Ultrastructural pathogenesis of the PRRS virus

Analysis of whole transcriptome reveals the immune response to porcine reproductive and respiratory syndrome virus infection and tylvalosin tartrate treatment in the porcine alveolar macrophages

Introduction

Porcine reproductive and respiratory syndrome virus (PRRSV) is a major pathogen that has caused severe economic losses in the swine industry. Screening key host immune-related genetic factors in the porcine alveolar macrophages (PAMs) is critical to improve the anti-virial ability in pigs.

Methods

In this study, an in vivo model was set to evaluate the anti-PRRSV effect of tylvalosin tartrates. Then, strand-specific RNA-sequencing (ssRNA-seq) and miRNA-sequencing (miRNA-seq) were carried out to profile the whole transcriptome of PAMs in the negative control, PRRSV-infected, and tylvalosin tartrates-treatment group.

Results

The ssRNA-seq identified 11740 long non-coding RNAs in PAMs. Based on our attention mechanism-improved graph convolutional network, 41.07% and 28.59% lncRNAs were predicted to be located in the nucleus and cytoplasm, respectively. The miRNA-seq revealed that tylvalosin tartrates-enhanced miRNAs might play roles in regulating angiogenesis and innate immune-related functions, and it rescued the expression of three anti-inflammation miRNAs (ssc-miR-30a-5p, ssc-miR-218-5p, and ssc-miR-218) that were downregulated due to PRRSV infection. The cytoplasmic lncRNAs enhanced by tylvalosin tartrates might form ceRNA networks with miRNAs to regulate PAM chemotaxis. While cytoplasmic lncRNAs that were rescued by tylvalosin tartrates might protect PAMs via efferocytosis-related ceRNA networks. On the other hand, the tylvalosin tartrates-rescued nuclear lncRNAs might negatively regulate T cell apoptosis and bind to key anti-inflammation factor IL37 to protect the lungs by cis- and trans-regulation.

Conclusions

Our data provides a catalog of key non-coding RNAs in response to PRRSV and tylvalosin tartrates and might enrich the genetic basis for future PRRSV prevention and control.

Activation of regulated cell death in the lung of piglets infected with virulent PRRSV-1 Lena strain occurs earlier and mediated by cleaved Caspase-8

PRRSV-1 virulent strains cause high fever, marked respiratory disease and severe lesions in lung and lymphoid organs. Regulated cell death (RCD), such as apoptosis, necroptosis and pyroptosis, is triggered by the host to interrupt viral replication eliminating infected cells, however, although it seems to play a central role in the immunopathogenesis of PRRSV, there are significant gaps regarding their sequence and activation upon PRRSV-infection. The present study evaluated RCD events by means of caspases expression in the lung of PRRSV-1-infected pigs and their impact on pulmonary macrophage subpopulations and lung lesion. Conventional piglets were intranasally inoculated with the virulent subtype 3 Lena strain or the low virulent subtype 1 3249 strain and euthanised at 1, 3, 6, 8 and 13 dpi. Lena-infected piglets showed severe and early lung damage with a high frequency of PRRSV-N-protein⁺ cells, depletion of CD163⁺ cells and high viral load in the lung. The number of TUNEL⁺ cells was significantly higher than cCasp3⁺ cells in Lena-infected piglets during the first week post-infection. cCasp8 and to a lesser extent cCasp9 were activated by both PRRSV-1 strains after one week post-infection together with a replenishment of both CD163⁺ and Arg-1⁺ pulmonary macrophages. These results highlight the induction of other forms of RCD beyond apoptosis, such as, necroptosis and pyroptosis during the first week post-infection followed by the activation of, mainly, extrinsic apoptosis during the second week post-infection. The recovery of CD163⁺ macrophages at the end of the study represents an attempt to restore pulmonary macrophage subpopulations lost during the early stages of the infection but also a macrophage polarisation into M2 macrophages.

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.

Autophagy involved in antiviral activity of sodium tanshinone IIA sulfonate against porcine reproductive and respiratory syndrome virus infection in vitro

BACKGROUND

In previous research, we have demonstrated that sodium tanshinone IIA sulfonate (STS) has anti-porcine reproductive and respiratory syndrome virus (PRRSV) activity, but whether autophagy is involved in this process is still unknown. In this study, the autophagy effect of STS against PRRSV infection was investigated in vitro.

