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

The role of plasmacytoid dendritic cells (pDCs) in immunity during viral infections and beyond

Type I and III interferons (IFNs) are essential for antiviral immunity and act through two different but complimentary pathways. First, IFNs activate intracellular antimicrobial programs by triggering the upregulation of a broad repertoire of viral restriction factors. Second, IFNs activate innate and adaptive immunity. Dysregulation of IFN production can lead to severe immune system dysfunction. It is thus crucial to identify and characterize the cellular sources of IFNs, their effects, and their regulation to promote their beneficial effects and limit their detrimental effects, which can depend on the nature of the infected or diseased tissues, as we will discuss. Plasmacytoid dendritic cells (pDCs) can produce large amounts of all IFN subtypes during viral infection. pDCs are resistant to infection by many different viruses, thus inhibiting the immune evasion mechanisms of viruses that target IFN production or their downstream responses. Therefore, pDCs are considered essential for the control of viral infections and the establishment of protective immunity. A thorough bibliographical survey showed that, in most viral infections, despite being major IFN producers, pDCs are actually dispensable for host resistance, which is achieved by multiple IFN sources depending on the tissue. Moreover, primary innate and adaptive antiviral immune responses are only transiently affected in the absence of pDCs. More surprisingly, pDCs and their IFNs can be detrimental in some viral infections or autoimmune diseases. This makes the conservation of pDCs during vertebrate evolution an enigma and thus raises outstanding questions about their role not only in viral infections but also in other diseases and under physiological conditions.

Macrophage phagocytosis of SARS-CoV-2-infected cells mediates potent plasmacytoid dendritic cell activation

Early and strong interferon type I (IFN-I) responses are usually associated with mild COVID-19 disease, whereas persistent or unregulated proinflammatory cytokine responses are associated with severe disease outcomes. Previous work suggested that monocyte-derived macrophages (MDMs) are resistant and unresponsive to SARS-CoV-2 infection. Here, we demonstrate that upon phagocytosis of SARS-CoV-2-infected cells, MDMs are activated and secrete IL-6 and TNF. Importantly, activated MDMs in turn mediate strong activation of plasmacytoid dendritic cells (pDCs), leading to the secretion of high levels of IFN-α and TNF. Furthermore, pDC activation promoted IL-6 production by MDMs. This kind of pDC activation was dependent on direct integrin-mediated cell‒cell contacts and involved stimulation of the TLR7 and STING signaling pathways. Overall, the present study describes a novel and potent pathway of pDC activation that is linked to the macrophage-mediated clearance of infected cells. These findings suggest that a high infection rate by SARS-CoV-2 may lead to exaggerated cytokine responses, which may contribute to tissue damage and severe disease.

Tracking Plasmacytoid Dendritic Cell Response to Physical Contact with Infected Cells

Dendritic cells (DCs) are key regulators of both innate and adaptive immunity via varied functions, including cytokine production and antigen presentation. Plasmacytoid DC (pDC) is a DC subset specialized in the production of type I and III interferons (IFNs). They are thus pivotal players of the host antiviral response during the acute phase of infection by genetically distant viruses. The pDC response is primarily triggered by the endolysosomal sensors Toll-like receptors, which recognize nucleic acids from pathogens. In some pathologic contexts, pDC response can also be triggered by host nucleic acids, hereby contributing to the pathogenesis of autoimmune diseases, such as, e.g., systemic lupus erythematosus. Importantly, recent in vitro studies from our laboratory and others uncovered that pDCs sense viral infections when a physical contact is established with infected cells. This specialized synapse-like feature enables a robust type I and III IFN secretion at the infected site. Therefore, this concentrated and confined response likely limits the correlated deleterious impacts of excessive cytokine production to the host, notably due to tissue damages. Here we provide a pipeline of methods for ex vivo studies of pDC antiviral functions, designed to address how pDC activation is regulated by cell-cell contact with virally infected cells and the current approaches enabling to decipher the underlying molecular events leading to an efficient antiviral response.

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.

