Classical swine fever virus suppresses maturation and modulates functions of monocyte-derived dendritic cells without activating nuclear factor kappa B

Activation of Dendritic Cells in Tonsils Is Associated with CD8 T Cell Responses following Vaccination with Live Attenuated Classical Swine Fever Virus

Classical swine fever (CSF) is a highly contagious disease caused by the classical swine fever virus (CSFV). The live attenuated C-strain vaccine is highly efficacious, initiating protection within several days of delivery. The vaccine strain is detected in the tonsil early after inoculation, yet little is known of the role that tonsillar immune cells might play in initiating protection. Comparing the C-strain vaccine with the pathogenic CSFV Alfort-187 strain, changes in the myeloid cell compartment of the tonsil were observed. CSFV infection led to the emergence of an additional CD163⁺CD14⁺ cell population, which showed the highest levels of Alfort-187 and C-strain infection. There was also an increase in both the frequency and activation status (as shown by increased MHC-II expression) of the tonsillar conventional dendritic cells 1 (cDC1) in pigs inoculated with the C-strain. Notably, the activation of cDC1 cells coincided in time with the induction of a local CSFV-specific IFN-γ⁺ CD8 T cell response in C-strain vaccinated pigs, but not in pigs that received Alfort-187. Moreover, the frequency of CSFV-specific IFN-γ⁺ CD8 T cells was inversely correlated to the viral load in the tonsils of individual animals. Accordingly, we hypothesise that the activation of cDC1 is key in initiating local CSFV-specific CD8 T cell responses which curtail early virus replication and dissemination.

Signature of genome wide gene expression in classical swine fever virus infected macrophages and PBMCs of indigenous vis-a-vis crossbred pigs

The genetic basis of differential host immune response vis-à-vis transcriptome profile was explored in PBMCs of indigenous (Ghurrah) and crossbred pigs after classical swine fever vaccination and in monocyte derived macrophages (MDMs) challenged with virulent classical swine fever (CSF) virus. The humoral immune response (E2 antibody) was higher (74.87%) in crossbred than indigenous pigs (58.20%) at 21st days post vaccination (21dpv). The rate of reduction of ratio of CD4⁺/CD8⁺ was higher in crossbred pigs than indigenous pigs at 7th days post vaccination (7dpv). The immune genes IFIT1, IFIT5, RELA, NFKB2, TNF and LAT2 were up regulated at 7dpv in RNA seq data set and was in concordance during qRT-PCR validation. The Laminin Subunit Beta 1 (LAMB1) was significantly (p ≤ 0.05) down-regulated in MDMs of indigenous pigs and consequently a significantly (p ≤ 0.01) higher copy number of virulent CSF virus was evidenced in macrophages of crossbred pigs than indigenous pigs. Activation of LXR:RXR pathway at 60 h post infection (60hpi) in MDMs of indigenous versus crossbred pigs inhibited nuclear translocation of NF-κB, resulted into transrepression of proinflammatory genes. But it helped in maintenance of HDL level by lowering down cholesterol/LDL level in MDMs of indigenous pigs. The key immune genes (TLR2, TLR4, IL10, IL8, CD86, CD54, CASP1) of TREM1 signaling pathway were upregulated at 7dpv in PBMCs but those genes were downregulated at 60hpi in MDMs indigenous pigs. Using qRT-PCR, the validation of differentially expressed, immunologically important genes (LAMB1, OAS1, TLR 4, TLR8 and CD86) in MDMs revealed that expression of these genes were in concordance with RNA-seq data.

Porcine Dendritic Cells and Viruses: An Update

Several viral infections of swine are responsible for major economic losses and represent a threat to the swine industry worldwide. New tools are needed to prevent and control endemic, emerging, and re-emerging viral diseases. Dendritic cells (DC) play a central role in linking the innate and adaptive arms of the immune system, so knowledge regarding their interaction with pathogens is necessary to understand the mechanisms underlying diseases pathogenesis and protection. In the first part of this review, we provide an update on the heterogeneous cell subsets that comprise the porcine DC family. In the second part of this review, we provide an overview of how three viruses, affecting pork production at a global level, African swine fever virus (ASFV), classical swine fever virus (CSFV), and porcine circovirus 2 (PCV2), modulate DC function.