METHODS

Quantitative real-time PCR (qRT-PCR) and western blot was used to evaluate the inhibition ability of STS on the mRNA expression levels on cell autophagy genes, that is Beclin1, ATG5 and ATG7. Simultaneously, the effect of STS on N protein/gene expression was assessed by indirect immuno-fluorescence assay (IFA), qRT-PCR and western blot.

RESULTS

The results indicated that STS inhibits autophagy induced by PRRSV. In addition, STS effectively suppresses PRRSV's N protein replication and N gene expression in Marc-145 cells infected with PRRSV in a time-dependent manner.

CONCLUSIONS

Our results suggest that STS exhibits anti-PRRSV activity in vitro by suppressing autophagy-related genes, which may provide a theoretical basis for further pharmacological agent development regarding PRRSV infection.

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

Porcine reproductive and respiratory syndrome virus (PRRSV) is the etiologic agent of porcine reproductive and respiratory syndrome (PRRS), a devastating disease of swine that poses a serious threat to the swine industry worldwide. The induction of apoptosis in host cells is suggested to be the key cellular mechanism that contributes to the pathogenesis of PRRS. Various signaling pathways have been identified to be involved in regulating PRRSV-induced apoptosis. In this review, we summarize the potential signaling pathways that contribute to PRRSV-induced apoptosis, and propose the issues that need to be addressed in future studies for a better understanding of the molecular basis underlying the pathogenesis of PRRS.

Evaluation of apoptosis markers in different cell lines infected with equine arteritis virus

Equine arteritis virus (EAV) induces apoptosis in infected cells. Cell death caused by EAV has been studied mainly using three cell lines, BHK-21, RK-13 and Vero cells. The mechanism of apoptosis varies among cell lines and results cannot be correlated owing to differences in EAV strains used. We evaluated different markers for apoptosis in BHK-21, RK-13 and Vero cell lines using the Bucyrus EAV reference strain. Acridine orange/ethidium bromide staining revealed morphological changes in infected cells, while flow cytometry indicated the extent of apoptosis. We also observed DNA fragmentation, but the DNA ladder was detected at different times post-infection depending on the cell line, i.e., 48, 72 and 96 h post-infection in RK-13, Vero and BHK-21 cells, respectively. Measurement of viral titers obtained with each cell line indicated that apoptosis causes interference with viral replication and therefore decreased viral titers. As an unequivocal marker of apoptosis, we measured the expression of caspase-3 and caspases-8 and -9 as extrinsic and intrinsic markers of apoptosis pathways, respectively. Caspase-8 in BHK-21 cells was the only protease that was not detected at any of the times assayed. We found that Bucyrus EAV strain exhibited a distinctive apoptosis pathway depending on the cell line.

Comparison of the immune competence of Turopolje, German Landrace × Turopolje, and German Landrace × Pietrain pigs after PRRSV vaccination

The competences of the immune systems of the ancient pig breed Turopolje (T×T), German Landrace × Turopolje (L×T) and 'modern' pig breed German Landrace × Pietrain (L×P) were compared in this study. All pigs were immunized with a modified live vaccine against 'Porcine Reproductive and Respiratory Syndrome' (PRRS) virus (Ingelvac PRRS MLV(®)) to simulate an infection. Antibody production against PRRS MLV was evaluated in serum. Elimination of the viral infectious fragments during the experimental period was monitored in serum, leukocytes and tonsils by RT-qPCR. Furthermore relevant immune marker genes were quantified either on gene expression level using RT-qPCR [toll like receptor (TLR) 7, TLR8, TRAF6, CD163, SIGLEC1, CD4, CD8, CD14, CD19, tumor necrosis factor alpha (TNFα), interleukin (IL) 1, IL2, IL6, IL12], and on protein level using ELISA [interleukin (IL)-1, IL-2, IL-6, and IL-12]. The three breeds showed individual inactivation efficiencies as a reaction to the PRRS MLV vaccination. T×T eliminated the virus in serum within 16 days, followed by L×T (28 days) and L×P (36 days). The antibody titers against PRRS MLV of L×T and L×P were significantly higher compared to T×T (p<0.05). The gene expression data and protein analysis of interleukins revealed that T×T reacted with a type 1 immune response. In contrast, the two other breeds (L×T and L×P) showed a type 2 immune response, which resulted in the higher synthesis of B-cells and an increased concentration of specific anti-PRRS MLV antibodies.