Type I Interferon Induction and Exhaustion during Viral Infection: Plasmacytoid Dendritic Cells and Emerging COVID-19 Findings

Type I Interferons (IFN-I) are a family of potent antiviral cytokines that act through the direct restriction of viral replication and by enhancing antiviral immunity. However, these powerful cytokines are a caged lion, as excessive and sustained IFN-I production can drive immunopathology during infection, and aberrant IFN-I production is a feature of several types of autoimmunity. As specialized producers of IFN-I plasmacytoid (p), dendritic cells (DCs) can secrete superb quantities and a wide breadth of IFN-I isoforms immediately after infection or stimulation, and are the focus of this review. Notably, a few days after viral infection pDCs tune down their capacity for IFN-I production, producing less cytokines in response to both the ongoing infection and unrelated secondary stimulations. This process, hereby referred to as "pDC exhaustion", favors viral persistence and associates with reduced innate responses and increased susceptibility to secondary opportunistic infections. On the other hand, pDC exhaustion may be a compromise to avoid IFN-I driven immunopathology. In this review we reflect on the mechanisms that initially induce IFN-I and subsequently silence their production by pDCs during a viral infection. While these processes have been long studied across numerous viral infection models, the 2019 coronavirus disease (COVID-19) pandemic has brought their discussion back to the fore, and so we also discuss emerging results related to pDC-IFN-I production in the context of COVID-19.

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.

Human plasmacytoid dendritic cells mount a distinct antiviral response to virus-infected cells

Plasmacytoid dendritic cells (pDCs) can rapidly produce interferons and other soluble factors in response to extracellular viruses or virus mimics such as CpG-containing DNA. pDCs can also recognize live cells infected with certain RNA viruses, but the relevance and functional consequences of such recognition remain unclear. We studied the response of primary DCs to the prototypical persistent DNA virus, human cytomegalovirus (CMV). Human pDCs produced high amounts of type I interferon (IFN-I) when incubated with live CMV-infected fibroblasts but not with free CMV; the response involved integrin-mediated adhesion, transfer of DNA-containing virions to pDCs, and the recognition of DNA through TLR9. Compared with transient polyfunctional responses to CpG or free influenza virus, pDC response to CMV-infected cells was long-lasting, dominated by the production of IFN-I and IFN-III, and lacked diversification into functionally distinct populations. Similarly, pDC activation by influenza-infected lung epithelial cells was highly efficient, prolonged, and dominated by interferon production. Prolonged pDC activation by CMV-infected cells facilitated the activation of natural killer cells critical for CMV control. Last, patients with CMV viremia harbored phenotypically activated pDCs and increased circulating IFN-I and IFN-III. Thus, recognition of live infected cells is a mechanism of virus detection by pDCs that elicits a unique antiviral immune response.

Distinctive Cellular and Metabolic Reprogramming in Porcine Lung Mononuclear Phagocytes Infected With Type 1 PRRSV Strains

Porcine reproductive and respiratory syndrome (PRRS) has an extensive impact on pig production. The causative virus (PRRSV) is divided into two species, PRRSV-1 (European origin) and PRRSV-2 (North American origin). Within PRRSV-1, PRRSV-1.3 strains, such as Lena, are more pathogenic than PRRSV-1.1 strains, such as Flanders 13 (FL13). To date, the molecular interactions of PRRSV with primary lung mononuclear phagocyte (MNP) subtypes, including conventional dendritic cells types 1 (cDC1) and 2 (cDC2), monocyte-derived DCs (moDC), and pulmonary intravascular macrophages (PIM), have not been thoroughly investigated. Here, we analyze the transcriptome profiles of in vivo FL13-infected parenchymal MNP subpopulations and of in vitro FL13- and Lena-infected parenchymal MNP. The cell-specific expression profiles of in vivo sorted cells correlated with their murine counterparts (AM, cDC1, cDC2, moDC) with the exception of PIM. Both in vivo and in vitro, FL13 infection altered the expression of a low number of host genes, and in vitro infection with Lena confirmed the higher ability of this strain to modulate host response. Machine learning (ML) and gene set enrichment analysis (GSEA) unraveled additional relevant genes and pathways modulated by FL13 infection that were not identified by conventional analyses. GSEA increased the cellular pathways enriched in the FL13 data set, but ML allowed a more complete comprehension of functional profiles during FL13 in vitro infection. Data indicates that cellular reprogramming differs upon Lena and FL13 infection and that the latter might keep antiviral and inflammatory macrophage/DC functions silent. Although the slow replication kinetics of FL13 likely contribute to differences in cellular gene expression, the data suggest distinct mechanisms of interaction of the two viruses with the innate immune system during early infection.

