Overexpression of programmed death ligands in naturally occurring postweaning multisystemic wasting syndrome

Immune Molecules' mRNA Expression in Porcine Alveolar Macrophages Co-Infected with Porcine Reproductive and Respiratory Syndrome Virus and Porcine Circovirus Type 2

Porcine reproductive and respiratory syndrome virus (PRRSV) and porcine circovirus 2 (PCV2) are economically important pathogens in swine, and pigs with dual infections of PCV2 and PRRSV consistently have more severe clinical symptoms and interstitial pneumonia. However, the synergistic pathogenesis mechanism induced by PRRSV and PCV2 co-infection has not yet been illuminated. Therefore, the aim of this study was to characterize the kinetic changes of immune regulatory molecules, inflammatory factors and immune checkpoint molecules in porcine alveolar macrophages (PAMs) in individuals infected or co-infected with PRRSV and/or PCV2. The experiment was divided into six groups: a negative control group (mock, no infected virus), a group infected with PCV2 alone (PCV2), a group infected with PRRSV alone (PRRSV), a PCV2-PRRSV co-infected group (PCV2-PRRSV inoculated with PCV2, followed by PRRSV 12 h later), a PRRSV-PCV2 co-infected group (PRRSV-PCV2 inoculated with PRRSV, followed by PCV2 12 h later) and a PCV2 + PRRSV co-infected group (PCV2 + PRRSV, inoculated with PCV2 and PRRSV at the same time). Then, PAM samples from the different infection groups and the mock group were collected at 6, 12, 24, 36 and 48 h post-infection (hpi) to detect the viral loads of PCV2 and PRRSV and the relative quantification of immune regulatory molecules, inflammatory factors and immune checkpoint molecules. The results indicated that PCV2 and PRRSV co-infection, regardless of the order of infection, had no effect on promoting PCV2 replication, while PRRSV and PCV2 co-infection was able to promote PRRSV replication. The immune regulatory molecules (IFN-α and IFN-γ) were significantly down-regulated, while inflammatory factors (TNF-α, IL-1β, IL-10 and TGF-β) and immune checkpoint molecules (PD-1, LAG-3, CTLA-4 and TIM-3) were significantly up-regulated in the PRRSV and PCV2 co-infection groups, especially in PAMs with PCV2 inoculation first followed by PRRSV. The dynamic changes in the aforementioned immune molecules were associated with a high viral load, immunosuppression and cell exhaustion, which may explain, at least partially, the underlying mechanism of the enhanced pulmonary lesions by dual infection with PCV2 and PRRSV in PAMs.

PRRSV-1 induced lung lesion is associated with an imbalance between costimulatory and coinhibitory immune checkpoints

Porcine reproductive and respiratory syndrome virus (PRRSV) induces a dysregulation on the innate and adaptive immune responses. T-cell activation requires a proper interaction and precise balance between costimulatory and coinhibitory molecules, commonly known as immune checkpoints. This study aims to evaluate the expression of immune checkpoints in lung and tracheobronchial lymph node from piglets infected with two PRRSV-1 strains of different virulence during the early stage of infection. Seventy 4-week-old piglets were grouped into three experimental groups: (i) control, (ii) 3249-infected group (low virulent strain), and (iii) Lena-infected group (virulent strain) and were euthanized at 1, 3, 6, 8, and 13 days post-infection (dpi). Lung and tracheobronchial lymph node were collected to evaluate histopathological findings, PRRSV viral load and mRNA expression of costimulatory (CD28, CD226, TNFRSF9, SELL, ICOS, and CD40) and coinhibitory (CTLA4, TIGIT, PD1/PDL1, TIM3, LAG3, and IDO1) molecules through RT-qPCR. Our findings highlight a mild increase of costimulatory molecules together with an earlier and stronger up-regulation of coinhibitory molecules in both organs from PRRSV-1-infected animals, especially in the lung from virulent Lena-infected animals. The simultaneous expression of coinhibitory immune checkpoints could work in synergy to control and limit the inflammation-induced tissue damage. Further studies should be addressed to determine the role of these molecules in later stages of PRRSV infection.