Glycoprotein 5 of porcine reproductive and respiratory syndrome virus strain SD16 inhibits viral replication and causes G2/M cell cycle arrest, but does not induce cellular apoptosis in Marc-145 cells

Cell apoptosis is common after infection with porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV GP5 has been reported to induce cell apoptosis. To further understand the role of GP5 in PRRSV induced cell apoptosis, we established Marc-145 cell lines stably expressing full-length GP5, GP5(Δ84-96) (aa 84-96 deletion), and GP5(Δ97-119) (aa 97-119 deletion). Cell proliferation, cell cycle progression, cell apoptosis and virus replication in these cell lines were evaluated. Neither truncated nor full-length GP5 induced cell apoptosis in Marc-145 cells. However, GP5(Δ97-119), but not full-length or GP5(Δ84-96), induced a cell cycle arrest at the G2/M phase resulting in a reduction in the growth of Marc-145 cells. Additionally, GP5(Δ84-96) inhibited the replication of PRRSV in Marc-145 cells through induction of IFN-β. These findings suggest that PRRSV GP5 is not responsible for inducing cell apoptosis in Marc-145 cells under these experimental conditions; however it has other important roles in virus/host cell biology.

Regulation and evasion of antiviral immune responses by porcine reproductive and respiratory syndrome virus

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Five PRRSV viral proteins are shown to inhibit type I IFN induction and signaling by targeting different intracellular signaling intermediates.

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PRRSV regulates the expression of IL-10 and TNFα.

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PRRSV modulates apoptosis during infection.

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MicroRNAs might play significant roles in subverting immunity for PRRSV.

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PRRSV escapes from adaptive immunity by impairing antigen presentation, activating Tregs, and ADE.

Virus infection of mammalian cells triggers host innate immune responses to restrict viral replication and induces adaptive immunity for viral elimination. In order to survive and propagate, viruses have evolved sophisticated mechanisms to subvert host defense system by encoding proteins that target key components of the immune signaling pathways. Porcine reproductive and respiratory syndrome virus (PRRSV), a RNA virus, impairs several processes of host immune responses including interfering with interferon production and signaling, modulating cytokine expression, manipulating apoptotic responses and regulating adaptive immunity. In this review, we highlight the molecular mechanisms of how PRRSV interferes with the different steps of initial antiviral host responses to establish persistent infection in pigs. Dissection of the PRRSV–host interaction is the key in understanding PRRSV pathogenesis and will provide a basis for the rational design of vaccines.

Overview: Replication of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus that causes significant losses in the pig industry, is one of the most important animal pathogens of global significance. Since the discovery of the virus, significant progress has been made in understanding its epidemiology and transmission, but no adequate control measures are yet available to eliminate infection with this pathogen. The genome replication of PRRSV is required to reproduce, within a few hours of infection, the millions of progeny virions that establish, disseminate, and maintain infection. Replication of the viral RNA genome is a multistep process involving a replication complex that is formed not only from components of viral and cellular origin but also from the viral genomic RNA template; this replication complex is embedded within particular virus-induced membrane vesicles. PRRSV RNA replication is directed by at least 14 replicase proteins that have both common enzymatic activities, including viral RNA polymerase, and also unusual and poorly understood RNA-processing functions. In this review, we summarize our current understanding of PRRSV replication, which is important for developing a successful strategy for the prevention and control of this pathogen.

Recombinant chimeric vaccine composed of PRRSV antigens and truncated Pseudomonas exotoxin A (PE-K13)

