Induction of anti-viral genes during acute infection with Viral hemorrhagic septicemia virus (VHSV) genogroup IVa in Pacific herring (Clupea pallasii)

Effects of temperature on viral load, inclusion body formation, and host response in Pacific Herring with viral erythrocytic necrosis (VEN)

OBJECTIVE

The primary objective of this study was to determine the effects of temperature on viral erythrocytic necrosis (VEN) progression under controlled conditions. Secondarily, this study was intended to evaluate the combined effects of temperature and VEN on the Pacific Herring Clupea palasii transcriptome.

METHODS

The effects of temperature on VEN progression were assessed by waterborne exposure of laboratory-reared, specific-pathogen-free Pacific Herring to tissues homogenates containing erythrocytic necrosis virus (ENV) at 6.9, 9.0, or 13.5°C.

RESULT

Exposure of Pacific Herring to ENV resulted in the establishment of infections characterized by high infection prevalence (89%; 40/45) and mean viral loads (5.5 log10 [gene copies/μg genomic DNA]) in kidney tissues at 44 days postexposure. Mean viral loads were significantly higher in fish from the ambient (mean = 9.0°C) and warm (mean = 13.5°C) treatments (6.1-6.2 log10 [gene copies/total genomic DNA]) than in fish from the cool (mean = 6.9°C) treatment (4.3 log10 [gene copies/μg genomic DNA]). Similarly, the peak proportion of diseased fish was directly related to temperature, with cytoplasmic inclusion bodies detected in 21% of fish from the cool treatment, 52% of fish from the ambient treatment, and 60% of fish from the warm treatment. The mean VEN load in each fish (enumerated as the percentage of erythrocytes with cytoplasmic inclusions) at 44 days postexposure increased with temperature from 15% in the cool treatment to 36% in the ambient treatment and 32% in the warm treatment. Transcriptional analysis indicated that the number of differentially expressed genes among ENV-exposed Pacific Herring increased with temperature, time postexposure, and viral load. Correlation network analysis of transcriptomic data showed robust activation of interferon and viral immune responses in the hepatic tissue of infected individuals independent of other experimental variables.

CONCLUSION

Results from this controlled laboratory study, combined with previous observations of natural epizootics in wild populations, support the conclusion that temperature is an important disease cofactor for VEN in Pacific Herring.

The emerging roles of leukocyte cell-derived chemotaxin-2 in immune diseases: From mechanisms to therapeutic potential

Leukocyte cell-derived chemotaxin-2 (LECT2, also named ChM-II), initially identified as a chemokine mediating neutrophil migration, is a multifunctional secreted factor involved in diverse physiological and pathological processes. The high sequence similarity of LECT2 among different vertebrates makes it possible to explore its functions by using comparative biology. LECT2 is associated with many immune processes and immune-related diseases via its binding to cell surface receptors such as CD209a, Tie1, and Met in various cell types. In addition, the misfolding LECT2 leads to the amyloidosis of several crucial tissues (kidney, liver, and lung, etc.) by inducing the formation of insoluble fibrils. However, the mechanisms of LECT2-mediated diverse immune pathogenic conditions in various tissues remain to be fully elucidated due to the functional and signaling heterogeneity. Here, we provide a comprehensive summary of the structure, the “double-edged sword” function, and the extensive signaling pathways of LECT2 in immune diseases, as well as the potential applications of LECT2 in therapeutic interventions in preclinical or clinical trials. This review provides an integrated perspective on the current understanding of how LECT2 is associated with immune diseases, with the aim of facilitating the development of drugs or probes against LECT2 for the theranostics of immune-related diseases.

Long-term shedding from fully convalesced individuals indicates that Pacific herring are a reservoir for viral hemorrhagic septicemia virus

Processes that allow viral hemorrhagic septicemia (VHS) virus to persist in the marine environment remain enigmatic, owing largely to the presence of covert and cryptic infections in marine fishes during typical sub-epizootic periods. As such, marine host reservoirs for VHS virus have not been fully demonstrated, nor have the mechanism(s) by which infected hosts contribute to virus perpetuation and transmission. Here, we demonstrate that after surviving VHS, convalesced Pacific herring continue to shed virus at a low rate for extended periods. Further, exposure of previously naïve conspecific sentinels to this shed virus can result in infections for at least 6 mo after cessation of overt disease. This transmission mechanism was not necessarily dependent on the magnitude of the disease outbreak, as prolonged transmission occurred from 2 groups of donor herring that experienced cumulative mortalities of 4 and 29%. The results further suggest that the virus persists in association with the gills of fully recovered individuals, and long-term viral shedding or shedding relapses are related to cooler or decreasing water temperatures. These results provide support for a new VHS virus perpetuation paradigm in the marine environment, whereby the virus can be maintained in convalesced survivors and trafficked from these carriers to sympatric susceptible individuals.

