How do microbes evade neutrophil killing?

Modulatory potential of resveratrol during lung inflammatory disease

Neutrophils are the first cells to achieve the sites of infection or inflammation in the lungs. The massive accumulation of these cells is associated with acute and chronic lung injury. Therefore, they have been implicated in the pathogenesis of many lung diseases through the release of reactive oxygen intermediates, proteolytic enzymes and Neutrophil Extracellular Traps (NETs). The excessive and continuous release of NETs, fibers composed by decondensed chromatin coated with neutrophil proteins, are associated to the impairment of lung function in different pathological settings. Flavonoids inhibit the respiratory burst of neutrophils in mammals. However, one of these flavonoids, resveratrol has a particular chemical property. It reduce Cu(II) to Cu(I) form with concomitant formation of reactive oxygen species, which can produce DNA breakage as reported in several in vitro models. We hypothesize that direct resveratrol administration in lungs can cleave DNA in NETs, improving lung function during acute airway infections or chronic inflammatory lung diseases. If the hypothesis is correct, the control of NET formation can be used to reduce the inflammatory environment in lung after neutrophil stimuli. Additionally, the production of proinflammatory cytokines by neutrophils could be also diminished by resveratrol administration. In this sense, this flavonoid provides a multifaceted opportunity for treatment of lung diseases with strong or chronic neutrophil activation.

The Phosphocarrier Protein HPr Contributes to Meningococcal Survival during Infection

Neisseria meningitidis is an exclusively human pathogen frequently carried asymptomatically in the nasopharynx but it can also provoke invasive infections such as meningitis and septicemia. N. meningitidis uses a limited range of carbon sources during infection, such as glucose, that is usually transported into bacteria via the phosphoenolpyruvate (PEP):sugar phosphotransferase system (PTS), in which the phosphocarrier protein HPr (encoded by the ptsH gene) plays a central role. Although N. meningitidis possesses an incomplete PTS, HPr was found to be required for its virulence. We explored the role of HPr using bioluminescent wild-type and ΔptsH strains in experimental infection in transgenic mice expressing the human transferrin. The wild-type MC58 strain was recovered at higher levels from the peritoneal cavity and particularly from blood compared to the ΔptsH strain. The ΔptsH strain provoked lower levels of septicemia in mice and was more susceptible to complement-mediated killing than the wild-type strain. We tested whether meningococcal structures impacted complement resistance and observed that only the capsule level was decreased in the ΔptsH mutant. We therefore compared the transcriptomic profiles of wild-type and ΔptsH strains and identified 49 differentially expressed genes. The HPr regulon contains mainly hypothetical proteins (43%) and several membrane-associated proteins that could play a role during host interaction. Some other genes of the HPr regulon are involved in stress response. Indeed, the ΔptsH strain showed increased susceptibility to environmental stress conditions. Our data suggest that HPr plays a pleiotropic role in host-bacteria interactions most likely through the innate immune response that may be responsible for the enhanced clearance of the ΔptsH strain from blood.

NOX2-dependent regulation of inflammation

NADPH oxidase (NOX) isoforms together have multiple functions that are important for normal physiology and have been implicated in the pathogenesis of a broad range of diseases, including atherosclerosis, cancer and neurodegenerative diseases. The phagocyte NADPH oxidase (NOX2) is critical for antimicrobial host defence. Chronic granulomatous disease (CGD) is an inherited disorder of NOX2 characterized by severe life-threatening bacterial and fungal infections and by excessive inflammation, including Crohn's-like inflammatory bowel disease (IBD). NOX2 defends against microbes through the direct antimicrobial activity of reactive oxidants and through activation of granular proteases and generation of neutrophil extracellular traps (NETs). NETosis involves the breakdown of cell membranes and extracellular release of chromatin and neutrophil granular constituents that target extracellular pathogens. Although the immediate effects of oxidant generation and NETosis are predicted to be injurious, NOX2, in several contexts, limits inflammation and injury by modulation of key signalling pathways that affect neutrophil accumulation and clearance. NOX2 also plays a role in antigen presentation and regulation of adaptive immunity. Specific NOX2-activated pathways such as nuclear factor erythroid 2-related factor 2 (Nrf2), a transcriptional factor that induces antioxidative and cytoprotective responses, may be important therapeutic targets for CGD and, more broadly, diseases associated with excessive inflammation and injury.

Escaping Underground Nets: Extracellular DNases Degrade Plant Extracellular Traps and Contribute to Virulence of the Plant Pathogenic Bacterium Ralstonia solanacearum

Plant root border cells have been recently recognized as an important physical defense against soil-borne pathogens. Root border cells produce an extracellular matrix of protein, polysaccharide and DNA that functions like animal neutrophil extracellular traps to immobilize pathogens. Exposing pea root border cells to the root-infecting bacterial wilt pathogen Ralstonia solanacearum triggered release of DNA-containing extracellular traps in a flagellin-dependent manner. These traps rapidly immobilized the pathogen and killed some cells, but most of the entangled bacteria eventually escaped. The R. solanacearum genome encodes two putative extracellular DNases (exDNases) that are expressed during pathogenesis, suggesting that these exDNases contribute to bacterial virulence by enabling the bacterium to degrade and escape root border cell traps. We tested this hypothesis with R. solanacearum deletion mutants lacking one or both of these nucleases, named NucA and NucB. Functional studies with purified proteins revealed that NucA and NucB are non-specific endonucleases and that NucA is membrane-associated and cation-dependent. Single ΔnucA and ΔnucB mutants and the ΔnucA/B double mutant all had reduced virulence on wilt-susceptible tomato plants in a naturalistic soil-soak inoculation assay. The ΔnucA/B mutant was out-competed by the wild-type strain in planta and was less able to stunt root growth or colonize plant stems. Further, the double nuclease mutant could not escape from root border cells in vitro and was defective in attachment to pea roots. Taken together, these results demonstrate that extracellular DNases are novel virulence factors that help R. solanacearum successfully overcome plant defenses to infect plant roots and cause bacterial wilt disease.

