Immunohistochemical staining of IFN-gamma positive cells in porcine reproductive and respiratory syndrome virus-infected lungs

Mechanisms of Adaptive Immunity to Porcine Reproductive and Respiratory Syndrome Virus

The adaptive immune response is necessary for the development of protective immunity against infectious diseases. Porcine reproductive and respiratory syndrome virus (PRRSV), a genetically heterogeneous and rapidly evolving RNA virus, is the most burdensome pathogen of swine health and wellbeing worldwide. Viral infection induces antigen-specific immunity that ultimately clears the infection. However, the resulting immune memory, induced by virulent or attenuated vaccine viruses, is inconsistently protective against diverse viral strains. The immunological mechanisms by which primary and memory protection are generated and used are not well understood. Here, we summarize current knowledge regarding cellular and humoral components of the adaptive immune response to PRRSV infection that mediate primary and memory immune protection against viruses.

An SH3 binding motif within the nucleocapsid protein of porcine reproductive and respiratory syndrome virus interacts with the host cellular signaling proteins STAMI, TXK, Fyn, Hck, and cortactin

Porcine reproductive and respiratory syndrome virus (PRRSV) causes an economically important global swine disease, and has a complicated virus-host immunomodulation that often leads to a weak Th2 immune response and viral persistence. In this study, we identified a Src homology 3 (SH3) binding motif, PxxPxxP, that is conserved within the N protein of PRRSV strains. Subsequently, we identified five host cellular proteins [signal transducing adaptor molecule (STAM)I, TXK tyrosine kinase (TXK), protein tyrosine kinase fyn (Fyn), hematopoietic cell kinase (Hck), and cortactin] that interact with this SH3 motif. We demonstrated that binding of SH3 proteins with PRRSV N protein depends on at least one intact PxxP motif as disruption of P53 within the motif significantly reduced interaction of each of the 5 proteins. The first PxxP motif appears to be more important for STAMI-N protein interactions whereas the second PxxP motif was more important for Hck interaction. Both STAMI and Hck interactions with PRRSV N protein required an unhindered C-terminal domain as the interaction was only observed with STAMI and Hck proteins with N-terminal but not C-terminal fluorescent tags. We showed that the P56 residue within the SH3 motif is critical for virus lifecycle as mutation resulted in a loss of virus infectivity, however the P50 and P53 mutations did not abolish virus infectivity suggesting that these highly conserved proline residues within the SH3 motif may provide a selective growth advantage through interactions with the host rather than a vital functional element. These results have important implications in understanding PRRSV-host interactions.

A comparative study of the local cytokine response in the lungs of pigs experimentally infected with different PRRSV-1 strains: upregulation of IL-1α in highly pathogenic strain induced lesions

Porcine reproductive and respiratory syndrome viruses (PRRSV) show high genetic differences both among and within genotypes. Recently, several highly pathogenic PRRSV (HP-PRRSV) strains have been described. This study compares and characterizes the production of cytokines by pulmonary macrophages in pigs experimentally infected with four different PRRSV-1 strains: two low-virulent strains, Lelystad (LV) and a British field strain (215-06); a HP strain (SU1-bel) from Belarus and the attenuated vaccine strain DV (Porcilis(®) PRRS). Animals were clinically monitored and post-mortem examinations were performed at 3, 7 and 35 days post-infection (dpi). Lung samples were processed for histopathological and immunohistochemical studies by using specific antibodies against PRRSV, IL1-α, IL-6, TNF-α, IL-10 and IFN-γ. SU1-bel infected animals presented the highest mean scores for clinical observations, gross and microscopic lesions as well as for PRRSV expression compared with the other infected groups (p≤0.027). These animals displayed the highest expression of IL1-α at 7dpi, together with the highest score for lung pathology, whereas LV, 215-06 and DV inoculated animals only showed a transient enhancement in some of these cytokines. SU1-bel-infected pigs showed a positive correlation between the amount of PRRSV antigen and IL-1α expression (r=0.645, p<0.001). The highest expression of IL-10 was detected in 215-06-infected animals (p≤0.004), with a positive correlation with the numbers of virus-infected cells (r=0.375, p≤0.013). In conclusion, the HP-PRRSV SU1-bel strain replicated more efficiently in the lung of infected animals and induced a higher expression of IL-1α than the other PRRSV-1-infected groups, which may have played a key role in the onset of the clinical signs and interstitial pneumonia.

