Porcine reproductive and respiratory syndrome virus (PRRSV) causes apoptosis during its replication in fetal implantation sites

Altered structural and transporter-related gene expression patterns in the placenta play a role in fetal demise during Porcine reproductive and respiratory syndrome virus infection

BACKGROUND

Porcine reproductive and respiratory syndrome virus (PRRSV) can be transmitted across the maternal-fetal-interface from an infected gilt to her fetuses. Although fetal infection status and disease outcomes vary, the mechanisms are not completely understood. The objective was to assess targeted placental structural and transporter-related gene expression patterns. At day 85 of gestation pregnant pigs were challenged with PRRSV, and at 12 days post maternal infection sows and fetuses were sacrificed, and the placental tissue was collected. Grouping of fetuses was by preservation status and PRRS viral load (VL): control (CTRL, n = 14), viable and low VL fetus (VIA_LVF, n = 15), viable and high VL fetus (VIA_HVF, n = 21), meconium mild and low VL fetus (MECm_LVF, n = 14), meconium mild and high VL fetus (MECm_HVF, n = 14), and meconium severe and high VL fetus (MECs_HVF, n = 13). NanoString was used to evaluate the expression of 86 genes: actin cytoskeleton signaling, arachidonic acid pathway, integrin signaling, intercellular junctions, transporters, and VEGF signaling. Statistical analyses were performed using Limma with P ≤ 0.05 considered significant.

RESULTS

We identified 1, 7, 0, 29, and 39 differentially expressed genes in VIA_LVF, VIA_HVF, MECm_LVF, MECm_HVF, and MECs_HVF, respectively, contrasted to CTRL. Placental transporter genes were significantly impacted (i.e., downregulation of SLC1A3, SLC1A5, SLC2A1, SLC2A3, SLC2A5, SLC2A10, SLC2A12, SLC7A4, SLC16A5, SLC16A10, and SLC27A6; and upregulation of SLC2A2, SLC16A3, and SLC27A4), compared to CTRL. Actin cytoskeleton signaling (ARHGEF6 and ARHGEF7), arachidonic acid (PTGES3 and PTGIS), integrin signaling (FN1 and ITGB6), intercellular junctions (CDH3 and CDH11), and VEGF signaling (MAPK3 and HPSE) gene groupings were significantly impacted, compared to CTRL.

CONCLUSION

Data reported here indicate that fetal PRRSV infection levels rather than fetal demise is necessary for transcriptional dysregulation of the fetal placenta, with a tendency towards more downregulation in the target gene sets among susceptible fetuses. These results generally support that in susceptible fetuses there is altered solute transportation, placental structural integrity, and reduced angiogenesis. The data described here is associated with fetal PRRS resistance/resilience and susceptibility.

Whole-genome analysis of resilience based on the stability of reproduction performance during a porcine reproductive and respiratory syndrome virus outbreak in sows

The Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) is a very prevalent viral pathogen that can induce reproductive failure in infected sows. PRRSV infection can result in smaller litters, foetal death, late-term abortions and retarded growth of infected piglets. Not all sows respond equally to the infection partly due to genetic factors. In this study, we aimed to characterise the genetic variability of pig resilience to PRRSV infection by using a stability reproductive performance (SRP) index as a proxy of resilience. By comparing reproductive data from 183 sows, we selected 48 sows with extreme SRP values, measured as the difference in piglets lost at farrowings before and during a PRRSV outbreak. Short-read DNA fragments were sequenced from selected sows using an Illumina platform. The analysis of whole-genome sequencing information identified 16 genome regions associated with the SRP classification (cut-off P-value < 10-6). Functional evaluation of the positional candidates by gene-ontology identifiers and their participation in biological pathways were used to identify genes involved in virus entry and replication (vimentin, RAC1 and OAZ2) but also in immune responses from the host (IRF1, and IL4, IL5 and IL13). Importantly, genes related to chemokines, extracellular proteins and cell-to-cell junction integrity might contribute to placental microseparations, facilitating the trafficking of viral particles from sow to foetus that takes place during the pathogenesis of transplacental PRRSV infection. However, given the small number of animals in the study, these results shall need to be validated in larger populations.

A Comprehensive Review on Porcine Reproductive and Respiratory Syndrome Virus with Emphasis on Immunity

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important pathogens in pig production worldwide and responsible for enormous production and economic losses. PRRSV infection in gestating gilts and sows induces important reproductive failure. Additionally, respiratory distress is observed in infected piglets and fattening pigs, resulting in growth retardation and increased mortality. Importantly, PRRSV infection interferes with immunity in the respiratory tract, making PRRSV-infected pigs more susceptible to opportunistic secondary pathogens. Despite the availability of commercial PRRSV vaccines for more than three decades, control of the disease remains a frustrating and challenging task. This paper provides a comprehensive overview of PRRSV, covering its history, economic and scientific importance, and description of the viral structure and genetic diversity. It explores the virus's pathogenesis, including cell tropism, viral entry, replication, stages of infection and epidemiology. It reviews the porcine innate and adaptative immune responses to comprehend the modulation mechanisms employed by PRRS for immune evasion.

Research Progress of Porcine Reproductive and Respiratory Syndrome Virus NSP2 Protein

Porcine reproductive and respiratory syndrome virus (PRRSV) is globally prevalent and seriously harms the economic efficiency of pig farming. Because of its immunosuppression and high incidence of mutant recombination, PRRSV poses a great challenge for disease prevention and control. Nonstructural protein 2 (NSP2) is the most variable functional protein in the PRRSV genome and can generate NSP2N and NSP2TF variants due to programmed ribosomal frameshifts. These variants are broad and complex in function and play key roles in numerous aspects of viral protein maturation, viral particle assembly, regulation of immunity, autophagy, apoptosis, cell cycle and cell morphology. In this paper, we review the structural composition, programmed ribosomal frameshift and biological properties of NSP2 to facilitate basic research on PRRSV and to provide theoretical support for disease prevention and control and therapeutic drug development.

Susceptibility of perivenous macrophages to PRRSV-1 subtype 1 LV and PRRSV-1 subtype 3 Lena using a new vein explant model

Vessel pathology such as increased permeability and blue discoloration is frequently observed with highly pathogenic PRRSV strains. However, data concerning the viral replication in the environment of blood vessels are absent. In the present study, ex vivo models with swine ear and hind leg vein explants were established to study the interaction of PRRSV-1 subtype 1 reference strain LV and highly pathogenic subtype 3 strain Lena with perivenous macrophages. The replication characteristics of these two strains were compared in vein explants by immunofluorescence analysis. The explants maintained a good viability during 48 hours of in vitro culture. We found that CD163-positive macrophages were mainly present around the veins and their number gradually decreased with increasing distance from the veins and longer incubation time. More CD163+Sn- cells than CD163+Sn+ cells (6.6 times more) were observed in the vein explants. The Lena strain demonstrated a higher replication level than the LV strain, with approximately 1.4-fold more infected cells in the surrounding areas of the ear vein and 1.1-fold more infected cells in the leg vein explants at 48 hours post inoculation. In both LV and Lena inoculated vein explants, most infected cells were identified as CD163+Sn+ (> 94%). In this study, an ex vivo vein model was successfully established, and our findings will contribute to a better understanding of the vein pathology during viral infections (e.g., PRRS, classical and African swine fever).

Epidemiological Investigation, Risk Factors, Spatial-Temporal Cluster, and Epidemic Trend Analysis of Pseudorabies Virus Seroprevalence in China (2017 to 2021)

Pseudorabies virus (PRV) is a double-stranded linear DNA virus capable of infecting various animals, including humans. We collected blood samples from 14 provinces in China between December 2017 and May 2021 to estimate PRV seroprevalence. The PRV gE antibody was detected using the enzyme-linked immunosorbent assay (ELISA). Logistic regression analysis identified potential risk factors associated with PRV gE serological status at the farm level. Spatial-temporal clusters of high PRV gE seroprevalence were explored using SaTScan 9.6 software. Time-series data of PRV gE seroprevalence were modeled using the autoregressive moving average (ARMA) method. A Monte Carlo sampling simulation based on the established model was performed to analyze epidemic trends of PRV gE seroprevalence using @RISK software (version 7.0). A total of 40,024 samples were collected from 545 pig farms across China. The PRV gE antibody positivity rates were 25.04% (95% confidence interval [CI], 24.61% to 25.46%) at the animal level and 55.96% (95% CI, 51.68% to 60.18%) at the pig farm level. Variables such as farm geographical division, farm topography, African swine fever (ASF) outbreak, and porcine reproductive and respiratory syndrome virus (PRRSV) control in pig farms were identified as risk factors for farm-level PRV infection. Five significant high-PRV gE seroprevalence clusters were detected in China for the first time, with a time range of 1 December 2017 to 31 July 2019. The monthly average change value of PRV gE seroprevalence was -0.826%. The probability of a monthly PRV gE seroprevalence decrease was 0.868, while an increase was 0.132. IMPORTANCE PRV is a critical pathogen threatening the global swine industry. Our research fills knowledge gaps regarding PRV prevalence, infection risk factors, spatial-temporal clustering of high PRV gE seroprevalence, and the epidemic trend of PRV gE seroprevalence in China in recent years. These findings are valuable for the clinical prevention and control of PRV infection and suggest that PRV infection is likely to be successfully controlled in China.

