Impact of genotype 1 and 2 of porcine reproductive and respiratory syndrome viruses on interferon-α responses by plasmacytoid dendritic cells

Plasmacytoid dendritic cell sensing of African swine fever virus-infected macrophages results in STING-dependent robust interferon-α production

While several African swine fever virus (ASFV)-encoded proteins potently interfere with the cGAS-STING (cyclic GMP-AMP synthetase-stimulator of interferon genes) pathway at different levels to suppress interferon (IFN) type I production in infected macrophages, systemic IFN-α is induced during the early stages of AFSV infection in pigs. The present study elucidates a mechanism by which such responses can be triggered, at least in vitro. We demonstrate that infection of monocyte-derived macrophages (MDMs) by ASFV genotype 2 strains is highly efficient but immunologically silent with respect to IFN type I, IFN-stimulated gene induction, and tumor necrosis factor production. Additionally, ASFV does not directly activate plasmacytoid dendritic cells (pDCs). However, coculturing pDCs with ASFV-infected MDMs results in a strong pDC response characterized by high levels of IFN-α and tumor necrosis factor. IFN type I, in turn, promoted interleukin-1 receptor antagonist production by macrophages. Similar to the sensing of infected cells by other viruses, pDC activation required integrin-mediated cognate interactions with ASFV-infected MDMs to form an interferogenic synapse. Inhibitor studies indicated that the activation of pDCs requires the STING pathway and the formation of gap junctions. While IL-4-polarized macrophages showed increased susceptibility, IFN-γ-polarized ASFV-infected macrophages induced higher pDC activation. Pretreatment of pDCs with IFN-β and IFN-γ also enhanced IFN-α production in response to ASFV-infected macrophages, highlighting the influence of the immunological microenvironment. These findings suggest that the IFN-α detected during ASFV infection in pigs may be a result of pDC sensing ASFV-infected macrophages.

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.

The scene of lung pathology during PRRSV-1 infection

Porcine reproductive and respiratory syndrome (PRRS) is one of the most economically important infectious diseases for the pig industry worldwide. The disease was firstly reported in 1987 and became endemic in many countries. Since then, outbreaks caused by strains of high virulence have been reported several times in Asia, America and Europe. Interstitial pneumonia, microscopically characterised by thickened alveolar septa, is the hallmark lesion of PRRS. However, suppurative bronchopneumonia and proliferative and necrotising pneumonia are also observed, particularly when a virulent strain is involved. This raises the question of whether the infection by certain strains results in an overstimulation of the proinflammatory response and whether there is some degree of correlation between the strain involved and a particular pattern of lung injury. Thus, it is of interest to know how the inflammatory response is modulated in these cases due to the interplay between virus and host factors. This review provides an overview of the macroscopic, microscopic, and molecular pathology of PRRSV-1 strains in the lung, emphasising the differences between strains of different virulence.

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.

Antibody-dependent enhancement of porcine reproductive and respiratory syndrome virus infection downregulates the levels of interferon-gamma/lambdas in porcine alveolar macrophages in vitro

Fc gamma receptor-mediated antibody-dependent enhancement (ADE) can promote virus invasion of target cells, sometimes exacerbating the severity of the disease. ADE may be an enormous hurdle to developing efficacious vaccines for certain human and animal viruses. ADE of porcine reproductive and respiratory syndrome virus (PRRSV) infection has been demonstrated in vivo and in vitro. However, the effect of PRRSV-ADE infection on the natural antiviral immunity of the host cells is yet to be well investigated. Specifically, whether the ADE of PRRSV infection affects the levels of type II (interferon-gamma, IFN-γ) and III (interferon-lambdas, IFN-λs) interferons (IFNs) remains unclear. In this study, our results showed that PRRSV significantly induced the secretion of IFN-γ, IFN-λ1, IFN-λ3, and IFN-λ4 in porcine alveolar macrophages (PAMs) in early infection, and weakly inhibited the production of IFN-γ, IFN-λ1, IFN-λ3, and IFN-λ4 in PAMs in late infection. Simultaneously, PRRSV infection significantly increased the transcription of interferon-stimulated gene 15 (ISG15), ISG56, and 2′, 5′-oligoadenylate synthetase 2 (OAS2) in PAMs. In addition, our results showed that PRRSV infection in PAMs via the ADE pathway not only significantly decreased the synthesis of IFN-γ, IFN-λ1, IFN-λ3, and IFN-λ4 but also significantly enhanced the generation of transforming growth factor-beta1 (TGF-β1). Our results also showed that the ADE of PRRSV infection significantly reduced the mRNAs of ISG15, ISG56, and OAS2 in PAMs. In conclusion, our studies indicated that PRRSV-ADE infection suppressed innate antiviral response by downregulating the levels of type II and III IFNs, hence facilitating viral replication in PAMs in vitro. The ADE mechanism demonstrated in the present study furthered our understanding of persistent pathogenesis following PRRSV infection mediated by antibodies.

