Neutrophils are important in early control of lung infection by Yersinia pestis

Yersinia pestis and pneumonic plague: Insight into how a lethal pathogen interfaces with innate immune populations in the lung to cause severe disease

Yersinia pestis is the causative agent of bubonic, septicemic and pneumonic plague. The historical importance and potential of plague to re-emerge as a threat worldwide are indisputable. The most severe manifestion of plague is pneumonic plague, which results in disease that is 100% lethal without treatment. Y. pestis suppresses host immune responses early in the lung to establish infection. The later stages of infection see the rapid onset of hyperinflammatory responses that prove lethal. The study of Y. pestis host/pathogen interactions have largely been investigated during bubonic plague and with attenuated strains in cell culture models. There remains a somewhat limited understanding of the interactions between virulent Y. pestis and immune populations in the lung that drive severe disease. In this review we give a broad overview of the progression of pneumonic plague and highlighting how Y. pestis interfaces with host innate immune populations in the lung to cause lethal disease.

The Metabolic Potential of the Human Lung Microbiome

The human lung microbiome remains largely underexplored, despite its potential implications in the pharmacokinetics of inhaled drugs and its involvement in lung diseases. Interactions within these bacterial communities and with the host are complex processes which often involve microbial small molecules. In this study, we employed a computational approach to describe the metabolic potential of the human lung microbiome. By utilizing antiSMASH and BiG-SCAPE software, we identified 1831 biosynthetic gene clusters for the production of specialized metabolites in a carefully compiled genome database of lung-associated bacteria and fungi. It was shown that RiPPs represent the largest class of natural products within the bacteriome, while NRPs constitute the largest class of natural products in the lung mycobiome. All predicted BGCs were further categorized into 767 gene cluster families, and a subsequent network analysis highlighted that these families are widely distributed and contain many uncharacterized members. Moreover, in-depth annotation allowed the assignment of certain gene clusters to putative lung-specific functions within the microbiome, such as osmoadaptation or surfactant synthesis. This study establishes the lung microbiome as a prolific source for secondary metabolites and lays the groundwork for detailed investigation of this unique environment.

Protection from plague via single dose administration of antibody to neutralize the type I interferon response

Yersinia pestis is a gram-negative bacterium and the causative agent for the plague. Yersinia spp . use effector proteins of the type III secretion system (T3SS) to skew the host immune response toward a bacterial advantage during infection. Previous work established that mice which lack the type I IFN receptor (IFNAR), exhibit resistance to pulmonary infection by Y. pestis . In this work, we addressed the efficacy of a single dose administration of neutralizing antibody to IFNAR (MAR1) as a preventive treatment for plague. We show that single dose administration of MAR1 provides protection from mortality due to secondary septicemic plague where it appears to reduce the production of serum TNFα during the disease phase. We further demonstrate that the T3SS effector protein YopJ is necessary for MAR1-induced protection, however IFNAR-dependent serum TNFα was observed independent of YopJ. We further define tissue-specific anti-bacterial roles of IFNAR that are blocked by YopJ activity indicating that YopJ and IFNAR work in parallel to promote disease. The combined data suggest that therapeutic targeting of IFNAR signaling may reduce the hyper-inflammatory response associated with plague.

Influenza virus infection augments susceptibility to respiratory Yersinia pestis exposure and impacts the efficacy of antiplague antibiotic treatments

Various respiratory viral infections in general and seasonal influenza in particular may increase the susceptibility to bacterial infections. Plague caused by Yersinia pestis endangers large populations during outbreaks or bioterrorism attacks. Recommended antibiotic countermeasures include well-established protocols based on animal studies and corroborated by effective treatment of human cases. Until now, prior exposure to viral respiratory infections was not taken into consideration when selecting the appropriate treatment for plague. Here, we show that as late as 25 days after exposure to influenza virus, convalescent mice still exhibited an increased susceptibility to sublethal doses of Y. pestis, presented with aberrant cytokine expression, and impaired neutrophil infiltration in the lungs. Increased levels of M2 alveolar macrophages and type II epithelial cells, as well as induction in metalloproteases expression and collagen and laminin degradation, suggested that the previous viral infection was under resolution, correlating with enhanced susceptibility to plague. Surprisingly, postexposure prophylaxis treatment with the recommended drugs revealed that ciprofloxacin was superior to doxycycline in mice recovering from influenza infection. These results suggest that after an influenza infection, the consequences, such as impaired immunity and lung tissue remodeling and damage, should be considered when treating subsequent Y. pestis exposure.

A Dual Role for the Plasminogen Activator Protease During the Preinflammatory Phase of Primary Pneumonic Plague

Early after inhalation, Yersinia pestis replicates to high numbers in the airways in the absence of disease symptoms or notable inflammatory responses to cause primary pneumonic plague. The plasminogen activator protease (Pla) is a critical Y. pestis virulence factor that is important for early bacterial growth in the lung via an unknown mechanism. In this article, we define a dual role for Pla in the initial stages of pulmonary infection. We show that Pla functions as an adhesin independent of its proteolytic function to suppress early neutrophil influx into the lungs, and that Pla enzymatic activity contributes to bacterial resistance to neutrophil-mediated bacterial killing. Our results suggest that the fate of Y. pestis infection of the lung is decided extremely early during infection and that Pla plays a dual role to tilt the balance in favor of the pathogen.