Infiltrated Macrophages Die of Pneumolysin-Mediated Necroptosis following Pneumococcal Myocardial Invasion

Streptococcus pneumoniae (the pneumococcus) is capable of invading the heart. Herein we observed that pneumococcal invasion of the myocardium occurred soon after development of bacteremia and was continuous thereafter. Using immunofluorescence microscopy (IFM), we observed that S. pneumoniae replication within the heart preceded visual signs of tissue damage in cardiac tissue sections stained with hematoxylin and eosin. Different S. pneumoniae strains caused distinct cardiac pathologies: strain TIGR4, a serotype 4 isolate, caused discrete pneumococcus-filled microscopic lesions (microlesions), whereas strain D39, a serotype 2 isolate, was, in most instances, detectable only using IFM and was associated with foci of cardiomyocyte hydropic degeneration and immune cell infiltration. Both strains efficiently invaded the myocardium, but cardiac damage was entirely dependent on the pore-forming toxin pneumolysin only for D39. Early microlesions caused by TIGR4 and microlesions formed by a TIGR4 pneumolysin-deficient mutant were infiltrated with CD11b(+) and Ly6G-positive neutrophils and CD11b(+) and F4/80-positive (F4/80(+)) macrophages. We subsequently demonstrated that macrophages in TIGR4-infected hearts died as a result of pneumolysin-induced necroptosis. The effector of necroptosis, phosphorylated mixed-lineage kinase domain-like protein (MLKL), was detected in CD11b(+) and F4/80(+) cells associated with microlesions. Likewise, treatment of infected mice and THP-1 macrophages in vitro with the receptor-interacting protein 1 kinase (RIP1) inhibitor necrostatin-5 promoted the formation of purulent microlesions and blocked cell death, respectively. We conclude that pneumococci that have invaded the myocardium are an important cause of cardiac damage, pneumolysin contributes to cardiac damage in a bacterial strain-specific manner, and pneumolysin kills infiltrated macrophages via necroptosis, which alters the immune response.

A review of the proposed role of neutrophils in rodent amebic liver abscess models

Host invasion by Entamoeba histolytica, the pathogenic agent of amebiasis, can lead to the development of amebic liver abscess (ALA). Due to the difficulty of exploring host and amebic factors involved in the pathogenesis of ALA in humans, most studies have been conducted with animal models (e.g., mice, gerbils, and hamsters). Histopathological findings reveal that the chronic phase of ALA in humans corresponds to lytic or liquefactive necrosis, whereas in rodent models there is granulomatous inflammation. However, the use of animal models has provided important information on molecules and mechanisms of the host/parasite interaction. Hence, the present review discusses the possible role of neutrophils in the effector immune response in ALA in rodents. Properly activated neutrophils are probably successful in eliminating amebas through oxidative and non-oxidative mechanisms, including neutrophil degranulation, the generation of free radicals (O₂⁻, H₂O₂, HOCl) and peroxynitrite, the activation of NADPH-oxidase and myeloperoxidase (MPO) enzymes, and the formation of neutrophil extracellular traps (NETs). On the other hand, if amebas are not eliminated in the early stages of infection, they trigger a prolonged and exaggerated inflammatory response that apparently causes ALAs. Genetic differences in animals and humans are likely to be key to a successful host immune response.

Cryptococcus and Phagocytes: Complex Interactions that Influence Disease Outcome

Cryptococcus neoformans and C. gattii are fungal pathogens that cause life-threatening disease. These fungi commonly enter their host via inhalation into the lungs where they encounter resident phagocytes, including macrophages and dendritic cells, whose response has a pronounced impact on the outcome of disease. Cryptococcus has complex interactions with the resident and infiltrating innate immune cells that, ideally, result in destruction of the yeast. These phagocytic cells have pattern recognition receptors that allow recognition of specific cryptococcal cell wall and capsule components. However, Cryptococcus possesses several virulence factors including a polysaccharide capsule, melanin production and secretion of various enzymes that aid in evasion of the immune system or enhance its ability to thrive within the phagocyte. This review focuses on the intricate interactions between the cryptococci and innate phagocytic cells including discussion of manipulation and evasion strategies used by Cryptococcus, anti-cryptococcal responses by the phagocytes and approaches for targeting phagocytes for the development of novel immunotherapeutics.

Genomic Comparative Study of Bovine Mastitis Escherichia coli

Escherichia coli, one of the main causative agents of bovine mastitis, is responsible for significant losses on dairy farms. In order to better understand the pathogenicity of E. coli mastitis, an accurate characterization of E. coli strains isolated from mastitis cases is required. By using phylogenetic analyses and whole genome comparison of 5 currently available mastitis E. coli genome sequences, we searched for genotypic traits specific for mastitis isolates. Our data confirm that there is a bias in the distribution of mastitis isolates in the different phylogenetic groups of the E. coli species, with the majority of strains belonging to phylogenetic groups A and B1. An interesting feature is that clustering of strains based on their accessory genome is very similar to that obtained using the core genome. This finding illustrates the fact that phenotypic properties of strains from different phylogroups are likely to be different. As a consequence, it is possible that different strategies could be used by mastitis isolates of different phylogroups to trigger mastitis. Our results indicate that mastitis E. coli isolates analyzed in this study carry very few of the virulence genes described in other pathogenic E. coli strains. A more detailed analysis of the presence/absence of genes involved in LPS synthesis, iron acquisition and type 6 secretion systems did not uncover specific properties of mastitis isolates. Altogether, these results indicate that mastitis E. coli isolates are rather characterized by a lack of bona fide currently described virulence genes.

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.

In the eye of the neutrophil swarm-navigation signals that bring neutrophils together in inflamed and infected tissues

Neutrophils are sentinel cells that express in higher vertebrates >30 chemokine and chemoattractant receptors to sense and quickly react to tissue damage signals. Intravital microscopy studies in mouse models of wounding, inflammation, and infection have revealed that neutrophils form cell swarms at local sites of tissue injury and cell death. This swarming response is choreographed by chemokines, lipids, and other chemoattractants, controlling sequential phases of highly coordinated chemotaxis, intercellular signal relay, and cluster formation among neutrophils. This review will give a brief overview about the basic principles and key molecules that have led to the refined multistep model of how neutrophils come together to isolate sites of tissue injury and microbial invasion from healthy tissue. Whereas auto- and paracrine signaling among neutrophils during later phases of swarming can provide a level of self-organization for robust navigation in diverse inflammatory settings, guidance factors from primary tissue lesions, resident bystander cells, and dying cells regulate the initial phases of the swarming response. This review will discuss how the specific environmental context and mixture of attractants at the locally inflamed site can lead to variants of the multistep attraction model and influence the extent of neutrophil swarming, ranging from accumulations of only few individual cells to the aggregation of several hundreds of neutrophils, as found in abscesses. Given the critical roles of neutrophils in both host protection and tissue destruction, novel insights on neutrophil swarming might provide useful for the therapeutic modulation of neutrophil-dependent inflammatory processes.

