Receptor (SLAM [CD150]) recognition and the V protein sustain swift lymphocyte-based invasion of mucosal tissue and lymphatic organs by a morbillivirus

Human SLAM-adapted canine distemper virus can enter human peripheral blood mononuclear cells and replicate in mice expressing human SLAM and defective for STAT1 expression

Canine distemper virus (CDV) is a member of the genus Morbillivirus with a worldwide distribution that causes fatal diseases in canids and marine mammals. In recent years, CDV has demonstrated the remarkable ability of pathogens to cross species barriers. The natural host range of CDV has expanded from Canidae to Primates, presumably attributed to ecological shifts and the emergence of viral variants. Therefore, it is important to investigate whether CDV can infect humans by adapting to the human signalling lymphocyte activation molecule (hSLAM) receptor to cross the species barrier. Through successive passaging and plaque cloning of a CDV wild-type strain (5804PeH) in Vero cells expressing hSLAM (Vero-hSLAM), we obtained an hSLAM adaptive strain, 5804PeH-VhS. The adapted CDV strain exhibited a D540G mutation within the receptor-binding domain (RBD) of the haemagglutinin (H) protein. The HD540G mutation has enhanced cell-cell fusion activity in Vero-hSLAM cells. This adaptation allowed the CDV strain to infect human peripheral blood mononuclear cells (PBMCs), particularly T lymphocytes and inhibited lymphocyte proliferation. Additionally, this strain could replicate in the lymphoid tissues of transgenic mice that express the hSLAM receptor, causing viraemia. However, the adapted strain did not spread to the epithelial cells or the central nervous system of the mice. While this adaptation indicates a potential risk, there is no definitive evidence that the virus can spread among humans.

Genome sequencing of canine distemper virus isolates from unvaccinated dogs in Mongolia

Canine distemper virus (CDV) triggers a severe, often fatal disease in dogs and wildlife known as canine distemper (CD). Prior research has noted significant genetic diversity and recombination among CDV isolates from different geographical regions, potentially contributing to vaccine failures. Despite this, no genetic characterization of Mongolian CDVs has been conducted. This study, isolated CDVs from three unvaccinated dogs: two 10-month-old mixed-breeds and an 18-month-old Samoyed. All exhibited CD symptoms and subsequently died. Virus isolation was conducted using Vero/dog SLAM cells, with genome sequencing performed via nanopore technology. The mixed-breed dogs were infected with non-recombinant CDV isolates, forming a sister clade to the Asia-1 lineage prevalent in Asia. The Samoyed was infected with a non-recombinant CDV isolate, classifying as Asia-4 lineage sporadically reported in some Asian countries. This sequencing data offers foundational information on genetic diversity, aiding CD control measure development and benefiting future Eurasia and Asian studies.

Genetic diversity accelerates canine distemper virus adaptation to ferrets

RNA viruses adapt rapidly to new host environments by generating highly diverse genome sets, so-called "quasispecies." Minor genetic variants promote their rapid adaptation, allowing for the emergence of drug-resistance or immune-escape mutants. Understanding these adaptation processes is highly relevant to assessing the risk of cross-species transmission and the safety and efficacy of vaccines and antivirals. We hypothesized that genetic memory within a viral genome population facilitates rapid adaptation. To test this, we investigated the adaptation of the Morbillivirus canine distemper virus to ferrets and compared an attenuated, Vero cell-adapted virus isolate with its recombinant derivative over consecutive ferret passages. Although both viruses adapted to the new host, the reduced initial genetic diversity of the recombinant virus resulted in delayed disease onset. The non-recombinant virus gradually increased the frequencies of beneficial mutations already present at very low frequencies in the input virus. In contrast, the recombinant virus first evolved de novo mutations to compensate for the initial fitness impairments. Importantly, while both viruses evolved different sets of mutations, most mutations found in the adapted non-recombinant virus were identical to those found in a previous ferret adaptation experiment with the same isolate, indicating that mutations present at low frequency in the original virus stock serve as genetic memory. An arginine residue at position 519 in the carboxy terminus of the nucleoprotein shared by all adapted viruses was found to contribute to pathogenesis in ferrets. Our work illustrates the importance of genetic diversity for adaptation to new environments and identifies regions with functional relevance.IMPORTANCEWhen viruses encounter a new host, they can rapidly adapt to this host and cause disease. How these adaptation processes occur remains understudied. Morbilliviruses have high clinical and veterinary relevance and are attractive model systems to study these adaptation processes. The canine distemper virus is of particular interest, as it exhibits a broader host range than other morbilliviruses and frequently crosses species barriers. Here, we compared the adaptation of an attenuated virus and its recombinant derivative to that of ferrets. Pre-existing mutations present at low frequency allowed faster adaptation of the non-recombinant virus compared to the recombinant virus. We identified a common point mutation in the nucleoprotein that affected the pathogenesis of both viruses. Our study shows that genetic memory facilitates environmental adaptation and that erasing this genetic memory by genetic engineering results in delayed and different adaptation to new environments, providing an important safety aspect for the generation of live-attenuated vaccines.

Rational attenuation of canine distemper virus (CDV) to develop a morbillivirus animal model that mimics measles in humans

Morbilliviruses are members of the family Paramyxoviridae and are known for their ability to cause systemic disease in a variety of mammalian hosts. The prototypic morbillivirus, measles virus (MeV), infects humans and still causes morbidity and mortality in unvaccinated children and young adults. Experimental infection studies in non-human primates have contributed to the understanding of measles pathogenesis. However, ethical restrictions call for the development of new animal models. Canine distemper virus (CDV) infects a wide range of animals, including ferrets, and its pathogenesis shares many features with measles. However, wild-type CDV infection is almost always lethal, while MeV infection is usually self-limiting. Here, we made five recombinant CDVs, predicted to be attenuated, and compared their pathogenesis to the non-attenuated recombinant CDV in a ferret model. Three viruses were insufficiently attenuated based on clinical signs, fatality, and systemic infection, while one virus was too attenuated. The last candidate virus caused a self-limiting infection associated with transient viremia and viral dissemination to all lymphoid tissues, was shed transiently from the upper respiratory tract, and did not result in acute neurological signs. Additionally, an in-depth phenotyping of the infected white blood cells showed lower infection percentages in all lymphocyte subsets when compared to the non-attenuated CDV. In conclusion, infection models using this candidate virus mimic measles and can be used to study pathogenesis-related questions and to test interventions for morbilliviruses in a natural host species.IMPORTANCEMorbilliviruses are transmitted via the respiratory route but cause systemic disease. The viruses use two cellular receptors to infect myeloid, lymphoid, and epithelial cells. Measles virus (MeV) remains an important cause of morbidity and mortality in humans, requiring animal models to study pathogenesis or intervention strategies. Experimental MeV infections in non-human primates are restricted by ethical and practical constraints, and animal morbillivirus infections in natural host species have been considered as alternatives. Inoculation of ferrets with wild-type canine distemper virus (CDV) has been used for this purpose, but in most cases, the virus overwhelms the immune system and causes highly lethal disease. Introduction of an additional transcription unit and an additional attenuating point mutation in the polymerase yielded a candidate virus that caused self-limiting disease with transient viremia and virus shedding. This rationally attenuated CDV strain can be used for experimental morbillivirus infections in ferrets that reflect measles in humans.

Canine Distemper Virus Pathogenesis in the Ferret Model

Canine distemper virus (CDV) is a highly contagious pathogen within the morbillivirus genus infecting a wide range of different carnivore species. The virus shares most biological features with other closely related morbilliviruses, including clinical signs, tissue tropism, and replication cycle in the respective host organisms.In the laboratory environment, experimental infections of ferrets with CDV were established as a potent surrogate model for the analysis of several aspects of the biology of the human morbillivirus, measles virus (MeV). The animals are naturally susceptible to CDV and display severe clinical signs resembling the disease seen in patients infected with MeV. As seen with MeV, CDV infects immune cells and is thus associated with a strong transient immunosuppression. Here we describe several methods to evaluate viral load and parameters of immunosuppression in blood-circulating immune cells isolated from CDV-infected animals.