Activation of Interleukin-1β Release by the Classical Swine Fever Virus Is Dependent on the NLRP3 Inflammasome, Which Affects Virus Growth in Monocytes

Classical swine fever virus (CSFV) is a classic Flavivirus that causes the acute, febrile, and highly contagious disease known as classical swine fever (CSF). Inflammasomes are molecular platforms that trigger the maturation of proinflammatory cytokines to engage innate immune defenses that are induced upon cellular infection or stress. However, the relationship between the inflammasome and CSFV infection has not been thoroughly characterized. To understand the function of the inflammasome response to CSFV infection, we infected porcine peripheral blood monocytes (PBMCs) with CSFV. Our results indicated that CSFV infection induced both the generation of pro-interleukin-1β (pro-IL-1β) and its processing in monocytes, leading to the maturation and secretion of IL-1β through the activation of caspase 1. Moreover, CSFV infection in PBMCs induced the production and cleavage of gasdermin D (GSDMD), which is an inducer of pyroptosis. Additional studies showed that CSFV-induced IL-1β secretion was mediated by NLRP3 and that CSFV infection could sufficiently activate the assembly of the NLRP3 inflammasome in monocytes. These results revealed that CSFV infection inhibited the expression of NLRP3, and knockdown of NLRP3 enhanced the replication of CSFV. In conclusion, these findings demonstrate that the NLRP3 inflammasome plays an important role in the innate immune response to CSFV infection.

Expression profiling of immune genes in classical swine fever vaccinated indigenous and crossbred piglets

Classical swine fever is a highly contagious disease of pigs which courses from life-threatening to asymptomatic, depending on the virulence of the virus strain and the immune-competence of the host. The present study was undertaken to investigate the expression of immunologically important genes, viz. IFNα, IFNβ, SLA, SLA-2, SLA-DR, Ii, SLA-DM, CSK and JUN and to ascertain genetic group differences on the basis of humoral immune response. Blood samples were collected from 5 indigenous and 6 crossbred piglets at pre-vaccination and after 28th day of classical swine fever (CSF) vaccination. On 28th day, the competitive Enzyme Linked Immunosorbent Assay (cELISA) revealed poor humoral immune response (E2 antibodies) in indigenous piglets (84.80%) as compared to crossbred piglets (98.33%) in response to CSF vaccination. The expression level of genes was analyzed in three ways, viz. indigenous 28th day post-vaccination (28dpv) versus pre-vaccination, crossbred 28th day post-vaccination versus pre-vaccination and crossbred 28th day post-vaccination versus indigenous 28th day post-vaccination. The study showed that IFNα, IFNβ, SLA, SLA-2, Ii, SLA-DM, CSK and JUN were significantly upregulated in crossbred piglets than indigenous piglets at 28th day post-vaccination. But the SLA-DR was significantly downregulated in CSF vaccinated crossbred over indigenous piglets.

Effect of Foot-and-Mouth Disease Virus Infection on the Frequency, Phenotype and Function of Circulating Dendritic Cells in Cattle

Foot-and-mouth disease virus (FMDV) is a highly contagious virus that causes one of the most devastating diseases in cloven-hoofed animals. Disease symptoms develop within 2 to 3 days of exposure and include fever and vesicular lesions on the tongue and hooves. Dendritic cells (DC) play an essential role in protective immune responses against pathogens. Therefore, investigating their role during FMDV infection would lead to a better understanding of host-pathogen interactions. In this study, following infection of cattle with FMDV, we investigated the frequency and function of conventional (cDC) and plasmacytoid DC (pDC) in blood by using multi-color flow cytometry. We show that the frequency of cDC and pDC increased following FMDV infection and peaked 3 to 4 days post-infection. During peak viremia, the cattle became lymphopenic, the expression of MHC class II molecules on cDC and pDC was dramatically down-regulated, the processing of exogenous antigen by cDC and pDC was impaired, and there was an increase in IL-10 production by DC and monocytes. Notably, after clearance of FMDV from the blood, MHC class II expression returned to pre-infection levels. Altogether, our study demonstrates that in cattle, FMDV inhibits the function of DC, thereby retarding the initiation of adaptive immune responses, potentially enhancing virus shedding during the acute phase of infection.

Thioredoxin 2 Is a Novel E2-Interacting Protein That Inhibits the Replication of Classical Swine Fever Virus

