Divergent antiviral roles of amphibian (Xenopus laevis) macrophages elicited by colony-stimulating factor-1 and interleukin-34

Ingestion of polyethylene terephthalate microplastic water contaminants by Xenopus laevis tadpoles negatively affects their resistance to ranavirus infection and antiviral immunity

Small plastic debris (0.1 μm-5 mm) or microplastics (MPs) have become major pollutants of aquatic ecosystems worldwide and studies suggest that MPs exposure can pose serious threats to human and wildlife health. However, to date the potential biological impacts of MPs accumulating in low amount in tissues during early life remains unclear. Here, for a more realistic assessment, we have used environmentally representative, mildly weathered, polyethylene terephthalate microplastics (PET MPs), cryomilled (1-100 μm) and fluorescently labelled. We leveraged the amphibian Xenopus laevis tadpoles as an animal model to define the biodistribution of PET MPs and determine whether exposure to PET MPs induce perturbations of antiviral immunity. Exposure to PET MPs for 1-14 days resulted in detectable PET MPs biodistribution in intestine, gills, liver, and kidney as determined by fluorescence microscopy on whole mount tissues. PET MPs accumulation rate in tissues was further evaluated via a novel in situ enzymatic digestion and subsequent filtration using silicon nanomembranes, which shows that PET MPs rapidly accumulate in tadpole intestine, liver and kidneys and persist over a week. Longer exposure (1 month) of tadpoles to relatively low concentration of PET MPs (25 μg/ml) significantly increased susceptibility to viral infection and altered innate antiviral immunity without inducing overt inflammation. This study provides evidence that exposure to MPs negatively impact immune defenses of aquatic vertebrates.

The amphibian immune system

Amphibians are at the forefront of bridging the evolutionary gap between mammals and more ancient, jawed vertebrates. Currently, several diseases have targeted amphibians and understanding their immune system has importance beyond their use as a research model. The immune system of the African clawed frog, Xenopus laevis, and that of mammals is well conserved. We know that several features of the adaptive and innate immune system are very similar for both, including the existence of B cells, T cells and innate-like T cells. In particular, the study of the immune system at early stages of development is benefitted by studying X. laevis tadpoles. The tadpoles mainly rely on innate immune mechanisms including pre-set or innate-like T cells until after metamorphosis. In this review we lay out what is known about the innate and adaptive immune system of X. laevis including the lymphoid organs as well as how other amphibian immune systems are similar or different. Furthermore, we will describe how the amphibian immune system responds to some viral, bacterial and fungal insults. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Amphibian myelopoiesis

Macrophage-lineage cells are indispensable to immunity and physiology of all vertebrates. Amongst these, amphibians represent a key stage in vertebrate evolution and are facing decimating population declines and extinctions, in large part due to emerging infectious agents. While recent studies indicate that macrophages and related innate immune cells are critically involved during these infections, much remains unknown regarding the ontogeny and functional differentiation of these cell types in amphibians. Accordingly, in this review we coalesce what has been established to date about amphibian blood cell development (hematopoiesis), the development of key amphibian innate immune cells (myelopoiesis) and the differentiation of amphibian macrophage subsets (monopoiesis). We explore the current understanding of designated sites of larval and adult hematopoiesis across distinct amphibian species and consider what mechanisms may lend to these species-specific adaptations. We discern the identified molecular mechanisms governing the functional differentiation of disparate amphibian (chiefly Xenopus laevis) macrophage subsets and describe what is known about the roles of these subsets during amphibian infections with intracellular pathogens. Macrophage lineage cells are at the heart of so many vertebrate physiological processes. Thus, garnering greater understanding of the mechanisms responsible for the ontogeny and functionality of these cells in amphibians will lend to a more comprehensive view of vertebrate evolution.

The amphibian invitrome: Past, present, and future contributions to our understanding of amphibian immunity

Many amphibian populations are declining worldwide, and infectious diseases are a leading cause. Given the eminent threat infectious diseases pose to amphibian populations, there is a need to understand the host-pathogen-environment interactions that govern amphibian susceptibility to disease and mortality events. However, using animals in research raises an ethical dilemma, which is magnified by the alarming rates at which many amphibian populations are declining. Thus, in vitro study systems such as cell lines represent valuable tools for furthering our understanding of amphibian immune systems. In this review, we curate a list of the amphibian cell lines established to date (the amphibian invitrome), highlight how research using amphibian cell lines has advanced our understanding of the amphibian immune system, anti-ranaviral defence mechanisms, and Batrachochytrium dendrobatidis replication in host cells, and offer our perspective on how future use of amphibian cell lines can advance the field of amphibian immunology.

A comparison of amphibian (Xenopus laevis) tadpole and adult frog macrophages

The amphibian declines are compounded by emerging pathogens that often preferentially target distinct amphibian developmental stages. While amphibian immune responses remain relatively unexplored, macrophage (Mφ)-lineage cells are believed to be important to both amphibian host defenses and to their pathogen infection strategies. As such, a greater understanding of tadpole and adult amphibian Mφ functionality is warranted. Mφ biology is interdependent of interleukin-34 (IL-34) and colony-stimulating factor-1 (CSF-1) cytokines and we previously showed that CSF-1- and IL-34-derived Mφs of the Xenopus laevis frog are morphologically, transcriptionally, and functionally distinct. Presently, we directly compared the cytology and transcriptomes of X. laevis tadpole and frog CSF-1- and IL-34-Mφs. Our results indicate that tadpole and frog CSF-1-Mφs possess greater non-specific esterase activity, typically associated with Mφ-lineage cells. By contrast, both tadpole and frog IL-34-Mφs have greater specific esterase activity, which is typically attributed to granulocyte-lineage cells. Our comparisons of tadpole CSF-1-Mφ transcriptomes with those of tadpole IL-34-Mφs indicate that the two tadpole populations possess significantly different transcriptional profiles of immune and non-immune genes. The frog CSF-1-Mφ gene expression profiles are likewise significantly disparate from those of frog IL-34-Mφs. Compared to their respective tadpole Mφ subtypes, frog CSF-1- and IL-34-Mφs exhibited greater expression of genes associated with antigen presentation. Conversely, compared to their frog Mφ counterparts, tadpole CSF-1- and IL-34-Mφs possessed greater levels of select Fc-like receptor genes. Presumably, these cytological and transcriptional differences manifest in distinct biological roles for these respective tadpole and frog Mφ subtypes.

