The 15N and 46R Residues of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Nucleocapsid Protein Enhance Regulatory T Lymphocytes Proliferation

Research progress on the N protein of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease caused by the porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV exhibits genetic diversity and complexity in terms of immune responses, posing challenges for eradication. The nucleocapsid (N) protein of PRRSV, an alkaline phosphoprotein, is important for various biological functions. This review summarizes the structural characteristics, genetic evolution, impact on PRRSV replication and virulence, interactions between viral and host proteins, modulation of host immunity, detection techniques targeting the N protein, and progress in vaccine development. The discussion provides a theoretical foundation for understanding the pathogenic mechanisms underlying PRRSV virulence, developing diagnostic techniques, and designing effective vaccines.

Highly pathogenic PRRSV upregulates IL-13 production through nonstructural protein 9-mediated inhibition of N6-methyladenosine demethylase FTO

Porcine reproductive and respiratory syndrome virus (PRRSV), a highly infectious virus, causes severe losses in the swine industry by regulating the inflammatory response, inducing tissue damage, suppressing the innate immune response, and promoting persistent infection in hosts. Interleukin-13 (IL-13) is a cytokine that plays a critical role in regulating immune responses and inflammation, particularly in immune-related disorders, certain types of cancer, and numerous bacterial and viral infections; however, the underlying mechanisms of IL-13 regulation during PRRSV infection are not well understood. In this study, we demonstrated that PRRSV infection elevates IL-13 levels in porcine alveolar macrophages. PRRSV enhances m6A-methylated RNA levels while reducing the expression of fat mass and obesity associated protein (FTO, an m6A demethylase), thereby augmenting IL-13 production. PRRSV nonstructural protein 9 (nsp9) was a key factor for this modulation. Furthermore, we found that the residues Asp567, Tyr586, Leu593, and Asp595 were essential for nsp9 to induce IL-13 production via attenuation of FTO expression. These insights delineate PRRSV nsp9's role in FTO-mediated IL-13 release, advancing our understanding of PRRSV's impact on host immune and inflammatory responses.

Streptococcus suis contributes to inguinal lymph node lesions in piglets after highly pathogenic porcine reproductive and respiratory syndrome virus infection

The swine pathogens porcine reproductive and respiratory syndrome virus (PRRSV) and Streptococcus suis have both been reported to cause damage to the immune organs. Inguinal lymph node (ILN) injury has been reported in PRRSV-infected pigs with secondary S. suis infection, but not much is known about the mechanism. In this study, secondary S. suis infection after highly pathogenic (HP)-PRRSV infection caused more severe clinical symptoms, mortality, and ILN lesions. Histopathological lesions were seen in ILNs with a marked decrease in lymphocyte numbers. Terminal deoxynucleotidyl transferase (TdT)-mediated de-oxyuridine triphosphate (dUTP)-biotin nick end-labeling (TUNEL) assays revealed that HP-PRRSV strain HuN4 alone induced ILN apoptosis, but dual-infection with S. suis strain BM0806 induced greater levels of apoptosis. Besides, we found that some HP-PRRSV-infected cells underwent apoptosis. Furthermore, anti-caspase-3 antibody staining confirmed that ILN apoptosis was mainly induced by a caspase-dependent pathway. Pyroptosis was also observed in HP-PRRSV-infected cells, and there was more pyroptosis in piglets infected with HP-PRRSV alone compared with those with secondary S. suis infection, and HP-PRRSV-infected cells underwent pyroptosis. Altogether, this is the first report to identify pyroptosis in ILNs and which signaling pathway is related to ILN apoptosis in single or dual-infected piglets. These results contribute to a better understanding of the pathogenic mechanisms during secondary S. suis infection.

