Signaling pathways involved in regulating apoptosis induction in host cells upon PRRSV infection

Pathological observation and transcriptomic analysis of thymus injury in PRRSV-infected piglets

The thymus, the central immune organ in mammals, plays an important role in immune defense. Porcine reproductive and respiratory syndrome virus (PRRSV) infection in piglets can cause thymus injury and immunosuppression. However, the mechanisms of thymus injury remain unknown. This study was aimed at investigating the specific manifestations of thymus injury through the construction of a PRRSV-infected piglet model and histopathological observation. In this study, fourteen 40-day-old PRRSV-free piglets were randomly divided into two groups, eleven of which were intramuscularly injected with 3 mL of PRRSV WUH3 virus suspension (106 PFU /mL) in the infection group, and three of which were sham-inoculated with 3 mL of RPMI-1640 medium in the control group. Clinical necropsy and samples collection were performed on day 8 after artificial infection. With the Illumina platform, the transcriptomes of piglet thymus tissues from infected and control piglets were sequenced to explore the relationships of differentially expressed genes (DEGs) and signaling pathways with thymus injury. The immune organs of PRRSV-infected piglets were severely damaged. The histopathological findings in the thymus indicated that PRRSV infection was associated with a large decrease in lymphocytes, cell necrosis and cell apoptosis; an increase in blood vessels and macrophages; thymic corpuscle hyperplasia; and interstitial widening of the thymic lobules. The transcriptomic analysis results revealed that the Gene Ontology functions of DEGs were enriched primarily in biological processes such as angiogenesis, regulation of angiogenesis and positive regulation of cell migration. Moreover, greater numbers of blood vessels and macrophages were observed in the thymus in PRRSV-infected than control piglets. KEGG pathway enrichment analysis revealed that the DEGs were significantly enriched in the Toll-like receptor signaling pathway, chemokine signaling pathway, IL-17 signaling pathway and TNF signaling pathway. The expression of TLR8, IRF5, the chemokines CCL2, CCL3L1 and CCL5; and their receptors CCR1, CCR2 and CCR5 was significantly up-regulated in PRRSV infection, thus suggesting that these cytokines were associated with the pathological processes of thymus injury.

Histopathological Pulmonary Lesions in 1st-Day Newborn Piglets Derived from PRRSV-1 MLV Vaccinated Sows at the Last Stage of Gestation

Modified live virus (MLV) vaccines for the control of porcine respiratory and reproductive syndrome virus (PRRSV) have been associated with the vertical and horizontal transmission of vaccine viruses. The present study aimed to describe pathological lung lesions in piglets born by gilts vaccinated with PRRSV-1 MLV. In total, 25 gilts were vaccinated at late gestation (100th day) and were divided into five groups according to the different vaccines (Vac) used: no vaccine-control group, Vac-1-strain DV, Vac-2-strain VP-046 BIS, Vac-3-strain 94881, Vac-4-strain 96V198. Within the first 0-9 h of the farrowing, blood samples were collected from all newborn piglets and lung samples were exanimated grossly, histopathologically and with scanning electron microscopy. PRRSV (RT-PCR-positive) and antibodies were detected in the serum of piglets from gilts vaccinated with Vac-2. In these piglets, moderate to severe interstitial pneumonia with thickened alveolar septa was noticed. Type II pneumocyte hyperplasia was also observed. The rest of the trial piglets showed unremarkable lung lesions. Phylogenetic analysis revealed the 98.7% similarity of the PRRSV field strain (GR 2019-1) to the PRRS MLV vaccine strain VP-046 BIS. In conclusion, the Vac-2 PRRSV vaccine strain can act as an infectious strain when vaccination is administrated at late gestation, causing lung lesions.