Evasion strategies of porcine reproductive and respiratory syndrome virus

During the co-evolution of viruses and their hosts, viruses have developed various strategies for overcoming host immunological defenses so that they can proliferate efficiently. Porcine reproductive and respiratory syndrome virus (PRRSV), a significant virus to the swine industry across the world, typically establishes prolonged infection via diverse and complicated mechanisms, which is one of the biggest obstacles for controlling the associated disease, porcine reproductive and respiratory syndrome (PRRS). In this review, we summarize the latest research on how PRRSV circumvents host antiviral responses from both the innate and adaptive immune systems and how this virus utilizes other evasion mechanisms, such as the manipulation of host apoptosis and microRNA. A thorough understanding of the exact mechanisms of PRRSV immune evasion will help with the development of novel antiviral strategies against PRRSV.

Quantitative Analysis of Inflammatory Uterine Lesions of Pregnant Gilts with Digital Image Analysis Following Experimental PRRSV-1 Infection

Reproductive disorders caused by porcine reproductive and respiratory syndrome virus-1 are not yet fully characterized. We report QuPath-based digital image analysis to count inflammatory cells in 141 routinely, and 35 CD163 immunohistochemically stained endometrial slides of vaccinated or unvaccinated pregnant gilts inoculated with a high or low virulent PRRSV-1 strain. To illustrate the superior statistical feasibility of the numerical data determined by digital cell counting, we defined the association between the number of these cells and endometrial, placental, and fetal features. There was strong concordance between the two manual scorers. Distributions of total cell counts and endometrial and placental qPCR results differed significantly between examiner1's endometritis grades. Total counts' distribution differed significantly between groups, except for the two unvaccinated. Higher vasculitis scores were associated with higher endometritis scores, and higher total cell counts were expected with high vasculitis/endometritis scores. Cell number thresholds of endometritis grades were determined. A significant correlation between fetal weights and total counts was shown in unvaccinated groups, and a significant positive correlation was found between these counts and endometrial qPCR results. We revealed significant negative correlations between CD163+ counts and qPCR results of the unvaccinated group infected with the highly virulent strain. Digital image analysis was efficiently applied to assess endometrial inflammation objectively.

Influence of PRRSV-1 vaccination and infection on mononuclear immune cells at the maternal-fetal interface

Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most devastating viruses for the global swine industry. Infection during late gestation causes reproductive failure but the local immune response in utero remains poorly understood. In this study, an experimental PRRSV-infection model with two different PRRSV-1 field isolates was used to investigate the immune cell phenotypes at the maternal-fetal interface during late gestation. In addition, phenotypic changes induced by a modified live virus (MLV, ReproCyc® PRRS EU) vaccine were studied. Vaccinated (n = 12) and non-vaccinated pregnant gilts (n = 12) were challenged with either one of the PRRSV-1 field isolates (low vs. high virulent, LV or HV) or sham-inoculated at day 84 of gestation. Twenty-one days post infection all gilts were euthanized and the fetal preservation status for all fetuses per litter was assessed. Leukocytes from the maternal-fetal interface were isolated and PRRSV-induced changes were investigated using ex vivo phenotyping by flow cytometry. PRRSV load in tissue from the maternal endometrium (ME) and fetal placenta (FP) was determined by RT-qPCR. In the ME, a vast increase in CD8β T cells with CD8αposCD27dim early effector phenotype was found for fetuses from the non-vaccinated LV and HV-challenged gilts, compared to non-treated and vaccinated-only controls. HV-challenged fetuses also showed significant increases of lymphocytes with effector phenotypes in the FP, including NKp46pos NK cells, CD8αhigh γδ T cells, as well as CD8αposCD27pos/dim CD4 and CD8 T cells. In vaccinated animals, this common activation of effector phenotypes was more confined and the fetal preservation status significantly improved. Furthermore, a negative correlation between the viral load and CD163highCD169pos mononuclear phagocytic cells was observed in the FP of HV-infected animals. These results suggest that the strong expansion of effector lymphocytes in gilts that were only infected causes immune-pathogenesis rather than protection. In contrast, the attenuated MLV seems to dampen this effect, yet presumably induces memory cells that limit reproductive failure. This work provides valuable insights into changes of local immune cell phenotypes following PRRSV vaccination and infection.

Angiotensin II Blood Serum Levels in Piglets, after Intra-Dermal or Intra-Muscular Vaccination against PRRSV

Simple Summary

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes massive financial losses in pig production worldwide. Vaccination is still the most cost-effective tool to handle PRRSV infection. PRRSV induces apoptosis in different organs. Angiotensin II (Ang II) participates in the inflammatory response, cell proliferation, migration, and apoptosis. The objective of the current study was to assess the concentration of Ang II in the serum of piglets following immunization against PRRSV through intradermal (ID) or intramuscular (IM) vaccination with a commercial PRRS modified live virus (MLV) vaccine. The results indicated differences in viremia of tested piglets at 7 weeks of age, while piglets at 10 weeks of age were all found qRT-PCR positive for PRRSV. Moreover, significant differences were noticed in Ang II in 7-week-old piglets. In conclusion, our study provides evidence that ID vaccination induces less tissue damage, based on the lower measurements of Ang II in the serum of ID vaccinated piglets.

Abstract

The Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) induces apoptosis in different organs. Angiotensin II (Ang II) is the main effector of the renin-angiotensin system and participates in apoptosis. Thus, this study aimed to investigate changes in piglet serum Ang II levels following intradermal (ID) and intramuscular (IM) vaccination with a commercial PRRS modified live virus (MLV) vaccine. The trial was conducted in a commercial pig farm, including 104 piglets which were randomly allocated to four groups: Group A—Porcilis PRRS ID, Group B—Porcilis PRRS IM, Group C—Diluvac ID and Group D—Diluvac IM. The study piglets were either vaccinated or injected at 2 weeks of age and they were tested by qRT-PCR for PRRSV and by ELISA for Ang II. The results indicated differences in viremia of tested piglets at 7 weeks of age, while piglets at 10 weeks of age were all found qRT-PCR positive for PRRSV. In addition, significant differences were noticed in Ang II in 7-week-old piglets. In conclusion, the present study provides evidence that ID vaccination induces less tissue damage, based on the lower measurements of Ang II in the serum of ID vaccinated piglets.

Effect of porcine reproductive and respiratory syndrome virus 2 on angiogenesis and cell proliferation at the maternal-fetal interface

Angiogenesis and cell proliferation in reproductive tissues are essential events for the maintenance of pregnancy, and alterations can lead to compromised fetal development and survival. Porcine reproductive and respiratory syndrome virus 2 (PRRSV-2) induces reproductive disease with negative financial and production impact on the swine industry. PRRSV-2 infection alters placental physiology through inflammatory and apoptotic pathways, yet fetal susceptibility varies. This study aimed to evaluate angiogenesis and cell proliferation in the porcine maternal-fetal interface (MFI) and determine if these physiological processes were altered by PRRSV-2 infection. Thirty-one pregnant gilts were inoculated with PRRSV-2 at gestation day 86 ± 0.4 (mean ± SD). Seven control gilts were sham-inoculated. All gilts were euthanized at 12 days postinoculation. Angiogenesis and cell proliferation were determined through the detection of vascular endothelial growth factor (VEGF) and Ki-67, respectively, using immunofluorescence of the MFI from 4 fetal resilience groups: uninfected (UNIF), high viral load–viable (HVL-VIA), and HVL-meconium-stained (MEC) from PRRSV-infected gilts, as well from sham-inoculated (CON) gilts. VEGF immunolabeling in the uterine submucosa was significantly lower in MEC compared with UNIF and HVL-VIA groups. Significantly greater Ki67 immunolabeling was detected in the trophoblasts of CON fetuses versus all other groups, and in uterine epithelium of CON and UNIF fetuses versus HVL-VIA and MEC. These results suggest that fetal resilience may be related to greater cell proliferation in uterine epithelium, and fetal compromise with reduced uterine submucosal angiogenesis, except fetuses with intrauterine growth restriction, in which inherently lower submucosal angiogenesis may be protective against PRRSV infection.

