Comparative Analysis of Immune Responses in Pigs to High and Low Pathogenic Porcine Reproductive and Respiratory Syndrome Viruses Isolated in China

Protection efficacy and the safety of the synergy between modified Bazhen powder and PRRSV modified-live virus vaccine against HP-PRRSV in piglets

The highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) poses a significant threat to the global swine industry. Vaccination is a preventive measure against viral infections. However, the use of vaccines in livestock healthcare programs faces the challenge of safety and delayed immune responses. Earlier studies have shown the potential of modified Bazhen powder as an immunomodulator with significant biological properties, but its effect on vaccines against HP-PRRSV is yet to be studied. This study elucidated how modified Bazhen powder could improve the safety and efficacy of the conventional PRRSV vaccine by evaluating T-cell responses, antibody levels, clinical symptoms, levels of viremia, organ health, and cytokine production. The results revealed that the oral application of modified Bazhen powder in combination with PRRS vaccination improved both cellular and humoral immunity, accelerated viremia clearance, improved lung injury scores, and reduced viral load in the tonsils. The modified Bazhen powder also effectively reduced inflammatory responses following a PRRSV challenge. These findings further highlight the pharmacological properties of modified Bazhen powder as a potential oral immunomodulatory agent that could enhance vaccine efficacy and ensure broad-spectrum protection against HP-PRRSV in pigs.

Differences in Immune Characteristics and Related Gene Expression in Spleen among Ningxiang, Berkshire Breeds and Their Hybrid Pigs

To investigate the differential immunology in Ningxiang and Berkshire pigs and their F1 offspring (F1 offspring), physiological and biochemical indicators in the plasma and spleen were analyzed. Then, transcriptomic analysis of the spleen identified 1348, 408, and 207 differentially expressed genes (DEGs) in comparisons of Ningxiang vs. Berkshire, Berkshire vs. F1 offspring, and Ningxiang vs. F1 offspring, respectively. In Ningxiang vs. Berkshire pigs, the gene ontology (GO) and the Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicated that the DEGs included CD163, MARCO, CXCL14, CCL19, and PPBP, which are associated with immunity. GO and KEGG analyses were also conducted comparing F1 offspring and their parents. The DEGs, including BPIFB1, HAVCR2, CD163, DDX3X, CCR5, and ITGB3, were enriched in immune-related pathways. Protein-protein interaction (PPI) analysis indicated that the EGFR and ITGA2 genes were key hub genes. In conclusion, this study identifies significant immune DEGs in different pig breeds, providing data to support the exploration of breeding strategies for disease resistance in local and crossbred pig populations.

Regulation of immune response against third-stage Gnathostoma spinigerum larvae by human genes

Background

Gnathostomiasis is an important zoonosis in tropical areas that is mainly caused by third-stage Gnathostoma spinigerum larvae (G. spinigerum L3).

Objectives

This study aimed to prove whether G. spinigerum L3 produces extracellular vesicles (EVs) and investigate human gene profiles related to the immune response against the larvae.

Methods

We created an immune cell model using normal human peripheral blood mononuclear cells (PBMCs) co-cultured with the larvae for 1 and 3 days, respectively. The PBMCs were harvested for transcriptome sequencing analysis. The EV ultrastructure was examined in the larvae and the cultured medium.

Results

Extracellular vesicle-like particles were observed under the larval teguments and in the pellets in the medium. RNA-seq analysis revealed that 2,847 and 3,118 genes were significantly expressed on days 1 and 3 after culture, respectively. The downregulated genes on day 1 after culture were involved in pro-inflammatory cytokines, the complement system and apoptosis, whereas those on day 3 were involved in T cell-dependent B cell activation and wound healing. Significantly upregulated genes related to cell proliferation, activation and development, as well as cytotoxicity, were observed on day 1, and genes regulating T cell maturation, granulocyte function, nuclear factor-κB and toll-like receptor pathways were predominantly observed on day 3 after culture.