Porcine Plasmacytoid Dendritic Cells Are Unique in Their Expression of a Functional NKp46 Receptor

The activating receptor NKp46 shows a unique expression pattern on porcine leukocytes. We showed already that in swine not all NK cells express NKp46 and that CD3⁺NKp46⁺ lymphocytes form a T-cell subset with unique functional properties. Here we demonstrate the expression of NKp46 on CD4^highCD14^-CD172a⁺ porcine plasmacytoid dendritic cells (pDCs). Multicolor flow cytometry analyses revealed that the vast majority of porcine pDCs (94.2% ± 4) express NKp46 ex vivo and have an increased expression on the single-cell level compared to NK cells. FSC/SSC^highCD4^highNKp46⁺ cells produced high levels of IFN-α after CpG ODN 2216 stimulation, a hallmark of pDC function. Following receptor triggering with plate-bound monoclonal antibodies against NKp46, phosphorylation of signaling molecules downstream of NKp46 was analyzed in pDCs and NK cells. Comparable to NK cells, NKp46 triggering led to an upregulation of the phosphorylated ribosomal protein S6 (pS6) in pDCs, indicating an active signaling pathway of NKp46 in porcine pDCs. Nevertheless, a defined effector function of the NK-associated receptor on porcine pDCs could not be demonstrated yet. NKp46-mediated cytotoxicity, as shown for NK cells, does not seem to occur, as NKp46⁺ pDCs did not express perforin. Yet, NKp46 triggering seems to contribute to cytokine production in porcine pDCs, as induction of TNF-α was observed in a small pDC subset after NKp46 cross-linking. To our knowledge, this is the first report on NKp46 expression on pDCs in a mammalian species, showing that this receptor contributes to pDC activation and function.

Systems Immunology Analyses Following Porcine Respiratory and Reproductive Syndrome Virus Infection and Vaccination

This study was initiated to better understand the nature of innate immune responses and the relatively weak and delayed immune response against porcine reproductive and respiratory syndrome virus (PRRSV). Following modified live virus (MLV) vaccination or infection with two PRRSV-2 strains, we analyzed the transcriptome of peripheral blood mononuclear cells collected before and at three and seven days after vaccination or infection. We used blood transcriptional modules (BTMs)-based gene set enrichment analyses. BTMs related to innate immune processes were upregulated by PRRSV-2 strains but downregulated by MLV. In contrast, BTMs related to adaptive immune responses, in particular T cells and cell cycle, were downregulated by PRRSV-2 but upregulated by MLV. In addition, we found differences between the PRRSV strains. Only the more virulent strain induced a strong platelet activation, dendritic cell activation, interferon type I and plasma cell responses. We also calculated the correlations of BTM with the neutralizing antibody and the T-cell responses. Early downregulation (day 0-3) of dendritic cell and B-cell BTM correlated to both CD4 and CD8 T-cell responses. Furthermore, a late (day 3-7) upregulation of interferon type I modules strongly correlated to helper and regulatory T-cell responses, while inflammatory BTM upregulation correlated more to CD8 T-cell responses. BTM related to T cells had positive correlations at three days but negative associations at seven days post-infection. Taken together, this work contributes to resolve the complexity of the innate and adaptive immune responses against PRRSV and indicates a fundamentally different immune response to the less immunogenic MLV compared to field strains which induced robust adaptive immune responses. The identified correlates of T-cell responses will facilitate a rational approach to improve the immunogenicity of MLV.

The Swine IFN System in Viral Infections: Major Advances and Translational Prospects

Interferons (IFNs) are a family of cytokines that play a pivotal role in orchestrating the innate immune response during viral infections, thus representing the first line of defense in the host. After binding to their respective receptors, they are able to elicit a plethora of biological activities, by initiating signaling cascades which lead to the transcription of genes involved in antiviral, anti-inflammatory, immunomodulatory and antitumoral effector mechanisms. In hindsight, it is not surprising that viruses have evolved multiple IFN escape strategies toward efficient replication in the host. Hence, in order to achieve insight into preventive and treatment strategies, it is essential to explore the mechanisms underlying the IFN response to viral infections and the constraints thereof. Accordingly, this review is focused on three RNA and three DNA viruses of major importance in the swine farming sector, aiming to provide essential data as to how the IFN system modulates the antiviral immune response, and is affected by diverse, virus-driven, immune escape mechanisms.

Reappraisal of PRRS Immune Control Strategies: The Way Forward

The control of porcine reproductive and respiratory syndrome (PRRS) is still a major issue worldwide in the pig farming sector. Despite extensive research efforts and the practical experience gained so far, the syndrome still severely affects farmed pigs worldwide and challenges established beliefs in veterinary virology and immunology. The clinical and economic repercussions of PRRS are based on concomitant, additive features of the virus pathogenicity, host susceptibility, and the influence of environmental, microbial, and non-microbial stressors. This makes a case for integrated, multi-disciplinary research efforts, in which the three types of contributing factors are critically evaluated toward the development of successful disease control strategies. These efforts could be significantly eased by the definition of reliable markers of disease risk and virus pathogenicity. As for the host's susceptibility to PRRSV infection and disease onset, the roles of both the innate and adaptive immune responses are still ill-defined. In particular, the overt discrepancy between passive and active immunity and the uncertain role of adaptive immunity vis-à-vis established PRRSV infection should prompt the scientific community to develop novel research schemes, in which apparently divergent and contradictory findings could be reconciled and eventually brought into a satisfactory conceptual framework.

