Changes in CD4(+), CD8(+), CD4(+) CD8(+), and immunoglobulin M-positive peripheral blood mononuclear cells of postweaning multisystemic wasting syndrome-affected pigs and age-matched uninfected wasted and healthy pigs correlate with lesions and porcine circovirus type 2 load in lymphoid tissues

Genomic composition and pathomechanisms of porcine circoviruses: A review

Porcine circovirus (PCV) belongs to the genus Circovirus within the family Circoviridae; it has the smallest genome and a complicated classification system comprising PCV1, PCV2, PCV3, and PCV4. Most types of these viruses can cause animals to develop serious diseases; in pigs in particular, it may manifest as postweaning multisystemic wasting syndrome (PMWS), reproductive failure, porcine dermatitis and nephropathy syndrome (PDNS), congenital tremors (CTs), proliferative and necrotizing pneumonia (PNP), lymphoid injury, and immunosuppression. Different types of PCVs cause different types of diseases and sometimes feature no pathogenicity; these various PCV types are associated with different pathomechanisms in animals. In this review, the genomic composition and systemic pathomechanisms of porcine circoviruses are introduced, and future research prospects are discussed.

Interaction between host cell proteins and open reading frames of porcine circovirus type 2

Postweaning multisystemic wasting syndrome (PMWS) is caused by a systemic inflammation after porcine circovirus type 2 (PCV2) infection. It was one of the most economically important pathogens affecting pig production worldwide before PCV2 vaccine was first introduced in 2006. After the development of a vaccine against PCV2a type, pig farms gradually restored enormous economic losses from PMWS. However, vaccine against PCV2a type could not be fully effective against several different PCV2 genotypes (PCV2b - PCV2h). In addition, PCV2a vaccine itself could generate antigenic drift of PCV2 capsid. Therefore, PCV2 infection still threats pig industry worldwide. PCV2 infection was initially found in local tissues including reproductive, respiratory, and digestive tracks. However, PCV2 infection often leads to a systemic inflammation which can cause severe immunosuppression by depleting peripheral lymphocytes in secondary lymphoid tissues. Subsequently, a secondary infection with other microorganisms can cause PMWS. Eleven putative open reading frames (ORFs) have been predicted to encode PCV2 genome. Among them, gene products of six ORFs from ORF1 to ORF6 have been identified and characterized to estimate its functional role during PCV2 infection. Acquiring knowledge about the specific interaction between each PCV2 ORF protein and host protein might be a key to develop preventive or therapeutic tools to control PCV2 infection. In this article, we reviewed current understanding of how each ORF of PCV2 manipulates host cell signaling related to immune suppression caused by PCV2.

TGF-β from the Porcine Intestinal Cell Line IPEC-J2 Induced by Porcine Circovirus 2 Increases the Frequency of Treg Cells via the Activation of ERK (in CD4+ T Cells) and NF-κB (in IPEC-J2)

Porcine circovirus 2 (PCV2) causes immunosuppression. Piglets infected with PCV2 can develop enteritis. Given that the gut is the largest immune organ, however, the response of the gut's immune system to PCV2 is still unclear. Here, IPEC-J2 cells with different treatments were co-cultured with PBMC or CD4⁺ T cells (Transwell). Flow cytometry and Western blotting revealed that PCV2-infected IPEC-J2 increased the frequency of CD4⁺ T cells among piglets' peripheral blood mononuclear cells (PBMCs) and caused CD4⁺ T cells to undergo a transformation into Foxp3⁺ regulatory T cells (Treg cells) via activating CD4⁺ T ERK. Cytokines production and an inhibitor assay showed that the induction of Tregs by PCV2-infected IPEC-J2 was dependent on TGF-β induced by PCV2 in IPEC-J2, which was associated with the activation of NF-κB. Taken together, PCV2-infected IPEC-J2 activated NF-κB to stimulate the synthesis of TGF-β, which enhanced the differentiation of CD4⁺ T cells into Treg cells through the activation of ERK in CD4⁺ T cells. This information sheds light on PCV2's function in the intestinal immune system and suggests a potential immunosuppressive mechanism for PCV2 infection.

Host immune response to infection with porcine circoviruses

Porcine circovirus type 2 (PCV2), which serves as a major causative agent of PCV2-associated diseases and causes severe loss to the pig industry worldwide, can dysregulate the immune response and induce immunosuppression in PCV2-infected pigs. Similar to PCV2, porcine circovirus type 3 (PCV3), a newly identified swine circovirus which might be closely associated with porcine dermatitis and nephropathy syndrome, reproductive disorder, and multisystemic inflammatory responses, also interferes with host immune defense. Interaction between host immune system and PCVs is considered to be a crucial determinant of pathogenicity in pigs. Here, we sought to briefly discuss the current knowledge regarding the interaction of porcine circovirus type 2 and/or 3 with host immune cells and immune responses to better depict the viral immunomodulatory capacity, pathogenic mechanisms, and the future research direction in host immune responses to infection with PCV2 and PCV3.