Differential Effects of Dietary Supplementation of Krill Meal, Soybean Meal, Butyrate, and Bactocell® on the Gene Expression of Atlantic Salmon Head Kidney

The head kidney is a key organ that plays a fundamental role in the regulation of the fish immune response and in the maintenance of endocrine homeostasis. Previous studies indicate that the supplementation of exogenous dietary components, such as krill meal (KM), soybean meal (SM), Bactocell® (BA), and butyrate (BU), can have a significant effect on the immune function of the head kidney. The aim of this study was to investigate the differential effect of these four dietary ingredients on the transcriptional profiles of the head kidney of the Atlantic salmon. This study revealed that just a small number of genes were responsive to the feeding regime after a long-term (12 weeks) treatment, and evidenced that the most significant alterations, both in terms of the number of affected genes and magnitude of changes in gene expression, were detectable in the BU- and KM-fed groups compared with controls, while the SM diet had a nearly negligible effect, and BA had no significant effects at all. Most of the differentially expressed genes were involved in the immune response and, in line with data previously obtained from pyloric caeca, major components of the complement system were significantly affected. These alterations were accompanied by an increase in the density of melanomacrophage centers in the KM- and SM-fed group and their reduction in the BU-fed group. While three types of dietary supplements (BU, KM, and SM) were able to produce a significant modulation of some molecular players of the immune system, the butyrate-rich diet was revealed as the one with the most relevant immune-stimulating properties in the head kidney. These preliminary results suggest that further investigations should be aimed towards the elucidation of the potential beneficial effects of butyrate and krill meal supplementation on farmed salmon health and growth performance.

Poly (I:C) and imiquimod induced immune responses and their effects on the survival of olive flounder (Paralichthys olivaceus) from viral haemorrhagic septicaemia

The stimulation of immune genes by polyinosinic:polycytidylic acid (poly (I:C)) and imiquimod in olive flounder (Paralichthys olivaceus) and their role in control of viral haemorrhagic septicaemia virus (VHSV) infection were examined. Poly (I:C) (100 μg/fish) treated olive flounder had very low mortality (5%) post VHSV infection, while the imiquimod treated group had 65% and 85% mortality at a dose of 100 μg/fish and 50 μg/fish, respectively. Though the imiquimod treated group had high mortality, it was lower than the untreated group, which had 90% mortality. In vivo experiments were conducted to determine effect of the two ligands on immune modulation in the head kidney of olive flounder. Poly (I:C) activated the immune genes (TLR-3, TLR-7, MDA-5, LGP-2, IRF-3, IRF-7, IL-1β type I IFN and Mx) very early, within 1 d post stimulation, faster and stronger than imiquimod. Though Mx levels were enhanced by imiquimod, the host was still susceptible to VHSV. The poly (I:C) treated group had a high immune response at the time of infection and 1 dpi, though it decreased at later stages. The imiquimod treated group and the unstimulated group had a higher immune response to VHSV compared to the poly (I:C) treated group. The nucleoprotein copies of VHSV were very low in the poly (I:C) treated group but interestingly, were high in both untreated and imiquimod treated fish. Thus, host survival from a viral infection does not only depend on the quantity of immune response but also the time of response. Although imiquimod enhanced immune gene expression in olive flounder, a delayed response could be the reason for high mortality to VHS compared with poly (I:C), which induced the immune system effectively and efficiently to protect the host.

Influence of Temperature on the Efficacy of Homologous and Heterologous DNA Vaccines against Viral Hemorrhagic Septicemia in Pacific Herring

Homologous and heterologous (genogroup Ia) DNA vaccines against viral hemorrhagic septicemia virus (genogroup IVa) conferred partial protection in Pacific Herring Clupea pallasii. Early protection at 2 weeks postvaccination (PV) was low and occurred only at an elevated temperature (12.6°C, 189 degree days), where the relative percent survival following viral exposure was similar for the two vaccines (IVa and Ia) and higher than that of negative controls at the same temperature. Late protection at 10 weeks PV was induced by both vaccines but was higher with the homologous vaccine at both 9.0°C and 12.6°C. Virus neutralization titers were detected among 55% of all vaccinated fish at 10 weeks PV. The results suggest that the immune response profile triggered by DNA vaccination of herring was similar to that reported for Rainbow Trout Oncorhynchus mykiss by Lorenzen and LaPatra in 2005, who found interferon responses in the early days PV and the transition to adaptive response later. However, the protective effect was far less prominent in herring, possibly reflecting different physiologies or adaptations of the two fish species. Received August 1, 2016; accepted March 10, 2017.