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

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

Interaction of Type 1 Porcine Reproductive and Respiratory Syndrome Virus With In Vitro Derived Conventional Dendritic Cells

The present study delineates the interaction of a typical PRRSV1.1 isolate 3267 (moderate virulence) with in vitro derived pig conventional dendritic cells, cDC1, cDC2, and a CD14⁺ population (designated as CD14⁺ DCs). cDC1 and cDC2 were not susceptible to 3267 infection, but a fraction of CD14⁺ DCs were infected. After exposure to the virus, all three DC types remained immature as determined by no increase of maturation molecules (MHC-I, MHC-II, CD80/86, CCR7), no release of cytokines, no modification of antigen presentation abilities, and no alteration of endocytic/phagocytic capabilities. However, when infected MARC-145 cells were used as a source of viral antigens, cDC2 and CD14⁺ DCs showed a significant increase in the expression of maturation molecules and substantial release of cytokines, notably IL-12/IL-23p40 (by both DC types) and IL-10 (by CD14⁺ DCs). To address the impact of PRRSV1 3267 on TLR3- and TLR7-mediated activation, cDC1, cDC2, and CD14⁺ DCs were inoculated by the virus (live or UV-inactivated) for 6 h prior to or simultaneously with the addition of poly I:C (TLR3 ligand) or gardiquimod (TLR7 ligand; not used for cDC1). Compared with using TLR ligand alone, combination with the virus did not result in any alteration to the maturation markers on all DC types but changed the cytokine response to either TLR3 or TLR7 ligand. Pre-exposure of cDC2 or CD14⁺ DCs to the live virus resulted in an increased production of IFN-α upon poly I:C stimulation, while pre-exposure to UV-inactivated virus tended to enhance the release of IL-10 upon gardiquimod stimulation. Simultaneous addition of the live virus and the TLR ligand either had no effect (mainly in cDC2) or impaired most of the cytokine release after gardiquimod stimulation (in CD14⁺ DCs). When used as antigen presenting cells, cDC2 pre-inoculated by the live virus before addition of gardiquimod impaired the proliferation of CD4^–CD8^– T cells. In the case of CD14⁺ DCs, pre-exposure to the live virus or simultaneously added with TLR3 or TLR7 ligand largely decreased the proliferation of CD4^–CD8⁺ and CD4^–CD8⁺ T-cell subsets. For cDC1, no significant changes were observed in cytokine responses or T-cell proliferation after poly I:C stimulation. Of note, cDC1 had a short life during in vitro culturing, for which the results obtained might be biased. Overall, exposure to PRRSV1 did not induce maturation of cDC1, cDC2, or CD14⁺ DCs, but modified TLR3 and TLR7-associated responses (except for cDC1), which may affect the development of adaptive immunity during PRRSV1 infection. Moreover, the sensing of infected cells was different from that of the free virus.

Activation of activating Fc gamma receptors down-regulates the levels of interferon β, interferon γ and interferon λ1 in porcine alveolar macrophages during PRRSV infection

Porcine activating Fc gamma receptors (FcγRI and FcγRIII) have been cloned and characterized for many years. However, their roles in interferon (IFN) antiviral immune response to porcine reproductive and respiratory syndrome virus (PRRSV) infection have not yet been investigated extensively. In this study, PRRSV infection assay showed that PRRSV increased significantly the transcription of IFN-β, IFN-γ and IFN-λ1 in porcine alveolar macrophages (PAMs) in early infection and decreased significantly the transcription of IFN-β, IFN-γ and IFN-λ1 in PAMs in late infection. Activation assay showed that specific activation of FcγRI or FcγRIII in PAMs decreased significantly the transcription of IFN-β, IFN-γ and IFN-λ1 and increased significantly the transcription of transforming growth factor β1 (TGF-β1). PRRSV infection assay mediated by FcγRI and FcγRIII showed that specific activation of FcγRI or FcγRIII in PAMs during PRRSV infection decreased significantly the transcription of IFN-β, IFN-γ and IFN-λ1, but increased significantly the transcription of TGF-β1 and enhanced significantly viral replication. In conclusion, our studies suggested that activating FcγR signaling inhibited the transcriptional levels of IFN-β, IFN-γ and IFN-λ1 in PAMs in response to PRRSV infection.

Cellular Innate Immunity against PRRSV and Swine Influenza Viruses

Porcine respiratory disease complex (PRDC) is a polymicrobial syndrome that results from a combination of infectious agents, such as environmental stressors, population size, management strategies, age, and genetics. PRDC results in reduced performance as well as increased mortality rates and production costs in the pig industry worldwide. This review focuses on the interactions of two enveloped RNA viruses—porcine reproductive and respiratory syndrome virus (PRRSV) and swine influenza virus (SwIV)—as major etiological agents that contribute to PRDC within the porcine cellular innate immunity during infection. The innate immune system of the porcine lung includes alveolar and parenchymal/interstitial macrophages, neutrophils (PMN), conventional dendritic cells (DC) and plasmacytoid DC, natural killer cells, and γδ T cells, thus the in vitro and in vivo interactions between those cells and PRRSV and SwIV are reviewed. Likewise, the few studies regarding PRRSV-SwIV co-infection are illustrated together with the different modulation mechanisms that are induced by the two viruses. Alterations in responses by natural killer (NK), PMN, or γδ T cells have not received much attention within the scientific community as their counterpart antigen-presenting cells and there are numerous gaps in the knowledge regarding the role of those cells in both infections. This review will help in paving the way for future directions in PRRSV and SwIV research and enhancing the understanding of the innate mechanisms that are involved during infection with these viruses.

