Chemokine-dependent neutrophil recruitment in a murine model of Legionella pneumonia: potential role of neutrophils as immunoregulatory cells

Dendritic cells activate pyroptosis and effector-triggered apoptosis to restrict Legionella infection

The innate immune system relies on pattern recognition receptors (PRRs) to detect pathogen-associated molecular patterns (PAMPs) and guard proteins to monitor pathogen disruption of host cell processes. How different immune cell types engage PRR- and guard protein-dependent defenses in response to infection is poorly understood. Here, we show that macrophages and dendritic cells (DCs) respond in distinct ways to bacterial virulence activities. In macrophages, the bacterial pathogen Legionella pneumophila deploys its Dot/Icm type IV secretion system (T4SS) to deliver effector proteins that facilitate its robust intracellular replication. In contrast, T4SS activity triggers rapid DC death that potently restricts Legionella replication within this cell type. Intriguingly, we found that infected DCs exhibit considerable heterogeneity at the single cell level. Initially, a subset of DCs activate caspase-11 and NLRP3 inflammasome-dependent pyroptosis and release IL-1 β early during infection. At later timepoints, a separate DC population undergoes apoptosis driven by T4SS effectors that block host protein synthesis, thereby depleting the levels of the pro-survival proteins Mcl-1 and cFLIP. Together, pyroptosis and effector-triggered apoptosis robustly restrict Legionella replication in DCs. Collectively, our work suggests a model where Mcl-1 and cFLIP guard host translation in DCs, and that macrophages and DCs distinctly employ innate immune sensors and guard proteins to mount divergent responses to Legionella infection.

Staphylococcus aureus in Inflammation and Pain: Update on Pathologic Mechanisms

Staphylococcus aureus (S. aureus) is a Gram-positive bacterium of significant clinical importance, known for its versatility and ability to cause a wide array of infections, such as osteoarticular, pulmonary, cardiovascular, device-related, and hospital-acquired infections. This review describes the most recent evidence of the pathogenic potential of S. aureus, which is commonly part of the human microbiota but can lead to severe infections. The prevalence of pathogenic S. aureus in hospital and community settings contributes to substantial morbidity and mortality, particularly in individuals with compromised immune systems. The immunopathogenesis of S. aureus infections involves intricate interactions with the host immune and non-immune cells, characterized by various virulence factors that facilitate adherence, invasion, and evasion of the host's defenses. This review highlights the complexity of S. aureus infections, ranging from mild to life-threatening conditions, and underscores the growing public health concern posed by multidrug-resistant strains, including methicillin-resistant S. aureus (MRSA). This article aims to provide an updated perspective on S. aureus-related infections, highlighting the main diseases linked to this pathogen, how the different cell types, virulence factors, and signaling molecules are involved in the immunopathogenesis, and the future perspectives to overcome the current challenges to treat the affected individuals.

Granulocyte colony stimulating factor promotes scarless tissue regeneration

Mammals typically heal with fibrotic scars, and treatments to regenerate human skin and hair without a scar remain elusive. We discovered that mice lacking C-X-C motif chemokine receptor 2 (CXCR2 knockout [KO]) displayed robust and complete tissue regeneration across three different injury models: skin, hair follicle, and cartilage. Remarkably, wild-type mice receiving plasma from CXCR2 KO mice through parabiosis or injections healed wounds scarlessly. A comparison of circulating proteins using multiplex ELISA revealed a 24-fold higher plasma level of granulocyte colony stimulating factor (G-CSF) in CXCR2 KO blood. Local injections of G-CSF into wild-type (WT) mouse wound beds reduced scar formation and increased scarless tissue regeneration. G-CSF directly polarized macrophages into an anti-inflammatory phenotype, and both CXCR2 KO and G-CSF-treated mice recruited more anti-inflammatory macrophages into injured areas. Modulating macrophage activation states at early time points after injury promotes scarless tissue regeneration and may offer a therapeutic approach to improve healing of human skin wounds.

BMAL1 plays a crucial role in immune homeostasis during sepsis-induced acute lung injury

Sepsis is a widespread and life-threatening disease characterised by infection-triggered immune hyperactivation and cytokine storms, culminating in tissue damage and multiple organ dysfunction syndrome. BMAL1 is a pivotal transcription factor in the circadian clock that plays a crucial role in maintaining immune homeostasis. BMAL1 dysregulation has been implicated in inflammatory diseases and immunodeficiency. However, the mechanisms underlying BMAL1 disruption in sepsis-induced acute lung injury (ALI) remain poorly understood. In vitro, we used THP1 and mouse peritoneal macrophages to elucidate the potential mechanism of BMAL1 function in sepsis. In vivo, an endotoxemia model was used to investigate the effect of BMAL1 on sepsis and the therapeutic role of targeting CXCR2. We showed that BMAL1 significantly affected the regulation of innate immunity in sepsis-induced ALI. BMAL1 deficiency in the macrophages exacerbated systemic inflammation and sepsis-induced ALI. Mechanistically, BMAL1 acted as a transcriptional suppressor and regulated the expression of CXCL2. BMAL1 deficiency in macrophages upregulated CXCL2 expression, increasing the recruitment of polymorphonuclear neutrophils and the formation of neutrophil extracellular traps (NETs) by binding to the chemokine receptor CXCR2, thereby intensifying lung injury in a sepsis model. Furthermore, a selective inhibitor of CXCR2, SB225002, exerted promising therapeutic effects by markedly reducing neutrophil infiltration and NETs formation and alleviating lung injury. Importantly, CXCR2 blockade mitigated multiple organ dysfunction. Collectively, these findings suggest that BMAL1 controls the CXCL2/CXCR2 pathway, and the therapeutic efficacy of targeting CXCR2 in sepsis has been validated, presenting BMAL1 as a potential therapeutic target for lethal infections.

Neutrophil heterogeneity and aging: implications for COVID-19 and wound healing

Neutrophils play a critical role in the immune response to infection and tissue injury. However, recent studies have shown that neutrophils are a heterogeneous population with distinct subtypes that differ in their functional properties. Moreover, aging can alter neutrophil function and exacerbate immune dysregulation. In this review, we discuss the concept of neutrophil heterogeneity and how it may be affected by aging. We then examine the implications of neutrophil heterogeneity and aging for COVID-19 pathogenesis and wound healing. Specifically, we summarize the evidence for neutrophil involvement in COVID-19 and the potential mechanisms underlying neutrophil recruitment and activation in this disease. We also review the literature on the role of neutrophils in the wound healing process and how aging and neutrophil heterogeneity may impact wound healing outcomes. Finally, we discuss the potential for neutrophil-targeted therapies to improve clinical outcomes in COVID-19 and wound healing.

Protein sociology of ProA, Mip and other secreted virulence factors at the Legionella pneumophila surface

The pathogenicity of L. pneumophila, the causative agent of Legionnaires' disease, depends on an arsenal of interacting proteins. Here we describe how surface-associated and secreted virulence factors of this pathogen interact with each other or target extra- and intracellular host proteins resulting in host cell manipulation and tissue colonization. Since progress of computational methods like AlphaFold, molecular dynamics simulation, and docking allows to predict, analyze and evaluate experimental proteomic and interactomic data, we describe how the combination of these approaches generated new insights into the multifaceted "protein sociology" of the zinc metalloprotease ProA and the peptidyl-prolyl cis/trans isomerase Mip (macrophage infectivity potentiator). Both virulence factors of L. pneumophila interact with numerous proteins including bacterial flagellin (FlaA) and host collagen, and play important roles in virulence regulation, host tissue degradation and immune evasion. The recent progress in protein-ligand analyses of virulence factors suggests that machine learning will also have a beneficial impact in early stages of drug discovery.

Sepsis causes neutrophil infiltration in muscle leading to muscle atrophy and weakness in mice

BACKGROUND

Sepsis-induced muscle atrophy leads to prolonged physical dysfunction. Although the interaction of muscle atrophy and macrophage has been reported in sepsis, the role of neutrophils in muscle atrophy has not been thoroughly investigated. This study sought to investigate the long-term changes in muscle-localized neutrophils after sepsis induction and their possible role in sepsis.

METHODS

Sepsis was induced in seven-week-old male C57BL/6J mice 8-12 (cecal slurry [CS] model) via intraperitoneal injection of 1 mg/g cecal slurry. The percentage change in body weight and grip strength was evaluated. The tibialis anterior muscles were dissected for microscopic examination of the cross-sectional area of myofibers or Fluorescence-activated cell sorting (FACS) analysis of immune cells. These changes were evaluated in the following conditions: (1) Longitudinal change until day 61, (2) CS concentration-dependent change on day 14 at the low (0.3 mg/g), middle (1.0 mg/g), and high (2.0 mg/g) concentrations, and (3) CS mice on day 14 treated with an anti-Ly6G antibody that depletes neutrophils.