Bacterial exploitation of phosphorylcholine mimicry suppresses inflammation to promote airway infection

Regulation of neutrophil activity is critical for immune evasion among extracellular pathogens, yet the mechanisms by which many bacteria disrupt phagocyte function remain unclear. Here, we have shown that the respiratory pathogen Streptococcus pneumoniae disables neutrophils by exploiting molecular mimicry to degrade platelet-activating factor (PAF), a host-derived inflammatory phospholipid. Using mass spectrometry and murine upper airway infection models, we demonstrated that phosphorylcholine (ChoP) moieties that are shared by PAF and the bacterial cell wall allow S. pneumoniae to leverage a ChoP-remodeling enzyme (Pce) to remove PAF from the airway. S. pneumoniae-mediated PAF deprivation impaired viability, activation, and bactericidal capacity among responding neutrophils. In the absence of Pce, neutrophils rapidly cleared S. pneumoniae from the airway and impeded invasive disease and transmission between mice. Abrogation of PAF signaling rendered Pce dispensable for S. pneumoniae persistence, reinforcing that this enzyme deprives neutrophils of essential PAF-mediated stimulation. Accordingly, exogenous activation of neutrophils overwhelmed Pce-mediated phagocyte disruption. Haemophilus influenzae also uses an enzyme, GlpQ, to hydrolyze ChoP and subvert PAF function, suggesting that mimicry-driven immune evasion is a common paradigm among respiratory pathogens. These results identify a mechanism by which shared molecular structures enable microbial enzymes to subvert host lipid signaling, suppress inflammation, and ensure bacterial persistence at the mucosa.

A bacterial regulatory RNA attenuates virulence, spread and human host cell phagocytosis

Staphylococcus aureus pathogenesis is directed by regulatory proteins and RNAs. We report the case of an RNA attenuating virulence and host uptake, possibly to sustain commensalism. A S. aureus sRNA, SprC (srn_3610), reduced virulence and bacterial loads in a mouse infection model. S. aureus deleted for sprC became more virulent and increased bacterial dissemination in colonized animals. Conversely, inducing SprC expression lowered virulence and the bacterial load. Without sprC, S. aureus phagocytosis by monocytes and macrophages was higher, whereas bacteria were internalized at lower yields when SprC expression was stimulated. Without sprC, higher internalization led to a greater number of extracellular bacteria, facilitating colonization. SprC expression decreased after phagocytosis, concurring with the facilitated growth of bacteria lacking the sRNA in the presence of an oxidant. The major staphylococcal autolysin facilitates S. aureus uptake by human phagocytes. ATL proved to be negatively regulated by SprC. The SprC domains involved in pairing with atl mRNA were analyzed. The addition of ATL reduced phagocytosis of bacteria lacking sprC with no effects on wild-type bacterial uptake, implying that SprC influences phagocytosis, at least in part, by controlling ATL. Since the control of SprC on ATL was modest, other factors must contribute to atl regulation.

Human dendritic cell subsets display distinct interactions with the pathogenic mould Aspergillus fumigatus

The mould Aspergillus fumigatus is primarily an opportunistic pathogen of immunocompromised patients. Once fungal spores have been inhaled they encounter cells of the innate immune system, which include dendritic cells (DCs). DCs are the key antigen-presenting cells of the immune system and distinct subtypes, which differ in terms of origin, morphology and function. This study has systematically compared the interactions between A. fumigatus and myeloid DCs (mDCs), plasmacytoid DCs (pDCs) and monocyte-derived DCs (moDCs). Analyses were performed by time-lapse video microscopy, scanning electron microscopy, plating assays, flow cytometry, 25-plex ELISA and transwell assays. The three subsets of DCs displayed distinct responses to the fungus with mDCs and moDCs showing the greatest similarities. mDCs and moDCs both produced rough convolutions and occasionally phagocytic cups upon exposure to A. fumigatus whereas pDCs maintained a smooth appearance. Both mDCs and moDCs phagocytosed conidia and germ tubes, while pDCs did not phagocytose any fungi. Analysis of cytokine release and maturation markers revealed specific differences in pro- and anti-inflammatory patterns between the different DC subsets. These distinct characteristics between the DC subsets highlight their differences and suggest specific roles of moDCs, mDCs and pDCs during their interaction with A. fumigatus in vivo.

Coinfection with an intestinal helminth impairs host innate immunity against Salmonella enterica serovar Typhimurium and exacerbates intestinal inflammation in mice

Salmonella enterica serovar Typhimurium is a Gram-negative food-borne pathogen that is a major cause of acute gastroenteritis in humans. The ability of the host to control such bacterial pathogens may be influenced by host immune status and by concurrent infections. Helminth parasites are of particular interest in this context because of their ability to modulate host immune responses and because their geographic distribution coincides with those parts of the world where infectious gastroenteritis is most problematic. To test the hypothesis that helminth infection may negatively regulate host mucosal innate immunity against bacterial enteropathogens, a murine coinfection model was established by using the intestinal nematode Heligmosomoides polygyrus and S. Typhimurium. We found that mice coinfected with S. Typhimurium and H. polygyrus developed more severe intestinal inflammation than animals infected with S. Typhimurium alone. The enhanced susceptibility to Salmonella-induced intestinal injury in coinfected mice was found to be associated with diminished neutrophil recruitment to the site of bacterial infection that correlated with decreased expression of the chemoattractants CXCL2/macrophage inflammatory protein 2 (MIP-2) and CXCL1/keratinocyte-derived chemokine (KC), poor control of bacterial replication, and exacerbated intestinal inflammation. The mechanism of helminth-induced inhibition of MIP-2 and KC expression involved interleukin-10 (IL-10) and, to a lesser extent, IL-4 and IL-13. Ly6G antibody-mediated depletion of neutrophils reproduced the adverse effects of H. polygyrus on Salmonella infection. Our results suggest that impaired neutrophil recruitment is an important contributor to the enhanced severity of Salmonella enterocolitis associated with helminth coinfection.

Phenol-soluble modulins: novel virulence-associated peptides of staphylococci

 Phenol-soluble modulins (PSMs), a novel class of small peptides with an amphipathic α-helical structure and strong surfactant-like properties, are produced by most staphylococci, especially pathogenic Staphylococcus aureus and Staphylococcus epidermidis. PSMs can: induce the production of proinflammatory cytokines; recruit, activate and lyse neutrophils to help staphylococci evade immune damage; lyse erythrocytes and are associated with the hemolysis of staphylococcal disease; facilitate the structuring and detachment of staphylococcal biofilms and disseminate biofilm-associated infection; and kill competing microbes and act as weapons in interbacterial warfare. Therefore, PSMs are considered to be critical virulence-associated factors and to play important roles in the pathogenesis of staphylococci. This review summarizes the classification, structure, expression regulation and biological functions of PSMs. The possible means to prevent PSM-associated diseases are also outlined in order to emphasize the need to investigate PSMs as potential targets for drug and vaccine design against staphylococcal infections.