Unraveling neutrophil- Yersinia interactions during tissue infection

The human and animal pathogens Yersinia pestis, which causes bubonic and pneumonic plague, and Yersinia pseudotuberculosis and Yersinia enterocolitica, which cause gastroenteritis, share a type 3 secretion system which injects effector proteins, Yops, into host cells. This system is critical for virulence of all three pathogens in tissue infection. Neutrophils are rapidly recruited to infected sites and all three pathogens frequently interact with and inject Yops into these cells during tissue infection. Host receptors, serum factors, and bacterial adhesins appear to collaborate to promote neutrophil- Yersinia interactions in tissues. The ability of neutrophils to control infection is mixed depending on the stage of infection and points to the efficiency of Yops and other bacterial factors to mitigate bactericidal effects of neutrophils. Yersinia in close proximity to neutrophils has higher levels of expression from yop promoters, and neutrophils in close proximity to Yersinia express higher levels of pro-survival genes than migrating neutrophils. In infected tissues, YopM increases neutrophil survival and YopH targets a SKAP2/SLP-76 signal transduction pathway. Yet the full impact of these and other Yops and other Yersinia factors on neutrophils in infected tissues has yet to be understood.

Yersinia pestis Exploits Early Activation of MyD88 for Growth in the Lungs during Pneumonic Plague

Yersinia pestis causes bubonic, pneumonic, and septicemic plague. Although no longer responsible for pandemic outbreaks, pneumonic plague continues to be a challenge for medical treatment and has been classified as a reemerging disease in some parts of the world. In the early stage of infection, inflammatory responses are believed to be suppressed by Y. pestis virulence factors in order to prevent clearance, while later, the hyperactivation of inflammation contributes to the progression of disease. In this work, we sought to identify the host factors that mediate this process and studied the role of the Toll/interleukin 1 (IL-1) receptor adapter and major inflammatory mediator myeloid differentiation primary response 88 (MyD88) in pneumonic plague. We show that pulmonary challenge of Myd88^-/- mice with wild-type (WT) Y. pestis results in significant loss of pro- and anti-inflammatory cytokines and chemokines, especially gamma interferon (IFN-γ) and KC, in the lungs compared to that in WT mice. Bacterial growth in the lungs occurred more rapidly in the WT mice, however, indicating a role for the MyD88 response in facilitating the primary lung infection. Nevertheless, Myd88^-/- mice were more sensitive to lethality from secondary septicemic plague. Together these findings indicate a central role for MyD88 during the biphasic inflammatory response to pulmonary Y. pestis infection. In the early phase, low-level MyD88-dependent chemokine expression limits initial growth but facilitates Y. pestis access to a protected replicative niche. The later hyperinflammatory phase is partially MyD88 dependent and ineffective in the lungs but controls systemic infection and reduces the progression of secondary septicemic plague.

Gain-of-Function Analysis Reveals Important Virulence Roles for the Yersinia pestis Type III Secretion System Effectors YopJ, YopT, and YpkA

Virulence of Yersinia pestis in mammals requires the type III secretion system, which delivers seven effector proteins into the cytoplasm of host cells to undermine immune responses. All seven of these effectors are conserved across Y. pestis strains, but three, YopJ, YopT, and YpkA, are apparently dispensable for virulence. Some degree of functional redundancy between effector proteins would explain both observations. Here, we use a combinatorial genetic approach to define the minimal subset of effectors required for full virulence in mice following subcutaneous infection. We found that a Y. pestis strain lacking YopJ, YopT, and YpkA is attenuated for virulence in mice and that addition of any one of these effectors to this strain increases lethality significantly. YopJ, YopT, and YpkA likely contribute to virulence via distinct mechanisms. YopJ is uniquely able to cause macrophage cell death in vitro and to suppress accumulation of inflammatory cells to foci of bacterial growth in deep tissue, whereas YopT and YpkA cannot. The synthetic phenotypes that emerge when YopJ, YopT, and YpkA are removed in combination provide evidence that each effector enhances Y. pestis virulence and that YopT and YpkA act through a mechanism distinct from that of YopJ.

Immunology of Yersinia pestis Infection

As a pathogen of plague, Yersinia pestis caused three massive pandemics in history that killed hundreds of millions of people. Yersinia pestis is highly invasive, causing severe septicemia which, if untreated, is usually fatal to its host. To survive in the host and maintain a persistent infection, Yersinia pestis uses several stratagems to evade the innate and the adaptive immune responses. For example, infections with this organism are biphasic, involving an initial "noninflammatory" phase where bacterial replication occurs initially with little inflammation and following by extensive phagocyte influx, inflammatory cytokine production, and considerable tissue destruction, which is called "proinflammatory" phase. In contrast, the host also utilizes its immune system to eliminate the invading bacteria. Neutrophil and macrophage are the first defense against Yersinia pestis invading through phagocytosis and killing. Other innate immune cells also play different roles, such as dendritic cells which help to generate more T helper cells. After several days post infection, the adaptive immune response begins to provide organism-specific protection and has a long-lasting immunological memory. Thus, with the cooperation and collaboration of innate and acquired immunity, the bacterium may be eliminated from the host. The research of Yersinia pestis and host immune systems provides an important topic to understand pathogen-host interaction and consequently develop effective countermeasures.