Molecular diversity and functional implication of amphibian interferon complex: Remarking immune adaptation in vertebrate evolution

Cross-species comparison of vertebrate genomes has unraveled previously unknown complexities of interferon (IFN) systems in amphibian species. Recent genomic curation revealed that amphibian species have evolved expanded repertoires of four types of intron-containing IFN genes akin to those seen in jawed fish, intronless type I IFNs and intron-containing type III IFNs akin to those seen in amniotes, as well as uniquely intronless type III IFNs. This appears to be the case with at least ten analyzed amphibian species; with distinct species encoding diverse repertoires of these respective IFN gene subsets. Amphibians represent a key stage in vertebrate evolution, and in this context offer a unique perspective into the divergent and converged pathways leading to the emergence of distinct IFN families and groups. Recent studies have begun to unravel the roles of amphibian IFNs during these animals' immune responses in general and during their antiviral responses, in particular. However, the pleiotropic potentials of these highly expanded amphibian IFN repertoires warrant further studies. Based on recent reports and our omics analyses using Xenopus models, we posit that amphibian IFN complex may have evolved novel functions, as indicated by their extensive molecular diversity. Here, we provide an overview and an update of the present understanding of the amphibian IFN complex in the context of the evolution of vertebrate immune systems. A greater understanding of the amphibian IFN complex will grant new perspectives on the evolution of vertebrate immunity and may yield new measures by which to counteract the global amphibian declines.

Interleukin 34 in Disease Progressions: A Comprehensive Review

IL-34, a cytokine, discovered a decade before and is known to be a colony stimulating factor CSF-1 receptor (CSF-1R) ligand. Along with CSF-1R, it also interacts with syndecan-1 receptors and protein-tyrosine phosphatase (PTP-ζ). Hence, IL-34 takes part in a number of biological activities owing to its involvement in different signaling pathways. This review was done to analyze the recent studies on the functions of IL-34 in progression of diseases. The role of IL-34 under the physiological and pathological settings is studied by reviewing current data. In the last ten years, studies suggested that the IL-34 was involved in the regulation of morbid states such as inflammatory diseases, infections, transplant rejection, autoimmune diseases, neurologic diseases, and cancer. In general, the involvement of IL-34 is observed in many serious health ailments like metabolic diseases, heart diseases, infections and even cancer. As such, IL-34 can be regarded as a therapeutic target, potential biomarker or as a therapeutic tool, which ought to be assessed in future research activities.

Endogenous Retroviruses Augment Amphibian (Xenopus laevis) Tadpole Antiviral Protection

The global amphibian declines are compounded by infections with members of the Ranavirus genus such as Frog Virus 3 (FV3). Premetamorphic anuran amphibians are believed to be significantly more susceptible to FV3 while this pathogen targets the kidneys of both pre- and postmetamorphic animals. Paradoxically, FV3-challenged Xenopus laevis tadpoles exhibit lower kidney viral loads than adult frogs. Presently, we demonstrate that X. laevis tadpoles are intrinsically more resistant to FV3 kidney infections than cohort-matched metamorphic and postmetamorphic froglets and that this resistance appears to be epigenetically conferred by endogenous retroviruses (ERVs). Using a X. laevis kidney-derived cell line, we show that enhancing ERV gene expression activates cellular double-stranded RNA-sensing pathways, resulting in elevated mRNA levels of antiviral interferon (IFN) cytokines and thus greater anti-FV3 protection. Finally, our results indicate that large esterase-positive myeloid-lineage cells, rather than renal cells, are responsible for the elevated ERV/IFN axis seen in the tadpole kidneys. This conclusion is supported by our observation that CRISPR-Cas9 ablation of colony-stimulating factor-3 results in abolished homing of these myeloid cells to tadpole kidneys, concurrent with significantly abolished tadpole kidney expression of both ERVs and IFNs. We believe that the manuscript marks an important step forward in understanding the mechanisms controlling amphibian antiviral defenses and thus susceptibility and resistance to pathogens like FV3. IMPORTANCE Global amphibian biodiversity is being challenged by pathogens like the Frog Virus 3 (FV3) ranavirus, underlining the need to gain a greater understanding of amphibian antiviral defenses. While it was previously believed that anuran (frog/toad) amphibian tadpoles are more susceptible to FV3, we demonstrated that tadpoles are in fact more resistant to this virus than metamorphic and postmetamorphic froglets. We showed that this resistance is conferred by large myeloid cells within the tadpole kidneys (central FV3 target), which possess an elevated expression of endogenous retroviruses (ERVs). In turn, these ERVs activate cellular double-stranded RNA-sensing pathways, resulting in a greater expression of antiviral interferon cytokines, thereby offering the observed anti-FV3 protection.

The Roles of Amphibian (Xenopus laevis) Macrophages during Chronic Frog Virus 3 Infections

Infections by Frog Virus 3 (FV3) and other ranavirus genus members are significantly contributing to global amphibian decline. The Xenopus laevis frog is an ideal research platform upon which to study the roles of distinct frog leukocyte populations during FV3 infections. Frog macrophages (MΦs) are integrally involved during FV3 infection, as they facilitate viral dissemination and persistence but also participate in immune defense against this pathogen. In turn, MΦ differentiation and functionality depend on the colony-stimulating factor-1 receptor (CSF-1R), which is ligated by CSF-1 and iterleukin-34 (IL-34) cytokines. Our past work indicated that X. laevis CSF-1 and IL-34 give rise to morphologically and functionally distinct frog MΦ subsets, and that these CSF-1- and IL-34-MΦs respectively confer susceptibility and antiviral resistance to FV3. Because FV3 targets the frog kidneys and establishes chronic infections therein, presently we examined the roles of the frog CSF-1- and IL-34-MΦs in seeding and maintaining these chronic kidney infections. Our findings indicate that the frog CSF-1-MΦs result in more prominent kidney FV3 infections, which develop into greater reservoirs of lingering FV3 marked by infiltrating leukocytes, fibrosis, and overall immunosuppressive states. Moreover, the antiviral effects of IL-34-MΦs are short-lived and are lost as FV3 infections progress.

Prior induction of cellular antiviral pathways limits frog virus 3 replication in two permissive Xenopus laevis skin epithelial-like cell lines

Frog virus 3 (FV3) causes mortality in a range of amphibian species. Despite the importance of the skin epithelium as a first line of defence against FV3, the interaction between amphibian skin epithelial cells and FV3 remains largely uncharacterized. Here, we used newly established Xenopus laevis skin epithelial-like cell lines, Xela DS2 and Xela VS2, to study the susceptibility and permissiveness of frog skin epithelial cells to FV3, and the innate immune antiviral and proinflammatory gene regulatory responses of these cells to FV3. Both cell lines are susceptible and permissive to FV3, yet do not exhibit appreciable transcript levels of scavenger receptors thought to be used by FV3 for cellular entry. Xela DS2 and Xela VS2 upregulate antiviral and proinflammatory cytokine transcripts in response to poly(I:C) but not to FV3 or UV-inactivated FV3. Poly(I:C) pretreatment limits FV3 replication and FV3-induced cytopathic effects in both cell lines. Thus, Xela DS2 and Xela VS2 can support FV3 replication, represent in vitro systems to investigate antiviral responses of frog skin epithelial cells, and can serve as novel tools for screening compounds that initiate effective antiviral programs to limit FV3 replication.