PCV2 Triggers PK-15 Cell Apoptosis Through the PLC-IP3R-Ca2+ Signaling Pathway

The endoplasmic reticulum (ER) plays an essential role in Ca²⁺ concentration balance and protein biosynthesis. During infection, the virus needs to complete its life process with the help of ER. At the same time, ER also produces ER stress (ERS), which induces apoptosis to resist virus infection. Our study explored the Ca²⁺ concentration, ERS, and the apoptosis mechanism after porcine circovirus 2 (PCV2) infection. We show here that PCV2 infection induces the increased cytoplasmic Ca²⁺ level and PK-15 cell ER swelling. The colocalization of phospholipase C (PLC) and inositol 1,4,5-trisphosphate receptor (IP3R) in the cytoplasm was observed by laser confocal microscopy. Western blot and quantitative polymerase chain reaction experiments confirmed that PLC and IP3R expression levels increased after PCV2 infection, and Ca²⁺ concentration in the cytoplasm increased after virus infection. These results suggest that PCV2 infection triggers ERS of PK-15 cells via the PLC–IP3R–Ca²⁺ signaling pathway to promote the release of intracellular Ca²⁺ and led to cell apoptosis.

Pathogenicity and immune response against porcine circovirus type 3 infection in caesarean-derived, colostrum-deprived pigs

Recently, a novel PCV species (PCV3) has been detected in cases associated with sow mortality, lesions consistent with porcine dermatitis and nephropathy syndrome, reproductive failure and multisystemic inflammation. The pathogenesis and clinical significance of PCV3 is still unclear. In this study, we investigated the immunopathogenesis of PCV3 in CD/CD pigs. Four treatment groups, PCV3 (n=6), PCV3-KLH (n=6), control (n=3) and control-KLH (n=3), were included with PCV3-positive tissue homogenate (gc=3.38×10¹² ml^-1 and gc=1.04×10¹¹ ml^-1), confirmed by quantitative PCR (qPCR) and next-generation sequencing. Clinical signs, viremia, viral shedding, systemic cytokines, humoral (IgG) and T-cellular response were evaluated for 42 days. At necropsy, tissues were collected for histological evaluation and PCV3 detection by qPCR and in situ hybridization. No significant clinical signs were observed through the study. Viremia was detected in both PCV3-inoculated groups from 3 days post-inoculation (p.i.) until the end of the study. Nasal shedding was detected from 3 to 28 days p.i. and faecal shedding was transient. PCV3 induced an early (7 days p.i.) and sustained (42 days p.i.) IgG response. No significant T-cell response was observed. Histological evaluation demonstrated lesions consistent with multisystemic inflammation and perivasculitis. All tissues evaluated were positive by qPCR and virus replication was confirmed by positive in situ hybridization. This study demonstrated the potential role of PCV3 in subclinical infection, producing a mild, multisystemic inflammatory response, prolonged viremia detectable for 42 days p.i., presence of IgG humoral response and viral shedding in nasal secretions. More research is required to understand and elucidate potential co-factors necessary in the manifestation and severity of clinical disease.

Evaluation of CVC1302 for Improved Efficacy of FMD-Inactivated Vaccine in Oxidative Stressed Mice Generated with PCV2 Infection

This study aimed to explore the effect of the immunopotentiator CVC1302 on foot-and-mouth disease (FMD) vaccination in animals placed under oxidative stress. We established oxidative stress models using porcine circovirus type 2 (PCV2)-infected PK-15 cells and mice model both in vitro and in vivo, respectively. The efficacy of CVC1302 on PK-15 cells or in addition to the FMD vaccine was evaluated by quantitative real-time polymerase chain reaction, histopathological and enzyme-linked immunosorbent assay (ELISA) analysis. CVC1302 affected apoptosis of PCV2-infected PK-15 cells and significantly inhibited PCV2 replication, while it had no effect on the viability for blank PK-15 cell in vitro test with varying dilutions of CVC1302. Results showed that PCV2 induced a strong oxidative stress response in mice. CVC1302 reduced the viral load in spleen of PCV2-infected mice and ameliorated the pathological injury of spleen. Furthermore, CVC1302 significantly increased IgG antibody titer, cytokine expression, superoxide dismutase activity, catalase concentrations, and glutathione content in mice immunized with FMD vaccine. In conclusion, CVC1302 inhibits PCV2 replication and regulates oxidative stress in PCV2-infected mice, which can improve the immune efficacy of the FMD vaccine, providing a safe and effective immune enhancement.

Therapeutic and immune function improvement of vitamin D combined with IFN-α on mouse with hepatitis B infection

