Immune response modulation upon sequential heterogeneous co-infection with Tetracapsuloides bryosalmonae and VHSV in brown trout (Salmo trutta)

Infection by the Parasite Myxobolus bejeranoi (Cnidaria: Myxozoa) Suppresses the Immune System of Hybrid Tilapia

Myxozoa (Cnidaria) is a large group of microscopic obligate endoparasites that can cause emerging diseases, affecting wild fish populations and fisheries. Recently, the myxozoan Myxobolus bejeranoi was found to infect the gills of hybrid tilapia (Nile tilapia (Oreochromis niloticus) × Jordan/blue tilapia (O. aureus)), causing high morbidity and mortality. Here, we used comparative transcriptomics to elucidate the molecular processes occurring in the fish host following infection by M. bejeranoi. Fish were exposed to pond water containing actinospores for 24 h and the effects of minor, intermediate, and severe infections on the sporulation site, the gills, and on the hematopoietic organs, head kidney and spleen, were compared. Enrichment analysis for GO and KEGG pathways indicated immune system activation in gills at severe infection, whereas in the head kidney a broad immune suppression included deactivation of cytokines and GATA3 transcription factor responsible for T helper cell differentiation. In the spleen, the cytotoxic effector proteins perforin and granzyme B were downregulated and insulin, which may function as an immunomodulatory hormone inducing systemic immune suppression, was upregulated. These findings suggest that M. bejeranoi is a highly efficient parasite that disables the defense mechanisms of its fish host hybrid tilapia.

Mitigating human impacts including climate change on proliferative kidney disease in salmonids of running waters

Over the last two decades, an increasing number of reports have identified a decline in salmonid populations, possibly linked to infection with the parasite Tetracapsuloides bryosalmonae and the corresponding disease, that is, proliferative kidney disease (PKD). The life cycle of this myxozoan parasite includes sessile bryozoan species as invertebrate host, which facilitates the distribution of the parasite in running waters. As the disease outcome is temperature dependent, the impact of the disease on salmonid populations is increasing with global warming due to climate change. The goal of this review is to provide a detailed overview of measures to mitigate the effects of PKD on salmonid populations. It first summarizes the parasite life cycle, temperature-driven disease dynamics and new immunological and molecular research into disease resistance and, based on this, discusses management possibilities. Sophisticated management actions focusing on local adaptation of salmonid populations, restoration of the riverine ecosystem and keeping water temperatures cool are necessary to reduce the negative effects of PKD. Such actions include temporary stocking with PKD-resistant salmonids, as this may assist in conserving current populations that fail to reproduce.

Uncovering the first occurrence of Tilapia parvovirus in Thailand in tilapia during co-infection with Tilapia tilapinevirus

The recently discovered Tilapia parvovirus (TiPV) was the first Parvovirus confirmed to infect fish, causing mortality outbreaks in farmed adult Nile tilapia in China. Severe mortality outbreaks caused by Tilapia tilapinevirus (TiLV) to farmed tilapia in Thailand revealed the concomitant occurrence of TiPV. Out of ten fish farms screened, TiPV was detected in one site rearing juvenile red hybrid tilapia. Clinical signs included abnormal swimming, scale protrusion, skin and muscle haemorrhaging, exophthalmia and generalized anaemia. Histological findings showed extensive infiltration of lymphocytes, with increased melanomacrophage centres in the anterior kidney and spleen, erythrocyte depletion in the spleen and hepatic syncytial cells. Both TiLV and TiPV were systemically distributed in the body of moribund fish. The analysis of the near-complete TiPV genome isolated from Thailand revealed 98.74% sequence identity to the formerly isolated from China, together with a highly conserved and comparable genomic organization and with a 3 nucleotides deletion in the 5-UTR. The viral genome structure was highly conserved for each of its components, with nucleotide and amino acid identity ranging from 100% for ORF1 to 97% for ORF2, and with conserved HuH and Walker loop motifs within NS1. Taken together, our results document the first detection of TiPV outside China, thus for the first time in Thailand. Moreover, TiPV was detected for the first time during a natural occurrence in farmed red hybrid tilapia and involved in co-infection pattern with TiLV. Diagnostic investigations during tilapia disease outbreaks should include the screening for TiPV. Further studies are needed to elucidate TiPV genomic variance, pathobiology, including focussing on the outcomes of TiLV-TiPV co-infection patterns, necessary to enable risk assessment for the worldwide spreading of TiPV and to design adequate control measures against these emerging viruses in tilapia.