Amphibian (Xenopus laevis) Tadpoles and Adult Frogs Differ in Their Antiviral Responses to Intestinal Frog Virus 3 Infections

The global amphibian declines are compounded by ranavirus infections such as Frog Virus 3 (FV3), and amphibian tadpoles more frequently succumb to these pathogens than adult animals. Amphibian gastrointestinal tracts represent a major route of ranavirus entry, and viral pathogenesis often leads to hemorrhaging and necrosis within this tissue. Alas, the differences between tadpole and adult amphibian immune responses to intestinal ranavirus infections remain poorly defined. As interferon (IFN) cytokine responses represent a cornerstone of vertebrate antiviral immunity, it is pertinent that the tadpoles and adults of the anuran Xenopus laevis frog mount disparate IFN responses to FV3 infections. Presently, we compared the tadpole and adult X. laevis responses to intestinal FV3 infections. Our results indicate that FV3-challenged tadpoles mount more robust intestinal type I and III IFN responses than adult frogs. These tadpole antiviral responses appear to be mediated by myeloid cells, which are recruited into tadpole intestines in response to FV3 infections. Conversely, myeloid cells bearing similar cytology already reside within the intestines of healthy (uninfected) adult frogs, possibly accounting for some of the anti-FV3 resistance of these animals. Further insight into the differences between tadpole and adult frog responses to ranaviral infections is critical to understanding the facets of susceptibility and resistance to these pathogens.

Herpesvirus-Associated Proliferative Skin Disease in Frogs and Toads: Proposed Pathogenesis

A comparative study was carried out on common and agile frogs (Rana temporaria and R. dalmatina) naturally infected with ranid herpesvirus 3 (RaHV3) and common toads (Bufo bufo) naturally infected with bufonid herpesvirus 1 (BfHV1) to investigate common pathogenetic pathways and molecular mechanisms based on macroscopic, microscopic, and ultrastructural pathology as well as evaluation of gene expression. Careful examination of the tissue changes, supported by in situ hybridization, at different stages of development in 6 frogs and 14 toads revealed that the skin lesions are likely transient, and part of a tissue cycle necessary for viral replication in the infected hosts. Transcriptomic analysis, carried out on 2 naturally infected and 2 naïve common frogs (Rana temporaria) and 2 naturally infected and 2 naïve common toads (Bufo bufo), revealed altered expression of genes involved in signaling and cell remodeling in diseased animals. Finally, virus transcriptomics revealed that both RaHV3 and BfHV1 had relatively high expression of a putative immunomodulating gene predicted to encode a decoy receptor for tumor necrosis factor in the skin of the infected hosts. Thus, the comparable lesions in infected frogs and toads appear to reflect a concerted epidermal and viral cycle, with presumptive involvement of signaling and gene remodeling host and immunomodulatory viral genes.

The Dual Role of Microglia in Blood-Brain Barrier Dysfunction after Stroke

It is well-known that stroke is one of the leading causes of death and disability all over the world. After a stroke, the blood-brain barrier subsequently breaks down. The BBB consists of endothelial cells surrounded by astrocytes. Microglia, considered the long-living resident immune cells of the brain, play a vital role in BBB function. M1 microglia worsen BBB disruption, while M2 microglia assist in repairing BBB damage. Microglia can also directly interact with endothelial cells and affect BBB permeability. In this review, we are going to discuss the mechanisms responsible for the dual role of microglia in BBB dysfunction after stroke.

Exploring the relationships between amphibian (Xenopus laevis) myeloid cell subsets

The differentiation of distinct leukocyte subsets is governed by lineage-specific growth factors that elicit disparate expression of transcription factors and markers by the developing cell populations. For example, macrophages (Mφs) and granulocytes (Grns) arise from common granulocyte-macrophage progenitors in response to distinct myeloid growth factors. In turn, myelopoiesis of the Xenopus laevis anuran amphibian appears to be unique to other studied vertebrates in several respects while the functional differentiation of amphibian Mφs and Grns from their progenitor cells remains poorly understood. Notably, the expression of colony stimulating factor-1 receptor (CSF-1R) or CSF-3R on granulocyte-macrophage progenitors marks their commitment to Mφ- or Grn-lineages, respectively. CSF-1R is activated by the colony stimulating factor-1 (CSF-1) and interleukin (IL-34) cytokines, resulting in morphologically and functionally distinct Mφ cell types. Conversely, CSF-3R is ligated by CSF-3 in a process indispensable for granulopoiesis. Presently, we explore the relationships between X. laevis CSF-1-Mφs, IL-34-Mφs and CSF-3-Grns by examining their expression of key lineage-specific transcription factor and myeloid marker genes as well as their enzymology. Our findings suggest that while the CSF-1- and IL-34-Mφs share some commonalities, the IL-34-Mφs possess transcriptional patterns more akin to the CSF-3-Grns. IL-34-Mφs also possess robust expression of dendritic cell-associated transcription factors and surface marker genes, further underlining the difference between this cell type and the CSF-1-derived frog Mφ subset. Moreover, the three myeloid populations differ in their respective tartrate-resistant acid phosphatase, specific- and non-specific esterase activity. Together, this work grants new insights into the developmental relatedness of these three frog myeloid subsets.