Modern Advances against Plague

Plague has been a scourge of humanity, responsible for the deaths of millions. The etiological agent, Yersinia pestis, has evolved relatively recently from an enteropathogen, Yersinia pseudotuberculosis. The evolution of the plague pathogen has involved a complex series of genetic acquisitions, deletions, and rearrangements in its transition from an enteric niche to becoming a systemic, flea-vectored pathogen. With the advent of modern molecular biology techniques, we are starting to understand how the organism adapts to the diverse niches it encounters and how to combat the threat it poses.

Bacterial Dose-Dependent Role of G Protein-Coupled Receptor Kinase 5 in Escherichia coli-Induced Pneumonia

G protein-coupled receptor kinase 5 (GRK5) is a serine/threonine kinase previously shown to mediate polymicrobial sepsis-induced inflammation. The goal of the present study was to examine the role of GRK5 in monomicrobial pulmonary infection by using an intratracheal Escherichia coli infection model of pneumonia. We used sublethal and lethal doses of E. coli to examine the mechanistic differences between low-grade and high-grade inflammation induced by E. coli infection. With a sublethal dose of E. coli, GRK5 knockout (KO) mice exhibited higher plasma CXCL1/KC levels and enhanced lung neutrophil recruitment early after infection, and lower bacterial loads, than wild-type (WT) mice. The inflammatory response was also diminished, and resolution of inflammation advanced, in the lungs of GRK5 KO mice. In contrast to the reduced bacterial loads in GRK5 KO mice following a sublethal dose, at a lethal dose of E. coli, the bacterial burdens remained high in GRK5 KO mice relative to those in WT mice. This occurred in spite of enhanced plasma CXCL1 levels as well as neutrophil recruitment in the KO mice. But the recruited neutrophils (following high-dose infection) exhibited decreased CD11b expression and reduced reactive oxygen species production, suggesting decreased neutrophil activation or increased neutrophil exhaustion in the GRK5 KO mice. In agreement with the increased bacterial burden, KO mice showed poorer survival than WT mice following E. coli infection at a lethal dose. Overall, our data suggest that GRK5 negatively regulates CXCL1/KC levels during bacterial pneumonia but that the role of GRK5 in the clinical outcome in this model is dependent on the bacterial dose.

A role for Toll-like receptor 4 in the host response to the lung infection of Yersinia pseudotuberculosis in mice

Although a Yersinia pseudotuberculosis (Yptb) lung infection model has been developed to study Y. pestis pathogenesis, it is still necessary to establish a new animal model to mimic the pathophysiological features induced by Y. pestis infection. Here, we provide a new lung infection model using the Yptb strain, IP2777, which displayed rapid spread of bacteria to the liver, spleen, and blood. In addition, we examined whether TLR4 is involved in Yptb-induced pathogenesis in the lung infection model of mice we generated. Following lung infection of WT and TLR4-deficient mice with the Yptb strain IP2777, the survival rate, bacterial colonization, histopathology, and level of cytokines and chemokines in the lung, spleen, liver, and blood were analyzed. TLR4-deficient mice had a lower survival rate than WT mice in response to Yptb lung infection. Although the bacterial colonization and pathology of the lung were comparable between WT and TLR4-deficient mice, those of the spleen and liver were more severe in TLR4-deficient mice. In addition, the levels of TNF-α and CXCL2 in the liver and IL-6 and CXCL2 in the blood were higher in TLR4-deficient mice than in WT mice. Our results demonstrate that TLR4 is necessary for optimal host protection against Yptb lung infection and TLR4-deficient mice may serve as a better genetic model of Yptb infection for mimicking Y. pestis infection.

Spatially distinct neutrophil responses within the inflammatory lesions of pneumonic plague

During pneumonic plague, the bacterium Yersinia pestis elicits the development of inflammatory lung lesions that continue to expand throughout infection. This lesion development and persistence are poorly understood. Here, we examine spatially distinct regions of lung lesions using laser capture microdissection and transcriptome sequencing (RNA-seq) analysis to identify transcriptional differences between lesion microenvironments. We show that cellular pathways involved in leukocyte migration and apoptosis are downregulated in the center of lung lesions compared to the periphery. Probing for the bacterial factor(s) important for the alteration in neutrophil survival, we show both in vitro and in vivo that Y. pestis increases neutrophil survival in a manner that is dependent on the type III secretion system effector YopM. This research explores the complexity of spatially distinct host-microbe interactions and emphasizes the importance of cell relevance in assays in order to fully understand Y. pestis virulence.

Yersinia pestis is a high-priority pathogen and continues to cause outbreaks worldwide. The ability of Y. pestis to be transmitted via respiratory droplets and its history of weaponization has led to its classification as a select agent most likely to be used as a biological weapon. Unrestricted bacterial growth during the initial preinflammatory phase primes patients to be infectious once disease symptoms begin in the proinflammatory phase, and the rapid disease progression can lead to death before Y. pestis infection can be diagnosed and treated. Using in vivo analyses and focusing on relevant cell types during pneumonic plague infection, we can identify host pathways that may be manipulated to extend the treatment window for pneumonic plague patients.

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.

Yersinia pestis targets neutrophils via complement receptor 3

Yersinia species display a tropism for lymphoid tissues during infection, and the bacteria select innate immune cells for delivery of cytotoxic effectors by the type III secretion system. Yet, the mechanism for target cell selection remains a mystery. Here we investigate the interaction of Yersinia pestis with murine splenocytes to identify factors that participate in the targeting process. We find that interactions with primary immune cells rely on multiple factors. First, the bacterial adhesin Ail is required for efficient targeting of neutrophils in vivo. However, Ail does not appear to directly mediate binding to a specific cell type. Instead, we find that host serum factors direct Y. pestis to specific innate immune cells, particularly neutrophils. Importantly, specificity towards neutrophils was increased in the absence of bacterial adhesins because of reduced targeting of other cell types, but this phenotype was only visible in the presence of mouse serum. Addition of antibodies against complement receptor 3 and CD14 blocked target cell selection, suggesting that a combination of host factors participate in steering bacteria towards neutrophils during plague infection.