RESULTS

Body weight and grip strength were significantly lower in the CS model until day 61 (body weight: 123.1% ± 1.8% vs. 130.3% ± 2.5%, p = 0.04; grip strength: 104.5% ± 3.8% vs. 119.3% ± 5.3%, p = 0.04). Likewise, cross-sectional muscle area gradually decreased until day 61 from the CS induction (895.6 [606.0-1304.9] μm2 vs. 718.8 [536.2-937.0] μm2, p < 0.01). The number of muscle-localized neutrophils increased from 2.3 ± 0.6 cell/mg on day 0 to 22.2 ± 13.0 cell/mg on day 14, and decreased thereafter. In terms of CS concentration-dependent change, cross-sectional area was smaller (484.4 ± 221.2 vs. 825.8 ± 436.2 μm2 [p < 0.001]) and grip strength was lower (71.4% ± 12.8% vs. 116.3% ± 7.4%, p = 0.01) in the CS High group compared with the control, with increased neutrophils (p = 0.03). Ly6G-depleted mice demonstrated significant increase of muscle cross-sectional area and grip strength compared with control mice (p < 0.01).

CONCLUSIONS

Sepsis causes infiltration of neutrophils in muscles, leading to muscle atrophy and weakness. Depletion of neutrophils in muscle reverses sepsis-induced muscle atrophy and weakness. These results suggest that neutrophils may play a critical role in sepsis-induced muscle atrophy and weakness.

Ethanol Intoxication Impairs Respiratory Function and Bacterial Clearance and Is Associated With Neutrophil Accumulation in the Lung After Streptococcus pneumoniae Infection

Alcohol consumption is commonplace in the United States and its prevalence has increased in recent years. Excessive alcohol use is linked to an increased risk of infections including pneumococcal pneumonia, mostly commonly caused by Streptococcus pneumoniae. In addition, pneumonia patients with prior alcohol use often require more intensive treatment and longer hospital stays due to complications of infection. The initial respiratory tract immune response to S. pneumoniae includes the production of pro-inflammatory cytokines and chemokines by resident cells in the upper and lower airways which activate and recruit leukocytes to the site of infection. However, this inflammation must be tightly regulated to avoid accumulation of toxic by-products and subsequent tissue damage. A majority of previous work on alcohol and pneumonia involve animal models utilizing high concentrations of ethanol or chronic exposure and offer conflicting results about how ethanol alters immunity to pathogens. Further, animal models often employ a high bacterial inoculum which may overwhelm the immune system and obscure results, limiting their applicability to the course of human infection. Here, we sought to determine how a more moderate ethanol exposure paradigm affects respiratory function and innate immunity in mice after intranasal infection with 10⁴ colony forming units of S. pneumoniae. Ethanol-exposed mice displayed respiratory dysfunction and impaired bacterial clearance after infection compared to their vehicle-exposed counterparts. This altered response was associated with increased gene expression of neutrophil chemokines Cxcl1 and Cxcl2 in whole lung homogenates, elevated concentrations of circulating granulocyte-colony stimulating factor (G-CSF), and higher neutrophil numbers in the lung 24 hours after infection. Taken together, these findings suggest that even a more moderate ethanol consumption pattern can dramatically modulate the innate immune response to S. pneumoniae after only 3 days of ethanol exposure and provide insight into possible mechanisms related to the compromised respiratory immunity seen in alcohol consumers with pneumonia.

Long Lasting Antibodies From Convalescent Pertussis Patients Induce ROS Production and Bacterial Killing by Human Neutrophils

Pertussis is a respiratory infection caused by the Gram-negative bacterium Bordetella pertussis. Despite high vaccination coverage this disease remains a public health concern worldwide. A better understanding of the protective immune responses to B. pertussis is required for the development of improved vaccines. The aim of this study was to determine the production of reactive oxygen species (ROS) by human neutrophils in response to B. pertussis and to determine the contribution of opsonizing antibodies from convalescent pertussis patients in this response. The serum samples from convalescent patients were taken at <3, 9, 18 and 36 months after diagnosis of pertussis. Also included were sera from healthy age-matched controls. We show that neutrophils produced high levels of ROS in response to opsonized, compared to non-opsonized, B. pertussis and that this effect was independent of the time the convalescent serum samples were taken. This indicates the presence of functional opsonizing antibodies up to 3 years after B. pertussis infection. While opsonization of B. pertussis with serum samples from uninfected controls also induced ROS production, sera from infected individuals induced significantly higher ROS levels. Spearman correlations analysis showed that IgG antibodies targeting fimbriae3 followed by pertactin, and BrkA correlate with ROS production. Additionally, we observed that neutrophils killed opsonized B. pertussis in a ROS-dependent manner. Searching for other antigen-specific antibodies from convalescent pertussis patients involved in ROS production by neutrophils may assist in the identification of novel antigens to improve the current pertussis vaccines.

Reduction of NETosis by targeting CXCR1/2 reduces thrombosis, lung injury, and mortality in experimental human and murine sepsis

BACKGROUND

Neutrophil extracellular traps (NETs) facilitate bacterial clearance but also promote thrombosis and organ injury in sepsis. We quantified ex vivo NET induction in septic humans and murine models of sepsis to identify signalling pathways that may be modulated to improve outcome in human sepsis.

METHODS

NET formation in human donor neutrophils was quantified after incubation with plasma obtained from patients with sepsis or systemic inflammation (double-blinded assessment of extracellular DNA using immunofluorescence microscopy). NET formation (% neutrophils forming NETs) was correlated with plasma cytokine levels (MultiPlex assay). Experimental sepsis (caecal ligation and puncture or intraperitoneal injection of Escherichia coli) was assessed in C57/BL6 male mice. The effect of pharmacological inhibition of CXCR1/2 signalling (reparixin) on NET formation, organ injury (hepatic, renal, and cardiac biomarkers), and survival in septic mice was examined.

RESULTS

NET formation was higher after incubation with plasma from septic patients (median NETs=25% [10.5-46.5%]), compared with plasma obtained from patients with systemic inflammation (14% [4.0-23.3%]; P=0.02). Similar results were observed after incubation of plasma from mice with neutrophils from septic non-septic mice. Circulating CXCR1/2 ligands correlated with NETosis in patients (interleukin-8; r=0.643) and mice (macrophage inflammatory protein-2; r=0.902). In experimental sepsis, NETs were primarily observed in the lungs, correlating with fibrin deposition (r=0.702) and lung injury (r=0.692). Inhibition of CXCR1/2 using reparixin in septic mice reduced NET formation, multi-organ injury, and mortality, without impairing bacterial clearance.

CONCLUSION

CXCR1/2 signalling-induced NET formation is a therapeutic target in sepsis, which may be guided by ex vivo NET assays.

Population analysis of Legionella pneumophila reveals a basis for resistance to complement-mediated killing

Legionella pneumophila is the most common cause of the severe respiratory infection known as Legionnaires' disease. However, the microorganism is typically a symbiont of free-living amoeba, and our understanding of the bacterial factors that determine human pathogenicity is limited. Here we carried out a population genomic study of 902 L. pneumophila isolates from human clinical and environmental samples to examine their genetic diversity, global distribution and the basis for human pathogenicity. We find that the capacity for human disease is representative of the breadth of species diversity although some clones are more commonly associated with clinical infections. We identified a single gene (lag-1) to be most strongly associated with clinical isolates. lag-1, which encodes an O-acetyltransferase for lipopolysaccharide modification, has been distributed horizontally across all major phylogenetic clades of L. pneumophila by frequent recent recombination events. The gene confers resistance to complement-mediated killing in human serum by inhibiting deposition of classical pathway molecules on the bacterial surface. Furthermore, acquisition of lag-1 inhibits complement-dependent phagocytosis by human neutrophils, and promoted survival in a mouse model of pulmonary legionellosis. Thus, our results reveal L. pneumophila genetic traits linked to disease and provide a molecular basis for resistance to complement-mediated killing.

Sublancin protects against methicillin-resistant Staphylococcus aureus infection by the combined modulation of innate immune response and microbiota

Methicillin-resistant Staphylococcus aureus (MRSA) is a major pathogen responsible for community and hospital bacterial infections. In the present study, the protective role of sublancin, an antimicrobial peptides, was explored in MRSA infection model. We report that sublancin directly induce macrophage migration through the chemotactic receptors. We further show that sublancin exhibits protection in a mouse MRSA infection model. This protection involved an immunomodulatory activity, but was blocked by depletion of monocyte/macrophages or neutrophils. Sublancin selectively up-regulates the levels of chemokines (C-X-C motif chemokine ligand 1, CXCL1 and monocyte chemoattractant protein-1, MCP-1) while reducing the production of pro-inflammatory cytokine (tumor necrosis factor-α, TNF-α). Meanwhile, sublancin regulated the microbiota composition disrupted by MRSA injection, increasing the abundance of Lactobacillus and decreasing that of Staphylococcus and Pseudomonas. Also, sublancin restored to normal levels of metabolic functional pathways, especially amino acid biosynthesis (e.g., branched amino acid, histidine and tryptophan), disrupted after injection, and this restoration was significantly correlated with neutrophils. These results demonstrates that sublancin stimulates the innate response and modulates the microbiota community to protect against MRSA infection.