Sulphiredoxin plays peroxiredoxin-dependent and -independent roles via the HOG signalling pathway in Cryptococcus neoformans and contributes to fungal virulence

Mechanisms of oxidative stress resistance are crucial virulence factors for survival and proliferation of fungal pathogens within the human host. In this study we have identified and functionally characterized the role of sulphiredoxin, Srx1, in oxidative stress resistance of Cryptococcus neoformans causing fungal meningoencephalitis and regulation of peroxiredoxins, Tsa1 and Tsa3, and thioredoxins, Trx1 and Trx2. The C. neoformans HOG (High Osmolarity Glycerol response) pathway was essential for the transcriptional regulation of SRX1 under peroxide stress conditions. A gene deletion study revealed that Srx1 was required for cells to counteract peroxide stress, but not other oxidative damaging agents. HOG1 was found to be essential for the induction of adaptive response to peroxide stress with concurrent repression of ergosterol biosynthesis in an SRX1-independent manner. Consistent with this, phosphorylation of C. neoformans Hog1 was modulated by both low and high doses of exogenous hydrogen peroxide treatment. Immunoblot analysis using the C. neoformans Tsa1 specific antibody revealed that both Srx1 and Trx1 were essential for recycling of oxidized Tsa1. In addition to its role in peroxide sensing and response C. neoformans Srx1 was also found to be required for a peroxiredoxin-independent function in promoting fungicide-dependent cell swelling and growth arrest. Finally we showed the importance of C. neoformans Srx1 in fungal pathogenesis by demonstrating its requirement for full virulence using a mouse infection model.

Streptococcus pneumoniae invades erythrocytes and utilizes them to evade human innate immunity

Streptococcus pneumoniae, a Gram-positive bacterium, is a major cause of invasive infection-related diseases such as pneumonia and sepsis. In blood, erythrocytes are considered to be an important factor for bacterial growth, as they contain abundant nutrients. However, the relationship between S. pneumoniae and erythrocytes remains unclear. We analyzed interactions between S. pneumoniae and erythrocytes, and found that iron ion present in human erythrocytes supported the growth of Staphylococcus aureus, another major Gram-positive sepsis pathogen, while it partially inhibited pneumococcal growth by generating free radicals. S. pneumoniae cells incubated with human erythrocytes or blood were subjected to scanning electron and confocal fluorescence microscopic analyses, which showed that the bacterial cells adhered to and invaded human erythrocytes. In addition, S. pneumoniae cells were found associated with human erythrocytes in cultures of blood from patients with an invasive pneumococcal infection. Erythrocyte invasion assays indicated that LPXTG motif-containing pneumococcal proteins, erythrocyte lipid rafts, and erythrocyte actin remodeling are all involved in the invasion mechanism. In a neutrophil killing assay, the viability of S. pneumoniae co-incubated with erythrocytes was higher than that without erythrocytes. Also, H2O2 killing of S. pneumoniae was nearly completely ineffective in the presence of erythrocytes. These results indicate that even when S. pneumoniae organisms are partially killed by iron ion-induced free radicals, they can still invade erythrocytes. Furthermore, in the presence of erythrocytes, S. pneumoniae can more effectively evade antibiotics, neutrophil phagocytosis, and H2O2 killing.

Neisseria gonorrhoeae phagosomes delay fusion with primary granules to enhance bacterial survival inside human neutrophils

Symptomatic infection with Neisseria gonorrhoeae (Gc) promotes inflammation driven by polymorphonuclear leucocytes (PMNs, neutrophils), yet some Gc survive PMN exposure during infection. Here we report a novel mechanism of gonococcal resistance to PMNs: Gc phagosomes avoid maturation into phagolysosomes by delayed fusion with primary (azurophilic) granules, which contain antimicrobial components including serine proteases. Reduced phagosome-primary granule fusion was observed in gonorrheal exudates and human PMNs infected ex vivo. Delayed phagosome-granule fusion could be overcome by opsonizing Gc with immunoglobulin. Using bacterial viability dyes along with antibodies to primary granules revealed that Gc survival in PMNs correlated with early residence in primary granule-negative phagosomes. However, when Gc was killed prior to PMN exposure, dead bacteria were also found in primary granule-negative phagosomes. These results suggest that Gc surface characteristics, rather than active bacterial processes, influence phagosome maturation and that Gc death inside PMNs occurs after phagosome-granule fusion. Ectopically increasing primary granule-phagosome fusion, by immunoglobulin opsonization or PMN treatment with lysophosphatidylcholine, reduced intracellular Gc viability, which was attributed in part to serine protease activity. We conclude that one method for Gc to avoid PMN clearance in acute gonorrhoea is by delaying primary granule-phagosome fusion, thus preventing formation of a degradative phagolysosome.

Control of neutrophil inflammation at mucosal surfaces by secreted epithelial products

The human intestine is a large and delicately balanced organ, responsible for efficiently absorbing nutrients and selectively eliminating disease-causing pathogens. The gut architecture consists of a single layer of epithelial cells that forms a barrier against the food antigens and resident microbiota within the lumen. This barrier is augmented by a thick layer of mucus on the luminal side and an underlying lamina propria containing a resident population of immune cells. Attempted breaches of the intestinal barrier by pathogenic bacteria result in the rapid induction of a coordinated innate immune response that includes release of antimicrobial peptides, activation of pattern recognition receptors, and recruitment of various immune cells. In recent years, the role of epithelial cells in initiating this immune response has been increasingly appreciated. In particular, epithelial cells are responsible for the release of a variety of factors that attract neutrophils, the body's trained bacterial killers. In this review we will highlight recent research that details a new understanding of how epithelial cells directionally secrete specific compounds at distinct stages of the inflammatory response in order to coordinate the immune response to intestinal microbes. In addition to their importance during the response to infection, evidence suggests that dysregulation of these pathways may contribute to pathologic inflammation during inflammatory bowel disease. Therefore, a continued understanding of the mechanisms by which epithelial cells control neutrophil migration into the intestine will have tremendous benefits in both the understanding of biological processes and the identification of potential therapeutic targets.

