Improving safety of a live attenuated Edwardsiella ictaluri vaccine against enteric septicemia of catfish and evaluation of efficacy

Transcriptome analysis reveals deep insights into the early immune response of turbot (Scophthalmus maximus) induced by inactivated Aeromonas salmonicida vaccine

Non-coding RNAs are a class of RNAs, including circRNA and miRNA, that cannot be translated into proteins, but play an important role in the regulation of the expression of protein-coding genes. More and more evidences showed that circRNA can regulate the expression of miRNA target genes by adsorbing miRNA and form the circRNA-miRNA-mRNA regulatory network. The inactivated Aeromonas salmonicida vaccine is a commercial vaccine for many teleost. Understanding the role of circRNA and miRNA in the early stage of vaccine injection will provide a new insight for the study of the early immune response process in teleost. In this study, the expression profiles of circRNA, miRNA and mRNA were analyzed by high-throughput sequencing at 6 h, 12 h, 24 h and 96 h after injection of inactivated Aeromonas salmonicida vaccine and normal turbot spleen. Compared with the control group, 111, 141 and 453 differentially expressed circRNAs, miRNAs and mRNAs were identified in the four vaccination groups, respectively. The targeting relationships of differentially expressed miRNA to circRNA and mRNA were predicted by using miRanda software, and the results showed that a variety of differentially expressed immune-related genes were targeted. A total of 53 differentially expressed circRNA-miRNA-mRNA regulatory networks were constructed according to circRNA-miRNA pairs and miRNA-mRNA pairs. Among them, cell adhesion molecule 3 and immunoglobulin superfamily member 21 were regulated by the same miRNA (novel_880) and circRNA (novel_circ_0000311/novel_circ_0005326). These suggest that these circRNA-miRNA-mRNA regulatory networks may be a multi-molecule regulatory network, and its regulatory mechanism needs to be further studied.

Edwardsiella ictaluri in a Colony of Zebrafish (Danio rerio) Used in a Teaching Laboratory

A small colony of zebrafish (Danio rerio) experienced 30% acute mortality within a few days after receipt from a commercial source. A few fish presented with small areas of raised scales or tissue necrosis, primarily near the caudal peduncle. Edwardsiella ictaluri (E. ictaluri) was identified by real-time PCR of pooled zebrafish and swabs of the pre-filter and fine filter pads, with subsequent sequence analysis. E. ictaluri is most commonly associated with an enteric septicemia in catfish species and can have significant economic impact on commercial catfish fisheries. However, several references report naturally occurring E. ictaluri infection of nonictalurid fishes, including zebrafish. Ours is the first report demonstrating the use of environmental sampling to identify E. ictaluri in a zebrafish colony by real-time PCR. Moreover, our report indicates that E. ictaluri is a relevant disease for institutions using zebrafish as research species and emphasizes the importance of carefully considering importation and quarantine practices.

Adaptive immune responses in channel catfish exposed to Edwardsiella ictaluri live attenuated vaccine and wild type strains through the specific gene expression profiles

We extend the previous findings on the differential activity of immune-related genes in the lymphoid organs of channel catfish in the 7 days post-challenge (dpc) with E. ictaluri live attenuated vaccines (LAVs) and wild type (WT) strains by assessing the expression of these genes in the 21 dpc. The expression of T and B cell-specific genes were significantly elevated in the spleen at 14 dpc and in the AK at 21 dpc in catfish treated with E. ictaluri WT and LAV strains compared to a non-treated control group. The gene expression of IFN-γ correlated with adaptive immunity genes in the lymphoid tissues of catfish. These data indicate that two novel LAVs were able to trigger the activation of T helper1 polarization cytokine IFN-γ gene and specific lymphocyte genes in the spleen followed by their activation in the AK of catfish without causing inflammation, thus providing protective immunity in E. ictaluri infection.

Live attenuated Edwardsiella ictaluri vaccines enhance the protective innate immune responses of channel catfish B cells

Edwardsiella ictaluri causes enteric septicemia of catfish. Our group developed two E. ictaluri live attenuated vaccines (LAVs). However, their effects on the innate functions of catfish B cells are still unexplored. We evaluated phagocytosis and killing of wild-type (WT) E. ictaluri opsonized with sera from vaccinated fish and the survival of B cells exposed to E. ictaluri strains. We assessed phagocytosis of the opsonized WT at 30 °C and 4 °C. B cells killed the internalized E. ictaluri opsonized with sera from vaccinated fish with LAVs more efficiently than other groups at 30 °C. However, catfish B cells were unable to destroy E. ictaluri at 4 °C. Furthermore, E. ictaluri opsonized with serum from fish exposed to WT induce apoptosis and decreased live B cells numbers. Results indicate that opsonization of E. ictaluri with sera from vaccinated fish enhanced phagocytosis and killing activity in B cells and inhibited apoptotic changes in the infected B cells.

