Neutrophil extracellular Taps play an important role in clearance ofStreptococcus suis in vivo

Encapsulated Streptococcus suis impairs optimal neutrophil functions which are not rescued by priming with colony-stimulating factors

The porcine pathogen and zoonotic agent Streptococcus suis induces an exacerbated inflammation in the infected hosts that leads to sepsis, meningitis, and sudden death. Several virulence factors were described for S. suis of which the capsular polysaccharide (CPS) conceals it from the immune system, and the suilysin exhibits cytotoxic activity. Although neutrophils are recruited rapidly upon S. suis infection, their microbicidal functions appear to be poorly activated against the bacteria. However, during disease, the inflammatory environment could promote neutrophil activation as mediators such as the granulocyte colony-stimulating factor granulocyte (G-CSF) and the granulocyte-macrophages colony-stimulating factor (GM-CSF) prime neutrophils and enhance their responsiveness to bacterial detection. Thus, we hypothesized that CPS and suilysin prevent an efficient activation of neutrophils by S. suis, but that G-CSF and GM-CSF rescue neutrophil activation, leading to S. suis elimination. We evaluated the functions of porcine neutrophils in vitro in response to S. suis and investigated the role of the CPS and suilysin on cell activation using isogenic mutants of the bacteria. We also studied the influence of G-CSF and GM-CSF on neutrophil response to S. suis by priming the cells with recombinant proteins. Our study confirmed that CPS prevents S. suis-induced activation of most neutrophil functions but participates in the release of neutrophil-extracellular traps (NETs). Priming with G-CSF did not influence cell activation, but GM-CSF strongly promote IL-8 release, indicating its involvement in immunomodulation. However, priming did not enhance microbicidal functions. Studying the interaction between S. suis and neutrophils-first responders in host defense-remains fundamental to understand the immunopathogenesis of the infection and to develop therapeutical strategies related to neutrophils' defense against this bacterium.

Fpr2 exacerbates Streptococcus suis-induced streptococcal toxic shock-like syndrome via attenuation of neutrophil recruitment

The life-threatening disease streptococcal toxic shock-like syndrome (STSLS), caused by the bacterial pathogen Streptococcus suis (S. suis). Proinflammatory markers, bacterial load, granulocyte recruitment, and neutrophil extracellular traps (NETs) levels were monitored in wild-type (WT) and Fpr2^-/- mice suffering from STSLS. LXA4 and AnxA1, anti-inflammatory mediators related to Fpr2, were used to identity a potential role of the Fpr2 in STSLS development. We also elucidated the function of Fpr2 at different infection sites by comparing the STSLS model with the S. suis-meningitis model. Compared with the WT mice, Fpr2^-/- mice exhibited a reduced inflammatory response and bacterial load, and increased neutrophil recruitment. Pretreatment with AnxA1 or LXA4 impaired leukocyte recruitment and increased both bacterial load and inflammatory reactions in WT but not Fpr2^-/- mice experiencing STSLS. These results indicated that Fpr2 impairs neutrophil recruitment during STSLS, and this impairment is enhanced by AnxA1 or LXA4. By comparing the functions of Fpr2 in different S. suis infection models, inflammation and NETs was found to hinder bacterial clearance in S. suis meningitis, and conversely accelerate bacterial clearance in STSLS. Therefore, interference with neutrophil recruitment could potentially be harnessed to develop new treatments for this infectious disease.

Comparative Genome Analysis of Streptococcus suis Serotype 9 Isolates from China, The Netherland, and the U.K

Streptococcus suis (S. suis) is an important swine pathogen and an emerging zoonotic agent worldwide. Serotype 9 is the most prevalent serotype in several European countries but it is relatively rare in China. In this study, through the investigation of the serotypes of 279 S. suis strains isolated from China from 2015 to 2017, it was found that serotype 9 is the second most prevalent serotype (43 out of 279), behind serotype 2 (83 out of 279). Next, the 43 serotype 9 isolates were sequenced and compared with those from the Netherland (28) and the U.K. (eight). For the purpose of comparison, the strain D12 (GCA_000231905), which has completed genome sequences, was also incorporated. Phylogenetic tree analysis showed that the strains from China and the U.K. were heterogeneous. In contrast, all but one from the Netherland belonged to the same clade. The dominant clades of Chinese strains (33) and strains from the Netherland (27) were very similar. Both of them may have originated from the same strain about 70 years ago. Then, the distributions of virulence-associated genes and antibiotic resistance genes among different clades and sources were analyzed. By comparison, strains from the Netherland carried more virulence-associated genes and those from the U.K. had more antibiotic resistance genes. Additionally, some virulence-associated genes (salK and salR) and antibiotic resistance genes (lincomycin and spectinomycin) existed only in several Chinese strains. In conclusion, our data displayed the population characteristics and differences of S. suis serotype 9 between China and Europe, suggesting that they have taken different evolutionary paths.