TRIM26-mediated degradation of nucleocapsid protein limits porcine reproductive and respiratory syndrome virus-2 infection

Porcine reproductive and respiratory syndrome (PRRS), caused by PRRSV, has ranked among the most economically important veterinary infectious diseases globally. Recently, tripartite motif (TRIMs) family members have arisen as novel restriction factors in antiviral immunity. Noteworthy, TRIM26 was reported as a binding partner of IRF3, TBK1, TAB1, and NEMO, yet its role in virus infection remains controversial. Herein, we showed that TRIM26 bound N protein by the C-terminal PRY/SPRY domain. Moreover, ectopic expression of TRIM26 impaired PRRSV replication and induced degradation of N protein. The anti-PRRSV activity was independent of the nuclear localization signal (NLS). Instead, deletion of the RING domain, or the PRY/SPRY portion, abrogated the antiviral function. Finally, siRNA depletion of TRIM26 resulted in enhanced production of viral RNA and virus yield in porcine alveolar macrophages (PAMs) after PRRSV infection. Overexpression of an RNAi-resistant TRIM26 rescue-plasmid led to the acquisition of PRRSV restriction in TRIM26-knockdown cells. Together, these data add TRIM26 as a potential target for drug design against PRRSV.

Activation of T-bet, FOXP3, and EOMES in Target Organs From Piglets Infected With the Virulent PRRSV-1 Lena Strain

Transcription factors (TFs) modulate genes involved in cell-type-specific proliferative and migratory properties, metabolic features, and effector functions. Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogen agents in the porcine industry; however, TFs have been poorly studied during the course of this disease. Therefore, we aimed to evaluate the expressions of the TFs T-bet, GATA3, FOXP3, and Eomesodermin (EOMES) in target organs (the lung, tracheobronchial lymph node, and thymus) and those of different effector cytokines (IFNG, TNFA, and IL10) and the Fas ligand (FASL) during the early phase of infection with PRRSV-1 strains of different virulence. Target organs from mock-, virulent Lena-, and low virulent 3249-infected animals humanely euthanized at 1, 3, 6, 8, and 13 days post-infection (dpi) were collected to analyze the PRRSV viral load, histopathological lesions, and relative quantification through reverse transcription quantitative PCR (RT-qPCR) of the TFs and cytokines. Animals belonging to both infected groups, but mainly those infected with the virulent Lena strain, showed upregulation of the TFs T-bet, EOMES, and FOXP3, together with an increase of the cytokine IFN-γ in target organs at the end of the study (approximately 2 weeks post-infection). These results are suggestive of a stronger polarization to Th1 cells and regulatory T cells (Tregs), but also CD4+ cytotoxic T lymphocytes (CTLs), effector CD8+ T cells, and γδT cells in virulent PRRSV-1-infected animals; however, their biological functionality should be the object of further studies.

On-site differential diagnostic detection of HP-PRRSV and C-PRRSV using EuNPs-mAb fluorescent probe-based immunoassay

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused worldwide economic losses in the swine industry. Pigs infected with highly pathogenic (HP)-PRRSV display more severe symptoms than those infected with classical (C)-PRRSV. A rapid, sensitive, and reliable detection method to distinguish between HP-PRRSV and C-PRRSV is needed. In this study, we prepared a monoclonal antibody from a hybridoma that can distinguish HP-PRRSV(including TP, QJ, LQ, JN-HS, and TY strain) from C-PRRSV (CH-1A strain) using cell surface-fluorescence immunosorbent assays (CSFIA). Based on this monoclonal antibody (4D5), we developed a europium microsphere-based lateral flow immunochromatographic strip (EuNPs-LFICS) for the differential diagnostic detection of HP-PRRSV and C-PRRSV. Under optimized conditions, the method was rapid (15 min), sensitive (LOD: 2.57 ng mL^-1, 606 TCID50/0.1 mL), selective for HP-PRRSV detection, and quantitative (DLR: 3.56-228 ng mL^-1). In clinical samples, the EuNPs-LFICS assay was largely consistent with PCR results, indicating its practical clinical application.