Intestinal Infection Is Associated With Impaired Lung Innate Immunity to Secondary Respiratory Infection

Background

Pneumonia and diarrhea are among the leading causes of death worldwide, and epidemiological studies have demonstrated that diarrhea is associated with an increased risk of subsequent pneumonia. Our aim was to determine the impact of intestinal infection on innate immune responses in the lung.

Methods

Using a mouse model of intestinal infection by Salmonella enterica serovar Typhimurium (S. Typhimurium [ST]), we investigated associations between gastrointestinal infections and lung innate immune responses to bacterial (Klebsiella pneumoniae) challenge.

Results

We found alterations in frequencies of innate immune cells in the lungs of intestinally infected mice compared with uninfected mice. On subsequent challenge with K. pneumoniae, we found that mice with prior intestinal infection have higher lung bacterial burden and inflammation, increased neutrophil margination, and neutrophil extracellular traps, but lower overall numbers of neutrophils, compared with mice without prior intestinal infection. Total numbers of dendritic cells, innate-like T cells, and natural killer cells were not different between mice with and without prior intestinal infection.

Conclusions

Together, these results suggest that intestinal infection impacts lung innate immune responses, most notably neutrophil characteristics, potentially resulting in increased susceptibility to secondary pneumonia.

Getting Your Neutrophil: Neutrophil Transepithelial Migration in the Lung

Neutrophil transepithelial migration is a fundamental process that facilitates the rapid trafficking of neutrophils to inflammatory foci and occurs across a diverse range of tissues. For decades there has been widespread interest in understanding the mechanisms that drive this migratory process in response to different pathogens and organ systems. This has led to the successful integration of key findings on neutrophil transepithelial migration from the intestines, lungs, liver, genitourinary tract, and other tissues into a single, cohesive model. However, recent studies have identified organ specific differences in neutrophil transepithelial migration. These findings support a model where the tissue in concert with the pro-inflammatory stimuli dictate a unique collection of signals that drive neutrophil trafficking. This review focuses on the mechanisms that drive neutrophil transepithelial migration in response to microbial infection of a single organ, the lung. Herein we provide a detailed analysis of the adhesion molecules and chemoattractants that contribute to the recruitment of neutrophil into the airways. We also highlight important advances in experimental models for studying neutrophil transepithelial migration in the lung over the last decade.

Neutrophil chemoattractant receptors in health and disease: double-edged swords

Neutrophils are frontline cells of the innate immune system. These effector leukocytes are equipped with intriguing antimicrobial machinery and consequently display high cytotoxic potential. Accurate neutrophil recruitment is essential to combat microbes and to restore homeostasis, for inflammation modulation and resolution, wound healing and tissue repair. After fulfilling the appropriate effector functions, however, dampening neutrophil activation and infiltration is crucial to prevent damage to the host. In humans, chemoattractant molecules can be categorized into four biochemical families, i.e., chemotactic lipids, formyl peptides, complement anaphylatoxins and chemokines. They are critically involved in the tight regulation of neutrophil bone marrow storage and egress and in spatial and temporal neutrophil trafficking between organs. Chemoattractants function by activating dedicated heptahelical G protein-coupled receptors (GPCRs). In addition, emerging evidence suggests an important role for atypical chemoattractant receptors (ACKRs) that do not couple to G proteins in fine-tuning neutrophil migratory and functional responses. The expression levels of chemoattractant receptors are dependent on the level of neutrophil maturation and state of activation, with a pivotal modulatory role for the (inflammatory) environment. Here, we provide an overview of chemoattractant receptors expressed by neutrophils in health and disease. Depending on the (patho)physiological context, specific chemoattractant receptors may be up- or downregulated on distinct neutrophil subsets with beneficial or detrimental consequences, thus opening new windows for the identification of disease biomarkers and potential drug targets.

Downregulation of FOXP3 in neutrophils by IL-8 promotes the progression of oral squamous cell carcinoma

The aim of the present study was to investigate the effects of the transcription factor forkhead box P3 (FOXP3) in neutrophils on the progression of oral squamous cell carcinoma (OSCC). Cancer tissue samples and paracarcinoma tissues were collected from 23 patients with OSCC for the current study. In addition, SCC-9, a human tongue carcinoma cell line, was co-cultured with primary human neutrophils and treated with recombinant interleukin 8 (IL-8). The effect of FOXP3 on the proliferation of SCC-9 cells was analyzed using a Cell Counting Kit 8 assay. FOXP3 expression in neutrophils was analyzed by quantitative PCR following IL-8 treatment. FOXP3 protein expression in neutrophils and the amount of IL-8 protein in the OSCC tumor microenvironment were determined by immunofluorescence analysis. The present study demonstrated that IL-8 downregulated FOXP3 mRNA expression in neutrophils. Neutrophils and peptide P60, a specific inhibitor of FOXP3, increased proliferation of SCC-9 cells. In patients with OSCC, FOXP3 protein expression in neutrophils of the stage IV group was significantly lower compared with that of the stage II and stage III groups, while IL-8 protein expression was higher in cancer tissues compared with that in paracarcinoma tissues. In summary, IL-8 in the tumor microenvironment may recruit neutrophils, and downregulation of FOXP3 in neutrophils by IL-8 may promote the progression of OSCC.

Viewing Legionella pneumophila Pathogenesis through an Immunological Lens

Legionella pneumophila is the causative agent of the severe pneumonia Legionnaires' disease. L. pneumophila is ubiquitously found in freshwater environments, where it replicates within free-living protozoa. Aerosolization of contaminated water supplies allows the bacteria to be inhaled into the human lung, where L. pneumophila can be phagocytosed by alveolar macrophages and replicate intracellularly. The Dot/Icm type IV secretion system (T4SS) is one of the key virulence factors required for intracellular bacterial replication and subsequent disease. The Dot/Icm apparatus translocates more than 300 effector proteins into the host cell cytosol. These effectors interfere with a variety of cellular processes, thus enabling the bacterium to evade phagosome-lysosome fusion and establish an endoplasmic reticulum-derived Legionella-containing vacuole, which facilitates bacterial replication. In turn, the immune system has evolved numerous strategies to recognize intracellular bacteria such as L. pneumophila, leading to potent inflammatory responses that aid in eliminating infection. This review aims to provide an overview of L. pneumophila pathogenesis in the context of the host immune response.

Dietary Polyunsaturated Fatty Acids Promote Neutrophil Accumulation in the Spleen by Altering Chemotaxis and Delaying Cell Death

Neutrophils are the most abundant circulating leukocytes in humans and are essential for the defense against invading pathogens. Like many other cells of an organism, neutrophils can be highly influenced by the diet. We have previously described that mice fed a high-fat diet rich in polyunsaturated fatty acids (HFD-P) present a higher frequency of neutrophils in bone marrow than mice fed a high-fat diet rich in saturated fatty acids (HFD-S). Interestingly, such an increase correlated with improved survival against bacterium-induced sepsis. In this study, we aimed to investigate the effects of dietary polyunsaturated and saturated fatty acids on neutrophil homeostasis. We found that HFD-P specifically induced the accumulation of neutrophils in the marginal pools of the spleen and liver. The accumulation of neutrophils in the spleen was a result of a dual effect of polyunsaturated fatty acids on neutrophil homeostasis. First, polyunsaturated fatty acids enhanced the recruitment of neutrophils from the circulation into the spleen via chemokine secretion. Second, they delayed neutrophil cell death in the spleen. Interestingly, these effects were not observed in mice fed a diet rich in saturated fatty acids, suggesting that the type of fat rather than the amount of fat mediates the alterations in neutrophil homeostasis. In conclusion, our results show that dietary polyunsaturated fatty acids have a strong modulatory effect on neutrophil homeostasis that may have future clinical applications.