Systematic Identification and Comparison of the Expressed Profiles of Exosomal MiRNAs in Pigs Infected with NADC30-like PRRSV Strain

Exosomes are biological vesicles secreted and released by cells that act as mediators of intercellular communication and play a unique role in virus infection, antigen presentation, and suppression/promotion of body immunity. Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most damaging pathogens in the pig industry and can cause reproductive disorders in sows, respiratory diseases in pigs, reduced growth performance, and other diseases leading to pig mortality. In this study, we used the PRRSV NADC30-like CHsx1401 strain to artificially infect 42-day-old pigs and isolate serum exosomes. Based on high-throughput sequencing technology, 305 miRNAs were identified in serum exosomes before and after infection, among which 33 miRNAs were significantly differentially expressed between groups (13 relatively upregulated and 20 relatively downregulated). Sequence conservation analysis of the CHsx1401 genome identified 8 conserved regions, of which a total of 16 differentially expressed (DE) miRNAs were predicted to bind to the conserved region closest to the 3' UTR of the CHsx1401 genome, including 5 DE miRNAs capable of binding to the CHsx1401 3' UTR (ssc-miR-34c, ssc-miR-375, ssc-miR-378, ssc-miR-486, ssc-miR-6529). Further analysis revealed that the target genes of differentially expressed miRNAs were widely involved in exosomal function-related and innate immunity-related signaling pathways, and 18 DE miRNAs (ssc-miR-4331-3p, ssc-miR-744, ssc-miR-320, ssc-miR-10b, ssc-miR-124a, ssc-miR-128, etc.) associated with PRRSV infection and immunity were screened as potential functional molecules involved in the regulation of PRRSV virus infection by exosomes.

Regulation of antiviral immune response by African swine fever virus (ASFV)

African swine fever (ASF) is a highly contagious and acute hemorrhagic viral disease with a high mortality approaching 100% in domestic pigs. ASF is an endemic in countries in sub-Saharan Africa. Now, it has been spreading to many countries, especially in Asia and Europe. Due to the fact that there is no commercial vaccine available for ASF to provide sustainable prevention, the disease has spread rapidly worldwide and caused great economic losses in swine industry. The knowledge gap of ASF virus (ASFV) pathogenesis and immune evasion is the main factor to limit the development of safe and effective ASF vaccines. Here, we will summarize the molecular mechanisms of how ASFV interferes with the host innate and adaptive immune responses. An in-depth understanding of ASFV immune evasion strategies will provide us with rational design of ASF vaccines.

Antiviral Mechanism of Tea Polyphenols against Porcine Reproductive and Respiratory Syndrome Virus

Neither inactivated nor attenuated vaccines can effectively prevent and control the infection and spread of porcine reproductive and respiratory syndrome virus (PRRSV). Therefore, it is necessary to broaden new horizons and to conceive effective preventive strategies. The main components of Tea polyphenol (TPP) are catechins and their derivatives. TPP has many physiological activities and has certain antiviral and antifungal effects. However, whether TPP shows anti-PRRSV activity remains unclear. We found that TPP effectively inhibited PRRSV infection in Marc-145 cells by suppressing the stages of viral attachment, internalization, replication, and release. TPP exhibited a potent anti-PRRSV effect regardless of pre-treatment or post-treatment. In addition, we demonstrated that TPP restrained PRRSV-induced p65 entry into the nucleus to suppress the activation of the NF-κB signaling pathway, which ultimately leads to the inhibition of the expression of inflammatory cytokines. Furthermore, TPP limited the synthesis of viral non-structural protein 2 (nsp2), the core component of viral replication transcription complexes, which may contribute to the inhibition of viral RNA replication. TPP has the potential to develop into an effective antiviral agent for PRRSV prevention and control in the future.