In vitro characterization of PRRSV isolates with different in vivo virulence using monocyte-derived macrophages

The recent emergence of highly pathogenic porcine reproductive and respiratory syndrome virus 1 (PRRSV-1) strains has caused severe economic losses. The biological elements defining virulence and pathogenicity are still unclear. In vitro characteristics using natural target cells of PRRSV provide important information to understand the basis of virulence at the cellular level, and provide a mean to reduce animal experimentations to achieve this goal. Here, we compared PRRSV strains from two geographically different regions, with varying in vivo characteristics, in terms of their interactions with monocyte-derived macrophages (MDMs). The strains included Lena and BOR59 from Belarus, and ILI6 from Russia, as well as PR11 and PR40, both from Italy. As a reference, we used a cell culture-adapted version of Lelystad, LVP. MDMs were pre-treated with IFNγ, IL-4 or IFNβ, in order to understand responses in polarized and antiviral MDMs. In general, independent of the geographical origin, the strains with high virulence infected a higher percentage of MDMs and replicated to higher titers. These virulence-dependent differences were most pronounced when the MDMs had been treated with IFNβ. Differentiation between intermediate and low virulent PRRSV was difficult, due to variations between different experiments, but LVP differed clearly from all field strains. IFNα and IL-10 were not detected in any experiment, but PR40 induced TNF and IL-1β. Taken together, these results validate the MDM model to understand pathogenicity factors of PRRSV and confirm the importance of the escape from type I and II IFN-mediated effects for PRRSV virulence.

Key Gaps in the Knowledge of the Porcine Respiratory Reproductive Syndrome Virus (PRRSV)

The porcine reproductive and respiratory syndrome virus (PRRSV) is one of the most important swine diseases in the world. It is causing an enormous economic burden due to reproductive failure in sows and a complex respiratory syndrome in pigs of all ages, with mortality varying from 2 to 100% in the most extreme cases of emergent highly pathogenic strains. PRRSV displays complex interactions with the immune system and a high mutation rate, making the development, and implementation of control strategies a major challenge. In this review, the biology of the virus will be addressed focusing on newly discovered functions of non-structural proteins and novel dissemination mechanisms. Secondly, the role of different cell types and viral proteins will be reviewed in natural and vaccine-induced immune response together with the role of different immune evasion mechanisms focusing on those gaps of knowledge that are critical to generate more efficacious vaccines. Finally, novel strategies for antigen discovery and vaccine development will be discussed, in particular the use of exosomes (extracellular vesicles of endocytic origin). As nanocarriers of lipids, proteins and nucleic acids, exosomes have potential effects on cell activation, modulation of immune responses and antigen presentation. Thus, representing a novel vaccination approach against this devastating disease.

In vitro immune responses of porcine alveolar macrophages reflect host immune responses against porcine reproductive and respiratory syndrome viruses

BACKGROUND

Currently, an in vitro immunogenicity screening system for the immunological assessment of potential porcine reproductive and respiratory syndrome virus (PRRSV) vaccine candidates is highly desired. Thus, in the present study, two genetically divergent PRRSVs were characterized in vitro and in vivo to identify an in vitro system and immunological markers that predict the host immune response. Porcine alveolar macrophages (PAMs) and peripheral blood mononuclear cells (PBMCs) collected from PRRSV-negative pigs were used for in vitro immunological evaluation, and the response of these cells to VR2332c or JA142c were compared with those elicited in pigs challenged with the same viruses.

RESULTS

Compared with VR2332c or mock infection, JA142c induced increased levels of type I interferons and pro-inflammatory cytokines (TNF-α, IL-1α/β, IL-6, IL-8, and IL-12) in PAMs, and these elevated levels were comparable to the cytokine induction observed in PRRSV-challenged pigs. Furthermore, significantly greater numbers of activated CD4⁺ T cells, type I helper T cells, cytotoxic T cells and total IFN-γ⁺ cells were observed in JA142c-challenged pigs than in VR2332c- or mock-challenged pigs.

CONCLUSIONS

Based on these results, the innate immune response patterns (particularly IFN-α, TNF-α and IL-12) to specific PRRSV strains in PAMs might reflect those elicited by the same viruses in pigs.

Porcine Reproductive and Respiratory Syndrome Virus Type 1.3 Lena Triggers Conventional Dendritic Cells 1 Activation and T Helper 1 Immune Response Without Infecting Dendritic Cells

Porcine Reproductive and Respiratory Syndrome virus (PRRSV) is an arterivirus responsible for highly contagious infection and huge economic losses in pig industry. Two species, PRRSV-1 and PRRSV-2 are distinguished, PRRSV-1 being more prevalent in Europe. PRRSV-1 can further be divided in subtypes. PRRSV-1.3 such as Lena are more pathogenic than PRRSV-1.1 such as Lelystad or Flanders13. PRRSV-1.3 viruses trigger a higher Th1 response than PRRSV-1.1, although the role of the cellular immune response in PRRSV clearance remains ill defined. The pathogenicity as well as the T cell response inductions may be differentially impacted according to the capacity of the virus strain to infect and/or activate DCs. However, the interactions of PRRSV with in vivo-differentiated-DC subtypes such as conventional DC1 (cDC1), cDC2, and monocyte-derived DCs (moDC) have not been thoroughly investigated. Here, DC subpopulations from Lena in vivo infected pigs were analyzed for viral genome detection. This experiment demonstrates that cDC1, cDC2, and moDC are not infected in vivo by Lena. Analysis of DC cytokines production revealed that cDC1 are clearly activated in vivo by Lena. In vitro comparison of 3 Europeans strains revealed no infection of the cDC1 and cDC2 and no or little infection of moDC with Lena, whereas the two PRRSV-1.1 strains infect none of the 3 DC subtypes. In vitro investigation of T helper polarization and cytokines production demonstrate that Lena induces a higher Th1 polarization and IFNγ secretion than FL13 and LV. Altogether, this work suggests an activation of cDC1 by Lena associated with a Th1 immune response polarization.