Exploring LIPIDs for their potential to improves bioavailability of lipophilic drugs candidates: A review

This review aims to provide a thorough examination of the benefits, challenges, and advancements in utilizing lipids for more effective drug delivery, ultimately contributing to the development of innovative approaches in pharmaceutical science. Lipophilic drugs, characterized by low aqueous solubility, present a formidable challenge in achieving effective delivery and absorption within the human body. To address this issue, one promising approach involves harnessing the potential of lipids. Lipids, in their diverse forms, serve as carriers, leveraging their unique capacity to enhance solubility, stability, and absorption of these challenging drugs. By facilitating improved intestinal solubility and selective lymphatic absorption of porously permeable drugs, lipids offer an array of possibilities for drug delivery. This versatile characteristic not only bolsters the pharmacological efficacy of drugs with low bioavailability but also contributes to enhanced therapeutic performance, ultimately reducing the required dose size and associated costs. This comprehensive review delves into the strategic formulation approaches that employ lipids as carriers to ameliorate drug solubility and bioavailability. Emphasis is placed on the critical considerations of lipid type, composition, and processing techniques when designing lipid-based formulations. This review meticulously examines the multifaceted challenges that come hand in hand with lipid-based formulations for lipophilic drugs, offering an insightful perspective on future trends. Regulatory considerations and the broad spectrum of potential applications are also thoughtfully discussed. In summary, this review presents a valuable repository of insights into the effective utilization of lipids as carriers, all aimed at elevating the bioavailability of lipophilic drugs.

Oncolytic virotherapies for pediatric tumors

INTRODUCTION

Many pediatric patients with malignant tumors continue to suffer poor outcomes. The current standard of care includes maximum safe surgical resection followed by chemotherapy and radiation which may be associated with considerable long-term morbidity. The emergence of oncolytic virotherapy (OVT) may provide an alternative or adjuvant treatment for pediatric oncology patients.

AREAS COVERED

We reviewed seven virus types that have been investigated in past or ongoing pediatric tumor clinical trials: adenovirus (AdV-tk, Celyvir, DNX-2401, VCN-01, Ad-TD-nsIL-12), herpes simplex virus (G207, HSV-1716), vaccinia (JX-594), reovirus (pelareorep), poliovirus (PVSRIPO), measles virus (MV-NIS), and Senecavirus A (SVV-001). For each virus, we discuss the mechanism of tumor-specific replication and cytotoxicity as well as key findings of preclinical and clinical studies.

EXPERT OPINION

Substantial progress has been made in the past 10 years regarding the clinical use of OVT. From our review, OVT has favorable safety profiles compared to chemotherapy and radiation treatment. However, the antitumor effects of OVT remain variable depending on tumor type and viral agent used. Although the widespread adoption of OVT faces many challenges, we are optimistic that OVT will play an important role alongside standard chemotherapy and radiotherapy for the treatment of malignant pediatric solid tumors in the future.

Meningoencephalitis caused by concurrent infection with canine distemper virus and a unique Sarcocystis sp. in a gray fox

A deceased 9-wk-old male gray fox (Urocyon cinereoargenteus) with a history of decreased ambulation and diarrhea was submitted to the Texas A&M Veterinary Medical Diagnostic Laboratory. No significant gross findings were evident on postmortem examination. Histologically, the cerebrum and brainstem had mild necrotizing meningoencephalitis with protozoal schizonts and merozoites. Additionally, glial cells contained intracytoplasmic and intranuclear viral inclusion bodies. Sections of the cerebrum were positive for canine distemper virus (CDV) and negative for Sarcocystis neurona on immunohistochemistry. Bayesian analysis revealed that this Sarcocystis sp. clustered most closely with a clade of unnamed Sarcocystis sp. found in viperid snakes, with a posterior probability of 99%. CDV likely played a significant role in the expression of clinical sarcocystosis in this gray fox.

Approaches to identify canine distemper virus with neurological symptoms on the basis of molecular characterization of hemagglutinin and fusion genes

Canine distemper virus (CDV), which causes severe infections in all domestic and wild carnivores, is transmitted by all secretions and excretions of infected animals. Despite the regular vaccination against it, CDV still manages to circulate in nature and is a worldwide problem in dogs. For many years in the world, the virus managed to circulate in nature. The current investigation aims to identify and characterize CDV in dogs with neurological symptoms and to determine whether CNS symptoms and phylogenetic data might be used to differentiate between CDV strains. The medical records of 35 dogs with central nervous system (CNS) symptoms were examined. An ELISA kit was used to identify CDV-specific IgG antibodies in all of the dogs' serum samples. RT-PCR confirmed the presence of CDV nucleic acid in 30 of these dogs. Of the RT-PCR-positive samples, 6 were randomly chosen for further sequencing, sequence comparisons, and phylogenetic reconstructions. Genes encoding the Hemagglutinin (H) and Fusion (F) proteins were partly sequenced and compared to other CDVs from throughout the world, including vaccine strains. The maximum likelihood method was used to build a phylogenetic tree using CDV H and F gene nucleotide sequences. According to phylogenetic analysis of partial H and F gene nucleotide sequences, the field CDVs in this investigation were unique and different from the vaccine strain. The phylogenetic analysis indicated that all Turkish CDV strains that induced CNS symptoms belonged to the European CDV clade. While the intricacy of the CNS and the complexities of glycosylation pathways may provide significant challenges to infections, future research will bring significant benefits by identifying evolutionarily conserved activities of N-glycosylation in CDV-infected dogs.

Canine Distemper Virus Alters Defense Responses in an Ex Vivo Model of Pulmonary Infection

Canine distemper virus (CDV), belonging to the genus Morbillivirus, is a highly contagious pathogen. It is infectious in a wide range of host species, including domestic and wildlife carnivores, and causes severe systemic disease with involvement of the respiratory tract. In the present study, canine precision-cut lung slices (PCLSs) were infected with CDV (strain R252) to investigate temporospatial viral loads, cell tropism, ciliary activity, and local immune responses during early infection ex vivo. Progressive viral replication was observed during the infection period in histiocytic and, to a lesser extent, epithelial cells. CDV-infected cells were predominantly located within the bronchial subepithelial tissue. Ciliary activity was reduced in CDV-infected PCLSs, while viability remained unchanged when compared to controls. MHC-II expression was increased in the bronchial epithelium on day three postinfection. Elevated levels of anti-inflammatory cytokines (interleukin-10 and transforming growth factor-β) were observed in CDV-infected PCLSs on day one postinfection. In conclusion, the present study demonstrates that PCLSs are permissive for CDV. The model reveals an impaired ciliary function and an anti-inflammatory cytokine response, potentially fostering viral replication in the lung during the early phase of canine distemper.

Persistent and Severe Viral Replication in PBMCs with Moderate Immunosuppression Served an Alternative Novel Pathogenic Mechanism for Canine Morbillivirus

Measles virus and canine distemper virus (CDV) cause lethal infections in their respective hosts characterized by severe immunosuppression. To furtherly acknowledge the attenuated mechanisms of the regionally ongoing epidemic CDV isolates and provide novel perspectives for designing new vaccines and therapeutic drugs, a recombinant CDV rHBF-vacH was employed with a vaccine hemagglutinin (H) gene replacement by reverse genetics based on an infectious cDNA clone for the CDV wild-type HBF-1 strain. Interestingly, unlike previously published reports that a vaccine H protein completely changed a pathogenic wild-type CDV variant to be avirulent, rHBF-vacH was only partially attenuated by alleviating the degree of viral immunosuppression, and still caused 66.7% lethality in ferrets with a prolonged period of disease. Further comparisons of pathogenic mechanisms proved that the weaker but necessary invasions into peripheral blood mononuclear cells (PBMCs) of rHBF-vacH, and subsequently persistent viral replications in PBMCs and multiple organs, together contributed to its 66.7% mortality. In addition, despite significantly higher titers than the parent viruses, rHBF-vacH would not be a suitable candidate for a live vaccine, with great invasion and infection potentials of PBMCs from 16 tested kinds of host species. Altogether, sustained and severe viral replication in PBMCs with moderate immunosuppression was first proven to be an alternative novel pathogenic mechanism for CDV, which might help us to understand possible reasons for CDV fatal infections among domestic dogs and the highly susceptible wild species during natural transmission. IMPORTANCE Despite widespread vaccine campaigns for domestic dogs, CDV remained an important infectious disease in vaccinated carnivores and wild species. In recent years, the regionally ongoing epidemic CDV isolates have emphasized conservation threats to, and potentially disastrous epidemics in, endangered species worldwide. However, little is known about how to deal with the CDV variants constantly regional epidemic. In this study, we employed a recombinant CDV rHBF-vacH with a vaccine H gene replacement in a CDV wild-type HBF-1 context to attenuate the epidemic CDV variant to design a new vaccine candidate. Interestingly, rHBF-vacH was only partially attenuated by alleviating the degree of viral immunosuppression, and still caused 66.7% lethality in ferrets by weaker but necessary invasions into PBMCs, and subsequently persistent and severe viral replications in PBMCs. Significantly higher virus titers of rHBF-vacH in vitro might indicate the rapid cell-to-cell spreads in vivo that indirectly contribute to fatal infections of rHBF-vacH in ferrets.