Conclusion

G. spinigerum L3 produces EV-like particles and releases them into the excretory-secretory products. Overall, genotypic findings during our 3-day observation revealed that most significant gene expressions were related to T and B cell signalling, driving T helper 2 cells related to chronic infection, immune evasion of the larvae, and the pathogenesis of gnathostomiasis. Further in-depth studies are necessary to clarify gene functions in the pathogenesis and immune evasion mechanisms of the infective larvae.

Oral Supplementation of Houttuynia cordata Extract Reduces Viremia in PRRSV-1 Modified-Live Virus-Vaccinated Pigs in Response to the HP-PRRSV-2 Challenge

This study evaluated the in vitro antiviral activities and the ex vivo immunomodulatory effects of Houttuynia cordata Thunb. (HC) ethanolic extracts in response to porcine reproductive and respiratory syndrome virus (PRRSV). In addition, this study evaluated the in vivo effects of oral supplementation of HC extract on immune responses to and cross-protective efficacy of PRRSV-1 modified-live virus (MLV) vaccine against the highly pathogenic (HP)-PRRSV-2 challenge. In vitro experiments demonstrated that HC extracted in either 50%, 70%, or 95% ethanol (referred to as HC50, HC70, and HC95, respectively) significantly interfered with PRRSV replication in MARC-145 cells. Ex vivo experiments revealed that all HC extracts significantly enhanced mRNA expressions of type I interferon-regulated genes, type I and II interferon (IFN), and pro- and anti-inflammatory cytokines in HP-PRRSV-2-inoculated monocyte-derived macrophages. An in vivo experiment included four groups of six pigs (4 weeks old; n = 24). Group 1 and group 2 were vaccinated with the PRRSV-1 MLV vaccine at 0 dpv (day post vaccination). Group 2 also received oral administration of HC50 extract at 0–49 dpv. Group 3 received the PRRSV-1 MLV vaccine solvent at 0 dpv, while group 4 served as strict control. Groups 1–3 were challenged intranasally with HP-PRRSV-2 at 28 dpv and immune-related and clinical parameters were monitored weekly until 49 dpv. Compared to group 1, group 2 demonstrated significantly increased IFN regulatory factor 3 mRNA expression of PRRSV-recalled peripheral blood mononuclear cells, and significantly reduced HP-PRRSV-2 viremia. No difference in PRRSV-specific antibody responses, rectal temperature, clinical scores, and average daily weight gain was detected. Our study reports the immunomodulatory and anti-PRRSV potentials of HC extract in PRRSV-1 MLV-vaccinated/HP-PRRSV-2 challenged pigs.

Evolution Characterization and Pathogenicity of a Porcine Reproductive and Respiratory Syndrome Virus Isolate from a Pig Farm in Shandong Province, China

In recent years, porcine reproductive and respiratory syndrome virus (PRRSV) strains have been experiencing extensive recombination in Chinese swine farms. This recombination usually happens in NADC30/34 strains and highly pathogenic (HP) PRRSV strains. This study identified a new PRRSV isolate that shared 99% and 99.1% nucleotide identity with CH-1a and CH-1R at the genomic level, respectively. After purification by viral plaque assay, this isolate was named PRRSV CSR1801. The isolate did not experience any recombination with other PRRSV strains common in swine herd epidemics in China, which means it still maintains the stable features of the classical PRRSV strain and did not easily recombine with other PRRSV strains. Further analysis of the pathogenicity of the PRRSV isolate CSR1801 was performed in piglets. The results indicated that none of the inoculated piglets showed the typical clinical manifestations of PRRS, which presented with runny noses, rough back hair, rectal temperatures always below 40.5 °C, and no deaths. Additionally, no obvious histopathological lesions such as severe interstitial pneumonia could be observed in the lungs of the piglets. Hence, the PRRSV isolate CSR1801 should be classified as a classical-like PRRSV strain. This classical PRRSV strain showed genetic stability and maintained low pathogenicity. This study may provide new clues for further understanding the genetic evolution and pathogenicity of PRRSV and may also be an important reference for the prevention and control of PRRS in swine farms.

