The alloherpesviral counterparts of interleukin 10 in European eel and common carp

Cytokines Studied in Carp (Cyprinus carpio L.) in Response to Important Diseases

Cytokines belong to the most widely studied group of intracellular molecules involved in the function of the immune system. Their secretion is induced by various infectious stimuli. Cytokine release by host cells has been extensively used as a powerful tool for studying immune reactions in the early stages of viral and bacterial infections. Recently, research attention has shifted to the investigation of cytokine responses using mRNA expression, an essential mechanism related to pathogenic and nonpathogenic-immune stimulants in fish. This review represents the current knowledge of cytokine responses to infectious diseases in the common carp (Cyprinus carpio L.). Given the paucity of literature on cytokine responses to many infections in carp, only select viral diseases, such as koi herpesvirus disease (KHVD), spring viremia of carp (SVC), and carp edema virus disease (CEVD), are discussed. Aeromonas hydrophila is one of the most studied bacterial pathogens associated with cytokine responses in common carp. Therefore, the cytokine-based immunoreactivity raised by this specific bacterial pathogen is also highlighted in this review.

Molecular characterization of Cyprinid herpesvirus 3 encoded viral interleukin10

Cyprinid herpesvirus 3 (CyHV-3), a virus that encodes an interleukin10 (IL-10) homologue, causes severe economic losses to the common carp and koi culture industry. The present study was devoted to this IL-10 homologue. Recombinant viral IL-10 (vIL-10) protein encoded by CyHV-3 ORF134 gene using prokaryotic expression system was obtained successfully. Bioinformatics analysis revealed that the amino acid sequence of CyHV-3 vIL-10 has low homology with other host IL-10 or viruses encoded IL-10s. However, their tertiary structure is quite similar, suggesting conservative biological functions between IL-10s and vIL-10s. The biological activity of CyHV-3 vIL-10 was detected by using CCK-8 kit and real time quantitative PCR. The results showed that CyHV-3 vIL-10 down regulate epithelioma papulosum cyprini (EPC) cellular activity at 72 h. Moreover, CyHV-3 vIL-10 inhibits the LPS-induced expression of proinflammatory genes, similar to common carp IL-10. Altogether, the results of this study demonstrate that a clear biological activity of CyHV-3 vIL-10 on its host cells and indicates CyHV-3 vIL-10 may play an important role in viral immune evasion.

Transcriptomic analysis of common carp anterior kidney during Cyprinid herpesvirus 3 infection: Immunoglobulin repertoire and homologue functional divergence

Cyprinid herpesvirus 3 (CyHV-3) infects koi and common carp and causes widespread mortalities. While the virus is a significant concern for aquaculture operations in many countries, in Australia the virus may be a useful biocontrol agent for pest carp. However, carp immune responses to CyHV-3, and the molecular mechanisms underpinning resistance, are not well understood. Here we used RNA-Seq on carp during different phases of CyHV-3 infection to detect the gene expression dynamics of both host and virus simultaneously. During acute CyHV-3 infection, the carp host modified the expression of genes involved in various immune systems and detoxification pathways. Moreover, the activated pathways were skewed toward humoral immune responses, which may have been influenced by the virus itself. Many immune-related genes were duplicated in the carp genome, and often these were expressed differently across the infection phases. Of particular interest were two interleukin-10 homologues that were not expressed synchronously, suggesting neo- or sub-functionalization. The carp immunoglobulin repertoire significantly diversified during active CyHV-3 infection, which was followed by the selection of high-affinity B-cells. This is indicative of a developing adaptive immune response, and is the first attempt to use RNA-Seq to understand this process in fish during a viral infection.