CXCL1 regulates neutrophil homeostasis in pneumonia-derived sepsis caused by Streptococcus pneumoniae serotype 3

Neutrophil migration to the site of bacterial infection is a critical step in host defense. Exclusively produced in the bone marrow, neutrophil release into the blood is tightly controlled. Although the chemokine CXCL1 induces neutrophil influx during bacterial infections, its role in regulating neutrophil recruitment, granulopoiesis, and neutrophil mobilization in response to lung infection-induced sepsis is unclear. Here, we used a murine model of intrapulmonary Streptococcus pneumoniae infection to investigate the role of CXCL1 in host defense, granulopoiesis, and neutrophil mobilization. Our results demonstrate that CXCL1 augments neutrophil influx to control bacterial growth in the lungs, as well as bacterial dissemination, resulting in improved host survival. This was shown in Cxcl1 ^-/- mice, which exhibited defective amplification of early neutrophil precursors in granulocytic compartments, and CD62L- and CD49d-dependent neutrophil release from the marrow. Administration of recombinant CXCL2 and CXCL5 after infection rescues the impairments in neutrophil-dependent host defense in Cxcl1 ^-/- mice. Taken together, these findings identify CXCL1 as a central player in host defense, granulopoiesis, and mobilization of neutrophils during Gram-positive bacterial pneumonia-induced sepsis.

Overlapping Roles for Interleukin-36 Cytokines in Protective Host Defense against Murine Legionella pneumophila Pneumonia

Legionella pneumophila causes life-threatening pneumonia culminating in acute lung injury. Innate and adaptive cytokines play an important role in host defense against L. pneumophila infection. Interleukin-36 (IL-36) cytokines are recently described members of the larger IL-1 cytokine family known to exert potent inflammatory effects. In this study, we elucidated the role for IL-36 cytokines in experimental pneumonia caused by L. pneumophila Intratracheal (i.t.) administration of L. pneumophila induced the upregulation of both IL-36α and IL-36γ mRNA and protein production in the lung. Compared to the findings for L. pneumophila-infected wild-type (WT) mice, the i.t. administration of L. pneumophila to IL-36 receptor-deficient (IL-36R^-/-) mice resulted in increased mortality, a delay in lung bacterial clearance, increased L. pneumophila dissemination to extrapulmonary organs, and impaired glucose homeostasis. Impaired lung bacterial clearance in IL-36R^-/- mice was associated with a significantly reduced accumulation of inflammatory cells and the decreased production of proinflammatory cytokines and chemokines. Ex vivo, reduced expression of costimulatory molecules and impaired M1 polarization were observed in alveolar macrophages isolated from infected IL-36R^-/- mice compared to macrophages from WT mice. While L. pneumophila-induced mortality in IL-36α- or IL-36γ-deficient mice was not different from that in WT animals, antibody-mediated neutralization of IL-36γ in IL-36α^-/- mice resulted in mortality similar to that observed in IL-36R^-/- mice, indicating redundant and overlapping roles for these cytokines in experimental murine L. pneumophila pneumonia.

Constitutive Interferon Maintains GBP Expression Required for Release of Bacterial Components Upstream of Pyroptosis and Anti-DNA Responses

Legionella pneumophila elicits caspase-11-driven macrophage pyroptosis through guanylate-binding proteins (GBPs) encoded on chromosome 3. It has been proposed that microbe-driven IFN upregulates GBPs to facilitate pathogen vacuole rupture and bacteriolysis preceding caspase-11 activation. We show here that macrophage death occurred independently of microbial-induced IFN signaling and that GBPs are dispensable for pathogen vacuole rupture. Instead, the host-intrinsic IFN status sustained sufficient GBP expression levels to drive caspase-1 and caspase-11 activation in response to cytosol-exposed bacteria. In addition, endogenous GBP levels were sufficient for the release of DNA from cytosol-exposed bacteria, preceding the cyclic GMP-AMP synthase/stimulator of interferon genes (cGAS/STING) pathway for Ifnb induction. Mice deficient for chromosome 3 GBPs were unable to mount a rapid IL-1/chemokine (C-X-C motif) ligand 1 (CXCL1) response during Legionella-induced pneumonia, with defective bacterial clearance. Our results show that rapid GBP activity is controlled by host-intrinsic cytokine signaling and that GBP activities precede immune amplification responses, including IFN induction, inflammasome activation, and cell death.

Autophagy Evasion and Endoplasmic Reticulum Subversion: The Yin and Yang of Legionella Intracellular Infection

The gram-negative bacterial pathogen Legionella pneumophila creates a novel organelle inside of eukaryotic host cells that supports intracellular replication. The L. pneumophila-containing vacuole evades fusion with lysosomes and interacts intimately with the host endoplasmic reticulum (ER). Although the natural hosts for L. pneumophila are free-living protozoa that reside in freshwater environments, the mechanisms that enable this pathogen to replicate intracellularly also function when mammalian macrophages phagocytose aerosolized bacteria, and infection of humans by L. pneumophila can result in a severe pneumonia called Legionnaires' disease. A bacterial type IVB secretion system called Dot/Icm is essential for intracellular replication of L. pneumophila. The Dot/Icm apparatus delivers over 300 different bacterial proteins into host cells during infection. These bacterial proteins have biochemical activities that target evolutionarily conserved host factors that control membrane transport processes, which results in the formation of the ER-derived vacuole that supports L. pneumophila replication. This review highlights research discoveries that have defined interactions between vacuoles containing L. pneumophila and the host ER. These studies reveal how L. pneumophila creates a vacuole that supports intracellular replication by subverting host proteins that control biogenesis and fusion of early secretory vesicles that exit the ER and host proteins that regulate the shape and dynamics of the ER. In addition to recruiting ER-derived membranes for biogenesis of the vacuole in which L. pneumophila replicates, these studies have revealed that this pathogen has a remarkable ability to interfere with the host's cellular process of autophagy, which is an ancient cell autonomous defense pathway that utilizes ER-derived membranes to target intracellular pathogens for destruction. Thus, this intracellular pathogen has evolved multiple mechanisms to control membrane transport processes that center on the involvement of the host ER.

The Goldilocks model of immune symbiosis with Mycobacteria and Candida colonizers

Mycobacteria and Candida species include significant human pathogens that can cause localized or disseminated infections. Although these organisms may appear to have little in common, several shared pathways of immune recognition and response are important for both control and infection-related pathology. In this article, we compare and contrast the innate and adaptive components of the immune system that pertain to these infections in humans and animal models. We also explore a relatively new concept in the mycobacterial field: biological commensalism. Similar to the well-established model of Candida infection, Mycobacteria species colonize their human hosts in equilibrium with the immune response. Perturbations in the immune response permit the progression to pathologic disease at the expense of the host. Understanding the immune factors required to maintain commensalism may aid with the development of diagnostic and treatment strategies for both categories of pathogens.

Neutrophils and Ly6Chi monocytes collaborate in generating an optimal cytokine response that protects against pulmonary Legionella pneumophila infection

Early responses mounted by both tissue-resident and recruited innate immune cells are essential for host defense against bacterial pathogens. In particular, both neutrophils and Ly6Chi monocytes are rapidly recruited to sites of infection. While neutrophils and monocytes produce bactericidal molecules, such as reactive nitrogen and oxygen species, both cell types are also capable of synthesizing overlapping sets of cytokines important for host defense. Whether neutrophils and monocytes perform redundant or non-redundant functions in the generation of anti-microbial cytokine responses remains elusive. Here, we sought to define the contributions of neutrophils and Ly6Chi monocytes to cytokine production and host defense during pulmonary infection with Legionella pneumophila, responsible for the severe pneumonia Legionnaires' disease. We found that both neutrophils and monocytes are critical for host defense against L. pneumophila. Both monocytes and neutrophils contribute to maximal IL-12 and IFNγ responses, and monocytes are also required for TNF production. Moreover, natural killer (NK) cells, NKT cells, and γδ T cells are sources of IFNγ, and monocytes direct IFNγ production by these cell types. Thus, neutrophils and monocytes cooperate in eliciting an optimal cytokine response that promotes effective control of bacterial infection.

The Influence of Programmed Cell Death in Myeloid Cells on Host Resilience to Infection with Legionella pneumophila or Streptococcus pyogenes

Pathogen clearance and host resilience/tolerance to infection are both important factors in surviving an infection. Cells of the myeloid lineage play important roles in both of these processes. Neutrophils, monocytes, macrophages, and dendritic cells all have important roles in initiation of the immune response and clearance of bacterial pathogens. If these cells are not properly regulated they can result in excessive inflammation and immunopathology leading to decreased host resilience. Programmed cell death (PCD) is one possible mechanism that myeloid cells may use to prevent excessive inflammation. Myeloid cell subsets play roles in tissue repair, immune response resolution, and maintenance of homeostasis, so excessive PCD may also influence host resilience in this way. In addition, myeloid cell death is one mechanism used to control pathogen replication and dissemination. Many of these functions for PCD have been well defined in vitro, but the role in vivo is less well understood. We created a mouse that constitutively expresses the pro-survival B-cell lymphoma (bcl)-2 protein in myeloid cells (CD68(bcl2tg), thus decreasing PCD specifically in myeloid cells. Using this mouse model we explored the impact that decreased cell death of these cells has on infection with two different bacterial pathogens, Legionella pneumophila and Streptococcus pyogenes. Both of these pathogens target multiple cell death pathways in myeloid cells, and the expression of bcl2 resulted in decreased PCD after infection. We examined both pathogen clearance and host resilience and found that myeloid cell death was crucial for host resilience. Surprisingly, the decreased myeloid PCD had minimal impact on pathogen clearance. These data indicate that the most important role of PCD during infection with these bacteria is to minimize inflammation and increase host resilience, not to aid in the clearance or prevent the spread of the pathogen.