A Pseudomonas exotoxin (PE-KDEL)-based chimeric subunit vaccine system was recently developed using a reverse vaccinology technique. In this study, the plasmids containing PE-PRRS chimeric subunits were constructed that composed of porcine reproductive and respiratory syndrome virus (PRRSV) antigen moieties, a ligand moiety and a Pseudomonas exotoxin A deleted domain III (PE (ΔIII)), and a carboxyl terminal moiety that includes a polypeptide with amino acid sequence KDEL (K3). The PE-PRRS combination vaccine can effectively induce not only PRRSV-specific INF-γ cellular immunity but also a slow-reacting and complement-requiring type serum neutralizing antibody in pigs. In a specific pathogen free (SPF) pig challenge model, body temperature (colonic temperature), occurrence of PRRSV viremia, nasal excretions, gross and histopathological appearances of pneumonia, and serum antibody activity (IFA and SN) titers significantly differed between the immunized group and the control group. The survey showed that a 0.3mg/dose PE-PRRS vaccine formula conferred protection against PRRSV. A field trial of PE-PRRS vaccine was performed to study the immune response of pregnant sows after vaccination in a PRRSV persist farm. The RT-PCR analysis of viremia and serological titers showed that the PE-PRRS vaccine not only increased sow reproductive performance and evoked its immune response to PRRS viremia, it also activated maternal immune protections to prevent piglets from inflicting viremia. In conclusion, we developed a novel and effective PRRS cytotoxic T-cells (CTLs)-based vaccine containing Pseudomonas exotoxin (PE-KDEL) carrier in combination with PRRSV conserved epitopes against PRRS virus.

Porcine reproductive and respiratory syndrome virus nonstructural protein 4 induces apoptosis dependent on its 3C-like serine protease activity

Porcine reproductive and respiratory syndrome (PRRS) is a highly contagious disease in pigs caused by PRRS virus (PRRSV). Although PRRSV infection-induced cell apoptosis has been established, the related viral protein is still unknown. Here, we reported that PRRSV nonstructural protein 4 (nsp4) was a critical apoptosis inducer. Nsp4 could activate caspase-3, -8, and -9. Using truncated constructs without different domains in nsp4, we demonstrated that the full-length of nsp4 structure was required for its apoptosis-inducing activity. Furthermore, using site-directed mutagenesis to inactivate the 3C-like serine protease activity of nsp4, we showed that nsp4-induced apoptosis was dependent on its serine protease activity. The ability of nsp4 to induce apoptosis was significantly impaired by His39, Asp64, and Ser118 mutations, suggesting that His39, Asp64, and Ser118 were essential for nsp4 to trigger apoptosis. In conclusion, our present work showed that PRRSV nsp4 could induce apoptosis in host cells and might be partially responsible for the apoptosis induced by PRRSV infection. PRRSV 3C-like protease-mediated apoptosis represents the first report in the genus Arterivirus, family Arteriviridae.

Involvement of unfolded protein response, p53 and Akt in modulation of porcine reproductive and respiratory syndrome virus-mediated JNK activation

Our previous study has shown that activation of JNK plays a critical role in Porcine reproductive and respiratory syndrome virus (PRRSV)-mediated apoptosis. In this follow-up study, we further investigated the mechanisms involved in modulation of PRRSV-mediated JNK activation and apoptosis. We found that unfolded protein response (UPR) was induced in response to PRRSV infection which in turn triggered JNK activation and apoptosis. We also found that p53 and Akt were activated at the early stage of infection and functioned as negative regulator of JNK activation to counteract the PRRSV-mediated apoptosis. Furthermore, induction of UPR, p53 and Akt was not only involved in modulation of PRRSV-mediated apoptosis, but also contributed to the virus replication. Our findings indicated that multiple signaling pathways were involved in modulation of PRRSV-mediated apoptosis of the host cells via regulating JNK signaling pathway and provided novel insights into understanding the mechanisms of pathogenesis of PRRSV infection.

Type 2 Porcine Reproductive and Respiratory Syndrome Virus infection mediated apoptosis in B- and T-cell areas in lymphoid organs of experimentally infected pigs

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection is characterized by persisting in lungs and lymphoid tissue, resulting in systemic lymphoid depletion. The aim of this study was to correlate the histological changes, viral antigen expression and apoptosis phenomena in tonsil, medial retropharyngeal and mediastinal lymph nodes of 12 pigs inoculated with a type 2 PRRSV isolate (Chilean strain 2402). Apoptosis phenomena were observed mainly in lymphocytes and secondly in macrophages of lymph nodes and tonsils of inoculated animals, showing a peak of both apoptotic cells and viral antigen expression at the end of the study (21 dpi). However, the number of apoptotic cells was higher than the number of PRRSV-positive cells at the end of the study. This finding together with the location of apoptotic cells and PRRSV-positive cells in different structures of lymphoid organs supports the hypothesis that PRRSV-positive macrophages might modulate the apoptosis phenomena in other cells, mainly lymphocytes, by means of an indirect mechanism. Furthermore, apoptotic cells were detected both in B- and T-cell areas of lymphoid organs, suggesting that apoptosis phenomena may play a role in the impairment of the host immune response during PRRS.

