Immunology of porcine circovirus type 2 (PCV2)

Coincidental detection of genomes of porcine parvoviruses and porcine circovirus type 2 infecting pigs in Japan

The infection status of 15 viruses in 120 pigs aged about 6 months was investigated based on tonsil specimens collected from a slaughterhouse. Only 5 species of porcine parvoviruses and porcine circovirus type 2 (PCV2) were detected at high frequencies; 67% for porcine parvovirus (PPV) (PPV-Kr or -NADL2 as the new abbreviation), 58% for PPV2 (CnP-PARV4), 39% for PPV3 (P-PARV4), 33% for PPV4 (PPV4), 55% for PBo-likeV (PBoV7) and 80% for PCV2. A phylogenetic analysis of PPV3 suggested that Japanese PPV3s showed a slight variation, and possibly, there were farms harboring homogeneous or heterogeneous PPV3s. Statistical analyses indicated that the detection of PCV2 was significantly coincidental with each detection of PPV, PPV2 and PPV3, and PPV and PPV4 were also coincidentally detected. The concurrent infection with PCV2 and porcine parvoviruses in the subclinically infected pigs may resemble the infection status of pigs with the clinical manifestations of porcine circovirus associated disease which occurs in 3–5 months old pigs and is thought to be primarily caused by the PCV2 infection.

Torque teno sus virus 1 and 2 viral loads in faeces of porcine circovirus 2-positive pigs

Recently, studies have suggested an association between the Torque teno sus virus (TTSuV) and the Porcine circovirus-2 (PCV2) in PCV2-associated disease cases. The aim of this study was to verify TTSuVs loads in pig faeces from PCV2-positive animals with and without diarrhea from PCVAD-affected and PCV2-unvaccinated herds. A total of 80 faecal samples were collected individually from nursery and grow-finish pigs with (n = 40) or without (n = 40) diarrhea. The samples were tested for PCV2 and TTSuVs by using DNA binding dye SYBR Green quantitative polymerase chain reaction (qPCR). Torque teno sus virus k2 (TTSuVk2) load in the faeces was significantly higher in the nursery pigs with diarrhea, and these pigs also exhibited significantly higher PCV2 (P < 0.01) faecal matter loads compared to the non-diarrheic animals from the same age group. Torque teno sus virus 1 (TTSuV1) viral loads were the same regardless of age group and disease condition. There were no correlations between PCV2 and TTSuV1 or TTSuVk2 and TTSuV1 viral loads; however, a weak correlation (r = 0.23, P = 0.03) was found between TTSuVk2 and PCV2 viral loads. In conclusion, TTSuVk2 viral loads were significantly higher in the diarrheic faeces from the nursery pigs. Additionally, the higher loads of PCV2 and TTSuVk2 in the nursery-diarrheic animals revealed that diarrhea might have an important role in the spread of both viruses in herds.

Porcine circovirus: a historical perspective

Porcine circoviruses (PCVs) belong to the genus Circovirus and the family Circoviridae, and they are the smallest known viruses that replicate autonomously in mammalian cells. They are nonenveloped, and they have characteristic single-stranded, negative-sense, circular DNA. Two types of divergent PCVs are recognized: PCV1 and PCV2. About 20 years ago, PCV2 began to emerge as a major pathogen of swine around the world, leading to burgeoning knowledge about the virus and porcine circovirus-associated diseases. However, much of the history of its discovery, including the controversy related to its importance, is not recorded. This review examines current issues related to the biology of PCV2 in the context of the original studies related to determining its causal association with disease and to the evolving understanding of the complex pathogenesis of PCV2 infections.

Overexpression of programmed death ligands in naturally occurring postweaning multisystemic wasting syndrome

Postweaning multisystemic wasting syndrome (PMWS) is regarded as an immunosuppressive disease in pigs caused by porcine circovirus type 2 (PCV2). Immune inhibitory receptors, particularly programmed death 1/programmed death-ligands (PD-1/PD-Ls) are presumably involved in the immunopathogenesis of PMWS. The aim of this investigation was to examine the relationship of immune inhibitory receptors and immunocompromised by PMWS. Nine 45-day-old conventional pigs were selected from a farm where pigs exhibited typical signs of PMWS (wasting and respiratory disorders) and tested positive for PCV2 infection by polymerase chain reaction (PCR). Six pigs were selected as controls due to their notably healthy state and absence of PCV2 infection. Heparinized blood samples were taken from each pig for pathogen detection and isolation of peripheral blood mononuclear cells (PBMCs), from which mRNA expression of immunomodulatory molecule (PD-1, PD-L1, PD-L2, PTEN, CTLA-4, LAG-3, and Foxp3) and cytokines (IL-10, IL-2, and IFN-γ) was determined. Proliferation of PBMCs was also assessed by flow cytometry utilizing cellular labeling dilutions for detection. The mRNA levels of PD-L1 (p<0.01), PD-L2 (p<0.05), and PTEN (p<0.01) were remarkably increased in the PBMCs of diseased pigs compared to healthy pigs, whereas no change was observed for PD-1, CTLA-4, LAG-3, and Foxp3 expression. Cytokine IL-10 mRNA levels were significantly elevated (p<0.01), while IL-2 and IFN-γ mRNA levels tended to be only slightly increased in the PBMCs of affected pigs compared to healthy controls. The proliferation of PBMCs was also decreased in diseased pigs. These data suggest that overexpression of PD-L1 and PD-L2 mRNA is one mechanism by which immunosupression of PMWS pigs occurs, supporting a new therapeutic strategy focused on PD-Ls for pigs suffering from PMWS.