This study aims to investigate the therapeutic effect of vitamin D (VD) combined with interferon (IFN) type I (IFN-α) on mice with hepatitis B and to explore the possible mechanism. The mice were divided into control group, model group, IFN-α group, and IFN-α ⁺ VD group. After 4 weeks, the mice of four groups were weighed, and the thymus and spleen indexes were calculated. The activity of alanine transaminase (ALT) and aspartate transaminase (AST) in the serum was detected while pathological changes of the liver were observed. The levels of interferon-γ (IFN-γ), interleukin-2 (IL-2), and interleukin-4 (IL-4) in serum were measured by enzyme-linked immunosorbent assay (ELISA). The proliferation of spleen lymphocytes was detected by MTT assay. Flow cytometry was used to detect the level of CD4⁺, CD8⁺, and CD4⁺/CD8⁺ in peripheral blood. The levels of ALT and AST in serum were significantly lower in the IFN-α ⁺ VD group than those in the IFN-α group, but the thymus and spleen indexes were significantly higher. Although both IFN-α and IFN-α ⁺ VD can improve the damaged structure of live tissue, IFN-α ⁺ VD achieved higher efficacy than IFN-α alone. The serum IFN-γ, TNF-α, and IL-2 levels were lower in the IFN-α ⁺ VD group compared with the IFN-α group, and no significant difference was found in IL-4. Compared with the IFN-α group, the percentage of CD4⁺ and the D4⁺/CD8⁺ ratio were significantly increased, but the percentage of CD8⁺ was reduced. The proliferation rate of splenic lymphocytes was higher in the IFN-α ⁺ VD group compared with the IFN-α group. IFN-α ⁺ VD was found to achieve higher efficacy than IFN-α alone for the treatment of hepatitis B in mice, possibly through increasing the immune level of mice.

Association between endotoxin levels in dust from indoor swine housing environments and the immune responses of pigs

Indoor animal husbandry environments are inevitably contaminated with endotoxins. Endotoxin exposure is associated with various inflammatory illnesses in animals. This cross-sectional study evaluated the relationship between the degree of endotoxin exposure and the cellular and humoral immune profiles of fattening pigs. Blood samples were taken from the jugular vein of 47 pigs from ten pig farms in Korea. Whole blood cell counts and plasma immunoglobulin (Ig) classes were determined. Peripheral-blood mononuclear cells were stimulated in vitro with concanavalin A for 48 h, and cytokines released into culture supernatants were measured. The barns in which the pigs lived were assessed for endotoxin levels in the total and respirable dust by using the limulus amebocyte lysate kinetic QCL method. Low and high endotoxin exposures were defined as ≤ 30 and > 30 EU/m³, respectively. Compared to pigs with low endotoxin exposure (n = 19), highly exposed pigs (n = 28) had higher circulating neutrophil and lymphocyte (particularly B cells) counts, IgG and IgE levels, interferon-gamma (IFNγ) and interleukin (IL)-4 productions, and lower IgA levels and tumor necrosis factor-alpha (TNFα) production. The IL-4, IFNγ, and TNFα levels significantly correlated with endotoxin level and/or pig age. Constant exposure of pigs to high levels of airborne endotoxins can lead to aberrant immune profiles.

Porcine Circovirus Type 2 (PCV2) Vaccines in the Context of Current Molecular Epidemiology

Porcine circovirus type 2 (PCV2) is an economically important swine pathogen and, although small, it has the highest evolution rate among DNA viruses. Since the discovery of PCV2 in the late 1990s, this minimalistic virus with a 1.7 kb single-stranded DNA genome and two indispensable genes has become one of the most important porcine pathogens, and presently is subjected to the highest volume of prophylactic intervention in the form of vaccines in global swine production. PCV2 can currently be divided into five different genotypes, PCV2a through PCV2e. It is well documented that PCV2 continues to evolve, which is reflected by changes in the prevalence of genotypes. During 2006, commercial vaccines for PCV2 were introduced on a large scale in a pig population mainly infected with PCV2b. Since 2012, the PCV2d genotype has essentially replaced the previously predominant PCV2b genotype in North America and similar trends are also documented in other geographic regions such as China and South Korea. This is the second major PCV2 genotype shift since the discovery of the virus. The potential increase in virulence of the emergent PCV2 genotype and the efficacy of the current vaccines derived from PCV2a genotype against the PCV2d genotype viruses has received considerable attention. This review attempts to synthesize the understanding of PCV2 biology, experimental studies on the antigenic variability, and molecular epidemiological analysis of the evolution of PCV2 genotypes.

Effect of sow vaccination against porcine circovirus type 2 (PCV2) on virological profiles in herds with or without PCV2 systemic disease

We investigated porcine circovirus type 2 (PCV2) virological profiles in herds affected (PCVAD-AH, n = 5) or non-affected (PCVAD-NAH, n = 4) by PCV2-associated diseases (PCVAD), before and after 1 y of PCV2 gilt and sow vaccination. Fresh feces from the floor (5 pens/age/farm) and 5 blood samples (1/pen) were collected at 3, 9, 15, 21 wk. Individual feces and blood samples were collected from 5 gilts and 15 sows. Sampling was repeated 1 y after vaccination. Quantitative PCR on feces, PCV2 antibodies in blood serum and cell-mediated immunity were investigated. Before vaccination, pigs of PCVAD-AH had higher viral load in feces (9 and 15 wk), lower IgG and higher IgM (3 wk) and lower lymphocyte counts (9 and 15 wk) suggesting immunosuppression. Vaccination reduced viral load in growers, increased IgG (3 wk) suggesting improved maternal immunity, reduced IgM (3 wk), increased total antibody titers in sows and increased CD79a cells in the pigs.

