Structural Perspectives of Beak and Feather Disease Virus and Porcine Circovirus Proteins

Structure of anellovirus-like particles reveal a mechanism for immune evasion

Anelloviruses are nonpathogenic viruses that comprise a major portion of the human virome. Despite being ubiquitous in the human population, anelloviruses (ANVs) remain poorly understood. Basic features of the virus, such as the identity of its capsid protein and the structure of the viral particle, have been unclear until now. Here, we use cryogenic electron microscopy to describe the first structure of an ANV-like particle. The particle, formed by 60 jelly roll domain-containing ANV capsid proteins, forms an icosahedral particle core from which spike domains extend to form a salient part of the particle surface. The spike domains come together around the 5-fold symmetry axis to form crown-like features. The base of the spike domain, the P1 subdomain, shares some sequence conservation between ANV strains while a hypervariable region, forming the P2 subdomain, is at the spike domain apex. We propose that this structure renders the particle less susceptible to antibody neutralization by hiding vulnerable conserved domains while exposing highly diverse epitopes as immunological decoys, thereby contributing to the immune evasion properties of anelloviruses. These results shed light on the structure of anelloviruses and provide a framework to understand their interactions with the immune system.

Establishment and preliminary application of duplex fluorescence quantitative PCR for porcine circoviruses type 2 and type 3

Porcine circovirus types 2 (PCV2) and 3 (PCV3) are the two most prevalent porcine circoviruses in China, all of which can infect swine herds and cause serious diseases. To detect coinfection with PCV2 and PCV3, primers and probes for duplex PCV2 and PCV3 real-time PCR were designed to target their cap genes based on the constructed plasmids pUC57-PCV2 and pUC57-PCV3. The established duplex PCV2 and PCV3 real-time PCRs were specific to PCV2 and PCV3 and showed no cross-reactions with other porcine viral pathogens. The limit of detection was 5 and 50 copies for the PCV2 and PCV3 plasmids, respectively. The intra- and interassay repeatability had coefficients of variation below 3 %. The established methods were used to analyze clinical samples from Liaoning and Jilin provinces of China. The coinfection rates of PCV2 and PCV3 in pigs extensively fed in Liaoning and Jilin, large-scale farmed pigs in Liaoning and large-scale farmed pigs in Jilin were 15.0 % (6/40), 36.7 % (11/30) and 35.4 % (62/175), respectively. This study established a useful duplex PCV2 and PCV3 real-time PCR method that can be used for the detection of PCV2 and PCV3 in local clinical samples.

Recombinantly expressed virus-like particles (VLPs) of canine circovirus for development of an indirect ELISA

Canine circovirus (CanineCV) is an emerging pathogen in domestic dogs, detected in multiple countries in association with varying clinical and pathological presentations including diarrhoea, vasculitis, granulomatous inflammation, and respiratory signs. Understanding the pathology of CanineCV is confounded by the fact that it has been detected in asymptomatic dogs as well as in diseased dogs concurrently infected with known pathogens. Recombinantly expressed self-assembling Virus-like particles (VLPs) lack viral genomic material but imitate the capsid surface conformations of wild type virion, allowing arrays of biological applications including subunit vaccine development and immunodiagnostics. In this study, full length CanineCV capsid gene was expressed in Escherichia coli followed by two-step purification process to yield soluble capsid protein in high concentration. Transmission electron microscopy (TEM) confirmed the capsid antigen self-assembled into 17-20 nm VLPs in glutathione S-transferase (GST) buffer, later utilised to develop an indirect enzyme-linked immunosorbent assay (iELISA). The respective sensitivity and specificity of the proposed iELISA were 94.10% and 88.40% compared with those obtained from Western blot. The mean OD450 value for western blot positive samples was 1.22 (range 0.12-3.39) and negative samples was 0.21 (range 0.07-0.41). An optimal OD450 cut-off of 0.35 was determined by ROC curve analysis. Median inter-assay and intra-assay validation revealed that the iELISA test results were reproducible with coefficients of variation 7.70 (range 5.6-11.9) and 4.21 (range 1.2-7.4). Our results demonstrated that VLP-based iELISA is a highly sensitive method for serological diagnosis of CanineCV infections in dogs, suitable for large-scale epidemiological studies.