The jigsaw of PRRSV virulence

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

Pathogenicity of NADC34-like PRRSV HLJDZD32-1901 isolated in China

Porcine reproductive and respiratory syndrome virus (PRRSV) causes a substantial economic loss to the swine industry. Recently, NADC34-like PRRSV was reported in the USA, China and Peru and consistently attributed to a large number of abortions in the clinic. In the USA, the pathogenicity of NADC34-like PRRSV in piglets is highly variable. However, the pathogenicity of NADC34-like PRRSV in China is unclear. In this study, an NADC34-like PRRSV strain, HLJDZD32-1901, was isolated in primary alveolar macrophage (PAM) cells from a sow blood sample collected from an abortive farm in China. HLJDZD32-1901, with no recombination, has a 100-aa deletion in the NSP2 protein corresponding to positions 328-427 in the VR2332 strain. Phylogenetic analysis based on open reading frame 5 (ORF5) indicated that HLJDZD32-1901 belongs to sublineage 1.5. Animal experiments showed that the weight loss of HLJDZD32-1901-infected piglets was significantly different from that of control piglets at 8-14 dpi. In addition, the challenge group had obvious histopathological lesions, including interstitial pneumonia and enlarged lymph nodes, and increased viremia and viral loads in three tissues. However, piglets in the challenge group had only mild clinical symptoms, with no death or fever. Our results showed that NADC34-like PRRSV HLJDZD32-1901 is a mildly pathogenic strain in piglets. However, we speculate that HLJDZD32-1901 may be a highly pathogenic strain in pregnant sows based on clinical morbidity.

A potential endemic strain in China: NADC34-like porcine reproductive and respiratory syndrome virus

Porcine respiratory and reproductive syndrome virus (PRRSV) causes an economically important disease affecting commercial pork production worldwide. NADC34-like PRRSV has had a strong impact on the U.S. and Peruvian pig industries in recent years and also emerged in northeastern China in 2017. However, the endemic status of NADC34-like PRRSV in China is unclear. In this study, we examined 650 tissue samples collected from 16 Provinces in China from 2018 to 2019. Six NADC34-like PRRSV strains were detected in samples from three Provinces, and the complete genomes of four of these strains were sequenced. Phylogenetic analysis showed that these novel PRRSV strains belong to sublineage 1.5 (or NADC34-like PRRSV), forming two groups in China. Sequence alignment suggested that these novel strains share the same 100-aa deletion in the Nsp2 protein that was identified in IA/2014/NADC34 isolated from the United States in 2014. Recombination analysis revealed that five of eight complete genome sequences are derived from recombination between IA/2014/NADC34 and ISU30 or NADC30. The number and distribution of NADC34-like PRRSVs is increasing in China. Importantly, compared with the currently endemic strain NADC30-like PRRSV, NADC34-like PRRSV has the potential to be an endemic strain in China. This study will help us understand the epidemic status of NADC34-like PRRSV in China and provide data for further monitoring this type of PRRSV in China.

Oral supplementation of quercetin in PRRSV-1 modified-live virus vaccinated pigs in response to HP-PRRSV-2 challenge

This study evaluated the immunomodulatory effect of quercetin on improving cross protection of porcine reproductive and respiratory syndrome virus-1 (PRRSV-1) modified-live virus (MLV) vaccine against highly pathogenic (HP)-PRRSV-2 challenge. Ex vivo experiments demonstrated that quercetin significantly enhanced type I interferon-regulated genes (IRGs) and type I and II interferon (IFN), and significantly decreased pro- and anti-inflammatory cytokine expressions in HP-PRRSV-inoculated monocyte-derived macrophages. In vivo experiments divided pigs (4-week-old; n = 24) into four groups of six pigs. Group 1 and group 2 were immunized with PRRSV-1 MLV vaccine at 0 dpv (day post vaccination). Group 2 also received oral administration of quercetin at 0-49 dpv. Group 3 was injected with PRRSV-1 MLV vaccine solvent at 0 dpv. Group 4 served as strict control. Group 1-3 were challenged intranasally with HP-PRRSV at 28 dpv and immune and clinical parameters were monitored weekly until 49 dpv. Group 1 demonstrated significantly reduced HP-PRRSV viremia, rectal temperature and clinical scores, and significantly improved average daily weight gain (ADWG), compared to group 3. Group 2 demonstrated significantly increased IFN regulatory factor 3, stimulator of IFN genes, IFNα, and significantly decreased transforming growth factor beta (TGFβ) mRNA expressions, compared to group 1. The animals demonstrated significantly reduced HP-PRRSV viremia, but did not demonstrate any further improved PRRSV-specific antibody responses, rectal temperature, clinical scores, and ADWG as compared to group 1. Our findings suggest that quercetin up-regulates IRGs, IFNα, and down-regulates TGFβ mRNA expressions which may contribute to further reducing number of viremic pigs and HP-PRRSV viremia which were conferred by PRRSV-1 MLV vaccine. Our findings also suggest that quercetin may serve as an effective oral immunomodulator for improving cell-mediated immune defense to HP-PRRSV.