Bacterial programming of host responses: coordination between type I interferon and cell death

During mammalian infection, bacteria induce cell death from an extracellular or intracellular niche that can protect or hurt the host. Data is accumulating that associate type I interferon (IFN) signaling activated by intracellular bacteria with programmed death of immune effector cells and enhanced virulence. Multiple pathways leading to IFN-dependent host cell death have been described, and in some cases it is becoming clear how these mechanisms contribute to virulence. Yet common mechanisms of IFN-enhanced bacterial pathogenesis are not obvious and no specific interferon stimulated genes have yet been identified that cause sensitivity to pathogen-induced cell death. In this review, we will summarize some bacterial infections caused by facultative intracellular pathogens and what is known about how type I IFN signaling may promote the replication of extracellular bacteria rather than stimulate protection. Each of these pathogens can survive phagocytosis but their intracellular life cycles are very different, they express distinct virulence factors and trigger different pathways of immune activation and crosstalk. These differences likely lead to widely varying amounts of type I IFN expression and a different inflammatory environment, but these may not be important to the pathologic effects on the host. Instead, each pathogen induces programmed cell death of key immune cells that have been sensitized by the activation of the type I IFN response. We will discuss how IFN-dependent host cell death may increase host susceptibility and try to understand common pathways of pathogenesis that lead to IFN-enhanced bacterial virulence.

The impact of "omic" and imaging technologies on assessing the host immune response to biodefence agents

Understanding the interactions between host and pathogen is important for the development and assessment of medical countermeasures to infectious agents, including potential biodefence pathogens such as Bacillus anthracis, Ebola virus, and Francisella tularensis. This review focuses on technological advances which allow this interaction to be studied in much greater detail. Namely, the use of “omic” technologies (next generation sequencing, DNA, and protein microarrays) for dissecting the underlying host response to infection at the molecular level; optical imaging techniques (flow cytometry and fluorescence microscopy) for assessing cellular responses to infection; and biophotonic imaging for visualising the infectious disease process. All of these technologies hold great promise for important breakthroughs in the rational development of vaccines and therapeutics for biodefence agents.

Prophylaxis and therapy of plague

Plague has been a scourge of mankind for centuries, and outbreaks continue to the present day. The virulence mechanisms employed by the etiological agent Yersinia pestis are reviewed in the context of the available prophylactic and therapeutic strategies for plague. Although antibiotics are available, resistance is emerging in this dangerous pathogen. Therapeutics used in the clinic are discussed and innovative approaches to the design and development of new therapeutic compounds are reviewed. Currently there is no licensed vaccine available for prevention of plague in the USA or western Europe, although both live attenuated strains and killed whole-cell extracts have been used historically. Live strains are still approved for human use in some parts of the world, such as the former Soviet Union, but poor safety profiles render them unacceptable to many countries. The development of safe, effective next-generation vaccines, including the recombinant subunit vaccine currently used in clinical trials is discussed.

Early host cell targets of Yersinia pestis during primary pneumonic plague

Inhalation of Yersinia pestis causes primary pneumonic plague, a highly lethal syndrome with mortality rates approaching 100%. Pneumonic plague progression is biphasic, with an initial pre-inflammatory phase facilitating bacterial growth in the absence of host inflammation, followed by a pro-inflammatory phase marked by extensive neutrophil influx, an inflammatory cytokine storm, and severe tissue destruction. Using a FRET-based probe to quantitate injection of effector proteins by the Y. pestis type III secretion system, we show that these bacteria target alveolar macrophages early during infection of mice, followed by a switch in host cell preference to neutrophils. We also demonstrate that neutrophil influx is unable to limit bacterial growth in the lung and is ultimately responsible for the severe inflammation during the lethal pro-inflammatory phase.

Inhalation of the bacterium Yersinia pestis results in primary pneumonic plague, a severe necrotizing pneumonia with mortality rates approaching 100% in the absence of timely antibiotic administration. Despite the notoriety of Y. pestis as a potential biological weapon and its well-established pandemic potential, very little is known regarding early host-pathogen interactions that lead to the progression of pulmonary infection. Y. pestis harbors a type III secretion system (T3SS) for delivery of Yersinia outer protein (Yop) effectors into host cells, an early and essential step in pathogenesis. In the work presented here, we identify the host cell targets of Y. pestis Yop secretion in the lung. We show that Y. pestis initially targets alveolar macrophages, followed by a shift in host cell preference to neutrophils. Through cellular depletion studies, we demonstrate that Y. pestis is highly resistant to macrophage- and neutrophil-mediated clearance, and that the accumulation of neutrophils in the lung is responsible for the severe necrotizing pneumonia that develops during the pro-inflammatory phase of pneumonic plague.

