeleton pathways in pathogen entry. The progress of phagosome formation was observed by transmission electron microscopy. Eight conserved integrin (ITG) subunit genes, belonging to the phagocytic receptors, were found in the transcriptomic sequence data. Additionally, quantitative real-time PCR showed that the mRNA expressions of most ITG subunit genes were related to the different infection times of E. tarda and the different bacterial pathogens. Further assays demonstrated that phagocytosis of FITC-labeled E. tarda by mudskipper MO/MФ was significantly reduced by the tetrapeptide Asp-Gly-Arg-Ser (RGDS). In summary, phagocytosis is one of the entry pathways into mudskipper MO/MΦ, and RGD-binding ITGs are involved in the phagosome formation process.

Gene identification and functional analysis of peptidoglycan recognition protein from the spotted sea bass (Lateolabrax maculatus)

Peptidoglycan recognition proteins (PGRPs), which are structurally conserved innate immune molecules in invertebrate and vertebrate animals, play the important roles in regulation of innate immune responses. In this paper, three PGRP genes of spotted sea bass, Lateolabrax maculatus, were cloned, designated as Ssb-PGRP2, Ssb-PGRP-L2 and Ssb-PGRP-SC2, respectively. Sequence analysis showed that the deduced amino acid sequences of Ssb-PGRP2, Ssb-PGRP-L2 and Ssb-PGRP-SC2 proteins contained respectively 468, 482 and 167 amino acid residues, and had the typical structural features of PGRPs, i.e. conserved PGRP domain and Zn²⁺ binding domain including four specific amino acid residues which were required for amidase activity. q-PCR analysis of total mRNA showed that the mRNA expression of three PGRP genes were detected in all the examined tissues and the expression patterns of Ssb-PGRP2, Ssb-PGRP-L2 and Ssb-PGRP-SC2 were different. After injected with LPS, Poly (I:C) and Edwardsiella tarda, there was a clear time-dependent expression pattern for each of the three PGRP genes in head kidney, spleen, intestine and gill of the spotted sea bass. In our study, three recombinant proteins corresponding to the three members of the peptidoglycan recognition protein family were expressed and purified. Moreover, all of the three recombinant PGRP proteins significantly inhibited bacterial survival and growth, and expressed bactericidal effects on Vibrio harveyi, Staphylococcus aureus and Edwardsiella tarda. In particular, it was firstly verified that their antimicrobial activity presented the superimposed effect. Overall, these findings indicated that three PGRP genes of spotted sea bass were at least involved in host defense against bacterial infections.

The Translocation and Assembly Module (TAM) of Edwardsiella tarda Is Essential for Stress Resistance and Host Infection

Translocation and assembly module (TAM) is a protein channel known to mediate the secretion of virulence factors during pathogen infection. Edwardsiella tarda is a Gram-negative bacterium that is pathogenic to a wide range of farmed fish and other hosts including humans. In this study, we examined the function of the two components of the TAM, TamA and TamB, of E. tarda (named tamA_Et and tamB_Et, respectively). TamA_Et was found to localize on the surface of E. tarda and be recognizable by TamA_Et antibody. Compared to the wild type, the tamA and tamB knockouts, TX01ΔtamA and TX01ΔtamB, respectively, were significantly reduced in motility, flagella formation, invasion into host cells, intracellular replication, dissemination in host tissues, and inducing host mortality. The lost virulence capacities of TX01ΔtamA and TX01ΔtamB were restored by complementation with the tamA_Et and tamB_Et genes, respectively. Furthermore, TX01ΔtamA and TX01ΔtamB were significantly impaired in the ability to survive under low pH and oxidizing conditions, and were unable to maintain their internal pH balance and cellular structures in acidic environments, which led to increased susceptibility to lysozyme destruction. Taken together, these results indicate that TamA_Et and TamB_Et are essential for the virulence of E. tarda and required for E. tarda to survive under stress conditions.

An Edwardsiella piscicida esaS mutant reveals contribution to virulence and vaccine potential

Edwardsiella piscicida is a Gram-negative pathogen that causes disease in diverse aquatic organisms. The disease leads to extensive losses in commercial aquaculture species, including farmed U.S. catfish. The type III secretion system (T3SS) often contributes to virulence of Gram-negative bacteria. The E. piscicida esaS gene encodes a predicted T3SS export apparatus protein. In the current study, an E. piscicida esaS mutant was constructed and characterized to increase our understanding of the role of T3SS in E. piscicida virulence. Deletion of esaS did not significantly affect biofilm formation and hemolytic activity of E. piscicida, but it had significant effects on expression of hemolysis and T3SS effector genes during biofilm growth. EpΔesaS showed significantly (P < 0.05) reduced virulence in catfish compared to the parent strain. No mortalities occurred in fish infected with EpΔesaS at 6.3 × 10⁵ and 1.26 × 10⁶ CFU/fish compared to 26% mortality in fish infected with wild-type E. piscicida at 7.5 × 10⁵ CFU/fish. Bioluminescence imaging indicated that EpΔesaS invades catfish and colonizes for a short period in the organs. Furthermore, catfish immunized with EpΔesaS at 6.3 × 10⁵ and 1.26 × 10⁶ CFU provided 47% and 87% relative percent survival, respectively. These findings demonstrated that esaS plays a role in E. piscicida virulence, and the deletion mutant has vaccine potential for protection against wild-type E. piscicida infection.