Survival of metazoan parasites in fish: Putting into context the protective immune responses of teleost fish

Defence mechanisms of fish can be divided into specific and non-specific that act in concert and are often interdependent. Most fish in both wild and cultured populations are vulnerable to metazoan parasites. Endoparasitic helminths include several species of digeneans, cestodes, nematodes, and acanthocephalans. Although they may occur in large numbers, helminth infections rarely result in fish mortality. Conversely, some ectoparasites cause mass mortality in farmed fish. Given the importance of fish innate immunity, this review addresses non-specific defence mechanisms of fish against metazoan parasites, with emphasis on granulocyte responses involving mast cells, neutrophils, macrophages, rodlet cells, and mucous cells. Metazoan parasites are important disease agents that affect wild and farmed fish and can induce high economic loss and, as pathogen organisms, deserve considerable attention. The paper will provide our light and transmission electron microscopy data on metazoan parasites-fish innate immune and neuroendocrine systems. Insights about the structure and functions of the cell types listed above and a brief account of the effects and harms of each metazoan taxon to specific fish apparati/organs will be presented.

Modulation of local and systemic immune responses in brown trout (Salmo trutta) following exposure to Myxobolus cerebralis

Myxobolus cerebralis, the etiological agent of Whirling Disease (WD), is a freshwater myxozoan parasite with considerable economic and ecological relevance for salmonids. There are differences in disease susceptibility between species and strains of salmonids. Recently, we have reported that the suppressor of cytokine signaling SOCS1 and SOCS3 are key in modulating rainbow trout (Oncorhynchus mykiss) immune responses and that resistant fish apparently exhibit effective Th17 cell response after exposure to M. cerebralis. It is unclear whether such molecules and pathways are also involved in the immune response of M. cerebralis infected brown trout (Salmo trutta). Hence, this study aimed to explore their role during immune modulation in infected brown trout, which is considered resistant to this parasite. Fish were exposed to the triactinomyxon (TAM) stages of M. cerebralis and quantitative real-time PCR (RT-qPCR) was carried out to examine local (caudal fin) and systemic (head kidney, spleen) immune transcriptional changes associated with WD over time in infected and control fish. All of the immune genes in the three tissues studied were differentially expressed in infected fish at multiple time points. Brown trout reduced the parasite load and demonstrated effective immune responses, likely by keeping pro-inflammatory and anti-inflammatory cytokines in balance whilst stimulating efficient Th17-mediated immunity. This study increases knowledge on the brown trout immune response to M. cerebralis and helps us to understand the underlying mechanisms of WD resistance.

Vaccination Route Determines the Kinetics and Magnitude of Nasal Innate Immune Responses in Rainbow Trout (Oncorhynchus mykiss)

Simple Summary

Many pathogens exploit the olfactory route to reach critical organs in the body such as the brain or lungs. Thus, effective onset of an early innate immune response in the nasal epithelium is key to stopping pathogen progression. The stimulation of nasal immunity by vaccines may depend on the type of vaccine and vaccination route. The goal of this study was to evaluate the ability of a live attenuated viral vaccine to stimulate innate immunity in the olfactory organ of rainbow trout, a teleost fish of commercial aquaculture value. The kinetics and magnitude of the innate immune response depended on the route of vaccination, with the strongest and fastest responses recorded in intranasally vaccinated fish. Injection vaccination had an intermediate effect, whereas immersion vaccination resulted in delayed and weak nasal innate immunity. Injection vaccination, even with the vehicle control, induced mortality in fingerlings, whereas nasal and immersion vaccines were safe. Challenge experiments with the live virus revealed that nasal and injected vaccines conferred very high and comparable levels of protection, but immersion vaccination only induced transient protection. In conclusion, the route of vaccination determines the type, magnitude and velocity of the innate immune response in the nasal epithelium of animals.