Comparison of some cytokines, acute phase proteins and citrulline levels in healthy and canine distemper infected dogs

Canine distemper virus (CDV) is the etiological agent of severe disease in domestic and wild carnivores. Clinical diagnosis of CDV is challenging because of its similarity to other canine respiratory and intestinal diseases. We aimed to determine certain cytokine (interleukin [IL]-1β, IL-2, IL-4, IL-6, IL-10, and tumor necrosis factor-α [TNF-α]), interferon (IFN)-γ, canine serum amyloid A (SAA), and canine citrulline (CIT) levels for the first time in CDV-positive dogs. For this purpose, 10 CDV-positive dogs with compatible clinical findings (i.e., neurological symptoms such as tremors and myoclonus, ocular and nasal discharge, and wheezing) and 10 healthy dogs based on the clinical examinations and rapid test results were enrolled. It was observed that the CIT, INF-γ, IL-1β, IL-2, IL-6, and TNF-α levels were significantly decreased in the CDV-positive dogs than that of the healthy ones (P<0.05). As a result, it was observed that CDV causes immunosuppression and accordingly, the inflammatory response might cause decreased cytokine and acute-phase protein synthesis. Therefore, it was concluded that further investigation of inflammatory pathways and CIT interactions may provide crucial clinical information at different stages of CDV, and aforementioned parameters may serve as important biomarkers for CDV in terms of demonstrating the presence of immunosuppression.

Multiple Receptors Involved in Invasion and Neuropathogenicity of Canine Distemper Virus: A Review

The canine distemper virus (CDV) is a morbillivirus that infects a broad range of terrestrial carnivores, predominantly canines, and is associated with high mortality. Similar to another morbillivirus, measles virus, which infects humans and nonhuman primates, CDV transmission from an infected host to a naïve host depends on two cellular receptors, namely, the signaling lymphocyte activation molecule (SLAM or CD150) and the adherens junction protein nectin-4 (also known as PVRL4). CDV can also invade the central nervous system by anterograde spread through olfactory nerves or in infected lymphocytes through the circulation, thus causing chronic progressive or relapsing demyelination of the brain. However, the absence of the two receptors in the white matter, primary cultured astrocytes, and neurons in the brain was recently demonstrated. Furthermore, a SLAM/nectin-4-blind recombinant CDV exhibits full cell-to-cell transmission in primary astrocytes. This strongly suggests the existence of a third CDV receptor expressed in neural cells, possibly glial cells. In this review, we summarize the recent progress in the study of CDV receptors, highlighting the unidentified glial receptor and its contribution to pathogenicity in the host nervous system. The reviewed studies focus on CDV neuropathogenesis, and neural receptors may provide promising directions for the treatment of neurological diseases caused by CDV. We also present an overview of other neurotropic viruses to promote further research and identification of CDV neural receptors.

Efficient Recovery of Attenuated Canine Distemper Virus from cDNA

To provide insights into the biology of the attenuated canine distemper virus (CDV) Onderstepoort (OP) strain (large plaque forming variant), design next-generation multivalent vaccines, or further investigate its promising potential as an oncolytic vector, we employed contemporary modifications to establish an efficient OP-CDV-based reverse genetics platform. Successful viral rescue was obtained however only upon recovery of a completely conserved charged residue (V13E) residing at the N-terminal region of the large protein (L). Although L-V13 and L-V13E did not display drastic differences in cellular localization and physical interaction with P, efficient polymerase complex (P+L) activity was recorded only with L-V13E. Interestingly, grafting mNeonGreen to the viral N protein via a P2A ribosomal skipping sequence (OP^neon) and its derivative V-protein-knockout variant (OP^neon-V^ko) exhibited delayed replication kinetics in cultured cells. Collectively, we established an efficient OP-CDV-based reverse genetics system that enables the design of various strategies potentially contributing to veterinary medicine and research.

Overcoming the Barrier of the Respiratory Epithelium during Canine Distemper Virus Infection

Canine distemper virus (CDV) is a highly contagious pathogen and is known to enter the host via the respiratory tract and disseminate to various organs. Current hypotheses speculate that CDV uses the homologous cellular receptors of measles virus (MeV), SLAM and nectin-4, to initiate the infection process. For validation, here, we established the well-differentiated air-liquid interface (ALI) culture model from primary canine tracheal airway epithelial cells. By applying the green fluorescent protein (GFP)-expressing CDV vaccine strain and recombinant wild-type viruses, we show that cell-free virus infects the airway epithelium mainly via the paracellular route and only after prior disruption of tight junctions by pretreatment with EGTA; this infection was related to nectin-4 but not to SLAM. Remarkably, when CDV-preinfected DH82 cells were cocultured on the basolateral side of canine ALI cultures grown on filter supports with a 1.0-μm pore size, cell-associated CDV could be transmitted via cell-to-cell contact from immunocytes to airway epithelial cultures. Finally, we observed that canine ALI cultures formed syncytia and started to release cell-free infectious viral particles from the apical surface following treatment with an inhibitor of the JAK/STAT signaling pathway (ruxolitinib). Our findings show that CDV can overcome the epithelial barrier through different strategies, including infection via immunocyte-mediated transmission and direct infection via the paracellular route when tight junctions are disrupted. Our established model can be adapted to other animals for studying the transmission routes and the pathogenicity of other morbilliviruses. IMPORTANCE Canine distemper virus (CDV) is not only an important pathogen of carnivores, but it also serves as a model virus for analyzing measles virus pathogenesis. To get a better picture of the different stages of infection, we used air-liquid interface cultures to analyze the infection of well-differentiated airway epithelial cells by CDV. Applying a coculture approach with DH82 cells, we demonstrated that cell-mediated infection from the basolateral side of well-differentiated epithelial cells is more efficient than infection via cell-free virus. In fact, free virus was unable to infect intact polarized cells. When tight junctions were interrupted by treatment with EGTA, cells became susceptible to infection, with nectin-4 serving as a receptor. Another interesting feature of CDV infection is that infection of well-differentiated airway epithelial cells does not result in virus egress. Cell-free virions are released from the cells only in the presence of an inhibitor of the JAK/STAT signaling pathway. Our results provide new insights into how CDV can overcome the barrier of the airway epithelium and reveal similarities and some dissimilarities compared to measles virus.

C Proteins: Controllers of Orderly Paramyxovirus Replication and of the Innate Immune Response

Particles of many paramyxoviruses include small amounts of proteins with a molecular weight of about 20 kDa. These proteins, termed “C”, are basic, have low amino acid homology and some secondary structure conservation. C proteins are encoded in alternative reading frames of the phosphoprotein gene. Some viruses express nested sets of C proteins that exert their functions in different locations: In the nucleus, they interfere with cellular transcription factors that elicit innate immune responses; in the cytoplasm, they associate with viral ribonucleocapsids and control polymerase processivity and orderly replication, thereby minimizing the activation of innate immunity. In addition, certain C proteins can directly bind to, and interfere with the function of, several cytoplasmic proteins required for interferon induction, interferon signaling and inflammation. Some C proteins are also required for efficient virus particle assembly and budding. C-deficient viruses can be grown in certain transformed cell lines but are not pathogenic in natural hosts. C proteins affect the same host functions as other phosphoprotein gene-encoded proteins named V but use different strategies for this purpose. Multiple independent systems to counteract host defenses may ensure efficient immune evasion and facilitate virus adaptation to new hosts and tissue environments.

A new canine distemper virus lineage identified from red pandas in China

Canine distemper virus (CDV) is a highly contagious virus that causes multi-systemic, sub-clinical to fatal diseases in a wide range of carnivore species. Based on the sequences of the haemagglutinin (H) gene, CDV strains have been classified into 18 major genetic lineages. In this study, we characterized the genomes of CDV isolated from the lungs of two dead red pandas in China. Histopathological and immunohistochemical analyses revealed damage due to viral infection in these lungs. The two strains showed a deep genetic distance from the other 18 recognized lineages (>4.6% at nucleotide level and >5.0% at amino acid level). The maximum clade credibility tree of the H- gene sequences showed that they belonged to an independent clade and had diverged a relatively long time ago from the Asia-4 lineage (since 1884). These results suggest that the analyzed strains belong to a new CDV lineage, which we designate as Asia-6. Our finding indicates that CDV infections in wildlife in China are complex and are a threat to endangered carnivores.