Development of fish vaccine in Southeast Asia: A challenge for the sustainability of SE Asia aquaculture

Southeast (SE) Asia plays an important role in global food security as this region has been regarded as one of the major producers of aquaculture product and, to date, freshwater fish accounted for one-third of the total aquaculture in SE Asia. The intensification of freshwater farming corresponding to increase of consumer demands has inevitably led to the emergence and re-emergence of diseases causing tremendous economic loss in the region. Nile tilapia (Oreochromis niloticus) and striped catfish (Pangasianodon hypophthalmus), the major freshwater fish species of SE Asia, have been reported susceptible to several bacterial pathogens, e.g. Streptococcus agalactiae, Edwardsiella ictalurid and Flavobacterium columnare. Since only a limited number of vaccines being registered and marketed, these pathogenic organisms still represent a severe threat to aquaculture industry in SE Asia. However, there is profound advancement in the understanding of disease epidemiology, pathogenic mechanisms, teleost mucosal immunity and vaccine delivery system over the last few years. This review aimed to summarize those recent findings which hopefully can provide novel insight into the future development of suitable vaccine and vaccination regime against bacterial infection in SE Asia region.

Effects of Live Attenuated Vaccine and Wild Type Strains of Edwardsiella ictaluri on Phagocytosis, Bacterial Killing, and Survival of Catfish B Cells

Edwardsiella ictaluri, a Gram-negative facultative intracellular pathogen, is the causative agent of enteric septicemia of catfish (ESC). The innate functions of B cells have been demonstrated in several teleost fish, including zebrafish, rainbow trout, and channel catfish. Recently, our group has developed several protective E. ictaluri live attenuated vaccines (LAVs). However, the innate role of catfish B cells to phagocytose and destroy E. ictaluri wild-type (WT) and live attenuated vaccine (LAV) strains has not been evaluated. In this study, we assessed the efficacy of E. ictaluri WT and two LAVs on phagocytosis, microbial killing, and survival of catfish anterior kidney (AK) B cells. Initially, we documented active uptake of E. ictaluri WT and two LAVs in B cells by flow cytometry and light microscopy. Then, we observed the E. ictaluri strains-induced phagosome and/or phagolysosome formation in the cytoplasm of catfish magnetically sorted IgM+ B cells. Furthermore, we demonstrated that AK B cells were able to destroy the internalized E. ictaluri WT and LAV strains efficiently. Finally, we documented early and late apoptotic/necrotic manifestations induced by E. ictaluri in catfish AK B cells. In conclusion, our results suggest that both LAVs and WT strain initiate similar innate immune responses such as active phagocytic uptake, induced bactericidal activity as well as promote early and late apoptotic changes in catfish B cells. Our data suggest that phagocytic and microbicidal B cells may serve as professional APCs in initiation of protective adaptive immune responses against ESC in channel catfish.

Assessment of the Live Attenuated and Wild-Type Edwardsiella ictaluri-Induced Immune Gene Expression and Langerhans-Like Cell Profiles in the Immune-Related Organs of Catfish

