Pathogenicity and immune modulation of porcine circovirus 3

Subunit vaccine of PCV3 capsid protein produced by sf9 cells with double knockout of Caspase-1 and Dronc induces strong immune response in mice

Porcine circovirus type 3 (PCV3) associated with multisystemic clinicopathological diseases in swine herds has caused economic losses and there is no available commercial vaccine. Production of PCV3 capsid protein (Cap) by Spodoptera frugiperda 9 (sf9) cells using baculovirus expression vector system (BEVS) is a valid strategy to develop vaccines. Here, we report that subunit vaccine of PCV3 produced by sf9 cells with double knockout of Caspase-1 and Dronc genes induces strong immune response in mice. Three kinds of knockout sf9 cells aimed at Caspase-1 gene, Dronc gene and both genes were successfully generated by clustered regularly interspaced short palindromic repeats-associated protein 9 (CRISPR-Cas9) system, and sequence analysis confirmed this. The anti-apoptosis ability of three kinds of knockout sf9 cells was assessed, and double knockout sf9 cells are the best. The expression of PCV3 Cap was enhanced in double knockout sf9 cells compared to wild type sf9 cells, and subunit vaccines were produced by PCV3 Cap expressed from double knockout sf9 cells and wild type cells, respectively. Results of immunological experiment in mice showed subunit vaccine of PCV3 Cap from double knockout sf9 cells induces higher level of serum antibody, stimulates lymphocyte proliferation and enhances expression of IL-2, IFN-γ, IL-4 and IL-10 compared to wild type cells. These results present knockout sf9 cells to enhance the expression of protein in BEVS, and provide a technical platform for vaccine development of PCV3.

Significant Detection of Porcine Circovirus 3 and Porcine Circovirus 4 in Wild Boars from Mid-Western Spain Without Apparent Sanitary Consequences

PCV-3 and PCV-4 are novel viruses that can infect domestic pigs and wild boars. Both viruses are associated with multiple disorders in domestic pigs (reproductive failure, respiratory damage, etc.). However, the clinical impact on wild boars remains unknown. This study aimed to assess the presence of these viruses in wild boars from mid-western Spain and their sanitary impact on the species. A total of 166 submandibular lymph nodes were collected from hunted wild boars, along with available information about their reproductive status, lung injuries, body condition, and tuberculosis status. The samples were used to detect PCV-3 and PCV-4 using real-time PCR. In total, 84.9% of the sampled animals were positive for PCV-3, and 33.7% were positive for PCV-4. The detection of PCV-4 was more frequent in wild boars that had received supplementary feeding, suggesting that the direct contact favored by this practice could increase the transmission of this virus in wild boar populations. The infections did not seem to influence the body condition, reproductive status, lung lesions, or TB lesion severity patterns in the studied animals. Thus, although these viruses have been widely detected throughout wild boar populations from the studied area, they do not seem to be a health threat to this species. Nevertheless, their monitoring in wild boars is recommended, as they are often in contact with extensively reared pigs, which are susceptible to these viruses.

Development and Application of a Fully Automated Chemiluminescence Enzyme Immunoassay for the Detection of Antibodies Against Porcine Circovirus 3 Cap

Porcine circovirus 3 (PCV3) is a small non-enveloped circovirus associated with porcine dermatitis and nephropathy syndrome (PDNS). It has occurred worldwide and poses a serious threat to the pig industry. However, there is no commercially available vaccine. PCV3 capsid protein (Cap) is an ideal antigen candidate for serodiagnosis. Here, a novel fully automated chemiluminescence enzyme immunoassay (CLEIA) was developed to detect antibodies (Abs) to Cap in porcine serum. Recombinant PCV3 Cap, self-assembled into virus-like particles (VLPs), was produced using baculovirus and coupled to magnetic particles (Cap-MPs) as carriers. Combined with an alkaline phosphatase (AP)-adamantane (AMPPD) system, Cap-Abs can be rapidly measured on a fully automated chemiluminescence analyzer. Under optimal conditions, a cut-off value of 31,508 was determined, with a diagnostic sensitivity of 96.8% and specificity of 97.3%. No cross-reactivity was observed with PCV1 and PCV2 and other common porcine pathogens, and both intra-assay and inter-assay coefficients were less than 5% and 10%, respectively. Prepared Cap-MPs can be stored at 4 °C for more than 6 months. Importantly, this CLEIA had a good agreement of 95.19% with the commercially available kit, demonstrating excellent analytical sensitivity and significantly reduced operating time and labor. A serological survey was then conducted, and showed that PCV3 continues to spread widely in South China. In conclusion, our CLEIA provides time and labor-saving, and a reliable tool for PCV3 epidemiological surveillance.

Human Circovirus in Patients with Hepatitis, Hong Kong

Circovirus human is a new viral species that includes the human circovirus (HCirV), which has been linked to hepatitis in immunocompromised persons. We investigated prevalence of HCirV infection in 278 patients with hepatitis and 184 asymptomatic persons using real-time PCR and sequencing assays. HCirV viremia and sequences were found in 8 (2.9%) hepatitis patients and no asymptomatic patients. Alternate causes of hepatitis (hepatitis E and cholangitis) were clearly identifiable in 2 HCirV-infected patients. HCirV could not be ruled out as a contributor to hepatitis in the remaining 6 patients, 4 of whom were immunocompromised. Persistent infections were documented in 3 patients, but only 1 had relapsing hepatitis. One HCirV patient displayed symptoms of an infectious mononucleosis-like syndrome. Isolates clustered with known HCirV strains from France and China. HCirV-derived virus-like particles bound to PLC/PRF/5 and Hep-G2 human hepatoma cells but not to lung epithelial cells, indicating hepatic tropism.

Antiviral role of cholesterol 25-hydroxylase in inhibiting porcine circovirus 3 replication

Cholesterol 25-hydroxylase (CH25H) has significant antiviral effects through the production of 25-hydroxycholesterol (25HC). In this study, we investigated the effects of CH25H, its catalytic product 25HC, and its catalytic mutant lacking hydroxylase activity (CH25H-M) on porcine circovirus 3 (PCV3) replication. By transfecting PCV3 persistently infected PK-15 cells with the pCAGGS-CH25H-Flag plasmid, the results demonstrated that overexpression of CH25H significantly inhibited PCV3 Cap protein expression, Cap mRNA levels, and viral titers in a dose-dependent manner. Moreover, its catalytic product 25HC inhibited PCV3 replication in PK-15 cells at concentrations below 10 µM without affecting cell viability. In contrast, knockdown of endogenous CH25H using small interfering RNA (siRNA) enhanced PCV3 replication, further confirming its antiviral role. Interestingly, the CH25H-M mutant also exhibited inhibitory effects on PCV3 replication, although the inhibition was much less effective compared with CH25H. In conclusion, CH25H plays a critical role in regulating PCV3 replication, and its antiviral effect is not entirely dependent on its enzymatic activity. These findings provide new insights into both the enzymatic and non-enzymatic antiviral mechanisms of CH25H and revealed some mechanistic immune evasion for PCV3.

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.