Roles of EvpP in Edwardsiella piscicida-Macrophage Interactions

Edwardsiella piscicida is found to be an important facultative intracellular pathogen with a broad host range. These organisms can replicate and survive within host macrophages to escape from the subversion of the immune defense. E. piscicida-macrophage interaction is very important in determining the outcome of edwardsiellasis. As an effector protein of E. piscicida T6SS, EvpP has been determined to be a very important virulence factor for E. piscicida, although its precise role in E. piscicida-macrophage interactions is not yet clear. In this study, the roles of EvpP in E. piscicida-macrophage interactions were characterized. Here, we constructed the deletion mutants of evpP (ΔevpP) and complementation (ΔevpP-C) by the allelic exchange method. Compared to wild type strain (WT), ΔevpP was found to be attenuated for growth within macrophages. In line with this observation, we found its survival capacity was lower than WT under oxidative and acid stress in vitro, which simulate conditions encountered in host macrophages. Attenuation of ΔevpP also correlated with enhanced activation of macrophages, as reflected by augmented NO production in ΔevpP-treated macrophages. Moreover, compared to WT, ΔevpP induced markedly increased apoptosis of macrophages, characterized by increased Annexin V binding and the activation of cleaved caspase-3. These findings provided strong evidence that EvpP is involved in the process of E. piscicida-macrophage interactions and is required for its survival and replication in macrophages. Thus, we propose that EvpP might be an important factor that controlling the fate of E. piscicida inside macrophages. To further exploring the underlying mechanism of EvpP action, the cDNA library was constructed from E. piscicida-infected macrophages and a yeast two-hybrid screen was performed to search for cellular proteins interacting with EvpP. Ribosomal protein S5 (RPS5) was identified as a target of EvpP. Furthermore, the interaction was validated with co-immunoprecipitation assay. This result implies that the observed effect of EvpP on macrophages might be related to RPS5-mediated regulation, contributing to a better understanding of the mechanisms of EvpP involved in E. piscicida-macrophage interactions.

Balanced role of T3SS and T6SS in contribution to the full virulence of Edwardsiella piscicida

Edwardsiella piscicida is an important pathogen that infects a wide range of hosts, from fish to human. Its infection leads to extensive losses in a diverse array of commercially important fish, like Japanese flounder, turbot, and tilapia. During the infection, type III secretion system (T3SS) and type VI secretion system (T6SS) of E. piscicida play significant roles, but how T3SS and T6SS cooperatively contribute to its virulence is still unknown. In this study, we first examined the roles of T3SS and T6SS in different processes during E. piscicida infection of host cells, and revealed that T3SS of E. piscicida is responsible for promoting bacterial invasion, the following intracellular replication and inducing cell death in host cells, while T6SS restrains E. piscicida intracellular replication and cell death in J774A.1 cells, which suggested that T3SS and T6SS antagonistically concert E. piscicida infection. Furthermore, we found an significant decrease in transcription level of IL-1β in zebrafish kidney infected with T3SS mutant and an drastically increase in transcription level of TNF- α infected with T6SS mutant when compared with the wild-type. Interestingly, both T3SS and T6SS mutants showed significant attenuated virulence in the zebrafish infection model when compared with the wild-type. Finally, considering the cooperative role of T3SS and T6SS, we generated a mutant strain WEDΔT6SS based on the existing live attenuated vaccine (LAV) WED which showed improved vaccine safety and comparable immune protection. Therefore, WEDΔT6SS could be used as an optimized LAV in the future. Taken together, this work suggested a bilateral role of T3SS and T6SS which respectively act as spear and shield during E. piscicida infection, together contribute to E. piscicida virulence.

Edwardsiella induces microtubule-severing in host epithelial cells

Edwardsiella bacteria cause economic losses to a variety of commercially important fish globally. Human infections are rare and result in a gastroenteritis-like illness. Because these bacteria are evolutionarily related to other Enterobacteriaceae and the host cytoskeleton is a common target of enterics, we hypothesized that Edwardsiella may cause similar phenotypes. Here we use HeLa and Caco-2 infection models to show that microtubules are severed during the late infections. This microtubule alteration phenotype was not dependant on the type III or type VI secretion system (T3SS and T6SS) of the bacteria as ΔT3SS and ΔT6SS mutants of E. piscicida EIB202 and E. tarda ATCC15947 that lacks both also caused microtubule disassembly. Immunolocalization experiments showed the host katanin catalytic subunits A1 and A like 1 proteins at regions of microtubule severing, suggesting their involvement in the microtubule disassembly events. To identify bacterial components involved in this phenotype, we screened a 2,758 transposon library of E. piscicida EIB202 and found that 4 single mutations in the atpFHAGDC operon disrupted microtubule disassembly in HeLa cells. We then constructed three atp deletion mutants; they all could not disassemble host microtubules. This work provides the first clear evidence of host cytoskeletal alterations during Edwardsiella infections.