Edwardsiella ictaluri is a Gram-negative intracellular pathogen that causes enteric septicemia of catfish (ESC). Successful vaccination against intracellular pathogens requires T cell priming by antigen presenting cells (APCs) that bridge innate and adaptive immunity. However, the evidence on immunological mechanisms that underscore E. ictaluri pathogenesis and the protective role of live attenuated vaccines (LAVs) is scarce. We assessed the expression of immune genes related to antigen presentation by real-time PCR and the distribution patterns of Langerhans-like (L/CD207⁺) cells by immunohistochemistry in the immune-related tissues of channel catfish challenged with two novel E. ictaluri LAVs, EiΔevpB, and ESC-NDKL1 and wild type (WT) strain. Our results indicated significantly elevated expression of IFN-γ gene in the anterior kidney (AK) and spleen of vaccinated catfish at the early stages of exposure, which correlated with increased numbers of L/CD207⁺ cells. In general, the ESC-NDKL1-induced IFN-γ gene expression patterns in the AK resembled that of the patterns induced by EiΔevpB. However the MHCII gene expression patterns differed between the strains with significant increases at 6 h post-challenge (pc) with the EiΔevpB and at 7 d pc with the ESC-NDKL1 strains, respectively. Significant increases in activity of T helper type polarization genes such as IFN-γ and T cell co-receptors after exposure to ESC-NDKL1, in combination with elevated numbers of L/CD207⁺ cells at 7 d pc with both LAVs compared to uninfected and the WT-exposed counterparts, were documented in the spleen. The dominant pro-inflammatory environment with dramatically overexpressed inflammatory genes in the AK and 7 d pc in the spleen in response to E. ictaluri was found in exposed catfish. In general, the pro-inflammatory gene expression profiles in the ESC-NDKL1 pc showed more similarities to the WT strain-induced gene profiles compared to the EiΔevpB counterpart. In addition, E. ictaluri WT significantly decreased the numbers of Langerhans-like L/CD207⁺ cells in the AK and spleen at 3 and 7 days pc. In conclusion, we report the differential framework of initiation of innate and adaptive immune responses between E. ictaluri strains with both LAVs having a potential of satisfying the stringent requirements for successful vaccines.

A Review of Molecular Responses of Catfish to Bacterial Diseases and Abiotic Stresses

Catfish is one of the major aquaculture species in the United States. However, the catfish industry is threatened by several bacterial diseases such as enteric septicemia of catfish (ESC), columnaris disease and Aeromonas disease, as well as by abiotic stresses such as high temperature and low oxygen. Research has been conducted for several decades to understand the host responses to these diseases and abiotic stresses. With the development of sequencing technologies, and the application of genome-wide association studies in aquaculture species, significant progress has been made. This review article summarizes recent progress in understanding the molecular responses of catfish after bacterial infection and stress challenges, and in understanding of genomic and genetic basis for disease resistance and stress tolerance.

Phagocytic and Bactericidal Properties of Channel Catfish Peritoneal Macrophages Exposed to Edwardsiella ictaluri Live Attenuated Vaccine and Wild-Type Strains

Edwardsiella ictaluri (E. ictaluri), a Gram-negative, intracellular, facultative bacterium, is the causative agent of enteric septicemia of catfish (ESC), which is one of the most significant diseases of farmed channel catfish. Macrophages have a critical role in major defense mechanisms against bacterial infections by migrating to the site of infection, engulfing and killing pathogens, and priming adaptive immune responses. Vaccination of catfish with E. ictaluri live attenuated vaccine (LAV) strains increased the efficiency of phagocytosis and bacterial killing in catfish peritoneal macrophages compared in vitro with macrophages from non-vaccinated fish. Recently, our group developed several protective LAV strains from E. ictaluri. However, their effects on the antigen uptake and bacterial killing in catfish macrophages have not been evaluated. In this study, we assessed the phagocytic and bactericidal activity of peritoneal macrophages in the uptake of E. ictaluri wild-type (WT) and two LAV strains. We found that phagocytosis of LAV strains was significantly higher compared to their WT counterpart in peritoneal macrophages. Moreover, the uptake of E. ictaluri opsonized with sera from vaccinated catfish was more efficient than when opsonized with sera from sham-vaccinated fish. Notably, catfish macrophages did not lose their phagocytic properties at 4°C, as described previously in mammalian and zebrafish models. Also, opsonization of E. ictaluri with inactivated sera from vaccinated and sham-vaccinated catfish decreased significantly phagocytic uptake of bacteria at 32°C, and virtually suppressed endocytosis at 4°C, suggesting the important role of complement-dependent mechanisms in catfish macrophage phagocytosis. In conclusion, our data on enhanced phagocytic capacity and effective killing ability in macrophages of vaccine strains suggested the LAVs’ advantage if processed and presented in the form of peptides to specific lymphocytes of an adaptive immune system and emphasize the importance of macrophage-mediated immunity against ESC. Furthermore, we showed the role of complement-dependent mechanisms in the phagocytic uptakes of E. ictaluri in catfish peritoneal macrophages at 4 and 32°C. Finally, LAV vaccine-induced bacterial phagocytosis and killing properties of peritoneal macrophages emphasized the importance of the innate immune responses in ESC.