Carp Il10a and Il10b exert identical biological activities in vitro, but are differentially regulated in vivo

We recently reported on the functional characterization of carp Il10. We showed that carp Il10 is able to downregulate proinflammatory activities by carp phagocytes and promote B cell proliferation, differentiation and antibody production as well as proliferation of memory T cells. Taking advantage of the recent annotation of the carp genome, we completed the sequence of a second il10 paralogue, named il10b, the presence of which was expected owing to the recent (8 million years ago) fourth round of whole genome duplication that occurred in common carp. In the present study we closely compared the two Il10 paralogues and show that Il10a and Il10b have almost identical gene structure, synteny, protein sequence as well as bioactivity on phagocytes. Although the two il10 paralogues show a large overlap in tissue expression, il10b has a low constitutive expression and is highly upregulated upon infection, whereas il10a is higher expressed under basal conditions but its gene expression remains constant during viral and parasitic infections. This differential regulation is most likely due to the observed differences in their promoter regions. Altogether our results demonstrate that gene duplication in carp, although recent, led to sub-functionalization and expression divergence rather than functional redundancy of the Il10 paralogues, yet with very similar protein sequences.

The Function of Fish Cytokines

What is known about the biological activity of fish cytokines is reviewed. Most of the functional studies performed to date have been in teleost fish, and have focused on the induced effects of cytokine recombinant proteins, or have used loss- and gain-of-function experiments in zebrafish. Such studies begin to tell us about the role of these molecules in the regulation of fish immune responses and whether they are similar or divergent to the well-characterised functions of mammalian cytokines. This knowledge will aid our ability to determine and modulate the pathways leading to protective immunity, to improve fish health in aquaculture.

Carp Il10 Has Anti-Inflammatory Activities on Phagocytes, Promotes Proliferation of Memory T Cells, and Regulates B Cell Differentiation and Antibody Secretion

In the current study, we investigated the effects of carp Il10 on phagocytes and lymphocytes. Carp Il10 shares several prototypical inhibitory activities on phagocytes with mammalian IL-10, including deactivation of neutrophils and macrophages, as shown by inhibition of oxygen and nitrogen radical production, as well as reduced expression of proinflammatory genes and mhc genes involved in Ag presentation. Similar to mammalian IL-10, carp Il10 acts through a signaling pathway involving phosphorylation of Stat3, ultimately leading to the early upregulation of socs3 expression. To our knowledge, this is the first study of the effects of Il10 on lymphocytes in fish. Although Il10 did not affect survival and proliferation of T cells from naive animals, it greatly promoted survival and proliferation of T cells in cultures from immunized animals, but only when used in combination with the immunizing Ag. Preliminary gene expression analysis suggests that, under these circumstances, carp Il10 stimulates a subset of CD8+ memory T cells while downregulating CD4+ memory Th1 and Th2 responses. In addition to the regulatory effect on T cells, carp Il10 stimulates proliferation, differentiation, and Ab secretion by IgM+ B cells. Overall, carp Il10 shares several prototypical activities with mammalian IL-10, including downregulation of the inflammatory response of phagocytes, stimulation of proliferation of subsets of memory T lymphocytes, and proliferation, differentiation, and Ab secretion by IgM+ B lymphocytes. To our knowledge, this is the first comprehensive analysis of biological activities of fish Il10 on both phagocytes and lymphocytes showing functional conservation of several properties of Il10.

Development and validation of a real-time PCR assay for the detection of anguillid herpesvirus 1

Anguillid herpesvirus 1 (AngHV1) causes a haemorrhagic disease with increased mortality in wild and farmed European eel, Anguilla anguilla (L.) and Japanese eel Anguilla japonica, Temminck & Schlegel). Detection of AngHV1 is currently based on virus isolation in cell culture, antibody-based typing assays or conventional PCR. We developed, optimized and concisely validated a diagnostic TaqMan probe based real-time PCR assay for the detection of AngHV1. The primers and probe target AngHV1 open reading frame 57, encoding the capsid protease and scaffold protein. Compared to conventional PCR, the developed real-time PCR is faster, less labour-intensive and has a reduced risk of cross-contamination. The real-time PCR assay was shown to be analytically sensitive and specific and has a high repeatability, efficiency and r(2) -value. The diagnostic performance of the assay was determined by testing 10% w/v organ suspensions and virus cultures from wild and farmed European eels from the Netherlands by conventional and real-time PCR. The developed real-time PCR assay is a useful tool for the rapid and sensitive detection of AngHV1 in 10% w/v organ suspensions from wild and farmed European eels.