Biology and Diseases of Swine

Swine are used in biomedical research as models for biomedical research and for teaching. This chapter covers normative biology and behavior along with common and emerging swine diseases. Xenotransplantation is discussed along with similarities and differences of swine immunology.

CD44 deficiency leads to decreased proinflammatory cytokine production in lung induced by PCV2 in mice

Porcine circovirus type 2 (PCV2) is the primary etiological agent of postweaning multisystemic wasting syndrome (PMWS). CD44 is a widely expressed class I transmembrane glycoprotein implicated in immunological and inflammatory responses. In previous studies, the role of CD44 in host defense against microorganism infection remains controversial. The role of CD44 in host defense against PCV2 infection has never been studied before. In this study, we investigated the role of CD44 in the development of pneumonia induced by PCV2 in mice model. Upon infection, CD44 mRNA level in lung tissue was upregulated, and we confirmed a detrimental role of CD44 in host defense against PCV2 infection. The results demonstrated that CD44 deficiency could result in decreased proinflammatory cytokine production in lung induced by PCV2 in mice, suggesting a previously unrecognized role for CD44 in the development of pneumonia response to PCV2 infection.

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.

Complex links between natural tuberculosis and porcine circovirus type 2 infection in wild boar

Individuals in natural populations are exposed to a diversity of pathogens which results in coinfections. The aim of this study was to investigate the relation between natural infection with tuberculosis (TB) due to infection by bacteria of the Mycobacterium tuberculosis complex and porcine circovirus type 2 (PCV2) in free-ranging Eurasian wild boar (Sus scrofa). Apparent prevalence for TB lesions and PCV2 infection was extremely high in all age classes, including piglets (51% for TB; 85.7% for PCV2). Modeling results revealed that the relative risk of young (less than 2 years old) wild boar to test positive to PCV2 PCR was negatively associated with TB lesion presence. Also, an interaction between TB, PCV2, and body condition was evidenced: in wild boar with TB lesions probability of being PCV2 PCR positive increased with body condition, whereas this relation was negative for wild boar without TB lesions. This study provides insight into the coinfections occurring in free-ranging host populations that are naturally exposed to several pathogens at an early age. Using TB and PCV2 as a case study, we showed that coinfection is a frequent event among natural populations that takes place early in life with complex effects on the infections and the hosts.

Functional production of a soluble and secreted single-chain antibody by a bacterial secretion system

Single-chain variable fragments (scFvs) serve as an alternative to full-length monoclonal antibodies used in research and therapeutic and diagnostic applications. However, when recombinant scFvs are overexpressed in bacteria, they often form inclusion bodies and exhibit loss of function. To overcome this problem, we developed an scFv secretion system in which scFv was fused with osmotically inducible protein Y (osmY), a bacterial secretory carrier protein, for efficient protein secretion. Anti-EGFR scFv (αEGFR) was fused with osmY (N- and C-termini) and periplasmic leader sequence (pelB) to generate αEGFR-osmY, osmY-αEGFR, and pelB-αEGFR (control), respectively. In comparison with the control, both the osmY-fused αEGFR scFvs were soluble and secreted into the LB medium. Furthermore, the yield of soluble αEGFR-osmY was 20-fold higher, and the amount of secreted protein was 250-fold higher than that of osmY-αEGFR. In addition, the antigen-binding activity of both the osmY-fused αEGFRs was 2-fold higher than that of the refolded pelB-αEGFR from inclusion bodies. Similar results were observed with αTAG72-osmY and αHer2-osmY. These results suggest that the N-terminus of osmY fused with scFv produces a high yield of soluble, functional, and secreted scFv, and the osmY-based bacterial secretion system may be used for the large-scale industrial production of low-cost αEGFR protein.