Intracellular Bacterial Infections: A Challenge for Developing Cellular Mediated Immunity Vaccines for Farmed Fish

Aquaculture is one of the most rapidly expanding farming systems in the world. Its rapid expansion has brought with it several pathogens infecting different fish species. As a result, there has been a corresponding expansion in vaccine development to cope with the increasing number of infectious diseases in aquaculture. The success of vaccine development for bacterial diseases in aquaculture is largely attributed to empirical vaccine designs based on inactivation of whole cell (WCI) bacteria vaccines. However, an upcoming challenge in vaccine design is the increase of intracellular bacterial pathogens that are not responsive to WCI vaccines. Intracellular bacterial vaccines evoke cellular mediated immune (CMI) responses that “kill” and eliminate infected cells, unlike WCI vaccines that induce humoral immune responses whose protective mechanism is neutralization of extracellular replicating pathogens by antibodies. In this synopsis, I provide an overview of the intracellular bacterial pathogens infecting different fish species in aquaculture, outlining their mechanisms of invasion, replication, and survival intracellularly based on existing data. I also bring into perspective the current state of CMI understanding in fish together with its potential application in vaccine development. Further, I highlight the immunological pitfalls that have derailed our ability to produce protective vaccines against intracellular pathogens for finfish. Overall, the synopsis put forth herein advocates for a shift in vaccine design to include CMI-based vaccines against intracellular pathogens currently adversely affecting the aquaculture industry.

Novel T3SS effector EseK in Edwardsiella piscicida is chaperoned by EscH and EscS to express virulence

Bacterium usually utilises type III secretion systems (T3SS) to deliver effectors directly into host cells with the aids of chaperones. Hence, it is very important to identify bacterial T3SS effectors and chaperones for better understanding of host-pathogen interactions. Edwardsiella piscicida is an invasive enteric bacterium, which infects a wide range of hosts from fish to human. Given E. piscicida encodes a functional T3SS to promote infection, very few T3SS effectors and chaperones have been identified in this bacterium so far. Here, we reported that EseK is a new T3SS effector protein translocated by E. piscicida. Bioinformatic analysis indicated that escH and escS encode two putative class I T3SS chaperones. Further investigation indicated that EscH and EscS can enhance the secretion and translocation of EseK. EscH directly binds EseK through undetermined binding domains, whereas EscS binds EseK via its N-terminal α-helix. We also found that EseK has an N-terminal chaperone-binding domain, which binds EscH and EscS to form a ternary complex. Zebrafish infection experiments showed that EseK and its chaperones EscH and EscS are necessary for bacterial colonisation in zebrafish. This work identified a new T3SS effector, EseK, and its two T3SS chaperones, EscH and EscS, in E. piscicida, which enriches our knowledge of bacterial T3SS effector-chaperone interaction and contributes to our understanding of bacterial pathogenesis.

Intracellular Trafficking Pathways of Edwardsiella tarda: From Clathrin- and Caveolin-Mediated Endocytosis to Endosome and Lysosome

Edwardsiella tarda is a Gram-negative bacterium that can infect a broad range of hosts including humans and fish. Accumulating evidences have indicated that E. tarda is able to survive and replicate in host phagocytes. However, the pathways involved in the intracellular infection of E. tarda are unclear. In this study, we examined the entry and endocytic trafficking of E. tarda in the mouse macrophage cell line RAW264.7. We found that E. tarda entered RAW264.7 and multiplied intracellularly in a robust manner. Cellular invasion of E. tarda was significantly impaired by inhibition of clathrin- and caveolin-mediated endocytic pathways and by inhibition of endosome acidification, but not by inhibition of macropinocytosis. Consistently, RAW264.7-infecting E. tarda was co-localized with clathrin, caveolin, and hallmarks of early and late endosomes, and intracellular E. tarda was found to exist in acid organelles. In addition, E. tarda in RAW264.7 was associated with actin and microtubule, and blocking of the functions of these cytoskeletons by inhibitors significantly decreased E. tarda infection. Furthermore, formaldehyde-killed E. tarda exhibited routes of cellular uptake and intracellular trafficking similar to that of live E. tarda. Together these results provide the first evidence that entry of live E. tarda into macrophages is probably a passive, virulence-independent process of phagocytosis effected by clathrin- and caveolin-mediated endocytosis and cytoskeletons, and that the intracellular traffic of E. tarda involves endosomes and endolysosomes.

Regulation of Type III Secretion of Translocon and Effector Proteins by the EsaB/EsaL/EsaM Complex in Edwardsiella tarda

The type III secretion system (T3SS) plays a crucial role in the pathogenesis of many Gram-negative bacteria, including Edwardsiella tarda, an important fish pathogen. Within the E. tarda T3SS, there are three proteins (EsaB/EsaL/EsaM) that are homologous to proteins present in many other bacteria, including SpiC/SsaL/SsaM in Salmonella, SepD/SepL/CesL in enteropathogenic Escherichia coli (EPEC) and enterohemorrhagic E. coli (EHEC), and YscB/YopN/SycN in Yersinia EsaL was found to interact with both EsaB and EsaM within the bacterial cell, as revealed by a coimmunoprecipitation assay. Moreover, EsaM is required for EsaB stability, and the two proteins interact with each other. EsaB, EsaL, and EsaM are all indispensable for the secretion of the T3SS translocon protein EseC into supernatants under pH 5.5 and pH 7.2 conditions. Unlike EseC, EseG is a T3SS effector whose secretion is suppressed by EsaL at pH 7.2 while it is promoted at pH 5.5 condition. Despite this finding, mutant strains lacking EsaB, EsaL, or EsaM (i.e., the ΔesaB, ΔesaL, or ΔesaM strain, respectively) were all outcompeted by wild-type E. tarda during a coinfection model. These results demonstrate that EsaB/EsaL/EsaM form a ternary complex controlling the secretion of T3SS translocon and effector proteins and contributing to E. tarda pathogenesis.