The E2 protein of classical swine fever virus (CSFV) is an envelope glycoprotein that is involved in virus attachment and entry. To date, the E2-interacting cellular proteins and their involvement in viral replication have been poorly documented. In this study, thioredoxin 2 (Trx2) was identified to be a novel E2-interacting partner using yeast two-hybrid screening from a porcine macrophage cDNA library. Trx2 is a mitochondrion-associated protein that participates in diverse cellular events. The Trx2-E2 interaction was further confirmed by glutathione S-transferase (GST) pulldown, in situ proximity ligation, and laser confocal assays. The thioredoxin domain of Trx2 and the asparagine at position 37 (N37) in the E2 protein were shown to be critical for the interaction. Silencing of the Trx2 expression in PK-15 cells by small interfering RNAs significantly promotes CSFV replication, and conversely, overexpression of Trx2 markedly inhibits viral replication of the wild-type (wt) CSFV and to a greater extent that of the CSFV N37D mutant, which is defective in binding Trx2. The wt CSFV but not the CSFV N37D mutant was shown to reduce the Trx2 protein expression in PK-15 cells. Furthermore, we demonstrated that Trx2 increases nuclear factor kappa B (NF-κB) promoter activity by promoting the nuclear translocation of the p65 subunit of NF-κB. Notably, activation of the NF-κB signaling pathway induced by tumor necrosis factor alpha (TNF-α) significantly inhibits CSFV replication in PK-15 cells, whereas blocking the NF-κB activation in Trx2-overexpressing cells no longer suppresses CSFV replication. Taken together, our findings reveal that Trx2 inhibits CSFV replication via the NF-κB signaling pathway.

IMPORTANCE

Thioredoxin 2 (Trx2) is a mitochondrion-associated protein that participates in diverse cellular events, such as antioxidative and antiapoptotic processes and the modulation of transcription factors. However, little is known about the involvement of Trx2 in viral replication. Here, we investigated, for the first time, the role of Trx2 in the replication of classical swine fever virus (CSFV), a devastating pestivirus of pigs. By knockdown and overexpression, we showed that Trx2 negatively regulates CSFV replication. Notably, we demonstrated that Trx2 inhibits CSFV replication by promoting the nuclear translocation of the p65 subunit of NF-κB, a key regulator of the host's innate immunity and inflammatory response. Our findings reveal a novel role of Trx2 in the host's antiviral response and provide new insights into the complex mechanisms by which CSFV interacts with the host cell.

PKR activation enhances replication of classical swine fever virus in PK-15 cells

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CSFV infection triggers PKR and eIF2α protein phosphorylation.

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PKR overexpression stimulates viral replication.

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PKR depletion blocks eIF2α phosphorylation and suppresses viral replication.

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PKR depletion increases IFN-β, and enhances the antiviral effect of IFN.

Classical swine fever (CSF) is a highly contagious swine disease that is responsible for economic losses worldwide. Protein kinase R (PK)R is an important protein in the host viral response; however, the role of PKR in CSFV infection remains unknown. This issue was addressed in the present study using the PK-15 swine kidney cell line. We found that CSFV infection increased the phosphorylation of eukaryotic translation initiation factor (eIF)2α and its kinase PKR. However, the expression of viral proteins continued to increase. Furthermore, PKR overexpression enhanced CSFV replication, while PKR inhibition resulted in reduced CSFV replication and an increase in interferon (IFN) induction. In addition, PKR was responsible for eIF2α phosphorylation in CSFV-infected cells. These results suggest that the activation of PKR during CSFV infection is beneficial to the virus. The virus is able to commandeer the host cell's translation machinery for viral protein synthesis while evading innate immune defenses.

Enhanced immune response of BMDCs pulsed with H9N2 AIV and CpG

Dendritic cells (DCs), professional antigen presenting cells, have demonstrated effective in controlling the initial of innate immune, while CpG could improve the performance of immune system. To explore the mechanism of CpG enhancing the immune response, we compared different stimulated mouse DCs with systemic approach microarrays. Analysis revealed 1840 differentially expressed genes in H9N2 stimulated group, more than 1728 altered genes in inactive H9N2 group. Investigation also proved that CpG/inactive H9N2 co-stimulation changed 2140 genes, more than that in H9N2 group, strongly demonstrated that CpG improved the performance of inactive H9N2 vaccination. Pathways analysis founded that DCs response rapid to shift in their maturation state, which involved Toll-like receptor (TLR) pathway significantly. Microarrays results were also verified by qRT-PCR with 14 elected representative genes. Further analysis proved that co-stimulatory molecules (CD40, CD80, CD86 and MHC-II), regulatory protein (IRF-7 and TRAF-6) and pro-inflammatory cytokines (IL-1, IL-6 and IL-12) were all changed and involved in DCs maturation. At last we demonstrated TLR signalling pathway in chicken bone marrow-derived dendritic cells (chBM-DCs) stimulated with CpG. The distinct transcriptional profiles of DCs pulsed with various stimuli expanded our understanding of how DCs respond and recognize influenza.