Cyprinid Herpesvirus 3 Il10 Inhibits Inflammatory Activities of Carp Macrophages and Promotes Proliferation of Igm+ B Cells and Memory T Cells in a Manner Similar to Carp Il10

Cyprinid herpesvirus 3 (CyHV-3) is the causative agent of a lethal disease of carp and encodes for an Il10 homolog (ORF134). Our previous studies with a recombinant ORF134-deleted strain and the derived revertant strain suggested that cyprinid herpesvirus 3 Il10 (CyHV-3 Il10 [cyhv3Il10]) is not essential for viral replication in vitro, or virulence in vivo. In apparent contrast, cyhv3Il10 is one of the most abundant proteins of the CyHV-3 secretome and is structurally very similar to carp Il10 and also human IL10. To date, studies addressing the biological activity of cyhv3Il10 on cells of its natural host have not been performed. To address the apparent contradiction between the presence of a structurally conserved Il10 homolog in the genome of CyHV-3 and the lack of a clear phenotype in vivo using recombinant cyhv3Il10-deleted viruses, we used an in vitro approach to investigate in detail whether cyhv3Il10 exerts any biological activity on carp cells. In this study, we provide direct evidence that cyhv3Il10 is biologically active and, similarly to carp Il10, signals via a conserved Stat3 pathway modulating immune cells of its natural host, carp. In vitro, cyhv3Il10 deactivates phagocytes with a prominent effect on macrophages, while also promoting proliferation of Igm(+) B cells and memory T cells. Collectively, this study demonstrates a clear biological activity of cyhv3Il10 on cells of its natural host and indicates that cyhv3Il10 is a true viral ortholog of carp Il10. Furthermore, to our knowledge, this is the first report on biological activities of a nonmammalian viral Il10 homolog.

Cyprinid Herpesvirus 3: An Archetype of Fish Alloherpesviruses

The order Herpesvirales encompasses viruses that share structural, genetic, and biological properties. However, members of this order infect hosts ranging from molluscs to humans. It is currently divided into three phylogenetically related families. The Alloherpesviridae family contains viruses infecting fish and amphibians. There are 12 alloherpesviruses described to date, 10 of which infect fish. Over the last decade, cyprinid herpesvirus 3 (CyHV-3) infecting common and koi carp has emerged as the archetype of fish alloherpesviruses. Since its first description in the late 1990s, this virus has induced important economic losses in common and koi carp worldwide. It has also had negative environmental implications by affecting wild carp populations. These negative impacts and the importance of the host species have stimulated studies aimed at developing diagnostic and prophylactic tools. Unexpectedly, the data generated by these applied studies have stimulated interest in CyHV-3 as a model for fundamental research. This review intends to provide a complete overview of the knowledge currently available on CyHV-3.

Differential effects of alloherpesvirus CyHV-3 and rhabdovirus SVCV on apoptosis in fish cells

Whilst Herpesviridae, which infect higher vertebrates, actively influence host immune responses to ensure viral replication, it is mostly unknown if Alloherpesviridae, which infect lower vertebrates, possess similar abilities. An important antiviral response is clearance of infected cells via apoptosis, which in mammals influences the outcome of infection. Here, we utilise common carp infected with CyHV-3 to determine the effect on the expression of genes encoding apoptosis-related proteins (p53, Caspase 9, Apaf-1, IAP, iNOS) in the pronephros, spleen and gills. The influence of CyHV-3 on CCB cells was also studied and compared to SVCV (a rhabdovirus) which induces apoptosis in carp cell lines. Although CyHV-3 induced iNOS expression in vivo, significant induction of the genetic apoptosis pathway was only seen in the pronephros. In vitro CyHV-3 did not induce apoptosis or apoptosis-related expression whilst SVCV did stimulate apoptosis. This suggests that CyHV-3 possesses mechanisms similar to herpesviruses of higher vertebrates to inhibit the antiviral apoptotic process.