Lymphopenia Caused by Virus Infections and the Mechanisms Beyond

Viral infections can give rise to a systemic decrease in the total number of lymphocytes in the blood, referred to as lymphopenia. Lymphopenia may affect the host adaptive immune responses and impact the clinical course of acute viral infections. Detailed knowledge on how viruses induce lymphopenia would provide valuable information into the pathogenesis of viral infections and potential therapeutic targeting. In this review, the current progress of viruses-induced lymphopenia is summarized and the potential mechanisms and factors involved are discussed.

C Protein is Essential for Canine Distemper Virus Virulence and Pathogenicity in Ferrets

Paramyxoviruses, including members of the genus Morbillivirus, express accessory proteins with ancillary functions during viral replication. One of these, the C protein, is expressed from an alternate open reading frame (ORF) located in the P gene. The measles virus (MeV) C protein has been implicated in modulation of interferon signaling, but has more recently been shown to play a vital role in regulation of viral transcription and replication, preventing the excessive production of double-stranded RNA. Failure to do so, as seen with C-deficient MeV, leads to early activation of innate immune responses resulting in restriction of viral replication and attenuation in the host. One puzzling aspect of morbillivirus C protein biology has been the finding that a C-deficient canine distemper virus (CDV) generated with a similar mutagenesis strategy displayed no attenuation in ferrets, an animal model commonly used to evaluate CDV pathogenesis. To resolve how virus lacking this protein could maintain virulence, we re-visited the CDV C protein and found that truncated C proteins are expressed from the CDV gene using alternative downstream start codons even when the first start codon was disrupted. We introduced an additional point mutation abrogating expression of these truncated C proteins. A new CDV with this mutation was attenuated in vitro and led to increased activation of protein kinase R. It was also strongly attenuated in ferrets, inducing only mild disease in infected animals, thus replicating the phenotype of C-deficient MeV. Our results demonstrate the crucial role of morbillivirus C proteins in pathogenesis.IMPORTANCE The measles (MeV) and canine distemper viruses (CDV) express accessory proteins that regulate the host immune response and enhance replication. The MeV C protein is critical in preventing the generation of excess immunostimulatory double-stranded RNA. C protein-deficient MeV is strongly attenuated compared to wild-type virus, whereas CDV with a similarly disrupted C open reading frame is fully pathogenic. Here we show that CDV can compensate the disrupting mutations by expression of truncated, but apparently functional C proteins from several alternative start codons. We generated a new recombinant CDV that does not express these truncated C protein. This virus was attenuated both in cell culture and in ferrets, and finally resolves the paradox of the MeV and CDV C proteins, showing that both in fact have similar functions important for viral pathogenesis.

Mink SLAM V-Region V74I Substitutions Contribute to the Formation of Syncytia Induced by Canine Distemper Virus

The Signal lymphatic activation molecule (SLAM, also known as CD150) as the cellular receptor of canine distemper virus (CDV) plays an important role in the virus-host interaction. However, it is still unknown whether amino acid differences in the SLAM variable (V) region affect the formation of syncytia. Here, using raccoon dog SLAM (rSLAM) and mink SLAM (mSLAM), we performed SLAM-V homologous modeling, site-directed mutagenesis, and surface expression analysis, as well as a cell fusion assay, to study the interaction between SLAM and CDV. More specifically, our investigation focused on two amino acid residues (74 and 129) of SLAM, previously predicted to play a relevant role in receptor-ligand interaction. Our results indicated that only residues at position 60, 74, and 129 were different between rSLAM and mSLAM among the 29 amino acids that might interact with CDV H, and residues 74 and 129 were located in the interface region interacting with CDV H. The amino acid substitution at the positions of 74 have a significant effect on the expression of mSLAM. The SLAM-V74I mutation in mink significantly improved the cell fusion efficiency of CDV. In contrast, the SLAM-I74V mutation in the raccoon dog significantly decreased cell fusion efficiency. We conclude that residue 74 of SLAM plays an important role during the the formation of syncytia. Only when implementing CDV infection analysis, the rSLAM-Q129R can significantly decreased the mean number of syncytia, but the mSLAM-R129Q can't. Additionally, residue 60 show variability between rSLAM and mSLAM. We believe that our study makes a significant contribution to the literature because we provide molecular data, partially accounting for the differences in host membrane and virus interaction laying the foundation for further molecular work.

Comparison of Growth Characteristics and Genomics of Two Canine Distemper Virus Strains Isolated From Minks in China

Canine distemper (CD), caused by the CDV variant strain with H^I542N/Y549H, has become an epidemic in fur-bearing animals in China since 2012. To well understand the genomic and replicated characteristics of the CDV variants, we determined the viral growth kinetics and completed the genome sequences of two CDV strains, namely SDZC(17)M2 and LNDL(17)M4, isolated from CDV-infected minks from Shandong and Liaoning province in China, in 2017. SDZC(17)M2 showed higher viral titers and extensive syncytia in BHK-minkSLAM (BMS) cells than LNDL(17)M4. Although both two strains belong to the Asia-1 genotype and clustered an independent clade in the phylogenetic tree, SDZC(17)M2, harboring I542N/Y549H substitutions in the H protein, shared high identity (99.3–99.6% nt) with the other variant strains, whereas LNDL(17)M4, with the only Y549H substitution, shared a lower identity (97.7%–97.9% nt) with the other variant strains. Furthermore, a novel R223K substitution was identified in the conserved cleavage site (RRQRR → RRQKR) of the F protein in the SDZC(17)M2 strain. However, it which did not significantly affect the cell to cell fusion activity when combined with the CDV H/minkSLAM in BHK-21 cells. The key variations in the genome contributed to the virulence and the evolutionary trend need to be determined in the future.

Identifying Candidate Genetic Markers of CDV Cross-Species Pathogenicity in African Lions

Canine distemper virus (CDV) is a multi-host pathogen with variable clinical outcomes of infection across and within species. We used whole-genome sequencing (WGS) to search for viral markers correlated with clinical distemper in African lions. To identify candidate markers, we first documented single-nucleotide polymorphisms (SNPs) differentiating CDV strains associated with different clinical outcomes in lions in East Africa. We then conducted evolutionary analyses on WGS from all global CDV lineages to identify loci subject to selection. SNPs that both differentiated East African strains and were under selection were mapped to a phylogenetic tree representing global CDV diversity to assess if candidate markers correlated with documented outbreaks of clinical distemper in lions (n = 3). Of 54 SNPs differentiating East African strains, ten were under positive or episodic diversifying selection and 20 occurred in the clinical strain despite strong purifying selection at those loci. Candidate markers were in functional domains of the RNP complex (n = 19), the matrix protein (n = 4), on CDV glycoproteins (n = 5), and on the V protein (n = 1). We found mutations at two loci in common between sequences from three CDV outbreaks of clinical distemper in African lions; one in the signaling lymphocytic activation molecule receptor (SLAM)-binding region of the hemagglutinin protein and another in the catalytic center of phosphodiester bond formation on the large polymerase protein. These results suggest convergent evolution at these sites may have a functional role in clinical distemper outbreaks in African lions and uncover potential novel barriers to pathogenicity in this species.

Cross-species transmission and evolutionary dynamics of canine distemper virus during a spillover in African lions of Serengeti National Park

The outcome of pathogen spillover from a reservoir to a novel host population can range from a "dead-end" when there is no onward transmission in the recipient population, to epidemic spread and even establishment in new hosts. Understanding the evolutionary epidemiology of spillover events leading to discrete outcomes in novel hosts is key to predicting risk and can lead to a better understanding of the mechanisms of emergence. Here we use a Bayesian phylodynamic approach to examine cross-species transmission and evolutionary dynamics during a canine distemper virus (CDV) spillover event causing clinical disease and population decline in an African lion population (Panthera leo) in the Serengeti Ecological Region between 1993 and 1994. Using 21 near-complete viral genomes from four species we found that this large-scale outbreak was likely  ignited by a single cross-species spillover event from a canid reservoir to noncanid hosts <1 year before disease detection and explosive spread of CDV in lions. Cross-species transmission from other noncanid species probably fuelled the high prevalence of CDV across spatially structured lion prides. Multiple lines of evidence suggest that spotted hyenas (Crocuta crocuta) could have acted as the proximate source of CDV exposure in lions. We report 13 nucleotide substitutions segregating CDV strains found in canids and noncanids. Our results are consistent with the hypothesis that virus evolution played a role in CDV emergence in noncanid hosts following spillover during the outbreak, suggest that host barriers to clinical infection can limit outcomes of CDV spillover in novel host species.