Neutrophils in Streptococcus suis Infection: From Host Defense to Pathology

Streptococcus suis is a swine pathogen and zoonotic agent responsible for economic losses to the porcine industry. Infected animals may develop meningitis, arthritis, endocarditis, sepsis and/or sudden death. The pathogenesis of the infection implies that bacteria breach mucosal host barriers and reach the bloodstream, where they escape immune-surveillance mechanisms and spread throughout the organism. The clinical manifestations are mainly the consequence of an exacerbated inflammation, defined by an exaggerated production of cytokines and recruitment of immune cells. Among them, neutrophils arrive first in contact with the pathogens to combat the infection. Neutrophils initiate and maintain inflammation, by producing cytokines and deploying their arsenal of antimicrobial mechanisms. Furthermore, neutrophilic leukocytosis characterizes S. suis infection, and lesions of infected subjects contain a large number of neutrophils. Therefore, this cell type may play a role in host defense and/or in the exacerbated inflammation. Nevertheless, a limited number of studies addressed the role or functions of neutrophils in the context of S. suis infection. In this review, we will explore the literature about S. suis and neutrophils, from their interaction at a cellular level, to the roles and behaviors of neutrophils in the infected host in vivo.

Screening of Virulence-Related Transcriptional Regulators in Streptococcus suis

Streptococcus suis (S.suis) is an important zoonotic pathogen that causes many severe diseases in pigs and humans. Virulence-related transcriptional regulators have been widely reported in pathogenic microorganisms, but only a few have been identified in S.suis. Our aim was to screen virulence-related transcriptional regulators in S.suis. A total of 89 such genes were predicted in the S.suis genome, of which 22 were up-regulated and 18 were down-regulated during S.suis infection in mice. To evaluate the roles of these differentially expressed factors in S.suis virulence, deletion mutants were constructed, and 10 mutants were successfully obtained. Among these genes, the deletion of comR, sitR, or sxvR caused significantly decreased virulence in mice, compared to that with the wild-type strain. Moreover, the survival of ΔcomR, ΔsitR, and ΔsxvR mutant strains in blood was significantly reduced both in vitro and in vivo. Furthermore, their pro-inflammatory abilities were also obviously decreased in vivo. The regulatory mechanisms of comR, sitR, and sxvR were then analyzed by whole transcriptome RNA sequencing (RNA-Seq). Results indicated that the absence of comR induced the down-regulation of 17 virulence factors or virulence-related factors, including genes involved in the synthesis of capsules, oxidative stress tolerance, immune evasion, and cell division. Furthermore, three and two virulence factors or virulence-related factors were down-regulated upon deletion of sitR and sxvR, respectively. Thus, this study reports the discovery of three virulence-associated transcriptional regulatory factors in S.suis. These factors could ultimately be targeted to control infection caused by these bacteria.

How Streptococcus suis serotype 2 attempts to avoid attack by host immune defenses

Streptococcus suis (S. suis) type 2 (SS2) is an important zoonotic pathogen that causes swine streptococcosis, a widespread infectious disease that occurs in pig production areas worldwide and causes serious economic losses in the pork industry. Hosts recognize pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs) to activate both innate and acquired immune responses. However, S. suis has evolved multiple mechanisms to escape host defenses. Pathogenic proteins, such as enolase, double-component regulatory systems, factor H-combining proteins and other pathogenic and virulence factors, contribute to immune escape by evading host phagocytosis, reactive oxygen species (ROS), complement-mediated immune destruction, etc. SS2 can prevent neutrophil extracellular trap (NET) formation to avoid being trapped by porcine neutrophils and disintegrate host immunoglobulins via IgA1 hydrolases and IgM proteases. Currently, the pathogenesis of arthritis and meningitis caused by SS2 infection remains unclear, and further studies are necessary to elucidate it. Understanding immune evasion mechanisms after SS2 infection is important for developing high-efficiency vaccines and targeted drugs.