Molecular mechanisms regulating NETosis in infection and disease

Neutrophils are the foot soldiers of the immune system. They home in to the site of infection and kill pathogens by phagocytosis, degranulation, and the release of web-like structures called neutrophil extracellular traps (NETs) that trap and kill a variety of microbes. NETs have been shown to play a multitude of additional roles in immunity but have also been implicated in inflammatory and autoimmune disease. Here, we discuss the role of NETs in these various contexts with a particular emphasis on the molecular mechanisms that regulate NET release and clearance. We highlight the comprehensive concepts and explore the important open questions in the field.

Cryptococcosis in cats: ABCD guidelines on prevention and management

OVERVIEW

Cryptococcosis is worldwide the most common systemic fungal disease in cats; it is caused by the Cryptococcus neoformans- Cryptococcus gattii species complex, which includes eight genotypes and some subtypes (strains) with varying geographical distribution, pathogenicity and antimicrobial susceptibility. Cats acquire the infection from a contaminated environment. The prognosis is favourable in most cases, provided a diagnosis is obtained sufficiently early and prolonged treatment is maintained.

INFECTION

Basidiospores are the infectious propagules of Cryptococcus species as they penetrate the respiratory system and induce primary infection. Asymptomatic colonisation of the respiratory tract is more common than clinical disease. Avian guanos, particularly pigeon droppings, offer favourable conditions for the reproduction of C neoformans. Both Cryptococcus species are associated with decaying vegetation.

DISEASE SIGNS

Cryptococcosis caused by C neoformans or C gattii is indistinguishable clinically. The disease can present in nasal, central nervous system (which can derive from the nasal form or occur independently), cutaneous and systemic forms.

DIAGNOSIS

An easy and reliable test for cryptococcosis diagnosis is antigen detection in body fluids. Only isolation and polymerase chain reaction allow identification of the species genotype.

DISEASE MANAGEMENT

Amphotericin B, ketoconazole, fluconazole and itraconazole have all been used to treat cats. Surgical excision of any nodules in the skin, nasal or oral mucosa assists recovery. Continued treatment is recommended until the antigen test is negative.

PREVENTION

Efficient preventive measures have not been demonstrated. Vaccines are not available.

In vitro resistance mechanisms of Neisseria meningitidis against neutrophil extracellular traps

Neisseria meningitidis (Nm) is a leading cause of septicemia in childhood. Nm septicemia is unique with respect to very quick disease progression, high in vivo bacterial replication rate and its considerable mortality. Nm circumvents major mechanisms of innate immunity such as complement system and phagocytosis. Neutrophil extracellular traps (NETs) are formed from neutrophils during systemic infection and are suggested to contain invading microorganisms. Here, we investigated the interaction of Nm with NETs. Both, meningococci and spontaneously released outer membrane vesicles (SOMVs) were potent NET inducers. NETs were unable to kill NET bound meningococci, but slowed down their proliferation rate. Using Nm as model organism we identified three novel mechanisms how bacteria can evade NET-mediated killing: (i) modification of lipid A of meningococcal LPS with phosphoethanolamine protected Nm from NET-bound cathepsin G; (ii) expression of the high-affinity zinc uptake receptor ZnuD allowed Nm to escape NET-mediated nutritional immunity; (iii) binding of SOMVs to NETs saved Nm from NET binding and the consequent bacteriostatic effect. Escape from NETs may contribute to the most rapid progression of meningococcal disease. The induction of NET formation by Nm in vivo might aggravate thrombosis in vessels ultimately directing to disseminated intravascular coagulation (DIC).

The Spr1875 protein confers resistance to the microglia-mediated killing of Streptococcus pneumoniae

By screening a whole-genome λ-display library of Streptococcus pneumoniae, we have previously identified a novel surface protein, named Spr1875, that exhibited immunogenic properties and was closely related to pneumococcal virulence. In the present study, we investigated the role of the Spr1875 antigen in the interaction of S. pneumoniae with microglia, the resident brain macrophages. By using an in vitro infection model, the BV2 microglial cell line was challenged with the S. pneumoniae strain DP1004 and its isogenic spr1875-deleted mutant (Δspr1875). Both strains were phagocytosed by microglia efficiently and to a similar extent; however, the DP1004 strain was more resistant than the Δspr1875 mutant to the intracellular killing, as assessed by antibiotic protection and phagosome maturation assays. Moreover, significant differences between the two strains were also observed in terms of susceptibility to microglia-mediated killing. Taken together, these results indicate that S. pneumoniae-microglial cell interplay is influenced by the presence of Spr1875, suggesting that this protein may play a role in the pathogenesis of pneumococcal meningitis.

Enhanced acute immune response in IL-12p35-/- mice is followed by accelerated distinct repair mechanisms in Staphylococcus aureus-induced murine brain abscess

BACKGROUND

Murine Staphylococcus aureus-mediated brain abscess comprises 2 major phases, an initial phase of cerebritis, followed by a healing phase characterized by capsule formation.

METHODS

C57BL/6 wild-type (WT) and IL-12p35(-/-) mice were intracerebrally infected with S. aureus to induce brain abscesses. Clinical disease activity and bacterial load were monitored. The cell populations that were involved, as well as their specific mediators, were analyzed by immunohistochemistry, quantitative real-time polymerase chain reaction, and flow cytometry.

RESULTS

In the acute phase, IL-12p35(-/-) mice were protected from disease. This was associated with enhanced recruitment of granulocytes, accompanied by upregulated expression of Il17a, Csf2 (which encodes granulocyte-macrophage colony-stimulating factor), Cxcl1, and Cxcl5, as well as increased expression of proinflammatory mediators, including Nos2 (which encodes inducible nitric oxide synthase), Ptgs2 (which encodes cyclooxygenase 2), and Tnf, that were primarily produced by granulocytes and activated microglia/macrophages. Furthermore, mechanisms associated with beneficial wound healing, including an accelerated formation of a fibrous capsule, were demonstrated by prominent VEGF-A production and collagen deposition driven by an earlier onset of T-helper 2 immunity in the absence of interleukin 12 (IL-12).

CONCLUSIONS

Brain abscess development is orchestrated by IL-12 at different stages of disease. Our data indicate that IL-12 has a nonprotective role in the acute phase and that IL-12 deficiency results in the accelerated formation of a protective capsule during the healing phase, which we consider crucial for early recovery from disease.