Proteomics on porcine haptoglobin and IgG/IgA show protein species distribution and glycosylation pattern to remain similar in PCV2-SD infection

Haptoglobin (Hp) and immunoglobulins are plasma glycoproteins involved in the immune reaction of the organism after infection and/or inflammation. Porcine circovirus type 2-systemic disease (PCV2-SD), formerly known as postweaning multisystemic wasting syndrome (PMWS), is a globally spread pig disease of great economic impact. PCV2-SD affects the immunological system of pigs causing immunosuppression. The aim of this work was to characterize the Hp protein species of healthy and PCV2-SD affected pigs, as well as the protein backbone and the glycan chain composition of porcine Hp. PCV2-SD affected pigs had an increased overall Hp level, but it did not affect the ratio between Hp species. Glycoproteomic analysis of the Hp β subunits confirmed that porcine Hp is N-glycosylated and, unexpectedly, O-glycosylated, a PTM that is not found on Hp from healthy humans. The glyco-profile of porcine IgG and IgA heavy chains was also characterized; decreased levels of both proteins were found in the investigated group of PCV2-SD affected pigs. Obtained results indicate that no significant changes in the N- and O-glycosylation patterns of these major porcine plasma glycoproteins were detectable between healthy and PCV2-SD affected animals.

BIOLOGICAL SIGNIFICANCE

PCV2-SD is a disease of great economic importance for pig production, characterized by a complex response of the immune system. In the search of a better diagnostic/prognostic marker for porcine PCV2-SD, extensive analyses of the Hp protein backbone and the glycan chains were thoroughly analyzed by various techniques. This resulted in detection and confirmation of Hp O-glycosylation and the glyco-profiling of porcine IgG and IgA. The N- and O-glycosylation of these major porcine plasma glycoproteins appears to be not affected by PCV2-SD infection. Interestingly, these data suggest that this viral infection, which significantly affects the immune responses of the host, leaves the biosynthetic glycosylation processes in the liver and immune cells unaffected. Lack of PTM changes is in contrast to findings in humans where for both proteins pattern changes have been reported in several chronic and inflammatory diseases. This underlines the importance of studying species in detail and not reaching to conclusions by analogy. Furthermore, since Hp is usually quantified by immunoassays in clinical routine analyses, our findings indicate that no bias in Hp determination capabilities due to an altered carbohydrate pattern is to be expected.

Nontransmission of porcine circovirus 2 (PCV2) by embryo transfer

Two experiments were conducted to determine the association of porcine circovirus type 2 (PCV2) with embryos and the risk of viral transmission by embryo transfer. In the first experiment, 240 embryos from uninfected donors were exposed to PCV2a 10(4)TCID50/mL in vitro before transfer to seronegative recipients; in the second experiment, 384 embryos recovered from infected donors, 10 days after donor inoculation with PCV2, were transferred to seronegative recipients. In total, 1120 embryos and/or ova were collected from 37 viral-free donors (experiment 1) and 1019 from 59 PCV2-infected donors (experiment 2) (P < 0.01). The washing and/or disinfection procedure recommended by the International Embryo Transfer Society was applied to embryos in both experiments. Transfer of embryos experimentally exposed in vitro to high titers of virus caused seroconversion of recipients (58%; N = 7/12) and their piglets (81%; N = 13/16). Postmortem, PCV2 DNA was detected in various organs of embryo transfer recipients and their embryo transfer-derived piglets. In contrast, the transfer of embryos recovered from infectious PCV2 donors did not result in the seroconversion of embryo recipients (N = 24) or their embryo transfer-derived piglets (N = 76). Neither PCV2 DNA nor infectious virus was detected in the tissues of either recipients or embryo transfer-derived piglets collected postmortem in the second experiment. The results obtained in this study indicate that the transmission of PCV2 from infected donors by embryo transfer is unlikely if the sanitary recommendations of the International Embryo Transfer Society are followed. In practical terms, this means that embryo transfer can be successfully used for the intentional elimination of PCV2 and to create virus-free offspring for the safe exchange of swine genetic materials.

Porcine circovirus type 2 (PCV2): pathogenesis and interaction with the immune system

Porcine circovirus type 2 (PCV2) is the primary causative agent of porcine circovirus-associated disease (PCVAD). The virus preferentially targets the lymphoid tissues, which leads to lymphoid depletion and immunosuppression in pigs. The disease is exacerbated by immunostimulation or concurrent infections with other pathogens. PCV2 resides in certain immune cells, such as macrophage and dendritic cells, and modulates their functions. Upregulation of IL-10 and proinflammatory cytokines in infected pigs may contribute to pathogenesis. Pig genetics influence host susceptibility to PCV2, but the viral genetic determinants for virulence remain unknown. PCV2 DNA and proteins interact with various cellular genes that control immune responses to regulate virus replication and pathogenesis. Both neutralizing antibodies and cell-mediated immunity are important immunological correlates of protection. Despite the availability of effective vaccines, variant strains of PCV2 continue to emerge. Although tremendous progress has been made toward understanding PCV2 pathogenesis and immune interactions, many important questions remain.