Molecular identification of erythrocytic necrosis virus (ENV) from the blood of Pacific herring (Clupea pallasii)

Viral erythrocytic necrosis (VEN) is a condition affecting the red blood cells of more than 20 species of marine and anadromous fishes in the North Atlantic and North Pacific Oceans. Among populations of Pacific herring (Clupea pallasii) on the west coast of North America the disease causes anemia and elevated mortality in periodic epizootics. Presently, VEN is diagnosed by observation of typical cytoplasmic inclusion bodies in stained blood smears from infected fish. The causative agent, erythrocytic necrosis virus (ENV), is unculturable and a presumed iridovirus by electron microscopy. In vivo amplification of the virus in pathogen-free laboratory stocks of Pacific herring with subsequent virus concentration, purification, DNA extraction, and high-throughput sequencing were used to obtain genomic ENV sequences. Fragments with the highest sequence identity to the family Iridoviridae were used to design four sets of ENV-specific polymerase chain reaction (PCR) primers. Testing of blood and tissue samples from experimentally and wild infected Pacific herring as well as DNA extracted from other amphibian and piscine iridoviruses verified the assays were specific to ENV with a limit of detection of 0.0003 ng. Preliminary phylogenetic analyses of a 1448 bp fragment of the putative DNA polymerase gene supported inclusion of ENV in a proposed sixth genus of the family Iridoviridae that contains other erythrocytic viruses from ectothermic hosts. This study provides the first molecular evidence of ENV's inclusion within the Iridoviridae family and offers conventional PCR assays as a means of rapidly surveying the ENV-status of wild and propagated Pacific herring stocks.

Modulation of proteome expression by F-type lectin during viral hemorrhagic septicemia virus infection in fathead minnow cells

Lectins found in fish tissues play an important role in the innate immune response against viral infection. A fucose-binding type lectin, RbFTL-3, from rock bream (Oplegnathus fasciatus) was identified using expressed sequence tag (EST) analysis. The expression of RbFTL-3 mRNA was higher in intestine than other tissues of rock bream. To determine the function of RbFTL-3, VHSV-susceptible fathead minnow (FHM) cells were transfected with pcDNA3.1(+) or pcDNA3.1(+)-RbFTL-3 and further infected with VHSV. The results show that the viability of FHM cells transfected with pcDNA3.1(+)-RbFTL-3 is higher than that of cells transfected with pcDNA3.1(+) (relative cell viability: 28.9% vs 56.2%). A comparative proteomic analysis, performed to explore the proteins related to the protective effect of RbFTL-3 in the cells during VHSV infection, identified 90 proteins differentially expressed in VHSV-infected FHM cells transfected with pcDNA3.1(+) or pcDNA3.1(+)-RbFTL-3. The expression of RbFTL-3 inhibits a vascular-sorting protein (SNF8) and diminishes the loss of prothrombin, which are closely associated with controlling viral budding and hemorrhage in fish cells, respectively. Subsequent Ingenuity Pathways Analysis enabled prediction of their biofunctional groupings and interaction networks. The results suggest RbFTL-3 modulates the expression of proteins related to viral budding (SNF8, CCT5 and TUBB) and thrombin signaling (F2) to increase the viability of VHSV infected cells.

Anti-viral immune responses in a primitive lung: characterization and expression analysis of interferon-inducible immunoproteasome subunits LMP2, LMP7 and MECL-1 in a sarcopterygian fish, the Nigerian spotted lungfish (Protopterus dolloi)

Lungfishes (Dipnoi) represent the closest ancestor of tetrapods. Dipnoi have dual breathing modes extracting oxygen from water and air. The primitive lungs of lungfishes are exposed to external antigens including viruses. To date, the immune response of lungfishes against viruses has not been investigated. During viral immune responses, cell exposure to type I interferon induces the replacement of the constitutive proteasome with LMP2, LMP7 and MECL-1 beta subunits forming the immunoproteasome and enhancing antigen presentation to MHC class I molecules. In order to study the immune defense system of the lungfish lung, we have characterized for the first time the three immunoproteasome subunits in the sarcopterygian fish, the Nigerian spotted lungfish (Protopterus dolloi). LMP2, LMP7 and MECL-1 were identified in P. dolloi and their sequences encoded predicted proteins of 216, 275 and 278 amino acids, respectively. The mRNA of these three genes was expressed in multiple tissues, including the lung, with the highest abundance observed in kidney and post-pyloric spleen. In vitro stimulation of lungfish lung and kidney primary cell cultures with PolyI:C for 4 and 12 h resulted in increased LMP2, LMP7 and MECL-1 expression in both tissues. These results suggest a central role of these genes in the activation of an antiviral immune response in lungfish. Importantly, they indicate that the primitive lung of the common ancestor of all tetrapods is capable of inducing the expression of these genes in response to viral stimulation.