Staphylococcus aureus virulence factors in evasion from innate immune defenses in human and animal diseases

In the last decades, Staphylococcus aureus acquired a dramatic relevance in human and veterinary medicine for different reasons, one of them represented by the increasing prevalence of antibiotic resistant strains. However, antibiotic resistance is not the only weapon in the arsenal of S. aureus. Indeed, these bacteria have plenty of virulence factors, including a vast ability to evade host immune defenses. The innate immune system represents the first line of defense against invading pathogens. This system consists of three major effector mechanisms: antimicrobial peptides and enzymes, the complement system and phagocytes. In this review, we focused on S. aureus virulence factors involved in the immune evasion in the first phases of infection: TLR recognition avoidance, adhesins affecting immune response and resistance to host defenses peptides and polypeptides. Studies of innate immune defenses and their role against S. aureus are important in human and veterinary medicine given the problems related to S. aureus antimicrobial resistance. Moreover, due to the pathogen ability to manipulate the immune response, these data are needed to develop efficacious vaccines or molecules against S. aureus.

Targeting macrophage activation for the prevention and treatment of Staphylococcus aureus biofilm infections

Biofilm infections often lead to significant morbidity due to their chronicity and recalcitrance to antibiotics. We have demonstrated that methicillin-resistant Staphylococcus aureus (MRSA) biofilms can evade macrophage (MΦ) antibacterial effector mechanisms by skewing MΦs toward an alternatively activated M2 phenotype. To overcome this immune evasion, we have used two complementary approaches. In the first, a proinflammatory milieu was elicited by local administration of classically activated M1 MΦs and in the second by treatment with the C5a receptor (CD88) agonist EP67, which invokes MΦ proinflammatory activity. Early administration of M1-activated MΦs or EP67 significantly attenuated biofilm formation in a mouse model of MRSA catheter-associated infection. Several proinflammatory mediators were significantly elevated in biofilm-infected tissues from MΦ- and EP67-treated animals, revealing effective reprogramming of the biofilm environment to a proinflammatory milieu. A requirement for MΦ proinflammatory activity was demonstrated by the fact that transfer of MyD88-deficient MΦs had minimal impact on biofilm growth. Likewise, neutrophil administration had no effect on biofilm formation. Treatment of established biofilm infections with M1-activated MΦs also significantly reduced catheter-associated biofilm burdens compared with antibiotic treatment. Collectively, these results demonstrate that targeting MΦ proinflammatory activity can overcome the local immune inhibitory environment created during biofilm infections and represents a novel therapeutic strategy.

Immunological aspects of Candida and Aspergillus systemic fungal infections

Patients with allogeneic stem cell transplantation (SCT) have a high risk of invasive fungal infections (IFIs) even after neutrophil regeneration. Immunological aspects might play a very important role in the IFI development in these patients. Some data are available supporting the identification of high-risk patients with IFI for example patients receiving stem cells from TLR4 haplotype S4 positive donors. Key defense mechanisms against IFI include the activation of neutrophils, the phagocytosis of germinating conidia by dendritic cells, and the fight of the cells of the innate immunity such as monocytes and natural killer cells against germlings and hyphae. Furthermore, immunosuppressive drugs interact with immune effector cells influencing the specific fungal immune defense and antimycotic drugs might interact with immune response. Based on the current knowledge on immunological mechanism in Aspergillus fumigatus, the first approaches of an immunotherapy using human T cells are in development. This might be an option for the future of aspergillosis patients having a poor prognosis with conventional treatment.

Granulocyte maturation determines ability to release chromatin NETs and loss of DNA damage response; these properties are absent in immature AML granulocytes

Terminally-differentiated cells cease to proliferate and acquire specific sets of expressed genes and functions distinguishing them from less differentiated and cancer cells. Mature granulocytes show lobular structure of cell nuclei with highly condensed chromatin in which HP1 proteins are replaced by MNEI. These structural features of chromatin correspond to low level of gene expression and the loss of some important functions as DNA damage repair, shown in this work and, on the other hand, acquisition of a new specific function consisting in the release of chromatin extracellular traps in response to infection by pathogenic microbes. Granulocytic differentiation is incomplete in myeloid leukemia and is manifested by persistence of lower levels of HP1γ and HP1β isoforms. This immaturity is accompanied by acquisition of DDR capacity allowing to these incompletely differentiated multi-lobed neutrophils of AML patients to respond to induction of DSB by γ-irradiation. Immature granulocytes persist frequently in blood of treated AML patients in remission. These granulocytes contrary to mature ones do not release chromatin for NETs after activation with phorbol myristate-12 acetate-13 and do not exert the neutrophil function in immune defence. We suggest therefore the detection of HP1 expression in granulocytes of AML patients as a very sensitive indicator of their maturation and functionality after the treatment. Our results show that the changes in chromatin structure underlie a major transition in functioning of the genome in immature granulocytes. They show further that leukemia stem cells can differentiate ex vivo to mature granulocytes despite carrying the translocation BCR/ABL.

Role of YopK in Yersinia pseudotuberculosis resistance against polymorphonuclear leukocyte defense

The enteropathogen Yersinia pseudotuberculosis can survive in the harsh environment of lymphoid compartments that abounds in immune cells. This capacity is dependent on the plasmid-encoded Yersinia outer proteins (Yops) that are delivered into the host cell via a mechanism involving the Yersinia type III secretion system. We show that the virulence protein YopK has a role in the mechanism by which Y. pseudotuberculosis avoids the polymorphonuclear leukocyte or neutrophil (PMN) defense. A yopK mutant, which is attenuated in the mouse infection model, where it fails to cause systemic infection, was found to colonize Peyer's patches and mesenteric lymph nodes more rapidly than the wild-type strain. Further, in mice lacking PMNs, the yopK mutant caused full disease with systemic spread and typical symptoms. Analyses of effects on PMNs revealed that both the wild-type strain and the yopK mutant inhibited internalization and reactive oxygen species production, as well as neutrophil extracellular trap formation by PMNs. However, the wild-type strain effectively avoided induction of PMN death, whereas the mutant caused a necrosis-like PMN death. Taken together, our results indicate that YopK is required for the ability of Yersinia to resist the PMN defense, which is critical for the virulence of the pathogen. We suggest a mechanism whereby YopK functions to prevent unintended Yop delivery and thereby PMN disruption, resulting in necrosis-like cell death, which would enhance the inflammatory response favoring the host.