Response of the cDC1 and cDC2 subtypes of tracheal dendritic cells to porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is the most important disease affecting the swine industry worldwide. Although monocytes and macrophages, especially tissue-resident and alveolar macrophages, are the primary target of PRRSV, monocyte- and bone marrow-derived dendritic cells (DCs) are also susceptible to PRRSV infection. It has been shown that lung DCs cannot be infected with PRRSV, but the response and susceptibility of bona fide conventional DC subtypes (cDCs; cDC1 and cDC2) is unknown. In this work, evaluation of the response of tracheal cDC1 and cDC2 subsets to PRRSV revealed differential cytokine expression, whereby cDC1 subsets expressed higher levels of IFN-α and cDC2 subsets more IL-10. Toll-like receptors (TLRs) were also affected: cDC2 cells induced greater upregulation of TLR2 and TLR4, and CD163⁺ cells showed TLR3 upregulation. However, we could not demonstrate under our experimental conditions that cDC1 and cCD2 subsets are susceptible to PRRSV infection. Our findings show the effects of PRRSV on cDC1 and cDC2 subsets and that these cells were not infected by PRRSV.

Porcine reproductive and respiratory syndrome virus suppresses post-transcriptionally the protein expression of IFN-β by upregulating cellular microRNAs in porcine alveolar macrophages in vitro

Porcine reproductive and respiratory syndrome virus (PRRSV) has been recognized to inhibit the response of type I interferon (IFN) both in vivo and in vitro. However, the post-transcriptional mechanism by which PRRSV suppresses type I IFN induction in virus-infected host cells remains unclear. The present study first demonstrated that PRRSV inhibited post-transcriptionally the protein induction of IFN-β in primary porcine alveolar macrophages (PAMs) during early infection, and the inhibition effect mediated by the Chinese highly pathogenic (HP)-PRRSV was stronger. Next, we analyzed the cellular microRNA (miRNA)-modulated protein expression of porcine IFN-β by dual firefly/Renilla luciferase reporter assay, transfection of miRNA mimics and inhibitor assay and polyinosinic-polycytidylic acid (poly I:C) treatment of PAMs, showing that porcine miRNAs including let-7b, miR-26a, miR-34a and miR-145 are able to inhibit IFN-β protein expression in primary PAMs by directly targeting sequences within the porcine IFN-β 3'UTR locating at 160-181, 9-31, 27-47 and 12-32 bp, respectively. Finally, we confirmed that let-7b, miR-26a, miR-34a and miR-145, were upregulated in PRRSV-infected PAMs early in vitro, and the expression level of these miRNAs in HP-PRRSV JXwn06-infected PAMs were higher than those in low pathogenic PRRSV HB-1/3.9-infected PAMs. The endogenous cellular miRNA-mediated inhibition of IFN-β induction in PRRSV-infected PAMs early could be relieved by miRNA antagonists. Taken together, our findings suggest for the first time that PRRSV can suppress post-transcriptionally protein expression of IFN-β by upregulating cellular miRNAs in PAMs in vitro, providing novel insight into mechanisms in relation to the PRRSV-mediated immunomodulation of porcine innate immunity.

Improved Vaccine against PRRSV: Current Progress and Future Perspective

Porcine reproductive and respiratory syndrome virus (PRRSV), one of the most economically significant pathogens worldwide, has caused numerous outbreaks during the past 30 years. PRRSV infection causes reproductive failure in sows and respiratory disease in growing and finishing pigs, leading to huge economic losses for the swine industry. This impact has become even more significant with the recent emergence of highly pathogenic PRRSV strains from China, further exacerbating global food security. Since new PRRSV variants are constantly emerging from outbreaks, current strategies for controlling PRRSV have been largely inadequate, even though our understanding of PRRSV virology, evolution and host immune response has been rapidly expanding. Meanwhile, practical experience has revealed numerous safety and efficacy concerns for currently licensed vaccines, such as shedding of modified live virus (MLV), reversion to virulence, recombination between field strains and MLV and failure to elicit protective immunity against heterogeneous virus. Therefore, an effective vaccine against PRRSV infection is urgently needed. Here, we systematically review recent advances in PRRSV vaccine development. Antigenic variations resulting from PRRSV evolution, identification of neutralizing epitopes for heterogeneous isolates, broad neutralizing antibodies against PRRSV, chimeric virus generated by reverse genetics, and novel PRRSV strains with interferon-inducing phenotype will be discussed in detail. Moreover, techniques that could potentially transform current MLV vaccines into a superior vaccine will receive special emphasis, as will new insights for future PRRSV vaccine development. Ultimately, improved PRRSV vaccines may overcome the disadvantages of current vaccines and minimize the PRRS impact to the swine industry.

The viral innate immune antagonism and an alternative vaccine design for PRRS virus

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PRRS virus has evolved to suppress the antiviral innate immunity during infection.