Animal morbilliviruses and their cross-species transmission potential

Like measles virus (MV), whose primary hosts are humans, non-human animal morbilliviruses use SLAM (signaling lymphocytic activation molecule) and PVRL4 (nectin-4) expressed on immune and epithelial cells, respectively, as receptors. PVRL4's amino acid sequence is highly conserved across species, while that of SLAM varies significantly. However, non-host animal SLAMs often function as receptors for different morbilliviruses. Uniquely, human SLAM is somewhat specific for MV, but canine distemper virus, which shows the widest host range among morbilliviruses, readily gains the ability to use human SLAM. The host range for morbilliviruses is also modulated by their ability to counteract the host's innate immunity, but the risk of cross-species transmission of non-human animal morbilliviruses to humans could occur if MV is successfully eradicated.

Viral Pathogenesis, Recombinant Vaccines, and Oncolytic Virotherapy: Applications of the Canine Distemper Virus Reverse Genetics System

Canine distemper virus (CDV) is a highly contagious pathogen transmissible to a broad range of terrestrial and aquatic carnivores. Despite the availability of attenuated vaccines against CDV, the virus remains responsible for outbreaks of canine distemper (CD) with significant morbidity and mortality in domesticated and wild carnivores worldwide. CDV uses the signaling lymphocytic activation molecule (SLAM, or CD150) and nectin-4 (PVRL4) as entry receptors, well-known tumor-associated markers for several lymphadenomas and adenocarcinomas, which are also responsible for the lysis of tumor cells and apparent tumor regression. Thus, CDV vaccine strains have emerged as a promising platform of oncolytic viruses for use in animal cancer therapy. Recent advances have revealed that use of the CDV reverse genetic system (RGS) has helped increase the understanding of viral pathogenesis and explore the development of recombinant CDV vaccines. In addition, genetic engineering of CDV based on RGS approaches also has the potential of enhancing oncolytic activity and selectively targeting tumors. Here, we reviewed the host tropism and pathogenesis of CDV, and current development of recombinant CDV-based vaccines as well as their use as oncolytic viruses against cancers.

Measles: An Overview of a Re-Emerging Disease in Children and Immunocompromised Patients

Despite the availability of a safe and effective vaccine, in 2018, around 350,000 measles cases were reported worldwide, which resulted in an estimate of 142,300 deaths from measles. Additionally, in 2017, global measles cases spiked, causing the death of 110,000 people, mostly children under the age of 5 years and immunocompromised adults. The increase in measles incidence is caused by the ongoing reduction of vaccination coverage. This event has triggered public and scientific interest. For this reason, we reviewed the pathophysiology of measles infection, focusing on mechanisms by which the virus spreads systemically through the host organism. By reaching the lymphocytes from the airways through a "trojan horse" strategy, measles induces an immunosuppression status. H and F glycoproteins, both expressed in the envelope, ensure attachment of the virus to host cells and spreading from one cell to another by binding to several receptors, as described in detail. The severity of the disease depends both on the age and underlying conditions of patients as well as the social and health context in which epidemics spread, and is often burdened by sequelae and complications that may occur several years after infection. Particular attention was paid to special groups that are more susceptible to severe or atypical measles. An overview of microbiology, symptoms, diagnosis, prevention, and treatment completes and enriches the review.

Receptor-mediated cell entry of paramyxoviruses: Mechanisms, and consequences for tropism and pathogenesis

Research in the last decade has uncovered many new paramyxoviruses, airborne agents that cause epidemic diseases in animals including humans. Most paramyxoviruses enter epithelial cells of the airway using sialic acid as a receptor and cause only mild disease. However, others cross the epithelial barrier and cause more severe disease. For some of these viruses, the host receptors have been identified, and the mechanisms of cell entry have been elucidated. The tetrameric attachment proteins of paramyxoviruses have vastly different binding affinities for their cognate receptors, which they contact through different binding surfaces. Nevertheless, all input signals are converted to the same output: conformational changes that trigger refolding of trimeric fusion proteins and membrane fusion. Experiments with selectively receptor-blinded viruses inoculated into their natural hosts have provided insights into tropism, identifying the cells and tissues that support growth and revealing the mechanisms of pathogenesis. These analyses also shed light on diabolically elegant mechanisms used by morbilliviruses, including the measles virus, to promote massive amplification within the host, followed by efficient aerosolization and rapid spread through host populations. In another paradigm of receptor-facilitated severe disease, henipaviruses, including Nipah and Hendra viruses, use different members of one protein family to cause zoonoses. Specific properties of different paramyxoviruses, like neurotoxicity and immunosuppression, are now understood in the light of receptor specificity. We propose that research on the specific receptors for several newly identified members of the Paramyxoviridae family that may not bind sialic acid is needed to anticipate their zoonotic potential and to generate effective vaccines and antiviral compounds.

Protective Immunity against Canine Distemper Virus in Dogs Induced by Intranasal Immunization with a Recombinant Probiotic Expressing the Viral H Protein

Canine distemper virus (CDV) elicits a severe contagious disease in a broad range of hosts. CDV mortality rates are 50% in domestic dogs and 100% in ferrets. Its primary infection sites are respiratory and intestinal mucosa. This study aimed to develop an effective mucosal CDV vaccine using a non-antibiotic marked probiotic pPG^ΔCm-T7g10-EGFP-H/L. casei 393 strain expressing the CDV H protein. Its immunogenicity in BALB/c mice was evaluated using intranasal and oral vaccinations, whereas in dogs the intranasal route was used for vaccination. Our results indicate that this probiotic vaccine can stimulate a high level of secretory immunoglobulin A (sIgA)-based mucosal and IgG-based humoral immune responses in mice. SIgA levels in the nasal lavage and lungs were significantly higher in intranasally vaccinated mice than those in orally vaccinated mice. Both antigen-specific IgG and sIgA antibodies were effectively elicited in dogs through the intranasal route and demonstrated superior immunogenicity. The immune protection efficacy of the probiotic vaccine was evaluated by challenging the immunized dogs with virulent CDV 42 days after primary immunization. Dogs of the pPG^ΔCm-T7g10-EGFP-H/L. casei 393 group were completely protected against CDV. The proposed probiotic vaccine could be promising for protection against CDV infection in dogs.

Canine and Phocine Distemper Viruses: Global Spread and Genetic Basis of Jumping Species Barriers

Canine distemper virus (CDV) and phocine distemper (PDV) are closely-related members of the Paramyxoviridae family, genus morbillivirus, in the order Mononegavirales. CDV has a broad host range among carnivores. PDV is thought to be derived from CDV through contact between terrestrial carnivores and seals. PDV has caused extensive mortality in Atlantic seals and other marine mammals, and more recently has spread to the North Pacific Ocean. CDV also infects marine carnivores, and there is evidence of morbillivirus infection of seals and other species in Antarctica. Recently, CDV has spread to felines and other wildlife species in the Serengeti and South Africa. Some CDV vaccines may also have caused wildlife disease. Changes in the virus haemagglutinin (H) protein, particularly the signaling lymphocyte activation molecule (SLAM) receptor binding site, correlate with adaptation to non-canine hosts. Differences in the phosphoprotein (P) gene sequences between disease and non-disease causing CDV strains may relate to pathogenicity in domestic dogs and wildlife. Of most concern are reports of CDV infection and disease in non-human primates raising the possibility of zoonosis. In this article we review the global occurrence of CDV and PDV, and present both historical and genetic information relating to these viruses crossing species barriers.

Contrasting Gene Expression Profiles of Monocytes and Lymphocytes From Peste-Des-Petits-Ruminants Virus Infected Goats

In this study, transcriptome analysis of PPRV infected PBMC subsets-T helper cells, T cytotoxic cells, monocytes, and B lymphocytes was done to delineate their role in host response. PPRV was found to infect lymphocytes and not monocytes. The established receptor for PPRV-SLAM was found downregulated in lymphocytes and non-differentially expressed in monocytes. A profound deviation in the global gene expression profile with a large number of unique upregulated genes (851) and downregulated genes (605) was observed in monocytes in comparison to lymphocytes. ISGs-ISG15, Mx1, Mx2, RSAD2, IFIT3, and IFIT5 that play a role in antiviral response and the genes for viral sensors-MDA5, LGP2, and RIG1, were found to be upregulated in lymphocytes and downregulated in monocytes. The transcription factors-IRF-7 and STAT-1 that regulate expression of most of the ISGs were found activated in lymphocytes and not in monocytes. Interferon signaling pathway and RIG1 like receptor signaling pathway were found activated in lymphocytes and not in monocytes. This contrast in gene expression profiles and signaling pathways indicated the predominant role of lymphocytes in generating the antiviral response against PPRV in goats, thus, giving us new insights into host response to PPRV.