Time-series transcriptomic analysis of bronchoalveolar lavage cells from virulent and low virulent PRRSV-1-infected piglets

Porcine reproductive and respiratory syndrome virus (PRRSV) has evolved to escape the immune surveillance for a survival advantage leading to a strong modulation of host’s immune responses and favoring secondary bacterial infections. However, limited data are available on how the immunological and transcriptional responses elicited by virulent and low-virulent PRRSV-1 strains are comparable and how they are conserved during the infection. To explore the kinetic transcriptional signature associated with the modulation of host immune response at lung level, a time-series transcriptomic analysis was performed in bronchoalveolar lavage cells upon experimental in vivo infection with two PRRSV-1 strains of different virulence, virulent subtype 3 Lena strain or the low-virulent subtype 1 3249 strain. The time-series analysis revealed overlapping patterns of dysregulated genes enriched in T-cell signaling pathways among both virulent and low-virulent strains, highlighting an upregulation of co-stimulatory and co-inhibitory immune checkpoints that were disclosed as Hub genes. On the other hand, virulent Lena infection induced an early and more marked “negative regulation of immune system process” with an overexpression of co-inhibitory receptors genes related to T-cell and NK cell functions, in association with more severe lung lesion, lung viral load, and BAL cell kinetics. These results underline a complex network of molecular mechanisms governing PRRSV-1 immunopathogenesis at lung level, revealing a pivotal role of co-inhibitory and co-stimulatory immune checkpoints in the pulmonary disease, which may have an impact on T-cell activation and related pathways. These immune checkpoints, together with the regulation of cytokine-signaling pathways, modulated in a virulence-dependent fashion, orchestrate an interplay among pro- and anti-inflammatory responses.

IMPORTANCE Porcine reproductive and respiratory syndrome virus (PRRSV) is one of the major threats to swine health and global production, causing substantial economic losses. We explore the mechanisms involved in the modulation of host immune response at lung level performing a time-series transcriptomic analysis upon experimental infection with two PRRSV-1 strains of different virulence. A complex network of molecular mechanisms was revealed to control the immunopathogenesis of PRRSV-1 infection, highlighting an interplay among pro- and anti-inflammatory responses as a potential mechanism to restrict inflammation-induced lung injury. Moreover, a pivotal role of co-inhibitory and co-stimulatory immune checkpoints was evidenced, which may lead to progressive dysfunction of T cells, impairing viral clearance and leading to persistent infection, favoring as well secondary bacterial infections or viral rebound. However, further studies should be conducted to evaluate the functional role of immune checkpoints in advanced stages of PRRSV infection and explore a possible T-cell exhaustion state.

Programmed death-ligand 1 expression in swine chronic infections and enhancement of interleukin-2 production via programmed death-1/programmed death-ligand 1 blockade

Introduction

Chronic infections lead to the functional exhaustion of T cells. Exhausted T cells are phenotypically differentiated by the surface expression of the immunoinhibitory receptor, such as programmed death‐1 (PD‐1). The inhibitory signal is produced by the interaction between PD‐1 and its PD‐ligand 1 (PD‐L1) and impairs the effector functions of T cells. However, the expression dynamics of PD‐L1 and the immunological functions of the PD‐1/PD‐L1 pathway in chronic diseases of pigs are still poorly understood. In this study, we first analyzed the expression of PD‐L1 in various chronic infections in pigs, and then evaluated the immune activation by the blocking assay targeting the swine PD‐1/PD‐L1 pathway.

Methods

In the initial experiments, anti‐bovine PD‐L1 monoclonal antibodies (mAbs) were tested for cross‐reactivity with swine PD‐L1. Subsequently, immunohistochemical analysis was conducted using the anti‐PD‐L1 mAb. Finally, we assessed the immune activation of swine peripheral blood mononuclear cells (PBMCs) by the blockade with anti‐PD‐L1 mAb.

Results

Several anti‐PD‐L1 mAbs tested recognized swine PD‐L1‐expressing cells. The binding of swine PD‐L1 protein to swine PD‐1 was inhibited by some of these cross‐reactive mAbs. In addition, immunohistochemical analysis revealed that PD‐L1 was expressed at the site of infection in chronic infections of pigs. The PD‐L1 blockade increased the production of interleukin‐2 from swine PBMCs.

Conclusions

These findings suggest that the PD‐1/PD‐L1 pathway could be also involved in immunosuppression in chronic infections in pigs. This study provides a new perspective on therapeutic strategies for chronic diseases in pigs by targeting immunosuppressive pathways.