Neutrophil extracellular traps exhibit antibacterial activity against burkholderia pseudomallei and are influenced by bacterial and host factors

Burkholderia pseudomallei is the causative pathogen of melioidosis, of which a major predisposing factor is diabetes mellitus. Polymorphonuclear neutrophils (PMNs) kill microbes extracellularly by the release of neutrophil extracellular traps (NETs). PMNs play a key role in the control of melioidosis, but the involvement of NETs in killing of B. pseudomallei remains obscure. Here, we showed that bactericidal NETs were released from human PMNs in response to B. pseudomallei in a dose- and time-dependent manner. B. pseudomallei-induced NET formation required NADPH oxidase activation but not phosphatidylinositol-3 kinase, mitogen-activated protein kinases, or Src family kinase signaling pathways. B. pseudomallei mutants defective in the virulence-associated Bsa type III protein secretion system (T3SS) or capsular polysaccharide I (CPS-I) induced elevated levels of NETs. NET induction by such mutants was associated with increased bacterial killing, phagocytosis, and oxidative burst by PMNs. Taken together the data imply that T3SS and the capsule may play a role in evading the induction of NETs. Importantly, PMNs from diabetic subjects released NETs at a lower level than PMNs from healthy subjects. Modulation of NET formation may therefore be associated with the pathogenesis and control of melioidosis.

Relationship among bacterial virulence, bladder dysfunction, vesicoureteral reflux and patterns of urinary tract infection in children

PURPOSE

We hypothesized that virulence levels of Escherichia coli isolates causing pediatric urinary tract infections differ according to severity of infection and also among various uropathies known to contribute to pediatric urinary tract infections. We evaluated these relationships using in vitro cytokine interleukin-6 elicitation.

MATERIALS AND METHODS

E. coli isolates were cultured from children presenting with urinary tract infections. In vitro cytokine (interleukin-6) elicitation was quantified for each isolate and the bacteria were grouped according to type of infection and underlying uropathy (neurogenic bladder, nonneurogenic bowel and bladder dysfunction, primary vesicoureteral reflux, no underlying etiology).

RESULTS

A total of 40 E. coli isolates were collected from children with a mean age of 61.5 months (range 1 to 204). Mean level of in vitro cytokine elicitation from febrile urinary tract infection producing E. coli was significantly lower than for nonfebrile strains (p = 0.01). The interleukin-6 response to E. coli in the neurogenic bladder group was also significantly higher than in the vesicoureteral reflux (p = 0.01) and no underlying etiology groups (p = 0.02).

CONCLUSIONS

In vitro interleukin-6 elicitation, an established marker to determine bacterial virulence, correlates inversely with clinical urinary tract infection severity. Less virulent, high cytokine producing E. coli were more likely to cause cystitis and were more commonly found in patients with neurogenic bladder and nonneurogenic bowel and bladder dysfunction, whereas higher virulence isolates were more likely to produce febrile urinary tract infections and to affect children with primary vesicoureteral reflux and no underlying etiology. These findings suggest that bacteria of different virulence levels may be responsible for differences in severity of pediatric urinary tract infections and may vary among different underlying uropathies.

Fhb, a novel factor H-binding surface protein, contributes to the antiphagocytic ability and virulence of Streptococcus suis

Streptococcus suis serotype 2 is a Gram-positive bacterium that causes sepsis and meningitis in piglets and humans. The mechanisms of S. suis serotype 2 invasive disease are not well understood. The surface proteins of pathogens usually play important roles in infection and bacterium-host interactions. Here, we identified a novel surface protein that contributed significantly to the virulence of S. suis serotype 2 in a piglet infection model. This protein showed little similarity to other reported proteins and exhibited strong binding activity to human factor H (hFH). It was designated Fhb (factor H-binding protein). The fhb genes found in S. suis serotypes 1, 2, 4, 7, and 9 exhibited molecular polymorphism. Fhb possessed two proline-rich repeat sequences and XPZ domains, and one repeat sequence exhibited a high homology to Bac, an IgA-binding protein of Streptococcus agalactiae. Evidence strongly indicated that fhb-deficient mutants had diminished phagocytosis resistance in bactericidal assays. In addition, Fhb plays important roles in complement-mediated immunity by interacting with hFH. These findings indicated that Fhb is a crucial surface protein contributing to the virulence of S. suis, with important functions in evading innate immune defenses by interaction with host complement regulatory factor hFH.

Afa/Dr diffusely adhering Escherichia coli strain C1845 induces neutrophil extracellular traps that kill bacteria and damage human enterocyte-like cells

We recently documented the neutrophil response to enterovirulent diffusely adherent Escherichia coli expressing Afa/Dr fimbriae (Afa/Dr DAEC), using the human myeloid cell line PLB-985 differentiated into fully mature neutrophils. Upon activation, particularly during infections, neutrophils release neutrophil extracellular traps (NETs), composed of a nuclear DNA backbone associated with antimicrobial peptides, histones, and proteases, which entrap and kill pathogens. Here, using fluorescence microscopy and field emission scanning electron microscopy, we observed NET production by PLB-985 cells infected with the Afa/Dr wild-type (WT) E. coli strain C1845. We found that these NETs were able to capture, immobilize, and kill WT C1845 bacteria. We also developed a coculture model of human enterocyte-like Caco-2/TC7 cells and PLB-985 cells previously treated with WT C1845 and found, for the first time, that the F-actin cytoskeleton of enterocyte-like cells is damaged in the presence of bacterium-induced NETs and that this deleterious effect is prevented by inhibition of protease release. These findings provide new insights into the neutrophil response to bacterial infection via the production of bactericidal NETs and suggest that NETs may damage the intestinal epithelium, particularly in situations such as inflammatory bowel diseases.

ETosis: A Microbicidal Mechanism beyond Cell Death

Netosis is a recently described type of neutrophil death occurring with the release to the extracellular milieu of a lattice composed of DNA associated with histones and granular and cytoplasmic proteins. These webs, initially named neutrophil extracellular traps (NETs), ensnare and kill microorganisms. Similarly, other cell types, such as eosinophils, mast cells, and macrophages, can also dye by this mechanism; thus, it was renamed as ETosis, meaning death with release of extracellular traps (ETs). Here, we review the mechanism of NETosis/etosis, emphasizing its role in diseases caused by protozoan parasites, fungi, and viruses.

CpsY influences Streptococcus iniae cell wall adaptations important for neutrophil intracellular survival

The ability of a pathogen to evade neutrophil phagocytic killing mechanisms is critically important for dissemination and establishment of a systemic infection. Understanding how pathogens overcome these innate defenses is essential for the development of optimal therapeutic strategies for invasive infections. CpsY is a conserved transcriptional regulator previously identified as an important virulence determinant for systemic infection of Streptococcus iniae. While orthologs of CpsY have been associated with the regulation of methionine metabolism and uptake pathways, CpsY additionally functions in protection from neutrophil-mediated killing. S. iniae does not alter neutrophil phagosomal maturation but instead is able to adapt to the extreme bactericidal environment of a mature neutrophil phagosome, a property dependent upon CpsY. This CpsY-dependent adaptation appears to involve stabilization of the cell wall through peptidoglycan O-acetylation and repression of cellular autolysins. Furthermore, S. iniae continues to be a powerful model for investigation of bacterial adaptations during systemic streptococcal infection.