T-cell reprogramming through targeted CD4-coreceptor and T-cell receptor expression on maturing thymocytes by latent Circoviridae family member porcine circovirus type 2 cell infections in the thymus

Although porcine circovirus type 2 (PCV2)-associated diseases have been evaluated for known immune evasion strategies, the pathogenicity of these viruses remained concealed for decades. Surprisingly, the same viruses that cause panzootics in livestock are widespread in young, unaffected animals. Recently, evidence has emerged that circovirus-like viruses are also linked to complex diseases in humans, including children. We detected PCV2 genome-carrying cells in fetal pig thymi. To elucidate virus pathogenicity, we developed a new pig infection model by in vivo transfection of recombinant PCV2 and the immunosuppressant cofactor cyclosporine A. Using flow cytometry, immunofluorescence and fluorescence in situ hybridization, we found evidence that PCV2 dictates positive and negative selection of maturing T cells in the thymus. We show for the first time that PCV2-infected cells reside at the corticomedullary junction of the thymus. In diseased animals, we found polyclonal deletion of single positive cells (SPs) that may result from a loss of major histocompatibility complex class-II expression at the corticomedullary junction. The percentage of PCV2 antigen-presenting cells correlated with the degree of viremia and, in turn, the severity of the defect in thymocyte maturation. Moreover, the reversed T-cell receptor/CD4-coreceptor expression dichotomy on thymocytes at the CD4⁺CD8^interm and CD4SP cell stage is viremia-dependent, resulting in a specific hypo-responsiveness of T-helper cells. We compare our results with the only other better-studied member of Circoviridae, chicken anemia virus. Our data show that PCV2 infection leads to thymocyte selection dysregulation, adding a valuable dimension to our understanding of virus pathogenicity.

The comparative profile of lymphoid cells and the T and B cell spectratype of germ-free piglets infected with viruses SIV, PRRSV or PCV2

Lymphocyte subsets isolated from germ-free piglets experimentally infected with swine influenza virus (SIV), porcine reproductive and respiratory syndrome virus (PRRSV) or porcine circovirus type 2 (PCV2) were studied and the profile of these subsets among these three infections was monitored. Germ-free piglets were used since their response could be directly correlated to the viral infection. Because SIV infections are resolved even by colostrum-deprived neonates whereas PRRSV and PCV2 infections are not, SIV was used as a benchmark for an effectively resolved viral infection. PRRSV caused a large increase in the proportion of lymphocytes at the site of infection and rapid differentiation of B cells leading to a high level of Ig-producing cells but a severe reduction in CD2^—CD21⁺ primed B cells. Unlike SIV and PCV2, PRRSV also caused an increase in terminally differentiated subset of CD2⁺CD8α⁺ γδ cells and polyclonal expansion of major Vβ families suggesting that non-specific helper T cells drive swift B cell activation. Distinct from infections with SIV and PRRSV, PCV2 infection led to the: (a) prevalence of MHC-II⁺ T cytotoxic cells, (b) restriction of the T helper compartment in the respiratory tract, (c) generation of a high proportion of FoxP3⁺ T cells in the blood and (d) selective expansion of IgA and IgE suggesting this virus elicits a mucosal immune response. Our findings suggest that PRRSV and PCV2 may negatively modulate the host immune system by different mechanisms which may explain their persistence.

Memory T cell proliferative responses and IFN-γ productivity sustain long-lasting efficacy of a Cap-based PCV2 vaccine upon PCV2 natural infection and associated disease

Porcine circovirus type 2 (PCV2) vaccination represents an important measure to cope with PCV2 infection; however, data regarding the modulation of the immune cell compartment are still limited, especially under field conditions. This study is aimed at investigating the features of the cellular immune response in conventional piglets induced by vaccination using a capsid (Cap) protein-based PCV2 vaccine compared to unvaccinated animals when exposed to PCV2 natural infection. Immune reactivity was evaluated by quantifying peripheral cell subsets involved in the anti-viral response and characterizing the interferon-gamma (IFN-γ) secreting cell (SC) responsiveness both in vivo and upon in vitro whole PCV2 recall. The vaccination triggered an early and intense IFN-γ secreting cell response and induced the activation of peripheral lymphocytes. The early increase of IFN-γ SC frequencies resulted in a remarkable and transient tendency to increased IFN-γ productivity in vaccinated pigs. In vaccinated animals, soon before the onset of infection occurred 15-16 weeks post-vaccination, the recalled PCV2-specific immune response was characterized by moderate PCV2-specific IFN-γ secreting cell frequencies and augmented productivity together with reactive CD4+CD8+ memory T cells. Conversely, upon infection, unvaccinated animals showed very high frequencies of IFN-γ secreting cells and a tendency to lower productivity, which paralleled with effector CD4-CD8+ cytotoxic cell responsiveness. The study shows that PCV2 vaccination induces a long-lasting immunity sustained by memory T cells and IFN-γ secreting cells that potentially played a role in preventing the onset of infection; the extent and duration of this reactivity can be an important feature for evaluating the protective immunity induced by vaccination.

Rapid Detection of Viruses Using Loop-Mediated Isothermal Amplification (LAMP): A Review

Most of the diseases caused by viral infection are found to be fatal, and the diagnosis is difficult due to confusion with other causative agents. So, a highly efficient molecular-based advance detection technique, i.e., loop-mediated isothermal amplification (LAMP) method, is developed for diagnosis of viral infections by various workers. It is based on amplification of DNA at very low level under isothermal conditions, using a set of four specifically designed primers and a DNA polymerase with strand displacement activity. This technique is found to be superior than most of the molecular techniques like PCR, RT-PCR, and real-time PCR due to its high specificity, sensitivity, and rapidity. Major advantage of LAMP method is its cost-effectiveness as it can be done simply by using water bath or dry bath. Here, in this review information regarding almost all the effective LAMP techniques which is developed so far for diagnosis of numerous viral pathogens is presented.