Abstract

Many pathogens infect animal hosts via the nasal route. Thus, understanding how vaccination stimulates early nasal immune responses is critical for animal and human health. Vaccination is the most effective method to prevent disease outbreaks in farmed fish. Nasal vaccination induces strong innate and adaptive immune responses in rainbow trout and was shown to be highly effective against infectious hematopoietic necrosis (IHN). However, direct comparisons between intranasal, injection and immersion vaccination routes have not been conducted in any fish species. Moreover, whether injection or immersion routes induce nasal innate immune responses is unknown. The goal of this study is to compare the effects of three different vaccine delivery routes, including intranasal (IN), intramuscular (i.m.) injection and immersion (imm) routes on the trout nasal innate immune response. Expression analyses of 13 immune-related genes in trout nasopharynx-associated lymphoid tissue (NALT), detected significant changes in immune expression in all genes analyzed in response to the three vaccination routes. However, nasal vaccination induced the strongest and fastest changes in innate immune gene expression compared to the other two routes. Challenge experiments 7 days post-vaccination (dpv) show the highest survival rates in the IN- and imm-vaccinated groups. However, survival rates in the imm group were significantly lower than the IN- and i.m.-vaccinated groups 28 dpv. Our results confirm that nasal vaccination of rainbow trout with live attenuated IHNV is highly effective and that the protection conferred by immersion vaccination is transient. These results also demonstrate for the first time that immersion vaccines stimulate NALT immune responses in salmonids.

Antimicrobial Peptides of Salmonid Fish: From Form to Function

Antimicrobial peptides (AMPs) are small, usually cationic, and amphiphilic molecules that play a crucial role in molecular and cellular host defense against pathogens, tissue damage, and infection. AMPs are present in all metazoans and several have been discovered in teleosts. Some teleosts, such as salmonids, have undergone whole genome duplication events and retained a diverse AMP repertoire. Salmonid AMPs have also been shown to possess diverse and potent antibacterial, antiviral, and antiparasitic activity and are induced by a variety of factors, including dietary components and specific molecules also known as pathogen-associated molecular patterns (PAMPs), which may activate downstream signals to initiate transcription of AMP genes. Moreover, a multitude of cell lines have been established from various salmonid species, making it possible to study host-pathogen interactions in vitro, and several of these cell lines have been shown to express various AMPs. In this review, the structure, function, transcriptional regulation, and immunomodulatory role of salmonid AMPs are highlighted in health and disease. It is important to characterize and understand how salmonid AMPs function as this may lead to a better understanding of host-pathogen interactions with implications for aquaculture and medicine.

A portrait of the immune response to proliferative kidney disease (PKD) in rainbow trout

Proliferative kidney disease (PKD), caused by the myxozoan Tetracapsuloides bryosalmonae, is one of the most serious parasitic diseases of salmonids in which outbreaks cause severe economic constraints for the aquaculture industry and declines of wild species throughout Europe and North America. Given that rainbow trout (Oncorhynchus mykiss) is one of the most widely farmed freshwater fish and an important model species for fish immunology, most of the knowledge on how the fish immune response is affected during PKD is from this organism. Once rainbow trout are infected, PKD pathogenesis results in a chronic kidney immunopathology mediated by decreasing myeloid cells and increasing lymphocytes. Transcriptional studies have revealed the regulation of essential genes related to T-helper (Th)-like functions and a dysregulated B-cell antibody type response. Recent reports have discovered unique details of teleost B-cell differentiation and functionality and characterized the differential immunoglobulin (Ig)-mediated response. These studies have solidified the rainbow trout T. bryosalmonae system as a sophisticated disease model capable of feeding key advances into mainstream immunology and have contributed essential information to design novel parasite disease prevention strategies. In our following perspective, we summarize these efforts to evaluate the immune mechanisms of rainbow trout during PKD pathogenesis.

