Increase of cells expressing PD-1 and PD-L1 and enhancement of IFN-γ production via PD-1/PD-L1 blockade in bovine mycoplasmosis

The role, relevance and management of immune exhaustion in bovine infectious diseases

Immune exhaustion is a state of immune cell dysfunction that occurs most commonly following chronic exposure to an antigen which persists after the immune response fails to remove it. Exhaustion has been studied most thoroughly with several cancers, but has also been observed in several chronic infectious diseases. The topic has mainly been studied with CD8+ T cells, but it can also occur with CD4+ T cells and other immune cell types too. Exhaustion is characterized by a hierarchical loss of effector cell functions, up-regulation of immuno-inhibitory receptors, disruption of metabolic activities, and altered chromatin landscapes. Exhaustion has received minimal attention so far in diseases of veterinary significance and this review's purpose is to describe examples where immune exhaustion is occurring in several bovine disease situations. We also describe methodology to evaluate immune exhaustion as well as the prospects of controlling exhaustion and achieving a more suitable outcome of therapy in some chronic disease scenarios.

DNA mismatch repair system regulates the expression of PD-L1 through DNMTs in cervical cancer

BACKGROUND

Cervical cancer (CC) is a potential clinical application of PD-1/PD-L1 inhibitor. We aimed to study the mechanism of DNA mismatch repair (MMR) system regulating the expression of PD-L1 in CC through DNA methyltransferase (DNMTs).

METHODS

We collected pathological specimens from 118 cases of CC to analyze the relationship between PD-L1 expression and DNMTs in different MMR states. RNA interference (RNAi) technique was used to simulate the formation of CC cell line with MMR deficiency (dMMR) state, and subcutaneous tumor formation experiment was carried out in nude mice to verify the relationship between PD-L1 expression and DNMTs in MMR state.

RESULTS

The PD-L1 positive rate in 118 cases of CC was 58.47%, while the microsatellite instability (MSI) status accounted for 5.93%. There was a significant difference in the expression of PD-L1 between patients within the dMMR and MMR proficient (pMMR) groups (χ2 = 21.405, P < 0.001). Subcutaneous inoculation after infection of Siha cells led to successful tumorigenesis in nude mice, accompanied by a significant increase in the level of PD-L1 expression in the mouse tumors, while the expression level of MLH1 and MSH2 protein decreased significantly. We also found that PD-L1 expression was closely related to the expression of DNMTs.

CONCLUSION

PD-L1 is universal expressed on the surface of CC cells, dMMR status enhances the expression of PD-L1 on the surface of CC cells, dMMR states of CC are related to the demethylation status of the PD-L1 gene promoter region.

Molecular characterization of immunoinhibitory factors PD-1/PD-L1 in sheep

Sheep have been used as a large animal experimental model for studying infectious diseases. However, due to a lack of staining antibodies and reagents, immunological studies on sheep have not progressed. The immunoinhibitory receptor programmed death-1 (PD-1) is expressed on T lymphocytes. The interaction of PD-1 with its ligand PD-ligand 1 (PD-L1) delivers inhibitory signals and impairs proliferation, cytokine production, and cytotoxicity of T cells. We previously reported that the PD-1/PD-L1 pathway was closely associated with T-cell exhaustion and disease progression in bovine chronic infections using anti-bovine PD-L1 monoclonal antibodies (mAbs). Furthermore, we found that blocking antibodies against PD-1 and PD-L1 restore T-cell functions and could be used in immunotherapy of cattle. However, the immunological role of the PD-1/PD-L1 pathway in chronic diseases of sheep remains unknown. In this study, we identified cDNA sequences of ovine PD-1 and PD-L1 and examined the cross-activity of anti-bovine PD-L1 mAbs against ovine PD-L1 as well as the expression of PD-L1 in ovine listeriosis. The amino acid sequences of ovine PD-1 and PD-L1 share a high degree of identity and similarity with homologs from ruminants and other mammalian species. Anti-bovine PD-L1 mAb recognized ovine PD-L1 on lymphocytes in the flow cytometric assay. Furthermore, an immunohistochemical staining confirmed the PD-L1 expression on macrophages in the brain lesions of ovine listeriosis. These findings indicated that our anti-PD-L1 mAb would be useful for analyzing the ovine PD-1/PD-L1 pathway. Further research is needed to determine the immunological role of PD-1/PD-L1 in chronic diseases such as BLV infection through experimental infection of sheep.

Expression profiles of the CD274 and PLEKHH2 gene and association of its polymorphism with hematologic parameters in sheep

CD274 and PLEKHH2 genes have been identified as immune- and multiple diseases-related genes, and have recently garnered significant interest. However, their role in regulating immune functions in sheep remains largely unexplored. In this study, we aimed to investigate the effects of polymorphisms in CD274 and PLEKHH2 on hematologic parameters in 915 sheep. Our results showed that the CD274 and PLEKHH2 genes were most highly expressed in the spleen and tail fat, respectively, as determined by qRT-PCR. We also identified a G to A mutation (g 0.11858 G > A) in the exon 4 region of CD274, and a C to G mutation (g 0.38384 C > G) in the intron 8 region of PLEKH2. Association analysis revealed that CD274 g 0.11858 G > A was significantly associated with RBC, HCT, MCHC, and MCV (P < 0.05), while PLEKHH2 g 0.38384 C > G was significantly associated with HCT, MPV, MCHC, and MCV (P < 0.05). These results suggest that CD274 and PLEKHH2 genes may play a role in regulating blood physiological indicators and could be potential functional candidates for influencing immune traits in sheep breeding programs.