Non-structural protein pORF 12 of cyprinid herpesvirus 3 is recognized by the immune system of the common carp Cyprinus carpio

Cyprinid herpesvirus 3 is an important pathogen and the causative agent of koi herpesvirus disease, which has been associated with mass mortalities in koi and common carp Cyprinus carpio. Currently, the only available commercial vaccine is an attenuated version of the virus. This has led to concerns about its risk to reversion to virulence. Furthermore, the vaccine is currently only available in Israel and the United States. In order to investigate the antigenic profile of the virus, western blot was performed using infected cell culture supernatant and sera from carp that had survived exposure to the virus. Only one antigen could be detected, and mass spectrometry analysis identified the corresponding protein as ORF 12, a putative secreted tumour necrosis factor receptor homologue. In other herpesviruses, such proteins have been associated with the viral infectious process in a number of ways, including the entry into the host cell and the inhibition of apoptosis in infected cells. The reason why only one antigen could be detected during this study is unknown.

Biology and host response to Cyprinid herpesvirus 3 infection in common carp

Viruses from the family Alloherpesviridae form an aquatic clade of herpesviruses infecting fish and amphibia. Diseases caused by these herpesviruses are of increasing importance because of the high morbidity and mortality associated with the infection, and the difficulties in diagnosing latently infected carriers. Cyprinid herpesvirus 3 (CyHV-3) induces a severe disease and mortality in common carp and thus greatly affects carp aquaculture and trade. This review summarises advancements in the understanding of the infection process and the current knowledge on immune responses of carp to CyHV-3. A focus is laid on host genetics and immunity responsible for resistance/survival from the disease and on the viral mechanisms accountable for evasion of carp immune responses. As current knowledge of immune responses to CyHV-3 is still limited, perspectives for future studies are outlined. Analysing CyHV-3 fish-host interactions will be useful and thought-provoking for a basic understanding of fish immune responses.

Pathogen manipulation of cIL-10 signaling pathways: opportunities for vaccine development?

Interleukin-10 (IL-10) is a tightly regulated, pleiotropic cytokine that has profound effects on all facets of the immune system, eliciting cell-type-specific responses within cells expressing the IL-10 receptor (IL-10R). It is considered a master immune regulator, and imbalances in IL-10 expression, resulting from either inherent or infectious etiologies, have far reaching clinical ramifications. Regarding infectious diseases, there has been accumulating recognition that many pathogens, particularly those that establish lifelong persistence, share a commonality of their natural histories: manipulation of IL-10-mediated signaling pathways. Multiple viral, bacterial, protozoal, and fungal pathogens appear to have evolved mechanisms to co-opt normal immune functions, including those involving IL-10R-mediated signaling, and immune effector pathways away from immune-mediated protection toward environments of immune evasion, suppression, and tolerance. As a result, pathogens can persist for the life of the infected host, many of whom possess otherwise competent immune systems. Because of pathogenic avoidance of immune clearance, persistent infections can exact incalculable physical and financial costs, and represent some of the most vexing challenges for improvements in human health. Enormous benefits could be gained by the development of efficient prevention and/or therapeutic strategies that block primary infection, or clear the infection. There are now precedents that indicate that modalities focusing on pathogen-mediated manipulation of IL-10 signaling may have clinical benefit.

IL-10 encoded by viruses: a remarkable example of independent acquisition of a cellular gene by viruses and its subsequent evolution in the viral genome

Many viruses have evolved strategies to deregulate the host immune system. These strategies include mechanisms to subvert or recruit the host cytokine network. IL-10 is a pleiotropic cytokine that has both immunostimulatory and immunosuppressive properties. However, its key features relate mainly to its capacity to exert potent immunosuppressive effects. Several viruses have been shown to upregulate the expression of cellular IL-10 (cIL-10) with, in some cases, enhancement of infection by suppression of immune functions. Other viruses encode functional orthologues of cIL-10, called viral IL-10s (vIL-10s). The present review is devoted to these virokines. To date, vIL-10 orthologues have been reported for 12 members of the family Herpesviridae, two members of the family Alloherpesviridae and seven members of the family Poxviridae. Study of vIL-10s demonstrated several interesting aspects on the origin and the evolution of these viral genes, e.g. the existence of multiple (potentially up to nine) independent gene acquisition events at different times during evolution, viral gene acquisition resulting from recombination with cellular genomic DNA or cDNA derived from cellular mRNA and the evolution of cellular sequence in the viral genome to restrict the biological activities of the viral orthologues to those beneficial for the virus life cycle. Here, various aspects of the vIL-10s described to date are reviewed, including their genetic organization, protein structure, origin, evolution, biological properties and potential in applied research.