No Evidence for a Role for Antibodies during Vaccination-Induced Enhancement of Porcine Reproductive and Respiratory Syndrome

Vaccination is one of the most important tools to protect pigs against infection with porcine reproductive and respiratory syndrome virus 1 (PRRSV-1). Although neutralizing antibodies are considered to represent an important mechanism of protective immunity, anti-PRRSV antibodies, in particular at subneutralizing concentrations, have also been reported to exacerbate PRRSV infection, probably through FcγR-mediated uptake of antibody-opsonized PRRSV, resulting in enhanced infection of, and replication in, target cells. Therefore, we investigated this pathway using sera from an animal experiment in which vaccine-mediated enhancement of clinical symptoms was observed. Three groups of six pigs were vaccinated with an inactivated PRRSV vaccine based on the PRRSV-1 subtype 3 strain Lena and challenged after a single or a prime-boost immunization protocol, or injected with PBS. We specifically tested if sera obtained from these animals can enhance macrophage infections, viral shedding, or cytokine release at different dilutions. Neither the presence of neutralizing antibodies nor general anti-PRRSV antibodies, mediated an enhanced infection, increased viral release or cytokine production by macrophages. Taken together, our data indicate that the exacerbated disease was not caused by antibodies.

Plasmacytoid Dendritic Cells and Infected Cells Form an Interferogenic Synapse Required for Antiviral Responses

Type I interferon (IFN-I) is critical for antiviral defense, and plasmacytoid dendritic cells (pDCs) are a predominant source of IFN-I during virus infection. pDC-mediated antiviral responses are stimulated upon physical contact with infected cells, during which immunostimulatory viral RNA is transferred to pDCs, leading to IFN production via the nucleic acid sensor TLR7. Using dengue, hepatitis C, and Zika viruses, we demonstrate that the contact site of pDCs with infected cells is a specialized platform we term the interferogenic synapse, which enables viral RNA transfer and antiviral responses. This synapse is formed via α_Lβ₂ integrin-ICAM-1 adhesion complexes and the recruitment of the actin network and endocytic machinery. TLR7 signaling in pDCs promotes interferogenic synapse establishment and provides feed-forward regulation, sustaining pDC contacts with infected cells. This interferogenic synapse may allow pDCs to scan infected cells and locally secrete IFN-I, thereby confining a potentially deleterious response.

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.

Plasmacytoid Dendritic Cells: Development, Regulation, and Function

Plasmacytoid dendritic cells (pDCs) are a unique sentinel cell type that can detect pathogen-derived nucleic acids and respond with rapid and massive production of type I interferon. This review summarizes our current understanding of pDC biology, including transcriptional regulation, heterogeneity, role in antiviral immune responses, and involvement in immune pathology, particularly in autoimmune diseases, immunodeficiency, and cancer. We also highlight the remaining gaps in our knowledge and important questions for the field, such as the molecular basis of unique interferon-producing capacity of pDCs. A better understanding of cell type-specific positive and negative control of pDC function should pave the way for translational applications focused on this immune cell type.

Monitoring of Interferon Response Triggered by Cells Infected by Hepatitis C Virus or Other Viruses Upon Cell-Cell Contact

Plasmacytoid dendritic cells (pDCs) constitute a unique DC subset specialized in rapid and massive secretion of cytokines, including type I interferon (i.e., IFNα and IFNβ), known to be pivotal for both innate immunity and the onset of adaptive response. The production of type I IFNs by pDCs is primarily induced by the recognition of viral nucleic acids through Toll-like receptor (TLR)-7 and -9 sensors located in the endolysosomal compartment. Importantly, in the context of hepatitis C virus (HCV) infection, pDC type I IFN response is triggered by the sensing of infected cells via physical cell-cell contact. Such a feature is also observed for many genetically distant viruses, including notably viruses of the Retroviridae, Arenaviridae, Flaviviridae, Picornaviridaea, Togaviridae families and observed for various infected cell types. Here, we described a set of experimental methods for the ex vivo studies of the regulation of pDC activation upon physical cell-cell contact with virally infected cells.

Infection, modulation and responses of antigen-presenting cells to African swine fever viruses

African swine fever (ASF) is a devastating viral disease of domestic pigs and wild boar for which there is no vaccine available. The aetiological agent ASF virus (ASFV) has a predilection for cells of the myeloid lineage. Macrophages provide a first line defence against pathogens and are the main target of ASFV, thus several studies analysed their response to infection in terms of cytokine/chemokine expression and modulation of functionality. These studies have typically used macrophages differentiated in vitro from blood or bone marrow progenitors and few studies have focused on responses of polarized macrophages (M1, M2) or functional macrophage subsets isolated from different tissues. ASFV can also infect dendritic cells (DC), but regardless of their central role in the induction of adaptive immune responses, their role in ASFV infection was only partially analysed. Future studies on ASFV-DC interaction are needed, which should take into consideration the heterogeneity within this family, composed of different subsets whose phenotype is also organ specific. Other porcine immune cells such as γδ-T cells, NK cells and fibrocytes, can act as 'non-conventional' antigen-presenting cells (APCs). In particular, γδ-T cells from ASFV immune pigs were shown to present viral antigens to T cells, but no studies have further explored the interaction of ASFV with this cell type or other non-conventional APCs. In this review we will provide an overview of the interaction of APCs with ASFV, describing the differences between virulent and attenuated strains, and suggesting areas for possible future studies.