Early apoptosis of macrophages modulated by injection of Yersinia pestis YopK promotes progression of primary pneumonic plague

Yersinia pestis causes pneumonic plague, a disease characterized by inflammation, necrosis and rapid bacterial growth which together cause acute lung congestion and lethality. The bacterial type III secretion system (T3SS) injects 7 effector proteins into host cells and their combined activities are necessary to establish infection. Y. pestis infection of the lungs proceeds as a biphasic inflammatory response believed to be regulated through the control of apoptosis and pyroptosis by a single, well-conserved T3SS effector protein YopJ. Recently, YopJ-mediated pyroptosis, which proceeds via the NLRP3-inflammasome, was shown to be regulated by a second T3SS effector protein YopK in the related strain Y. pseudotuberculosis. In this work, we show that for Y. pestis, YopK appears to regulate YopJ-mediated apoptosis, rather than pyroptosis, of macrophages. Inhibition of caspase-8 blocked YopK-dependent apoptosis, suggesting the involvement of the extrinsic pathway, and appeared cell-type specific. However, in contrast to yopJ, deletion of yopK caused a large decrease in virulence in a mouse pneumonic plague model. YopK-dependent modulation of macrophage apoptosis was observed at 6 and 24 hours post-infection (HPI). When YopK was absent, decreased populations of macrophages and dendritic cells were seen in the lungs at 24 HPI and correlated with resolution rather than progression of inflammation. Together the data suggest that Y. pestis YopK may coordinate the inflammatory response during pneumonic plague through the regulation of apoptosis of immune cells.

In this work, we studied the mechanism whereby bacteria manipulate innate immune responses by controlling host cell death. Yersinia pestis, the causative agent of plague, requires effector Yops of the Type III Secretion System (T3SS) to evade the innate immune system during infection. We show that Yersinia induces apoptosis of macrophages through two distinct mechanisms, each through the activity of the well-characterized T3SS effector YopJ, yet regulated in an opposing manner through the activity of a second effector protein YopK. In a murine pneumonic plague model, we found evidence that YopK regulates apoptosis of macrophages during the early stage of infection, leading to uncontrolled inflammation and disease. In contrast, the absence of YopK-regulated apoptosis allowed recruitment of lymphocytes and CCR2⁺ immune cells which led to bacterial clearance and resolution of inflammation. Together the data suggest that Yersinia YopK modulates apoptosis of immune cells to control the inflammatory response during plague.

The influence of cigarette smoking on viral infections: translating bench science to impact COPD pathogenesis and acute exacerbations of COPD clinically

COPD is a complex syndrome that poses a serious health threat to >1.1 billion smokers worldwide. The stable disease is punctuated by episodes of acute exacerbation, which are predominantly the result of viral and bacterial infections. Despite their devastating health impact, mechanisms underlying disease exacerbations remain poorly understood. Mounting evidence suggests that cigarette smoke profoundly affects the immune system, compromising the host's ability to mount appropriate immune and inflammatory responses against microbial agents. This review highlights recent advances in our understanding of the impact of cigarette smoke on type 1 interferon and IL-1 signaling cascades. The immune defects caused by cigarette smoke on these two key pathways contribute to the seemingly contradictory nature of cigarette smoke as both a damaging and a proinflammatory factor as well as an immunosuppressive factor. Understanding the impact of cigarette smoke on the immune system may unravel novel targets for therapies that could affect acute exacerbations and COPD pathogenesis.

Fibrin facilitates both innate and T cell-mediated defense against Yersinia pestis

The Gram-negative bacterium Yersinia pestis causes plague, a rapidly progressing and often fatal disease. The formation of fibrin at sites of Y. pestis infection supports innate host defense against plague, perhaps by providing a nondiffusible spatial cue that promotes the accumulation of inflammatory cells expressing fibrin-binding integrins. This report demonstrates that fibrin is an essential component of T cell-mediated defense against plague but can be dispensable for Ab-mediated defense. Genetic or pharmacologic depletion of fibrin abrogated innate and T cell-mediated defense in mice challenged intranasally with Y. pestis. The fibrin-deficient mice displayed reduced survival, increased bacterial burden, and exacerbated hemorrhagic pathology. They also showed fewer neutrophils within infected lung tissue and reduced neutrophil viability at sites of liver infection. Depletion of neutrophils from wild-type mice weakened T cell-mediated defense against plague. The data suggest that T cells combat plague in conjunction with neutrophils, which require help from fibrin to withstand Y. pestis encounters and effectively clear bacteria.

Early sensing of Yersinia pestis airway infection by bone marrow cells

Bacterial infection of the lungs triggers a swift innate immune response that involves the production of cytokines and chemokines that promote recruitment of immune cells from the bone marrow (BM) into the infected tissue and limit the ability of the pathogen to replicate. Recent in vivo studies of pneumonic plague in animal models indicate that the pulmonary pro-inflammatory response to airway infection with Yersinia pestis is substantially delayed in comparison to other pathogens. Consequently, uncontrolled proliferation of the pathogen in the lungs is observed, followed by dissemination to internal organs and death. While the lack of an adequate early immune response in the lung is well described, the response of BM-derived cells is poorly understood. In this study, we show that intranasal (i.n.) infection of mice with a fully virulent Y. pestis strain is sensed early by the BM compartment, resulting in a reduction in CXCR4 levels on BM neutrophils and their subsequent release into the blood 12 hours (h) post infection. In addition, increased levels of BM-derived hematopoietic stem and progenitor cells (HSPC) were detected in the blood early after infection. Mobilization of both immature and mature cells was accompanied by the reduction of BM SDF-1 (CXCL-12) levels and the reciprocal elevation of SDF-1 in the blood 24 h post infection. RT-PCR analysis of RNA collected from total BM cells revealed an early induction of myeloid-associated genes, suggesting a prompt commitment to myeloid lineage differentiation. These findings indicate that lung infection by Y. pestis is sensed by BM cells early after infection, although bacterial colonization of the BM occurs at late disease stages, and point on a potential cross-talk between the lung and the BM at early stages of pneumonic plague.