Threonine and tryptophan supplementation enhance porcine respiratory and reproductive syndrome (PRRS) vaccine-induced immune responses of growing pigs

The aim of the present study was to investigate influences of threonine and tryptophan supplementation (TTS) on immune response of growing pigs inoculated with modified live porcine reproductive and respiratory syndrome virus (PRRSV) vaccine. Twenty growing barrows (Landrace × Yorkshire) were randomly assigned to four groups according to the PRRS vaccination and TTS. Serum samples were collected from all pigs at days 0, 7, 14, 21, 28, 35, 49 post-vaccination (day 0 defined as the day of vaccination). Pigs were euthanized and samples collected at day 49 post-vaccination. The results showed that TTS tended to increase weight gain and average daily gain (ADG) of pigs (P < 0.1). PRRS vaccine enhanced serum PRRSV-specific antibody, serum virus neutralizing (SVN) antibody and interferon-γ, interleukin (IL)-10 and IL-1β concentrations (P < 0.05). The expression of TLR3 and TLR7 mRNA in lymph nodes were higher in TTS than in the control group after PRRS vaccine inoculation (P < 0.05). TTS diet mitigated lung damage which is induced by PRRS vaccination from microscopic evaluation. These results suggest that dietary TTS could improve growth performance of growing pigs, which may be ascribed to the improved immune response and mitigated lung damage.

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

The CH-1a and HuN4 strains of porcine reproductive and respiratory syndrome virus (PRRSV) show different pathogenicities in pigs. To understand host immune responses against these viruses, we investigated the dynamic changes in cytokine levels produced in peripheral blood of piglets infected with the highly pathogenic PRRSV HuN4 strain or the CH-1a strain. Clinical signs, virus loads and serum cytokine levels [interferon(IFN)-α, Interleukin (IL)-1, TNF-α, IL-6, IL-12, IFN-γ, IL-10 and TGF-β] were tested. The results showed that while piglets developed effective cellular immune responses against CH-1a infection, those infected with HuN4 displayed ineffective cellular immunity, organ lesions and persistent elevated levels of immunoregulatory cytokines (IL-10 and TGF-β), which delayed the development of PRRSV-specific immune responses. These results demonstrated that HuN4 infection induced higher cytokine levels than that of CH-1a infection induced. The changes in inflammatory cytokines intensified the inflammatory reaction and damaged the tissues and organs.

Oral Administration of EC-12 Increases the Baseline Gene Expression of Antiviral Cytokine Genes, IFN-γ and TNF-α, in Splenocytes and Mesenteric Lymph Node Cells of Weaning Piglets

Weaning piglets are continuously exposed to various viruses. The antiviral effects of lactic acid bacteria (LAB) have been confirmed mainly in humans and mice, while few studies have been conducted in livestock. In this study, we evaluated the effect of oral administration of Enterococcus faecalis strain EC-12 (EC-12) on the gene expressions of antiviral cytokines in weaning piglets. Piglets were allocated to the EC-12-administered group (E group) and the no-treatment control group (C group). The small intestinal tissue, the mesenteric lymph node (MLN) cells and the splenocytes were collected from the piglets. The tissue and cells were co-cultured with a live vaccine of porcine reproductive respiratory syndrome virus, porcine epidemic diarrhea virus or EC-12. After the incubation, the gene expressions of IFN-γ and TNF-α in the tissue and cells were evaluated. The gene expressions of IFN-γ in the MLN cells and TNF-α in the splenocytes were significantly higher in the E group than in the C group. However, the increase in the gene expression of antiviral cytokines was observed independently of the antigen treatments. The results of the present study suggest that oral administration of EC-12 did not increase the response of immune cells to specific viral antigens but increased the baseline gene expression of antiviral cytokines.