Porcine reproductive and respiratory syndrome virus activates the transcription of interferon alpha/beta (IFN-α/β) in monocyte-derived dendritic cells (Mo-DC)

Porcine reproductive and respiratory syndrome virus (PRRSV) is known to be a poor inducer of interferon alpha/beta (IFN-α/β), which may contribute to the delayed development of adaptive immunity and the resultant viral persistence. However, the exact mechanism by which PRRSV inhibits the induction of IFN-α/β during infection of its natural host cells remains less well defined. Here, we show that PRRSV efficiently activates the transcription of IFN-α/β in porcine monocyte-derived dendritic cells (Mo-DC) in a time-dependent and transient manner; and this effect is dependent on the activation of phosphatidylinositol 3-kinase (PI3K). Despite the abundant IFN-α transcripts detected in PRRSV-infected Mo-DC, little or no detectable IFN-α is found in the supernatants and cell lysates of PRRSV-infected Mo-DC, suggesting that PRRSV either blocks the translation of IFN-α or inhibits the RNA processing and transport. Furthermore, we observed that PRRSV infection significantly reduced the induction of IFN-α by Poly I:C treatment; and virus replication is essential to the effect since heat-inactivated PRRSV has no effect on IFN-α induction by Poly I:C. Overall, our data provide evidence for the possible role of PI3K in the activation of the transcription of IFN-α/β by PRRSV. We conclude that PRRSV inhibits the induction of IFN-α in Mo-DC by as yet undefined post-transcriptional mechanisms.

Detection of asymptomatic antigenemia in pigs infected by porcine reproductive and respiratory syndrome virus (PRRSV) by a novel capture immunoassay with monoclonal antibodies against the nucleocapsid protein of PRRSV

Routine surveillance for porcine reproductive and respiratory syndrome virus (PRRSV) infections is crucial for the epidemiological control of this disease. Antibody tests are widely used but cannot differentiate between vaccination and reinfection. We developed a PRRSV antigen capture enzyme-linked immunosorbent assay (ELISA) using well-characterized monoclonal antibodies (MAbs) raised against the nucleocapsid (N) protein of North American and European PRRSV. This antigen assay detected purified N protein from both genotypes at levels as low as 0.4 and 0.8 ng, respectively. The specificity and sensitivity of the N antigen assay were evaluated with ground lung tissues from 8 PRRSV-infected and 16 healthy swine, and culture supernatants from six PRRSV isolates as well as other swine viruses were confirmed by reverse transcriptase PCR (RT-PCR). Antigen assays were positive in all eight infected tissues and with six different PRRSV isolates, with no false positives among healthy tissues and other swine viruses (i.e., pseudorabies and foot and mouth disease viruses). A number of sera, field collected from 466 vaccinated and asymptomatic pigs in Guangdong, China, between 2008 and 2009, tested positive by the N antigen assay (12.45%), RT-PCR (15.02%), and a commercial test for antibodies against PRRSV (78.97%). Of the 466 sera, 47 were positive by both antigen and RT-PCR tests, 11 by antigen test only, and 23 by RT-PCR only; the two assays had an overall agreement of 92.7%, indicating a significant percentage of active PRRSV in asymptomatic pigs despite previous immunization. These findings suggest that the antigen assay is a valuable field tool for the epidemiological control of PRRSV that can be used for rapid screening, particularly in asymptomatic animals.

Changes in lymphocyte subsets and cytokines during European porcine reproductive and respiratory syndrome: increased expression of IL-12 and IL-10 and proliferation of CD4(-)CD8(high)