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.

Molecular dissection of plasmacytoid dendritic cell activation in vivo during a viral infection

Plasmacytoid dendritic cells (pDC) are the major source of type I interferons (IFN-I) during viral infections, in response to triggering of endosomal Toll-like receptors (TLRs) 7 or 9 by viral single-stranded RNA or unmethylated CpG DNA, respectively. Synthetic ligands have been used to disentangle the underlying signaling pathways. The adaptor protein AP3 is necessary to transport molecular complexes of TLRs, synthetic CpG DNA, and MyD88 into endosomal compartments allowing interferon regulatory factor 7 (IRF7) recruitment whose phosphorylation then initiates IFN-I production. High basal expression of IRF7 by pDC and its further enhancement by positive IFN-I feedback signaling appear to be necessary for robust cytokine production. In contrast, we show here that in vivo during mouse cytomegalovirus (MCMV) infection pDC produce high amounts of IFN-I downstream of the TLR9-to-MyD88-to-IRF7 signaling pathway without requiring IFN-I positive feedback, high IRF7 expression, or AP3-driven endosomal routing of TLRs. Hence, the current model of the molecular requirements for professional IFN-I production by pDC, established by using synthetic TLR ligands, does not strictly apply to a physiological viral infection.

Transcriptional profiles of PBMCs from pigs infected with three genetically diverse porcine reproductive and respiratory syndrome virus strains

Porcine reproductive and respiratory syndrome virus is the cause of reproductive failure in sows and respiratory disease in young pigs, which has been considered as one of the most costly diseases to the worldwide pig industry for almost 30 years. This study used microarray-based transcriptomic analysis of PBMCs from experimentally infected pigs to explore the patterns of immune dysregulation after infection with two East European PRRSV strains from subtype 2 (BOR and ILI) in comparison to a Danish subtype 1 strain (DAN). Transcriptional profiles were determined at day 7 post infection in three tested groups of pigs and analysed in comparison with the expression profile of control group. Microarray analysis revealed differential regulation (> 1.5-fold change) of 4253 and 7335 genes in groups infected with BOR and ILI strains, respectively, and of 12518 genes in pigs infected with Danish strain. Subtype 2 PRRSV strains showed greater induction of many genes, especially those involved in innate immunity, such as interferon stimulated antiviral genes and inflammatory markers. Functional analysis of the microarray data revealed a significant up-regulation of genes involved in processes such as acute phase response, granulocyte and agranulocyte adhesion and diapedesis, as well as down-regulation of genes enrolled in pathways engaged in protein synthesis, cell division, as well as B and T cell signaling. This study provided an insight into the host response to three different PRRSV strains at a molecular level and demonstrated variability between strains of different pathogenicity level.

Antagonizing cytokine-mediated JAK-STAT signaling by porcine reproductive and respiratory syndrome virus

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PRRSV inhibits the interferon-activated JAK-STAT1/STAT2 signaling.

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PRRSV nsp1β induces degradation of KPNA1 to block the STAT1 nuclear translocation.

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PRRSV nsp5 reduces STAT3 to inhibit the JAK-STAT3 signaling.

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PRRSV may interfere with other STAT signaling and antagonizes ISGs.

Janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway is activated by myriad cytokines, which are involved in regulation of cell growth, proliferation, differentiation, apoptosis, angiogenesis, immunity and inflammatory response. Because of its significance in immune response, JAK-STAT pathway is often targeted by pathogens, including porcine reproductive and respiratory syndrome virus (PRRSV). PRRSV causes reproductive failure in sows and respiratory disease in pigs of all ages. A typical feature of the immune response to PRRSV infection in pigs is delayed production and low titer of virus neutralizing antibodies, and weak cell-mediated immune response. One of the possible reasons for the weak protective immune response is that PRRSV interferes with cytokine-mediated JAK-STAT signaling. PRRSV inhibits interferon-activated JAK-STAT signaling by blocking nuclear translocation of STAT1 and STAT2. The mechanism is that PRRSV non-structural protein 1β (nsp1β) induces degradation of karyopherin α1 (KPNA1), a critical adaptor in nucleo-cytoplasmic transport. PRRSV also antagonizes IL6-activated JAK-STAT3 signaling via inducing degradation of STAT3. In this review, we briefly introduce JAK-STAT signaling, summarize the PRRSV interference with it, and provide perspective on the perturbation in the context of PRRSV-elicited immune response.