Molecular Epidemiology of Porcine Parvovirus Type 1 (PPV1) and the Reactivity of Vaccine-Induced Antisera Against Historical and Current PPV1 Strains

Porcine Parvovirus Type 1 (PPV1) contributes to important losses in the swine industry worldwide. During a PPV1 infection, embryos and fetuses are targeted, resulting in stillbirth, mummification, embryonic death, and infertility (SMEDI syndrome). Even though vaccination is common in gilts and sows, strains mainly belonging to the 27a-like group have been spreading in Europe since early 2000s, resulting in SMEDI problems and requiring in-depth studies into the molecular epidemiology and vaccination efficacy of commercial vaccines. Here, we show that PPV1 has evolved since 1855 [1737, 1933] at a rate of 4.71 × 10⁻⁵ nucleotide substitutions per site per year. Extensive sequencing allowed evaluating and reassessing the current PPV1 VP1-based classifications, providing evidence for the existence of four relevant phylogenetic groups. While most European strains belong to the PPV1a (G1) or PPV1b (G2 or 27a-like) group, most Asian and American G2 strains and some European strains were divided into virulent PPV1c (e.g. NADL-8) and attenuated PPV1d (e.g. NADL-2) groups. The increase in the swine population, vaccination degree, and health management (vaccination and biosafety) influenced the spread of PPV1. The reactivity of anti-PPV1 antibodies from sows vaccinated with Porcilis^© Parvo, Eryseng^© Parvo, or ReproCyc^© ParvoFLEX against different PPV1 field strains was the highest upon vaccination with ReproCyc^© ParvoFLEX, followed by Eryseng^© Parvo, and Porcilis^© Parvo. Our findings contribute to the evaluation of the immunogenicity of existing vaccines and support the development of new vaccine candidates. Finally, the potential roles of cluster-specific hallmark amino acids in elevated pathogenicity and viral entry are discussed.

Maternal Immune Activation and Dietary Soy Isoflavone Supplementation Influence Pig Immune Function but not Muscle Fiber Formation

The goals of this study were to determine the impact of maternal PRRSV infection on offspring muscle and immune development and the potential of dietary soy isoflavones to mitigate those effects. Thirteen first-parity gilts (“gilts”) were randomly allotted into one of three treatments: not infected and fed a diet devoid of isoflavones (CON), infected with porcine reproductive and respiratory syndrome virus (PRRSV) and fed the control diet (POS) or that supplemented with 1,500 mg/kg soy-derived isoflavones (ISF). Gilts were inoculated with PRRSV intranasally on gestational day (GD) 70. After farrowing (GD 114 ± 2), 1-2 offspring (“pigs”) closest to the average litter weight were selected either at birth (3 ± 2 d of age) or weaning (21 ±2 d of age) to determine body, muscle, and organ weights as well as muscle cell number and size. Four weaned pigs of average body weight within each litter were selected for postnatal immune challenge. At PND 52, pigs were injected with 5 µg/kg BW lipopolysaccharide (LPS) intraperitoneally. Serum was collected at 0, 4, and 8 h following LPS administration to analyze tumor necrosis factor alpha (TNF-α). At PND 59, pigs were administered a novel vaccine to elicit an adaptive immune response. At PND 59, 66, and 73, peripheral blood mononuclear cells were isolated and T-cell populations determined by flow cytometry. Both POS and ISF pigs exhibited persistent PRRSV infections throughout the study (PND 1-73). At PND 3, whole body, muscle, and organ weights were not different (P &gt; 0.22) between groups, with the exception of relative liver weight, which was increased (P &lt; 0.05) in POS compared with CON pigs. At PND 21, ISF pigs had reduced (P ≤ 0.05) whole body and muscle weights, but greater (P &lt; 0.05) kidney weight compared with CON, and greater (P &lt; 0.05) relative liver weight compared with CON and POS. Muscle fiber number and size were not different (P &gt; 0.39) between groups at birth or weaning. After LPS administration, TNF-α was greatest in ISF pigs (P &lt; 0.05) at both 0 and 8 h post-challenge. At the peak time-point of 4 h post-challenge, ISF pigs had the greatest concentration of TNF-α and CON pigs had the lowest, with POS pigs being intermediate (P = 0.01). After vaccination, ISF offspring had shifts in T-cell populations indicating an impaired immune response. These data indicate that maternal PRRSV infection may impact offspring organ growth and immune function, particularly when the dam is supplemented with isoflavones.

Phenotypic Characterization of a Virulent PRRSV-1 Isolate in a Reproductive Model With and Without Prior Heterologous Modified Live PRRSV-1 Vaccination

Reproductive disorders induced by porcine reproductive and respiratory syndrome virus (PRRSV) cause high economic losses in the pig industry worldwide. In this study, we aimed to phenotypically characterize a virulent PRRSV-1 subtype 1 isolate (AUT15-33) in a reproductive model. Furthermore, the protective effect of a heterologous modified live virus vaccine (ReproCyc® PRRS EU) was evaluated. In addition, PRRSV AUT15-33 was genotypically compared to other well-characterized isolates. Sixteen gilts were equally divided into four groups: a vaccinated and infected group (V–I), a vaccinated and non-infected group (V–NI), a non-vaccinated and infected group (NV–I), and a non-vaccinated and non-infected (NV–NI) group. After PRRSV infection on gestation day 84, all gilts were clinically examined on a daily basis, and blood samples were taken at five timepoints. Necropsy was performed 3 weeks after infection. The fetal preservation status was assessed, and PRRSV RNA concentrations were measured in the blood and tissue samples from all gilts and fetuses. After infection, all four gilts in the NV–I group were viremic throughout 17 days post-infection (dpi), whereas two gilts in the V–I group were viremic at only one timepoint at 6 dpi. The viral load was significantly higher in gilt serum, tracheobronchial lymph nodes, uterine lymph nodes, maternal endometrium, and fetal placenta of NV–I gilts compared to the V–I ones (p &lt; 0.05). Moreover, the preservation status of the fetuses derived from NV–I gilts was significantly impaired (55.9% of viable fetuses) compared to the other groups (p &lt; 0.001). Upon comparison with other known isolates, the phylogenetic analyses revealed the closest relation to a well-characterized PRRSV-1 subtype 1 field isolate from Belgium. In conclusion, the high virulence of AUT15-33 was phenotypically confirmed in an experimental reproductive model. The vaccination of the gilts showed promising results in reducing viremia, fetal damage, and transplacental transmission of the PRRSV-1 strain characterized in this study.

Investigation of Fas (APO-1)-Related Apoptosis in Piglets Intradermally or Intramuscularly Vaccinated with a Commercial PRRSV MLV

Porcine reproductive and respiratory syndrome virus (PRRSV) induces apoptosis through the activation of death receptors, including cell-surface Fas receptor. The aim of this study was to investigate the impact of intradermal (ID) and intramuscular (IM) vaccination with a commercial PRRSV-modified live vaccine in piglets on Fas-related apoptosis. The study included 104 suckling piglets from a commercial farrow-to-finish pig farm, suffering from positive unstable PRRSV status. Animals were assigned in four groups: group A-Porcilis PRRS ID-vaccinated pigs, group B-Porcilis PRRS IM-vaccinated pigs, group C-Diluvac ID adjuvant-administered pigs, and group D-Diluvac IM adjuvant-administered pigs. Vaccines were administered at 2 weeks of age. Blood samples were collected from the same pigs at 4, 7, and 10 weeks of age. Sera were examined by quantitative real-time reverse transcription-PCR (qRT-PCR) for PRRSV and by ELISA for soluble Fas (sFas). At 4 weeks of age, all groups were negative qRT-PCR for PRRSV; at 7 weeks only group A was negative; and at 10 weeks all groups were positive. sFas was significantly increased in groups C (4 vs. 7, 4 vs. 10, and 7 vs. 10 weeks) and D (7 vs. 10 weeks). Significant differences among groups were noticed only at 10 weeks (A vs. C, A vs. D, B vs. C, B vs. D). A significant positive and moderate correlation between PRRSV viral load and Fas level was observed. In unvaccinated piglets, increased serum sFas levels reveal apoptotic suppression compared with vaccinated piglets. In the latter, vaccine-derived antibodies limit the infection and may attribute to the reduced Fas expression, suggesting a weak induction of lymphocyte-mediated cytotoxicity.