What the SIF Is Happening-The Role of Intracellular Salmonella-Induced Filaments

A common strategy among intracellular bacterial pathogens is to enter into a vacuolar environment upon host cell invasion. One such pathogen, Salmonella enterica, resides within the Salmonella-containing vacuole (SCV) inside epithelial cells and macrophages. Salmonella hijacks the host endosomal system to establish this unique intracellular replicative niche, forming a highly complex and dynamic network of Salmonella-induced filaments (SIFs). SIFs radiate outwards from the SCV upon onset of bacterial replication. SIF biogenesis is dependent on the activity of bacterial effector proteins secreted by the Salmonella-pathogenicity island-2 (SPI-2) encoded type III secretion system. While the presence of SIFs has been known for almost 25 years, their precise role during infection remains elusive. This review summarizes our current knowledge of SCV maturation and SIF biogenesis, and recent advances in our understanding of the role of SIFs inside cells.

In vitro model to estimate Edwardsiella tarda-macrophage interactions using RAW264.7 cells

Edwardsiella tarda has been recognized as an important facultative intracellular pathogen of fish with capability of survival and replication within macrophages. E. tarda-macrophage interactions play a very important role in the defense mechanism of fish against infection. The mechanisms that E. tarda use to infect and persist inside macrophages are not well characterized. To gain insight concerning this process, RAW264.7 cells was used to investigate the interactions between E. tarda and macrophages. Using an in vitro model involving RAW264.7 cells, internalization assay demonstrated that MOIs of 10:1 and 100:1 could result in a satisfactory infection rate after a 2 h infection period. Consistent with the performance in fish macrophages, E. tarda could survive, replicate and induce iNOS-mediated NO production in RAW264.7 cells. Light and electron microscopy confirmed the internalization and replication of E. tarda in RAW264.7 cells, showing once inside macrophages, numberous bacteria may be destroyed within phagolysosomes and those that successfully subvert phagocyte defenses are capable of extensively replicating within the vacuolar-like compartment in macrophages. In addition, E. tarda-induced apoptosis was observed in RAW264.7 cells in a dose-and time-dependent manner, characterized by increased Annexin V binding and the activation of caspase-3. The results described here indicate that RAW264.7 cells could model the behavior of fish macrophages in response to E. tarda in many ways and may serve as a cell model for study on interactions between E. tarda and macrophages. The successful establishment of E. tarda-invaded RAW264.7 cells model may contribute to providing a basis for more detailed understanding of E. tarda pathogenesis.

Molecular characterization reveals involvement of four caspases in the antibacterial immunity of tongue sole (Cynoglossus semilaevis)

Caspases are a family of proteases involved in many important biological processes including apoptosis and inflammation. In this study, we analyzed in a comparative manner the expression patterns and immune effects of four different types of caspases, i.e. caspase-1 (CsCas1), caspase-2 (CsCas2), caspase-3 (CsCas3), and caspase-9 (CsCas9), from the teleost fish tongue sole (Cynoglossus semilaevis). CsCas1, CsCas2, CsCas3, and CsCas9 share 35.4%-58.5%, 61.2%-75.3%, 52.3%-65.6%, and 63.0%-76.2% overall sequence identities, respectively, with their counterparts in teleost species. CsCas1, CsCas2, CsCas3, and CsCas9 possess the caspase domain and catalytic site conserved in known caspases. The expressions of the four caspases were detected in a wide range of tissues, however, the expression levels varied between different tissues and caspases. Following bacterial infection, the expressions of the four caspases were upregulated or downregulated to significant extents in a time- and tissue-dependent manner. In vivo analysis showed that overexpression of each of the caspases in tongue sole significantly enhanced the ability of the fish to resist bacterial dissemination in and colonization of tissues. These results indicate an involvement of fish caspases in bacteria-induced immune response and demonstrate for the first time that caspase-1, 2, 3, and 9 are essential to the optimal defense against bacterial infection in fish.

EseE of Edwardsiella tarda Augments Secretion of Translocon Protein EseC and Expression of the escC-eseE Operon

Edwardsiella tarda is an important Gram-negative pathogen that employs a type III secretion system (T3SS) to deliver effectors into host cells to facilitate bacterial survival and replication. These effectors are translocated into host cells through a translocon complex composed of three secreted proteins, namely, EseB, EseC, and EseD. The secretion of EseB and EseD requires a chaperone protein called EscC, whereas the secretion of EseC requires the chaperone EscA. In this study, we identified a novel protein (EseE) that also regulates the secretion of EseC. An eseE deletion mutant secreted much less EseC into supernatants, accompanied by increased EseC levels within bacterial cells. We also demonstrated that EseE interacted directly with EseC in a pulldown assay. Interestingly, EseC, EseE, and EscA were able to form a ternary complex, as revealed by pulldown and gel filtration assays. Of particular importance, the deletion of eseE resulted in decreased levels of EseB and EseD proteins in both the bacterial pellet and supernatant fraction. Furthermore, real-time PCR assays showed that EseE positively regulated the transcription of the translocon operon escC-eseE, comprising escC, eseB, escA, eseC, eseD, and eseE These effects of EseE on the translocon components/operon appeared to have a functional consequence, since the ΔeseE strain was outcompeted by wild-type E. tarda in a mixed infection in blue gourami fish. Collectively, our results demonstrate that EseE not only functions as a chaperone for EseC but also acts as a positive regulator controlling the expression of the translocon operon escC-eseE, thus contributing to the pathogenesis of E. tarda in fish.