Effects of PRRSV Infection on the Porcine Thymus

Porcine reproductive and respiratory syndrome virus (PRRSV) dramatically affects the thymus and its ability to carry out its normal functions. In particular, infection incapacitates PRRSV-susceptible CD14^pos antigen-presenting cells (APCs) in the thymus and throughout the body. PRRSV-induced autophagy in thymic epithelial cells modulates the development of T cells, and PRRSV-induced apoptosis in CD4^posCD8^pos thymocytes modulates cellular immunity against PRRSV and other pathogens. Pigs are less able to resist and/or eliminate secondary infectious agents due the effect of PRRSV on the thymus, and this susceptibility phenomenon is long recognized as a primary characteristic of PRRSV infection.

Efficacy of live attenuated porcine reproductive and respiratory syndrome virus 2 strains to protect pigs from challenge with a heterologous Vietnamese PRRSV 2 field strain

Background

Effective vaccines against porcine reproductive and respiratory syndrome virus (PRRSV), especially against highly pathogenic (HP) PRRSV are still missing. The objective of this study was to evaluate the protective efficacy of an experimental live attenuated PRRSV 2 vaccine, composed of two strains, against heterologous challenge with a Vietnamese HP PRRSV 2 field strain. For this reason, 20 PRRSV negative piglets were divided into two groups. The pigs of group 1 were vaccinated with the experimental vaccine, group 2 remained unvaccinated. All study piglets received an intranasal challenge of the HP PRRSV 2 on day 0 of the study (42 days after vaccination). Blood samples were taken on days 7 and 21 after vaccination and on several days after challenge. On day 28 after challenge, all piglets were euthanized and pathologically examined.

Results

On days 7 and 21 after vaccination, a PRRSV 2 viraemia was seen in all piglets of group 1 which remained detectable in seven piglets up to 42 days after vaccination. On day 3 after challenge, all piglets from both groups were positive in PRRSV 2 RT-qPCR. From day 7 onwards, viral load and number of PRRSV 2 positive pigs were lower in group 1 than in group 2. All pigs of group 1 seroconverted after PRRSV 2 vaccination. PRRSV antibodies were detected in serum of all study pigs from both groups from day 14 after challenge onwards. In group 2, moderate respiratory symptoms with occasional coughing were seen following the challenge with HP PRRSV 2. Pigs of group 1 remained clinically unaffected. Interstitial pneumonia was found in four piglets of group 1 and in all ten piglets of group 2. Histopathological findings were more severe in group 2.

Conclusions

It was thus concluded that the used PRRSV 2 live experimental vaccine provided protection from clinical disease and marked reduction of histopathological findings and viral load in pigs challenged with a Vietnamese HP PRRSV 2 field strain.