Swine Dendritic Cell Response to Porcine Reproductive and Respiratory Syndrome Virus: An Update

Dendritic cells (DCs) are the most potent antigen-presenting cells, unique to initiate and coordinate the adaptive immune response. In pigs, conventional DCs (cDCs), plasmacytoid DCs (pDCs), and monocyte-derived DCs (moDCs) have been described in blood and tissues. Different pathogens, such as viruses, could infect these cells, and in some cases, compromise their response. The understanding of the interaction between DCs and viruses is critical to comprehend viral immunopathological responses. Porcine reproductive and respiratory syndrome virus (PRRSV) is the most important respiratory pathogen in the global pig population. Different reports support the notion that PRRSV modulates pig immune response in addition to their genetic and antigenic variability. The interaction of PRRSV with DCs is a mostly unexplored area with conflicting results and lots of uncertainties. Among the scarce certainties, cDCs and pDCs are refractory to PRRSV infection in contrast to moDCs. Additionally, response of DCs to PRRSV can be different depending on the type of DCs and maybe is related to the virulence of the viral isolate. The precise impact of this virus-DC interaction upon the development of the specific immune response is not fully elucidated. The present review briefly summarizes and discusses the previous studies on the interaction of in vitro derived bone marrow (bm)- and moDCs, and in vivo isolated cDCs, pDCs, and moDCs with PRRSV1 and 2.

The jigsaw of PRRSV virulence

Porcine reproductive and respiratory syndrome virus (PRRSV) is the causative agent of the, probably, most economically important disease for the pig industry worldwide. This disease, characterised by producing reproductive failure in sows and respiratory problems in growing pigs, appeared in the late 1980s in the United States and Canada. Since its appearance, strains capable of producing higher mortality rates as well as greater severity in clinical signs and lesions than classical strains have been identified. However, since the first reports of these "virulent" PRRSV outbreaks, no homogeneity and consensus in their description have been established. Moreover, to the authors' knowledge, there is no published information related to the criteria that a PRRSV strain should fulfil to be considered as a "virulent" strain. In this review, we revise the terminology used and gather the information related to the main characteristics and differences in clinical signs, lesions, viral replication and tropism as well as immunological parameters between virulent and classical PRRSV strains and propose a first approximation to the criteria to define a virulent PRRSV strain.

S100A9 regulates porcine reproductive and respiratory syndrome virus replication by interacting with the viral nucleocapsid protein

Porcine reproductive and respiratory syndrome virus (PRRSV) has caused huge economic losses to the pig industry worldwide over the last 30 years, yet the associated viral-host interactions remain poorly understood. S100A9 is a damage-associated molecular pattern of the S100 protein family. Here, we found that PRRSV infection stimulated S100A9 expression in porcine alveolar macrophages (PAMs) and Marc-145 cells. S100A9 inhibited PRRSV replication via cellular Ca²⁺ dependent manner. The viral nucleocapsid (N) protein co-localized with S100A9 in the cytoplasm, and directly interacted at amino acid 78 of S100A9 and amino acids 36-37 of N protein. Moreover, we also found that the mutant S100A9 (E78Q) protein exhibited decreased antiviral activity against PRRSV compared with the parent S100A9. Recombinant PRRSV rBB (36/37) with two mutations in amino acid 36-37 in the N protein exhibited greater replication than the parent PRRSV BB0907 in S100A9-overexpressed PAM and Marc-145 cells. Thus, S100A9 may restrict PRRSV proliferation by interacting with the viral N protein.