Immunopathogenesis of porcine reproductive and respiratory syndrome in the respiratory tract of pigs

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) impairs local pulmonary immune responses by damaging the mucociliary transport system, impairing the function of porcine alveolar macrophages and inducing apoptosis of immune cells. An imbalance between pro- and anti-inflammatory cytokines, including tumour necrosis factor-α and interleukin-10, in PRRS may impair the immune response of the lung. Pulmonary macrophage subpopulations have a range of susceptibilities to different PRRSV strains and different capacities to express cytokines. Infection with PRRSV decreases the bactericidal activity of macrophages, which increases susceptibility to secondary bacterial infections. PRRSV infection is associated with an increase in concentrations of haptoglobin, which may interact with the virus receptor (CD163) and induce the synthesis of anti-inflammatory mediators. The balance between pro- and anti-inflammatory cytokines modulates the expression of CD163, which may affect the pathogenicity and replication of the virus in different tissues. With the emergence of highly pathogenic PRRSV, there is a need for more information on the immunopathogenesis of different strains of PRRS, particularly to develop more effective vaccines.

Porcine reproductive and respiratory syndrome virus induces interleukin-15 through the NF-κB signaling pathway

Porcine reproductive and respiratory syndrome virus (PRRSV) mainly infects macrophages/dendritic cells and modulates cytokine expression in these cells. Interleukin-15 (IL-15) is a pleiotropic cytokine involved in wide range of biological activities. It has been shown to be essential for the generation, activation, and proliferation of NK and NKT cells and for the survival and activation of CD8(+) effector and memory T cells. In this study, we discovered that PRRSV infection upregulated IL-15 production at both the mRNA and protein levels in porcine alveolar macrophages (PAMs), blood monocyte-derived macrophages (BMo), and monocyte-derived dendritic cells (DCs). We subsequently demonstrated that the NF-κB signaling pathway was essential for PRRSV infection-induced IL-15 production. First, addition of an NF-κB inhibitor drastically reduced PRRSV infection-induced IL-15 production. We then found that NF-κB was indeed activated upon PRRSV infection, as evidenced by IκB phosphorylation and degradation. Moreover, we revealed an NF-κB binding motif in the cloned porcine IL-15 (pIL-15) promoter, deletion of which abrogated the pIL-15 promoter activity in PRRSV-infected alveolar macrophages. In addition, we demonstrated that PRRSV nucleocapsid (N) protein had the ability to induce IL-15 production in porcine alveolar macrophage cell line CRL2843 by transient transfection, which was mediated by its multiple motifs, and it also activated NF-κB. These data indicated that PRRSV infection-induced IL-15 production was likely through PRRSV N protein-mediated NF-κB activation. Our findings provide new insights into the molecular mechanisms underling the IL-15 production induced by PRRSV infection.

The interaction between PRRSV and the late gestation pig fetus

Porcine reproductive and respiratory syndrome virus (PRRSV) crosses the placenta during late gestation and productively infects the fetus. Virus replication and cytokine responses were measured in tissues of fetuses recovered at 109–112 days of gestation, just prior to parturition. At the time of recovery, gross anatomical abnormalities were evident in both infected and non-infected fetuses from the infected dams. Virus isolation and immunohistochemistry identified the thymus as the primary site of virus replication. Steady state RT-PCR amplification of inflammatory, Th1 and Th2 cytokines, showed elevated IFN-γ and TNF-α mRNAs in tissues from infected fetuses, which corresponded to elevated cytokine proteins in serum but not amniotic fluid. Further evidence for induction of immunity was found in the hyperplastic response of lymph nodes, which included the development of germinal centers occupied CDw75+ B cells. Collectively, these data support the notion that the immunocompetent fetus is capable of initiating an antiviral response, which is compartmentalized within the infected fetus. Furthermore, fetal pathology may not be a direct result of virus replication in the fetus.

