Caspase-dependent induction of apoptosis in barramundi, Lates calcarifer (Bloch), muscle cells by grouper iridovirus

Aquatic animal viruses mediated immune evasion in their host

Viruses are important and lethal pathogens that hamper aquatic animals. The result of the battle between host and virus would determine the occurrence of diseases. The host will fight against virus infection with various responses such as innate immunity, adaptive immunity, apoptosis, and so on. On the other hand, the virus also develops numerous strategies such as immune evasion to antagonize host antiviral responses. Here, We review the research advances on virus mediated immune evasions to host responses containing interferon response, NF-κB signaling, apoptosis, and adaptive response, which are executed by viral genes, proteins, and miRNAs from different aquatic animal viruses including Alloherpesviridae, Iridoviridae, Nimaviridae, Birnaviridae, Reoviridae, and Rhabdoviridae. Thus, it will facilitate the understanding of aquatic animal virus mediated immune evasion and potentially benefit the development of novel antiviral applications.

Delineating the roles of cellular and innate antiviral immune parameters mediating ranavirus susceptibility using rainbow trout cell lines

Frog virus 3 is the type species of the Ranavirus genus and the causative agent of massive mortalities of aquatic species worldwide. A critical step in limiting virus replication, particularly early in infection, is the innate immune response. Presently, little is known regarding what innate immune strategies limit FV3 at the cellular level. To this end, the present study uses two rainbow trout cell lines, RTG-2 and RTgutGC, which demonstrate susceptible and relatively resistant phenotypes to FV3 infection, to elucidate susceptibility factors to FV3. RTG-2 demonstrated a lower LD₅₀ and significantly higher virus transcript production compared to RTgutGC. The mode of cell death appeared to be apoptosis for both cell lines; however, RTG-2 did not demonstrate fragmented nuclei typical of apoptosis in cell culture. Next, the source of RTG-2's enhanced susceptibility was pursued, in hopes of highlighting unique features of this virus-host interaction that would predispose a cell to susceptibility. The type I interferon (IFN) response is the keystone mechanism used by the innate immune system to limit virus replication. FV3 induced very low to no levels of IFNs and interferon stimulated genes (ISGs) in either cell line, nor did inducing IFNs prior to infection inhibit virus-induced cell death. A dsRNA-induced antiviral state did reduce virus replication however. UV-inactivated FV3 was also able to kill RTG-2; thus, susceptibility to FV3-induced cell death observed in RTG-2 was independent of virus replication or the cell's ability, or lack thereof, to produce an IFN response. Importantly, RTG-2 showed greater viral entry compared to RTgutGC, suggesting non-innate immune factors, such as surface receptor expression or endocytic mechanism rates, may be key contributors to FV3 susceptibility. These findings contribute to our understanding of cell-level susceptibility to this environmentally important aquatic animal pathogen.

Production and characterization of a monoclonal antibody against a late gene encoded by grouper iridovirus 64L

Viral envelope proteins play important roles in viral infection and assembly. The grouper iridovirus ORF 64L (GIV-64L) was predicted to encode an envelope protein and was conserved in all sequenced Ranaviruses. In this study, the complete nucleotide sequence of the GIV-64L gene (1215 bp) was cloned into the isopropyl β-D-1-thiogalactopyranoside (IPTG) induction prokaryotic expression vector pET23a. The approximately 50.2 kDa recombinant GIV-64L-His protein was induced, purified and used as an immunogen to immunize BALB/c mice. Three monoclonal antibodies (mAbs), all IgG1 class antibodies against GIV-64L protein, were produced by enzyme-linked immunosorbent assay. Reverse transcription polymerase chain reaction analyses revealed GIV-64L to be a late gene when expressed in grouper kidney cells during GIV infection with cycloheximide (an inhibitor of protein synthesis) or cytosine arabinoside (an inhibitor of DNA synthesis) present. Finally, one of the established mAbs, GIV-64L-mAb-17, was used in Western blotting and an immunofluorescence assay, which showed that GIV-64L protein was expressed at 24 h post-infection and localized only in the cytoplasm in GIV-infected cells, packed into a whole virus particle. The presently characterized GIV-64L mAbs should have widespread applications in GIV immunodiagnostics and other research, and these results should offer important insights into the pathogenesis of GIV.