Mudskipper interleukin-34 modulates the functions of monocytes/macrophages via the colony-stimulating factor-1 receptor 1

Interleukin-34 (IL-34) is a novel cytokine that plays an important role in innate immunity and inflammatory processes by binding to the colony-stimulating factor-1 receptor (CSF-1R). However, information on the function of IL-34 in fish remains limited. In the present study, we identified an IL-34 homolog from mudskippers (Boleophthalmus pectinirostris). In silico analysis showed that the mudskipper IL-34 (BpIL-34) was similar to other known IL-34 variants in sequence and structure and was most closely related to an orange-spotted grouper (Epinephelus coioides) homolog. BpIL-34 transcripts were constitutively expressed in various tissues, with the highest level of expression found in the brain. Edwardsiella tarda infection significantly up-regulated the mRNA expression of BpIL-34 in the mudskipper tissues. The recombinant mature BpIL-34 peptide (rBpIL-34) was purified and used to produce anti-rBpIL-34 IgG. Western blot analysis combined with PNGase F digestion revealed that native BpIL-34 in monocytes/macrophages (MOs/MФs) was N-glycosylated. In vitro, rBpIL-34 treatment enhanced the phagocytotic and bactericidal activity of mudskipper MOs/MФs, as well as the mRNA expression of pro-inflammatory cytokines like tumor necrosis factor α (BpTNF-α) and BpIL-1β in these cells. Furthermore, the knockdown of mudskipper CSF-1R1 (BpCSF-1R1), but not mudskipper BpCSF-1R2, significantly inhibited the rBpIL-34-mediated enhanced effect on MO/MФ function. In conclusion, our results indicate that mudskipper BpIL-34 modulates the functions of MOs/MФs via BpCSF-1R1.

Water Contaminants Associated With Unconventional Oil and Gas Extraction Cause Immunotoxicity to Amphibian Tadpoles

Chemicals associated with unconventional oil and gas (UOG) operations have been shown to contaminate surface and ground water with a variety of endocrine disrupting compounds (EDCs) inducing multiple developmental alteration in mice. However, little is known about the impacts of UOG-associated contaminants on amphibian health and resistance to an emerging ranavirus infectious disease caused by viruses in the genus Ranavirus, especially at the vulnerable tadpole stage. Here we used tadpoles of the amphibian Xenopus laevis and the ranavirus Frog virus 3 (FV3) as a model relevant to aquatic environment conservation research for investigating the immunotoxic effects of exposure to a mixture of 23 UOG-associated chemicals with EDC activity. Xenopus tadpoles were exposed to an equimass mixture of 23 UOG-associated chemicals (range from 0.1 to 10 µg/l) for 3 weeks prior to infection with FV3. Our data show that exposure to the UOG chemical mixture is toxic for tadpoles at ecological doses of 5 to 10 µg/l. Lower doses significantly altered homeostatic expression of myeloid lineage genes and compromised tadpole responses to FV3 through expression of TNF-α, IL-1β, and Type I IFN genes, correlating with an increase in viral load. Exposure to a subset of 6 UOG chemicals was still sufficient to perturb the antiviral gene expression response. These findings suggest that UOG-associated water pollutants at low but environmentally relevant doses have the potential to induce acute alterations of immune function and antiviral immunity.

The amphibian (Xenopus laevis) colony-stimulating factor-1 and interleukin-34-derived macrophages possess disparate pathogen recognition capacities

Pathogens such as the Frog Virus 3 (FV3) ranavirus are contributing to the worldwide amphibian declines. While amphibian macrophages (Mϕs) are central to the immune defenses against these viruses, the pathogen recognition capacities of disparate amphibian Mϕ subsets remain unexplored. In turn, Mϕ differentiation and functionality are interdependent on the colony-stimulating factor-1 receptor (CSF-1R), which is ligated by colony-stimulating factor-1 (CSF-1) and the unrelated interleukin-34 (IL-34) cytokines. Notably, the Xenopus laevis frog CSF-1- and IL-34-derived Mϕs are functionally distinct, and while the CSF-1-Mϕs are more susceptible to FV3, the IL-34-Mϕs are highly resistant to this pathogen. Here, we elucidate the pathogen recognition capacities of CSF-1- and IL-34-differentiated Mϕs by evaluating their baseline transcript levels of key pathogen pattern recognition receptors (PRRs). Compared to the frog CSF-1-Mϕs, their IL-34-Mϕs exhibited greater expression of PRR genes associated with viral recognition as well as PRR genes known for recognizing bacterial pathogen-associated molecular patterns (PAMPs). By contrast, the CSF-1-Mϕs displayed greater expression of toll-like receptors (TLRs) that are absent in humans. Moreover, although the two Mϕ types possessed similar expression of most downstream PRR signaling components, they exhibited distinct outcomes upon stimulation with hallmark PAMPs, as measured by their tumor necrosis factor-alpha and interferon-7 gene expression. Remarkably, stimulation with a TLR2/6 agonist conferred FV3 resistance to the otherwise susceptible CSF-1-Mϕs while treatment with a TLR9 agonist significantly ablated the IL-34-Mϕ resistance to FV3. These changes in Mϕ-FV3 susceptibility and resistance appeared to be linked to changes in their expression of key immune genes. Greater understanding of the amphibian macrophage pathogen-recognition capacities will lend to further insights into the pathogen-associated causes of the amphibian declines.

Functional characterisation of interleukin 34 in grass carp Ctenopharyngodon idella

Interleukin (IL) 34 plays an important role in regulating macrophage functions and inflammation process. IL-34 homologues have recently been discovered in fish but the functions have not been studied. In this study, an IL-34 homologue was identified in grass carp Ctenopharyngodon idella and its bioactivities were investigated. The grass carp IL-34 was constitutively expressed in tissues, with the highest expression detected in spleen. It could be up-regulated in spleen after infection with F. cloumnare and grass carp reovirus II, and in primary head kidney leucocytes by recombinant IL-4/13B. The recombinant IL-34 produced in bacteria and HEK293T cells showed stimulatory effect on the expression of IL-1β, IL-6 and IL-8 but inhibited expression of IL-10 and TGF-β1 in primary head kidney macrophages. The results demonstrate that IL-34 is a proinflammatory cytokine in grass carp.

Critical Role of an MHC Class I-Like/Innate-Like T Cell Immune Surveillance System in Host Defense against Ranavirus (Frog Virus 3) Infection

Besides the central role of classical Major Histocompatibility Complex (MHC) class Ia-restricted conventional Cluster of Differentiation 8 (CD8) T cells in antiviral host immune response, the amphibian Xenopus laevis critically rely on MHC class I-like (mhc1b10.1.L or XNC10)-restricted innate-like (i)T cells (iVα6 T cells) to control infection by the ranavirus Frog virus 3 (FV3). To complement and extend our previous reverse genetic studies showing that iVα6 T cells are required for tadpole survival, as well as for timely and effective adult viral clearance, we examined the conditions and kinetics of iVα6 T cell response against FV3. Using a FV3 knock-out (KO) growth-defective mutant, we found that upregulation of the XNC10 restricting class I-like gene and the rapid recruitment of iVα6 T cells depend on detectable viral replication and productive FV3 infection. In addition, by in vivo depletion with XNC10 tetramers, we demonstrated the direct antiviral effector function of iVα6 T cells. Notably, the transitory iV6 T cell defect delayed innate interferon and cytokine gene response, resulting in long-lasting negative inability to control FV3 infection. These findings suggest that in Xenopus and likely other amphibians, an immune surveillance system based on the early activation of iT cells by non-polymorphic MHC class-I like molecules is important for efficient antiviral immune response.