Detection of circovirus in free-ranging brown rats (Rattus norvegicus)

Accidentally found, two poisoned brown rats from Hungary were surveyed for presence of circoviral DNA, using specific nested primers, designed against the rep gene of the virus. Both specimens were positive. The whole genomes were amplified using inverse PCR based on the Rep sequence parts and sequenced by the primer walking method. Genomic analyses revealed that these novel rat viruses, together with tawny owl-associated circovirus reported by Italian researchers in 2022, are sequence variations of the same virus from genus Circovirus. In phylogenetic reconstructions, these circovirus strains detected from brown rats clustered closest to circoviruses derived from faeces samples of various predatory mammals. Molecular data as well as the phylogenetic analyses of the complete derived replication-associated protein and the capsid protein, as well as the prey preference of the host species of the recently described tawny owl-associated virus suggest that brown rat could be the evolutionary adapted host of the viruses described in this paper (brown rat circovirus types 1 and 2) and the previously reported tawny owl-associated virus. Possible pathogenic and zoonotic role of these viruses need further studies.

Molecular characterization of the gastrointestinal eukaryotic virome in elderly people in Belem, Para, Brazil

Acute gastroenteritis is one of the main causes of mortality and morbidity worldwide, affecting mainly children, the immunocompromised and elderly people. Enteric viruses, especially rotavirus A, are considered important etiological agents, while long-term care facilities are considered favorable environments for the occurrence of sporadic cases and outbreaks of acute gastroenteritis. Therefore, it is important to monitor the viral agents present in nursing homes, especially because studies involving the elderly population in Brazil are scarce, resulting in a lack of available virological data. As a result, the causative agent remains unidentified in a large number of reported acute gastroenteritis cases. However, the advent of next-generation sequencing provides new opportunities for viral detection and discovery. The aim of this study was to identify the viruses that circulate among elderly people with and without acute gastroenteritis, living in residential care homes in Belém, Pará, Brazil, between 2017 and 2019. Ninety-three samples were collected and screened by immunochromatography and qPCR. After, the samples were analyzed individually or in pools by next generation sequencing to identify the viruses circulating in this population. In 26 sequenced samples, members of 13 eukaryotic virus families were identified. The most abundantly present virus families were Parvoviridae, Genomoviridae and Smacoviridae. Contigs displaying similarity to pegiviruses were also detected. Furthermore, a near-complete rotavirus A genome was obtained and could be classified as G3P[8] genotype with the equine DS-1-like genetic background. Complete sequences of the VP4 and VP7 genes of a rotavirus C were also detected, belonging to G4P[2]. This study demonstrates the first characterization of the gastrointestinal virome in elderly in Northern Brazil. A diversity of viruses was found to be present in patients with and without diarrhea, reinforcing the need to monitor elderly people residing in long-term care facilities, especially in cases of acute gastroenteritis.

Novel Circoviruses from Birds Share Common Evolutionary Roots with Fish Origin Circoviruses

Circoviruses occur in a variety of animal species and are common pathogens of mammalian and avian hosts. In our study internal organ samples of wild birds were processed for screening of circoviral sequences. Two novel viruses were identified and characterized in specimens of a little bittern and a European bee-eater that suffered from wing injuries, were weakened, had liver or kidney failures, and finally succumbed at a rescue station. The 1935 nt and 1960 nt long viral DNA genomes exhibited a genomic structure typical for circoviruses and were predicted to encode replication-associated protein in the viral strand, and a capsid protein in the complementary strand of the replicative intermediate DNA form. The genome of the newly described viruses showed 37.6% pairwise identity with each other and ≤41.5% identity with circovirus sequences, and shared a common branch with fish, human and Weddel seal circoviruses in the phylogenetic tree, implying evolutionary relationship among the ancestors of these viruses. Based on the results the little bittern and European bee-eater circoviruses represent two distinct species of the Circovirus genus, Circoviridae family.

Detection and Molecular Characterization of a Novel Species of Circovirus in a Tawny Owl (Strix aluco) in Southern Italy

Simple Summary

The genus Circovirus groups some of the smallest viruses capable of autonomous replication, including some notable swine and avian pathogens. Among domestic and wild birds, circoviruses are often associated with immunosuppression and integumental disorders, but, despite their relevance, nothing is known about their circulation in birds of prey. By conducting molecular analyses on samples from birds of prey recovered by a wildlife rescue centre in Italy, we identified a new viral species in the spleen of a tawny owl (Strix aluco). However, there is contrasting evidence regarding its definitive host. On one hand, the virus was discovered to be phylogenetically closer to mammalian rather than avian circoviruses, which allows speculations on its host being a micromammal preyed by the tawny owl, rather than the bird itself. On the other hand, its detection in the spleen, a lymphoid organ in which other avian circoviruses are often detected, supports the tawny owl being its actual host, perhaps following a spillover event associated with predation. Adding to the growing number of circoviruses found in recent years in a diverse range of hosts, this discovery represents another step forward in the characterization of this genus of remarkable veterinary importance.