Comparison of the pathogenicity of porcine reproductive and respiratory syndrome virus (PRRSV)-1 and PRRSV-2 in pregnant sows

To date, few studies related to the evaluation of the pathogenicity of different PRRSV isolates using a reproductive model have been undertaken, and the main focus has remained on respiratory models using young pigs. This study aimed to evaluate the pathogenicity of two PRRSV-1 isolates (D40 and CBNU0495) and two PRRSV-2 isolates (K07-2273 and K08-1054) in a reproductive model. Pregnant sows were experimentally infected with PRRSV at gestational day 93 or used as an uninfected negative control. Sera were collected at 0, 3, 7, 14, and 19 days post-challenge (dpc) for virological and serological assays. At 19 dpc, all sows were euthanized, and their fetuses were recovered by performing cesarean section and immediately euthanized for sample collection. Here, compared to the other isolates, the CBNU0495 isolate replicated most efficiently in the pregnant sows, and K07-2273 produced the highest rate of reproductive failure even though it did not replicate as efficiently as the other isolates in sows and fetuses, indicating that vertical transmission and reproductive failure due to PRRSV infection do not have any significant correlation with the viral loads in samples from sows and fetuses. Similarly, the viral loads and the histopathological lesions did not show any correlation with each other, as the PRRSV-2-infected groups displayed more prominent and frequent histopathological lesions with lower viral loads than the PRRSV-1-infected groups. However, viral loads in the myometrium/endometrium might be related to the spreading of PRRSV in the fetuses, which affected the birth weight of live fetuses. This study contributes to a better understanding of the pathogenicity of the most prevalent Korean PRRSVs in a reproductive model.

Decreased tight junction protein intensity in the placenta of porcine reproductive and respiratory syndrome virus-2 infected fetuses

INTRODUCTION

Existing strategies to control porcine reproductive and respiratory syndrome (PRRS) are not completely effective and require alternative approaches. Although intrauterine growth restricted (IUGR) fetuses are more resilient to transplacental PRRS virus-2 (PRRSV2) infection compared to normal fetuses, the exact mechanisms are unknown. The objective of this research was to assess abundance and localization of a subset of tight junction (TJ) proteins in the maternal-fetal interface and any alterations that may affect the movement of nutrients or PRRSV2 across the epitheliochorial placenta.

METHODS

Paraffin-embedded samples of placenta from non-infected control (CTRL) and PRRSV2 infected fetuses (IUGR, non(N)-IUGR, meconium-stained (MEC) (n = 6 per group) were randomly selected from a large challenge trial and immunostained for claudins (CLDN) 1, 3, 4, 7 and tight junction protein 1 (TJP1). Immunostaining intensity was semi-subjectively scored by region.

RESULTS

Intensity of CLDN1 was lower in placenta of IUGR, MEC, and N-IUGR fetuses compared to CTRL, mainly in fetal epithelium and maternal endothelial cells (MECL). CLDN4 intensity was lower in MECL of IUGR compared to CTRL and MEC fetuses. TJP1 intensity was lower in maternal and fetal epithelia of placenta within IUGR, MEC, and N-IUGR fetuses versus CTRL.

DISCUSSION

Differences were mainly observed between PRRSV2 infected and non-infected groups indicating TJ integrity was affected by PRRSV2 infection. These results provide insights into the potential mechanisms of transplacental transmission of PRRSV2; however, since only CLDN4 differed amongst the infected groups, PRRSV2 induced changes in TJ integrity do not appear to explain variation in fetal outcomes after infection.

Fetal hypoxia and apoptosis following maternal porcine reproductive and respiratory syndrome virus (PRRSV) infection

Background

Mechanisms of fetal death following maternal PRRSV2 infection remain uncharacterized, although hypoxia from umbilical cord lesions and/or placental detachment due to apoptosis are hypothesized. We performed two experiments examining hypoxia and apoptosis in PRRSV-infected and non-infected, third-trimester fetuses to elucidate possible associations with fetal death. Fetuses were selected based on four phenotypic infection groups: fetuses from non-challenged control gilts (CTRL); low viral load fetuses (LVL; Exp 1) or uninfected fetuses (UNINF; Exp 2) from inoculated gilts; viable high viral load fetuses (HVL-VIA); and HVL meconium-stained fetuses (HVL-MEC).

Results

In experiment 1, paraffin embedded fetal tissues collected 21 days post maternal infection (DPI) were examined for DNA fragmentation associated with apoptosis. Positively stained foci were larger and more numerous (P < 0.05) in heart, liver, and thymus of HVL-VIA and HVL-MEC compared to CTRL and LVL fetuses. In experiment 2, group differences in gene expression within the hypoxia (HIF1a, IDO1, VEGFa, LDHA, NOS2, NOX1) and apoptosis (CASP3, CASP7, CASP8, CASP9, RIPK1, RIPK3) pathways were assessed by RT-qPCR in fetal tissues collected at 12 DPI. High viral load fetuses showed differential expression relative to the CTRL and UNINF (P < 0.05 for all). Brain tissue from HVL-VIA and HVL-MEC fetuses presented increased expression of CASP7, CASP8, RIPK3, HIF1a and IDO1. Fetal heart showed increased expression of CASP8, HIF1a, IDO and NOX1 and a decrease in NOS2 expression in infected groups. CASP7, CASP9, RIPK1 and RIPK3 were only increased in the heart of HVL-VIA while VEGFa was only increased for HVL-MEC fetuses. Thymus from HVL-MEC had decreased expression of CASP9 and there was increased IDO1 in all infected fetuses.

Conclusions

There is strong evidence of apoptosis occurring in the heart, liver and thymus of highly viral load fetuses at 21 DPI. Furthermore, there was clear upregulation of apoptotic genes in the heart of high viral load infected fetuses and less prominent upregulation in the brain of PRRSV-infected fetuses, whereas thymus appears to be spared at 12 DPI. There was no strong evidence of hypoxia at 12 DPI in brain and thymus but some indication of hypoxia occurring in fetal heart.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-021-02883-0.

Samples sizes required to accurately quantify viral load and histologic lesion severity at the maternal-fetal interface of PRRSV-inoculated pregnant gilts

Porcine reproductive and respiratory syndrome virus (PRRSV) is transmitted vertically, causing fetal death in late gestation. Spatiotemporal distribution of virus at the maternal-fetal interface (MFI) is variable, and accurate assessment of viral concentration and lesions is thus subject to sampling error. Our objectives were: 1) to assess whether viral load and lesion severity in a single sample of endometrium (END) and placenta (PLC), collected near the base of the umbilical cord (the current standard), are representative of the entire organ; and 2) to compare sampling strategies and evaluate if spatial variation in viral load can be overcome by pooling of like-tissues. Spatially distinct pieces of END and PLC of 24 fetuses from PRRSV-2-infected dams were collected. PRRSV RNA quantified by RT-qPCR was compared in 5 individual pieces per fetus and in respective pools of tissue and extracted RNA. Three distinct pieces of MFI were assessed for histologic severity. Concordance correlation and kappa inter-rater agreement were used to characterize agreement among individual samples and pools. The viral load of individual samples and pools of END had greater concordance to a referent standard than did samples of PLC. Larger pool sizes had greater concordance than smaller pool sizes. Average viral load and lesion severity did not differ by location sampled, and no technical advantages of pooling tissues versus RNA extracts were found. We conclude that multiple pieces of MFI tissues must be evaluated to accurately assess lesion severity and viral load. Three pieces per fetus provided a reasonable balance of cost and logistic feasibility.

Commercial PRRS Modified-Live Virus Vaccines

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) presents one of the challenging viral pathogens in the global pork industry. PRRS is characterized by two distinct clinical presentations; reproductive failure in breeding animals (gilts, sows, and boars), and respiratory disease in growing pigs. PRRSV is further divided into two species: PRRSV-1 (formerly known as the European genotype 1) and PRRSV-2 (formerly known as the North American genotype 2). A PRRSV-2 modified-live virus (MLV) vaccine was first introduced in North America in 1994, and, six years later, a PRRSV-1 MLV vaccine was also introduced in Europe. Since then, MLV vaccination is the principal strategy used to control PRRSV infection. Despite the fact that MLV vaccines have shown some efficacy, they were problematic as the efficacy of vaccine was often unpredictable and depended highly on the field virus. This paper focused on the efficacy of commercially available MLV vaccines at a global level based on respiratory disease in growing pigs, and maternal and paternal reproductive failure in breeding animals.