Marine Morbilliviruses: Diversity and Interaction with Signaling Lymphocyte Activation Molecules

Epidemiological reports of phocine distemper virus (PDV) and cetacean morbillivirus (CeMV) have accumulated since their discovery nearly 30 years ago. In this review, we focus on the interaction between these marine morbilliviruses and their major cellular receptor, the signaling lymphocyte activation molecule (SLAM). The three-dimensional crystal structure and homology models of SLAMs have demonstrated that 35 residues are important for binding to the morbillivirus hemagglutinin (H) protein and contribute to viral tropism. These 35 residues are essentially conserved among pinnipeds and highly conserved among the Caniformia, suggesting that PDV can infect these animals, but are less conserved among cetaceans. Because CeMV can infect various cetacean species, including toothed and baleen whales, the CeMV-H protein is postulated to have broader specificity to accommodate more divergent SLAM interfaces and may enable the virus to infect seals. In silico analysis of viral H protein and SLAM indicates that each residue of the H protein interacts with multiple residues of SLAM and vice versa. The integration of epidemiological, virological, structural, and computational studies should provide deeper insight into host specificity and switching of marine morbilliviruses.

Tropism and molecular pathogenesis of canine distemper virus

Background

Canine distemper virus (CDV), currently termed Canine morbillivirus, is an extremely contagious disease that affects dogs. It is identified as a multiple cell tropism pathogen, and its host range includes a vast array of species. As a member of Mononegavirales, CDV has a negative, single-stranded RNA genome, which encodes eight proteins.

Main body

Regarding the molecular pathogenesis, the hemagglutinin protein (H) plays a crucial role both in the antigenic recognition and the viral interaction with SLAM and nectin-4, the host cells’ receptors. These cellular receptors have been studied widely as CDV receptors in vitro in different cellular models. The SLAM receptor is located in lymphoid cells; therefore, the infection of these cells by CDV leads to immunosuppression, the severity of which can lead to variability in the clinical disease with the potential of secondary bacterial infection, up to and including the development of neurological signs in its later stage.

Conclusion

Improving the understanding of the CDV molecules implicated in the determination of infection, especially the H protein, can help to enhance the biochemical comprehension of the difference between a wide range of CDV variants, their tropism, and different steps in viral infection. The regions of interaction between the viral proteins and the identified host cell receptors have been elucidated to facilitate this understanding. Hence, this review describes the significant molecular and cellular characteristics of CDV that contribute to viral pathogenesis.

Electronic supplementary material

The online version of this article (10.1186/s12985-019-1136-6) contains supplementary material, which is available to authorized users.

Stronger together: Multi-genome transmission of measles virus

Measles virus (MeV) is an immunosuppressive, extremely contagious RNA virus that remains a leading cause of death among children. MeV is dual-tropic: it replicates first in lymphatic tissue, causing immunosuppression, and then in epithelial cells of the upper airways, accounting for extremely efficient contagion. Efficient contagion is counter-intuitive because the enveloped MeV particles are large and relatively unstable. However, MeV particles can contain multiple genomes, which can code for proteins with different functional characteristics. These proteins can cooperate to promote virus spread in tissue culture, prompting the question of whether multi-genome MeV transmission may promote efficient MeV spread also in vivo. Consistent with this hypothesis, in well-differentiated primary human airway epithelia large genome populations spread rapidly through intercellular pores. In another line of research, it was shown that distinct lymphocytic-adapted and epithelial-adapted genome populations exist; cyclical adaptation studies indicate that suboptimal variants in one environment may constitute a low frequency reservoir for adaptation to the other environment. Altogether, these observations suggest that, in humans, MeV spread relies on en bloc genome transmission, and that genomic diversity is instrumental for rapid MeV dissemination within hosts.

Genetic and evolutionary analysis of a new Asia-4 lineage and naturally recombinant canine distemper virus strains from Thailand

Canine distemper virus (CDV), a pathogen causing fatal disease in a wide range of carnivores, can be classified into several geographically-related lineages. It is unclear how genetic recombination contributed to the evolution and emergence of the novel CDV strains and the evolutions of these strains are not fully yet investigated. In this study, the complete genome sequences of eight CDV viruses, isolated from domestic dogs in Thailand, were investigated. Interestingly, most of the identified CDV strains (CDV1-3, -5, -8 TH/2014) clustered as a novel Asia-4 lineage, while the CDV4, -6, -7 TH/2014 belonged to the Asia-1 lineage. Recombination analysis revealed that the CDV4 TH/2014 is a putative recombinant virus from the Asia-1 and America-2 parent viruses. In contrast, no recombination events were detected in the Asia-4 lineage, indicating that it is a distinctive lineage. Evolutionary analysis suggested that the CDV Asia-4 lineage had emerged since 1924 and shared common ancestor with the America-2 lineage. Pressure analysis revealed that CDV nucleotides were under negative selection pressure for its rapid adaptation. These findings demonstrate the evolution of CDV Asia-4 lineage and identified the Asia-1 recombination event. The information regarding genetic diversity of CDVs is essential for further CDV's research and monitoring.

Phylogenetic analysis of canine distemper virus in South African wildlife

Canine distemper virus (CDV) causes a severe contagious disease in a broad range of hosts. This is the first study to genetically characterise CDV strains from four different wildlife species in South Africa. The phylogenetic diversity of CDV is examined, using the haemagglutinin gene. The South African wildlife CDV isolates showed a high degree of similarity to CDV in South African domestic dogs. Phylogenetic analyses confirmed the presence of 12 geographical lineages with CDV strains from South African wildlife falling within the Southern African lineage. The study reveals two possible co-circulating sub-genotypes corresponding to the northern and southern regions of South Africa respectively. CDV strains from the non-canid species were distinct, but similar to CDV isolates from domestic dog and wild canids. Residues at amino acid sites of the SLAM binding region support the notion that CDV strains encoding 519I / 549H are better adapted to non-canid species than canid species. The amino acids present at site 530 are conserved regardless of host species. Strains from South African wild carnivores showed no difference between host species with all strains presenting 530N. All non-canid strains in this study presented the combination 519I/549H. No evidence of host adaptation or lineage grouping was observed for the Nectin-4 binding region. Further studies should include CDV strains isolated from various hosts from a wider geographical range in South Africa.

Canine Distemper Virus Spread and Transmission to Naive Ferrets: Selective Pressure on Signaling Lymphocyte Activation Molecule-Dependent Entry

Upon infection, morbilliviruses such as measles virus, rinderpest virus, and canine distemper virus (CDV) initially target immune cells via the signaling lymphocyte activation molecule (SLAM) before spreading to respiratory epithelia through the adherens junction protein nectin-4. However, the roles of these receptors in transmission from infected to naive hosts have not yet been formally tested. To experimentally addressing this question, we established a model of CDV contact transmission between ferrets. We show here that transmission of wild-type CDV sometimes precedes the onset of clinical disease. In contrast, transmission was not observed in most animals infected with SLAM- or nectin-4-blind CDVs, even though all animals infected with the nectin-4-blind virus developed sustained viremia. There was an unexpected case of transmission of a nectin-4-blind virus, possibly due to biting. Another unprecedented event was transient viremia in an infection with a SLAM-blind virus. We identified three compensatory mutations within or near the SLAM-binding surface of the attachment protein. A recombinant CDV expressing the mutated attachment protein regained the ability to infect ferret lymphocytes in vitro, but its replication was not as efficient as that of wild-type CDV. Ferrets infected with this virus developed transient viremia and fever, but there was no transmission to naive contacts. Our study supports the importance of epithelial cell infection and of sequential CDV H protein interactions first with SLAM and then nectin-4 receptors for transmission to naive hosts. It also highlights the in vivo selection pressure on the H protein interactions with SLAM.IMPORTANCE Morbilliviruses such as measles virus, rinderpest virus, and canine distemper virus (CDV) are highly contagious. Despite extensive knowledge of how morbilliviruses interact with their receptors, little is known about how those interactions influence viral transmission to naive hosts. In a ferret model of CDV contact transmission, we showed that sequential use of the signaling lymphocytic activation molecule (SLAM) and nectin-4 receptors is essential for transmission. In one animal infected with a SLAM-blind CDV, we documented mild viremia due to the acquisition of three compensatory mutations within or near the SLAM-binding surface. The interaction, however, was not sufficient to cause disease or sustain transmission to naive contacts. This work confirms the sequential roles of SLAM and nectin-4 in morbillivirus transmission and highlights the selective pressure directed toward productive interactions with SLAM.

