Apoptosis in postweaning multisystemic wasting syndrome (PMWS) hepatitis in pigs naturally infected with porcine circovirus type 2 (PCV2)

Rescue and characterization of PCV4 infectious clones: pathogenesis and immune response in piglets

Porcine circovirus 4 (PCV4) was first identified in 2019, categorized within the genus Circovirus in the family Circoviridae. To date, the virus has not been isolated from clinical samples. Meanwhile, many aspects of the biology and pathogenic mechanisms of PCV4 infection remain unknown. In this study, PCV4 was successfully rescued from an infectious clone. We utilized a PCV4 virus stock derived from this infectious clone to intranasally inoculate 4-week-old specific-pathogen-free piglets to evaluate PCV4 pathogenesis. The rescued PCV4 was capable of replicating in both PK-15 cells and piglets, with the virus detectable in nearly all collected samples from the challenge groups. Pathological lesions and PCV4-specific antigens were observed in various tissues and organs, including the lungs, kidneys, lymph nodes, spleen, and liver, in the inoculated piglets. Additionally, the levels of pro-inflammatory cytokines in the serum of the PCV4-inoculated group were significantly elevated compared to the control group, indicating that the induced inflammatory response may contribute to tissue damage associated with PCV4 infection. These findings offer new insights into the pathogenesis and inflammatory responses associated with PCV4-related diseases.

A Triple Gene-Deleted Pseudorabies Virus-Vectored Subunit PCV2b and CSFV Vaccine Protects Pigs against PCV2b Challenge and Induces Serum Neutralizing Antibody Response against CSFV

Porcine circovirus type 2 (PCV2) is endemic worldwide. PCV2 causes immunosuppressive infection. Co-infection of pigs with other swine viruses, such as pseudorabies virus (PRV) and classical swine fever virus (CSFV), have fatal outcomes, causing the swine industry significant economic losses in many if not all pig-producing countries. Currently available inactivated/modified-live/vectored vaccines against PCV2/CSFV/PRV have safety and efficacy limitations. To address these shortcomings, we have constructed a triple gene (thymidine kinase, glycoprotein E [gE], and gG)-deleted (PRVtmv) vaccine vector expressing chimeric PCV2b-capsid, CSFV-E2, and chimeric E^rns-fused with bovine granulocytic monocyte-colony stimulating factor (E^rns-GM-CSF), designated as PRVtmv+, a trivalent vaccine. Here we compared this vaccine’s immunogenicity and protective efficacy in pigs against wild-type PCV2b challenge with that of the inactivated Zoetis Fostera Gold PCV commercial vaccine. The live PRVtmv+ prototype trivalent subunit vaccine is safe and highly attenuated in pigs. Based on PCV2b-specific neutralizing antibody titers, viremia, viral load in lymphoid tissues, fecal-virus shedding, and leukocyte/lymphocyte count, the PRVtmv+ yielded better protection for vaccinated pigs than the commercial vaccine after the PCV2b challenge. Additionally, the PRVtmv+ vaccinated pigs generated low to moderate levels of CSFV-specific neutralizing antibodies.

Mitochondrial Localization Signal of Porcine Circovirus Type 2 Capsid Protein Plays a Critical Role in Cap-Induced Apoptosis

Porcine circovirus 2 (PCV2), considered one of the most globally important porcine pathogens, causes postweaning multisystemic wasting syndrome (PMWS). This virus is localized in the mitochondria in pigs with PMWS. Here, we identified, for the first time, a mitochondrial localization signal (MLS) in the PCV2 capsid protein (Cap) at the N-terminus. PK-15 cells showed colocalization of the MLS-EGFP fusion protein with mitochondria. Since the PCV2 Cap also contained a nuclear localization signal (NLS) that mediated entry into the nucleus, we inferred that the subcellular localization of the PCV2 Cap is inherently complex and dependent on the viral life cycle. Furthermore, we also determined that deletion of the MLS attenuated Cap-induced apoptosis. More importantly, the MLS was essential for PCV2 replication, as absence of the MLS resulted in failure of virus rescue from cells infected with infectious clone DNA. In conclusion, the MLS of the PCV2 Cap plays critical roles in Cap-induced apoptosis, and MLS deletion of Cap is lethal for virus rescue.