Yersinia enterocolitica YopH-Deficient Strain Activates Neutrophil Recruitment to Peyer's Patches and Promotes Clearance of the Virulent Strain

Yersinia enterocolitica evades the immune response by injecting Yersinia outer proteins (Yops) into the cytosol of host cells. YopH is a tyrosine phosphatase critical for Yersinia virulence. However, the mucosal immune mechanisms subverted by YopH during in vivo orogastric infection with Y. enterocolitica remain elusive. The results of this study revealed neutrophil recruitment to Peyer's patches (PP) after infection with a YopH-deficient mutant strain (Y. enterocolitica ΔyopH). While the Y. enterocolitica wild-type (WT) strain in PP induced the major neutrophil chemoattractant CXCL1 mRNA and protein levels, infection with the Y. enterocolitica ΔyopH mutant strain exhibited a higher expression of the CXCL1 receptor, CXCR2, in blood neutrophils, leading to efficient neutrophil recruitment to the PP. In contrast, migration of neutrophils into PP was impaired upon infection with Y. enterocolitica WT strain. In vitro infection of blood neutrophils revealed the involvement of YopH in CXCR2 expression. Depletion of neutrophils during Y. enterocolitica ΔyopH infection raised the bacterial load in PP. Moreover, the clearance of WT Y. enterocolitica was improved when an equal mixture of Y. enterocolitica WT and Y. enterocolitica ΔyopH strains was used in infecting the mice. This study indicates that Y. enterocolitica prevents early neutrophil recruitment in the intestine and that the effector protein YopH plays an important role in the immune evasion mechanism. The findings highlight the potential use of the Y. enterocolitica YopH-deficient strain as an oral vaccine carrier.

Chemokine-Releasing Microparticles Improve Bacterial Clearance and Survival of Anthrax Spore-Challenged Mice

In this study the hydrogel microparticles (MPs) were used to enhance migration of neutrophils in order to improve outcome of anthrax infection in a mouse model. Two MP formulations were tested. In the first one the polyacrylamide gel MPs were chemically coupled with Cibacron Blue (CB) affinity bait. In the second one the bait molecules within the MPs were additionally loaded with neutrophil-attracting chemokines (CKs), human CXCL8 and mouse CCL3. A non-covalent interaction of the bait with the CKs provided their gradual release after administration of the MPs to the host. Mice were challenged into footpads with Bacillus anthracis Sterne spores and given a dose of MPs a few hours before and/or after the spores. Pre-treatment with a single dose of CK-releasing MPs without any additional intervention was able to induce influx of neutrophils to the site of spore inoculation and regional lymph nodes correlating with reduced bacterial burden and decreased inflammatory response in footpads. On average, in two independent experiments, up to 53% of mice survived over 13 days. All control spore-challenged but MP-untreated mice died. The CB-coupled particles were also found to improve survival likely due to the capacity to stimulate release of endogenous CKs, but were less potent at decreasing the inflammatory host response than the CK-releasing MPs. The CK post-treatment did not improve survival compared to the untreated mice which died within 4 to 6 days with a strong inflammation of footpads, indicating quick dissemination of spores though the lymphatics after challenge. This is the first report on the enhanced innate host resistance to anthrax in response to CKs delivered and/or endogenously induced by the MPs.

The life stage-specific pathometabolism of Legionella pneumophila

The genus Legionella belongs to Gram-negative bacteria found ubiquitously in aquatic habitats, where it grows in natural biofilms and replicates intracellularly in various protozoa (amoebae, ciliates). L. pneumophila is known as the causative agent of Legionnaires' disease, since it is also able to replicate in human alveolar macrophages, finally leading to inflammation of the lung and pneumonia. To withstand the degradation by its host cells, a Legionella-containing vacuole (LCV) is established for intracellular replication, and numerous effector proteins are secreted into the host cytosol using a type four B secretion system (T4BSS). During intracellular replication, Legionella has a biphasic developmental cycle that alternates between a replicative and a transmissive form. New knowledge about the host-adapted and life stage-dependent metabolism of intracellular L. pneumophila revealed a bipartite metabolic network with life stage-specific usages of amino acids (e.g. serine), carbohydrates (e.g. glucose) and glycerol as major substrates. These metabolic features are associated with the differentiation of the intracellular bacteria, and thus have an important impact on the virulence of L. pneumophila.

IFN-γ induction by neutrophil-derived IL-17A homodimer augments pulmonary antibacterial defense

The role of interleukin-17A (IL-17A) in host defense against Legionella pneumophila remains elusive. To address this issue, we used Il17a(-/-), Il17f(-/-), and Il17a/Il17f(-/-) mice on a C57Bl/6 (non-permissive) background and IL-17 neutralizing Abs in mice on an A/J (permissive) background. Higher bacterial (L. pneumophila) counts in the lung and blood along with reduced neutrophil recruitment were detected in Il17a(-/-), but not Il17f(-/-), mice. We found that neutrophils produce IL-17A homodimer (IL-17A) during L. pneumophila infection, and hematopoietic cell-derived IL-17A is known to be important for bacterial clearance. Thus, intratracheal administration of wild-type neutrophils or recombinant IL-17A restored bacterial clearance and neutrophil recruitment in Il17a(-/-) mice. Furthermore, neutrophil-depleted Rag2(-/-) and Rag2/Il-2rγ(-/-) mice exhibited increased bacterial burden, reduced neutrophil influx and IL-17A production in the lung. Recombinant IFN-γ administration in Il17a(-/-) mice augmented bacterial elimination, whereas IL-17A administration in Ifnγ(-/-) mice did not augment bacterial clearance. IFN-γ is produced by T cells, but not neutrophils or macrophages, suggesting that neutrophil-derived IL-17A induces IFN-γ in a paracrine fashion. Human pneumonic lungs and human neutrophils challenged with L. pneumophila exhibited increased numbers of IL-17A producing cells. These findings display a novel function of neutrophil-derived IL-17A in antibacterial defense via the induction of IFN-γ in a paracrine manner.

Neutrophil and Alveolar Macrophage-Mediated Innate Immune Control of Legionella pneumophila Lung Infection via TNF and ROS

Legionella pneumophila is a facultative intracellular bacterium that lives in aquatic environments where it parasitizes amoeba. However, upon inhalation of contaminated aerosols it can infect and replicate in human alveolar macrophages, which can result in Legionnaires' disease, a severe form of pneumonia. Upon experimental airway infection of mice, L. pneumophila is rapidly controlled by innate immune mechanisms. Here we identified, on a cell-type specific level, the key innate effector functions responsible for rapid control of infection. In addition to the well-characterized NLRC4-NAIP5 flagellin recognition pathway, tumor necrosis factor (TNF) and reactive oxygen species (ROS) are also essential for effective innate immune control of L. pneumophila. While ROS are essential for the bactericidal activity of neutrophils, alveolar macrophages (AM) rely on neutrophil and monocyte-derived TNF signaling via TNFR1 to restrict bacterial replication. This TNF-mediated antibacterial mechanism depends on the acidification of lysosomes and their fusion with L. pneumophila containing vacuoles (LCVs), as well as caspases with a minor contribution from cysteine-type cathepsins or calpains, and is independent of NLRC4, caspase-1, caspase-11 and NOX2. This study highlights the differential utilization of innate effector pathways to curtail intracellular bacterial replication in specific host cells upon L. pneumophila airway infection.

Caspase Exploitation by Legionella pneumophila

Legionella pneumophila remains a major health concern, especially for hospitalized patients. L. pneumophila in the environment can survive extracellular or as protozoan parasite within amoeba. After human infection it efficiently replicates in alveolar macrophages without activating inflammasome assembly and cleavage of caspase-1. In contrast murine macrophages actively recognize intracellular L. pneumophila via inflammasome components which initiate pro-inflammatory cytokine secretion, phagosomal maturation and pyroptotic cell death thereby leading to bacterial restriction. During this process flagellin-dependent and -independent signaling pathways trigger the canonical as well as the non-canonical inflammasome. This review describes the current knowledge about L. pneumophila-induced inflammasome pathways in permissive and restrictive host cells.