Characterization of systemic and pneumonic murine models of plague infection using a conditionally virulent strain

Yersinia pestis causes bubonic and pneumonic plague in humans. The pneumonic infection is the most severe and invariably fatal if untreated. Because of its high virulence, ease of delivery and precedent of use in warfare, Y. pestis is considered as a potential bioterror agent. No licensed plague vaccine is currently available in the US. Laboratory research with virulent strains requires appropriate biocontainment (i.e., Biosafety Level 3 (BSL-3) for procedures that generate aerosol/droplets) and secure facilities that comply with federal select agent regulations. To assist in the identification of promising vaccine candidates during the early phases of development, we characterized mouse models of systemic and pneumonic plague infection using the Y. pestis strain EV76, an attenuated human vaccine strain that can be rendered virulent in mice under in vivo iron supplementation. Mice inoculated intranasally or intravenously with Y. pestis EV76 in the presence of iron developed a systemic and pneumonic plague infection that resulted in disease and lethality. Bacteria replicated and severely compromised the spleen, liver and lungs. Susceptibility was age dependent, with younger mice being more vulnerable to pneumonic infection. We used these models of infection to assess the protective capacity of newly developed Salmonella-based plague vaccines. The protective outcome varied depending on the route and dose of infection. Protection was associated with the induction of specific immunological effectors in systemic/mucosal compartments. The models of infection described could serve as safe and practical tools for identifying promising vaccine candidates that warrant further potency evaluation using fully virulent strains in BSL-3 settings.

Yersinia pestis intracellular parasitism of macrophages from hosts exhibiting high and low severity of plague

BACKGROUND

Yersinia pestis causes severe disease in natural rodent hosts, but mild to inapparent disease in certain rodent predators such as dogs. Y. pestis initiates infection in susceptible hosts by parasitizing and multiplying intracellularly in local macrophages prior to systemic dissemination. Thus, we hypothesize that Y. pestis disease severity may depend on the degree to which intracellular Y. pestis overcomes the initial host macrophage imposed stress.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis, the progression of in vitro infection by Y. pestis KIM62053.1+ of mouse splenic and RAW264.7 tissue culture macrophages and dog peripheral blood-derived and DH82 tissue culture macrophages was studied using microscopy and various parameters of infection. The study showed that during the early stage of infection, intracellular Y. pestis assumed filamentous cellular morphology with multiple copies of the genome per bacterium in both mouse and dog macrophages. Later, in mouse macrophages, the infection elicited spacious vacuolar extension of Yersinia containing vacuoles (YCV), and the filamentous Y. pestis reverted to coccobacillary morphology with genomic equivalents approximately equaling colony forming units. In contrast, Y. pestis infected dog macrophages did not show noticeable extension of YCV, and intracellular Y. pestis retained the filamentous cellular morphology for the entire experiment in DH82 cells or were killed by blood-derived macrophages. In addition, during the later stage of infection, Y. pestis infected mouse macrophages exhibited cell lysis whereas dog macrophages did not.

CONCLUSION/SIGNIFICANCE

Overall, these results support our hypothesis that Y. pestis in mouse macrophages can overcome the initial intracellular stress necessary for subsequent systemic infection. However, in dogs, failure of Y. pestis to overcome macrophage imposed stress may result in mild or in apparent disease in dogs.

Opposing roles for interferon regulatory factor-3 (IRF-3) and type I interferon signaling during plague

Type I interferons (IFN-I) broadly control innate immunity and are typically transcriptionally induced by Interferon Regulatory Factors (IRFs) following stimulation of pattern recognition receptors within the cytosol of host cells. For bacterial infection, IFN-I signaling can result in widely variant responses, in some cases contributing to the pathogenesis of disease while in others contributing to host defense. In this work, we addressed the role of type I IFN during Yersinia pestis infection in a murine model of septicemic plague. Transcription of IFN-β was induced in vitro and in vivo and contributed to pathogenesis. Mice lacking the IFN-I receptor, Ifnar, were less sensitive to disease and harbored more neutrophils in the later stage of infection which correlated with protection from lethality. In contrast, IRF-3, a transcription factor commonly involved in inducing IFN-β following bacterial infection, was not necessary for IFN production but instead contributed to host defense. In vitro, phagocytosis of Y. pestis by macrophages and neutrophils was more effective in the presence of IRF-3 and was not affected by IFN-β signaling. This activity correlated with limited bacterial growth in vivo in the presence of IRF-3. Together the data demonstrate that IRF-3 is able to activate pathways of innate immunity against bacterial infection that extend beyond regulation of IFN-β production.

