Background: Porcine circovirus (PCV) disease caused by PCV type 2 (PCV2) is mainly attributed to immunosuppression and immune damage. PCV2 can infect vascular endothelial cells and induce high expression of endothelial IL-8. Dendritic cells (DCs), as professional antigen-presenting cells, can not only present antigens but also activate naïve T-cells, causing an immune response. Methods: To demonstrate whether endothelial IL-8 is the main factor inhibiting the maturation and related functions of dendritic cells during PCV2 infection, monocyte-derived DCs (MoDCs) and porcine iliac artery endothelial cells (PIECs) processed by different methods were co-cultured in two ways. Flow cytometry, molecular probe labeling, fluorescence quantitative PCR, and the MTS assay were used to detect the changes in related functions and molecules of MoDCs. Results: Compared to those in the PIEC-DC group, the endothelial IL-8 upregulation co-culture group showed significantly lower double-positive rates for CD80/86 and MHC-II of MoDCs and significantly increased endocytosis of MoDCs. Meanwhile, the adhesion rate and average fluorescence intensity of MoDCs were significantly downregulated in migration and adhesion experiments. Furthermore, the MHC-I and LAMP7 mRNA levels in MoDCs and the proliferation of MoDC-stimulated T-cells were markedly reduced. However, the changes in MoDCs of the endothelial IL-8 downregulation co-culture group were the opposite. Conclusions: PCV2-induced endothelial IL-8 reduces the adhesion and migration ability of MoDCs, resulting in a decreased maturation rate of MoDCs, and further inhibits antigen presentation by DCs. These results may explain the immunosuppressive mechanism of PCV2 from the perspective of the interaction between endothelial cells and DCs in vitro.

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.

Porcine circovirus-associated disease is caused by porcine circovirus type 2 (PCV2) infection, which targets iliac artery endothelial cells (PIECs); it leads to severe immunopathologies and is associated with major economic losses in the porcine industry. Here, we report that in vitro PCV2 infection of PIECs causes cell injury, which affects DC function as well as adaptive immunity. Specifically, PCV2 infection downregulated PIEC antigen-presenting molecule expression, upregulated cytokines involved in the immune and inflammatory response causing cell damage and repair, and altered the migratory capacity of PIECs. In addition, PCV2-infected PIECs inhibited DC maturation, enhanced the endocytic ability of DCs, and weakened the stimulatory effect of DCs on T lymphocytes. Together, these findings indicate that profound functional impairment of DCs in the presence of PCV2-infected PIECs may be a potential pathogenic mechanism associated with PCV2-induced porcine disease.

Shi F, et al. J Vet Sci. 2019 Oct;21:e78. https://doi.org/10.4142/jvs.2020.21.e78

Porcine circovirus 2 (PCV2) is one of the most important endemic swine pathogens, inducing immunosuppression in pigs and predisposing them to secondary bacterial or viral infections. Our previous studies show that PCV2 infection stimulated pig intestinal epithelial cells (IPEC-J2) to produce the secretory transforming growth factor-β (TGF-β), which, in turn, caused CD4⁺ T cells to differentiate into regulatory T cells (T_regs). This may be one of the key mechanisms by which PCV2 induces immunosuppression. Here, we attempt to identify the viral proteins that affect the TGF-β secretion, as well as the key amino acids that are primarily responsible for this occurrence. The three amino acids C35, S36 and V39 of the ORF4 protein are the key sites at which PCV2 induces a large amount of TGF-β production in IPEC-J2 and influences the frequency of T_regs. This may elucidate the regulatory effect of PCV2 on the T_regs differentiation from the perspective of virus structure and intestinal epithelial cell interaction, laying a theoretical foundation for improving the molecular mechanism of PCV2-induced intestinal mucosal immunosuppression in piglets.