Biological functions and clinical implications of interleukin-34 in inflammatory diseases

Interleukin (IL)-34 is a recently discovered cytokine and ligand of the colony-stimulating factor (CSF)-1 receptor. Although CSF-1 and IL-34 share similar biological properties, their expression patterns and downstream signaling pathways are distinct. IL-34 can influence differentiation and has functions in multiple cell types (e.g., dendritic cells, monocytes, macrophages). In the pathological conditions, IL-34 is induced by pro-inflammatory stimuli (e.g., cytokines, pathogen-associated molecular patterns, and infection). Current evidence shows that IL-34 is a critical player in inflammatory response and is involved in the pathogenesis of inflammatory autoimmune dysfunction. Therefore, IL-34 may be a promising clinical biomarker and therapeutic target for treating inflammatory related disorders. In this article, we review the advances in biological functions of IL-34 and our understanding of its role in the development of inflammatory diseases as well as therapeutic applications.

Class A Scavenger Receptors Are Used by Frog Virus 3 During Its Cellular Entry

Frog virus 3 (FV3) is the type species of the genus Ranavirus (family Iridoviridae). FV3 and FV3-like viruses are globally distributed infectious agents with the capacity to replicate in three vertebrate classes (teleosts, amphibians, and reptiles). At the cellular level, FV3 and FV3-like viruses can infect cells from virtually all vertebrate classes. To date, the cellular receptors that are involved in the FV3 entry process are unknown. Class A scavenger receptors (SR-As) are a family of evolutionarily conserved cell-surface receptors that bind a wide range of chemically distinct polyanionic ligands and can function as cellular receptors for other DNA viruses, including vaccinia virus and herpes simplex virus. The present study aimed to determine whether SR-As are involved in FV3 cellular entry. By using well-defined SR-A competitive and non-competitive ligand-blocking assays and absolute qPCR, we demonstrated that the SR-A competitive ligands drastically reduced the quantities of cell-associated viral loads in frog cells. Moreover, inducing the expression of a human SR-AI in an SR-A null cell line significantly increased FV3⁻cell association. Together, our results indicate that SR-As are utilized by FV3 during the cellular entry process.

Human interleukin-34-derived macrophages have increased resistance to HIV-1 infection

The establishment of latent HIV-1 reservoirs in terminally differentiated cells represents a major impediment to the success of antiretroviral therapies. Notably, macrophages (Mϕs) are susceptible to HIV-1 infection and recent evidence suggests that they may be involved in long-term HIV-1 persistence. While the extensive functional heterogeneity seen across the Mϕ cell lineage parallels the spectrum of HIV-1 susceptibility reported across these cell subsets, the facets of Mϕ HIV-1 resistance and susceptibility remain to be fully defined. Notably, the differentiation of most Mϕ subsets depends on signaling through the macrophage colony-stimulating factor receptor (M-CSFR), which in addition to M-CSF, is now known to bind the unrelated interleukin-34 (IL-34) cytokine. The biological need for two M-CSFR ligands awaits full elucidation. Here, we report that Mϕs differentiated from human peripheral blood monocytes with IL-34 are substantially more resistant to HIV-1 infection than M-CSF-derived Mϕs. Moreover, while both Mϕ subsets express comparable surface protein levels of the HIV-1 receptor and co-receptor, CD4 and CCR5 respectively, the IL-34-Mϕs express significantly greater levels of pertinent restriction factor genes, potentially accounting for their greater resistance to HIV-1 infection than that observed in M-CSF-Mϕs. Together, our findings underline previously unexplored differentiation pathways resulting in HIV-1-susceptible and resistant Mϕ subsets and pave the way for further research that may overcome one of the last major hurdles in developing more successful antiretroviral therapy.

Interleukin-34, a comprehensive review

IL-34 is a novel cytokine that was identified in 2008 in a comprehensive proteomic analysis as a tissue-specific ligand of CSF-1 receptor (CSF-1R). IL-34 exists in all vertebrates including fish, amphibians, birds, and mammals, showing high conservation among species. Structurally, IL-34 belongs to the short-chain helical hematopoietic cytokine family but shows no apparent consensus structural domains, motifs, or sequence homology with other cytokines. IL-34 is synthesized as a secreted homodimeric glycoprotein that binds to the extracellular domains of CSF-1R and receptor-type protein-tyrosine phosphatase-zeta (PTP-ζ) in addition to the chondroitin sulfate chains of syndecan-1. These interactions result in activating several signaling pathways that regulate major cellular functions, including proliferation, differentiation, survival, metabolism, and cytokine/chemokine expression in addition to cellular adhesion and migration. In the steady state, IL-34 contributes to the development and maintenance of specific myeloid cell subsets in a tissue-specific manner: Langerhans cells in the skin and microglia in the brain. In pathological conditions, changes in IL-34 expression-increased or decreased-are involved in disease pathogenesis and correlate with progression, severity, and chronicity. One decade after its discovery, IL-34 has been introduced as a newcomer to the big family of interleukins with specific physiological functions, critical pathological roles, and promising clinical applications in disease diagnosis and treatment. In this review, we celebrate the 10th anniversary of IL-34 discovery, introducing its biological characteristics, and discussing the importance of IL-34 signaling network in health and disease.

Flow Cytometric Analysis of Xenopus Immune Cells

Flow cytometry is a versatile analytical platform capable of multiparameter analysis of more than a thousand individual cells per second. This technique is used to measure the physical and chemical characteristics of individual cells in a heterogeneous cell suspension as they pass through one or multiple lasers. Physical properties, such as size and internal complexity, are recorded as light scattering at different angles and are expressed as forward- and side-scatter, respectively. Following light excitation, fluorochromes conjugated to antibodies or intercalated with different cellular components reemit light at distinct wavelengths. This can identify a broad array of cell specific antigens, further defining distinct cell subsets based on activation, lineage, and developmental stage. The combination of labels that can be used depends on the laser used to excite the fluorochromes and on the detector and available antibodies. With the growing number of Xenopus-specific antibodies, flow cytometry can be used to identify, isolate, and characterize distinct immune cell subsets. In this protocol, different methods to obtain single-cell suspensions from various X. laevis tissues are described. A standard three-parameter procedure defining viability and two cell-surface markers is then described.