Differential responses in placenta and fetal thymus at 12 days post infection elucidate mechanisms of viral level and fetal compromise following PRRSV2 infection

BACKGROUND

A pregnant gilt infected with porcine reproductive and respiratory syndrome virus (PRRSV) can transmit the virus to her fetuses across the maternal-fetal-interface resulting in varying disease outcomes. However, the mechanisms leading to variation in fetal outcome in response to PRRSV infection are not fully understood. Our objective was to assess targeted immune-related gene expression patterns and pathways in the placenta and fetal thymus to elucidate the molecular mechanisms involved in the resistance/tolerance and susceptibility of fetuses to PRRSV2 infection. Fetuses were grouped by preservation status and PRRS viral load (VL): mock infected control (CTRL), no virus detected (UNINF), virus detected in the placenta only with viable (PLCO-VIA) or meconium-stained fetus (PLCO-MEC), low VL with viable (LVL-VIA) or meconium-stained fetus (LVL-MEC), and high VL with viable (HVL-VIA) or meconium-stained fetus (HVL-MEC).

RESULTS

The host immune response was initiated only in fetuses with detectable levels of PRRSV. No differentially expressed genes (DEG) in either the placenta or thymus were identified in UNINF, PLCO-VIA, and PLCO-MEC when compared to CTRL fetuses. Upon fetal infection, a set of core responsive IFN-inducible genes (CXCL10, IFIH1, IFIT1, IFIT3, ISG15, and MX1) were strongly upregulated in both tissues. Gene expression in the thymus is a better differentiator of fetal VL; the strong downregulation of several innate and adaptive immune pathways (e.g., B Cell Development) are indicative of HVL. Gene expression in the placenta may be a better differentiator of fetal demise than the thymus, based-on principle component analysis clustering, gene expression patterns, and dysregulation of the Apoptosis and Ubiquitination pathways.

CONCLUSION

Our data supports the concept that fetal outcome in response to PRRSV2 infection is determined by fetal, and more significantly placental response, which is initiated only after fetal infection. This conceptual model represents a significant step forward in understanding the mechanisms underpinning fetal susceptibility to the virus.

Resilience Effects of SGK1 and TAP1 DNA Markers during PRRSV Outbreaks in Reproductive Sows

The porcine reproductive and respiratory syndrome virus (PRRSV) is a major infectious stressor that causes serious health problems and productivity drops. Based on previous genome-wide analyses, we selected SGK1 and TAP1 as candidate genes for resilience, and genotyped three mutations, including a 3'UTR variant SGK1_rs338508371 and two synonymous variants TAP1_rs1109026889 and TAP1_rs80928141 in 305 Landrace × Large White sows. All polymorphisms affected the reproductive performance in the outbreak, but not during the endemic phase, thereby indicating a potential use of these markers for resilience. Moreover, some genotypes were associated with a stable performance across PRRSV phases. Thus, in the outbreak, the SGK1_rs338508371 AA sows had less piglets born alive (p < 0.0001) and more stillborns (p < 0.05) while other sows were able to keep their productivity. During the outbreak, TAP1_rs80928141 GG sows had less piglets born alive (p < 0.05) and both TAP1 polymorphisms influenced the number of mummies in an additive manner (p < 0.05). Remarkably, TAP1_rs80928141 AA sows had around one mummy more than GG sows (p < 0.01). Resilience to PRRSV could be improved by including the SGK1 and TAP1 markers in crossbreeding and/or selection schemes, as they contribute to maintaining a stable number of piglets born alive and lost, particularly mummies, despite the outbreak.

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.

Maternal and fetal thyroid dysfunction following porcine reproductive and respiratory syndrome virus2 infection

To better understand the host response to porcine reproductive and respiratory virus-2 (PRRSV2) we evaluated circulating thyroid hormone and associated gene expression in a late gestation challenge model. Pregnant gilts were inoculated at gestation day 85 and fetal samples collected at either 12 or 21 days post-infection (dpi). A subset of fetuses was selected for analysis based on viability and viral load categorized as either uninfected-viable (UNIF), high viral load viable (HV-VIA) or high viral load meconium stained (HV-MEC) and were compared with gestational age matched controls (CON). In dams, circulating levels of total T3 and T4 decreased in the acute period following infection and rebounded by 21 dpi. A similar effect was observed in fetuses, but was largely restricted to HV-VIA and HV-MEC, with minimal decrease noted in UNIF relative to CON at 21 dpi. Gene expression in fetal heart at 12 dpi showed significant decompensatory transcription of thyroid hormone transporters (SLC16A2) and deiodinases (DIO2, DIO3), which was not observed in brain. Correspondingly, genes associated with cell cycle progression (CDK1,2,4) were downregulated in only the heart of highly infected fetuses, while expression of their inhibitor (CDKN1A) was upregulated in both tissues. Finally, expression of genes associated with cardiac stress including CAMKD and AGT were upregulated in the hearts of highly infected fetuses, and a shift in expression of MYH6 to MYH7 was observed in HV-MEC fetuses specifically. Collectively, the results suggest PRRSV2 infection causes a hypothyroid state that disproportionally impacts the fetal heart over the brain.

A Porcine Model of Zika Virus Infection to Profile the In Utero Interferon Alpha Response

Pigs are highly relevant to model human in utero Zika virus (ZIKV) infection because both species have similar physiology, genetics, immunity, fetal brain development, and postnatal brain growth. The virus causes persistent in utero infection and replicates in the fetal brain, fetal membranes, and placenta. Subclinical persistent in utero infection in mid-gestation also increases interferon alpha (IFN-α) levels in fetal blood plasma and amniotic fluid. Moreover, we demonstrated altered IFN-α responses in porcine offspring affected with subclinical in utero ZIKV infection. Elevated levels of in utero type I interferons were suggested to play a role in fetal pathology. Thus, the porcine model may provide an understanding of ZIKV-induced immunopathology in fetuses and sequelae in offspring, which is important for the development of targeted interventions. Here, we describe surgery, ultrasound-guided in utero injection, postoperative monitoring, sampling, and cytokine testing protocols.

The African strain of Zika virus causes more severe in utero infection than Asian strain in a porcine fetal transmission model

Studies in mice showed that African Zika virus (ZIKV) strains cause more damage in embryos. These studies, however, were limited to the mouse-adapted African MR766 strain or infection at early gestation. Here, we compared infection of Asian and African strains in the fetal pig model at midgestation. Both strains caused fetal infection. ZIKV was detected in placenta, amniotic membrane, amniotic fluid, fetal blood, and brain. The African strain produced more vigorous in utero infection as represented by more efficient virus transmission between siblings, and higher viral loads in fetal organs and membranes. Infection with both strains was associated with reduced fetal brain weight and increased number of placental CD163-positive cells, as well as elevated in utero interferon alpha and cortisol levels. This is the first large animal model study which demonstrated that African strain of ZIKV, with no passage history in experimental animals, can cause persistent infection in fetuses and fetal membranes at midgestation. Our studies also suggest that similar to Asian strains, ZIKV of African lineage might cause silent pathology which is difficult to identify in deceptively healthy fetuses. The findings emphasize the need for further studies to highlight the impact of ZIKV heterogeneity on infection outcomes during pregnancy.

Subclinical in utero Zika virus infection is associated with interferon alpha sequelae and sex-specific molecular brain pathology in asymptomatic porcine offspring

Zika virus (ZIKV) infection during human pregnancy may lead to severe fetal pathology and debilitating impairments in offspring. However, the majority of infections are subclinical and not associated with evident birth defects. Potentially detrimental life-long health outcomes in asymptomatic offspring evoke high concerns. Thus, animal models addressing sequelae in offspring may provide valuable information. To induce subclinical infection, we inoculated selected porcine fetuses at the mid-stage of development. Inoculation resulted in trans-fetal virus spread and persistent infection in the placenta and fetal membranes for two months. Offspring did not show congenital Zika syndrome (e.g., microcephaly, brain calcifications, congenital clubfoot, arthrogryposis, seizures) or other visible birth defects. However, a month after birth, a portion of offspring exhibited excessive interferon alpha (IFN-α) levels in blood plasma in a regular environment. Most affected offspring also showed dramatic IFN-α shutdown during social stress providing the first evidence for the cumulative impact of prenatal ZIKV exposure and postnatal environmental insult. Other eleven cytokines tested before and after stress were not altered suggesting the specific IFN-α pathology. While brains from offspring did not have histopathology, lesions, and ZIKV, the whole genome expression analysis of the prefrontal cortex revealed profound sex-specific transcriptional changes that most probably was the result of subclinical in utero infection. RNA-seq analysis in the placenta persistently infected with ZIKV provided independent support for the sex-specific pattern of in utero-acquired transcriptional responses. Collectively, our results provide strong evidence that two hallmarks of fetal ZIKV infection, altered type I IFN response and molecular brain pathology can persist after birth in offspring in the absence of congenital Zika syndrome.