A novel PCV2 ORF5-interacting host factor YWHAB inhibits virus replication and alleviates PCV2-induced cellular response

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YWHAB is a PCV2 ORF5-interacting host factor.

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YWHAB expression is activated by PCV2 infection and ORF5 transfection.

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YWHAB inhibits PCV2 replication.

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YWHAB alleviates PCV2 infection induced ERS, autophagy, ROS production and apoptosis.

Porcine circovirus type 2 (PCV2) infection causes porcine circovirus associated diseases (PCVAD) worldwide. Identification of host factors that interact with viral proteins is a fundamental step to understand the pathogenesis of PCV2. Our previous study reported that ORF5, a newly identified PCV2 viral protein supports PCV2 replication and interacts with multiple host factors. Here, we showed that a host factor YWHAB is an ORF5-interacting protein and plays essential roles during PCV2 infection. By using protein-protein interaction assays, we confirmed that YWHAB directly interacts with PCV2-ORF5 protein. We further showed that YWHAB expression was potently induced upon ORF5 overexpression and PCV2 infection. Remarkably, we found that the YWHAB strongly inhibited PCV2 replication, suggesting its role in defending PCV2 infection. By using the ectopic overexpression and gene knockdown approaches, we revealed that YWHAB inhibits PCV2-induced endoplasmic reticulum stress (ERS), autophagy, reactive oxygen species (ROS) production and apoptosis, suggesting its vital role in alleviating PCV2-induced cellular damage. Together, this study demonstrated that an ORF5-interacting host factor YWHAB affects PCV2 infection and PCV2-induced cellular response, which expands the current understanding of YWHAB biological function and might serves as a new therapeutic target to manage PCV2 infection-associated diseases.

Identification of lncRNAs Involved in PCV2 Infection of PK-15 Cells

Porcine circovirus type 2 (PCV2) can cause severe disease in infected pigs, resulting in massive economic loss for the swine industry. Transcriptomic and proteomic approaches have been widely employed to identify the underlying molecular mechanisms of the PCV2 infection. Numerous differentially expressed mRNAs, miRNAs, and proteins, together with their associated signaling pathways, have been identified during PCV2 infection, paving the way for analysis of their biological functions. Long noncoding RNAs (lncRNAs) are important regulators of multiple biological processes. However, little is known regarding their role in the PCV2 infection. Hence, in our study, RNA-seq was performed by infecting PK-15 cells with PCV2. Analysis of the differentially expressed genes (DEGs) suggested that the cytoskeleton, apoptosis, cell division, and protein phosphorylation were significantly disturbed. Then, using stringent parameters, six lncRNAs were identified. Additionally, potential targets of the lncRNAs were predicted using both cis- and trans-prediction methods. Interestingly, we found that the HOXB (Homeobox B) gene cluster was probably the target of the lncRNA LOC106505099. Enrichment analysis of the target genes showed that numerous developmental processes were altered during PCV2 infection. Therefore, our study revealed that lncRNAs might affect porcine embryonic development through the regulation of the HOXB genes.

Haemato-biochemical alterations and oxidative stress associated with naturally occurring porcine circovirus2 infection in pigs