Social status mediates the fitness costs of infection with canine distemper virus in Serengeti spotted hyenas

The extent to which the fitness costs of infection are mediated by key life-history traits such as age or social status is still unclear. Within populations, individual heterogeneity in the outcome of infection is the result of two successive processes; the degree of contact with the pathogen (exposure) and the immune response to infection. In social mammals, because individuals holding high social status typically interact more frequently with group members, they should be more often in contact with infected individuals than those of low social status. However, when access to resources is determined by social status, individuals with a high social status are often better nourished, have a greater opportunity to allocate resources to immune processes and therefore should have a smaller chance of succumbing to infection than individuals with low social status.We investigated the risk and fitness costs of infection during a virulent epidemic of canine distemper virus (CDV) in a social carnivore, the spotted hyena, in the Serengeti National Park. We analysed two decades of detailed life-history data from 625 females and 816 males using a multi-event capture-mark-recapture model that accounts for uncertainty in the assignment of individual infection states.Cubs of mothers with a high social status had a lower probability of CDV infection and were more likely to survive infection than those with low social status. Subadult and adult females with high social status had a higher infection probability than those with low social status. Subadult females and pre-breeder males that had recovered from CDV infection had a lower survival than susceptible ones.Our study disentangles the relative importance of individual exposure and resource allocation to immune processes, demonstrates fitness costs of infection for juveniles, particularly for those with low social status, shows that patterns of infection can be driven by different mechanisms among juveniles and adults and establishes a negative relationship between infection and fitness in a free-ranging mammal. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13059/suppinfo is available for this article.

The Unstructured Paramyxovirus Nucleocapsid Protein Tail Domain Modulates Viral Pathogenesis through Regulation of Transcriptase Activity

The paramyxovirus replication machinery comprises the viral large (L) protein and phosphoprotein (P-protein) in addition to the nucleocapsid (N) protein, which encapsidates the single-stranded RNA genome. Common to paramyxovirus N proteins is a C-terminal tail (Ntail). The mechanistic role and relevance for virus replication of the structurally disordered central Ntail section are unknown. Focusing initially on members of the Morbillivirus genus, a series of measles virus (MeV) and canine distemper virus (CDV) N proteins were generated with internal deletions in the unstructured tail section. N proteins with large tail truncations remained bioactive in mono- and polycistronic minireplicon assays and supported efficient replication of recombinant viruses. Bioactivity of Ntail mutants extended to N proteins derived from highly pathogenic Nipah virus. To probe an effect of Ntail truncations on viral pathogenesis, recombinant CDVs were analyzed in a lethal CDV/ferret model of morbillivirus disease. The recombinant viruses displayed different stages of attenuation ranging from ameliorated clinical symptoms to complete survival of infected animals, depending on the molecular nature of the Ntail truncation. Reinfection of surviving animals with pathogenic CDV revealed robust protection against a lethal challenge. The highly attenuated virus was genetically stable after ex vivo passaging and recovery from infected animals. Mechanistically, gradual viral attenuation coincided with stepwise altered viral transcriptase activity in infected cells. These results identify the central Ntail section as a determinant for viral pathogenesis and establish a novel platform to engineer gradual virus attenuation for next-generation paramyxovirus vaccine design.IMPORTANCE Investigating the role of the paramyxovirus N protein tail domain (Ntail) in virus replication, we demonstrated in this study that the structurally disordered central Ntail region is a determinant for viral pathogenesis. We show that internal deletions in this Ntail region of up to 55 amino acids in length are compatible with efficient replication of recombinant viruses in cell culture but result in gradual viral attenuation in a lethal canine distemper virus (CDV)/ferret model. Mechanistically, we demonstrate a role of the intact Ntail region in the regulation of viral transcriptase activity. Recombinant viruses with Ntail truncations induce protective immunity against lethal challenge of ferrets with pathogenic CDV. This identification of the unstructured central Ntail domain as a nonessential paramyxovirus pathogenesis factor establishes a foundation for harnessing Ntail truncations for vaccine engineering against emerging and reemerging members of the paramyxovirus family.

Subacute sclerosing panencephalitis and immune thrombocytopenia: More than a coincidence?

Subacute sclerosing panencephalitis (SSPE) is a progressive disease caused by persistent measles virus (MV). It has an incidence of 0.4-2.0/million in Turkey. Immune thrombocytopenia (ITP) is a bleeding disorder whose estimated incidence is 4.2/100.000 person/years in the pediatric age group. We observed three cases with ITP in our cohort of 315 pediatric SSPE cases, an incidence higher than coincidentally expected in the general population. We hypothesize an association between SSPE and ITP. Our three cases had measles 1-2 years before the onset of ITP and 8-10 years before first symptoms of SSPE. A common immunogenetic background creating susceptibility to infection and autoimmunity might play a role. Alternatively, chronic antigenic stimulation by the MV leading to synthesis of cross-reacting antibodies against platelets, or treatment of ITP with immunoglobulins or steroids might affect or alter the development and manifestation of SSPE. The co-occurrence of these two disorders of viral and immune pathogenesis may draw attention to similar observations and provide clues for their mechanisms.

Genome Sequences of Three Vaccine Strains and Two Wild-Type Canine Distemper Virus Strains from a Recent Disease Outbreak in South Africa

Canine distemper virus causes global multihost infectious disease. This report details complete genome sequences of three vaccine and two new wild-type strains. The wild-type strains belong to the South African lineage, and all three vaccine strains to the America 1 lineage. This constitutes the first genomic sequences of this virus from South Africa.

Delineating morbillivirus entry, dissemination and airborne transmission by studying in vivo competition of multicolor canine distemper viruses in ferrets

Identification of cellular receptors and characterization of viral tropism in animal models have vastly improved our understanding of morbillivirus pathogenesis. However, specific aspects of viral entry, dissemination and transmission remain difficult to recapitulate in animal models. Here, we used three virologically identical but phenotypically distinct recombinant (r) canine distemper viruses (CDV) expressing different fluorescent reporter proteins for in vivo competition and airborne transmission studies in ferrets (Mustela putorius furo). Six donor ferrets simultaneously received three rCDVs expressing green, red or blue fluorescent proteins via conjunctival (ocular, Oc), intra-nasal (IN) or intra-tracheal (IT) inoculation. Two days post-inoculation sentinel ferrets were placed in physically separated adjacent cages to assess airborne transmission. All donor ferrets developed lymphopenia, fever and lethargy, showed progressively increasing systemic viral loads and were euthanized 14 to 16 days post-inoculation. Systemic replication of virus inoculated via the Oc, IN and IT routes was detected in 2/6, 5/6 and 6/6 ferrets, respectively. In five donor ferrets the IT delivered virus dominated, although replication of two or three different viruses was detected in 5/6 animals. Single lymphocytes expressing multiple fluorescent proteins were abundant in peripheral blood and lymphoid tissues, demonstrating the occurrence of double and triple virus infections. Transmission occurred efficiently and all recipient ferrets showed evidence of infection between 18 and 22 days post-inoculation of the donor ferrets. In all cases, airborne transmission resulted in replication of a single-colored virus, which was the dominant virus in the donor ferret. This study demonstrates that morbilliviruses can use multiple entry routes in parallel, and co-infection of cells during viral dissemination in the host is common. Airborne transmission was efficient, although transmission of viruses expressing a single color suggested a bottleneck event. The identity of the transmitted virus was not determined by the site of inoculation but by the viral dominance during dissemination.

Modulation of Host Immunity by the Human Metapneumovirus

Globally, as a leading agent of acute respiratory tract infections in children <5 years of age and the elderly, the human metapneumovirus (HMPV) has gained considerable attention. As inferred from studies comparing vaccinated and experimentally infected mice, the acquired immune response elicited by this pathogen fails to efficiently clear the virus from the airways, which leads to an exaggerated inflammatory response and lung damage. Furthermore, after disease resolution, there is a poor development of T and B cell immunological memory, which is believed to promote reinfections and viral spread in the community. In this article, we discuss the molecular mechanisms that shape the interactions of HMPV with host tissues that lead to pulmonary pathology and to the development of adaptive immunity that fails to protect against natural infections by this virus.

Advances in canine distemper virus pathogenesis research: a wildlife perspective

Canine distemper virus (CDV) has emerged as a significant disease of wildlife, which is highly contagious and readily transmitted between susceptible hosts. Initially described as an infectious disease of domestic dogs, it is now recognized as a global multi-host pathogen, infecting and causing mass mortalities in a wide range of carnivore species. The last decade has seen the effect of numerous CDV outbreaks in various wildlife populations. Prevention of CDV requires a clear understanding of the potential hosts in danger of infection as well as the dynamic pathways CDV uses to gain entry to its host cells and its ability to initiate viral shedding and disease transmission. We review recent research conducted on CDV infections in wildlife, including the latest findings on the causes of host specificity and cellular receptors involved in distemper pathogenesis.