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Type I interferons are potent antiviral cytokines and function to stimulate the adaptive immune responses.

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Six viral proteins have been identified as interferon antagonists and characterized for their molecular actions.

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Interferon antagonism-negative viruses are attenuated and have been proven induce protective immunity.

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Interferon suppression-negative PRRS virus may serve as an alternative vaccine for PRRS.

Porcine reproductive and respiratory syndrome (PRRS) remains one of the most economically significant diseases in the swine industry worldwide. The current vaccines are less satisfactory to confer protections from heterologous infections and long-term persistence, and the need for better vaccines are urgent. The immunological hallmarks in PRRSV-infected pigs include the unusually poor production of type I interferons (IFNs-α/β) and the aberrant and delayed adaptive immune responses, indicating that PRRSV has the ability to suppress both innate and adaptive immune responses in the host. Type I IFNs are the potent antiviral cytokines and recent studies reveal their pleiotropic functions in the priming of expansion and maturation of adaptive immunity. Thus, IFN antagonism-negative PRRSV is hypothesized to be attenuated and to build effective and broad- spectrum innate and adaptive immune responses in pigs. Such vaccines are promising alternatives to traditional vaccines for PRRSV.

Sensing of Porcine Reproductive and Respiratory Syndrome Virus-Infected Macrophages by Plasmacytoid Dendritic Cells

Porcine reproductive and respiratory syndrome virus (PRRSV) represents a macrophage (MØ)-tropic virus which is unable to induce interferon (IFN) type I in its target cells. Nevertheless, infected pigs show a short but prominent systemic IFN alpha (IFN-α) response. A possible explanation for this discrepancy is the ability of plasmacytoid dendritic cells (pDC) to produce IFN-α in response to free PRRSV virions, independent of infection. Here, we show that the highly pathogenic PRRSV genotype 1 strain Lena is unique in not inducing IFN-α production in pDC, contrasting with systemic IFN-α responses found in infected pigs. We also demonstrate efficient pDC stimulation by PRRSV Lena-infected MØ, resulting in a higher IFN-α production than direct stimulation of pDC by PRRSV virions. This response was strain-independent, required integrin-mediated intercellular contact, intact actin filaments in the MØ and was partially inhibited by an inhibitor of neutral sphingomyelinase. Although infected MØ-derived exosomes stimulated pDC, an efficient delivery of the stimulatory component was dependent on a tight contact between pDC and the infected cells. In conclusion, with this mechanism the immune system can efficiently sense PRRSV, resulting in production of considerable quantities of IFN-α. This is adding complexity to the immunopathogenesis of PRRSV infections, as IFN-α should alert the immune system and initiate the induction of adaptive immune responses, a process known to be inefficient during infection of pigs.

Genome-wide analysis of the transcriptional response to porcine reproductive and respiratory syndrome virus infection at the maternal/fetal interface and in the fetus

Background

Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) infection of pregnant pigs can result in congenital infection and ultimately fetal death. Little is known about immune responses to infection at the maternal-fetal interface and in the fetus itself, or the molecular events behind virus transmission and disease progression in the fetus. To investigate these processes, RNA-sequencing of two sites, uterine endothelium with adherent placental tissue and fetal thymus, was performed 21 days post-challenge on four groups of fetuses selected from a large PRRSV challenge experiment of pregnant gilts: control (CON), uninfected (UNINF), infected (INF), and meconium-stained (MEC) (n = 12/group). Transcriptional analyses consisted of multiple contrasts between groups using two approaches: differential gene expression analysis and weighted gene co-expression network analysis (WGCNA). Biological functions, pathways, and regulators enriched for differentially expressed genes or module members were identified through functional annotation analyses. Expression data were validated by reverse transcription quantitative polymerase chain reaction (RTqPCR) carried out for 16 genes of interest.

Results

The immune response to infection in endometrium was mainly adaptive in nature, with the most upregulated genes functioning in either humoral or cell-mediated immunity. In contrast, the expression profile of infected fetal thymus revealed a predominantly innate immune response to infection, featuring the upregulation of genes regulated by type I interferon and pro-inflammatory cytokines. Fetal infection was associated with an increase in viral load coupled with a reduction in T cell signaling in the endometrium that could be due to PRRSV-controlled apoptosis of uninfected bystander cells. There was also evidence for a reduction in TWIST1 activity, a transcription factor involved in placental implantation and maturation, which could facilitate virus transmission or fetal pathology through dysregulation of placental function. Finally, results suggested that events within the fetus could also drive fetal pathology. Thymus samples of meconium-stained fetuses exhibited an increase in the expression of pro-inflammatory cytokine and granulocyte genes previously implicated in swine infectious disease pathology.

Conclusions

This study identified major differences in the response to PRRSV infection in the uterine endometrium and fetus at the gene expression level, and provides insight into the molecular basis of virus transmission and disease progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2720-4) contains supplementary material, which is available to authorized users.