Streptococcus Suis Serotype 2 Stimulates Neutrophil Extracellular Traps Formation via Activation of p38 MAPK and ERK1/2

Streptococcus suis serotype 2 is a major pathogen of swine streptococcicosis, which result in serious economic loss worldwide. SS2 is an important zoonosis causing meningitis and even death in humans. Neutrophil extracellular traps (NETs) constitute a significant bactericidal strategy of innate immune. The battle between SS2 and NETs may account for the pathogenicity of SS2. However, the molecular mechanism underlying release of SS2-induced NETs remains unclear. In this study, SS2 was found to induce NETs within 2–4 h, and was dependent on reactive oxygen species (ROS) from NADPH oxidase. Moreover, SS2 could activate neutrophil p38 MAPK and ERK1/2. Blockage of p38 MAPK or ERK1/2 activation decreased SS2-induced NETs formation by 65 and 85%, respectively. In addition, NADPH oxidase derived ROS inhibition negatively affected phosphorylation of p38 MAPK and ERK1/2 in SS2 induced neutrophils. Both TLR2 and TLR4 were significantly up-regulated by SS2 infection in blood cells in vivo and neutrophils in vitro, which indicates these two receptors are involved in SS2 recognition. Blocking TLR4 signaling could further inhibit the activation of ERK1/2, but not p38 MAPK; however, TLR4 signaling inhibition reduced NETs formation induced by SS2. In conclusion, SS2 could be recognized by TLR2 and/or TLR4, initiating NETs formation signaling pathways in a NADPH oxidase derived ROS dependent manner. ROS will activate p38 MAPK and ERK1/2, which ultimately induces NETs formation.

A Label-Free Quantitative Proteomic Analysis of Mouse Neutrophil Extracellular Trap Formation Induced by Streptococcus suis or Phorbol Myristate Acetate (PMA)

Streptococcus suis (S. suis) ranks among the five most important porcine pathogens worldwide and occasionally threatens human health, particularly in people who come into close contact with pigs or pork products. An S. suis infection induces the formation of neutrophil extracellular traps (NETs) in vitro and in vivo, and the NET structure plays an essential role in S. suis clearance. However, the signaling pathway by which S. suis induces NET formation remains to be elucidated. In the present study, we used a label-free quantitative proteomic analysis of mouse NET formation induced by S. suis or phorbol myristate acetate (PMA), a robust NET inducer. Greater than 50% of the differentially expressed proteins in neutrophils infected by S. suis showed similar changes as observed following PMA stimulation, and PKC, NADPH oxidase, and MPO were required for NET formation induced by both stimuli. Because PMA induced robust NET formation while S. suis (MOI = 2) induced only weak NET formation, the association between the inducer and NET formation was worth considering. Interestingly, proteins involved in peptidase activity showed significant differential changes in response to each inducer. Of these peptidases, MMP-8 expression was obviously decreased in response to PMA, but it was not significantly changed in response to S. suis. A subsequent study further confirmed that MMP-8 activity was inversely correlated with NET formation induced by both stimuli. Therefore, the present study provides potentially important information about the manner by which neutrophils responded to the inducers to form NETs.

Air pollution as a determinant of rheumatoid arthritis

Pollution has long been incriminated in many cardiovascular and respiratory diseases. More recently, studies evaluated the potential role for particulate pollutants in autoimmune diseases, including rheumatoid arthritis (RA). The incidence of RA was found to be higher in urban areas. Living near air pollution emitters was associated with higher risks of developing RA and of producing RA-specific autoantibodies. Nevertheless, no strong epidemiological evidence exists to link one or more specific air pollution particles to RA. The presence in the bronchi of lymphoid satellite islands (inducible bronchus-associated lymphoid tissue, iBALT) is strongly associated with both inflammatory lung disease and RA-associated lung disease. Diesel exhaust particles can stimulate iBALT formation. The induction by air pollution of an inflammatory environment with high citrullination levels in the lung may induce iBALT formation, thereby causing a transition toward a more specific immune response via the production of anti-citrullinated peptide antibodies. Air pollution not only triggers innate immune responses at the molecular level, increasing the levels of proinflammatory cytokines and reactive oxygen species, but is also involved in adaptive immune responses. Thus, via the aryl hydrocarbon receptor (AHR), diesel exhaust particles can trigger a T-cell switch to the Th17 profile. Finally, in the murine collagen-induced arthritis model, animals whose lymphocytes lack the AHR develop milder arthritis.

Potential roles of neutrophils in regulating intestinal mucosal inflammation of inflammatory bowel disease

Inflammatory bowel diseases (IBD), comprising of ulcerative colitis and Crohn's disease, are inflammatory disorders of the gastrointestinal tract characterized by chronically relapsing mucosal inflammation. Neutrophils, as the effector cells of acute inflammation, have long been reported to play a role in the maintenance of intestinal homeostasis and pathogenesis of IBD. At the early stage of mucosal inflammation in patients with IBD, neutrophils flood into intestinal mucosa, phagocytose pathogenic microbes, and promote mucosal healing and resolution of inflammation. However, large numbers of neutrophils infiltrating in the inflamed mucosa and accumulating in the epithelia cause damage of mucosal architecture, compromised epithelial barrier and production of inflammatory mediators. In this review we discuss the critical roles of neutrophils in modulating innate and adaptive immune responses in intestinal mucosa, and, importantly, clarify the potential roles of neutrophils related to their production of inflammatory mediators, transenthothelial and transepithelial migration into intestinal mucosa, and the underlying mechanisms in regulating mucosal inflammation of IBD. Moreover, we also describe a new subset of neutrophils (i.e., CD177⁺ neutrophils) and illustrate its protective role in modulating intestinal mucosal immune responses in IBD.