The PD-1/PD-L1 Pathway: A Perspective on Comparative Immuno-Oncology

Simple Summary

The programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) pathway inhibits the function of activated immune cells. This mediates immune tolerance and prevents immune-mediated tissue destruction. The malfunction of this pathway is involved in the pathogenesis of chronic infections, autoimmunity, and cancer. The PD-1/PD-L1 pathway is an excellent example of the research benefits of comparative pathology and attests to the importance of the “one health one medicine” concept. Pioneering research was mainly focused on the examination of cells and tissues of human and mouse origin. It mainly revealed that PD-L1-positive tumor cells can paralyze PD-1-bearing immune cells, which prevents immunological destruction of cancer cells. This led to a major breakthrough in cancer treatment, i.e., the use of antibodies that block the interaction of these molecules and restore anti-cancer immune defense (immune checkpoint therapy). Further studies provided more detailed information on the tissue-specific context and fine-tuning of this pathway. The most recent research has extended the investigations to the examination of several animal species with the aim of improving disease diagnostics and treatment for certain animal diseases, in particular cancer, which is a major cause of disease and death in companion animals.

Abstract

The programmed cell death protein 1/programmed death-ligand 1 (PD-1/PD-L1) pathway mainly attracted attention in immuno-oncology, leading to the development of immune checkpoint therapy. It has, however, much broader importance for tissue physiology and pathology. It mediates basic processes of immune tolerance and tissue homeostasis. In addition, it is involved in the pathogenesis of chronic infectious diseases, autoimmunity, and cancer. It is also an important paradigm for comparative pathology as well as the “one health one medicine” concept. The aim of this review is to provide an overview of novel research into the diverse facets of the PD-1/PD-L1 pathway and to give insights into its fine-tuning homeostatic role in a tissue-specific context. This review details early translational research from the discovery phase based on mice as animal models for understanding pathophysiological aspects in human tissues to more recent research extending the investigations to several animal species. The latter has the twofold goal of comparing this pathway between humans and different animal species and translating diagnostic tools and treatment options established for the use in human beings to animals and vice versa.

Brief Research Report: Expression of PD-1 and CTLA-4 in T Lymphocytes and Their Relationship With the Periparturient Period and the Endometrial Cytology of Dairy Cows During the Postpartum Period

The present study sought to evaluate the expression of PD-1 and CTLA-4 in blood T lymphocytes during the periparturient period and their relationship with uterine health in dairy cows, as determined by endometrial cytology and serum concentrations of β-hydroxybutyrate (BHB) and non-esterified fatty acids (NEFAs), which are indicators of a negative energy balance. The second objective of this study was to investigate whether the expression of PD-1 and CTLA-4 in T lymphocytes is associated with the serum acute phase-protein haptoglobin concentration during the periparturient period. To address these objectives, 26 clinically healthy dairy cows were used. Peripheral blood was collected 14 days prepartum (T-14), at calving (T0), and 30 days postpartum (T30) to measure the expression of PD-1 and CTLA-4 in blood T lymphocytes by flow cytometry. In addition, we collected blood at T0, 10 days after parturition (T10), and T30 to obtain serum and determine the serum concentrations of NEFA, BHB, and Hp. Endometrial cytology was performed at T10, 20 days after parturition (T20), and T30. In the present study, we observed higher expression of CTLA-4 and PD-1 in T lymphocytes at parturition and in the prepartum period, which could indicate a relationship between these immune checkpoints and immunological tolerance during gestation in dairy cattle. In addition, a negative association between the expression of these immune checkpoints prepartum or at parturition and endometrial cytology at T20 and T30 was observed, indicating the negative implications of these immune response regulators in susceptibility to infections. This finding was further corroborated by the relationship between the serum concentration of haptoglobin and the expression of CTLA-4 and PD-1 by T lymphocytes. However, we did not observe a relationship between the indicators of negative energy balance, evaluated by the serum concentrations of BHB and NEFA, and the expression of the immune checkpoint markers studied. Thus, our findings represent an initial step that paves the way for the development of new therapeutic alternatives directed by the host with the objective of increasing the resistance of dairy cattle to infections in this critical period of life.