Concurrent porcine circovirus type 2a (PCV2a) or PCV2b infection increases the rate of amino acid mutations of porcine reproductive and respiratory syndrome virus (PRRSV) during serial passages in pigs

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The genetic variability of PRRSV during serial passage in PCV2-infected pigs was investigated.

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PRRSV structural genes ORF6 and ORF7 mutated at various degrees over time in vivo.

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A significantly higher mutation rate was observed when pigs were co-infected with PCV2.

Porcine reproductive and respiratory syndrome virus (PRRSV) has a high degree of genetic and antigenic variability. The purpose of this study was to determine if porcine circovirus type 2 (PCV2) infection increases genetic variability of PRRSV during serial passages in pigs and to determine if there is a difference in the PRRSV mutation rate between pigs concurrently infected with PCV2a or PCV2b. After 8 consecutive passages of PRRSV alone (group 1), PRRSV with PCV2a (group 2), or PCV2b (group 3) in pigs, the sequences of PRRSV structural genes for open reading frame (ORF) 5, ORF6, ORF7 and the partial non-structural protein gene (Nsp) 2 were determined. The total number of identified amino acid mutations in ORF5, ORF6, ORF7 and Nsp2 sequences was 30 for PRRSV infection only, 63 for PRRSV/PCV2a concurrent infection, and 77 for PRRSV/PCV2b concurrent infection when compared with the original VR2385 virus used to infect the passage 1 pigs. Compared to what occurred in pigs infected with PRRSV only, the mutation rates in ORF5 and ORF6 were significantly higher for concurrent PRRSV/PCV2b infected pigs. The PRRSV/PCV2a pigs had a significantly higher mutation rate in ORF7. The results from this study indicated that, besides ORF5 and Nsp2, the PRRSV structural genes ORF6 and ORF7 were shown to mutate at various degrees when the PRRSV was passaged over time in vivo. Furthermore, a significantly higher mutation rate of PRRSV was observed when pigs were co-infected with PCV2 highlighting the importance of concurrent infections on PRRSV evolution and control.

Identification and functional analysis of the novel ORF4 protein encoded by porcine circovirus type 2

Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated diseases in pigs. To date, viral proteins Cap, Rep, Rep', and ORF3, encoded by the PCV2 genome, have been described. Here, transcription and translation of a novel viral gene within the PCV2 genome (designated ORF4) was determined and functionally analyzed in vitro and in vivo. Northern blot analysis indicated that the RNA transcribed from the ORF4 gene is about 180 bp in length and overlaps ORF3 in the same direction. Site-directed mutagenesis confirmed that the viral ORF4 protein is not essential for virus replication in PK-15 cells and in mice infected with an ORF4-deficient PCV2 (PCV2Δ). PCV2Δ triggered higher activity levels of caspase-3 and -8 than wild-type PCV2 (wPCV2) in PK-15 cells. The antigenic epitopes of two mouse monoclonal antibodies (MAbs) raised against the viral ORF4 protein were mapped to the same 19KSSASPR25 peptide. Expression of ORF4 was confirmed using the specific MAbs in wPCV2-infected PK-15 cells and mice. Mice infected with PCV2Δ had a higher serum viral load (genomic copies) and more severe lymphoid tissue damage in the spleen than those infected with wPCV2. Meanwhile, flow-cytometric analysis indicated that the PCV2Δ infection caused a significant decrease of CD4(+) and CD8(+) T lymphocytes. Our results demonstrate that ORF4 is a newly discovered viral protein that is not essential for PCV2 replication but plays a role in suppressing caspase activity and regulating CD4(+) and CD8(+) T lymphocytes during PCV2 infection.

Loop-mediated isothermal amplification for detection of porcine circovirus type 2

BACKGROUND

Porcine circovirus type 2 (PCV2) is the primary causative agent of the emerging swine disease known as postweaning multisystemic wasting syndrome (PMWS). Nowadays, polymerase chain reaction (PCR) is still the most widespread technique in pathogen detection. Loop-mediated isothermal amplification (LAMP), a novel nucleic acid amplification method developed in 2000, will possibly replace PCR in the field of detection. To establish a LAMP method for rapid detection of PCV2, two pairs of primers were designed specially from the open reading frame 2 (ORF2) sequences of PCV2. A LAMP method for rapid detection of PCV2 was established. To compare with PCR, sensitivity and specificity of LAMP were evaluated using the optimized reaction system. The LAMP products could be determined by agarose gel electrophoresis or adding SYBR Green I dye.

RESULTS

The amplification of LAMP could be obtained at 63°C for 60 min. The detection limit was nearly 1 copy of DNA plasmid, more sensitive than PCR. There was no cross-reaction with porcine circovirus type 1 (PCV1), porcine pseudorabies virus (PRV) and porcine parvovirus (PPV) under the same conditions.

CONCLUSIONS

LAMP is an useful rapid detection method with high sensitivity and specificity for PCV2.