Mechanisms of Fish Macrophage Antimicrobial Immunity

Overcrowding conditions and temperatures shifts regularly manifest in large-scale infections of farmed fish, resulting in economic losses for the global aquaculture industries. Increased understanding of the functional mechanisms of fish antimicrobial host defenses is an important step forward in prevention of pathogen-induced morbidity and mortality in aquaculture setting. Like other vertebrates, macrophage-lineage cells are integral to fish immune responses and for this reason, much of the recent fish immunology research has focused on fish macrophage biology. These studies have revealed notable similarities as well as striking differences in the molecular strategies by which fish and higher vertebrates control their respective macrophage polarization and functionality. In this review, we address the current understanding of the biological mechanisms of teleost macrophage functional heterogeneity and immunity, focusing on the key cytokine regulators that control fish macrophage development and their antimicrobial armamentarium.

Differentiation-dependent antiviral capacities of amphibian (Xenopus laevis) macrophages

Infections by ranaviruses such as Frog virus 3 (Fv3), are significantly contributing to worldwide amphibian population declines. Notably, amphibian macrophages (Mφs) are important to both the Fv3 infection strategies and the immune defense against this pathogen. However, the mechanisms underlying amphibian Mφ Fv3 susceptibility and resistance remain unknown. Mφ differentiation is mediated by signaling through the colony-stimulating factor-1 receptor (CSF-1R) which is now known to be bound not only by CSF-1, but also by the unrelated interleukin-34 (IL-34) cytokine. Pertinently, amphibian (Xenopus laevis) Mφs differentiated by CSF-1 and IL-34 are highly susceptible and resistant to Fv3, respectively. Accordingly, in the present work, we elucidate the facets of this Mφ Fv3 susceptibility and resistance. Because cellular resistance to viral replication is marked by expression of antiviral restriction factors, it was intuitive to find that IL-34-Mφs possess significantly greater mRNA levels of select restriction factor genes than CSF-1-Mφs. Xenopodinae amphibians have highly expanded repertoires of antiviral interferon (IFN) cytokine gene families, and our results indicated that in comparison with the X. laevis CSF-1-Mφs, the IL-34-Mφs express substantially greater transcripts of representative IFN genes, belonging to distinct gene family clades, as well as their cognate receptor genes. Finally, we demonstrate that IL-34-Mφ-conditioned supernatants confer IFN-mediated anti-Fv3 protection to the virally susceptible X. laevis kidney (A6) cell line. Together, this work underlines the differentiation pathways leading to Fv3-susceptible and -resistant amphibian Mφ populations and defines the molecular mechanisms responsible for these differences.

Immunoregulatory properties of the cytokine IL-34

Interleukin-34 is a cytokine with only partially understood functions, described for the first time in 2008. Although IL-34 shares very little homology with CSF-1 (CSF1, M-CSF), they share a common receptor CSF-1R (CSF-1R) and IL-34 has also two distinct receptors (PTP-ζ) and CD138 (syndecan-1). To make the situation more complex, IL-34 has also been shown as pairing with CSF-1 to form a heterodimer. Until now, studies have demonstrated that this cytokine is released by some tissues that differ to those where CSF-1 is expressed and is involved in the differentiation and survival of macrophages, monocytes, and dendritic cells in response to inflammation. The involvement of IL-34 has been shown in areas as diverse as neuronal protection, autoimmune diseases, infection, cancer, and transplantation. Our recent work has demonstrated a new and possible therapeutic role for IL-34 as a Foxp3+ Treg-secreted cytokine mediator of transplant tolerance. In this review, we recapitulate most recent findings on IL-34 and its controversial effects on immune responses and address its immunoregulatory properties and the potential of targeting this cytokine in human.

Immune roles of amphibian (Xenopus laevis) tadpole granulocytes during Frog Virus 3 ranavirus infections

Infections by Frog Virus 3 (FV3) and other ranaviruses (RVs) are contributing to the amphibian declines, while the mechanisms controlling anuran tadpole susceptibility and adult frog resistance to RVs, including the roles of polymorphonuclear granulocytes (PMNs) during anti-FV3 responses, remain largely unknown. Since amphibian kidneys represent an important FV3 target, the inability of amphibian (Xenopus laevis) tadpoles to mount effective kidney inflammatory responses to FV3 is thought to contribute to their susceptibility. Here we demonstrate that a recombinant X. laevis granulocyte colony-stimulating factor (G-CSF) generates PMNs with hallmark granulocyte morphology. Tadpole pretreatment with G-CSF prior to FV3 infection reduces animal kidney FV3 loads and extends their survival. Moreover, G-CSF-derived PMNs are resistant to FV3 infection and express high levels of TNFα in response to this virus. Notably, FV3-infected tadpoles fail to recruit G-CSFR expressing granulocytes into their kidneys, suggesting that they lack an integral inflammatory effector population at this site.

Xenopus-FV3 host-pathogen interactions and immune evasion

We first review fundamental insights into anti-ranavirus immunity learned with the Xenopus laevis/ranavirus FV3 model that are generally applicable to ectothermic vertebrates. We then further investigate FV3 genes involved in immune evasion. Focusing on FV3 knockout (KO) mutants defective for a putative viral caspase activation and recruitment domain-containing (CARD)-like protein (Δ64R-FV3), a β-hydroxysteroid dehydrogenase homolog (Δ52L-FV3), and an immediate-early18kDa protein (FV3-Δ18K), we assessed the involvement of these viral genes in replication, dissemination and interaction with peritoneal macrophages in tadpole and adult frogs. Our results substantiate the role of 64R and 52L as critical immune evasion genes, promoting persistence and dissemination in the host by counteracting type III IFN in tadpoles and type I IFN in adult frogs. Comparably, the substantial accumulation of genome copy numbers and exacerbation of type I and III IFN gene expression responses but deficient release of infectious virus suggests that 18K is a viral regulatory gene.

Amphibian (Xenopus laevis) tadpoles and adult frogs mount distinct interferon responses to the Frog Virus 3 ranavirus

Infections of amphibians by Frog Virus 3 (FV3) and other ranavirus genus members are significantly contributing to the amphibian declines, yet much remains unknown regarding amphibian antiviral immunity. Notably, amphibians represent an important step in the evolution of antiviral interferon (IFN) cytokines as they are amongst the first vertebrates to possess both type I and type III IFNs. Accordingly, we examined the roles of type I and III IFNs in the skin of FV3-challenged amphibian Xenopus laevis) tadpoles and adult frogs. Interestingly, FV3-infected tadpoles mounted type III IFN responses, whereas adult frogs relied on type I IFN immunity. Subcutaneous administration of type I or type III IFNs offered short-term protection of tadpoles against FV3 and these type I and type III IFNs induced the expression of distinct antiviral genes in the tadpole skin. Moreover, subcutaneous injection of tadpoles with type III IFN significantly extended their survival and reduced FV3 dissemination.