Effects of intra-articular inoculation with Mycoplasma bovis on immunological responses in calf joints

Mycoplasma arthritis that caused by Mycoplasma bovis exhibit severe lameness. This disease is difficult to cure with antibiotics, but the detailed pathological mechanisms have not been fully clarified. In this study, we examined the effects of intra-articular inoculation with M. bovis on immunological responses in calf joints. We inoculated three calves each with M. bovis or phosphate buffer saline (control) into the right stifle joint and dissected them at 15 days postinoculation. Mycoplasma bovis-inoculated calves exhibited swelling of the stifle joint, increases in synovial fluid, fibrin deposition, and cartilage thinning. Intracellular M. bovis was detected in synovial tissues analyzed by immunohistochemistry and transmission electron microscopy. Messenger RNA expressions of interleukin (IL)-1β, IL-6, IL-8, IL-12p40, and IL-17A in synovial fluid cells and synovial tissues from M. bovis-inoculated calves were significantly higher than those from control calves. Protein levels of these cytokines in synovial fluid from M. bovis-inoculated calves were markedly higher than those from control calves. Our study clarified that inoculation with M. bovis into the stifle joint induced the production of inflammatory cytokines by synovial fluid cells and synovial tissues, causing a severe inflammatory response in joints. Additionally, M. bovis could invade cells in synovial tissues, which may have aided it in evading antibiotics and host immune surveillance.

Enhancement of interleukin-2 production by bovine peripheral blood mononuclear cells treated with the combination of anti-programmed death-ligand 1 and cytotoxic T lymphocyte antigen 4 chimeric monoclonal antibodies

Our previous studies demonstrate the therapeutic efficacy against bovine diseases of an anti-bovine programmed death-ligand 1 (PD-L1) chimeric antibody. In humans, PD-1 and PD-L1 antibodies are more effective when combined with an antibody targeting cytotoxic T lymphocyte antigen 4 (CTLA-4) and these combination therapies are therefore clinically used. Here we generated an anti-bovine CTLA-4 chimeric antibody (chAb) to enhance the therapeutic efficacy of the PD-L1 antibody. We further analyzed the effects of dual blockade of CTLA-4 and PD-1 pathways on T-cell responses. The established anti-bovine CTLA-4 chAb showed comparable blocking activity on the binding of bovine CTLA-4 to CD80 and CD86 as the anti-bovine CTLA-4 mouse monoclonal antibody. Anti-bovine CTLA-4 chAb also significantly increased IL-2 production from bovine peripheral blood mononuclear cells (PBMCs). Further, the combination of anti-CTLA-4 chAb with anti-PD-L1 chAb significantly upregulated IL-2 production by PBMCs. These results suggest that the combination of antibodies have higher potential to enhance immune responses against pathogens compared with single administration.

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.

Immune Evasion of Mycoplasma bovis

Mycoplasma bovis (M. bovis) causes various chronic inflammatory diseases, including mastitis and bronchopneumonia, in dairy and feed cattle. It has been found to suppress the host immune response during infection, leading to the development of chronic conditions. Both in vitro and in vivo studies have confirmed that M. bovis can induce proinflammatory cytokines and chemokines in the host. This consists of an inflammatory response in the host that causes pathological immune damage, which is essential for the pathogenic mechanism of M. bovis. Additionally, M. bovis can escape host immune system elimination and, thus, cause chronic infection. This is accomplished by preventing phagocytosis and inhibiting key responses, including the neutrophil respiratory burst and the development of nitric oxide (NO) and inducible nitric oxide synthase (iNOS) that lead to the creation of an extracellular bactericidal network, in addition to inhibiting monocyte and alveolar macrophage apoptosis and inducing monocytes to produce anti-inflammatory factors, thus inducing the apoptosis of peripheral blood mononuclear cells (PBMCs), inhibiting their proliferative response and resulting in their invasion. Together, these conditions lead to long-term M. bovis infection. In terms of the pathogenic mechanism, M. bovis may invade specific T-cell subsets and induce host generation of exhausted T-cells, which helps it to escape immune clearance. Moreover, the M. bovis antigen exhibits high-frequency variation in size and expression period, which allows it to avoid activation of the host humoral immune response. This review includes some recent advances in studying the immune response to M. bovis. These may help to further understand the host immune response against M. bovis and to develop potential therapeutic approaches to control M. bovis infection.

Neutrophils Encompass a Regulatory Subset Suppressing T Cells in Apparently Healthy Cattle and Mice

Neutrophils that reside in the bone marrow are swiftly recruited from circulating blood to fight infections. For a long time, these first line defenders were considered as microbe killers. However their role is far more complex as cross talk with T cells or dendritic cells have been described for human or mouse neutrophils. In cattle, these new roles are not documented yet. We identified a new subset of regulatory neutrophils that is present in the mouse bone marrow or circulate in cattle blood under steady state conditions. These regulatory neutrophils that display MHC-II on the surface are morphologically indistinguishable from classical MHC-II^neg neutrophils. However MHC-II^pos and MHC-II^neg neutrophils display distinct transcriptomic profiles. While MHC-II^neg and MHC-II^pos neutrophils display similar bacterial phagocytosis or killing activity, MHC-II^pos only are able to suppress T cell proliferation under contact-dependent mechanisms. Regulatory neutrophils are highly enriched in lymphoid organs as compared to their MHC-II^neg counterparts and in the mouse they express PDL-1, an immune checkpoint involved in T-cell blockade. Our results emphasize neutrophils as true partners of the adaptive immune response, including in domestic species. They open the way for discovery of new biomarkers and therapeutic interventions to better control cattle diseases.