Canine distemper virus in the Serengeti ecosystem: molecular adaptation to different carnivore species

Was the 1993/1994 fatal canine distemper virus (CDV) epidemic in lions and spotted hyaenas in the Serengeti ecosystem caused by the recent spillover of a virulent domestic dog strain or one well adapted to these noncanids? We examine this question using sequence data from 13 'Serengeti' strains including five complete genomes obtained between 1993 and 2011. Phylogenetic and haplotype network analyses reveal that strains from noncanids during the epidemic were more closely related to each other than to those from domestic or wild canids. All noncanid 'Serengeti' strains during the epidemic encoded: (1) one novel substitution G134S in the CDV-V protein; and (2) the rare amino acid combination 519I/549H at two sites under positive selection in the region of the CDV-H protein that binds to SLAM (CD 150) host cell receptors. Worldwide, only a few noncanid strains in the America II lineage encode CDV-H 519I/549H. All canid 'Serengeti' strains during the epidemic coded CDV-V 134G, and CDV-H 519R/549Y, or 519R/549H. A functional assay of cell entry revealed the highest performance by CDV-H proteins encoding 519I/549H in cells expressing lion SLAM receptors, and the highest performance by proteins encoding 519R/549Y, typical of dog strains worldwide, in cells expressing dog SLAM receptors. Our findings are consistent with an epidemic in lions and hyaenas caused by CDV variants better adapted to noncanids than canids, but not with the recent spillover of a dog strain. Our study reveals a greater complexity of CDV molecular epidemiology in multihost environments than previously thought.

Morbillivirus Experimental Animal Models: Measles Virus Pathogenesis Insights from Canine Distemper Virus

Morbilliviruses share considerable structural and functional similarities. Even though disease severity varies among the respective host species, the underlying pathogenesis and the clinical signs are comparable. Thus, insights gained with one morbillivirus often apply to the other members of the genus. Since the Canine distemper virus (CDV) causes severe and often lethal disease in dogs and ferrets, it is an attractive model to characterize morbillivirus pathogenesis mechanisms and to evaluate the efficacy of new prophylactic and therapeutic approaches. This review compares the cellular tropism, pathogenesis, mechanisms of persistence and immunosuppression of the Measles virus (MeV) and CDV. It then summarizes the contributions made by studies on the CDV in dogs and ferrets to our understanding of MeV pathogenesis and to vaccine and drugs development.

Systems Perspective of Morbillivirus Replication

Systems biology refers to system-wide changes in biological components such as RNA/DNA (genomics), protein (proteomics) and lipids (lipidomics). In this review, we provide comprehensive information about morbillivirus replication. Besides discussing the role of individual viral/host proteins in virus replication, we also discuss how systems-level analyses could improve our understanding of morbillivirus replication, host-pathogen interaction, immune response and disease resistance. Finally, we discuss how viroinformatics is likely to provide important insights for understanding genome-genome, genome-protein and protein-protein interactions.

Nipah Virus C and W Proteins Contribute to Respiratory Disease in Ferrets

Nipah virus (NiV) is a highly lethal paramyxovirus that recently emerged as a causative agent of febrile encephalitis and severe respiratory disease in humans. The ferret model has emerged as the preferred small-animal model with which to study NiV disease, but much is still unknown about the viral determinants of NiV pathogenesis, including the contribution of the C protein in ferrets. Additionally, studies have yet to examine the synergistic effects of the various P gene products on pathogenesis in animal models. Using recombinant NiVs (rNiVs), we examine the sole contribution of the NiV C protein and the combined contributions of the C and W proteins in the ferret model of NiV pathogenesis. We show that an rNiV void of C expression resulted in 100% mortality, though with limited respiratory disease, like our previously reported rNiV void of W expression; this finding is in stark contrast to the attenuated phenotype observed in previous hamster studies utilizing rNiVs void of C expression. We also observed that an rNiV void of both C and W expression resulted in limited respiratory disease; however, there was severe neurological disease leading to 60% mortality, and the surviving ferrets demonstrated sequelae similar to those for human survivors of NiV encephalitis.

IMPORTANCE

Nipah virus (NiV) is a human pathogen capable of causing lethal respiratory and neurological disease. Many human survivors have long-lasting neurological impairment. Using a ferret model, this study demonstrated the roles of the NiV C and W proteins in pathogenesis, where lack of either the C or the W protein independently decreased the severity of clinical respiratory disease but did not decrease lethality. Abolishing both C and W expression, however, dramatically decreased the severity of respiratory disease and the level of destruction of splenic germinal centers. These ferrets still suffered severe neurological disease: 60% succumbed to disease, and the survivors experienced long-term neurological impairment, such as that seen in human survivors. This new ferret NiV C and W knockout model may allow, for the first time, the examination of interventions to prevent or mitigate the neurological damage and sequelae experienced by human survivors.

Inhibition of the host antiviral response by Nipah virus: current understanding and future perspectives

Nipah virus (NiV) is a lethal paramyxovirus that has recently emerged as a human pathogen capable of causing acute respiratory disease and encephalitis. Like many viral pathogens, NiV has developed multiple means of antagonizing the host antiviral response. The viral proteins responsible for this antiviral inhibition are encoded in the NiV P gene and include the P, V, W and C proteins, which contain various unique and overlapping roles. This review examines the current data on inhibition of the host antiviral response for each of these proteins gathered from viral protein expression systems, in vitro data using recombinant NiV mutants and from in vivo studies using recombinant NiV mutants, as well as a future perspective regarding the direction of the field.

In Vitro Exposure of Harbor Seal Immune Cells to Aroclor 1260 Alters Phocine Distemper Virus Replication

In the last 30 years, several large-scale marine mammal mortality events have occurred, often in close association with highly polluted regions, leading to suspicions that contaminant-induced immunosuppression contributed to these epizootics. Some of these recent events also identified morbillivirus as a cause of or contributor to death. The role of contaminant exposures regarding morbillivirus mortality is still unclear. The results of this study aimed to address the potential for a mixture of polychlorinated biphenyls (PCBs), specifically Aroclor 1260, to alter harbor seal T-lymphocyte proliferation and to assess if exposure resulted in increased likelihood of phocine distemper virus (PDV USA 2006) to infect susceptible seals in an in vitro system. Exposure of peripheral blood mononuclear cells to Aroclor 1260 did not significantly alter lymphocyte proliferation (1, 5, 10, and 20 ppm). However, using reverse transcription-quantitative polymerase chain reaction (RT-qPCR), lymphocytes exposed to 20 ppm Aroclor 1260 exhibited a significant decrease in PDV replication at day 7 and a significant increase at day 11 compared with unexposed control cells. Similar and significant differences were apparent on exposure to Aroclor 1260 in monocytes and supernatant. The results here indicate that in harbor seals, Aroclor 1260 exposure results in a decrease in virus early during infection and an increase during late infection. The consequences of this contaminant-induced infection pattern in a highly susceptible host could result in a greater potential for systemic infection with greater viral load, which could explain the correlative findings seen in wild populations exposed to a range of persistent contaminants that suffer from morbillivirus epizootics.

Saxitoxin increases phocine distemper virus replication upon in-vitro infection in harbor seal immune cells

Several marine mammal epizootics have been closely linked to infectious diseases, as well as to the biotoxins produced by harmful algal blooms (HABs). In two of three saxitoxin (STX) associated mortality events, dolphin morbillivirus (DMV) or phocine distemper virus (PDV) was isolated in affected individuals. While STX is notorious for its neurotoxicity, immunotoxic effects have also been described. This study investigated the role of STX in altering immune function, specifically T lymphocyte proliferation, in harbor seals (Phoca vitulina concolor) upon in-vitro exposure. In addition, the study also examined whether exposure to STX could alter the susceptibility of harbor seal immune cells to PDV infection upon in-vitro exposure. STX caused an increase in harbor seal lymphocyte proliferation at 10ppb and exposure to STX significantly increased the amount of virus present in lymphocytes. These results suggest that low levels of STX within the range of those reported in northeast U.S. seals may affect the likelihood of systemic PDV infection upon in-vivo exposure in susceptible seals. Given the concurrent increase in morbillivirus epizootics and HAB events in the last 25 years, the relationship between low level toxin exposure and host susceptibility to morbillivirus needs to be further explored.