A number of studies showed that Zika virus (ZIKV) can cause severe abnormalities in fetuses, e.g., brain lesions, and subsequently life-long developmental and cognitive impairment in children. However, the majority of infections in pregnant women are subclinical and are not associated with developmental abnormalities in fetuses and newborns. It is known that disruptions to the in utero environment during fetal development can program increased risks for disease in adulthood. For this reason, children affected in utero even by mild ZIKV infection can appear deceptively healthy at birth but develop immune dysfunction and brain abnormalities during postnatal development. Here, we used the porcine model of subclinical fetal ZIKV infection to determine health sequelae in offspring which did not show apparent signs of the disease. We demonstrated that subclinical fetal infection was associated with abnormal immunological responses in apparently healthy offspring under normal environmental conditions and during social stress. We also showed silent sex-specific brain pathology as represented by altered gene expression. Our study provides new insights into potential outcomes of subclinical in utero ZIKV infection. It also emphasizes that further attempts to better understand silent pathology and develop alleviative interventions in ZIKV-affected offspring should take into account interactions of host factors, like sex, and environmental insults, like social stress.

An Escherichia coli-Expressed Porcine Reproductive and Respiratory Syndrome Virus Chimeric Protein Induces a Specific Immunoglobulin G Response in Immunized Piglets

Porcine reproductive and respiratory syndrome virus (PRRSV) still poses a threat to the swine industry worldwide. Currently, commercial vaccines against PRRSV, which consist of modified live or inactivated virus, reduce symptoms and viremia in immunized pigs, but efficacy against heterologous strains is variable. This has led to the development of subunit vaccines that contain viral antigens that show the highest variability. In this work, a chimeric protein comprising short amino acid sequences from glycoprotein 3 (GP3), glycoprotein 4 (GP4), glycoprotein 5 (GP5), and M (matrix protein) proteins of PRRSV was designed and expressed in Escherichia coli. This protein, designated as PRRSVchim, was purified by immobilized metal affinity chromatography and evaluated. PRRSVchim was identified by immunoglobulin G (IgG) presence in serum samples from PRRSV-positive pigs. Also, the protein probed to be antigenic in immunized mice and piglets and provided some degree of protection against challenge with a PRRSV field isolate. These results show the potential of PRRSVchim protein for both PRRSV diagnostic and immunoprophylaxis.

Fetal cytokine response to porcine reproductive and respiratory syndrome virus-2 infection

To understand the fetal immune response to porcine reproductive and respiratory virus-2 (PRRSV) and to evaluate the association with fetal viability, pregnant gilts were challenged on gestation day 85 and euthanized 21 days post infection. Based on preservation status and viral load in serum and thymus, fetuses were classified as either uninfected-viable (UNIF), high viral load viable (HV-VIA), or high viral load meconium stained (HV-MEC) and were compared with age matched control (CON) fetuses derived from mock infected gilts. Gene expression of IFNB, IFNG, CCL2, CCL5, CXCL10 and IL10, were all found to be significantly upregulated in the thymus and spleen of both high viral load groups. UNIF fetuses remained largely unaffected, with only small upregulations in IFNA and IL10 in the thymus, and IFNA, CCL5 and CXCL10 in the spleen. Regarding fetal viability, expression of CCL5 was significantly elevated in the thymus and spleen of HV-MEC compared to HV-VIA fetuses. The concentrations of IFNα, IFNγ, TNFα and CCL2 were elevated in the sera of all infected fetuses, whereas IFNβ was below the detection limit in all fetal sera. Additional gene expression analysis in the thymus showed significant downregulation of CDK1, CDK2 and CDK4, and upregulation of the inhibitor CDKN1A, suggesting altered regulation of cell cycle progression. Collectively, these results show near complete compartmentalization of the fetal immune response to infected fetuses and suggest this immune response is not a major contributor to fetal death.

The porcine trophoblast cell line PTr2 is susceptible to porcine reproductive and respiratory syndrome virus-2 infection

INTRODUCTION

Porcine reproductive and respiratory syndrome virus-2 (PRRSV-2) breaches the maternal-fetal interface (MFI) to infect porcine fetuses, yet the exact mechanism(s) of transmission is not understood. The objective of this study was to determine the susceptibility of porcine trophoblast cell line (PTr2) to PRRSV-2 infection to understand the potential role of the trophoblast in viral transmission to fetuses in vivo.

METHODS

PTr2 cells were exposed in vitro to PRRSV-2 and then subjected to immunofluorescence analysis (IF), flow cytometry (FCM), real-time quantitative PCR (RT-qPCR), transmission electron microscopy (TEM) and immunogold electron microscopy (IEM) to assess viral infection. The effects of PRRSV-2 on PTr2 cell cycle progression and apoptosis, as well as the ability of PTr2 cells to produce infectious viral particles were also examined.

RESULTS

PRRSV-2 was readily detected in PTr2 cells by IF, FCM, RT-qPCR, TEM and IEM techniques. RT-qPCR and FCM results of a time course of infection of PTr2 cells indicated PRRSV-2 load decreased over time after initial infection up to 72 h. PRRSV-2 infection altered PTr2 cell cycle with a selective increase of cells within the G2/M phase and also induced apoptosis. TEM and IEM demonstrated PRRSV-2 within and on the surface of PTr2 cells and PRRSV-2 virions released from PTr2 cells infected naïve MARC-145 cells inducing cytopathic effects.

DISCUSSION

Trophoblast cells are susceptible to PRRSV-2 infection and release live virions capable of inducing cytopathic effects in naïve cells. This suggests a possible mechanism by which PRRSV-2 can breach the MFI resulting in fetal infection and death.

Spatiotemporal immunofluorescent evaluation of porcine reproductive and respiratory syndrome virus transmission across the maternal-fetal interface

Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes severe reproductive failure characterized by high fetal morbidity and mortality leading to substantial economic losses to the swine industry. Evaluation of spatiotemporal transmission of PRRSV at the maternal-fetal interface (MFI) is critical for understanding fetal infection. Localization of PRRSV-2 strain NVSL 97-7895 at different regions of the MFI in 20 pregnant gilts at 2, 5, 8, 12 and 14 days post-inoculation (dpi) were analyzed by immunofluorescence (IF). Samples of MFI were collected from 15 inoculated and 5 control gilts and transplacental PRRSV transmission assessed in randomly selected fetuses from each litter. Localization of NVSL 97-7895 antigen immunoreactivity in the MFI was focused in three major areas: endometrial connective tissues (ENDO), the feto-maternal junction (FMJ) and fetal placenta (PLC). NVSL 97-7895 was detected at the FMJ by 2 dpi. At 2, 5 and 8 dpi, NVSL 97-7895 was localized within the ENDO and FMJ, whereas at 12 and 14 dpi, it was mainly localized in the PLC. Using a novel IF strategy for counting and size sorting NVSL 97-7895 viral antigen in situ, results of this study indicate that non-cell-associated mechanisms are involved in PRRSV transmission across the MFI.

Establishment and evaluation of a PRRSV-sensitive porcine endometrial epithelial cell line by transfecting SV40 large T antigen

Background

PRRSV is an infectious illness causing lung injury and abortion in sows. Cells apoptosis in the interface between the endometrium and fetal placenta is a crucial factor causing abortion. Previous study confirmed PRRSV could cause apoptosis of macrophages but rarely produced an obvious change in porcine endometrial epithelial cells (PECs). Recently, PRRSV-induced abortion was attributed to fetal placental and endometrium epithelial cells (Sn⁺ and CD163⁺) apoptosis. However, the mechanism of abortion is still unrevealed because of the limit of porcine endometrium epithelial cells (PEC). The aim of this study was to establish a stable immortalized PECs lines and use it to reveal the abortion mechanism.

Results

In this study, highly purified primary PECs were harvested through differential digestion, and their characteristics were confirmed by CK18, ERɑ and PR staining. Cells were then immortalized by transfecting a lentiviral vector that expressed SV40 large T antigen. PECs lines were obtained after puromycin screening. Proliferation of cell line was evaluated by cell growth curve and cell cycle assays. Cell lines exhibited faster proliferation capacity than primary cells. Biological characteristics of cell line were assessed by Western blot, karyotype analysis and staining, which confirmed that the cell line retained the endometrium characteristics. Finally, PRRSV sensitivity was assessed; expression of Sn and CD163 indicated that primary PECs and cell lines were all potentially sensitive to PRRSV. PRRSV infection tests showed an obvious increase in apoptotic rate in the infected PEC cell line, which suggested its susceptibility.