Cytokine expression by macrophages in the lung of pigs infected with the porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is caused by a virus that predominantly replicates in alveolar macrophages. The aim of the present study was to characterize the production of cytokines by subpopulations of pulmonary macrophages in pigs infected by the PRRS virus (PRRSV). Expression of interleukin (IL) 1α, IL-6 and tumour necrosis factor (TNF)-α correlated with the severity of pulmonary pathology and the numbers of pulmonary macrophages. Significant correlations were observed between PRRSV infection and the expression of IL-10, between the expression of IL-12p40 and interferon (IFN)-γ, and between the expression of TNF-α and IFN-γ. These findings suggest that PRRSV modulates the immune response by the up-regulation of IL-10, which may in turn reduce expression of cytokines involved in viral clearance (e.g. IFN-α, IFN-γ, IL-12p40 and TNF-α). The results also suggest that expression of IFN-γ is stimulated by IL-12p40 and TNF-α, but not by IFN-α. All of these cytokines were expressed mainly by septal macrophages with weaker expression by alveolar macrophages, lymphocytes and neutrophils. There appears to be differential activation of septal and alveolar macrophages in PRRSV infection, with septal macrophages being the major source of cytokines.

Differences in clinical disease and immune response of pigs challenged with a high-dose versus low-dose inoculum of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) continues to be an economically important infectious disease of swine. Mechanisms governing activation of the innate immune response to PRRSV remain to be elucidated. Virulence differences observed between PRRSV isolates have been attributed to replication ability in vivo, though immunogenic differences likely contribute to virulence also. The current study utilized a single PRRSV isolate given at two different challenge doses to investigate the effect of viral replication and load on immune responses, including type I interferon activation. Body temperature, viral load, antibody levels, cellular infiltration into pulmonary tissue, and the interferon response were measured in animals receiving either a low (10(2) CCID(50)) or high (10(6) CCID(50)) dose of inoculum to understand the role of challenge dose in acute immune responses. Initial PRRSV dose did not correlate with serum levels of PRRSV vRNA or antibody titers during the acute stage of infection (days 2-12 PI), but did have an effect on the immune response and mortality. Type I interferon responses, measured by transcriptional changes in IFN-beta, IFN-alpha, Mx, and PKR, were uniquely different when assessed relative to viral dose or cell type, but no overall trend existed to discern responses based on challenge dose. Serum IFN-gamma levels correlated with serum viral RNA load at day 19 PI. Overall, between days 2 and 12 PI, serum vRNA load was not significantly different between pigs challenged with a low or high dose of PRRSV. Animals receiving high-dose inoculum were viremic longer and eventually succumbed to respiratory disease. IFN-gamma may play a role in PRRSV pathogenesis, as serum levels increased significantly in pigs challenged with the high dose of PRRSV.

Immune response against porcine reproductive and respiratory syndrome virus during acute and chronic infection

A significant obstacle to the prevention and control of porcine reproductive and respiratory syndrome virus (PRRSV) is the inability of current diagnostic tests to provide information concerning the stage of PRRSV infection. To explore possible prognostic combinations of cell-mediated and humoral immune responses, 3-week-old pigs (n=10) were intramuscularly (IM) inoculated with PRRSV isolate VR-2332 and followed for 193 days post-inoculation (DPI). Negative control pigs (n=10) were IM inoculated with minimum essential medium (MEM). At approximately 2-week intervals, blood samples were collected from all animals and tested for the number of interferon (IFN)-gamma-secreting peripheral blood mononuclear cells (enzyme-linked immunosorbent spot, Elispot), PRRSV viremia (quantitative reverse-transcriptase polymerase chain reaction, qRT-PCR), and serum antibodies using PRRSV protein ELISAs (N, GP5 3', GP5 5', M 5', M 3', GP5-M, and nsp2p) and a commercial PRRSV ELISA (IDEXX Laboratories Inc.). All pigs were viremic by 7 days post-inoculation, with 50% of the pigs resolving viremia by 56 DPI. A PRRSV-specific IFN-gamma response was detected at DPI 28, reached a plateau at 42 DPI, declined slightly, and remained relatively stable from 56 to 193 DPI. On the basis of ROC area under the curve (AUC) analysis, the ELISAs that most reliably differentiated PRRSV-inoculated pigs from negative control pigs were the commercial ELISA (AUC=0.97), the N ELISA (AUC=0.96), and the M 3' ELISA (AUC=0.93). Multivariate analyses were performed to evaluate the relationship between the immune response and the duration and level of viremia. With all antibody assays and Elispot included in the models, the analysis determined that the serum-virus neutralizing antibody response was the best predictor of both level and duration of viremia. It was concluded that humoral antibody responses, particularly the commercial ELISA, N ELISA, and M 3' ELISA were good predictors of prior exposure to PRRSV, but provided little information regarding the ontogeny of the protective immune response. Likewise, cell-mediated immunity based on the number of IFN-gamma-secreting lymphocytes was a poor prognosticator of PRRSV infection status.