Identification and characterization of a late gene encoded by grouper iridovirus 2L (GIV-2L)

Grouper iridovirus (GIV) belongs to the Ranavirus genus and is one of the most important viral pathogens in grouper, particularly at the fry and fingerling stages. In this study, we identified and characterized the GIV-2L gene, which encodes a protein of unknown function. GIV-2L is 1242 bp in length, with a predicted protein mass of 46.2 kDa. It displayed significant identity only with members of the Ranavirus and Iridovirus genera. We produced mouse monoclonal antibodies against the GIV-2L protein by immunizing mice with GIV-2L-His-tag recombinant protein. By inhibiting de novo protein and DNA synthesis in GIV-infected cells, we showed that GIV-2L was a late gene during the viral replication. Finally, immunofluorescence microscopy revealed that GIV-2L protein accumulated in both the nucleus and cytoplasm of infected cells. These results offer important insights into the pathogenesis of GIV.

Identification and characterization of a late gene of grouper iridovirus 61l and antibody production against the protein encoded by it

In this study, a late gene encoded by grouper iridovirus, giv-61L, was identified and classified, and mouse monoclonal antibodies (mAbs) were raised against this protein. Giv-61L homologues were found only in the genus Ranavirus. Three mAbs to Giv-61L protein were produced. In drug inhibition assays, giv-61L was identified as a late gene. Finally, GIV-61L-mAb-8 was used in western blotting and immunofluorescence assays to demonstrate that Giv-61L protein was included in the GIV particle, expressed at 18 h, and localized only in the cytoplasm of GIV-infected cells. The results of this study provide insight into GIV pathogenesis and GIV-61L-mAbs will have broad applications in GIV immunodiagnostics.

Monoclonal antibody against a putative myristoylated membrane protein encoded by grouper iridovirus 59L gene

Groupers (Epinephelus spp.) are economically important fish species worldwide, and ranaviruses are major viral pathogens causing heavy economic losses in grouper aquaculture. In this study, the 59L gene of grouper iridovirus (GIV-59L) was cloned and characterized. This gene is 1521 bp and encodes a protein of 506 amino acids with a predicted molecular mass of 53.9 kDa. Interestingly, GIV-59L and its homologs are found in all genera of the family Iridoviridae. A mouse monoclonal antibody specific for the C-terminal domain (amino acid positions 254-506) of the GIV-59L protein, GIV-59L(760-1518)-MAb-21, was produced and proved to be well suited for use in a number of GIV immunoassays. RT-PCR, Western blotting, and cycloheximide and cytosine arabinoside drug inhibition analyses indicated that GIV-59L is a viral late gene in GIV-infected grouper kidney cells. Immunofluorescence analysis revealed that GIV-59L protein mainly accumulates in the cytoplasm of infected cells and is finally packed into a whole virus particle. The GIV-59L(760-1518)-MAb-21 characterized in this study could have widespread application in GIV immunodiagnostics and other research on GIV. In addition, the results presented here offer important insights into the pathogenesis of GIV.

Development and characterization of two monoclonal antibodies against grouper iridovirus 55L and 97L proteins

Grouper iridovirus (GIV) is one of the most important viral pathogens in grouper, particularly at the fry and fingerling stages. The study of GIV pathogenicity has been hampered by the lack of proper immunological reagents to study the expression and function of viral proteins in the infected cells. In this study, two mouse monoclonal antibodies (mAbs) against GIV 55L and 97L proteins were produced. Enzyme-linked immunosorbent assay (ELISA) and Western blotting were used to screen these hybridomas, resulting in the identification of two high-affinity mAbs named GIV55L-mAb-2 and GIV97L-mAb-3, respectively. Both mAbs belong to the IgG1 isotype and were effective in detecting their respective target viral protein. Reverse-transcription polymerase chain reaction (RT-PCR) and Western blot analyses of GIV-infected GK cells revealed that GIV 97L is an immediate early gene, whereas GIV 55L a late one. The localization of 55L and 97L in GIV-infected cells was further characterized by immunofluorescence microscopy with the mAbs. The 55L protein mainly aggregated in the cytoplasm while 97L distributed in both the nucleus and cytoplasm of the infected cells. These studies demonstrate the validity of the two mAbs as immunodiagnostic and research reagents.