Conclusion

The newly constructed cell line is a useful tool for studying the mechanism of abortion caused by PRRSV.

Deletion of CD163 Exon 7 Confers Resistance to Highly Pathogenic Porcine Reproductive and Respiratory Viruses on Pigs

Porcine reproductive and respiratory syndrome (PRRS) caused by PRRS virus (PRRSV) is a severe infectious disease in the swine industry. PRRSV infection is mediated by porcine CD163 (pCD163). Scavenger receptor cysteine-rich domain 5 coded by exon 7 of pCD163 is essential for PRRSV infection. In this study, we generated CD163 exon 7 deleted (CD163E7D) pigs using CRISPR/Cas9 mediated homologous recombination and somatic cell nuclear transfer (SCNT). The deletion of exon 7 had no adverse effects on CD163-associated functions. Pigs were further challenged with a highly pathogenic PRRSV (HP-PRRSV) strain. The CD163E7D pigs exhibited mild clinical symptoms and had decreased viral loads in blood. All CD163E7D pigs survived the viral challenge, while all the WT pigs displayed severe symptoms, and 2 out of 6 WT pigs died during the challenge. Our results demonstrated that CD163 exon 7 deletion confers resistance to HP-PRRSV infection without impairing the biological functions of CD163.

Global miRNA, lncRNA, and mRNA Transcriptome Profiling of Endometrial Epithelial Cells Reveals Genes Related to Porcine Reproductive Failure Caused by Porcine Reproductive and Respiratory Syndrome Virus

Porcine reproductive and respiratory syndrome virus (PRRSV) can cause respiratory disease and reproductive failure in pregnant pigs. Previous transcriptome analyses in susceptive cells have mainly concentrated on pulmonary alveolar macrophages (PAM) and Marc-145 cells, and on the respiratory system. Some studies reported that apoptosis of placental cells and pig endometrial epithelial cells (PECs) is an obvious sign linked to reproductive failure in pregnant sows, but the mechanism is still unknown. In this study, Sn-positive PECs were isolated and apoptosis rates were assessed by flow cytometry. PRRSV-infected PECs exhibited apoptosis, indicative of their susceptibility to PRRSV. Subsequently, the whole transcriptome was compared between mock- and PRRSV-infected PECs and 54 differentially expressed microRNAs (DEmiRNAs), 104 differentially expressed genes (DEGs), 22 differentially expressed lncRNAs (DElncRNAs), and 109 isoforms were obtained, which were mainly enriched in apoptosis, necroptosis, and p53 signal pathways. Integration analysis of DEmiRNA and DEG profiles revealed two microRNAs (ssc-miR-339-5p and ssc-miR-181d-5p) and five genes (SLA-DQB1, THBS1, SLC3A1, ZFP37, and LOC100517161) participating in the apoptosis signal, of which THBS1 and SLC3A1 were mainly linked to the p53 pathway. Integration analysis of DEGs with DElncRNA profiles identified genes involved in apoptosis signal pathway are regulated by LTCONS_00010766 and LTCONS_00045988. Pathway enrichment revealed that the phagosome and p53 pathways are the two main signals causing apoptosis of PECs, and functional analysis revealed a role of miR-339-5p in regulating apoptosis of PECs after PRRSV inoculation.

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.

A Triple Amino Acid Substitution at Position 88/94/95 in Glycoprotein GP2a of Type 1 Porcine Reproductive and Respiratory Syndrome Virus (PRRSV1) Is Responsible for Adaptation to MARC-145 Cells

The Meat Animal Research Center-145 (MARC-145) cell line has been proven to be valuable for viral attenuation regarding vaccine development and production. Cell-adaptation is necessary for the efficient replication of porcine reproductive and respiratory syndrome virus (PRRSV) in these cells. Multiple sequence analysis revealed consistent amino acid substitutions in GP2a (V88F, M94I, F95L) of MARC-145 cell-adapted strains. To investigate the putative effect of these substitutions, mutations at either position 88, 94, 95, and their combinations were introduced into two PRRSV1 (13V091 and IVI-1173) infectious clones followed by the recovery of viable recombinants. When comparing the replication kinetics in MARC-145 cells, a strongly positive effect on the growth characteristics of the 13V091 strain (+2.1 log10) and the IVI-1173 strain (+1.7 log10) compared to wild-type (WT) virus was only observed upon triple amino acid substitution at positions 88 (V88F), 94 (M94I), and 95 (F95L) of GP2a, suggesting that the triple mutation is a determining factor in PRRSV1 adaptation to MARC-145 cells.

Porcine reproductive and respiratory syndrome virus inhibits MARC-145 proliferation via inducing apoptosis and G2/M arrest by activation of Chk/Cdc25C and p53/p21 pathway

Porcine reproductive and respiratory syndrome virus(PRRSV) is an important immunosuppressive virus which can suppresses infected cells proliferation. In this work, we examined PRRSV ability to manipulate cell cycle progression of MARC-145 cells and explored the potential molecular mechanisms. The results showed that PRRSV infection imposed a growth-inhibitory effect on MARC-145 cells by inducing cell cycle arrest at G2/M phase. This arrest was due to the significant decrease of Cdc2-cyclinB1 complex activity in PRRSV-infected cells and the activity reduction was a result of Cdc2 Tyr15 phosphorylation and the accumulation of Cdc2 and cyclinB1 in the nucleus. Not only elevated Wee1 and Myt1 expression and inactivated Cdc25C, but also increase of p21 and 14–3-3σ in a p53-dependent manner caused the inhibitory Tyr15 phosphorylation of Cdc2. PRRSV infection also activated Chk1. Our data suggest PRRSV infection induces G2/M arrest via various molecular regulatory mechanisms. These results provide a new insights for PRRSV pathogenesis.

Pigs Lacking the Scavenger Receptor Cysteine-Rich Domain 5 of CD163 Are Resistant to Porcine Reproductive and Respiratory Syndrome Virus 1 Infection

Porcine reproductive and respiratory syndrome virus (PRRSV) has a narrow host cell tropism, limited to cells of the monocyte/macrophage lineage. CD163 protein is expressed at high levels on the surface of specific macrophage types, and a soluble form is circulating in blood. CD163 has been described as a fusion receptor for PRRSV, with the scavenger receptor cysteine-rich domain 5 (SRCR5) region having been shown to be the interaction site for the virus. As reported previously, we have generated pigs in which exon 7 of the CD163 gene has been deleted using CRISPR/Cas9 editing in pig zygotes. These pigs express CD163 protein lacking SRCR5 (ΔSRCR5 CD163) and show no adverse effects when maintained under standard husbandry conditions. Not only was ΔSRCR5 CD163 detected on the surface of macrophage subsets, but the secreted, soluble protein can also be detected in the serum of the edited pigs, as shown here by a porcine soluble CD163-specific enzyme-linked immunosorbent assay (ELISA). Previous results showed that primary macrophage cells from ΔSRCR5 CD163 animals are resistant to PRRSV-1 subtype 1, 2, and 3 as well as PRRSV-2 infection in vitro Here, ΔSRCR5 pigs were challenged with a highly virulent PRRSV-1 subtype 2 strain. In contrast to the wild-type control group, ΔSRCR5 pigs showed no signs of infection and no viremia or antibody response indicative of a productive infection. Histopathological analysis of lung and lymph node tissue showed no presence of virus-replicating cells in either tissue. This shows that ΔSRCR5 pigs are fully resistant to infection by the virus.IMPORTANCE Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) is the etiological agent of PRRS, causing late-term abortions, stillbirths, and respiratory disease in pigs, incurring major economic losses to the worldwide pig industry. The virus is highly mutagenic and can be divided into two species, PRRSV-1 and PRRSV-2, each containing several subtypes. Current control strategies mainly involve biosecurity measures, depopulation, and vaccination. Vaccines are at best only partially protective against infection with heterologous subtypes and sublineages, and modified live vaccines have frequently been reported to revert to virulence. Here, we demonstrate that a genetic-control approach results in complete resistance to PRRSV infection in vivo CD163 is edited so as to remove the viral interaction domain while maintaining protein expression and biological function, averting any potential adverse effect associated with protein knockout. This research demonstrates a genetic-control approach with potential benefits in animal welfare as well as to the pork industry.