Correlation of cell-mediated immunity against porcine reproductive and respiratory syndrome virus with protection against reproductive failure in sows during outbreaks of porcine reproductive and respiratory syndrome in commercial herds

OBJECTIVE

To determine whether cell-mediated immunity against porcine reproductive and respiratory syndrome (PRRS) virus is correlated with protection against reproductive failure in sows during clinical outbreaks of PRRS in commercial herds.

DESIGN

Outbreak investigation in 4 swine breeding herds.

ANIMALS

97 sows.

PROCEDURES

On each farm, blood samples were collected from sows with clinical signs (abortion or increased fetal death; case sows) and from clinically normal sows (control sows). The intensity of the cell-mediated immune (CMI) response was determined by use of an interferon-gamma enzyme-linked immunospot (ELISPOT) assay. Multiple logistic regression analyses and t tests were used to compare ELISPOT assay values between case and control sows. Multiple linear regression was used to investigate associations between cell-mediated immunity and the magnitude of clinical signs.

RESULTS

In 2 farms, case sows had lower ELISPOT assay values than control sows. A negative association between the intensity of the CMI response and the number of pigs born dead per litter was detected on 1 farm. In 1 farm, no association was detected between the intensity of the CMI response and protection against reproductive failure.

CONCLUSIONS AND CLINICAL RELEVANCE

Evidence that a strong CMI response was correlated with protection against clinical PRRS was detected in 3 of 4 farms. However, farms and sows within farms varied considerably in their immune responsiveness and in the degree to which they were protected clinically. Increasing cell-mediated immunity within infected herds has the potential to decrease clinical reproductive disease, but only if the sources of intra- and interfarm variation in the intensity of cell-mediated immunity to PRRS virus can be identified.

Deciphering the involvement of innate immune factors in the development of the host response to PRRSV vaccination

The natural response of pigs to porcine reproductive and respiratory syndrome virus (PRRSV) infections and vaccinations needs to be altered so that better protection is afforded against both homologous and heterologous challenges by this pathogen. To address this problem, real-time gene expression assays were coupled with cytokine Elispot and protein analyses to assess the nature of the anti-PRRSV response of pigs immunized with modified live virus (MLV) vaccine. Although T helper 1 (Th1) immunity was elicited in all vaccinated animals, as evidenced by the genesis of PRRSV-specific interferon-gamma secreting cells (IFNG SC), the overall extent of the memory response was variable and generally weak. Peripheral blood mononuclear cells (PBMC) isolated from these pigs responded to PRRSV exposure with a limited increase in their expression of the Th1 immune markers, IFNG, tumor necrosis factor-alpha and interleukin-15 (IL15), and a reduction in the quantity of mRNAs encoding the innate and inflammatory proteins, IL1B, IL8 and IFNA. Efforts to enhance Th1 immunity, by utilizing an expression plasmid encoding porcine IFNA (pINA) as an adjuvant, resulted in a temporary increase in the frequency of PRRSV-specific IFNG SC but only minor changes overall in the expression of Th1 associated cytokine or innate immune marker mRNA by virus-stimulated PBMC. Administration of pINA, however, did correlate with decreased IL1B secretion by cultured, unstimulated PBMC but had no effect on their ability to release IFNG. Thus, while exogenous addition of IFNA during PRRSV vaccination has an impact on the development of a Th1 immune response, other alterations will be required for substantial boosting of virus-specific protection.