Type I interferons (IFN-I) broadly stimulate innate immunity against viral, bacterial and parasitic pathogens. Many bacterial pathogens induce IFN-I through phosphorylation of Interferon Regulatory Factor 3 (IRF-3) allowing it to bind promoters containing Interferon Stimulated Response Elements (ISRE) which include IFN-β and pro-inflammatory cytokines and chemokines. Secreted IFN-β is taken up by the IFN-αβ receptor (IFNAR), triggering activation of the JAK-STAT pathway which also activates ISRE-containing genes. In this work, we have discovered a novel anti-bacterial function of IRF-3. We show that the respiratory pathogen, Yersinia pestis, the causative agent of plague, activates IRF-3 and the IFN-I response and that these two events cause opposite outcomes in the host. While IRF-3 is necessary for an early stage of phagocytosis, IFNAR signaling promotes the infection and may directly contribute to neutrophil depletion during infection. These results demonstrate that an IFN-independent function of IRF-3 is important to host defense against bacterial infection.

The natural history and incidence of Yersinia pestis and prospects for vaccination

Plague is an ancient, serious, infectious disease which is still endemic in regions of the modern world and is a potential biothreat agent. This paper discusses the natural history of the bacterium and its evolution into a flea-vectored bacterium able to transmit bubonic plague. It reviews the incidence of plague in the modern world and charts the history of vaccines which have been used to protect against the flea-vectored disease, which erupts as bubonic plague. Current approaches to vaccine development to protect against pneumonic, as well as bubonic, plague are also reviewed. The considerable challenges in achieving a vaccine which is licensed for human use and which will comprehensively protect against this serious human pathogen are assessed.

Leukotriene B(4) receptors BLT1 and BLT2 are involved in interleukin-8 production in human neutrophils induced by Trichomonas vaginalis-derived secretory products

OBJECTIVE AND METHOD

Trichomonas vaginalis is a flagellated protozoan parasite that causes human trichomoniasis. Although T. vaginalis itself can secrete lipid mediator leukotriene (LT) B(4) leading to neutrophil activation, information regarding the signaling mechanism involved in neutrophil activation induced by T. vaginalis-secreted LTB(4) is limited. We investigated whether LTB(4) contained in the T. vaginalis-derived secretory products (TvSP) is closely involved in interleukin (IL)-8 production in human neutrophils via LTB(4) receptors BLT1 or BLT2.

RESULTS

T. vaginalis produced more than 714 pg/ml of LTB(4) per 1 × 10(7) trichomonads. The ability of trichomonads to secrete LTB(4) was inhibited by treatment of trichomonads with the 5-lipo-oxygenease inhibitor AA861, but not the cyclo-oxygenease I inhibitor FR122047. When neutrophils were incubated with TvSP obtained from 1 × 10(7) trichomonads, IL-8 protein secretion was significantly increased compared to results for cells incubated with medium alone. The stimulatory effect of TvSP on IL-8 production was strongly inhibited by pretreatment of TvSP with lipase, although pretreatment with heat or proteinase K showed little inhibitory effect. Moreover, TvSP-induced IL-8 production was efficiently inhibited when trichomonads were pretreated with AA861 or when neutrophils were pretreated with antagonists for BLT1 or BLT2.

CONCLUSION

Our results suggest that LTB(4) receptors BLT1 and BLT2 are involved in IL-8 production in neutrophils induced by T. vaginalis.

Resistance to Yersinia pestis infection decreases with age in B10.T(6R) mice

We demonstrate that 2-month-old female B10.T(6R) mice are highly resistant to systemic infection with the KIM5 strain of Yersinia pestis and that B10.T(6R) mice become susceptible to Y. pestis infection by the age of 5 months. In this study, young (2-month-old) and middle-aged (5- to 12-month-old) B10.T(6R) mice were infected with equal CFU counts of Y. pestis. The 50% lethal dose (LD(50)) for young B10.T(6R) mice was ∼1.4 × 10(4) CFU, while middle-aged B10.T(6R) mice exhibited an LD(50) of ∼60 CFU. Elevated bacterial burdens were found in the spleens of middle-aged mice at 24 and 60 h and in the livers at 60 h postinfection. Immune cell infiltration was greater in the livers of resistant young mice than in those of middle-aged mice and mice of the susceptible C57BL/6N strain. Unlike susceptible mice, young B10.T(6R) mice did not develop necrotic lesions throughout the liver. Instead, livers from young B10.T(6R) mice contained granuloma-like structures. Immunohistochemical staining of liver sections from these mice at 60 h postinfection revealed that the majority of immune cells present in these structures were neutrophils. These findings suggest that resistance to plague in B10.T(6R) mice correlates with early formation of neutrophilic lesions in the liver.

Novel highly sensitive IL-10-beta-lactamase reporter mouse reveals cells of the innate immune system as a substantial source of IL-10 in vivo

In this study, we report on a novel, highly sensitive IL-10 reporter mouse based on the reporter enzyme β-lactamase and the fluorescence resonance energy transfer substrate coumarin-cephalosporin-fluorescein (4). In contrast to an IL-10 reporter mouse model that we generated by using enhanced GFP as reporter and allowed tracking IL-10 expression only in T cells, the IL-10-β-lactamase reporter (ITIB) mouse enables us to easily analyze and quantify IL-10 production at the single-cell level in all myeloid and lymphoid cell types. Furthermore, the ITIB mouse allows studying of the kinetics of IL-10 expression on a single-cell basis and provides a valuable tool for in vivo screening of cell type-specific IL-10-modulating drugs. Remarkably, the ITIB mouse revealed that, although a significant portion of each myeloid and lymphoid cell type produces IL-10, macrophages represent the major IL-10 producer population in several organs of naive mice. Moreover, using the examples of bacterial infection and transplantable skin melanoma models, we demonstrate the exceptional applicability of the ITIB mouse for the identification of IL-10-producing cells during immune responses in vivo. In this study, we identified tumor-infiltrating F4/80(+) macrophages as the major source for IL-10 in B16-F10 melanoma in vivo. During systemic infection with Yersinia enterocolitica, although the proportion of IL-10(+) cells increased in each myeloid and lymphoid cell type population, infiltrating CD11b(+)Ly6G(+) neutrophils represent a majority among IL-10-producing cells at the site of infection. We conclude that cells of the innate immune system that are involved in immune homeostasis or immune responses are substantial sources of IL-10.