Subversion of Cell-Autonomous Immunity and Cell Migration by Legionella pneumophila Effectors

Bacteria trigger host defense and inflammatory processes, such as cytokine production, pyroptosis, and the chemotactic migration of immune cells toward the source of infection. However, a number of pathogens interfere with these immune functions by producing specific so-called “effector” proteins, which are delivered to host cells via dedicated secretion systems. Air-borne Legionella pneumophila bacteria trigger an acute and potential fatal inflammation in the lung termed Legionnaires’ disease. The opportunistic pathogen L. pneumophila is a natural parasite of free-living amoebae, but also replicates in alveolar macrophages and accidentally infects humans. The bacteria employ the intracellular multiplication/defective for organelle trafficking (Icm/Dot) type IV secretion system and as many as 300 different effector proteins to govern host–cell interactions and establish in phagocytes an intracellular replication niche, the Legionella-containing vacuole. Some Icm/Dot-translocated effector proteins target cell-autonomous immunity or cell migration, i.e., they interfere with (i) endocytic, secretory, or retrograde vesicle trafficking pathways, (ii) organelle or cell motility, (iii) the inflammasome and programed cell death, or (iv) the transcription factor NF-κB. Here, we review recent mechanistic insights into the subversion of cellular immune functions by L. pneumophila.

Type 1 Diabetes Prone NOD Mice Have Diminished Cxcr1 mRNA Expression in Polymorphonuclear Neutrophils and CD4+ T Lymphocytes

In humans, CXCR1 and CXCR2 are two homologous proteins that bind ELR+ chemokines. Both receptors play fundamental roles in neutrophil functions such as migration and reactive oxygen species production. Mouse Cxcr1 and Cxcr2 genes are located in an insulin-dependent diabetes genetic susceptibility locus. The non obese diabetic (NOD) mouse is a spontaneous well-described animal model for insulin-dependent type 1 diabetes. In this disease, insulin deficiency results from the destruction of insulin-producing beta cells by autoreactive T lymphocytes. This slow-progressing disease is dependent on both environmental and genetic factors. Here, we report descriptive data about the Cxcr1 gene in NOD mice. We demonstrate decreased expression of mRNA for Cxcr1 in neutrophils and CD4+ lymphocytes isolated from NOD mice compared to other strains, related to reduced NOD Cxcr1 gene promoter activity. Looking for Cxcr1 protein, we next analyze the membrane proteome of murine neutrophils by mass spectrometry. Although Cxcr2 protein is clearly found in murine neutrophils, we did not find evidence of Cxcr1 peptides using this method. Nevertheless, in view of recently-published experimental data obtained in NOD mice, we argue for possible Cxcr1 involvement in type 1 diabetes pathogenesis.

Distinct Contributions of Neutrophils and CCR2+ Monocytes to Pulmonary Clearance of Different Klebsiella pneumoniae Strains

Klebsiella pneumoniae is a common respiratory pathogen, with some strains having developed broad resistance to clinically available antibiotics. Humans can become infected with many different K. pneumoniae strains that vary in genetic background, antibiotic susceptibility, capsule composition, and mucoid phenotype. Genome comparisons have revealed differences between K. pneumoniae strains, but the impact of genomic variability on immune-mediated clearance of pneumonia remains unclear. Experimental studies of pneumonia in mice have used the rodent-adapted 43816 strain of K. pneumoniae and demonstrated that neutrophils are essential for optimal host defense. It remains unclear, however, whether CCR2(+) monocytes contribute to K. pneumoniae clearance from the lung. We selectively depleted neutrophils, CCR2(+) monocytes, or both from immunocompetent mice and determined susceptibility to infection by the 43816 strain and 4 newly isolated clinical K. pneumoniae strains. The clinical K. pneumoniae strains, including one carbapenem-resistant ST258 strain, are less virulent than 43816. Optimal clearance of each of the 5 strains required either neutrophils or CCR2(+) monocytes. Selective neutrophil depletion markedly worsened infection with K. pneumoniae strain 43816 and three clinical isolates but did not increase susceptibility of mice to infection with the carbapenem-resistant K. pneumoniae ST258 strain. Depletion of CCR2(+) monocytes delayed recovery from infection with each of the 5 K. pneumoniae strains, revealing a contribution of these cells to bacterial clearance from the lung. Our findings demonstrate strain-dependent variation in the contributions of neutrophils and CCR2(+) monocytes to clearance of K. pneumoniae pulmonary infection.

Circumventing Y. pestis Virulence by Early Recruitment of Neutrophils to the Lungs during Pneumonic Plague

Pneumonic plague is a fatal disease caused by Yersinia pestis that is associated with a delayed immune response in the lungs. Because neutrophils are the first immune cells recruited to sites of infection, we investigated the mechanisms responsible for their delayed homing to the lung. During the first 24 hr after pulmonary infection with a fully virulent Y. pestis strain, no significant changes were observed in the lungs in the levels of neutrophils infiltrate, expression of adhesion molecules, or the expression of the major neutrophil chemoattractants keratinocyte cell-derived chemokine (KC), macrophage inflammatory protein 2 (MIP-2) and granulocyte colony stimulating factor (G-CSF). In contrast, early induction of chemokines, rapid neutrophil infiltration and a reduced bacterial burden were observed in the lungs of mice infected with an avirulent Y. pestis strain. In vitro infection of lung-derived cell-lines with a YopJ mutant revealed the involvement of YopJ in the inhibition of chemoattractants expression. However, the recruitment of neutrophils to the lungs of mice infected with the mutant was still delayed and associated with rapid bacterial propagation and mortality. Interestingly, whereas KC, MIP-2 and G-CSF mRNA levels in the lungs were up-regulated early after infection with the mutant, their protein levels remained constant, suggesting that Y. pestis may employ additional mechanisms to suppress early chemoattractants induction in the lung. It therefore seems that prevention of the early influx of neutrophils to the lungs is of major importance for Y. pestis virulence. Indeed, pulmonary instillation of KC and MIP-2 to G-CSF-treated mice infected with Y. pestis led to rapid homing of neutrophils to the lung followed by a reduction in bacterial counts at 24 hr post-infection and improved survival rates. These observations shed new light on the virulence mechanisms of Y. pestis during pneumonic plague, and have implications for the development of novel therapies against this pathogen.

Regulation of neutrophilic inflammation by proteinase-activated receptor 1 during bacterial pulmonary infection

Neutrophils are key effector cells of the innate immune response to pathogenic bacteria, but excessive neutrophilic inflammation can be associated with bystander tissue damage. The mechanisms responsible for neutrophil recruitment to the lungs during bacterial pneumonia are poorly defined. In this study, we focus on the potential role of the major high-affinity thrombin receptor, proteinase-activated receptor 1 (PAR-1), during the development of pneumonia to the common lung pathogen Streptococcus pneumoniae. Our studies demonstrate that neutrophils were indispensable for controlling S. pneumoniae outgrowth but contributed to alveolar barrier disruption. We further report that intra-alveolar coagulation (bronchoalveolar lavage fluid thrombin-antithrombin complex levels) and PAR-1 immunostaining were increased in this model of bacterial lung infection. Functional studies using the most clinically advanced PAR-1 antagonist, SCH530348, revealed a key contribution for PAR-1 signaling in influencing neutrophil recruitment to lung airspaces in response to both an invasive and noninvasive strain of S. pneumoniae (D39 and EF3030) but that PAR-1 antagonism did not impair the ability of the host to control bacterial outgrowth. PAR-1 antagonist treatment significantly decreased pulmonary levels of IL-1β, CXCL1, CCL2, and CCL7 and attenuated alveolar leak. Ab neutralization studies further demonstrated a nonredundant role for IL-1β, CXCL1, and CCL7 in mediating neutrophil recruitment in response to S. pneumoniae infection. Taken together, these data demonstrate a key role for PAR-1 during S. pneumoniae lung infection that is mediated, at least in part, by influencing multiple downstream inflammatory mediators.