Differences of immune responses between Tongcheng (Chinese local breed) and Large White pigs after artificial infection with highly pathogenic porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) is one of the severest infectious diseases of pigs throughout the world. Pigs of different breeds infected with PRRS virus (PRRSV) have been reported to vary in their immune responses. Here, the differences of immune responses to highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV) were investigated by artificially infecting Tongcheng (TC) pigs (a Chinese indigenous breed) and Large White (LW) pigs with PRRSV WUH3. Compared to LW pigs, TC pigs showed less severe symptoms and lower level of viral load. The routine blood test results indicated that TC pigs were relatively steady in terms of erythrocyte, leukocyte and platelet. Additionally, PRRSV infection induced higher IFN-γ activity in TC pigs, but stimulated an excessive level of IL-10 and IL-12p40 in LW pigs. Our study provides direct evidence that TC pigs have stronger resistance to early PRRSV infection than LW pigs, suggesting that the resistance of pigs to PRRSV is likely associated with breed differences.

C-reactive protein, haptoglobin and Pig-Major acute phase protein profiles of pigs infected experimentally by different isolates of porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV) is the etiologic agent of PRRS, one of the most important diseases in swine worldwide. In the present work, the effects of different PRRSV strains were tested on a piglet experimental model to study the induced acute phase response. For this purpose, pigs (n=15 for each group) were intranasally inoculated with one of five PRRSV strains (isolates EU10, 12, 17, 18 from genotype 1 and isolate JA-142 from genotype 2). The acute phase response was monitored by measuring acute phase proteins (APPs). Specifically, the serum concentration of haptoglobin (Hp), C-reactive protein (CRP) and Pig-Major Acute Protein (Pig-MAP) was determined at 0, 3, 6, 9, 12, 15, 18 and 21 days p.i. Clinical signs and growth performance were also monitored during the experiment. All animals became viremic after inoculation during the study period. The APP response was heterogeneous and dependent on the strain, being strains EU10, EU 18 and JA-142 those that induced the highest response and the strongest clinical signs. In general, Hp was the most sensitive biomarker for PRRSV infection, CRP behaved as moderate and Pig-MAP was the less responsive during the course of PRRSV experimental infection. Hp and CRP were significantly discriminatory between infected and control pigs, but not Pig-MAP.

Innate and adaptive immunity against Porcine Reproductive and Respiratory Syndrome Virus

Many highly effective vaccines have been produced against viruses whose virulent infection elicits strong and durable protective immunity. In these cases, characterization of immune effector mechanisms and identification of protective epitopes/immunogens has been informative for the development of successful vaccine programs. Diseases in which the immune system does not rapidly clear the acute infection and/or convalescent immunity does not provide highly effective protection against secondary challenge pose a major hurdle for clinicians and scientists. Porcine reproductive and respiratory syndrome virus (PRRSV) falls primarily into this category, though not entirely. PRRSV causes a prolonged infection, though the host eventually clears the virus. Neutralizing antibodies can provide passive protection when present prior to challenge, though infection can be controlled in the absence of detectable neutralizing antibodies. In addition, primed pigs (through natural exposure or vaccination with a modified-live vaccine) show some protection against secondary challenge. While peripheral PRRSV-specific T cell responses have been examined, their direct contribution to antibody-mediated immunity and viral clearance have not been fully elucidated. The innate immune response following PRRSV infection, particularly the antiviral type I interferon response, is meager, but when provided exogenously, IFN-α enhances PRRSV immunity and viral control. Overall, the quality of immunity induced by natural PRRSV infection is not ideal for informing vaccine development programs.

The epitopes necessary for protection may be identified through natural exposure or modified-live vaccines and subsequently applied to vaccine delivery platforms to accelerate induction of protective immunity following vaccination. Collectively, further work to identify protective B and T cell epitopes and mechanisms by which PRRSV eludes innate immunity will enhance our ability to develop more effective methods to control and eliminate PRRS disease.

Host-pathogen interactions during porcine reproductive and respiratory syndrome virus 1 infection of piglets

Porcine reproductive and respiratory syndrome (PRRS) is a major disease affecting pigs worldwide and resulting in considerable economic losses. While PRRS is a global phenomenon, the causative viruses PRRSV-1 (first detected in Europe) and PRRSV-2 (isolated in North America) are genetically and biologically distinct. In addition, the disease outcome is directly linked to co-infections associated with the porcine respiratory disease complex and the host response is variable between different breeds of pigs. It is therefore warranted when studying the pathogenesis of PRRS to consider each viral genotype separately and apply careful consideration to the disease model studied. We here review the respiratory pig model for PRRSV-1, with a focus on a recent set of studies conducted with carefully selected virus strains and pigs, which may serve as both a baseline and benchmark for future investigation.

Porcine reproductive and respiratory syndrome (PRRS): an immune dysregulatory pandemic

Porcine reproductive and respiratory disease syndrome (PRRS) is a viral pandemic that especially affects neonates within the “critical window” of immunological development. PRRS was recognized in 1987 and within a few years became pandemic causing an estimated yearly $600,000 economic loss in the USA with comparative losses in most other countries. The causative agent is a single-stranded, positive-sense enveloped arterivirus (PRRSV) that infects macrophages and plasmacytoid dendritic cells. Despite the discovery of PRRSV in 1991 and the publication of >2,000 articles, the control of PRRS is problematic. Despite the large volume of literature on this disease, the cellular and molecular mechanisms describing how PRRSV dysregulates the host immune system are poorly understood. We know that PRRSV suppresses innate immunity and causes abnormal B cell proliferation and repertoire development, often lymphopenia and thymic atrophy. The PRRSV genome is highly diverse, rapidly evolving but amenable to the generation of many mutants and chimeric viruses for experimental studies. PRRSV only replicates in swine which adds to the experimental difficulty since no inbred well-defined animal models are available. In this article, we summarize current knowledge and apply it toward developing a series of provocative and testable hypotheses to explain how PRRSV immunomodulates the porcine immune system with the goal of adding new perspectives on this disease.