Cellular Immune Responses in the Lungs of Pigs Infectedin Uterowith PRRSV: An Immunohistochemical Study

The cellular response in the lungs of pigs transplacentally infected with porcine reproductive and respiratory syndrome virus (PRRSV) was examined by immunohistochemistry. Double staining for the T-cell marker antigen CD3 and PRRSV demonstrated that the appearance and distribution of T-cells homing to the lungs of infected pigs correlated well with the presence and location of virusinfected cells. Single stainings showed that cells positive for the CD2 and CD8 antigen were almost as numerous in pneumonic lesions as CD3 positive cells whereas cells expressing the CD4 antigen were rare. The morphology and the distribution pattern of the CD2 and CD8 positive cells were similar to that of the CD3 positive cells suggesting coexpression of all three antigens within the majority of the recruited T-lymphocytes. The presence of cells consistent with the phenotype of cytotoxic T-lymphocytes (CTL) close to virus infected macrophages strongly indicate an important role of CTLs in the PRRSV-specific pulmonary immune response.

Interleukin-10, interleukin-12, and interferon-gamma levels in the respiratory tract following mycoplasma hyopneumoniae and PRRSV infection in pigs

The cytokine profile associated with either a T helper 1 (Th1) or Th2 response in a porcine respiratory disease model was assessed by measuring IL-12, IL-10 and IFN-gamma using RT-PCR and ELISA, respectively. IL-10, IL-12, and IFN-gamma levels in pulmonary alveolar macrophages and bronchial lavage fluid were increased in pigs infected with porcine reproductive and respiratory syndrome virus (PRRSV), Mycoplasma hyopneumoniae, or both pathogens. At 10 days post-infection (DPI), both IL-10 and IL-12 mRNA levels were increased in both groups infected with PRRSV. The IL-12 levels were increased in pigs infected with both pathogens and IFN-gamma protein levels were increased in pigs infected with PRRSV alone and only numerically increased in the dual infection. At 28 DPI, IL-12 mRNA levels and IL-10 protein levels were increased in all infected groups. The mRNA level of IL-12 remained elevated in the group infected with both pathogens at 42 DPI. Production of IFN-gamma did not appear to be closely correlated with elimination of virus from the respiratory tract. However, when the virus existed in the lung, the local IFN-gamma production appeared to increase. Although IL-12 mRNA levels were significantly elevated in the pigs infected with both pathogens, the increased protein levels of IL-12 may compromise the immune system's ability to clear PRRSV from the lung. This could explain the prolonged presence of PRRSV, IFN-gamma production and the increased pneumonia observed in the lungs of dual-infected pigs. The increased levels of cytokines associated with both Th1 and Th2 responses in the respiratory tract of pigs infected with PRRSV and M. hyopneumoniae provides valuable information on the pathogenesis of these diseases.

Upregulation of interleukin-10 gene expression in the leukocytes of pigs infected with porcine reproductive and respiratory syndrome virus

Recent studies suggest that porcine reproductive and respiratory syndrome virus (PRRSV) may have immunomodulatory effects on the host immune system by upregulating interleukin (IL)-10 gene expression. To determine the effect of PRRSV on porcine cytokine gene expression in vivo, we infected pigs with either the European or North American strain of PRRSV and monitored cytokine gene expression in peripheral blood mononuclear cells (PBMC) and bronchoalveolar lavage cells (BALC) using a multiplex PCR assay. Our results showed that both European and North American strains of PRRSV significantly upregulated IL-10 gene expression in PBMC of infected pigs from 5 days post-infection (p.i.). In addition, upregulation of IL-10 and interferon (IFN)-gamma gene expression was observed in BALC starting from 9 days p.i. The upregulation of cytokine gene expression in BALC was observed concurrent with an increased percentage of lymphocytes in the BALC population, suggesting a role for peripheral leukocytes in cytokine production in lungs. Our results showed that PRRSV infection resulted in an upregulation of IL-10 gene expression in vivo and that both European and North American strains induced comparable levels of IL-10 gene expression in the infected pigs, despite differences in the clinical signs. Our data support the notion that induction of IL-10 production may be one of the strategies used by PRRSV to modulate the host's immune responses, and this may contribute to the unique clinical picture observed following PRRSV infection.