CXCR1/CXCR2 antagonist CXCL8(3-74)K11R/G31P blocks lung inflammation in swine barn dust-instilled mice

Inhalation of agricultural occupational dusts from swine confinement facilities can result in lung inflammation. The innate immune response to organic barn dusts results in production of a number of pro-inflammatory factors in the lungs of barn workers such as cytokines, chemokines, and an influx of neutrophils. Many of these inflammatory factors are influenced by the chemokine CXCL8/IL-8 (KC or MIP-2 in mice). Previously, we have demonstrated that an endotoxin-independent component of swine barn dust extract (SBE) elevates lung chemokines in a protein kinase C (PKC)-dependent manner resulting in the significant formation of lung inflammatory cell infiltrates in a mouse model of SBE injury. In this study we test the ability of a CXCR1/CXCR2 antagonist, CXCL8(3-74)K11R/G31P (G31P) to block many of the features of lung-inflammation in response to challenge with SBE in an established mouse exposure system. Injection of G31P concurrent with SBE nasal instillation over a course of 3 weeks significantly reduced neutrophil accumulation in the lungs of barn dust exposed animals compared to those given SBE alone. There was a similar reduction in pro-inflammatory cytokines and chemokines IL-6, KC, and MIP-2 in SBE plus G31P-treated mice. In addition to excreted products, the receptors ICAM-1, CXCR1, and CXCR2, which all were elevated with SBE exposure, were also decreased with G31P treatment. SBE activation of PKCα and PKCε was reduced as well with G31P treatment. Thus, G31P was found to be highly effective at reducing several features of lung inflammation in mice exposed to barn dust extracts.

Chemokine receptors and their therapeutic opportunities in diseased lung: far beyond leukocyte trafficking

Chemokine receptors and their chemokine ligands, key mediators of inflammatory and immune cell trafficking, are involved in the regulation of both physiological and pathological processes in the lung. The discovery that chemokine receptors/chemokines, typically expressed by inflammatory and immune cells, are also expressed in structural lung tissue cells suggests their role in mediating the restoration of lung tissue structure and functions. Thus, chemokine receptors/chemokines contribute not only to inflammatory and immune responses in the lung but also play a critical role in the regulation of lung tissue repair, regeneration, and remodeling. This review aims to summarize current state-of-the-art on chemokine receptors and their ligands in lung diseases such as chronic obstructive pulmonary disease, asthma/allergy, pulmonary fibrosis, acute lung injury, and lung infection. Furthermore, the therapeutic opportunities of chemokine receptors in aforementioned lung diseases are discussed. The review also aims to delineate the potential contribution of chemokine receptors to the processes leading to repair/regeneration of the lung tissue.

Neutrophil priming occurs in a sequential manner and can be visualized in living animals by monitoring IL-1β promoter activation

Rapid enhancement of phagocyte functionality is a hallmark of neutrophil priming. GeneChip analyses unveiled elevated CD54, dectin-2, and IL-1β mRNA expression by neutrophils isolated from inflammatory sites. In fact, CD54 and dectin-2 protein expression was detected on neutrophils recovered from skin, peritoneal, and lung inflammation lesions but not on those in bone marrow or peripheral blood. Neutrophils increased CD54 and dectin-2 mRNA during migration in Boyden chambers and acquired CD54 and dectin-2 surface expression after subsequent exposure to GM-CSF. Neutrophils purified from IL-1β promoter-driven DsRed-transgenic mice acquired DsRed signals during cell migration or exposure to GM-CSF. CD54 and dectin-2 were expressed by DsRed(+) (but not DsRed(-)) neutrophils in GM-CSF-supplemented cultures, and neutrophils recovered from inflammatory sites exhibited strong DsRed signals. The dynamic process of neutrophil priming was studied in chemically induced inflammatory skin lesions by monitoring DsRed expression using confocal microscopy. A majority (>80%) of Ly6G(+) neutrophils expressed DsRed, and those DsRed(+)/Ly6G(+) cells exhibited crawling motion with a higher velocity compared with their DsRed(-)/Ly6G(+) counterparts. This report unveils motile behaviors of primed neutrophils in living animals. We propose that neutrophil priming occurs in a sequential manner with rapid enhancement of phagocyte functionality, followed by CD54 and dectin-2 mRNA and protein expression, IL-1β promoter activation, and accelerated motility. Not only do these findings provide a new conceptual framework for our understanding of the process of neutrophil priming, they also unveil new insights into the pathophysiology of many inflammatory disorders that are characterized by neutrophil infiltration.

Defective neutrophil recruitment in leukocyte adhesion deficiency type I disease causes local IL-17-driven inflammatory bone loss

Leukocyte adhesion deficiency type I (LAD-I), a disease syndrome associated with frequent microbial infections, is caused by mutations on the CD18 subunit of β₂ integrins. LAD-I is invariably associated with severe periodontal bone loss, which historically has been attributed to the lack of neutrophil surveillance of the periodontal infection. We provide an alternative mechanism by showing that the cytokine interleukin-17 (IL-17) plays a major role in the oral pathology of LAD-I. Defective neutrophil recruitment in LAD-I patients or in LFA-1 (CD11a/CD18)-deficient mice--which exhibit the LAD-I periodontal phenotype--was associated with excessive production of predominantly T cell-derived IL-17 in the periodontal tissue, although innate lymphoid cells also contributed to pathological IL-17 elevation in the LFA-1-deficient mice. Local treatment with antibodies to IL-17 or IL-23 in LFA-1-deficient mice not only blocked inflammatory periodontal bone loss but also caused a reduction in the total bacterial burden, suggesting that the IL-17-driven pathogenesis of LAD-I periodontitis leads to dysbiosis. Therefore, our findings support an IL-17-targeted therapy for periodontitis in LAD-I patients.

A soybean and fish oil mixture with different n-6/n-3 PUFA ratios modulates the inflammatory reaction in mice with dextran sulfate sodium-induced acute colitis

BACKGROUND & AIMS

Inflammatory bowel disease is a recurrent disease of the gastrointestinal tract. n-3 polyunsaturated fatty acids (PUFAs) are proved to have anti-inflammatory and immunomodulatory properties. This study evaluated the effects of different dietary n-6/n-3 PUFA ratios on the mechanism of alleviating the inflammatory response in mice with dextran sulfate sodium (DSS)-induced colitis.

METHODS

Mice were randomly assigned to 6 groups including 3 non-colitis groups (C, LF, and HF) and 3 colitis groups (DC, DLF, and DHF). Mice in the C and DC groups were fed a common semipurified diet with soybean oil as the fat source. The other groups received an identical component except that part of the soybean oil was replaced by different amounts of fish oil. The n-6/n-3 PUFA ratio of the LF and DLF groups was 4:1, the ratio of the HF and DHF groups was 2:1. After feeding the respective diets for 2 weeks, the colitis groups were given distilled water containing 2% DSS, while the non-colitis groups were given distilled water for 5 days. After that, all mice were sacrificed at the recovery phase after drinking distilled water for another 5 days.

RESULTS

Colitis resulted in higher expressions of colonic inflammatory mediators in colon tissues and colon lavage fluid. Also, colonic peroxisome proliferator-activated receptor (PPAR)-γ and the IκBα/nuclear factor (NF)-κB p65 ratio were lower than those of the non-colitis groups. Compared to the DC group, fish oil-enriched colitis groups had lower inflammatory mediator expressions and higher PPAR-γ protein levels and IκBα/NF-κB p65 ratios in colon tissues. The DHF group had even lower colonic inflammatory gene and higher PPAR-γ protein expressions than did the DLF group.

CONCLUSIONS

These findings suggest that diets enriched with fish oil upregulated PPAR-γ and decreased NF-κB activation that may consequently have reduced luminal inflammatory mediator production. Compared to a n-6/n-3 PUFA ratio 4:1, a ratio of 2:1 was more effective in reducing inflammatory reactions in DSS-induced colitis.

Alveolar macrophages and neutrophils are the primary reservoirs for Legionella pneumophila and mediate cytosolic surveillance of type IV secretion

Legionella pneumophila, an intracellular pathogen responsible for the severe pneumonia Legionnaires' disease, uses its dot/icm-encoded type IV secretion system (T4SS) to translocate effector proteins that promote its survival and replication into the host cell cytosol. However, by introducing bacterial products into the host cytosol, L. pneumophila also activates cytosolic immunosurveillance pathways, thereby triggering robust proinflammatory responses that mediate the control of infection. Thus, the pulmonary cell types that L. pneumophila infects not only may act as an intracellular niche that facilitates its pathogenesis but also may contribute to the immune response against L. pneumophila. The identity of these host cells remains poorly understood. Here, we developed a strain of L. pneumophila producing a fusion protein consisting of β-lactamase fused to the T4SS-translocated effector RalF, which allowed us to track cells injected by the T4SS. Our data reveal that alveolar macrophages and neutrophils both are the primary recipients of T4SS-translocated effectors and harbor viable L. pneumophila during pulmonary infection of mice. Moreover, both alveolar macrophages and neutrophils from infected mice produced tumor necrosis factor and interleukin-1α in response to T4SS-sufficient, but not T4SS-deficient, L. pneumophila. Collectively, our data suggest that alveolar macrophages and neutrophils are both an intracellular reservoir for L. pneumophila and a source of proinflammatory cytokines that contribute to the host immune response against L. pneumophila during pulmonary infection.