HP1330 Contributes to Streptococcus suis Virulence by Inducing Toll-Like Receptor 2- and ERK1/2-Dependent Pro-inflammatory Responses and Influencing In Vivo S. suis Loads

Streptococcus suis 2 (SS2) has evolved into a highly invasive pathogen responsible for two large-scale outbreaks of streptococcal toxic shock-like syndrome (STSLS) in China. Excessive inflammation stimulated by SS2 is considered a hallmark of STSLS, even it also plays important roles in other clinical symptoms of SS2-related disease, including meningitis, septicemia, and sudden death. However, the mechanism of SS2-caused excessive inflammation remains poorly understood. Here, a novel pro-inflammatory protein was identified (HP1330), which could induce robust expression of pro-inflammatory cytokines (TNF-α, MCP-1, and IL-1β) in RAW264.7 macrophages. To evaluate the role of HP1330 in SS2 virulence, an hp1330-deletion mutant (Δhp1330) was constructed. In vitro, hp1330 disruption led to a decreased pro-inflammatory ability of SS2 in RAW 264.7 macrophages. In vivo, Δhp1330 showed reduced lethality, pro-inflammatory activity, and bacterial loads in mice. To further elucidate the mechanism of HP1330-induced pro-inflammatory cytokine production, antibody blocking and gene-deletion experiments with macrophages were performed. The results revealed that the pro-inflammatory activity of HP1330 depended on the recognition of toll-like receptor 2 (TLR2). Furthermore, a specific inhibitor of the extracellular signal-regulated kinase 1/2 (ERK1/2) pathways could significantly decrease HP1330-induced pro-inflammatory cytokine production, and western blot analysis showed that HP1330 could induce activation of the ERK1/2 pathway. Taken together, our findings demonstrate that HP1330 contributes to SS2 virulence by inducing TLR2- and ERK1/2-dependent pro-inflammatory cytokine production and influencing in vivo bacterial loads, implying that HP1330 may be associated with STSLS caused by SS2.

Neutrophil extracellular trap formation in the Streptococcus suis-infected cerebrospinal fluid compartment

Streptococcus suis is an important meningitis-causing pathogen in pigs and humans. Neutrophil extracellular traps (NETs) have been identified as host defense mechanism against different pathogens. Here, NETs were detected in the cerebrospinal fluid (CSF) of S. suis-infected piglets despite the presence of active nucleases. To study NET-formation and NET-degradation after transmigration of S. suis and neutrophils through the choroid plexus epithelial cell barrier, a previously described model of the human blood-CSF barrier was used. NETs and respective entrapment of streptococci were recorded in the "CSF compartment" despite the presence of active nucleases. Comparative analysis of S. suis wildtype and different S. suis nuclease mutants did not reveal significant differences in NET-formation or bacterial survival. Interestingly, transcript expression of the human cathelicidin LL-37, a NET-stabilizing factor, increased after transmigration of neutrophils through the choroid plexus epithelial cell barrier. In good accordance, the porcine cathelicidin PR-39 was significantly increased in CSF of piglets with meningitis. Furthermore, we confirmed that PR-39 is associated with NETs in infected CSF and inhibits neutrophil DNA degradation by bacterial nucleases. In conclusion, neutrophils form NETs after breaching the infected choroid plexus epithelium, and those NETs may be protected by antimicrobial peptides against bacterial nucleases.

Inducible Bronchus-Associated Lymphoid Tissue: Taming Inflammation in the Lung

Following pulmonary inflammation, leukocytes that infiltrate the lung often assemble into structures known as inducible Bronchus-Associated Lymphoid Tissue (iBALT). Like conventional lymphoid organs, areas of iBALT have segregated B and T cell areas, specialized stromal cells, high endothelial venules, and lymphatic vessels. After inflammation is resolved, iBALT is maintained for months, independently of inflammation. Once iBALT is formed, it participates in immune responses to pulmonary antigens, including those that are unrelated to the iBALT-initiating antigen, and often alters the clinical course of disease. However, the mechanisms that govern immune responses in iBALT and determine how iBALT impacts local and systemic immunity are poorly understood. Here, we review our current understanding of iBALT formation and discuss how iBALT participates in pulmonary immunity.