Evolutionary Aspects of Macrophages Polarization

Macrophages constitute a heterogeneous population of myeloid cells that are essential for maintaining homeostasis and as a first line of innate responders controlling and organizing host defenses against pathogens. Monocyte-macrophage lineage cells are among the most functionally diverse and plastic cells of the immune system. They undergo specific activation into functionally distinct phenotypes in response to immune signals and microbial products. In mammals, macrophage functional heterogeneity is defined by two activation states, M1 and M2, which represent two polar ends of a continuum exhibiting pro-inflammatory and tissue repair activities, respectively. While the ancient evolutionary origin of macrophages as phagocytic defenders is well established, the evolutionary roots of the specialized division of macrophages into subsets with polarized activation phenotypes is less well defined. Accordingly, this chapter focuses on recent advances in the understanding of the evolution of macrophage polarization and functional heterogeneity with a focus on ectothermic vertebrates.

Regulation of Embryonic and Postnatal Development by the CSF-1 Receptor

Macrophages are found in all tissues and regulate tissue morphogenesis during development through trophic and scavenger functions. The colony stimulating factor-1 (CSF-1) receptor (CSF-1R) is the major regulator of tissue macrophage development and maintenance. In combination with receptor activator of nuclear factor κB (RANK), the CSF-1R also regulates the differentiation of the bone-resorbing osteoclast and controls bone remodeling during embryonic and early postnatal development. CSF-1R-regulated macrophages play trophic and remodeling roles in development. Outside the mononuclear phagocytic system, the CSF-1R directly regulates neuronal survival and differentiation, the development of intestinal Paneth cells and of preimplantation embryos, as well as trophoblast innate immune function. Consistent with the pleiotropic roles of the receptor during development, CSF-1R deficiency in most mouse strains causes embryonic or perinatal death and the surviving mice exhibit multiple developmental and functional deficits. The CSF-1R is activated by two dimeric glycoprotein ligands, CSF-1, and interleukin-34 (IL-34). Homozygous Csf1-null mutations phenocopy most of the deficits of Csf1r-null mice. In contrast, Il34-null mice have no gross phenotype, except for decreased numbers of Langerhans cells and microglia, indicating that CSF-1 plays the major developmental role. Homozygous inactivating mutations of the Csf1r or its ligands have not been reported in man. However, heterozygous inactivating mutations in the Csf1r lead to a dominantly inherited adult-onset progressive dementia, highlighting the importance of CSF-1R signaling in the brain.

The unique myelopoiesis strategy of the amphibian Xenopus laevis

Myeloid progenitors reside within specific hematopoietic organs and commit to progenitor lineages bearing megakaryocyte/erythrocyte (MEP) or granulocyte/macrophage potentials (GMP) within these sites. Unlike other vertebrates, the amphibian Xenopus laevis committed macrophage precursors are absent from the hematopoietic subcapsular liver and instead reside within their bone marrow. Presently, we demonstrate that while these frogs' liver-derived cells are unresponsive to recombinant forms of principal X. laevis macrophage (colony-stimulating factor-1; CSF-1) and granulocyte (CSF-3) growth factors, bone marrow cells cultured with CSF-1 and CSF-3 exhibit respectively archetypal macrophage and granulocyte morphology, gene expression and functionalities. Moreover, we demonstrate that liver, but not bone marrow cells possess erythropoietic capacities when stimulated with a X. laevis erythropoietin. Together, our findings indicate that X. laevis retain their MEP within the hematopoietic liver while sequestering their GMP to the bone marrow, thus marking a very novel myelopoietic strategy as compared to those seen in other jawed vertebrate species.

Water Temperature Affects Susceptibility to Ranavirus

The occurrence of emerging infectious diseases in wildlife populations is increasing, and changes in environmental conditions have been hypothesized as a potential driver. For example, warmer ambient temperatures might favor pathogens by providing more ideal conditions for propagation or by stressing hosts. Our objective was to determine if water temperature played a role in the pathogenicity of an emerging pathogen (ranavirus) that infects ectothermic vertebrate species. We exposed larvae of four amphibian species to a Frog Virus 3 (FV3)-like ranavirus at two temperatures (10 and 25°C). We found that FV3 copies in tissues and mortality due to ranaviral disease were greater at 25°C than at 10°C for all species. In a second experiment with wood frogs (Lithobates sylvaticus), we found that a 2°C change (10 vs. 12°C) affected ranaviral disease outcomes, with greater infection and mortality at 12°C. There was evidence that 10°C stressed Cope's gray tree frog (Hyla chrysoscelis) larvae, which is a species that breeds during summer-all individuals died at this temperature, but only 10% tested positive for FV3 infection. The greater pathogenicity of FV3 at 25°C might be related to faster viral replication, which in vitro studies have reported previously. Colder temperatures also may decrease systemic infection by reducing blood circulation and the proportion of phagocytes, which are known to disseminate FV3 through the body. Collectively, our results indicate that water temperature during larval development may play a role in the emergence of ranaviruses.

Amphibian macrophage development and antiviral defenses

Macrophage lineage cells represent the cornerstone of vertebrate physiology and immune defenses. In turn, comparative studies using non-mammalian animal models have revealed that evolutionarily distinct species have adopted diverse molecular and physiological strategies for controlling macrophage development and functions. Notably, amphibian species present a rich array of physiological and environmental adaptations, not to mention the peculiarity of metamorphosis from larval to adult stages of development, involving drastic transformation and differentiation of multiple new tissues. Thus it is not surprising that different amphibian species and their respective tadpole and adult stages have adopted unique hematopoietic strategies. Accordingly and in order to establish a more comprehensive view of these processes, here we review the hematopoietic and monopoietic strategies observed across amphibians, describe the present understanding of the molecular mechanisms driving amphibian, an in particular Xenopus laevis macrophage development and functional polarization, and discuss the roles of macrophage-lineage cells during ranavirus infections.