Macrophage Polarization in Virus-Host Interactions

Macrophage involvement in viral infections and antiviral states is common. However, this involvement has not been well-studied in the paradigm of macrophage polarization, which typically has been categorized by the dichotomy of classical (M1) and alternative (M2) statuses. Recent studies have revealed the complexity of macrophage polarization in response to various cellular mediators and exogenous stimuli by adopting a multipolar view to revisit the differential process of macrophages, especially those re-polarized during viral infections. Here, through examination of viral infections targeting macrophages/monocytic cells, we focus on the direct involvement of macrophage polarization during viral infections. Type I and type III interferons (IFNs) are critical in regulation of viral pathogenesis and host antiviral infection; thus, we propose to incorporate IFN-mediated antiviral states into the framework of macrophage polarization. This view is supported by the multifunctional properties of type I IFNs, which potentially elicit and regulate both M1- and M2-polarization in addition to inducing the antiviral state, and by the discoveries of viral mechanisms to adapt and modulate macrophage polarization. Indeed, several recent studies have demonstrated effective prevention of viral diseases through manipulation of macrophage immune statuses.

Cytokine profiles in pregnant gilts experimentally infected with porcine reproductive and respiratory syndrome virus and relationships with viral load and fetal outcome

In spite of extensive research, immunologic control mechanisms against Porcine Reproductive and Respiratory Syndrome virus (PRRSv) remain poorly understood. Cytokine responses have been exhaustively studied in nursery pigs and show contradictory results. Since no detailed reports on cytokine responses to PRRSv in pregnant females exist, the objectives of this study were to compare host cytokine responses between PRRSv-infected and non-infected pregnant gilts, and to investigate relationships between cytokine levels in infected gilts and viral load or fetal mortality rate. Serum samples and supernatants of peripheral blood mononuclear cells (PBMC) either stimulated with PRRSv or phorbol myristate acetate/Ionomycin (PMA/Iono) were analyzed for cytokines/chemokines: interleukins (IL) 1-beta (IL1β), IL4, IL8, IL10, IL12, chemokine ligand 2 (CCL2), interferon alpha (IFNα) and interferon gamma (IFNγ). Three cytokines (IFNα, CCL2, IFNγ) in gilt serum differed significantly in inoculated versus control gilts over time. In supernatants of PRRSv stimulated PBMC from PRRSv-infected gilts, levels of IFNα were significantly decreased, while IL8 secretion was significantly increased. PRRSv infection altered the secretion of all measured cytokines, with the exception of IFNα, from PBMC after mitogen stimulation, indicating a possible immunomodulatory effect of PRRSv. IFNα, CCL2, and IFNγ in serum, and IFNγ in supernatants of PMA/Iono stimulated PBMC were significantly associated with viral load in tissues, serum or both. However, only IFNα in supernatants of PRRSv stimulated PBMC was significantly associated with fetal mortality rate. We conclude that of the eight cytokines tested in this study IFNα was the best indicator of viral load and severity of reproductive PRRSv infection.

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.

Modulation of innate immune signaling by nonstructural protein 1 (nsp1) in the family Arteriviridae

Arteriviruses infect immune cells and may cause persistence in infected hosts. Inefficient induction of pro-inflammatory cytokines and type I IFNs are observed during infection of this group of viruses, suggesting that they may have evolved to escape the host immune surveillance for efficient survival. Recent studies have identified viral proteins regulating the innate immune signaling, and among these, nsp1 (nonstructural protein 1) is the most potent IFN antagonist. For porcine reproductive and respiratory syndrome virus (PRRSV), individual subunits (nsp1α and nsp1β) of nsp1 suppress type I IFN production. In particular, PRRSV-nsp1α degrades CREB (cyclic AMP responsive element binding)-binding protein (CBP), a key component of the IFN enhanceosome, whereas PRRSV-nsp1β degrades karyopherin-α1 which is known to mediate the nuclear import of ISGF3 (interferon-stimulated gene factor 3). All individual subunits of nsp1 of PRRSV, equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV) appear to contain IFN suppressive activities. As with PRRSV-nsp1α, CBP degradation is evident by LDV-nsp1α and partly by SHFV-nsp1γ. This review summarizes the biogenesis and the role of individual subunits of nsp1 of arteriviruses for innate immune modulation.

Antagonizing interferon-mediated immune response by porcine reproductive and respiratory syndrome virus

Interferons (IFNs) are important components in innate immunity involved in the first line of defense to protect host against viral infection. Porcine reproductive and respiratory syndrome virus (PRRSV) leads to severe economic losses for swine industry since being first identified in early 1990s. PRRSV interplays with host IFN production and IFN-activated signaling, which may contribute to the delayed onset and low level of neutralizing antibodies, as well as weak cell-mediated immune response in infected pigs. PRRSV encodes several proteins that act as antagonists for the IFN signaling. In this review, we summarized the various strategies used by PRRSV to antagonize IFN production and thwart IFN-activated antiviral signaling, as well as the variable interference with IFN-mediated immune response by different PRRSV strains. Thorough understanding of the interaction between PRRSV and host innate immune response will facilitate elucidation of PRRSV pathogenesis and development of a better strategy to control PRRS.

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.