Porcine circovirus type 2 (PCV2): genetic variation and newly emerging genotypes in China

Background

Porcine circovirus type 2 (PCV2), the causative agent of postweaning multisystemic wasting syndrome (PMWS), is a serious economic problem for the swine industry in China. In this study, we investigated the genetic variation of PCV2 in China using strains isolated from 2004-2008. Viruses were isolated from samples collected from pigs with multi-systemic lesions and clinical signs of PMWS from different regions of China, and the genomes of these viruses were sequenced. The assembled sequences were used to define the genotypes of these strains; PCR-RFLP methodology was used to distinguish isolates and capture ELISA was used to demonstrate the antigenic changes resulted from ORF2 gene mutation of the isolates.

Results

We identified 19 PCV2 isolates, including four newly emerging PCV2 mutant strains. The 19 isolates were designated into three genotypes (PCV2a, PCV2b and PCV2d). PCV2d represented a novel genotype and a shift from PCV2a to PCV2b as the predominant genotype in China was identified. This is the first report of 1766 nt PCV2 harboring a base deletion at other new different positions. Amino acid sequence analysis identified two novel ORF2 mutations (resulting in ORF2 sequences 705 and 708 nt in length) in three deletion strains (1766 nt) and one strain with a genome 1767 nt in length. Finding of two amino acids elongation of the ORF2-encoded Cap protein is firstly observed among PCV2 strains all over the world. The isolates were distinguished into different genotypes by PCR-RFLP methodology and antigenic changes were present in Cap protein of mutation isolates by capture ELISA.

Conclusions

The results of this study provide evidence that PCV2 is undergoing constant genetic variation and that the predominant strain in China as well as the antigenic situation has changed in recent years. Furthermore, the PCR-RFLP method presented here may be useful for the differential identification of PCV2 strains in future studies.

Transient correlation between viremia levels and IL-10 expression in pigs subclinically infected with porcine circovirus type 2 (PCV2)

Immunological impairment by porcine circovirus type 2 (PCV2) infection is well documented in pigs suffering from postweaning multisystemic wasting syndrome. Nonetheless, little is known about immune status of pigs that remain PCV2 subclinically infected. Thus, seven pigs successfully infected in an experimental inoculation and without developing disease and nine control non-inoculated pigs were examined. Serological, virological and immunological determinations were done throughout ten weeks post-infection (PI). At week 3 PI, inoculated animals presented the peak of viremia and produced higher levels of IL-10 than the controls; correlation between viral load and IL-10 amounts was observed (p < 0.05). Also, the ratio IgM/IgG suffered a shift skewing IgM production towards an IgG response. By 10 weeks PI, levels of IL-10 disappeared and the viremia decreased. In summary, subclinically PCV2-infected pigs developed a transient PCV2-specific IL-10 response during the viremic phase of infection which coincided with the inversion of the IgM/IgG ratio.

Porcine circovirus type 2 infection decreases the efficacy of a modified live porcine reproductive and respiratory syndrome virus vaccine

Porcine reproductive and respiratory syndrome virus (PRRSV)-induced pneumonia is a major problem, and vaccination is used to reduce losses associated with PRRSV. Porcine circovirus type 2 (PCV2) causes lymphoid depletion, and there is concern that this adversely affects the immune response. The objective of this study was to investigate the effect of PCV2 infection on the efficacy of modified live virus (MLV) PRRSV vaccine. Sixty-nine 2-week-old pigs were randomly assigned to one of seven groups of 9 to 10 pigs each. At 6 weeks of age, pigs in groups 4, 5, and 6 were inoculated intranasally with PCV2 ISU-40895. At 8 weeks of age, groups 3, 4, 6, and 7 were vaccinated with a PRRSV MLV vaccine. At 12 weeks of age, groups 2, 3, and 4 were challenged with PRRSV SDSU73. All pigs were necropsied 14 days after PRRSV challenge. PCV2-infected, PRRSV-vaccinated, and PRRSV-challenged pigs had significantly (P < 0.05) more-severe macroscopic lung lesions than did the PRRSV-vaccinated and PRRSV-challenged pigs that were not exposed to PCV2 prior to PRRSV vaccination. Nonvaccinated PRRSV-infected pigs had a significantly (P < 0.001) higher incidence of PRRSV antigen in lungs than did all other groups except the group infected with PCV2 prior to PRRSV vaccination and challenge. The nonvaccinated PRRSV-challenged group and the group challenged with PCV2 prior to PRRSV vaccination and challenge had significantly (P < 0.001) lower average daily weight gain than did the control and the vaccinated groups. This work suggests that PCV2 infection has an adverse effect on the development of protective immunity induced by PRRSV vaccine.