Chemokine receptor CXCR2 mediates bacterial clearance rather than neutrophil recruitment in a murine model of pneumonic plague

Pulmonary infection by Yersinia pestis causes pneumonic plague, a necrotic bronchopneumonia that is rapidly lethal and highly contagious. Acute pneumonic plague accompanies the up-regulation of pro-inflammatory cytokines and chemokines, suggesting that the host innate immune response may contribute to the development of disease. To address this possibility, we sought to understand the consequences of neutrophil recruitment during pneumonic plague, and we studied the susceptibility of C3H-HeN mice lacking the CXC chemokine KC or its receptor CXC receptor 2 (CXCR2) to pulmonary Y. pestis infection. We found that without Kc or Cxcr2, disease progression was accelerated both in bacterial growth and development of primary bronchopneumonia. When examined in an antibody clearance model, Cxcr2(-/-) mice were not protected by neutralizing Y. pestis antibodies, yet bacterial growth in the lungs was delayed in a manner associated with a neutrophil-mediated inflammatory response. After this initial delay, however, robust neutrophil recruitment in Cxcr2(-/-) mice correlated with bacterial growth and the development of fulminant pneumonic and septicemic plague. In contrast, attenuated Y. pestis lacking the conserved pigmentation locus could be cleared from the lungs in the absence of Cxcr2 indicating virulence factors within this locus may inhibit CXCR2-independent pathways of bacterial killing. Together, the data suggest CXCR2 uniquely induces host defense mechanisms that are effective against virulent Y. pestis, raising new insight into the activation of neutrophils during infection.

Molecular recognition of chymotrypsin by the serine protease inhibitor ecotin from Yersinia pestis

Resistance to antibiotics is a problem not only in terms of healthcare but also biodefense. Engineering of resistance into a human pathogen could create an untreatable biothreat pathogen. One such pathogen is Yersinia pestis, the causative agent of plague. Previously, we have used a bioinformatic approach to identify proteins that may be suitable targets for antimicrobial therapy and in particular for the treatment of plague. The serine protease inhibitor ecotin was identified as one such target. We have carried out mutational analyses in the closely related Yersinia pseudotuberculosis, validating that the ecotin gene is a virulence-associated gene in this bacterium. Y. pestis ecotin inhibits chymotrypsin. Here, we present the structure of ecotin in complex with chymotrypsin to 2.74 Å resolution. The structure features a biologically relevant tetramer whereby an ecotin dimer binds to two chymotrypsin molecules, similar to what was observed in related serine protease inhibitor structures. However, the vast majority of the interactions in the present structure are distinctive, indicating that the broad specificity of the inhibitor for these proteases is based largely on its capacity to recognize features unique to each of them. These findings will have implications for the development of small ecotin inhibitors for therapeutic use.

Yersinia pseudotuberculosis is resistant to killing by human neutrophils

The interaction between human neutrophils and the Gram negative gastrointestinal pathogen Yersinia pseudotuberculosis was investigated in vitro. Despite the wealth of data describing how Yersinia can affect the function of neutrophils, there are no published studies describing if neutrophil cells can affect the viability of Y. pseudotuberculosis. The wild-type IP32953 strain of Y. pseudotuberculosis was found to be resistant to killing by human neutrophils. Confocal examination and flow-cytometric analysis of this interaction revealed that bacteria were taken up.

IL-17 contributes to cell-mediated defense against pulmonary Yersinia pestis infection

Pneumonic plague is one of the world's most deadly infectious diseases. The causative bacterium, Yersinia pestis, has the potential to be exploited as a biological weapon, and no vaccine is available. Vaccinating B cell-deficient mice with D27-pLpxL, a live attenuated Y. pestis strain, induces cell-mediated protection against lethal pulmonary Y. pestis challenge. In this article, we demonstrate that prime/boost vaccination with D27-pLpxL confers better protection than prime-only vaccination. The improved survival does not result from enhanced bacterial clearance but is associated with increased levels of IL-17 mRNA and protein in the lungs of challenged mice. The boost also increases pulmonary numbers of IL-17-producing CD4 T cells. Interestingly, most of these cells simultaneously produce canonical type 1 and type 17 cytokines; most produce IL-17 and TNF-α, and many produce IL-17, TNF-α, and IFN-γ. Neutralizing IL-17 counteracts the improved survival associated with prime/boost vaccination without significantly impacting bacterial burden. Thus, IL-17 appears to mediate the enhanced protection conferred by booster immunization. Although neutralizing IL-17 significantly reduces neutrophil recruitment to the lungs of mice challenged with Y. pestis, this impact is equally evident in mice that receive one or two immunizations with D27-pLpxL, suggesting it cannot suffice to account for the improved survival that results from booster immunization. We conclude that IL-17 plays a yet to be identified role in host defense that enhances protection against pulmonary Y. pestis challenge, and we suggest that pneumonic plague vaccines should aim to induce mixed type 1 and type 17 cellular responses.