The changes in peripheral blood mononuclear cells (PBMCs) have been studied in several reports in an attempt to determine the immune response against porcine reproductive and respiratory syndrome virus (PRRSV) infection. However, how these changes are evoked after PRRSV infection has not yet been clarified. The aim of this study was to analyze the changes seen in lymphocyte subsets and immunomodulatory cytokine expression in pigs after an acute experimental infection with a European PRRSV field isolate. Pigs were inoculated intramuscularly with PRRSV field isolate 2982. Samples from blood, medial retropharyngeal and tracheobronchial lymph nodes, and spleen were collected at different time points for flow cytometry studies and for cytokine expression by ELISA. CD21(+) cell counts increased in PBMCs and tracheobronchial lymph node cells from 17 to 24 dpi, coinciding with an increase in PRRSV-specific antibody titer in blood. CD3(+) T-cell counts increased mainly due to an enhancement of CD4(-)CD8(high) and CD4(+)CD8(+) T cells. CD4(-)CD8(low) T cells were decreased in all organs studied, whereas CD4(+)CD8(-) T cells decreased only in the spleen. The drop in viremia correlated with an enhancement of CD4(-)CD8(high) T cells, and with a higher expression of interleukin-10 (IL-10) and interleukin-12 p40 (IL-12 p40). No efficient interferon-gamma (IFN-gamma) response was detected during the acute phase of the infection, and the expression of interferon-alpha (IFN-alpha) was late and reached its maximum expression once the viremia decreased. These results point to IL-10 and IL-12 as cytokines that might play a significant role in the PRRSV immune response, as may CD4(-)CD8(high) T cells.

Emergence of fatal PRRSV variants: unparalleled outbreaks of atypical PRRS in China and molecular dissection of the unique hallmark

Porcine reproductive and respiratory syndrome (PRRS) is a severe viral disease in pigs, causing great economic losses worldwide each year. The causative agent of the disease, PRRS virus (PRRSV), is a member of the family Arteriviridae. Here we report our investigation of the unparalleled large-scale outbreaks of an originally unknown, but so-called "high fever" disease in China in 2006 with the essence of PRRS, which spread to more than 10 provinces (autonomous cities or regions) and affected over 2,000,000 pigs with about 400,000 fatal cases. Different from the typical PRRS, numerous adult sows were also infected by the "high fever" disease. This atypical PRRS pandemic was initially identified as a hog cholera-like disease manifesting neurological symptoms (e.g., shivering), high fever (40-42 degrees C), erythematous blanching rash, etc. Autopsies combined with immunological analyses clearly showed that multiple organs were infected by highly pathogenic PRRSVs with severe pathological changes observed. Whole-genome analysis of the isolated viruses revealed that these PRRSV isolates are grouped into Type II and are highly homologous to HB-1, a Chinese strain of PRRSV (96.5% nucleotide identity). More importantly, we observed a unique molecular hallmark in these viral isolates, namely a discontinuous deletion of 30 amino acids in nonstructural protein 2 (NSP2). Taken together, this is the first comprehensive report documenting the 2006 epidemic of atypical PRRS outbreak in China and identifying the 30 amino-acid deletion in NSP2, a novel determining factor for virulence which may be implicated in the high pathogenicity of PRRSV, and will stimulate further study by using the infectious cDNA clone technique.

Host immune status influences the development of attaching and effacing lesions in weaned pigs

Attaching and effacing Escherichia coli (AEEC) has been associated with naturally occurring attaching and effacing (A/E) lesions in weaned pigs, and although A/E lesions have been experimentally reproduced in newborn piglets, such lesions have been much more difficult to induce in older conventional pigs. Hence, the aim of this study was to examine the effect of oral administration of dexamethasone on the development of A/E lesions in weaned pigs challenged with a porcine enteropathogenic E. coli (PEPEC) strain and to investigate the involvement of local intestinal cytokine response. Dexamethasone, given orally at a dosage of 3 mg kg of body weight(-1), significantly enhanced both the colonization of the challenge strain and the prevalence of foci of intimately adherent bacteria, resulting in extensive A/E lesions in the ileum, cecum, and colon of challenged pigs. We also confirmed the expression of both intimin and Tir by PEPEC strain ECL1001 in A/E lesions in vivo, which is, to our knowledge, the first report of the involvement of the latter proteins in any AEEC infections in vivo. Moreover, semiquantitative reverse transcription-PCR demonstrated that interleukin 1beta (IL-1beta), IL-6, IL-8, and, to a lesser extent, IL-12p40 are significantly upregulated in the ileum following challenge with strain ECL1001, whereas dexamethasone blocks such upregulation. Taken together, our results strongly suggested that host immune status influences the development of A/E lesions in weaned pigs, and it appears that IL-1beta, IL-6, IL-8, and, to a lesser extent, IL-12p40 are expressed during infection of weaned pigs by PEPEC and may contribute to the natural resistance of the host against PEPEC infection.