Mechanisms of amphibian macrophage development: characterization of the Xenopus laevis colony-stimulating factor-1 receptor

Macrophage-lineage cells are indispensable to vertebrate homeostasis and immunity. In turn, macrophage development is largely regulated through colony-stimulating factor-1 (CSF1) binding to its cognate receptor (CSF1R). To study amphibian monopoiesis, we identified and characterized the X. laevis CSF1R cDNA transcript. Quantitative analysis revealed that CSF1R tissue gene expression increased with X. laevis development, with greatest transcript levels detected in the adult lung, spleen and liver tissues. Notably, considerable levels of CSF1R mRNA were also detected in the regressing tails of metamorphosing animals, suggesting macrophage involvement in this process, and in the adult bone marrow; corroborating the roles for this organ in Xenopus monopoiesis. Following animal infections with the ranavirus Frog Virus 3 (FV3), both tadpole and adult X. laevis exhibited increased kidney CSF1R gene expression. Conversely, while FV3-infected tadpoles increased their spleen and liver CSF1R mRNA levels, the FV3-challenged adults did not. Notably, FV3 induced elevated bone marrow CSF1R expression, and while stimulation of tadpoles with heat-killed E. coli had no transcriptional effects, bacterial stimulation of adult frogs resulted in significantly increased spleen, liver and bone marrow CSF1R expression. We produced the X. laevis CSF1R in recombinant form (rXlCSF1R) and determined, via in vitro cross-linking studies, that two molecules of rXlCSF1R bound the dimeric rXlCSF1. Finally, administration of rXlCSF1R abrogated the rXlCSF1-induced tadpole macrophage recruitment and differentiation as well as bacterial and FV3-elicited peritoneal leukocyte accumulation. This work marks a step towards garnering greater understanding of the unique mechanisms governing amphibian macrophage biology.

Distinct functional roles of amphibian (Xenopus laevis) colony-stimulating factor-1- and interleukin-34-derived macrophages

Although Mϕ represent the most primordial immune cell subsets, the mechanisms governing their functional heterogeneity remain poorly defined. However, it is well established that the CSF-1 cytokine contributes to monopoiesis and to this heterogeneity, whereas the unrelated IL-34 also binds the CSF-1R toward poorly understood immunologic roles. To delineate the molecular and evolutionary basis behind vertebrate Mϕ functional heterogeneity, we performed comprehensive transcriptional and functional studies of amphibian (Xenopus laevis) BM (in vitro) and PER (in vivo) Mϕ derived by rXlCSF-1 and rXlIL-34. Our findings indicate that these amphibian cytokines promote morphologically and functionally distinct Mϕ populations. Mϕ induced by rXlCSF-1 possess more robust iNOS gene expression, are substantially more phagocytic, display greater NO responses, and exhibit enhanced bactericidal capacities. By contrast, rXlIL-34-derived Mϕ express greater levels of Arg-1 and NADPH oxidase components and possess greater respiratory burst responses. Most notably, whereas CSF-1 Mϕ are highly susceptible to the emerging FV3 ranavirus, rXlIL-34 Mϕ exhibit potent antiviral activity against this pathogen, which is dependent on reactive oxygen production. This work marks an advance in our understanding of the possible mechanisms governing vertebrate Mϕ functional heterogeneity.

Nonclassical MHC-Restricted Invariant Vα6 T Cells Are Critical for Efficient Early Innate Antiviral Immunity in the Amphibian Xenopus laevis

Nonclassical MHC class Ib-restricted invariant T (iT) cell subsets are attracting interest because of their potential to regulate immune responses against various pathogens. The biological relevance and evolutionary conservation of iT cells have recently been strengthened by the identification of iT cells (invariant Vα6 [iVα6]) restricted by the nonclassical MHC class Ib molecule XNC10 in the amphibian Xenopus laevis. These iVα6 T cells are functionally similar to mammalian CD1d-restricted invariant NKT cells. Using the amphibian pathogen frog virus 3 (FV3) in combination with XNC10 tetramers and RNA interference loss of function by transgenesis, we show that XNC10-restricted iVα6 T cells are critical for early antiviral immunity in adult X. laevis. Within hours following i.p. FV3 infection, iVα6 T cells were specifically recruited from the spleen into the peritoneum. XNC10 deficiency and concomitant lack of iVα6 T cells resulted in less effective antiviral and macrophage antimicrobial responses, which led to impaired viral clearance, increased viral dissemination, and more pronounced FV3-induced kidney damage. Together, these findings imply that X. laevis XNC10-restricted iVα6 T cells play important roles in the early anti-FV3 response and that, as has been suggested for mammalian invariant NKT cells, they may serve as immune regulators polarizing macrophage effector functions toward more effective antiviral states.

Use of cell lines and primary cultures to explore the capacity of rainbow trout to be a host for frog virus 3 (FV3)

The capacity of rainbow trout, Oncorhynchus mykiss, to be a host for frog virus 3 (FV3) was evaluated at the cellular level. Cell cultures from this species were tested for their ability to express FV3 major capsid protein (MCP) gene, to develop cytopathic effect (CPE), and to produce FV3. After FV3 addition, MCP transcripts were detected in six of six cell lines and in primary macrophage cultures. CPE developed in all cell culture systems, except primary lymphocytes. For the macrophage cell line, RTS11, and primary macrophages, cell death was by apoptosis because DNA laddering and Annexin staining were detected. By contrast, markers of apoptosis did not accompany CPE in three epithelial cell lines from the gill (RTgill-W1), intestine (RTgut-GC), and liver (RTL-W1) and in two fibroblast cell lines from gonads (RTG-2) and skin (RTHDF). Therefore, FV3 was able to enter and begin replicating in several cell types. Yet, FV3 was produced in only two cell lines, RTG-2 and RTL-W1, and only modestly. Overall, these results suggest that if tissue accessibility were possible, FV3 would have the capacity to induce injury, but the ability to replicate would be limited, likely making rainbow trout a poor host for FV3.

Inflammation-induced reactivation of the ranavirus Frog Virus 3 in asymptomatic Xenopus laevis

Natural infections of ectothermic vertebrates by ranaviruses (RV, family Iridoviridae) are rapidly increasing, with an alarming expansion of RV tropism and resulting die-offs of numerous animal populations. Notably, infection studies of the amphibian Xenopus laevis with the ranavirus Frog Virus 3 (FV3) have revealed that although the adult frog immune system is efficient at controlling RV infections, residual quiescent virus can be detected in mononuclear phagocytes of otherwise asymptomatic animals following the resolution of RV infections. It is noteworthy that macrophage-lineage cells are now believed to be a critical element in the RV infection strategy. In the present work, we report that inflammation induced by peritoneal injection of heat-killed bacteria in asymptomatic frogs one month after infection with FV3 resulted in viral reactivation including detectable viral DNA and viral gene expression in otherwise asymptomatic frogs. FV3 reactivation was most prominently detected in kidneys and in peritoneal HAM56+ mononuclear phagocytes. Notably, unlike adult frogs that typically clear primary FV3 infections, a proportion of the animals succumbed to the reactivated FV3 infection, indicating that previous exposure does not provide protection against subsequent reactivation in these animals.