Tick saliva-induced programmed death-1 and PD-ligand 1 and its related host immunosuppression

The tick Rhipicephalus microplus is a harmful parasite of cattle that causes considerable economic losses to the cattle breeding industry. Although R. microplus saliva (Rm-saliva) contains several immunosuppressants, any association between Rm-saliva and the expression of immunoinhibitory molecules, such as programmed death (PD)-1 and PD-ligand 1 (PD-L1), has not been described. In this study, flow cytometric analyses revealed that Rm-saliva upregulated PD-1 expression in T cells and PD-L1 expression in CD14⁺ and CD11c⁺ cells in cattle. Additionally, Rm-saliva decreased CD69 expression in T cells and Th1 cytokine production from peripheral blood mononuclear cells. Furthermore, PD-L1 blockade increased IFN-γ production in the presence of Rm-saliva, suggesting that Rm-saliva suppresses Th1 responses via the PD-1/PD-L1 pathway. To reveal the upregulation mechanism of PD-1/PD-L1 by Rm-saliva, we analyzed the function of prostaglandin E₂ (PGE₂), which is known as an inducer of PD-L1 expression, in Rm-saliva. We found that Rm-saliva contained a high concentration of PGE₂, and PGE₂ treatment induced PD-L1 expression in CD14⁺ cells in vitro. Immunohistochemical analyses revealed that PGE₂ and PD-L1 expression was upregulated in tick-attached skin in cattle. These data suggest that PGE₂ in Rm-saliva has the potential to induce the expression of immunoinhibitory molecules in host immune cells.

Inflammatory cytokine mRNA and protein levels in the synovial fluid of Mycoplasma arthritis calves

Bovine Mycoplasma arthritis (MA) is caused by Mycoplasma bovis and exhibits severe clinical symptoms. However, the pathophysiology of bovine MA is incompletely understood. In this study, we examined the cytokine mRNA expression of synovial fluid (SF) cells and cytokine concentrations in the SF of MA calves. The SF was isolated from five clinically healthy (control) and seven MA calves. mRNA and protein levels of interleukin (IL)-1β, IL-6, IL-8, IL-12, and IL-17 in the SF from MA calves were significantly higher than those from control calves. Our results indicate that SF cells produce inflammatory cytokines, which mainly contribute to the severe inflammatory response in the joints of the MA calves.

Invasion of Mycoplasma bovis into bovine synovial cells utilizing the clathrin-dependent endocytosis pathway

Mycoplasma bovis causes chronic arthritis in cattle, accompanied by a severe inflammatory reaction of the joints. Recent studies demonstrated that M. bovis can invade bovine non-phagocytic cells, but the mechanism of M. bovis internalization in the cells remains unclear. In this study, we examined the mechanism by which M. bovis invades synovial cells, including the pathway of cell invasion. Using fluorescence and electron microscopy, multiple M. bovis were observed to adhere to and be internalized in cultured bovine synovial cells. The number of M. bovis colocalized with clathrin heavy chain (CLTC) per cell was significantly higher than the number of M. bovis colocalized with caveolin-1 (Cav-1). The internalized ratio of M. bovis in synovial cells treated with clathrin-dependent endocytosis inhibitor and small interfering RNA (siRNA) against CLTC was significantly lower than that in control cells. In contrast, the internalized ratio of M. bovis in synovial cells was unaffected by siRNA against Cav-1. These findings provide the first evidence that clathrin-dependent endocytosis is one of the major pathways by which M. bovis invades into synovial cells.

Transcriptome analysis of Mycoplasma bovis stimulated bovine peripheral blood mononuclear cells

Mycoplasma bovis is a pathogenic bacterium in bovines that causes huge global economic losses. Numerous factors play important roles in M. bovis pathogenesis; however, the host immune response involved in M. bovis infection has not been fully elucidated. We aimed to determine the characteristics of the host immune response to Mycoplasma infection. We evaluated the responsiveness of bovine peripheral blood mononuclear cells (PBMCs) stimulated with M. bovis via microarray analysis. The transcriptional abundance of innate immune-related genes IL-36A, IL-27, IFN-γ, and IL-17 in PBMCs increased after M. bovis exposure. Upon M. bovis infection, there was increased expression of the lymphocyte activated genes basic leucine zipper transcription factor (BATF) and signaling lymphocytic activation molecule family members 1 and 7 (SLAMF 1 and SLAMF 7) in PBMCs compared with that in unstimulated cells. The study revealed that the transcriptional abundance of innate immunity genes in PBMCs increased during M. bovis infection. This induced the activation of PBMCs, giving rise to an immune response, which is followed by the development of the inflammatory response. The results from this study could be used as the basis for the development of novel vaccine candidates against M. bovis.