The antiviral innate immune response in fish: evolution and conservation of the IFN system

Innate immunity constitutes the first line of the host defense after pathogen invasion. Viruses trigger the expression of interferons (IFNs). These master antiviral cytokines induce in turn a large number of interferon-stimulated genes, which possess diverse effector and regulatory functions. The IFN system is conserved in all tetrapods as well as in fishes, but not in tunicates or in the lancelet, suggesting that it originated in early vertebrates. Viral diseases are an important concern of fish aquaculture, which is why fish viruses and antiviral responses have been studied mostly in species of commercial value, such as salmonids. More recently, there has been an interest in the use of more tractable model fish species, notably the zebrafish. Progress in genomics now makes it possible to get a relatively complete image of the genes involved in innate antiviral responses in fish. In this review, by comparing the IFN system between teleosts and mammals, we will focus on its evolution in vertebrates.

Impaired TLR2 and TLR7 response in olive flounder infected with viral haemorrhagic septicaemia virus at host susceptible 15 °C but high at non-susceptible 20 °C

The olive flounder (Paralichthys olivaceus) is susceptible to viral haemorrhagic septicaemia virus (VHSV) at 15 °C but no mortality is observed at 20 °C even though the virus can grow profusely in vitro. Thus, we designed an experiment to better understand the immune response of olive flounder to VHSV when the host reared at 15 °C or 20 °C and infected with the virus. Olive flounder (18-22 g) reared at 15 ± 0.5 °C or 20 ± 0.5 °C were intra-peritoneally injected with VHSV (10(7.8) TCID50/fish) and sampled (n = 5) for head kidney at 3, 6, 12 hpi, 1, 2, 4 and 7 dpi; similarly, mock injected control groups (n = 5). Real-time PCR-based absolute quantification method was followed to quantify copies of VHSV gRNA and mRNA, while the immune gene expression of the olive flounder was quantified relative to internal control, β-actin. Viral infection resulted in a cumulative mortality of 24% in olive flounder reared at 15 °C, but no mortality was recorded in the 20 °C group or control groups. TLR2 and TLR7 expression at 15 °C was enhanced during early-infection phase (3-6 hpi) and recovery phase (4-7 dpi) when viral transcription was low, but expression was significantly reduced (12 hpi-1 dpi) at peak-infection period. However, the 20 °C group showed low viral transcription and expressed high level of TLR7 and a moderately higher unchanged level of TLR2. In both the groups, TLR3 expression was unaffected. Nevertheless, expression of MDA5 and LGP2 increased significantly irrespective of rearing temperature at the time of peak infection, hence at 15 °C VHSV down-regulated expression of TLR2 and TLR7 but not MDA5 or LGP2. Comparatively, at 15 °C IRF3 expressed high but IRF7 remained very low. Interleukins (IL-1β, IL-6 and IL-8) were significantly elevated in both the groups, but quicker and for a shorter period at 20 °C. In the 15 °C group, an extended period of expression of ILs could create an unsafe prolonged inflammatory condition. The olive flounders expressed high ISGs at 15 °C but were lagging by 12 h than 20 °C group. Based on these findings, we concluded that viral-mediated disruption of TLR2 and TLR7 expression in the 15 °C group could have delayed the host interferon response and provided a window for high viral growth. However, an effective host immune response at 20 °C contained VHSV from reaching the critical limit.

Production and characterization of monoclonal antibodies to IgM of Pacific herring (Clupea pallasii)

Pacific herring (Clupea pallasii) have a central role in the North Pacific ecosystem as a forage fish species and are natural reservoirs of several important finfish pathogens, including Viral hemorrhagic septicemia virus (VHSV). Here, we report the identification of the gene encoding the immunoglobulin mu (IgM) heavy chain, as well as the development and characterization of monoclonal antibodies (MAbs) that specifically react with Pacific herring IgM. Pacific herring immunoglobulin was purified and consisted of heavy and light chains of approximately 80 and 25 kDa. Three hybridoma clones were initially identified by ELISA as reactive with purified immunoglobulin but only one clone was able to detect an 80 kDa protein in Pacific and Atlantic herring (Clupea harengus) whole plasma by denaturing western blot. However, all three MAbs were able to precipitate an 80 kDa protein from Pacific herring and LCMS sequencing of peptide fragments derived from this protein matched the predicted amino acid sequence of the cloned, heavy chain gene. In addition, two of the MAbs stained cells within the putative lymphocyte gates for the spleen, anterior kidney and posterior kidney but were not reactive for myeloid/granulocyte gates, which is consistent with these MAbs reacting with surface IgM⁺ B-cells. To our knowledge, this is the first report of IgM-related gene sequences and anti-IgM monoclonal antibodies from any member of the family Clupeidae. The antibodies produced in this study are critical for achieving our long-term goal of conducting serological surveillance to assess pathogen exposure in natural populations of Pacific herring.