Immunological responses of swine to porcine reproductive and respiratory syndrome virus infection

The immunology of porcine reproductive and respiratory syndrome virus (PRRS) begins with an initial encounter of PRRSV with the pig. Regardless of the route of entry of PRRSV--via inhalation, intramuscular vaccination, insemination, or other routes--productive infection occurs predominately in alveolar macrophages of the lung. Thus, innate responses of the lung and the alveolar macrophage comprise the initial defense against PRRSV. The virus appears not to elicit innate interferon and cytokine responses characteristic of other strongly immunogenic viral pathogens, and its effects are consistent with induction of a weak adaptive immune response. Humoral and cell-mediated immunity is induced in due course, and results in clearance of virus from the circulation but not from lymphoid tissues, where the infection becomes persistent. Subsequent reexposure to PRRSV elicits an anamnestic response that is partially to completely protective. Within this unconventional picture of anti-PRRSV immunity lie a variety of unresolved issues, including the nature of protective immunity within individual pigs and among pigs in commercial populations, the efficacy of protective immunity against genetically different PRRSV isolates, the effects of developmental age, sex, genetics, and other host factors on the immune response to PRRSV, and the possible suppression of host immunity to other pathogens.

Adjuvant danger signals increase the immune response to porcine reproductive and respiratory syndrome virus

The immune response of swine to vaccination with a live, attenuated PRRSV was assessed in the presence and absence of cytokine adjuvants or cholera toxin (CT) to address the hypothesis that adjuvant danger signals, that is, inflammatory cytokines and bacterial extoxin, stimulate a more robust immune response. Animals received four injections of recombinant porcine IL-1 and IL-6, IL-12 alone, or CT alone within 1 week of intramuscular administration of a vaccine strain of PRRSV, Ingelvac MLV. Serological and cell-mediated responses were monitored for 42 days after vaccination and for a further 10 days after challenge with the virulent VR2332 strain of PRRSV. First, the principal observation was an enhancing effect of IL-12 on the interferon gamma response to PRRSV, with a more rapid and heightened PRRSV-specific interferon gamma ELISPOT response in peripheral blood mononuclear cells. The more rapid and robust development of a cell-mediated immune response, as determined by this assay, suggests that IL-12 may influence the induction of antigen-specific T cell responses. Second, animals that received CT adjuvant displayed a more robust antibody response to GP5, the major envelope glycoprotein. Anti-GP5 titers peaked at 21 days after vaccination at more than twice the level of any other treatment, for which the peak response was at 28 days. Third, there was no evidence of PRRSV immunosuppression of immunity to unrelated antigens, including circovirus. CT is a potent mucosal adjuvant, particularly for antibody responses. It acts in part through the production of IL-1 in macrophages, but its effect was not replaced by combination treatment with IL-1 and IL-6. In sum, the results suggest that cytokine adjuvants, particularly IL-12, and CT have the potential to enhance immune responses to live viral vaccines.

Porcine reproductive and respiratory syndrome virus-induced cell death exhibits features consistent with a nontypical form of apoptosis

Porcine reproductive and respiratory syndrome virus (PRRSV), an arterivirus, belongs to a group of RNA viruses that are cytopathic for macrophages and establish persistent infections. Apoptosis is the presumed mechanism of cell death in monkey kidney cell lines and porcine alveolar macrophages after infection with European PRRSV isolates. However, evidence from in vivo and in vitro studies using North American strains have failed to identify apoptosis in cells supporting virus replication and suggest that apoptosis is present in only uninfected bystander cells. The purpose of this study was to evaluate the mechanism of cell death following the infection of MARC-145 cells with wild-type (P6) and a cell culture-adapted (P136) strains derived from the North American isolate SDSU-23983. At 2 days after infection with P136, cytoplasmic blebbing and nuclear condensation were absent in monolayers containing almost 90% infected cells. By day 3, these infected cells detached and showed evidence of apoptosis, including nuclear condensation and inter-nucleosomal DNA fragmentation. Apoptosis in single infected floating cells was confirmed by the co-localization of FITC-anti-digoxigenin antibody, used to detect uridine-digoxigenin-labled nuclear DNA in a terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end-labeling assay, and Texas red-labeled PRRSV antibody. A majority of infected floating cells were also positive for the uptake of trypan blue, an indicator of necrotic cell death. These results demonstrate that apoptosis does occur in PRRSV infected cells, but is a late event during PRRSV replication and rapidly culminates in a necrotic-like death.