Interaction of CSFV E2 protein with swine host factors as detected by yeast two-hybrid system

E2 is one of the envelope glycoproteins of pestiviruses, including classical swine fever virus (CSFV) and bovine viral diarrhea virus (BVDV). E2 is involved in several critical functions, including virus entry into target cells, induction of a protective immune response and virulence in swine. However, there is no information regarding any host binding partners for the E2 proteins. Here, we utilized the yeast two-hybrid system and identified fifty-seven host proteins as positive binding partners which bound E2 from both CSFV and BVDV with the exception of two proteins that were found to be positive for binding only to CSFV E2. Alanine scanning of CSFV E2 demonstrated that the binding sites for these cellular proteins on E2 are likely non-linear binding sites. The possible roles of the identified host proteins are discussed as the results presented here will be important for future studies to elucidate mechanisms of host protein-virus interactions during pestivirus infection. However, due to the limitations of the yeast two hybrid system, the proteins identified is not exhaustive and each interaction identified needs to be confirmed by independent experimental approaches in the context of virus-infected cells before any definitive conclusion can be drawn on relevance for the virus life cycle.

Classical swine fever virus triggers RIG-I and MDA5-dependent signaling pathway to IRF-3 and NF-κB activation to promote secretion of interferon and inflammatory cytokines in porcine alveolar macrophages

BACKGROUND

Classical swine fever (CSF) caused by CSF virus (CSFV) is a highly contagious disease of pigs. The RNA helicases retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA-5) are differentially involved in the detection of various RNA viruses. In present study, we investigated the roles of RIG-I and MDA-5 in eliciting antiviral and inflammatory responses to CSFV shimen strain in Porcine alveolar macrophages (PAMs).

METHODS

CSFV Shimen strain was used as challenge virus in this study and PAMs were cultured in vitro. Interferon regulatory factor (IRF)-3 and nuclear factor-kappa B (NF-κB) translocation was detected using immunofluorescent staining; RIG-I, MDA5, interferon promoter-stimulating factor 1 (IPS-1), IRF-3 and NF-κB expression was measured by Western Blotting; Interferon beta (IFN-β), IFN-α, interleukin-1beta (IL-1β), IL-6 and tumor necrosis factor (TNF-α) expression was tested by Enzyme-linked immunosorbent assays (ELISA) and shRNA-mediated knockdown of MDA5 or RIG-I was performed.

RESULTS

The findings suggested that the initial response to CSFV infection resulted in the higher expression of RIG-I and MDA5 leading to the activation of IPS-1, IRF-3 and NF-κB in a dose-dependent manner. Evaluation of IFN-α, IFN-β, IL-1β, IL-6 or TNF-α expressed by PAMs showed significant differences between infected and uninfected cells. CSFV infected cells induced to express high levels of IFN-α, IFN-β, IL-1β, IL-6 and TNF-α in a dose-dependent way within 24 h post-infection (hpi). At the same time, CSFV improved the nuclear translocation of IRF-3 and NF-κB. We also directly compared and assessed the roles of RIG-I and MDA5 in triggering innate immune actions during CSFV infection through shRNA-mediated knockdown of MDA5 or RIG-I. We found that, compared to the control, the production of IFN-α, IFN-β, IL-1β, IL-6 and TNF-α in response to CSFV infection was heavily reduced in RIG-I knockdown cells while it was moderately decreased in MDA5 knockdown cells. PAMs derived from knockdown of both RIG-I and MDA5 almost failed to produce IFNs and inflammatory cytokines.

CONCLUSIONS

It indicates that CSFV can be recognized by both RIG-I and MDA5 to initiate the RIG-I signaling pathway to trigger innate defenses against infection.

In vitro infection with classical swine fever virus inhibits the transcription of immune response genes

Background

Classical swine fever virus (CSFV) can evade the immune response and establish chronic infection under natural and experimental conditions. Some genes related to antigen processing and presentation and to cytokine regulation are known to be involved in this response, but the precise mechanism through which each gene responds to CSFV infection remains unclear.

Results

In this study, the amplification standard curve and corresponding linear regression equations for the genes SLA-2, TAP1, SLA-DR, Ii, CD40, CD80, CD86, IFN-α, and IFN-β were established successfully. Real-time RT-PCR was used to quantify the immune response gene transcription in PK-15 cells post CSFV infection. Results showed that: (1) immune response genes were generally down-regulated as a result of CSFV infection, and (2) the expression of SLA-2, SLA-DR, Ii and CD80 was significantly decreased (p<0.001).

Conclusion

We conclude that in vitro infection with CSFV inhibits the transcription of host immune response genes. These findings may facilitate the development of effective strategies for controlling CSF.