Abstract

Thanks to recent developments in molecular methods, many new species have been discovered within the genus Circovirus, which comprises viruses of veterinary relevance found in a broad range of hosts. In particular, several circoviruses are known to infect birds, often causing immunosuppression and feathering disorders. Nonetheless, nothing is known about their circulation in birds of prey. In this study, samples from 61 birds of prey representing ten different species, recovered by a wildlife rescue centre in Southern Italy, were taken at necropsy and analysed by PCR with pan-Circovirus primers. Only one sample, collected from a tawny owl (Strix aluco), tested positive. Its genome, sequenced by primer walking, displays the typical features of circoviruses. Based on demarcation criteria, the detected strain qualifies as a novel species, which was named “tawny owl-associated circovirus” (ToCV). Phylogenetically, ToCV clustered with mammalian rather than avian circoviruses, and its closeness to a rodent circovirus suggests that its host may have been a micromammal eaten by the tawny owl. On the other hand, its detection in the spleen fits with the tropism of other avian circoviruses. Little can be therefore said on its biology and pathogenicity, and further efforts are needed to better characterize its epidemiology.

Porcine circovirus 2 manipulates PERK-ERO1α axis of endoplasmic reticulum in favor of its replication by derepressing viral DNA from HMGB1 sequestration within nuclei

Porcine circovirus type 2 (PCV2) causes several disease syndromes in grower pigs. PCV2 infection triggers endoplasmic reticulum (ER) stress, autophagy and oxidative stress, all of which support PCV2 replication. We have recently reported that nuclear HMGB1 is an anti-PCV2 factor by binding to viral genomic DNA. However, how PCV2 manipulates host cell responses to favor its replication has not been explored. Here, we demonstrate that PCV2 infection increased expression of ERO1α, generation of ROS and nucleocytoplasmic migration of HMGB1 via PERK activation in PK-15 cells. Inhibition of PERK or ERO1α repressed ROS production in PCV2-infected cells and increased HMGB1 retention within nuclei. These findings indicate that PCV2-induced activation of the PERK-ERO1α axis would lead to enhanced generation of ROS sufficient to decrease HMGB1 retention in the nuclei, thus derepressing viral DNA from HMGB1 sequestration. The viral Rep and Cap proteins were able to induce PERK-ERO1α-mediated ROS accumulation. Cysteine residues 107 and 305 of Rep or 108 of Cap played important roles in PCV2-induced PERK activation and distribution of HMGB1. Of the mutant viruses, only the mutant PCV2 with substitution of all three cysteine residues failed to activate PERK with reduced ROS generation and decreased nucleocytoplasmic migration of HMGB1. Collectively, this study offers novel insight into the mechanism of enhanced viral replication in which PCV2 manipulates ER to perturb its redox homeostasis via the PERK-ERO1α axis and the ER-sourced ROS from oxidative folding is sufficient to reduce HMGB1 retention in the nuclei, hence the release of HMGB1-bound viral DNA for replication. IMPORTANCE Considering the fact that clinical PCVAD mostly results from activation of latent PCV2 infection by confounding factors such as co-infection or environmental stresses, we propose that such confounding factors might impose oxidative stress to the animals where PCV2 in infected cells might utilize the elevated ROS to promote HMGB1 migration out of nuclei in favor of its replication. An animal infection model with a particular stressor could be approached with or without antioxidant treatment to examine the relationship among the stressor, ROS level, HMGB1 distribution in target tissues, virus replication and severity of PCVAD. This will help decide the use of antioxidants in the feeding regime on pig farms that suffer from PCVAD. Further investigation could examine if similar strategies are employed by DNA viruses, such as PCV3 and BFDV and if there is cross-talk among ER stress, autophagy/mitophagy and mitochondria-sourced ROS in favor of PCV2 replication.

Genomic Diversity of CRESS DNA Viruses in the Eukaryotic Virome of Swine Feces

Replication-associated protein (Rep)-encoding single-stranded DNA (CRESS DNA) viruses are a diverse group of viruses, and their persistence in the environment has been studied for over a decade. However, the persistence of CRESS DNA viruses in herds of domestic animals has, in some cases, serious economic consequence. In this study, we describe the diversity of CRESS DNA viruses identified during the metagenomics analysis of fecal samples collected from a single swine herd with apparently healthy animals. A total of nine genome sequences were assembled and classified into two different groups (CRESSV1 and CRESSV2) of the Cirlivirales order (Cressdnaviricota phylum). The novel CRESS DNA viral sequences shared 85.8–96.8% and 38.1–94.3% amino acid sequence identities for the Rep and putative capsid protein sequences compared to their respective counterparts with extant GenBank record. Data presented here show evidence for simultaneous infection of swine herds with multiple novel CRESS DNA viruses, including po-circo-like viruses and fur seal feces-associated circular DNA viruses. Given that viral genomes with similar sequence and structure have been detected in swine fecal viromes from independent studies, investigation of the association between presence of CRESS DNA viruses and swine health conditions seems to be justified.