The Suppression of Th1 Response by Inducing TGF-β1 From Regulatory T Cells in Bovine Mycoplasmosis

Regulatory T cells (Tregs) regulate immune responses and maintain host immune homeostasis. Tregs contribute to the disease progression of several chronic infections by oversuppressing immune responses via the secretion of immunosuppressive cytokines, such as transforming growth factor (TGF)-β and interleukin-10. In the present study, we examined the association of Tregs with Mycoplasma bovis infection, in which immunosuppression is frequently observed. Compared with uninfected cattle, the percentage of Tregs, CD4⁺CD25^highFoxp3⁺ T cells, was increased in M. bovis-infected cattle. Additionally, the plasma of M. bovis-infected cattle contained the high concentrations of TGF-β1, and M. bovis infection induced TGF-β1 production from bovine immune cells in in vitro cultures. Finally, we analyzed the immunosuppressive effects of TGF-β1 on bovine immune cells. Treatment with TGF-β1 significantly decreased the expression of CD69, an activation marker, in T cells, and Th1 cytokine production in vitro. These results suggest that the increase in Tregs and TGF-β1 secretion could be one of the immunosuppressive mechanisms and that lead to increased susceptibility to other infections in terms of exacerbation of disease during M. bovis infection.

PD-L1 expression in equine malignant melanoma and functional effects of PD-L1 blockade

Programmed death-1 (PD-1) is an immunoinhibitory receptor expressed on lymphocytes. Interaction of PD-1 with its ligand PD-ligand 1 (PD-L1) delivers inhibitory signals and impairs proliferation, cytokine production, and cytotoxicity of T cells. In our previous studies, we have developed anti-bovine PD-L1 monoclonal antibodies (mAbs) and reported that the PD-1/PD-L1 pathway was closely associated with T-cell exhaustion and disease progression in bovine chronic infections and canine tumors. Furthermore, we found that blocking antibodies that target PD-1 and PD-L1 restore T-cell functions and could be used in immunotherapy in cattle and dogs. However, the immunological role of the PD-1/PD-L1 pathway for chronic equine diseases, including tumors, remains unclear. In this study, we identified cDNA sequences of equine PD-1 (EqPD-1) and PD-L1 (EqPD-L1) and investigated the role of anti-bovine PD-L1 mAbs against EqPD-L1 using in vitro assays. In addition, we evaluated the expression of PD-L1 in tumor tissues of equine malignant melanoma (EMM). The amino acid sequences of EqPD-1 and EqPD-L1 share a considerable identity and similarity with homologs from non-primate species. Two clones of the anti-bovine PD-L1 mAbs recognized EqPD-L1 in flow cytometry, and one of these cross-reactive mAbs blocked the binding of equine PD-1/PD-L1. Of note, immunohistochemistry confirmed the PD-L1 expression in EMM tumor tissues. A cultivation assay revealed that PD-L1 blockade enhanced the production of Th1 cytokines in equine immune cells. These findings showed that our anti-PD-L1 mAbs would be useful for analyzing the equine PD-1/PD-L1 pathway. Further research is warranted to discover the immunological role of PD-1/PD-L1 in chronic equine diseases and elucidate a future application in immunotherapy for horses.

Outer Membrane Vesicles From Brucella melitensis Modulate Immune Response and Induce Cytoskeleton Rearrangement in Peripheral Blood Mononuclear Cells

Similar to what has been described in other Gram-negative bacteria, Brucella melitensis releases outer membrane vesicles (OMVs). OMVs from B. melitensis 16M and the rough-mutant B. melitensis VTRM1 were able to induce a protective immune response against virulent B. melitensis in mice models. The presence of some proteins which had previously been reported to induce protection against Brucella were found in the proteome of OMVs from B. melitensis 16M. However, the proteome of OMVs from B. melitensis VTRM1 had not previously been determined. In order to be better understand the role of OMVs in host-cell interactions, the aim of this work was to compare the proteomes of OMVs from B. melitensis 16M and the derived rough-mutant B. melitensis VTRM1, as well as to characterize the immune response induced by vesicles on host cells. Additionally, the effect of SDS and proteinase K on the stability of OMVs was analyzed. OMVs from B. melitensis 16M (smooth strain) and the B. melitensis VTRM1 rough mutant (lacking the O-polysaccharide side chain) were analyzed through liquid chromatography-mass spectrometry (LC-MS/MS). OMVs were treated with proteinase K, sodium deoxycholate, and SDS, and then their protein profile was determined using SDS-PAGE. Furthermore, PBMCs were treated with OMVs in order to measure their effect on cytoskeleton, surface molecules, apoptosis, DNA damage, proliferation, and cytokine-induction. A total of 131 proteins were identified in OMVs from B. melitensis16M, and 43 in OMVs from B. melitensis VTRM1. Proteome comparison showed that 22 orthologous proteins were common in vesicles from both strains, and their core proteome contained Omp31, Omp25, GroL, and Omp16. After a subsequent detergent and enzyme treatment, OMVs from B. melitensis VTRM1 exhibited higher sensitive compared to OMVs from the B. melitensis 16M strain. Neither OMVs induced IL-17, proliferation, apoptosis or DNA damage. Nonetheless, OMVs from the smooth and rough strains induced overproduction of TNFα and IL-6, as well as actin and tubulin rearrangements in the cytoskeleton. Moreover, OMVs from both strains inhibited PD-L1 expression in T-cells. These data revealed significant differences in OMVs derived from the rough and smooth Brucella strains, among which, the presence or absence of complete LPS appeared to be crucial to protect proteins contained within vesicles and to drive the immune response.