Porcine circovirus2 (PCV2) infection in pigs is one of the major causes of economic loss to the farmers in terms of low production, slow growth and increase post-weaning mortality rate. The effect of PCV2 infection on haemogram, serum biochemical profile and oxidant/anti-oxidant status is not well established in pigs. In the present study, haemogram, serum biochemical profile and oxidant/anti-oxidant status were assessed in pigs confirmed positive for PCV2 infections as evidenced by commercially available enzyme-linked immunosorbent assay kit (n = 151) and polymerase chain reaction (PCR) (n = 42) among a total of 306 number of pigs included in the study. Non-infected healthy pigs (n = 6) served as healthy control. The total erythrocyte count (TEC), haemoglobin (Hb), packed cell volume (PCV), total leukocyte count (TLC), differential leukocyte count (DLC) and thrombocyte count were measured. The levels of total protein, albumin, globulin, total bilirubin, direct bilirubin, blood urea nitrogen (BUN), creatinine and glucose and enzymes viz. alanine transaminase (ALT), aspartate transaminase (AST), gamma-glutamyl transferase (GGT) and alkaline phosphatase (ALP) were measured. Oxidative stress indicators such as plasma malondialdehyde (MDA) and total anti-oxidant activity (TAOA) were measured using commercially available kits. The mean values of TLC, lymphocytes and thrombocyte count were significantly (P < 0.05) low in PCV2-infected pigs. The levels of globulin, AST, GGT, BUN and creatinine were significantly increased (P < 0.05) whereas levels of albumin and glucose significantly (P < 0.05) decreased in PCV2-infected pigs. The significant increase (P < 0.05) in MDA level and significant decrease (P < 0.05) in TAOA level were noticed in PCV2-infected animals as compared with healthy control. The present study supports immunosuppression, possible multiple organ damage and oxidative stress associated with naturally occurring PCV2 infection in pigs. Timely vaccination and managemental practices can reduce PCV2 infection in farms. In spite of many research studies, there is still paucity of detailed systemic study on haemato-biochemical alteration and oxidative stress associated with PCV2 infection.

Regulation of Apoptosis During Porcine Circovirus Type 2 Infection

Apoptosis, an indispensable innate immune mechanism, regulates cellular homeostasis by removing unnecessary or damaged cells. It contains three signaling pathways: the mitochondria-mediated pathway, the death receptor pathway and the endoplasmic reticulum pathway. The importance of apoptosis in host defenses is stressed by the observation that multiple viruses have evolved various strategies to inhibit apoptosis, thereby blunting the host immune responses and promoting viral propagation. Porcine Circovirus type 2 (PCV2) utilizes various strategies to induce or inhibit programmed cell death. In this article, we review the latest research progress of the apoptosis mechanisms during infection with PCV2, including several proteins of PCV2 regulate apoptosis via interacting with host proteins and multiple signaling pathways involved in PCV2-induced apoptosis, which provides scientific basis for the pathogenesis and prevention of PCV2.

Caspase-Dependent Apoptosis Induction via Viral Protein ORF4 of Porcine Circovirus 2 Binding to Mitochondrial Adenine Nucleotide Translocase 3

Apoptosis is an essential strategy of host defense responses and is used by viruses to maintain their life cycles. However, the apoptotic signals involved in virus replication are poorly known. In the present study, we report the molecular mechanism of apoptotic induction by the viral protein ORF4, a newly identified viral protein of porcine circovirus type 2 (PCV2). Apoptosis detection revealed not only that the activity of caspase-3 and -9 is increased in PCV2-infected and ORF4-transfected cells but also that cytochrome c release from the mitochondria to the cytosol is upregulated. Subsequently, ORF4 protein colocalization with adenine nucleotide translocase 3 (ANT3) was observed using structured illumination microscopy. Moreover, coimmunoprecipitation and pulldown analyses confirmed that the ORF4 protein interacts directly with mitochondrial ANT3 (mtANT3). Binding domain analysis further confirmed that N-terminal residues 1 to 30 of the ORF4 protein, comprising a mitochondrial targeting signal, are essential for the interaction with ANT3. Knockdown of ANT3 markedly inhibited the apoptotic induction of both ORF4 protein and PCV2, indicating that ANT3 plays an important role in ORF4 protein-induced apoptosis during PCV2 infection. Taken together, these data indicate that the ORF4 protein is a mitochondrial targeting protein that induces apoptosis by interacting with ANT3 through the mitochondrial pathway.IMPORTANCE The porcine circovirus type 2 (PCV2) protein ORF4 is a newly identified viral protein; however, little is known about its functions. Apoptosis is an essential strategy of the host defense response and is used by viruses to maintain their life cycles. In the present study, we report the molecular mechanism of the apoptosis induced by the ORF4 protein. The ORF4 protein contains a mitochondrial targeting signal and is an unstable protein that is degraded by the proteasome-dependent pathway. Viral protein ORF4 triggers caspase-3- and -9-dependent cellular apoptosis in mitochondria by directly binding to ANT3. We conclude that the ORF4 protein is a mitochondrial targeting protein and reveal a mechanism whereby circovirus recruits ANT3 to induce apoptosis.