Edwardsiella tarda-Induced Inhibition of Apoptosis: A Strategy for Intracellular Survival

Edwardsiella tarda is a Gram-negative bacterial pathogen that can infect a wide range of freshwater and marine fish. One salient feature of E. tarda is the ability to survive and replicate in various host cells. In this study, we observed that E. tarda replicated robustly in the zebrafish cell line ZF4, and that E. tarda-infected cells exhibited no detectable signs of apoptosis. Global transcriptome analysis and quantitative real-time RT-PCR revealed that E. tarda infection generally significantly downregulated pro-apoptotic genes and upregulated anti-apoptotic genes. To investigate the role of apoptosis in E. tarda infection, two upregulated anti-apoptotic genes (Fech and Prx3) and two downregulated pro-apoptotic genes (Brms1a and Ivns1a) were overexpressed in zebrafish. Subsequent infection study showed that Fech and Prx3 overexpression significantly promoted E. tarda dissemination in and colonization of fish tissues, while Brms1a and Ivns1a overexpression significantly reduced E. tarda dissemination and colonization. Consistently, when Fech and Prx3 were knocked down in zebrafish, E. tarda infection was significantly inhibited, whereas Brms1a and Ivns1a knockdown significantly enhanced E. tarda infection. These results indicate for the first time that E. tarda prevents apoptosis in teleost as a strategy for intracellular survival, and that some putative apoptotic genes of teleost function in the apoptosis pathway probably in a manner similar to that in mammalian systems.

ORF13 in the Type III secretion system gene cluster of Edwardsiella tarda binds to the mammalian factor Cugbp2

The Type III secretion system (TTSS) is essential for the intracellular replication of Edwardsiella tarda in phagocytes of fish and mammals, and a hypothetical gene (orf13) located in the TTSS gene cluster is required for intracellular replication and virulence of E. tarda. Here, we show that under TTSS-inducing conditions, the protein ORF13 was secreted into culture supernatant. Then, using a yeast 2-hybrid screen, we show that the mammalian factor Cugbp2, which regulates apoptosis in breast cancer cells, directly interacts with ORF13. A pull-down assay revealed that ORF13 binds to the C-terminal region of Cugbp2. Our results suggest that ORF13 may facilitate E. tarda replication in phagocytes by binding to Cugbp2.

Intramacrophage Infection Reinforces the Virulence of Edwardsiella tarda

Edwardsiella tarda is an important pathogenic bacterium that can replicate in macrophages. However, how the intramacrophage infection process affects the virulence of this bacterium is essentially unknown. Here, we show that E. tarda replicates and induces a caspase-1-dependent cell pyroptosis in a murine macrophage model. Via pyroptosis, intracellular E. tarda escapes to the extracellular milieu, forming a unique bacterial population. Being different from the bacteria cultured alone, this unique population possesses a reprogrammed transcriptional profile, particularly with upregulated type III secretion system (T3SS)/T6SS cluster genes. Subsequent studies revealed that the macrophage-released population gains enhanced infectivity for host epithelial cells and increases resistance to multiple host defenses and hence displays significantly promoted virulence in vivo Further studies indicated that T3SS is essentially required for the macrophage infection process, while T6SS contributes to infection-induced bacterial virulence. Altogether, this work demonstrates that E. tarda can utilize macrophages as a niche for virulence priming and for spreading infection, suggesting a positive role for intramacrophage infection in bacterial pathogenesis.

IMPORTANCE

Many pathogens can replicate in macrophages, which is crucial for their pathogenesis. To survive in the macrophage cell, pathogens are likely to require fitness genes to counteract multiple host-killing mechanisms. Here, Edwardsiella tarda is proved to exit from macrophages during infection. This macrophage-released population displays a reprogrammed transcriptional profile with significantly upregulated type III secretion system (T3SS)/T6SS-related genes. Furthermore, both enhanced infectivity in epithelial cells and activated resistance to complex host defenses were conferred on this macrophage-primed population, which consequently promoted the full virulence of E. tarda in vivo Our work provides evidence that E. tarda can utilize macrophages as a niche for virulence priming and for spreading infection, highlighting the importance of the intramacrophage infection cycle for the pathogenesis of E. tarda.