Distinctive Cellular and Metabolic Reprogramming in Porcine Lung Mononuclear Phagocytes Infected With Type 1 PRRSV Strains

Porcine reproductive and respiratory syndrome (PRRS) has an extensive impact on pig production. The causative virus (PRRSV) is divided into two species, PRRSV-1 (European origin) and PRRSV-2 (North American origin). Within PRRSV-1, PRRSV-1.3 strains, such as Lena, are more pathogenic than PRRSV-1.1 strains, such as Flanders 13 (FL13). To date, the molecular interactions of PRRSV with primary lung mononuclear phagocyte (MNP) subtypes, including conventional dendritic cells types 1 (cDC1) and 2 (cDC2), monocyte-derived DCs (moDC), and pulmonary intravascular macrophages (PIM), have not been thoroughly investigated. Here, we analyze the transcriptome profiles of in vivo FL13-infected parenchymal MNP subpopulations and of in vitro FL13- and Lena-infected parenchymal MNP. The cell-specific expression profiles of in vivo sorted cells correlated with their murine counterparts (AM, cDC1, cDC2, moDC) with the exception of PIM. Both in vivo and in vitro, FL13 infection altered the expression of a low number of host genes, and in vitro infection with Lena confirmed the higher ability of this strain to modulate host response. Machine learning (ML) and gene set enrichment analysis (GSEA) unraveled additional relevant genes and pathways modulated by FL13 infection that were not identified by conventional analyses. GSEA increased the cellular pathways enriched in the FL13 data set, but ML allowed a more complete comprehension of functional profiles during FL13 in vitro infection. Data indicates that cellular reprogramming differs upon Lena and FL13 infection and that the latter might keep antiviral and inflammatory macrophage/DC functions silent. Although the slow replication kinetics of FL13 likely contribute to differences in cellular gene expression, the data suggest distinct mechanisms of interaction of the two viruses with the innate immune system during early infection.

TREM2 suppresses the proinflammatory response to facilitate PRRSV infection via PI3K/NF-κB signaling

Triggering receptor expressed on myeloid cells 2 (TREM2) serves as an anti-inflammatory receptor, negatively regulating the innate immune response. TREM2 is mainly expressed on dendritic cells and macrophages, the target cells of porcine reproductive and respiratory syndrome virus (PRRSV). Thus, we investigated the potential role of TREM2 in PRRSV infection in porcine alveolar macrophages (PAMs). We found that there was an increased expression of TREM2 upon PRRSV infection in vitro. TREM2 silencing restrained the replication of PRRSV, whereas TREM2 overexpression facilitated viral replication. The cytoplasmic tail domain of TREM2 interacted with PRRSV Nsp2 to promote infection. TREM2 downregulation led to early activation of PI3K/NF-κB signaling, thus reinforcing the expression of proinflammatory cytokines and type I interferons. Due to the enhanced cytokine expression, a disintegrin and metalloproteinase 17 was activated to promote the cleavage of membrane CD163, which resulted in suppression of infection. Furthermore, exogenous soluble TREM2 (sTREM2)-mediated inhibition of PRRSV attachment might be attributed to its competitive binding to viral envelope proteins. In pigs, following PRRSV challenge in vivo, the expression of TREM2 in lungs and lymph nodes as well as the production of sTREM2 were significantly increased. These novel findings indicate that TREM2 plays a role in regulating PRRSV replication via the inflammatory response. Therefore, our work describes a novel antiviral mechanism against PRRSV infection and suggests that targeting TREM2 could be a new approach in the control of the PRRSV infection.

Molecular and Cellular Mechanisms for PRRSV Pathogenesis and Host Response to Infection

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PRRSV has evolved to arm with various strategies to modify host antiviral response.

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Viral modulation of homeostatic cellular processes provides favorable conditions for PRRSV survival during infection.

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PRRSV modulation of cellular processes includes pathways for interferons, apoptosis, microRNAs, cytokines, autophagy, and viral genome recombination.

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused tremendous amounts of economic losses to the swine industry for more than three decades, but its control is still unsatisfactory. A significant amount of information is available for host cell-virus interactions during infection, and it is evident that PRRSV has evolved to equip various strategies to disrupt the host antiviral system and provide favorable conditions for survival. The current study reviews viral strategies for modulations of cellular processes including innate immunity, apoptosis, microRNAs, inflammatory cytokines, and other cellular pathways.