Comparative cardiac pathological changes of Atlantic salmon (Salmo salar L.) affected with heart and skeletal muscle inflammation (HSMI), cardiomyopathy syndrome (CMS) and pancreas disease (PD)

The heart is considered the powerhouse of the cardiovascular system. Heart and skeletal muscle inflammation (HSMI), cardiomyopathy syndrome (CMS) and pancreas disease (PD) are cardiac diseases of marine farmed Atlantic salmon (Salmo salar) which commonly affect the heart in addition to the skeletal muscle, liver and pancreas. The main findings of these diseases are necrosis and inflammatory cells infiltrates affecting different regions of the heart. In order to better characterize the cardiac pathology, study of the inflammatory cell characteristics and cell cycle protein expression was undertaken by immunohistochemistry. Immunohistochemistry was performed on paraffin embedded hearts from confirmed diseased cases applying specific antibodies. The inflammatory cells were predominantly CD3(+) T lymphocytes. The PD diseased hearts exhibited moderate hypoxia inducible factor-1α (HIF1α) immuno-reaction that suggested tissue hypoxia while recombinant tumor necrosis factor-α (rTNFα) antibody identified putative macrophages and eosinophilic granulocytes (EGCs) in addition to endocardial cells around lesions. There were strong to low levels of major histocompatibility complex (MHC) class II immunostaining in the diseased hearts associated with macrophage-like and lymphocyte-like cells. The diseased hearts expressed strong to low levels of apoptotic cells identified by caspase 3 and terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) staining. The strong signals for proliferative cell nuclear antigen (PCNA) and TUNEL, and moderate levels of caspase 3 immuno-reactivity suggested a high cell turnover where DNA damage/repair might be occurring in the diseased hearts. Interestingly, the apparently similar cardiac diseases exhibited differences in the immunopathological responses in Atlantic salmon.

Cardiac pathological changes of Atlantic salmon (Salmo salar L.) affected with heart and skeletal muscle inflammation (HSMI)

Heart and skeletal muscle inflammation (HSMI) is a disease of marine farmed Atlantic salmon where the pathological changes associated with the disease involve necrosis and an infiltration of inflammatory cells into different regions of the heart and skeletal muscle. The aim of this work was to characterize cardiac changes and inflammatory cell types associated with a clinical HSMI outbreak in Atlantic salmon using immunohistochemistry. Different immune cells and cardiac tissue responses associated with the disease were identified using different markers. The spectrum of inflammatory cells associated with the cardiac pathology consisted of mainly CD3(+) T lymphocytes, moderate numbers of macrophages and eosinophilic granulocytes. Proliferative cell nuclear antigen (PCNA) immuno-reaction identified significantly increased nuclear and cytoplasmic staining as well as identifying hypertrophic nuclei. Strong immunostaining was observed for major histocompatibility complex (MHC) class II in HSMI hearts. Although low in number, a few positive cells in diseased hearts were detected using the mature myeloid cell line granulocytes/monocytes antibody indicating more positive cells in diseased than non-diseased hearts. The recombinant tumor necrosis factor-α (TNFα) antibody identified stained macrophage-like cells and endothelial cells around lesions in addition to eosinophilic granular cells (EGCs). These findings suggested that the inflammatory response in diseased hearts comprised of mostly CD3(+) T lymphocytes and eosinophilic granular cells and hearts exhibited high cell turnover where DNA damage/repair might be the case (as identified by PCNA, caspase 3 and terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) reactivity).