Generation and evaluation of virulence attenuated mutants of Edwardsiella tarda as vaccine candidates to combat edwardsiellosis in flounder (Paralichthys olivaceus)

Edwardsiella tarda is an intracellular pathogen that causes edwardsiellosis in fish. The development of a live attenuated vaccine may be an effective approach for preventing this disease in fish. In this study, we introduced deletions of esrB, esaC, evpH, rpoS, and purA into the E. tarda LSE40ΔaroA strain, thereby generating five double-gene mutants (ΔaroAΔesrB, ΔaroAΔesaC, ΔaroAΔrpoS, ΔaroAΔevpH, and ΔaroAΔpurA) and two triple-gene mutants (ΔaroAΔesrBΔevpH and ΔaroAΔesaCΔevpH). When blue gourami (Trichogaster trichopterus) was used as a fish model for the primary screening and evaluation of the vaccine candidates, all mutants were attenuated significantly by more than 2 to 3 logs in terms of the 50% lethal dose (LD(50)). Five double-gene mutants yielded relative percentage survival (RPS) rates of 26.1-82.6% after challenge with wild-type E. tarda. The ΔaroAΔesrB mutant that conferred the highest RPS (82.6%) in blue gourami was also evaluated in flounder (Paralichthys olivaceus). After vaccination via intramuscular (i.m.) injection or immersion, this mutant could persist in the flounder for 14-35 days and it induced higher serum antibody titers than the control fish (P < 0.01). Flounder vaccinated via i.m. injection at doses of 10(3)-10(7) CFU/fish had RPS rates of 14.3-66.7% after i.m. challenge with 10(4) CFU/fish using wild-type E. tarda. Flounder vaccinated via immersion at a dose of 10(7) CFU/ml exhibited 100% RPS against immersion challenge with 10(7) CFU/ml using wild-type E. tarda. These results indicate that the ΔaroAΔesrB mutant could be used as an effective live vaccine to combat edwardsiellosis in flounder.

Identification and functional characterization of the novel Edwardsiella tarda effector EseJ

Edwardsiella tarda is a Gram-negative enteric pathogen that causes hemorrhagic septicemia in fish and gastro- and extraintestinal infections in humans. The type III secretion system (T3SS) of E. tarda has been identified as a key virulence factor that contributes to pathogenesis in fish. However, little is known about the associated effectors translocated by this T3SS. In this study, by comparing the profile of secreted proteins of the wild-type PPD130/91 and its T3SS ATPase ΔesaN mutant, we identified a new effector by matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry. This effector consists of 1,359 amino acids, sharing high sequence similarity with Orf29/30 of E. tarda strain EIB202, and is renamed EseJ. The secretion and translocation of EseJ depend on the T3SS. A ΔeseJ mutant strain adheres to epithelioma papillosum of carp (EPC) cells 3 to 5 times more extensively than the wild-type strain does. EseJ inhibits bacterial adhesion to EPC cells from within bacterial cells. Importantly, the ΔeseJ mutant strain does not replicate efficiently in EPC cells and fails to replicate in J774A.1 macrophages. In infected J774A.1 macrophages, the ΔeseJ mutant elicits higher production of reactive oxygen species than wild-type E. tarda. The replication defect is consistent with the attenuation of the ΔeseJ mutant in the blue gourami fish model: the 50% lethal dose (LD50) of the ΔeseJ mutant is 2.34 times greater than that of the wild type, and the ΔeseJ mutant is less competitive than the wild type in mixed infection. Thus, EseJ represents a novel effector that contributes to virulence by reducing bacterial adhesion to EPC cells and facilitating intracellular bacterial replication.

Type III secretion system genes of Edwardsiella tarda associated with intracellular replication and virulence in zebrafish

The Type III secretion system (T3SS) is essential for intracellular replication of Edwardsiella tarda in phagocytes of fish and mammals. Two possible effector candidate genes (eseE and eseG) and 7 hypothetical genes (esaB, escC, orf13, orf19, orf26, orf29, and orf30) located in the T3SS gene cluster were inactivated by an allelic exchange method, and we found that E. tarda strains carrying insertion mutations in escC, orf13, orf19, orf29, and orf30 were unable to replicate within J774 macrophages and HEp-2 epithelial cells. Furthermore, the virulence of these mutants to zebrafish was severely attenuated as well. These data suggest that the escC, orf13, orf19, orf29, and orf30 genes are required for intracellular replication and virulence of E. tarda.

Edwardsiella tarda-Induced cytotoxicity depends on its type III secretion system and flagellin

Many Gram-negative bacteria utilize a type III secretion system (T3SS) to translocate virulence proteins into host cells to cause diseases. In responding to infection, macrophages detect some of the translocated proteins to activate caspase-1-mediated cell death, called pyroptosis, and secretion of proinflammatory cytokines to control the infection. Edwardsiella tarda is a Gram-negative enteric pathogen that causes hemorrhagic septicemia in fish and both gastrointestinal and extraintestinal infections in humans. In this study, we report that the T3SS of E. tarda facilitates its survival and replication in murine bone marrow-derived macrophages, and E. tarda infection triggers pyroptosis of infected macrophages from mice and fish and increased secretion of the cytokine interleukin 1β in a T3SS-dependent manner. Deletion of the flagellin gene fliC of E. tarda results in decreased cytotoxicity for infected macrophages and does not attenuate its virulence in a fish model of infection, whereas upregulated expression of FliC in the fliC mutant strain reduces its virulence. We propose that the host controls E. tarda infection partially by detecting FliC translocated by the T3SS, whereas the bacteria downregulate the expression of FliC to evade innate immunity.