Gene expression signature for biliary atresia and a role for interleukin-8 in pathogenesis of experimental disease

Biliary atresia (BA) is a progressive fibroinflammatory obstruction of extrahepatic bile ducts that presents as neonatal cholestasis. Due to the overlap in clinical, biochemical, and histological features with other causes of cholestasis, the diagnosis requires an intraoperative cholangiogram. Thus, we determined whether diseased livers express a gene expression signature unique to BA. Applying stringent statistical analysis to a genome-wide liver expression platform of 64 infants with BA at the time of diagnosis, 14 age-appropriate subjects with intrahepatic cholestasis as diseased controls and seven normal controls, we identified 15 genes uniquely expressed in BA with an accuracy of 92.3%. Among these genes, IL8 and LAMC2 were sufficient to classify subjects with BA distinctly from diseased controls with an area under the curve of 0.934 (95% confidence interval [CI]: 0.84-1.03), sensitivity of 96.9%, and specificity of 85.7% using their combined first principal component. Direct measurement of interleukin (IL)8 protein in the serum, however, was not different between the two groups. To investigate whether the liver-restricted increase in IL8 was relevant to disease pathogenesis, we inactivated the signaling of IL8 homologs by genetic targeting of the Cxcr2 receptor in a murine model of experimental BA. Disruption of Cxcr2 shortened the duration of cholestasis, decreased the incidence of bile duct obstruction, and improved survival above wild-type neonatal mice.

CONCLUSION

The hepatic expression of IL8 and LAMC2 has high sensitivity for BA at diagnosis and may serve as a biomarker of disease, with an important role for the IL8 signaling in experimental BA.

Identification of genomic loci associated with Rhodococcus equi susceptibility in foals

Pneumonia caused by Rhodococcus equi is a common cause of disease and death in foals. Although agent and environmental factors contribute to the incidence of this disease, the genetic factors influencing the clinical outcomes of R. equi pneumonia are ill-defined. Here, we performed independent single nucleotide polymorphism (SNP)- and copy number variant (CNV)-based genome-wide association studies to identify genomic loci associated with R. equi pneumonia in foals. Foals at a large Quarter Horse breeding farm were categorized into 3 groups: 1) foals with R. equi pneumonia (clinical group [N = 43]); 2) foals with ultrasonographic evidence of pulmonary lesions that never developed clinical signs of pneumonia (subclinical group [N = 156]); and, 3) foals without clinical signs or ultrasonographic evidence of pneumonia (unaffected group [N = 49]). From each group, 24 foals were randomly selected and used for independent SNP- and CNV-based genome-wide association studies (GWAS). The SNP-based GWAS identified a region on chromosome 26 that had moderate evidence of association with R. equi pneumonia when comparing clinical and subclinical foals. A joint analysis including all study foals revealed a 3- to 4-fold increase in odds of disease for a homozygous SNP within the associated region when comparing the clinical group with either of the other 2 groups of foals or their combination. The region contains the transient receptor potential cation channel, subfamily M, member 2 (TRPM2) gene, which is involved in neutrophil function. No associations were identified in the CNV-based GWAS. Collectively, these data identify a region on chromosome 26 associated with R. equi pneumonia in foals, providing evidence that genetic factors may indeed contribute to this important disease of foals.

Diverse macrophage populations mediate acute lung inflammation and resolution

Acute respiratory distress syndrome (ARDS) is a devastating disease with distinct pathological stages. Fundamental to ARDS is the acute onset of lung inflammation as a part of the body's immune response to a variety of local and systemic stimuli. In patients surviving the inflammatory and subsequent fibroproliferative stages, transition from injury to resolution and recovery is an active process dependent on a series of highly coordinated events regulated by the immune system. Experimental animal models of acute lung injury (ALI) reproduce key components of the injury and resolution phases of human ARDS and provide a methodology to explore mechanisms and potential new therapies. Macrophages are essential to innate immunity and host defense, playing a featured role in the lung and alveolar space. Key aspects of their biological response, including differentiation, phenotype, function, and cellular interactions, are determined in large part by the presence, severity, and chronicity of local inflammation. Studies support the importance of macrophages to initiate and maintain the inflammatory response, as well as a determinant of resolution of lung inflammation and repair. We will discuss distinct roles for lung macrophages during early inflammatory and late resolution phases of ARDS using experimental animal models. In addition, each section will highlight human studies that relate to the diverse role of macrophages in initiation and resolution of ALI and ARDS.

Fibrates ameliorate the course of bacterial sepsis by promoting neutrophil recruitment via CXCR2

Bacterial sepsis results in high mortality rates, and new therapeutics to control infection are urgently needed. Here, we investigate the therapeutic potential of fibrates in the treatment of bacterial sepsis and examine their effects on innate immunity. Fibrates significantly improved the survival from sepsis in mice infected with Salmonella typhimurium, which was paralleled by markedly increased neutrophil influx to the site of infection resulting in rapid clearance of invading bacteria. As a consequence of fibrate-mediated early control of infection, the systemic inflammatory response was repressed in fibrate-treated mice. Mechanistically, we found that fibrates preserve chemotaxis of murine neutrophils by blocking LPS-induced phosphorylation of ERK. This results in a decrease of G protein-coupled receptor kinase-2 expression, thereby inhibiting the LPS-mediated downregulation of CXCR2, a chemokine receptor critical for neutrophil recruitment. Accordingly, application of a synthetic CXCR2 inhibitor completely abrogated the protective effects of fibrates in septicemia in vivo. Our results unravel a novel function of fibrates in innate immunity and host response to infection and suggest fibrates as a promising adjunct therapy in bacterial sepsis.

Attenuated Bordetella pertussis vaccine protects against respiratory syncytial virus disease via an IL-17-dependent mechanism

RATIONALE

We attenuated virulent Bordetella pertussis by genetically eliminating or detoxifying three major toxins. This strain, named BPZE1, is being developed as a possible live nasal vaccine for the prevention of whooping cough. It is immunogenic and safe when given intranasally in adult volunteers.

OBJECTIVES

Before testing in human infants, we wished to examine the potential effect of BPZE1 on a common pediatric infection (respiratory syncytial virus [RSV]) in a preclinical model.

METHODS

BPZE1 was administered before or after RSV administration in adult or neonatal mice. Pathogen replication, inflammation, immune cell recruitment, and cytokine responses were measured.

MEASUREMENTS AND MAIN RESULTS

BPZE1 alone did not cause overt disease, but induced efflux of neutrophils into the airway lumen and production of IL-10 and IL-17 by mucosal CD4(+) T cells. Given intranasally before RSV infection, BPZE1 markedly attenuated RSV, preventing weight loss, reducing viral load, and attenuating lung cell recruitment. Given neonatally, BPZE1 also protected against RSV-induced weight loss even through to adulthood. Furthermore, it markedly increased IL-17 production by CD4(+) T cells and natural killer cells and recruited regulatory cells and neutrophils after virus challenge. Administration of anti-IL-17 antibodies ablated the protective effect of BPZE1 on RSV disease.

CONCLUSIONS

Rather than enhancing RSV disease, BPZE1 protected against viral infection, modified viral responses, and enhanced natural mucosal resistance. Prevention of RSV infection by BPZE1 seems in part to be caused by induction of IL-17. Clinical trial registered with www.clinicaltrials.gov (NCT 01188512).

CXCL5-secreting pulmonary epithelial cells drive destructive neutrophilic inflammation in tuberculosis

Successful host defense against numerous pulmonary infections depends on bacterial clearance by polymorphonuclear leukocytes (PMNs); however, excessive PMN accumulation can result in life-threatening lung injury. Local expression of CXC chemokines is critical for PMN recruitment. The impact of chemokine-dependent PMN recruitment during pulmonary Mycobacterium tuberculosis infection is not fully understood. Here, we analyzed expression of genes encoding CXC chemokines in M. tuberculosis-infected murine lung tissue and found that M. tuberculosis infection promotes upregulation of Cxcr2 and its ligand Cxcl5. To determine the contribution of CXCL5 in pulmonary PMN recruitment, we generated Cxcl5(-/-) mice and analyzed their immune response against M. tuberculosis. Both Cxcr2(-/-) mice and Cxcl5(-/-) mice, which are deficient for only one of numerous CXCR2 ligands, exhibited enhanced survival compared with that of WT mice following high-dose M. tuberculosis infection. The resistance of Cxcl5(-/-) mice to M. tuberculosis infection was not due to heightened M. tuberculosis clearance but was the result of impaired PMN recruitment, which reduced pulmonary inflammation. Lung epithelial cells were the main source of CXCL5 upon M. tuberculosis infection, and secretion of CXCL5 was reduced by blocking TLR2 signaling. Together, our data indicate that TLR2-induced epithelial-derived CXCL5 is critical for PMN-driven destructive inflammation in pulmonary tuberculosis.