Persistence in Livestock Mycoplasmas—a Key Role in Infection and Pathogenesis

Purpose of Review

Mycoplasma, economically important pathogens in livestock, often establishes immunologically complex persistent infections that drive their pathogenesis and complicate prophylaxis and therapy of the caused diseases. In this review, we summarize some of the recent findings concerning cellular and molecular persistence mechanisms related to the pathogenesis of mycoplasma infections in livestock.

Recent Findings

Data from recent studies prove several mechanisms including intracellular lifestyle, immune dysregulation, and autoimmunity as well as microcolony and biofilm formation and apoptosis of different host cell types as important persistence mechanisms in several clinically significant Mycoplasma species, i.e., M. bovis, M. gallisepticum, M. hyopneumoniae, and M. suis.

Summary

Evasion of the immune system and the establishment of persistent infections are key features in the pathogenesis of livestock mycoplasmas. In-depth knowledge of the underlying mechanisms will provide the basis for the development of therapy and prophylaxis strategies against mycoplasma infections.

Effect of TIPE1 on Immune Function of Dendritic Cells and Its Signaling Pathway in Septic Mice

Dendritic cell (DC) dysfunction plays a pivotal role in sepsis-induced immunosuppression. Tumor necrosis factor α (TNF-α)-induced protein 8 like-1 (TIPE1), a new member of the tumor necrosis factor α-induced protein 8 family, may be related to cell death. The aim of the present study was to elucidate the effect of TIPE1 on the immune function of DCs and its regulatory mechanism via PD-L1/PD-1 signaling in mice. Sepsis was induced in adult C57BL/6 male mice via cecal ligation and puncture. In vitro, we found that expression of CD80, CD86, and major histocompatibility complex class II in DCs and levels of cytokines, including tumor necrosis factor α and interleukin 12p40, were elevated; similarly, T-cell proliferation and differentiation were promoted when the gene expressing TIPE1 was silenced. Next, we examined the in vivo role of TIPE1 in a cecal ligation and puncture animal model system. Flow cytometry of the immune functional status in DCs revealed negative regulation of TIPE1 on DC maturation, as well as activation. Moreover, changes in PD-L1/PD-1 levels confirmed the negative effect of TIPE1 in DCs. Collectively, we report that TIPE1 might exert negative regulation in sepsis, at least in part by inhibiting DC maturation and subsequent T-cell-mediated immunity via PD-L1/PD-1 signaling.

Highly pathogenic H7N9 avian influenza virus infection associated with up-regulation of PD-1/PD-Ls pathway-related molecules

To investigate the main transcriptional and biological changes of human host during low and highly pathogenic avian H7N9 influenza virus infection and to analyze the possible causes of escalated virulence and the systematic progression of H7N9 virus infection, we utilized whole transcriptome sequencing (RNA-chip and RNA-seq) and other biomolecular methods to analyze and verify remarkable changes of host cells during these two subtypes of H7N9 influenza viruses infection. Whole transcriptome analysis showed the global profiles of differentially expressed genes (DEGs) and identified 458 DEGs associated with major changes in biological processes of the host cells after infection with 2017 HPAI H7N9 virus versus 2013 LPAI H7N9 virus, mainly including drastically increased defense responses to viruses (e.g. negative regulation of viral gene replication), IFNs related pathways, immune response/native immune response, and inflammatory response. Genes of programmed cell death 1 (PD-1) pathways were found changed remarkably and several highly correlated non-coding RNAs were identified. The results suggested that HPAI H7N9 virus induces stronger immune response and suppressing response than LPAI H7N9. Meanwhile, PD-1/PD-Ls signaling pathways work together in regulating host responses including antiviral defense, lethal inflammation caused by the virus and immune response, thus contribute to the high pathogenicity of 2017H7N9 virus that can be regulated by non-coding RNAs. The present study represents a comprehensive understanding and good reference of regulation of pathogenicity of H7N9 virus even other fatal viruses and correlated host immune responses.

Upregulation of PD-L1 Expression by Prostaglandin E2 and the Enhancement of IFN-γ by Anti-PD-L1 Antibody Combined With a COX-2 Inhibitor in Mycoplasma bovis Infection

Bovine mycoplasmosis caused by Mycoplasma bovis results in pneumonia and mastitis in cattle. We previously demonstrated that the programmed death 1 (PD-1)/PD-ligand 1 (PD-L1) pathway is involved in immune dysfunction during M. bovis infection and that prostaglandin E₂ (PGE₂) suppressed immune responses and upregulated PD-L1 expression in Johne's disease, a bacterial infection in cattle. In this study, we investigated the role of PGE₂ in immune dysfunction and the relationship between PGE₂ and the PD-1/PD-L1 pathway in M. bovis infection. In vitro stimulation with M. bovis upregulated the expressions of PGE₂ and PD-L1 presumably via Toll-like receptor 2 in bovine peripheral blood mononuclear cells (PBMCs). PGE₂ levels of peripheral blood in infected cattle were significantly increased compared with those in uninfected cattle. Remarkably, plasma PGE₂ levels were positively correlated with the proportions of PD-L1⁺ monocytes in M. bovis-infected cattle. Additionally, plasma PGE₂ production in infected cattle was negatively correlated with M. bovis-specific interferon (IFN)-γ production from PBMCs. These results suggest that PGE₂ could be one of the inducers of PD-L1 expression and could be involved in immunosuppression during M. bovis infection. In vitro blockade assays using anti-bovine PD-L1 antibody and a cyclooxygenase 2 inhibitor significantly upregulated the M. bovis-specific IFN-γ response. Our study findings might contribute to the development of novel therapeutic strategies for bovine mycoplasmosis that target PGE₂ and the PD-1/PD-L1 pathway.

