Establishment of a brain cell line obtained from hybrids of Channa argus ×Channa maculata for the detection of tilapia lake virus

Development of a New Marine Fish Continuous Cell Line Derived from Brain of Red Sea Bream (Pagrosomus major) and Its Application to Fish Virology and Heavy Metal Toxicology

Red sea bream (Pagrosomus major) is one of the most popular farmed marine teleost fish species. Fish cell lines are becoming important research tool in the aquaculture field, and they are suitable models to study fish virology, immunology and toxicology. To obtain a Pagrosomus major cell line for biological studies, a continuous cell line from brain of red sea bream (designated as RSBB cell line) was established and has been successfully subcultured over 100 passages. The RSBB cell line predominantly consisted of fibroblast-like cells and multiplied well in M199 medium supplemented with 10% fetal bovine serum at 28 °C. Karyotyping analysis indicated that the modal chromosome numbers of RSBB cells was 48. After transfection with pEGFP-N1, RSBB cells showed bright green fluorescence with a transfection efficiency approaching 8%. For toxicology study, it was demonstrated that metal Cd could induce cytotoxic effects of RSBB cells, accompanied with a dose-dependent MTT conversion capacity. Morphologically, cells treated with metal Cd produced rounding, shrinking and detaching and induced both cell apoptosis and necrosis. For virology study, the RSBB cells were highly susceptible to Nervous necrosis virus (NNV) and Singapore grouper iridovirus (SGIV) with steady titers (i.e., 108.0~8.3 TCID50 mL-1 and 107.0~7.2 TCID50 mL-1 respectively). Furthermore, an obvious cytopathic effect (CPE) could be observed in RSBB cells infected with Infectious spleen and kidney necrosis virus (ISKNV) and Siniperca chuatsi rhabdoviruses (SCRV). Meanwhile, all the infections were confirmed by polymerase chain reaction. The new brain cell line developed and characterized from red sea bream in this study could be used as an in vitro model for fish studies in the fields of toxicology and virology.

Development and characterization of a spleen cell line from yellowfin seabream Acanthopagrus latus and its susceptibility to Mandarinfish ranavirus

Yellowfin seabream (Acanthopagrus latus) is one of the most commercially important marine fish in China. In this study, a new continuous cell line, named ALS cells, was developed from the spleen tissue of A. latus. The cell line was maintained in Dulbecco's modified Eagle medium/Nutrient Mixture F-12 Ham (DMEM/F-12) supplemented with 10% fetal bovine serum (FBS) and successfully cultured up to 50 passages. The cell line was authenticated by amplifying and sequencing mitochondrial cytochrome C oxidase subunit-I (coi-I) gene. The ALS cell line had the maximum growth rate in DMEM/F-12 medium containing 20% FBS at 27°C. Chromosome number analysis showed that the ALS cells have a modal diploid chromosome number of 34. The ALS cell line was transfected with the pEGFP-N1 plasmid, and green fluorescence was observed. The ALS cell line was used for testing Mandarinfish ranavirus (MRV) susceptibility, and the cytopathic effects in the cell line were observed at 4 days post-infection (dpi). Furthermore, the susceptibility of the ALS cell line to MRV and the levels of MRV mRNA and viral loads were found to be significantly increased at 1-7 dpi. This study revealed that the ALS cell line could be useful for molecular, virological, and biotechnological studies on yellowfin seabream.

Characterization of three novel cell lines derived from the brain of spotted sea bass: Focusing on cell markers and susceptibility toward iridoviruses

Despite tens of cell lines originating from fish brain tissue have been constructed, little is known about the definite cell types they belong to. Whether fish cell lines derived from the brain shares similar characteristics is not well-answered yet. Here, we constructed three cell lines designated as LMB-S, LMB-M, LMB-L using brain tissue of spotted sea bass (Lateolabrax maculatus). Among them, LMB-L was identified as astroglia-like cells considering the high expression of GFAP, DCX, PTX, S100b, which are regarded as astrocyte-specific or astrocyte-associated cell markers. LMB-M exhibited smooth muscle-like features showing strong expression of LMOD1, SLAMP, M-cadherin, MGP, which are confirmed as muscle-restricted or myogenesis-involved cell markers. Although LMB-S was not definitely identified, it appeared an activation of WNT/β-catenin pathway. Besides the distinct expression profiles of cell markers, the three cell lines also presented differences in transfection efficiency and susceptibility to iridovirus infection. Relying on the established cell lines, a novel megalocytivirus, named LMIV (Lateolabrax maculatus iridovirus), was first isolated from diseased spotted sea bass. Genetic analysis of major capsid protein (MCP) and adenosine triphosphatase (ATPase) manifested that LMIV was clearly distinguishable from other representative teleost iridoviruses. Further investigations revealed that LMIV could replicate most efficiently in LMB-L cells obtaining the highest viral load (2.16 × 10¹⁰ copy/mL). By contrast, LMB-S cells gave rise to the highest viral load up to 3.86 × 10⁸ copy/mL, when the three cell lines were infected with MRV, a newly emerged ranavirus. Moreover, LMIV infection caused lots of cells to be detached from monolayers, generating adherent and non-adherent cells. An opposite expression profiling of type I IFN pathway-related genes (JAK1, STAT1, STAT2, IRF9, Mx1) was found between adherent and non-adherent cells. Combined with the analysis of MCP gene expression, it is speculated that inhibiting type I IFN pathway in non-adherent cells allowed the facilitation of virus duplication. Taken together, the present study broadens our understanding about the diversity of cell lines derived from fish brain tissue and screening cells more susceptible to virus is not only meaningful for the development of vaccine, but also provide clues for further clarification of cell-iridovirus interactions.