Two Residues in NSP9 Contribute to the Enhanced Replication and Pathogenicity of Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus

Highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) possesses greater replicative capacity and pathogenicity than classical PRRSV. However, the factors that lead to enhanced replication and pathogenicity remain unclear. In our study, an alignment of all available full-length sequences of North American-type PRRSVs (n = 204) revealed two consistent amino acid mutations that differed between HP-PRRSV and classical PRRSV and were located at positions 519 and 544 in nonstructural protein 9. Next, a series of mutant viruses with either single or double amino acid replacements were generated from HP-PRRSV HuN4 and classical PRRSV CH-1a infectious cDNA clones. Deletion of either of the amino acids led to a complete loss of virus viability. In both Marc-145 and porcine alveolar macrophages, the replicative efficiencies of mutant viruses based on HuN4 were reduced compared to the parent, whereas the replication level of CH-1a-derived mutant viruses was increased. Plaque growth assays showed clear differences between mutant and parental viruses. In infected piglets, the pathogenicity of HuN4-derived mutant viruses, assessed through clinical symptoms, viral load in sera, histopathology examination, and thymus atrophy, was reduced. Our results indicate that the amino acids at positions 519 and 544 in NSP9 are involved in the replication efficiency of HP-PRRSV and contribute to enhanced pathogenicity. This study is the first to identify specific amino acids involved in PRRSV replication or pathogenicity. These findings will contribute to understanding the molecular mechanisms of PRRSV replication and pathogenicity, leading to better therapeutic and prognostic options to combat the virus.IMPORTANCE Porcine reproductive and respiratory syndrome (PRRS), caused by porcine reproductive and respiratory syndrome virus (PRRSV), is a significant threat to the global pig industry. Highly pathogenic PRRSV (HP-PRRSV) first emerged in China in 2006 and has subsequently spread across Asia, causing considerable damage to local economies. HP-PRRSV strains possess a greater replication capacity and higher pathogenicity than classical PRRSV strains, although the mechanisms that underlie these characteristics are unclear. In the present study, we identified two mutations in HP-PRRSV strains that distinguish them from classical PRRSV strains. Further experiments that swapped the two mutations in an HP-PRRSV strain and a classical PRRSV strain demonstrated that they are involved in the replication efficiency of the virus and its virulence. Our findings have important implications for understanding the molecular mechanisms of PRRSV replication and pathogenicity and also provide new avenues of research for the study of other viruses.

Investigating the financial impact of porcine reproductive and respiratory syndrome on smallholder pig farmers in Cambodia

Porcine reproductive and respiratory syndrome (PRRS) was first identified in Cambodia in 2010, causing serious problems on affected farms, although the costs of the disease have not been well defined. The household financial impact of a PRRS outbreak in Cambodia was investigated using partial budget analysis, examining the economic benefit of three proposed interventions: (i) quarterly PRRS vaccine use, (ii) biosecurity implementation, and (iii) implementation of vaccination and biosecurity. The analyses were applied to three farm models: (i) a two-sow breeder; (ii) a five-pig fattener; and (iii) a single-sow, three-pig farrow-to-finish/breeder. Data was derived from a knowledge, attitude, and practice survey of 240 smallholder farmers (61 with pigs) from 16 villages across 5 provinces, plus case studies of 12 farmers selected for more detailed financial analysis. The study indicated that financial losses associated with PRRS were severe, with a 25% mean loss to the annual household income of 61 interviewed farmers. Partial budget analysis identified a strongly positive incentive for vaccination and biosecurity to be implemented in combination, with the highest annual net benefit of USD 357.10 realised by the breeder system. However, due to current scarcity of the PRRS vaccine and its high cost to smallholders, biosecurity interventions may be more cost-effective, especially for low PRRS incidence regions. It was concluded that PRRS critically constrains the profitability of smallholder pig farms and that these findings will assist development of village-level livestock disease risk management programmes that encourage adoption of vaccination and biosecurity practices to enhance farmer livelihoods in Cambodia.

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

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

Dynamic changes in bronchoalveolar macrophages and cytokines during infection of pigs with a highly or low pathogenic genotype 1 PRRSV strain