Edwardsiella tarda mutant disrupted in type III secretion system and chorismic acid synthesis and cured of a plasmid as a live attenuated vaccine in turbot

Edwardsiella tarda is an intractable Gram-negative pathogen in many fish species to cause edwardsiellosis. Its infection leads to extensive losses in a diverse array of commercially important fish. The type III secretion system (T3SS) has been considered as one of the major virulence factors and plays important roles in its intracellular lifestyle. In this study, an E. tarda EIB202 mutant WED with deletions in the T3SS genes for EseB, EseC, EseD and EscA, along with the aroC gene for the biosynthesis of chorismic acid, as well as the curing of endogenous plasmid pEIB202 was constructed by allelic exchange strategy. Compared to the wild-type EIB202 which was highly virulent towards turbot (Scophthamus maximus) via intraperitoneal (i.p.), intramuscular (i.m.) injection or immersion and caused systemic infection in turbot as well as the unexpected red mouth symptom when immersion challenged, WED was highly attenuated when inoculated into turbot via i.m., i.p. and immersion routes, and exhibited significantly impaired capacity to survive in fish tissues. WED showed 5700-fold higher 50% lethal dose (LD50) than that of the wild type when i.m. or i.p. challenged. Inoculation with WED by i.p. or immersion injection routes elicited significant protection against the challenge of the wild-type E. tarda after 5 weeks of vaccination. The vaccinated fish produced low while significant level of specific antibody and showed increased expression of immune-related factors including IL-1β, IFN-γ, MHC II, MHC-I and CD8, indicating that WED possesses significant immunoprotective potential. Furthermore, our data indicated that a single dose of i.p. and immersion vaccination with WED could produce significant protection as long as 12 and 6 months, respectively. These results demonstrated the feasibility of WED as a live attenuated vaccine in turbot against edwardsiellosis by immersion or i.p. injection routes.

Gene expression profiling in live attenuated Edwardsiella tarda vaccine immunized and challenged zebrafish: insights into the basic mechanisms of protection seen in immunized fish

Despite the importance and success of vaccine immunization against bacterial diseases in fish, little is known about the molecular mechanisms of vaccine-induced immune protection in teleost fish. In this study, the live attenuated Edwardsiella tarda vaccine strain WED, which has been shown to evoke efficacious protection against edwardsiellosis and ascites diseases in fish, was extensively evaluated for multiple parameters in a 5-week immunization and challenge experiment in zebrafish. The parameters evaluated included the immunologic potency (relative percent survival, RPS), the specific IgM antibody titers and the expression profiles of multiple immune-related gene markers at multiple time points following immunization and challenge. During the 4-week immunization phase, the toll-like receptor (TLR) 5 signaling pathway, the MHC-I antigen processing pathway and cytotoxic T lymphocyte (CTL) responses were activated in succession. In contrast, the MHC-II antigen processing pathway and the markers of CD4(+) T lymphocyte activation were down-regulated, and IgM transcription and specific IgM antibody titers were not significantly induced following immunization. During the 1-week challenge phase, the induction of MHC-I and CTL responses and the inhibition of MHC-II and CD4(+) T cell responses were similarly observed in immunized zebrafish following challenge with wild E. tarda. With the 5-week immunization and challenge model, our data suggest the basic mechanism that underlying the long-lasting protective immunity elicited by WED in zebrafish. This mechanism involved the induction of the TLR-5 signaling pathway, the MHC-I antigen processing pathway and CTL effector function, and CTL function seems play a major role in the protection against E. tarda infection in zebrafish.

RNA-seq liver transcriptome analysis reveals an activated MHC-I pathway and an inhibited MHC-II pathway at the early stage of vaccine immunization in zebrafish

Background

Zebrafish (Danio rerio) is a prominent vertebrate model of human development and pathogenic disease and has recently been utilized to study teleost immune responses to infectious agents threatening the aquaculture industry. In this work, to clarify the host immune mechanisms underlying the protective effects of a putative vaccine and improve its immunogenicity in the future efforts, high-throughput RNA sequencing technology was used to investigate the immunization-related gene expression patterns of zebrafish immunized with Edwardsiella tarda live attenuated vaccine.

Results

Average reads of 18.13 million and 14.27 million were obtained from livers of zebrafish immunized with phosphate buffered saline (mock) and E. tarda vaccine (WED), respectively. The reads were annotated with the Ensembl zebrafish database before differential expressed genes sequencing (DESeq) comparative analysis, which identified 4565 significantly differentially expressed genes (2186 up-regulated and 2379 down-regulated in WED; p<0.05). Among those, functional classifications were found in the Gene Ontology database for 3891 and in the Kyoto Encyclopedia of Genes and Genomes database for 3467. Several pathways involved in acute phase response, complement activation, immune/defense response, and antigen processing and presentation were remarkably affected at the early stage of WED immunization. Further qPCR analysis confirmed that the genes encoding the factors involved in major histocompatibility complex (MHC)-I processing pathway were up-regulated, while those involved in MHC-II pathway were down-regulated.

Conclusion

These data provided insights into the molecular mechanisms underlying zebrafish immune response to WED immunization and might aid future studies to develop a highly immunogenic vaccine against gram-negative bacteria in teleosts.