Characterization of the pathogenesis and immune response to a highly virulent Edwardsiella tarda strain responsible for mass mortality in the hybrid snakehead (Channa maculate ♀ × Channa argus ♂)

Edwardsiella tarda is reported as the causative agent of the systemic disease Edwardsiellosis in fish, which lead to huge economic losses in aquaculture. The pathogenicity and immune response to a highly virulent E. tarda isolate responsible for mass mortality in hybrid snakehead were performed. After species identification, morphology and virulence gene detection of Edwardsiella isolated from hybrid snakehead, the pathogenicity of the strain and histopathological changes in infected fish were analyzed. The infected fish exhibited typical acute hemorrhagic symptoms and enlarged internal organs. Histopathology revealed that the liver, spleen, kidney and intestinal tissues of diseased fish exhibited marked inflammatory with vacuolar degeneration and cell necrosis. Subsequently, humoral immune factors such as superoxide dismutase, lysozyme and acid phosphatase activities were detected as serum indicators, and real-time quantitative PCR was used to investigate immune-related genes (STAT1, HSP70, IgM, IL-6, IL-8, TRAF2, CD40, HLA-DMA and LCK) expression patterns in liver, spleen and head kidney. The results showed that these enzyme activity indicators and immune-related gene expression were significantly activated compared with healthy fish. These data provide insight into the pathogenic mechanisms and host immune responses of E. tarda, which could be useful for the future prevention and treatment of Edwardsiellosis in fish.

Immunogenicity and efficacy of two DNA vaccines encoding antigenic PspA and TerD against Nocardia seriolae in hybrid snakehead

Fish nocardiosis is a widespread chronic granulomatous disease in aquatic environment, which was particularly caused by Nocardia seriolae. The phage shock protein A (PspA) and tellurium resistance protein D (TerD) were identified to be the immunodominant antigens of the wild-type N. seriolae strain ZJ0503 in our previous study. In an attempt to develop effective DNA vaccines against this pathogen, PspA and TerD were used as candidates to ligate with pcDNA3.1-Flag plasmids, respectively. In addition, the abilities of these two DNA vaccines to elicit various immune responses in hybrid snakehead and supply protective efficacy against artificial challenge with N. seriolae were determined in the present study. The results showed that intramuscular injection with pcDNA-PspA and pcDNA-TerD did not exhibit cytotoxic activities in hybrid snakehead via histopathological examination. Besides, hybrid snakehead immunization with pcDNA-PspA and pcDNA-TerD could increase several non-specific immune paraments in serum, including LYZ, POD, ACP, AKP and SOD activities. Meanwhile, the pcDNA-TerD DNA vaccine could induce strongly specific antibody (IgM) titer in hybrid snakehead with a relative percent of survival (RPS) value of 83.14% against N. seriolae, while that of pcDNA-PspA DNA vaccine was displayed comparably low IgM titer with RPS value of 57.83%. Furthermore, quantitative real-time PCR assays presented that the expression of immune-related genes (MHCIα, MHCIIα, CD4, CD8α, IL-1β and TNFα) were up-regulated to various degrees after vaccination with pcDNA-PspA or pcDNA-TerD, indicating that these two DNA vaccines were able to boost humoral and cell-mediated immune responses in hybrid snakehead. Taken together, both the pcDNA-PspA and pcDNA-TerD DNA vaccines were proved to be safe, immunogenic and effective in protecting hybrid snakehead against N. seriolae infection, which can promote the development and application of DNA vaccines to control fish nocardiosis in aquaculture.

Tilapia lake virus: The story so far

Tilapia lake virus (TiLV) is a highly contagious pathogen that has detrimental effects on tilapia farming. This virus was discovered in 2014 and has received tremendous global attention from the aquaculture sector due to its association with high fish mortalities and its strong economic impact on the tilapia aquaculture industry. Currently, TiLV has been reported in 16 countries, and this number is continuing to rise due to improved diagnostic assays and surveillance activities around the world. In this review, we summarize the up-to-date knowledge of TiLV with regard to TiLV host species, the clinical signs of a TiLV infection, the affected tissues, pathogenesis and potential disease risk factors. We also describe the reported information concerning the virus itself: its morphology, genetic make-up and transmission pathways. We review the current methods for virus detection and potential control measures. We close the review of the TiLV story so far, by offering a commentary on the major TiLV research gaps, why these are delaying future TiLV research and why the TiLV field needs to come together and proceed as a more collaborative scientific community if there is any hope limiting the impact of this serious virus.