Nuclear localization signal in TRIM22 is essential for inhibition of type 2 porcine reproductive and respiratory syndrome virus replication in MARC-145 cells

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes one of the most economically important swine diseases worldwide. Tripartite motif-containing 22 (TRIM22), a TRIM family protein, has been identified as a crucial restriction factor that inhibits a group of human viruses. Currently, the role of cellular TRIM22 in PRRSV infection remains unclear. In the present study, we analyzed the effect of TRIM22 on PRRSV replication in vitro and explored the underlying mechanism. Ectopic expression of TRIM22 impaired the viral replication, while TRIM22-RNAi favored the replication of PRRSV in MARC-145 cells. Additionally, we observed that TRIM22 deletion SPRY domain or Nuclear localization signal (NLS) losses the ability to inhibit PRRSV replication. Finally, Co-IP analysis identified that TRIM22 interacts with PRRSV nucleocapsid (N) protein through the SPRY domain, while the NLS2 motif of N protein is involved in interaction with TRIM22. Although the concentration of PRRSV N protein was not altered in the presence of TRIM22, the abundance of N proteins from simian hemorrhagic fever virus (SHFV), equine arteritis virus (EAV), and murine lactate dehydrogenase-elevating virus (LDV) diminished considerably with increasing TRIM22 expression. Together, our findings uncover a previously unrecognized role for TRIM22 and extend the antiviral effects of TRIM22 to arteriviruses.

Negative Immunomodulatory Effects of Type 2 Porcine Reproductive and Respiratory Syndrome Virus-Induced Interleukin-1 Receptor Antagonist on Porcine Innate and Adaptive Immune Functions

Impaired innate and adaptive immune responses are evidenced throughout the course of PRRSV infection. We previously reported that interleukin-1 receptor antagonist (IL-1Ra) was involved in PRRSV-induced immunosuppression during an early phase of infection. However, the exact mechanism associated with PRRSV-induced IL-1Ra immunomodulation remains unknown. To explore the immunomodulatory properties of PRRSV-induced IL-1Ra on porcine immune functions, monocyte-derived dendritic cells (MoDC) and leukocytes were cultured with type 2 PRRSV, and the immunological role of IL-1Ra was assessed by addition of anti-porcine IL-1Ra Ab. The results demonstrated that PRRSV-induced IL-1Ra reduced phagocytosis, surface expression of MHC II (SLA-DR) and CD86, as well as downregulation of IFNA and IL1 gene expression in the MoDC culture system. Interestingly, IL-1Ra secreted by the PRRSV-infected MoDC also inhibited T lymphocyte differentiation and proliferation, but not IFN-γ production. Although PRRSV-induced IL-1Ra was not directly linked to IL-10 production, it contributed to the differentiation of regulatory T lymphocytes (Treg) within the culture system. Taken together, our results demonstrated that PRRSV-induced IL-1Ra downregulates innate immune functions, T lymphocyte differentiation and proliferation, and influences collectively with IL-10 in the Treg induction. The immunomodulatory roles of IL-1Ra elucidated in this study increase our understanding of the immunobiology of PRRSV.

Nsp1α of Porcine Reproductive and Respiratory Syndrome Virus Strain BB0907 Impairs the Function of Monocyte-Derived Dendritic Cells via the Release of Soluble CD83