Porcine reproductive and respiratory syndrome virus (PRRSV) replicates primarily in pulmonary alveolar macrophages (PAMs) and the resulting lung damage is influenced by strain virulence. To better understand the pathogenesis of PRRSV infection, we performed a longitudinal study of the PAM population and lung cytokines in specific pathogen-free pigs infected either with the highly pathogenic Lena strain or with the low pathogenic Finistere strain in comparison to uninfected pigs. Bronchoalveolar lavage fluid (BALF) and blood were collected to follow viral, cellular and cytokine changes in lung with respect to clinical signs and systemic events. Compared to Finistere-infected pigs, Lena-infected pigs exhibited more severe clinical signs and 10- to 100-fold higher viral loads in BALF and blood. Similarly, they showed an earlier drop in BALF cell viability and phagocytic activity along with a decrease in the macrophage count. From 8 to 15 days post-infection (dpi), monocytes increased both in BALF and blood from Lena-infected pigs. BALF and blood showed contrasting cytokine patterns, with low increase of IFN-α and TNF-α levels and high increase for IL-1α and IL-8 in BALF after Lena-infection. In contrast, in the blood, the increase was marked for IFN-α and TNF-α but limited for IL-1β and IL-8. Down-regulation of PAM functions combined with inflammatory cytokine and monocyte recruitment may promote lung pathogenesis and virus replication in PRRSV infections with the highly pathogenic Lena strain. In contrast, the low pathogenic Finistere strain showed prolonged viral replication in lung, possibly related to the weak IFN-γ response.

The Chinese highly pathogenic porcine reproductive and respiratory syndrome virus infection suppresses Th17 cells response in vivo

Porcine reproductive and respiratory syndrome virus (PRRSV) has been shown to immunomodulate innate and adaptive immunity of pigs. The Chinese highly pathogenic PRRSV (HP-PRRSV) infection causes severe bacterial secondary infection in pigs. However, the mechanism in relation to the bacterial secondary infection induced by HP-PRRSV remains unknown. In the present study, Th17 cells response in peripheral blood, lungs, spleens and lymph nodes of piglets were analyzed, and bacterial loads in lungs of piglets were examined upon HP-PRRSV infection. Meanwhile the changes of CD4(+) and CD8(+) T cells in peripheral blood of the inoculated piglets were analyzed. The results showed that HP-PRRSV-inoculated piglets exhibited a suppressed Th17 cells response in peripheral blood and a reduced number of Th17 cells in lungs, and higher bacterial loads in lungs, compared with low pathogenic PRRSV. Moreover, HP-PRRSV obviously resulted in severe depletion of porcine T cells in peripheral blood at the early stage of infection. These findings indicate that HP-PRRSV infection suppresses the response of Th17 cells that play an important role in combating bacterial infections, suggesting a possible correlation between the suppression of Th17 cells response in vivo and bacterial secondary infection induced by HP-PRRSV. Our present study adds a novel insight into better understanding of the pathogenesis of the Chinese HP-PRRSV.

Highly Pathogenic Porcine Reproductive and Respiratory Syndrome Virus Infection Induced Apoptosis and Autophagy in Thymi of Infected Piglets

Previously, we demonstrated that the highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) HuN4 strain causes obvious thymic atrophy and thymocytes apoptosis in infected piglets after birth, which is more severe than that induced by classical PRRSV. In this study, we investigated apoptosis and autophagy in the thymus of piglets infected with the HP-PRRSV HuN4 strain, and found that both apoptosis and autophagy occurred in the thymus of piglets infected with HP-PRRSV. In addition to a few virus-infected cells, CD14⁺ cells, the main autophagic cells in the thymus were thymic epithelial cells. These findings demonstrated that HP-PRRSV induces apoptosis in bystander cells, and induces autophagy in both infected and bystander cells in the thymus of infected piglets. Herein, we first present new data on the thymic lesions induced by HP-PRRSV, and show that apoptosis and autophagy are key mechanisms involved in cell survival and determinants of the severity of thymic atrophy in infected piglets. Finally, future studies of the mechanism underlying immune responses are proposed based on our current understanding of PRRSV-host interactions.