The ORF3 protein of porcine circovirus type 2 is involved in viral pathogenesis in vivo

Porcine circovirus type 2 (PCV2) is the primary causative agent of an emerging swine disease, postweaning multisystemic wasting syndrome. We previously showed that a novel identified protein, ORF3, was not essential for PCV2 replication in cultured PK15 cells and played a major role in virus-induced apoptosis. To evaluate the role of the ORF3 protein in viral pathogenesis in vivo, we inoculated 8-week-old BALB/c mice that have been developed for PCV2 replication with ORF3-deficient mutant PCV2 (mPCV2). By 42 days postinoculation, all of the mice inoculated with the mPCV2, as well as with wild-type PCV2 (wPCV2), had seroconverted to PCV2 capsid antibody, and the mutant induced levels of PCV2 antibodies that were higher than those of the wPCV2. The PCV2 genomic copy numbers in serum were significantly higher (P<0.05) in the wPCV2-inoculated mice than in mice inoculated with the mPCV2. Also, the wPCV2 caused microscopic lesions characterized by lymphocyte depletion with histiocytic infiltration of lymphoid organs, but the mutant virus failed to induce any obvious pathological lesions. In situ hybridization and immunohistochemical analyses also showed that larger amounts of viral DNA and antigens were detected in the lymph nodes of the wPCV2-inoculated than mPCV2-inoculated mice. Furthermore, animals of the wPCV2-inoculated group showed significant downshifts of CD8(+) T-cell subsets of peripheral blood lymphocytes compared to the control mice (P<0.05) at various time points postinoculation. Also, the proportions of the CD4(+) and CD4(+) CD8(+) cells were significantly reduced in wPCV2-inoculated mice at some time points postinoculation. In contrast, there are some reductions in the proportions of these subsets in the mutant virus-inoculated mice, but the proportions do not decrease significantly. Taken together, these results demonstrate that the ORF3 protein is also dispensable for viral replication in vivo and that it plays an important role in viral pathogenesis.

Serological investigation and genomic characterization of PCV2 isolates from different geographic regions of Zhejiang province in China

Sera collected from 46 swine farms in Zhejiang province were evaluated for the presence of antibodies to PCV2 using an indirect-fluorescent antibody procedure. In addition PCV2 isolated from superficial inguinal lymph node samples collected from 40-to 90-day-old pigs with clinical signs of post-weaning multisystemic wasting syndrome (PMWS) using the PK-15 cell line were sequenced and compared. Overall seroprevalence of PCV2 antibody averaged 58.34% for all samples. Breakdown of serology by groups was as follows: 59.38% for sows, 57.41% for post-weaning piglets, 44.83% for Landrace sows and 64.28% for Landrace piglets. The seroprevalence of Landrace sows was higher than that of Yorkshire and Duroc sows, but non-significant (p > 0.05). Serological analysis also showed that seroprevalence of PCV2 antibody was a negative correlation to that of PRRSV antibody. The complete genomes of five PCV2 isolates identified in the herds with PMWS consisted of 1767nt, containing the 11 potential ORFs. Genome of the virus isolates shared 93.8% to 99.8% identity with PCV2 reference strains from GenBank, 76.6% to 77.9% identity with PCV1. Phylogenetic analysis indicated that there were two subgenotypes within PCV2: subgenotype I (1767 nt) and subgenotype II (1768 nt).

Correlation Between Type of Adaptive Immune Response Against Porcine Circovirus Type 2 and Level of Virus Replication

Porcine circovirus 2 (PCV2) replication is characterized by high variation among infected pigs. This study investigated the role of immunologic responses in causing this variation. Twelve gnotobiotic pigs were inoculated with PCV2. Four of these pigs were treated with cyclosporin A (CysA) to monitor the effect of the adaptive immunity on the development of the PCV2 infection. Through lymph node biopsies at 10, 15, and 21 days postinoculation (DPI), PCV2 replication in lymphoid tissues was monitored. The production of total PCV2-specific and PCV2-neutralizing antibodies was followed, together with interferon-gamma (IFN-gamma) mRNA expression levels in peripheral blood monocytes as a marker for cellular immunity. In general, the CysA-treated pigs showed the highest PCV2 titers, indicating that the adaptive immunity is necessary to restrain PCV2 replication. Three different PCV2 replication patterns were observed in non-CysA-treated pigs. Pattern 1: In two pigs, PCV2 was not detected. They had the highest neutralizing antibody titers, appearing from 15 DPI. In these pigs a good cellular response was indicated by a peak in IFN-gamma mRNA at 15 DPI. Pattern 2: Five pigs contained low to moderate PCV2 titers at 15 DPI, remaining constant or decreasing towards 21 DPI. Lower neutralizing antibody titers were observed and no rise in IFN-gamma was detected. Pattern 3: In one pig, a low PCV2 titer at 15 DPI dramatically increased toward 21 DPI. Although an antibody response against PCV2 was mounted, no PCV2-neutralizing antibodies were detected. This pig also showed no rise in IFN-gamma. The study findings indicate that variation in the onset of the adaptive immunity may account for variation in PCV2 replication among pigs. Absence of PCV2-neutralizing antibodies may be an important factor in the development of an increased virus replication.

Use of a polymerase chain reaction assay and an ELISA to monitor porcine circovirus type 2 infection in pigs from farms with and without postweaning multisystemic wasting syndrome

OBJECTIVE

To determine whether correlations exist between viremia with porcine circovirus type 2 (PCV2) and serum antibody profiles and between detection of PCV2 in nasal cavities and viremia of pigs from farms with and without postweaning multisystemic wasting syndrome (PMWS).

ANIMALS

495 pigs, ranging from the late nursery stage to the early grower-finisher stage of production.