Optimization on Preparation Conditions of Salidroside Liposome and Its Immunological Activity on PCV-2 in Mice

The aim of this study was to optimize the preparation conditions of salidroside liposome with high encapsulation efficiency (EE) and to study the immunological enhancement activity of salidroside liposome as porcine circovirus type 2 virus (PCV-2) vaccine adjuvant. Response surface methodology (RSM) was selected to optimize the conditions for the preparation of salidroside liposome using Design-Expert V8.0.6 software. Three kinds of salidroside liposome adjuvants were prepared to study their adjuvant activity. BALB/c mice were immunized with PCV-2 encapsulated in different kinds of salidroside liposome adjuvants. The PCV-2-specific IgG in immunized mice serum was determined with ELISA. The results showed that when the concentration of ammonium sulfate was 0.26 mol·L⁻¹, ethanol volume 6.5 mL, temperature 43°C, ethanol injection rate 3 mL·min⁻¹, and salidroside liposome could be prepared with high encapsulation efficiency of 94.527%. Salidroside liposome as adjuvant could rapidly induce the production of PCV-2-specific IgG and salidroside liposome I adjuvant proved to provide the best effect among the three kinds of salidroside liposome adjuvants.

Activation of extrinsic- and Daxx-mediated pathways in lymphoid tissue of PRRSV-infected pigs

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is a major infectious pathogen in pigs leading to huge economical losses worldwide. PRRSV is able to escape from host immunity and causes transient infections. In the present study, expression of different apoptotic markers and its connection with PRRSV were assessed in tonsil and mediastinal lymph node from PRRSV-infected pigs. Cleaved caspase (CCasp)8, CCasp9, Fas, Daxx, CCasp3 and PRRSV expression were analyzed by immunohistochemistry. An up-regulation of CCasp8, Fas and CCasp3 expression in lymphocytes and macrophages from both organs was found during PRRSV infection, indicating the activation of the extrinsic-mediated pathway of apoptosis. Moreover, Daxx expression was also enhanced in macrophages of both organs, suggesting a simultaneous caspase-independent pathway of apoptosis. A correlation between the expression of the different apoptotic markers and IL-10, IL-6 and TGF-β but not with PRRSV antigen was found in our study, which supports the hypothesis of an indirect mechanism in PRRSV-induced apoptosis.

Regulatory role of ASK1 in porcine circovirus type 2-induced apoptosis

Porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome which is an emerging and important swine disease, can induce apoptotic responses in vitro and in vivo. However, the molecular mechanism of PCV2-induced apoptosis is not well understood. Apoptosis signal-regulating kinase 1 (ASK1), serves as an upstream enzyme that activates the JNK and p38 MAPK pathways, has been shown to play a target role in the regulation of apoptosis. Here, we showed that PCV2 infection induced ASK1 phosphorylation which preceded activation of JNK1/2 and p38 signaling in the cultured cells, and that the phosphatidylinostol 3-kinase (PI3K)/Akt signaling triggered by PCV2 infection limited the extent of JNK1/2 and p38 activation and thereby apoptotic cell death. Furthermore, inhibition of JNK and p38 activation is associated with PI3K-dependent negative-regulation of ASK1 in the PCV2-infected cells. These results indicate an important regulatory role of ASK1 in PCV2-induced apoptotic responses.