Health Surveillance of Wild Brown Trout (Salmo trutta fario) in the Czech Republic Revealed a Coexistence of Proliferative Kidney Disease and Piscine Orthoreovirus-3 Infection

The population of brown trout (Salmo trutta fario) in continental Europe is on the decline, with infectious diseases confirmed as one of the causative factors. However, no data on the epizootiological situation of wild fish in the Czech Republic are currently available. In this study, brown trout (n = 260) from eight rivers were examined for the presence of viral and parasitical pathogens. Salmonid alphavirus-2, infectious pancreatic necrosis virus, piscine novirhabdovirus (VHSV) and salmonid novirhabdovirus (IHNV) were not detected using PCR. Cell culturing showed no viruses as well, and serological analysis of 110 sera did not detect any specific antibodies against VHSV or IHNV. Fish from two rivers were positive for the presence of piscine orthoreovirus-3 (PRV-3), subtype PRV-3b. However, none of the PRV-3-positive fish showed gross pathologies typically associated with PRV infections. By far the most widespread pathogen was Tetracapsuloides bryosalmonae which was confirmed in each of the examined locations, with a prevalence of up to 65% and 100%, as established by immunohistochemistry and PCR, respectively. Furthermore, up to 43.8% of fish showed signs of proliferative kidney disease caused by T. bryosalmonae, suggesting that this parasite is a main health challenge for brown trout in the Czech Republic.

Comparative effects of Novirhabdovirus genes on modulating constitutive transcription and innate antiviral responses, in different teleost host cell types

BACKGROUND

Infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV) are highly contagious, pathogenic Novirhabdoviruses affecting fish and are thusly notifiable diseases with the World Organization for Animal Health. This study assessed the relative capacities of IHNV and VHSV genes to modulate host general transcription and explores the abilities of specific IHNV genes to interfere with the interferon pathway in heterogenous teleost cell-lines.

METHODS

Optimized protocols allowed for efficient transient transfections in EPC, BF-2, RTG-2 and RTgill-W1 cell lines of plasmids encoding IHNV (M genogroup) and VHSV (-IVb genotype) genes, including N, P, M, G and NV. Their impact on general cellular transcription was measured 48 hours post transfection (hpt) with luciferase constructs driven by a modified β-Actin promoter (pCAG). Their modulation of the innate antiviral immune response was characterized 72 hpt, using luciferase constructs measuring rainbow trout Type I IFN or MX-1 promoter augmentation, upon MAVS co-transfection.

RESULTS

M was generally confirmed as the strongest constitutive transcriptional suppressor while IHNV P, but not VHSV P, augmented constitutive transcription in fibroblastic cell types. Cell-specific effects were observed for viral G gene, with VHSV G exhibiting suppression of basal transcription in EPC and BF-2 but not in trout cells; while IHNV G was stimulatory in RTG-2, but inhibitory in RTgill-W1. NV consistently stimulated constitutive transcription, with higher augmentation patterns seen in fibroblastic compared to epithelial cells, and for IHNV NV compared to VHSV NV. The innate antiviral immune response, focusing on the IFN pathway, was silenced by IHNV M in all cell lines tested. IHNV N showed a dose-dependent suppression of type I IFN, but with minor effects on MX-1. IHNV P and G played minor IFN-inhibitory roles, consistent and dose-dependent only for G in rainbow trout cells. IHNV NV mediated a consistent stimulatory effect on either Type I IFN or MX-1, but much less pronounced in RTgill-W1.

CONCLUSIONS

This study extends our understanding of Novirhabdoviruses-host interaction, showing differential innate immune responses in heterogenous cell types. Viral regulators of innate immune signaling are identified, either as dose-dependent suppressors (such as M and N) or stimulators (mainly NV), indicating novel targets for the design of more efficient vaccination strategies.