Mycoplasma bovis: Interactions with the Immune System and Failure to Generate an Effective Immune Response

Host responses are often ineffective at clearing Mycoplasma bovis infection and may contribute to the pathogenesis of disease. M bovis possesses a surprisingly large repertoire of strategies to evade and modulate host responses. Unopsonized M bovis impairs phagocytosis and killing by neutrophils and macrophages. Apoptosis of neutrophils and lymphocytes is enhanced, whereas it is delayed in macrophages. Both proinflammatory and antiinflammatory cytokines are stimulated during M bovis infection depending on the cell type and location, and overall systemic responses tend to have a T-helper 2 bias. M bovis reduces proliferation of T cells and, in chronic infection, causes T-cell exhaustion.

Cooperation of PD-1 and LAG-3 in the exhaustion of CD4+ and CD8+ T cells during bovine leukemia virus infection

Bovine leukemia virus (BLV) is a retrovirus that infects B cells in cattle and causes bovine leukosis after a long latent period. Progressive exhaustion of T cell functions is considered to facilitate disease progression of BLV infection. Programmed death-1 (PD-1) and lymphocyte activation gene-3 (LAG-3) are immunoinhibitory receptors that contribute to T-cell exhaustion caused by BLV infection in cattle. However, it is unclear whether the cooperation of PD-1 and LAG-3 accelerates disease progression of BLV infection. In this study, multi-color flow cytometric analyses of PD-1- and LAG-3-expressing T cells were performed in BLV-infected cattle at different stages of the disease. The frequencies of PD-1+LAG-3+ heavily exhausted T cells among CD4+ and CD8+ T cells was higher in the blood of cattle with B-cell lymphoma over that of BLV-uninfected and BLV-infected cattle without lymphoma. In addition, blockade assays of peripheral blood mononuclear cells were performed to examine whether inhibition of the interactions between PD-1 and LAG-3 and their ligands by blocking antibodies could restore T-cell function during BLV infection. Single or dual blockade of the PD-1 and LAG-3 pathways reactivated the production of Th1 cytokines, interferon-γ and tumor necrosis factor-α, from BLV-specific T cells of the infected cattle. Taken together, these results indicate that PD-1 and LAG-3 cooperatively mediate the functional exhaustion of CD4+ and CD8+ T cells and are associated with the development of B-cell lymphoma in BLV-infected cattle.

Prostaglandin E2 Induction Suppresses the Th1 Immune Responses in Cattle with Johne's Disease

Johne's disease, caused by Mycobacterium avium subsp. paratuberculosis, is a bovine chronic infection that is endemic in Japan and many other countries. The expression of immunoinhibitory molecules is upregulated in cattle with Johne's disease, but the mechanism of immunosuppression is poorly understood. Prostaglandin E₂ (PGE₂) is immunosuppressive in humans, but few veterinary data are available. In this study, functional and kinetic analyses of PGE₂ were performed to investigate the immunosuppressive effect of PGE₂ during Johne's disease. In vitro PGE₂ treatment decreased T-cell proliferation and Th1 cytokine production and upregulated the expression of immunoinhibitory molecules such as interleukin-10 and programmed death ligand 1 (PD-L1) in peripheral blood mononuclear cells (PBMCs) from healthy cattle. PGE₂ was upregulated in sera and intestinal lesions of cattle with Johne's disease. In vitro stimulation with Johnin purified protein derivative (J-PPD) induced cyclooxygenase-2 (COX-2) transcription, PGE₂ production, and upregulation of PD-L1 and immunoinhibitory receptors in PBMCs from cattle infected with M. avium subsp. paratuberculosis Therefore, Johnin-specific Th1 responses could be limited by the PGE₂ pathway in cattle. In contrast, downregulation of PGE₂ with a COX-2 inhibitor promoted J-PPD-stimulated CD8⁺ T-cell proliferation and Th1 cytokine production in PBMCs from the experimentally infected cattle. PD-L1 blockade induced J-PPD-stimulated CD8⁺ T-cell proliferation and interferon gamma production in vitro Combined treatment with a COX-2 inhibitor and anti-PD-L1 antibodies enhanced J-PPD-stimulated CD8⁺ T-cell proliferation in vitro, suggesting that the blockade of both pathways is a potential therapeutic strategy to control Johne's disease. The effects of COX-2 inhibition warrant further study as a novel treatment of Johne's disease.