Porcine reproductive and respiratory syndrome virus (PRRSV), a virulent pathogen of swine, suppresses the innate immune response and induces persistent infection. One mechanism used by viruses to evade the immune system is to cripple the antigen-processing machinery in monocyte-derived dendritic cells (MoDCs). In this study, we show that MoDCs infected by PRRSV express lower levels of the major histocompatibility complex (MHC)-peptide complex proteins TAP1 and ERp57 and are impaired in their ability to stimulate T cell proliferation and increase their production of CD83. Neutralization of sCD83 removes the inhibitory effects of PRRSV on MoDCs. When MoDCs are incubated with exogenously added sCD83 protein, TAP1 and ERp57 expression decreases and T lymphocyte activation is impaired. PRRSV nonstructural protein 1α (Nsp1α) enhances CD83 promoter activity. Mutations in the ZF domain of Nsp1α abolish its ability to activate the CD83 promoter. We generated recombinant PRRSVs with mutations in Nsp1α and the corresponding repaired PRRSVs. Viruses with Nsp1α mutations did not decrease levels of TAP1 and ERp57, impair the ability of MoDCs to stimulate T cell proliferation, or increase levels of sCD83. We show that the ZF domain of Nsp1α stimulates the secretion of CD83, which in turn inhibits MoDC function. Our study provides new insights into the mechanisms of immune suppression by PRRSV.IMPORTANCE PRRSV has a severe impact on the swine industry throughout the world. Understanding the mechanisms by which PRRSV infection suppresses the immune system is essential for a robust and sustainable swine industry. Here, we demonstrated that PRRSV infection manipulates MoDCs by interfering with their ability to produce proteins in the MHC-peptide complex. The virus also impairs the ability of MoDCs to stimulate cell proliferation, due in large part to the enhanced release of soluble CD83 from PRRSV-infected MoDCs. The viral nonstructural protein 1 (Nsp1) is responsible for upregulating CD83 promoter activity. Amino acids in the ZF domain of Nsp1α (L5-2A, rG45A, G48A, and L61-6A) are essential for CD83 promoter activation. Viruses with mutations at these sites no longer inhibit MoDC-mediated T cell proliferation. These findings provide novel insights into the mechanism by which the adaptive immune response is suppressed during PRRSV infection.

Serine 105 and 120 are important phosphorylation sites for porcine reproductive and respiratory syndrome virus N protein function

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We identified one novel phophorylation site of the PRRSV N protein.

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We firstly found that the mutated the residue 105 and 120 could down-regulate the N-induced IL-10.

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We firstly found that mutating the residue 105 could impair the virus growth ability.

The nucleocapsid (N) protein is the most abundant protein of porcine reproductive and respiratory syndrome virus (PRRSV). It has been shown to be multiphosphorylated. However, the phosphorylation sites are still unknown. In this study, we used liquid chromatography tandem mass spectrometry (LC–MS/MS) to analyze the phosphorylation sites of N protein expressed in Sf9 cells. The results showed that N protein contains two phosphorylation sites. Since N protein can regulate IL-10, which may facilitate PRRSV replication, we constructed four plasmids (pCA-XH-GD, pCA-A105, pCA-A120 and pCA-A105-120) and transfected them into Pig alveolar macrophages (PAMs,3D4/2). The qPCR results showed that mutations at residues 105 and 120 were associated with down-regulation of the IL-10 mRNA level, potentially decreasing the viral growth ability. Then, we mutated the phosphorylation sites (S105A and S120A) and rescued three mutated viruses, namely, A105, A120 and A105-120. Compared with wild-type virus titers, the titers of the mutated viruses at 48 hpi were significantly decreased. The Nsp(non-structural protein) 9 qPCR results were consistent with the multistep growth kinetics results. The infected PAMs (primary PAMs) results were similar with Marc-145.The findings indicated that the mutations impaired the viral replication ability. The confocal microscopy results suggested that mutations to residues 105 and 120 did not affect N protein distribution. Whether the mutations affected other functions of N protein and what the underlying mechanisms are need further investigation. In conclusion, our results show that residues 105 and 120 are phosphorylation sites and are important for N protein function and for viral replication ability.

Interaction of porcine reproductive and respiratory syndrome virus proteins with SUMO-conjugating enzyme reveals the SUMOylation of nucleocapsid protein

SUMOylation is a reversible post-translational modification that regulates the function of target protein. In this study, we first predicted by software that the multiple proteins of porcine reproductive and respiratory syndrome virus (PRRSV) could be sumoylated. Next, we confirmed that Nsp1β, Nsp4, Nsp9, Nsp10 and nucleocapsid (N) protein of PRRSV could interact with the sole SUMO E2 conjugating enzyme Ubc9, and Ubc9 could be co-localized with Nsp1β, Nsp4, Nsp9 and Nsp10 in the cytoplasm, while with N protein in both the cytoplasm and nucleus. Finally, we demonstrated that N protein could be sumoylated by either SUMO1 or SUMO2/3. In addition, the overexpression of Ubc9 could inhibit viral genomic replication at early period of PRRSV infection and the knockdown of Ubc9 by siRNA could promote the virus replication. These findings reveal the SUMOylation property of PRRSV N protein and the involvement of Ubc9 in PRRSV replication through interaction with multiple proteins of PRRSV. To our knowledge, this is the first study indicating the interplay between SUMO modification system and PRRSV.