The perils of pathogen discovery: origin of a novel parvovirus-like hybrid genome traced to nucleic acid extraction spin columns

Next-generation sequencing was used for discovery and de novo assembly of a novel, highly divergent DNA virus at the interface between the Parvoviridae and Circoviridae. The virus, provisionally named parvovirus-like hybrid virus (PHV), is nearly identical by sequence to another DNA virus, NIH-CQV, previously detected in Chinese patients with seronegative (non-A-E) hepatitis. Although we initially detected PHV in a wide range of clinical samples, with all strains sharing ∼99% nucleotide and amino acid identity with each other and with NIH-CQV, the exact origin of the virus was eventually traced to contaminated silica-binding spin columns used for nucleic acid extraction. Definitive confirmation of the origin of PHV, and presumably NIH-CQV, was obtained by in-depth analyses of water eluted through contaminated spin columns. Analysis of environmental metagenome libraries detected PHV sequences in coastal marine waters of North America, suggesting that a potential association between PHV and diatoms (algae) that generate the silica matrix used in the spin columns may have resulted in inadvertent viral contamination during manufacture. The confirmation of PHV/NIH-CQV as laboratory reagent contaminants and not bona fide infectious agents of humans underscores the rigorous approach needed to establish the validity of new viral genomes discovered by next-generation sequencing.

Activation of NF-κB contributes to production of pig-major acute protein and serum amyloid A in pigs experimentally infected with porcine circovirus type 2

Acute phase proteins (APPs) have protective and regulatory roles in the inflammatory response. Previous studies indicate that APPs in serum change after pigs are infected with porcine circovirus type 2 (PCV2), but the mechanisms underlying APP production have remained unclear. In this present study, 35-day-old pigs were challenged with PCV2 and responses compared to an uninfected control group. To investigate the concentrations of APPs in serum and the activity of NF-κB in the liver, five pigs in the PCV2-infected group were euthanized at 14, 21 and 35days post inoculation (dpi) while four pigs were sacrificed in the control group at 0, 14, 21 and 35 days, respectively. The concentrations of pig-major acute protein (Pig-MAP), C-reactive protein (CRP) and serum amyloid A (SAA) in infected animals were increased at 14 and 21 dpi, while the concentration of alpha-1 acid glycoprotein (AGP) was lower at 35 dpi, indicating that PCV2 induced the production of APPs. Moreover, the DNA binding activity of NF-κB and expression levels of NF-κB p65 subunit (NF-κB p65) from the cytoplasm to nucleus were increased at 14 and 21 dpi in the liver of infected pigs, while the phosphorylation of IκBα (p-IκBα) in the liver was also increased at 21dpi. This demonstrated that PCV2 infection induced the activation of NF-κB. Both SAA and Pig-MAP concentrations correlated significantly with expression levels of NF-κB p65, indicating that activation of NF-κB contributes to the production of SAA and Pig-MAP.

Activation of the phosphatidylinositol 3-kinase/Akt signaling pathway during porcine circovirus type 2 infection facilitates cell survival and viral replication

Virus infection activates host cellular signaling pathways, including the phosphatidylinositol 3-kinase (PI3K)/Akt pathway, which regulates diverse cellular activities related to cell growth, survival, and apoptosis. The present study demonstrated for the first time that porcine circovirus type 2 (PCV2), a major causative agent of postweaning multisystemic wasting syndrome, which is an emerging and important swine disease, can transiently induce the PI3K/Akt pathway in cultured cells at an early step during PCV2 infection. Activation of the PI3K/Akt signal was also induced by UV-irradiated PCV2, indicating that virus replication was not required for this induction. Inhibition of PI3K activation leads to reduced virus yield, which is associated with decreased viral DNA replication and lower virus protein expression. However, inhibition of PI3K activation greatly enhanced apoptotic responses as evidenced by the cleavage of poly-ADP ribose polymerase and caspase-3 as well as DNA fragmentation using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end-labeling staining during the early stage of PCV2 infection. Furthermore, the pancaspase inhibitor zVAD.fmk alleviated the reduction in Akt phosphorylation levels by inhibiting PI3K activation, indicating that the signaling promotes cell survival and thereby favors viral replication. These results reveal that an antiapoptotic role for the PI3K/Akt pathway induced by PCV2 infection to suppress premature apoptosis for improved virus growth after infection, extending our understanding of the molecular mechanism of PCV2 infection.