Mycoplasma bovis-Induced Inhibition of Bovine Peripheral Blood Mononuclear Cell Proliferation Is Ameliorated after Blocking the Immune-Inhibitory Programmed Death 1 Receptor

Mycoplasma bovis-induced immune suppression is a major obstacle faced by the host for controlling infections. M. bovis impairment of antigen-specific T-cell responses is achieved through inhibiting the proliferation of peripheral blood mononuclear cells (PBMCs). This impairment may contribute to the persistence of M. bovis infection in various sites, including lungs, and its systemic spread to various organs such as joints, with the underlying mechanisms remaining elusive. Here, we elucidated the role of the immune-inhibitory receptor programmed death 1 (PD-1) and its ligand (PD-L1) in M. bovis infection. Flow cytometry (FCM) analyses revealed an upregulation of PD-L1 expression on tracheal and lung epithelial cell lines after M. bovis infection. In addition, we found increased PD-L1 expression on purified lung lavage macrophages following M. bovis infection by FCM and determined its localization by immunofluorescence analysis comparing infected and control lung tissue sections. Moreover, M. bovis infection increased the expression of the PD-1 receptor on total PBMCs and in gated CD4⁺ and CD8⁺ T-cell subpopulations. We demonstrated that M. bovis infection induced a significant decrease in CD4⁺ PD-1^INT and CD8⁺ PD-1^INT subsets with intermediate PD-1 expression, which functioned as progenitor pools giving rise to CD4⁺ PD-1^HIGH and CD8⁺ PD-1^HIGH subsets with high PD-1 expression levels. We blocked PD-1 receptors on PBMCs using anti-PD-1 antibody at the beginning of infection, leading to a significant restoration of the proliferation of PBMCs. Taken together, our data indicate a significant involvement of the PD-1/PD-L1 inhibitory pathway during M. bovis infection and its associated immune exhaustion, culminating in impaired host immune responses.

Longitudinal assessment of T cell inhibitory receptors in liver transplant recipients and their association with posttransplant infections

Current immunosuppression regimens in organ transplantation primarily inhibit T cells. However, T cells are also critical in protective immunity, especially in immune-compromised patients. In this study, we examined the association of T cell dysfunction, as marked by expression of T cell exhaustion molecules, and posttransplant infections in a cohort of liver transplant patients. We focused on Programmed Death 1 (PD-1) and T cell Ig- and mucin-domain molecule 3 (Tim-3), which are potent co-inhibitory receptors, and their persistent expression often leads to T cell dysfunction and compromised protective immunity. We found that patients with the highest expression of PD-1 +Tim-3+ T cells in the memory compartment before transplantation had increased incidence of infections after liver transplantation, especially within the first 90 days. Longitudinal analysis in the first year showed a strong association between variability of PD-1 and Tim-3 expression by T cells and infectious episodes in transplant patients. Furthermore, T cells that expressed PD-1 and Tim-3 had a significantly reduced capacity in producing interferon (IFN)-γ in vitro, and this reduced IFN-γ production could be partially reversed by blocking PD-1 and Tim-3. Interestingly, the percentage of Foxp3+ regulatory T cells in liver transplant patients was stable in the study period. We concluded that the functional status of T cells before and after liver transplantation, as shown by PD-1 and Tim-3 expression, may be valuable in prognosis and management of posttransplant infections.

Anti-Bovine Programmed Death-1 Rat-Bovine Chimeric Antibody for Immunotherapy of Bovine Leukemia Virus Infection in Cattle

Blockade of immunoinhibitory molecules, such as programmed death-1 (PD-1)/PD-ligand 1 (PD-L1), is a promising strategy for reinvigorating exhausted T cells and preventing disease progression in a variety of chronic infections. Application of this therapeutic strategy to cattle requires bovinized chimeric antibody targeting immunoinhibitory molecules. In this study, anti-bovine PD-1 rat–bovine chimeric monoclonal antibody 5D2 (Boch5D2) was constructed with mammalian expression systems, and its biochemical function and antiviral effect were characterized in vitro and in vivo using cattle infected with bovine leukemia virus (BLV). Purified Boch5D2 was capable of detecting bovine PD-1 molecules expressed on cell membranes in flow cytometric analysis. In particular, Biacore analysis determined that the binding affinity of Boch5D2 to bovine PD-1 protein was similar to that of the original anti-bovine PD-1 rat monoclonal antibody 5D2. Boch5D2 was also capable of blocking PD-1/PD-L1 binding at the same level as 5D2. The immunomodulatory and therapeutic effects of Boch5D2 were evaluated by in vivo administration of the antibody to a BLV-infected calf. Inoculated Boch5D2 was sustained in the serum for a longer period. Boch5D2 inoculation resulted in activation of the proliferation of BLV-specific CD4⁺ T cells and decrease in the proviral load of BLV in the peripheral blood. This study demonstrates that Boch5D2 retains an equivalent biochemical function to that of the original antibody 5D2 and is a candidate therapeutic agent for regulating antiviral immune response in vivo. Clinical efficacy of PD-1/PD-L1 blockade awaits further experimentation with a large number of animals.