Back From the Brink: Alterations in B and T Cell Responses Modulate Recovery of Rainbow Trout From Chronic Immunopathological Tetracapsuloides bryosalmonae Infection

Proliferative kidney disease (PKD) caused by the myxozoan parasite Tetracapsuloides bryosalmonae is one of the most serious infectious diseases negatively impacting farmed and wild salmonids throughout Europe and North America. PKD pathogenesis results in a massive B cell proliferation and dysregulation with aberrant immunoglobulin production and plasma cell differentiation along with a decrease in myeloid cells and inhibition of innate pathways. Despite the huge immunopathological reaction in the kidney during infection, under specific conditions, fish can survive and return to full fitness. Fish are unique in this ability to recover renal structure and functionality from extensive tissue damage in contrast to mammals. However, only limited knowledge exists regarding the host immune response coinciding with PKD recovery. Moreover, almost no studies of the immune response during disease recovery exist in fish. We utilized the rainbow trout-T. bryosalmonae system as an immunological model of disease recovery. Our results demonstrated that recovery is preceded by an intense immune response at the transcript level, decreasing parasite burden, and an increased degree of kidney inflammation. Later in the recovery phase, the immune response transpired with a significant decrease in lymphocytes and an increase in myeloid cells. These lymphocytes populations contained lower levels of B cells comparative to the control in the anterior and posterior kidney. Additionally, there was downregulation of several transcripts used as markers for plasma cells (blimp1, igt sec, igm sec, igd sec, and cd38) and T cell subsets (cd4, cd8α, cd8β, and tcrβ). The decrease in these T cell transcripts significantly correlated with decreasing parasite intensity. Alternatively, there was strong upregulation of pax-5 and igt mem. This suggests a change in B cell processes during the recovery phase relative to clinical PKD may be necessary for the host to re-establish homeostasis in terms of an arrest in the dominant antibody like response transitioning to a transcriptional profile associated with resting B cells. The knowledge generated here in combination with earlier studies illuminates the full power of analyzing the entire trajectory of disease from the normal healthy state to recovery enabling the measurement of an immune response to pinpoint a specific disease stage.

Proliferative kidney disease in Alaskan salmonids with evidence that pathogenic myxozoans may be emerging north

Proliferative kidney disease (PKD) of salmonids, a chronic immunopathology caused by the myxozoan parasite Tetracapsuloides bryosalmonae, is exacerbated by increased water temperatures. PKD causes economic concerns to trout farmers and contributes to the decline of wild salmonid populations in North America and Europe. The parasite occurs as far north as Norway and Iceland in Europe and was confirmed from California to southern British Columbia in the American continent. In mid-September 2011 adult chum salmon (Oncorhynchus keta) were sampled from Kantishna River, a tributary to Yukon River in Alaska. Clinical PKD was diagnosed based on the macroscopic appearance of mottled kidneys that were uniformly swollen and by the detection of tumultuous histozoic extrasporogonic and coelozoic sporogonic stages of T. bryosalmonae in renal tissue by histopathology. Archived samples provided the molecular confirmation and local strain identification, representing the first confirmed case of PKD in wild adult chum salmon, also co-infected with Parvicapsula minibicornis that represents another novel myxozoan detection in Alaska. Our investigation was extended to another case from August/September 1997, with mortality following furunculosis and ectoparasite co-infections, in sockeye salmon (Oncorhynchus nerka) pre-smolts net-pen reared in English Bay Lakes, Alaska. Immunohistochemistry on archived histological preparations confirmed T. bryosalmonae sporogonic and extrasporogonic stages, indicating a severe to resolving PKD, with concomitant Chloromyxum spp. infection. Those cases provide the first documentation that this parasite is present in Alaska and causes PKD in wild and cultured salmonids in the region. The known geographic range of T. bryosalmonae can be extended to ~267 km south of the Arctic Circle, representing the northernmost detection in America. Given the vast size of Alaska and small resident population, it is likely that T. bryosalmonae remained undetected, but more recently became evident due to the clinical manifestation of PKD, possibly linked to increasing water temperatures reported at the sample locations.