The Nucleocapsid Protein and Nonstructural Protein 10 of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Enhance CD83 Production via NF-κB and Sp1 Signaling Pathways

Porcine reproductive and respiratory syndrome, caused by porcine reproductive and respiratory syndrome virus (PRRSV), is a panzootic disease that is one of the most economically costly diseases to the swine industry. A key aspect of PRRSV virulence is that the virus suppresses the innate immune response and induces persistent infection, although the underlying mechanisms are not well understood. The dendritic cell (DC) marker CD83 belongs to the immunoglobulin superfamily and is associated with DC activation and immunosuppression of T cell proliferation when expressed as soluble CD83 (sCD83). In this study, we show that PRRSV infection strongly stimulates CD83 expression in porcine monocyte-derived DCs (MoDCs) and that the nucleocapsid (N) protein and nonstructural protein 10 (nsp10) of PRRSV enhance CD83 promoter activity via the NF-κB and Sp1 signaling pathways. R43A and K44A amino acid substitution mutants of the N protein suppress the N protein-mediated increase of CD83 promoter activity. Similarly, P192-5A and G214-3A mutants of nsp10 (with 5 and 3 alanine substitutions beginning at residues P192 and G214, respectively) abolish the nsp10-mediated induction of the CD83 promoter. Using reverse genetics, four mutant viruses (rR43A, rK44A, rP192-5A, and rG214-3A) and four revertants [rR43A(R), rK44A(R), rP192-5A(R), and rG214-3A(R)] were generated. Decreased induction of CD83 in MoDCs was observed after infection by mutants rR43A, rK44A, rP192-5A, and rG214-3A, in contrast to the results obtained using rR43A(R), rK44A(R), rP192-5A(R), and rG214-3A(R). These findings suggest that PRRSV N and nsp10 play important roles in modulating CD83 signaling and shed light on the mechanism by which PRRSV modulates host immunity.IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most economically costly pathogens affecting the swine industry. It is unclear how PRRSV inhibits the host's immune response and induces persistent infection. The dendritic cell (DC) marker CD83 belongs to the immunoglobulin superfamily and has previously been associated with DC activation and immunosuppression of T cell proliferation and differentiation when expressed as soluble CD83 (sCD83). In this study, we found that PRRSV infection induces sCD83 expression in porcine MoDCs via the NF-κB and Sp1 signaling pathways. The viral nucleocapsid protein, nonstructural protein 1 (nsp1), and nsp10 were shown to enhance CD83 promoter activity. Amino acids R43 and K44 of the N protein, as well as residues 192 to 196 (P192-5) and 214 to 216 (G214-3) of nsp10, play important roles in CD83 promoter activation. These findings provide new insights into the molecular mechanism of immune suppression by PRRSV.

Pathogenesis and control of the Chinese highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) has remained a major threat to the worldwide swine industry ever since its first discovery in the early 1990s. Under the selective pressures in the field, this positive-stranded RNA virus undergoes rapid genetic evolution that eventually leads to emergence in 2006 of the devastating Chinese highly pathogenic PRRSV (HP-PRRSV). The atypical nature of HP-PRRSV has caused colossal economic losses to the swine producers in China and the surrounding countries. In this review, we summarize the recent advances in our understanding of the pathogenesis, evolution and ongoing field practices on the control of this troubling virus in China.