The in vivo extracellular life of facultative intracellular bacterial parasites: role in pathogenesis

Classically labeled facultative intracellular pathogens are characterized by the ability to have an intracellular phase in the host, which is required for pathogenicity, while capable of extracellular growth in vitro. The ability of these bacteria to replicate in cell-free conditions is usually assessed by culture in artificial bacteriological media. However, the extracellular growth ability of these pathogens may also be expressed by a phase of extracellular infection in the natural setting of the host with pathologic consequences, an ability that adds to the pathogenic potential of the infectious agent. This infective capability to grow in the extracellular sites of the host represents an additional virulence attribute of those pathogens which may lead to severe outcomes. Here we discuss examples of infectious diseases where the in vivo infective extracellular life is well documented, including infections by Francisella tularensis, Yersinia pestis, Burkholderia pseudomallei, Burkholderia cenocepacia, Salmonella enterica serovar Typhimurium and Edwardsiella tarda. The occurrence of a phase of systemic dissemination with extracellular multiplication during progressive infections by facultative intracellular bacterial pathogens has been underappreciated, with most studies exclusively centered on the intracellular phase of the infections. The investigation of the occurrence of a dual lifestyle in the host among bacterial pathogens in general should be extended and likely will reveal more cases of infectious diseases with a dual infective intracellular/extracellular pattern.

Classical labeling of bacterial pathogens according to their lifestyle in the host: inconsistencies and alternatives

An ample understanding of the complex interactions between host and pathogen will improve our ability to develop new prophylactic and therapeutic measures against infection. Precise classification of infectious agents in regards to their infective lifestyles in the host and corresponding pathogenic implications are required because clear concepts are essential to plan fruitful research. Classically, pathogenic bacteria are classified as extracellular, facultative intracellular, and obligate intracellular. In my opinion, this classification is inadequate because, as concluded from data here discussed, it is based on inconsistencies and hyper-valorizes the capacity of the infectious agent replicate in vitro in cell-free media. For a microbial pathogen, what matters is whether intra- or extracellularity is in the context of the in vivo life and in association with pathogenicity. When living as a pathogen in association with its host, what is relevant in microbiological terms is not the ability to grow in artificial cell-free bacteriological media or in environmental niches but whether the intracellular infectious agent, besides the phase of intracellular growth which is behind its label, also is able to live extracellularly in the natural settings of the extracellular territories of their hosts. To eliminate the inconsistencies associated with the classical labeling of bacterial pathogens, I propose that bacterial pathogens be labeled exclusive extracellular, dual intracellular/extracellular and exclusive intracellular based on their infective lifestyle in the host, not in the ability to grow in artificial bacteriological media.

Anthrax lethal toxin-induced gene expression changes in mouse lung

A major virulence factor of Bacillus anthracis is the anthrax Lethal Toxin (LeTx), a bipartite toxin composed of Protective Antigen and Lethal Factor. Systemic administration of LeTx to laboratory animals leads to death associated with vascular leakage and pulmonary edema. In this study, we investigated whether systemic exposure of mice to LeTx would induce gene expression changes associated with vascular/capillary leakage in lung tissue. We observed enhanced susceptibility of A/J mice to death by systemic LeTx administration compared to the C57BL/6 strain. LeTx-induced groups of both up- and down-regulated genes were observed in mouse lungs 6 h after systemic administration of wild type toxin compared to lungs of mice exposed to an inactive mutant form of the toxin. Lungs of the less susceptible C57BL/6 strain showed 80% fewer differentially expressed genes compared to lungs of the more sensitive A/J strain. Expression of genes known to regulate vascular permeability was modulated by LeTx in the lungs of the more susceptible A/J strain. Unexpectedly, the largest set of genes with altered expression was immune specific, characterized by the up-regulation of lymphoid genes and the down-regulation of myeloid genes. Transcripts encoding neutrophil chemoattractants, modulators of tumor regulation and angiogenesis were also differentially expressed in both mouse strains. These studies provide new directions for the investigation of vascular leakage and pulmonary edema induced by anthrax LeTx.

Regulation of the Edwardsiella ictaluri type III secretion system by pH and phosphate concentration through EsrA, EsrB, and EsrC

A recently described Edwardsiella ictaluri type III secretion system (T3SS) with functional similarity to the Salmonella pathogenicity island 2 T3SS is required for replication in channel catfish head-kidney-derived macrophages (HKDM) and virulence in channel catfish. Quantitative PCR and Western blotting identified low pH and phosphate limitation as conducive to expression of the E. ictaluri T3SS, growth conditions that mimic the phagosomal environment. Mutagenesis studies demonstrated that expression is under the control of the EsrAB two-component regulatory system. EsrB also induces upregulation of the AraC-type regulatory protein EsrC, which enhances expression of the EscB/EseG chaperone/effector operon in concert with EsrB and induces expression of the pEI1-encoded effector, EseH. EsrC also induces expression of a putative type VI secretion system translocon protein, EvpC, which is secreted under the same low-pH conditions as the T3SS translocon proteins. The pEI2-encoded effector, EseI, was upregulated under low-pH and low-phosphate conditions but not in an EsrB- or EsrC-dependent manner. Mutations of EsrA and EsrB both resulted in loss of the ability to replicate in HKDM and full attenuation in the channel catfish host. Mutation of EsrC did not affect intracellular replication but did result in attenuation in catfish. Although EsrB is the primary transcriptional regulator for E. ictaluri genes within the T3SS pathogenicity island, EsrC regulates expression of the plasmid-carried effector eseH and appears to mediate coordinated expression of the T6SS with the T3SS.