PROCEDURE

Serum antibodies to PCV2 were studied with an ELISA that detects the ORF2 viral protein. Nasal swab specimens and serum samples were tested with a PCV2-specific PCR assay.

RESULTS

PCV2 DNA and serum antibodies to PCV2 were detected in pigs from all farms, although in different proportions. Overall, PCV2 DNA was detected in greater percentages in serum samples and nasal swab specimens of pigs from farms with PMWS. Although viral DNA was detected in both serum samples and nasal swab specimens, PCV2 detection in nasal swab specimens was higher than in serum samples of pigs from all farms. Serum antibodies to PCV2 were detected in a greater percentage of pigs from farms with PMWS, compared with farms without PMWS.

CONCLUSIONS AND CLINICAL RELEVANCE

A high prevalence of PCV2 infection was found in pigs from farms with and without PMWS. Besides the presence of PCV2, unknown additional factors may be necessary to induce the full expression of PMWS.

Dendritic cells harbor infectious porcine circovirus type 2 in the absence of apparent cell modulation or replication of the virus

Dendritic cells (DCs) play crucial roles in innate and adaptive immune responses, rendering them critical targets for virus infections. Porcine circovirus type 2 (PCV2) is associated with the development of postweaning multisystemic wasting syndrome (PMWS) in piglets. We demonstrate here that 80 to 90% of monocyte-derived and bone marrow-derived DCs interact with PCV2 similar to the early stages of an infection. There was no evidence for virus replication, but the virus did persist in DCs without loss of infectivity nor the induction of cell death. This could reflect an abortive infection, but there was no evidence of virus uncoating-the infectivity remained intact for at least 5 days. Alternatively, the results may reflect DC endocytosis of antigenic material. However, there was no modulation of DC surface major histocompatibility complex class I and class II, CD80/86, CD25, CD16, or CD14. Furthermore, infected DC did not transmit virus to syngeneic T lymphocytes, even when the latter were activated. Such coculture did not induce PCV2 replication or death of the lymphocytes or DCs. These results demonstrate that PCV2 can persist in DCs in the absence of virus replication or degradation. Such a silent virus infection presents a novel mechanism of not only immune evasion but also escaping the DC degradation pathway. Because of their migratory capacity, infection of DCs thus provides a potent vehicle for transport of the virus throughout the host without the need for replication. In addition, the lymphopenia seen in PMWS is not a direct effect of the virus on lymphocytes but would require additional events, as proposed by others.

Immunohistochemical characterisation of PCV2 associate lesions in lymphoid and non-lymphoid tissues of pigs with natural postweaning multisystemic wasting syndrome (PMWS)

The lymphoid, renal, pulmonary, and hepatic lesions of naturally occurring postweaning multisystemic wasting syndrome (PMWS) affected pigs have been studied by means of immunohistology. Ten conventionally reared pigs showing acute clinical signs of PMWS were selected from a farm on which animal were seronegative to porcine reproductive and respiratory virus and to Aujeszky's disease virus. All pigs were positive in tests for porcine circovirus type 2 by ISH and IHC. Monoclonal and polyclonal antibodies to CD3, CD79alpha, CD45RA (3C3/9), lysozyme, SLA-II-DQ (BL2H5), and MAC387 were used to characterise cells in PMWS lesions. The most relevant changes were reduction or loss of B and T lymphocytes, increased numbers of macrophages, and partial loss and redistribution of antigen presenting cells throughout lymphoid tissues compared to uninfected controls. The characteristics of lymphoid lesions in the present study strongly suggest an immunosuppressive effect of PMWS in affected pigs.

Infection with chicken anaemia virus impairs the generation of pathogen-specific cytotoxic T lymphocytes

Infection with chicken anaemia virus (CAV), a circovirus, can result in immunosuppression and subsequent increased susceptibility to secondary infections. This is the first report of impairment of pathogen-specific cytotoxic T lymphocytes (CTL) after natural and experimental infection of chickens with CAV and Marek's disease virus (MDV) or reticuloendotheliosis virus (REV). MDV- and REV-specific CTL were generated at 7 days post infection by 9-30-day-old-chickens that were positive for maternal antibodies to CAV at 9-17 days of age. Replication of CAV could not be demonstrated in these chickens using quantitative real-time polymerase chain reaction (PCR) and reverse transcriptase (RT)-PCR assays. In contrast, REV-specific CTL failed to develop when chickens negative for maternal antibodies at 9-17 days of age were infected. Infection with CAV at 45 days of age after CAV maternal antibodies had waned also caused a decreased REV-specific CTL response. In these chickens increased levels of CAV DNA of up to 107 copy numbers per micro g DNA and increased relative transcript levels of CAV by up to a factor of 106 were detected by quantitative real-time PCR and RT-PCR. Interleukin (IL)-1beta and IL-2 mRNA levels were not significantly affected by CAV infection at 7 or 14 days p.i. Similar assays for interferon-gamma (IFN-gamma) transcripts demonstrated a 10-fold increase in IFN-gamma mRNA levels at 7 days post infection following REV or REV + CAV infection, while CAV alone caused a two- to fourfold increase. These results show a strong link between CAV antibody status, CAV replication, and the ability to generate REV-specific CTL. It is likely that the immunosuppressive effects of subclinical infection have previously been underestimated.