Cyprinid herpesvirus 3: an interesting virus for applied and fundamental research

Cyprinid herpesvirus 3 (CyHV-3), a member of the family Alloherpesviridae is the causative agent of a lethal, highly contagious and notifiable disease in common and koi carp. The economic importance of common and koi carp industries together with the rapid spread of CyHV-3 worldwide, explain why this virus became soon after its isolation in the 1990s a subject of applied research. In addition to its economic importance, an increasing number of fundamental studies demonstrated that CyHV-3 is an original and interesting subject for fundamental research. In this review, we summarized recent advances in CyHV-3 research with a special interest for studies related to host-virus interactions.

The IL-10 homologue encoded by cyprinid herpesvirus 3 is essential neither for viral replication in vitro nor for virulence in vivo

Cyprinid herpesvirus 3 (CyHV-3), a member of the family Alloherpesviridae, is the causative agent of a lethal disease in common and koi carp. CyHV-3 ORF134 encodes an interleukin-10 (IL-10) homologue. The present study was devoted to this ORF. Transcriptomic analyses revealed that ORF134 is expressed as a spliced gene belonging to the early-late class. Proteomic analyses of CyHV-3 infected cell supernatant demonstrated that the ORF134 expression product is one of the most abundant proteins of the CyHV-3 secretome. To investigate the role of ORF134 in viral replication in vitro and in virulence in vivo, a deleted strain and a derived revertant strain were produced using BAC cloning technologies. The recombinant ORF134 deleted strain replicated in vitro comparably to the parental and the revertant strains. Infection of fish by immersion in water containing the virus induced comparable CyHV-3 disease for the three virus genotypes tested (wild type, deleted and revertant). Quantification of viral DNA by real time TaqMan PCR (in the gills and the kidney) and analysis of carp cytokine expression (in the spleen) by RT-qPCR at different times post-infection did not revealed any significant difference between the groups of fish infected with the three virus genotypes. Similarly, histological examination of the gills and the kidney of infected fish revealed no significant differences between fish infected with ORF134 deleted virus versus fish infected with the control parental or revertant strains. All together, the results of the present study demonstrate that the IL-10 homologue encoded by CyHV-3 is essential neither for viral replication in vitro nor for virulence in common carp.

Genome-wide gene expression analysis of anguillid herpesvirus 1

Background

Whereas temporal gene expression in mammalian herpesviruses has been studied extensively, little is known about gene expression in fish herpesviruses. Here we report a genome-wide transcription analysis of a fish herpesvirus, anguillid herpesvirus 1, in cell culture, studied during the first 6 hours of infection using reverse transcription quantitative PCR.

Results

Four immediate-early genes – open reading frames 1, 6A, 127 and 131 – were identified on the basis of expression in the presence of a protein synthesis inhibitor and unique expression profiles during infection in the absence of inhibitor. All of these genes are located within or near the terminal direct repeats. The remaining 122 open reading frames were clustered into groups on the basis of transcription profiles during infection. Expression of these genes was also studied in the presence of a viral DNA polymerase inhibitor, enabling classification into early, early-late and late genes. In general, clustering by expression profile and classification by inhibitor studies corresponded well. Most early genes encode enzymes and proteins involved in DNA replication, most late genes encode structural proteins, and early-late genes encode non-structural as well as structural proteins.

Conclusions

Overall, anguillid herpesvirus 1 gene expression was shown to be regulated in a temporal fashion, comparable to that of mammalian herpesviruses.