Immune Control of PRRS: Lessons to be Learned and Possible Ways Forward

Porcine reproductive and respiratory syndrome (PRRS) is an elusive model of host/virus relationship in which disease is determined by virus pathogenicity, pig breed susceptibility and phenotype, microbial infectious pressure, and environmental conditions. The disease can be controlled by farm management programs, which can be supported by vaccination or conditioning of animals to circulating PRRS virus (PRRSV) strains. Yet, PRRS still represents a cause of heavy losses for the pig industry worldwide. Immunological control strategies are often compounded by poor and late development of adaptive immunity in both vaccinated and infected animals. Also, there is evidence that results of field trials can be worse than those of experimental studies in isolation facilities. Neutralizing antibody (NA) was shown to prevent PRRSV infection. Instead, the role of NA and adaptive immunity on the whole in virus clearance after established PRRSV infections is still contentious. Pigs eventually eliminate PRRSV infection, which may be correlated with an “educated,” innate immune response, which may also develop following vaccination. In addition to vaccination, an immunomodulation strategy for PRRS can be reasonably advocated in pig “problem” farms, where a substantial control of disease prevalence and disease-related losses is badly needed. This is not at odds with vaccination, which should be preferably restricted to PRRSV-free animals bound for PRRSV-infected farm units. Oral, low-dose, interferon-α treatments proved effective on farm for the control of respiratory and reproductive disease outbreaks, whereas the results were less clear in isolation facilities. Having in mind the crucial interaction between PRRSV and bacterial lipopolysaccharides for occurrence of respiratory disease, the strong control actions of low-dose type I interferons on the inflammatory response observed in vitro and in vivo probably underlie the rapid clinical responses observed in field trials.

Nsp9 and Nsp10 contribute to the fatal virulence of highly pathogenic porcine reproductive and respiratory syndrome virus emerging in China

Atypical porcine reproductive and respiratory syndrome (PRRS), which is caused by the Chinese highly pathogenic PRRS virus (HP-PRRSV), has resulted in large economic loss to the swine industry since its outbreak in 2006. However, to date, the region(s) within the viral genome that are related to the fatal virulence of HP-PRRSV remain unknown. In the present study, we generated a series of full-length infectious cDNA clones with swapped coding regions between the highly pathogenic RvJXwn and low pathogenic RvHB-1/3.9. Next, the in vitro and in vivo replication and pathogenicity for piglets of the rescued chimeric viruses were systematically analyzed and compared with their backbone viruses. First, we swapped the regions including the 5′UTR+ORF1a, ORF1b, and structural proteins (SPs)-coding region between the two viruses and demonstrated that the nonstructural protein-coding region, ORF1b, is directly related to the fatal virulence and increased replication efficiency of HP-PRRSV both in vitro and in vivo. Furthermore, we substituted the nonstructural protein (Nsp) 9-, Nsp10-, Nsp11- and Nsp12-coding regions separately; or Nsp9- and Nsp10-coding regions together; or Nsp9-, Nsp10- and Nsp11-coding regions simultaneously between the two viruses. Our results indicated that the HP-PRRSV Nsp9- and Nsp10-coding regions together are closely related to the replication efficiency in vitro and in vivo and are related to the increased pathogenicity and fatal virulence for piglets. Our findings suggest that Nsp9 and Nsp10 together contribute to the fatal virulence of HP-PRRSV emerging in China, helping to elucidate the pathogenesis of this virus.

PRRS is a considerable threat to the pig industry worldwide. A large-scale atypical PRRS caused by highly pathogenic PRRSV (HP-PRRSV) that emerged in 2006 has resulted in considerable economic loss to Chinese pig production. The disease is characterized by a high body temperature (41°C–42°C), morbidity and by mortality of the affected pigs. Although the genomic marker, the 30-amino-acid deletion in its Nsp2-coding region has been previously verified to have no relation to its increased pathogenicity, the genomic region(s) associated with the fatal virulence of HP-PRRSV remain unclear. A series of chimeric viruses with swapped coding regions between HP- and LP-PRRSV were constructed, and their growth abilities and pathogenicities in piglets were analyzed. Our results demonstrated that Nsp9 and Nsp10 together contribute to the replication efficiency and the fatal virulence of HP-PRRSV for piglets. Our finding is not only the first unambiguous illumination concerning the key virulence determinant of Chinese HP-PRRSV but it also provides a novel insight for understanding the molecular pathogenesis of this virus and for designing new drugs and vaccines against PRRSV infection in the future.