A multiplex reverse transcription PCR and automated electronic microarray assay for detection and differentiation of seven viruses affecting swine

Microarray technology can be useful for pathogen detection as it allows simultaneous interrogation of the presence or absence of a large number of genetic signatures. However, most microarray assays are labour-intensive and time-consuming to perform. This study describes the development and initial evaluation of a multiplex reverse transcription (RT)-PCR and novel accompanying automated electronic microarray assay for simultaneous detection and differentiation of seven important viruses that affect swine (foot-and-mouth disease virus [FMDV], swine vesicular disease virus [SVDV], vesicular exanthema of swine virus [VESV], African swine fever virus [ASFV], classical swine fever virus [CSFV], porcine respiratory and reproductive syndrome virus [PRRSV] and porcine circovirus type 2 [PCV2]). The novel electronic microarray assay utilizes a single, user-friendly instrument that integrates and automates capture probe printing, hybridization, washing and reporting on a disposable electronic microarray cartridge with 400 features. This assay accurately detected and identified a total of 68 isolates of the seven targeted virus species including 23 samples of FMDV, representing all seven serotypes, and 10 CSFV strains, representing all three genotypes. The assay successfully detected viruses in clinical samples from the field, experimentally infected animals (as early as 1 day post-infection (dpi) for FMDV and SVDV, 4 dpi for ASFV, 5 dpi for CSFV), as well as in biological material that were spiked with target viruses. The limit of detection was 10 copies/μl for ASFV, PCV2 and PRRSV, 100 copies/μl for SVDV, CSFV, VESV and 1,000 copies/μl for FMDV. The electronic microarray component had reduced analytical sensitivity for several of the target viruses when compared with the multiplex RT-PCR. The integration of capture probe printing allows custom onsite array printing as needed, while electrophoretically driven hybridization generates results faster than conventional microarrays that rely on passive hybridization. With further refinement, this novel, rapid, highly automated microarray technology has potential applications in multipathogen surveillance of livestock diseases.

Histologic Changes Associated With Placental Separation in Gilts Infected with Porcine Reproductive and Respiratory Syndrome Virus

The placenta is a vital organ providing the developing fetus with nutrient and gas exchange, thermoregulation, and waste elimination necessary for fetal development, as well as producing hormones to maintain pregnancy. It is hypothesized that fetal pig death in porcine reproductive and respiratory syndrome may be attributed to pathology of the maternal-fetal interface leading to premature placental separation. This study was designed to evaluate the chronologic progression of porcine reproductive and respiratory syndrome virus (PRRSV)–induced lesions at the maternal-fetal interface, with particular focus on placental separation in experimentally challenged third-trimester gilts. Fifteen gilts were inoculated with a virulent strain of PRRSV-2 on gestation day 86 ± 0.4. On multiple days postinoculation, 3 gilts along with 1 sham-inoculated control per time point were euthanized, and uterine and fetal placental tissues corresponding to each fetus were collected for histopathologic evaluation. The presence of any fetal lesion was 23 times more likely in compromised (meconium-stained and decomposed) compared with viable fetuses ( P < .001). In PRRSV-infected gilts, endometritis was more severe than placentitis, and the severity of endometrial inflammation and vasculitis increased progressively from 2 to 14 days postinoculation. Neither placental vasculitis nor a chronologic progression in the severity of placental detachment was observed. Severe placental detachment was more frequently present in PRRSV-infected compared with noninfected samples and was most significantly associated with placental inflammation, compared with other uterine lesions, viral load, or termination day. The results of this study suggest that placental separation by itself is not sufficient to significantly compromise fetal viability in reproductive porcine reproductive and respiratory syndrome.

A modified-live porcine reproductive and respiratory syndrome virus (PRRSV)-1 vaccine protects late-term pregnancy gilts against heterologous PRRSV-1 but not PRRSV-2 challenge

The objective of this study was to determine the efficacy of a commercially available porcine reproductive and respiratory syndrome virus (PRRSV)-1 modified-live virus (MLV) vaccine against PRRSV-1 and PRRSV-2 challenge in late-term pregnancy gilts. Gilts were vaccinated with the PRRSV-1 MLV vaccine at 4 weeks prior to breeding and then challenged intranasally with PRRSV-1 or PRRSV-2 at 93 days of gestation. After PRRSV-1 challenge, vaccinated pregnant gilts had a significantly longer gestation period, significantly higher numbers of live-born and weaned piglets and a significantly lower number of stillborn piglets at birth compared to unvaccinated pregnant gilts. No significant improvement in reproductive performance was observed between vaccinated and unvaccinated pregnant gilts following PRRSV-2 challenge. Vaccinated pregnant gilts also exhibited a significantly improved reproductive performance after challenge with PRRSV-1 compared to vaccinated pregnant gilts following PRRSV-2 challenge. The PRRSV-1 MLV vaccine was able to reduce PRRSV-1 but not PRRSV-2 viremia in pregnant gilts. Vaccinated gilts also showed a significantly higher number of PRRSV-1-specific IFN-γ-secreting cells (IFN-γ-SC) compared to PRRSV-2-specific IFN-γ-SC. The data presented here suggest that the vaccination of pregnant gilts with a PRRSV-1 MLV vaccine provides good protection against PRRSV-1 but only limited protection against PRRSV-2 challenge in late-term pregnancy gilts based on improvement of reproductive performance, reduction in viremia and induction of IFN-γ-SC.

Reproductive effects of arteriviruses: equine arteritis virus and porcine reproductive and respiratory syndrome virus infections

Equine arteritis virus (EAV) and porcine reproductive and respiratory syndrome virus (PRRSV) are the most economically important members of the family Arteriviridae. EAV and PRRSV cause reproductive and respiratory disease in equids and swine, respectively and constitute a significant economic burden to equine and swine industries around the world. Furthermore, they both cause abortion in pregnant animals and establish persistent infection in their natural hosts, which fosters viral shedding in semen leading to sexual transmission. The primary focus of this article is to provide an update on the effects of these two viruses on the reproductive tract of their natural hosts and provide a comparative analysis of clinical signs, virus-host interactions, mechanisms of viral pathogenesis and viral persistence.

Zika Virus Causes Persistent Infection in Porcine Conceptuses and may Impair Health in Offspring

Outcomes of Zika virus (ZIKV) infection in pregnant women vary from the birth of asymptomatic offspring to abnormal development and severe brain lesions in fetuses and infants. There are concerns that offspring affected in utero and born without apparent symptoms may develop mental illnesses. Therefore, animal models are important to test interventions against in utero infection and health sequelae in symptomatic and likely more widespread asymptomatic offspring. To partially reproduce in utero infection in humans, we directly inoculated selected porcine conceptuses with ZIKV. Inoculation resulted in rapid trans-fetal infections, persistent infection in conceptuses, molecular pathology in fetal brains, fetal antibody and type I interferon responses. Offspring infected in utero showed ZIKV in their fetal membranes collected after birth. Some in utero affected piglets were small, depressed, had undersized brains, and showed seizures. Some piglets showed potentially increased activity. Our data suggest that porcine model of persistent in utero ZIKV infection has a strong potential for translational research and can be used to test therapeutic interventions in vivo.

Type 2 porcine reproductive and respiratory syndrome virus infection increases apoptosis at the maternal-fetal interface in late gestation pregnant gilts

The pathogenesis of fetal death associated with porcine reproductive and respiratory syndrome (PRRS) is hypothesized to be a consequence of PRRS virus-induced apoptosis at the maternal-fetal interface (MFI). The objectives of this study were to evaluate distribution and degree of apoptosis in the uterine and fetal placental tissues during the experimental type 2 PRRS virus (PRRSV) infection and determine associations between apoptosis at the MFI, PRRSV RNA concentration and antigen staining intensity, PRRSV-induced microscopic lesions, and fetal preservation status. A total of 114 naïve, high-health pregnant gilts were inoculated with type 2 PRRSV on gestation day 85±1 with euthanasia 21 days later; 19 sham-inoculated gilts served as controls. Two hundred and fifty samples of uterine tissue with fetal placenta were selected based on negative, low PRRSV RNA, and high PRRSV RNA concentration (0, < or > 2.7 log₁₀ copies/mg, respectively). TUNEL assay was used to detect apoptosis in the endometrium and at the MFI. PRRSV RNA concentration and numbers of PRRSV immunopositive cells in uterine and placental tissue were positively associated with the severity of apoptosis in the endometrium and the MFI (P<0.001, P<0.05 and P<0.001, respectively). The number of TUNEL positive cells at the MFI was also positively associated with the severity (P<0.001) of vasculitis, but not total numbers of inflammatory cells in the endometrium. Increased numbers of TUNEL positive cells at the MFI were associated with PRRSV load in the fetal thymus, and greater odds of meconium staining of the fetus at 21 days post infection (P<0.001 for both). These findings suggest an important role of apoptosis in the pathogenesis of uterine epithelial and trophoblastic cell death at the MFI. Moreover, apoptosis at the MFI is significantly associated with fetal demise during in utero type 2 PRRSV infection.