Up-Regulation of Immune Checkpoints in the Thymus of PRRSV-1-Infected Piglets in a Virulence-Dependent Fashion

Virulent porcine reproductive and respiratory syndrome virus (PRRSV) strains, such as the Lena strain, have demonstrated a higher thymus tropism than low virulent strains. Virulent PRRSV strains lead to severe thymus atrophy, which could be related to marked immune dysregulation. Impairment of T-cell functions through immune checkpoints has been postulated as a strategy executed by PRRSV to subvert the immune response, however, its role in the thymus, a primary lymphoid organ, has not been studied yet. Therefore, the goal of this study was to evaluate the expression of selected immune checkpoints (PD1/PDL1, CTLA4, TIM3, LAG3, CD200R1 and IDO1) in the thymus of piglets infected with two different PRRSV-1 strains. Thymus samples from piglets infected with the low virulent 3249 strain, the virulent Lena strain and mock-infected were collected at 1, 3, 6, 8 and 13 days post-infection (dpi) to analyze PRRSV viral load, relative quantification and immunohistochemical staining of immune checkpoints. PD1/PDL1, CTLA4, TIM3, LAG3 and IDO1 immune checkpoints were significantly up-regulated in the thymus of PRRSV infected piglets, especially in those infected with the virulent Lena strain from 6 dpi onwards. This up-regulation was associated with disease progression, high viral load and cell death. Co-expression of these molecules can affect T-cell development, maturation and selection, negatively regulating the host immune response against PRRSV.

Induction of Porcine Dermatitis and Nephropathy Syndrome in Piglets by Infection with Porcine Circovirus Type 3

Porcine circovirus type 3 (PCV3) is an emerging porcine circovirus that has been associated with porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs, reproductive failure, cardiac pathologies, and multisystemic inflammation in piglets and sows. Many aspects of PCV3 infection biology and pathogenesis, however, remain unknown. Here, we used a PCV3 virus stock from the rescue of an infectious PCV3 DNA clone to intranasally inoculate 4- and 8-week-old specific-pathogen-free piglets for evaluation of PCV3 pathogenesis. For 4-week-old piglets, typical clinical signs resembling those of PDNS-like disease were observed when piglets were inoculated with PCV3 alone or PCV3 combined with immunostimulation by keyhole limpet hemocyanin, with a mortality of 40% (2/5) for both types of inoculated piglets during a 28-day observation period postinoculation. Both types of inoculated piglets showed similar progressive increases in viral loads in the sera and had seroconverted to PCV3 capsid antibody after inoculation. Pathological lesions and PCV3-specific antigen were detected in various tissues and organs, including the lung, heart, kidney, lymph nodes, spleen, liver, and small intestine, in both types of inoculated piglets. The levels of proinflammatory cytokines and chemokines, including interleukin 1 beta (IL-1β), IL-6, IL-23α, gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and chemokine ligand 5 (CCL5), were significantly upregulated in both groups of inoculated piglets. Eight-week-old piglets also exhibited a similar PDNS-like disease but without death after PCV3 inoculation, as evidenced by pathological lesions and PCV3 antigen in various tissues and organs. These results show for the first time successful reproduction of PDNS-like disease by PCV3 infection and further provide significant information regarding the pathogenesis of PCV3 in piglets.IMPORTANCE Porcine circovirus type 3 (PCV3), an emerging porcine circovirus, is considered the cause of porcine dermatitis and nephropathy syndrome (PDNS)-like clinical signs and other systemic diseases in piglets and sows. To evaluate the pathogenesis of PCV3 infection in vivo, we used a PCV3 virus stock from the rescue of an infectious PCV3 DNA clone to intranasally inoculate 4- and 8-week-old specific-pathogen-free piglets and demonstrated successful reproduction of PDNS-like disease in animals that were inoculated with PCV3 alone or PCV3 combined with immunostimulation by keyhole limpet hemocyanin. Both 4- and 8-week-old PCV3-inoculated piglets showed similar increases in viral loads in the sera and had seroconverted to PCV3 capsid antibody. Pathological lesions and PCV3-specific antigen were detected in various tissues and organs, while numerous proinflammatory cytokines and chemokines in the sera were significantly upregulated after PCV3 inoculation. These results will provide significant information regarding the pathogenesis of PCV3 in piglets.

Immune exhaustion during chronic infections in cattle

Recently, dysfunction of antigen-specific T cells is well documented as T-cell exhaustion and has been defined by the loss of effector functions during chronic infections and cancer in human. The exhausted T cells are characterized phenotypically by the surface expression of immunoinhibitory receptors, such as programmed death 1 (PD-1), lymphocyte activation gene 3 (LAG-3), T-cell immunoglobulin and mucin domain-containing protein 3 (Tim-3) and cytotoxic T-lymphocyte antigen 4 (CTLA-4). However, there is still a fundamental lack of knowledge about the immunoinhibitory receptors in the fields of veterinary medicine. In particular, very little is known about mechanism of T cell dysfunction in chronic infection in cattle. Recent our studies have revealed that immunoinhibitory molecules including PD-1/ programmed death-ligand 1 (PD-L1) play critical roles in immune exhaustion and disease progression in case of bovine leukemia virus (BLV) infection, Johne's disease and bovine anaplasmosis. This review includes some recent data from us.