Comparative genomics of carp herpesviruses

Three alloherpesviruses are known to cause disease in cyprinid fish: cyprinid herpesviruses 1 and 3 (CyHV1 and CyHV3) in common carp and koi and cyprinid herpesvirus 2 (CyHV2) in goldfish. We have determined the genome sequences of CyHV1 and CyHV2 and compared them with the published CyHV3 sequence. The CyHV1 and CyHV2 genomes are 291,144 and 290,304 bp, respectively, in size, and thus the CyHV3 genome, at 295,146 bp, remains the largest recorded among the herpesviruses. Each of the three genomes consists of a unique region flanked at each terminus by a sizeable direct repeat. The CyHV1, CyHV2, and CyHV3 genomes are predicted to contain 137, 150, and 155 unique, functional protein-coding genes, respectively, of which six, four, and eight, respectively, are duplicated in the terminal repeat. The three viruses share 120 orthologous genes in a largely colinear arrangement, of which up to 55 are also conserved in the other member of the genus Cyprinivirus, anguillid herpesvirus 1. Twelve genes are conserved convincingly in all sequenced alloherpesviruses, and two others are conserved marginally. The reference CyHV3 strain has been reported to contain five fragmented genes that are presumably nonfunctional. The CyHV2 strain has two fragmented genes, and the CyHV1 strain has none. CyHV1, CyHV2, and CyHV3 have five, six, and five families of paralogous genes, respectively. One family unique to CyHV1 is related to cellular JUNB, which encodes a transcription factor involved in oncogenesis. To our knowledge, this is the first time that JUNB-related sequences have been reported in a herpesvirus.

Koi herpesvirus encodes and expresses a functional interleukin-10

Koi herpesvirus (KHV) (species Cyprinid herpesvirus 3) ORF134 was shown to transcribe a spliced transcript encoding a 179-amino-acid (aa) interleukin-10 (IL-10) homolog (khvIL-10) in koi fin (KF-1) cells. Pairwise sequence alignment indicated that the expressed product shares 25% identity with carp IL-10, 22 to 24% identity with mammalian (including primate) IL-10s, and 19.1% identity with European eel herpesvirus IL-10 (ahvIL-10). In phylogenetic analyses, khvIL-10 fell in a divergent position from all host IL-10 sequences, indicating extensive structural divergence following capture from the host. In KHV-infected fish, khvIL-10 transcripts were observed to be highly expressed during the acute and reactivation phases but to be expressed at very low levels during low-temperature-induced persistence. Similarly, KHV early (helicase [Hel] and DNA polymerase [DNAP]) and late (intercapsomeric triplex protein [ITP] and major capsid protein [MCP]) genes were also expressed at high levels during the acute and reactivation phases, but only low-level expression of the ITP gene was detected during the persistent phase. Injection of khvIL-10 mRNA into zebrafish (Danio rerio) embryos increased the number of lysozyme-positive cells to a similar degree as zebrafish IL-10. Downregulation of the IL-10 receptor long chain (IL-10R1) using a specific morpholino abrogated the response to both khvIL-10 and zebrafish IL-10 transcripts, indicating that, despite the structural divergence, khvIL-10 functions via this receptor. This is the first report describing the characteristics of a functional viral IL-10 gene in the Alloherpesviridae.

Anguillid herpesvirus 1 transcriptome

We used deep sequencing of poly(A) RNA to characterize the transcriptome of an economically important eel virus, anguillid herpesvirus 1 (AngHV1), at a stage during the lytic life cycle when infectious virus was being produced. In contrast to the transcription of mammalian herpesviruses, the overall level of antisense transcription from the 248,526-bp genome was low, amounting to only 1.5% of transcription in predicted protein-coding regions, and no abundant, nonoverlapping, noncoding RNAs were identified. RNA splicing was found to be more common than had been anticipated previously. Counting the 10,634-bp terminal direct repeat once, 100 splice junctions were identified, of which 58 were considered likely to be involved in the expression of functional proteins because they represent splicing between protein-coding exons or between 5' untranslated regions and protein-coding exons. Each of the 30 most highly represented of these 58 splice junctions was confirmed by RT-PCR. We also used deep sequencing to identify numerous putative 5' and 3' ends of AngHV1 transcripts, confirming some and adding others by rapid amplification of cDNA ends (RACE). The findings prompted a revision of the AngHV1 genome map to include a total of 129 protein-coding genes, 5 of which are duplicated in the terminal direct repeat. Not counting duplicates, 11 genes contain integral, spliced protein-coding exons, and 9 contain 5' untranslated exons or, because of alternative splicing, 5' untranslated and 5' translated exons. The results of this study sharpen our understanding of AngHV1 genomics and provide the first detailed view of a fish herpesvirus transcriptome.