Immune subversion and quorum-sensing shape the variation in infectious dose among bacterial pathogens

Many studies have been devoted to understand the mechanisms used by pathogenic bacteria to exploit human hosts. These mechanisms are very diverse in the detail, but share commonalities whose quantification should enlighten the evolution of virulence from both a molecular and an ecological perspective. We mined the literature for experimental data on infectious dose of bacterial pathogens in humans (ID50) and also for traits with which ID50 might be associated. These compilations were checked and complemented with genome analyses. We observed that ID50 varies in a continuous way by over 10 orders of magnitude. Low ID50 values are very strongly associated with the capacity of the bacteria to kill professional phagocytes or to survive in the intracellular milieu of these cells. Inversely, high ID50 values are associated with motile and fast-growing bacteria that use quorum-sensing based regulation of virulence factors expression. Infectious dose is not associated with genome size and shows insignificant phylogenetic inertia, in line with frequent virulence shifts associated with the horizontal gene transfer of a small number of virulence factors. Contrary to previous proposals, infectious dose shows little dependence on contact-dependent secretion systems and on the natural route of exposure. When all variables are combined, immune subversion and quorum-sensing are sufficient to explain two thirds of the variance in infectious dose. Our results show the key role of immune subversion in effective human infection by small bacterial populations. They also suggest that cooperative processes might be important for successful infection by bacteria with high ID50. Our results suggest that trade-offs between selection for population growth-related traits and selection for the ability to subvert the immune system shape bacterial infectiousness. Understanding these trade-offs provides guidelines to study the evolution of virulence and in particular the micro-evolutionary paths of emerging pathogens.

Every pathogen is unique and uses distinctive combinations of specific mechanisms to exploit the human host. Yet, several common themes in the ways pathogens use these mechanisms can be found among distantly related bacteria. The understanding of these common themes provides useful concepts and uncovers important principles in pathogenesis. Here, we have made a cross-species analysis of traits thought to be relevant for virulence of bacterial pathogens. We have found that the infectious dose of pathogens is much lower when they are able to kill professional phagocytes of the immune system or to survive in the intracellular milieu of these cells. On the other hand, bacteria requiring higher infectious dose are more likely to be motile, fast-growing and regulate the expression of virulence factors when the population quorum is high enough to be effective in starting an infection. This suggests that infectious dose results from a trade-off between selection for fast coordinated growth and the ability to subvert the immune system. This trade-off may underlie other traits such as the ability of a pathogen to live outside the association from a host. Understanding the patterns shaping infectious dose will facilitate the prediction of evolutionary paths of emerging pathogens.

A bacterial siren song: intimate interactions between Neisseria and neutrophils

Neisseria gonorrhoeae and Neisseria meningitidis are Gram-negative bacterial pathogens that are exquisitely adapted for growth at human mucosal surfaces and for efficient transmission between hosts. One factor that is essential to neisserial pathogenesis is the interaction between the bacteria and neutrophils, which are recruited in high numbers during infection. Although this vigorous host response could simply reflect effective immune recognition of the bacteria, there is mounting evidence that in fact these obligate human pathogens manipulate the innate immune response to promote infectious processes. This Review summarizes the mechanisms used by pathogenic neisseriae to resist and modulate the antimicrobial activities of neutrophils. It also details some of the major outstanding questions about the Neisseria-neutrophil relationship and proposes potential benefits of this relationship for the pathogen.

The neutrophil in vascular inflammation

Here we focus on how neutrophils have a key regulatory role in vascular inflammation. Recent studies using advanced imaging techniques have yielded new insights into the mechanisms by which neutrophils contribute to defense against bacterial infections and also against sterile injury. In these settings, neutrophils are recruited by various mechanisms depending on the situation. We also describe how these processes may be disrupted in systemic infections, with a particular emphasis on mouse models of sepsis. Neutrophils are often immobilized in the lungs and liver during systemic infections, and this immobilization may be a mechanism through which bacteria can evade the innate immune response or allow neutrophils to form neutrophil extracellular traps that trap and kill bacteria in blood. The platelet is also an important player in sepsis, and we describe how it collaborates with neutrophils in the formation of neutrophil extracellular traps.

Innate and adaptive immune responses against Staphylococcus aureus skin infections

Staphylococcus aureus is an important human pathogen that is responsible for the vast majority of bacterial skin and soft tissue infections in humans. S. aureus can also become more invasive and cause life-threatening infections such as bacteremia, pneumonia, abscesses of various organs, meningitis, osteomyelitis, endocarditis, and sepsis. These infections represent a major public health threat due to the enormous numbers of these infections and the widespread emergence of methicillin-resistant S. aureus (MRSA) strains. MSRA is endemic in hospitals worldwide and is rapidly spreading throughout the normal human population in the community. The increasing frequency of MRSA infections has complicated treatment as these strains are more virulent and are increasingly becoming resistant to multiple different classes of antibiotics. The important role of the immune response against S. aureus infections cannot be overemphasized as humans with certain genetic and acquired immunodeficiency disorders are at an increased risk for infection. Understanding the cutaneous immune responses against S. aureus is essential as most of these infections occur or originate from a site of infection or colonization of the skin and mucosa. This review will summarize the innate immune responses against S. aureus skin infections, including antimicrobial peptides that have direct antimicrobial activity against S. aureus as well as pattern recognition receptors and proinflammatory cytokines that promote neutrophil abscess formation in the skin, which is required for bacterial clearance. Finally, we will discuss the recent discoveries involving IL-17-mediated responses, which provide a key link between cutaneous innate and adaptive immune responses against S. aureus skin infections.

Cryptococcus neoformans modulates extracellular killing by neutrophils

We recently established a key role for host sphingomyelin synthase (SMS) in regulating the killing activity of neutrophils against Cryptococcus neoformans. In this paper, we studied the effect of C. neoformans on the killing activity of neutrophils and whether SMS would still be a player against C. neoformans in immunocompromised mice lacking T and natural killer (NK) cells (Tgε26 mice). To this end, we analyzed whether C. neoformans would have any effect on neutrophil survival and killing in vitro and in vivo. We show that unlike Candida albicans, neither the presence nor the capsule size of C. neoformans cells have any effect on neutrophil viability. Interestingly, melanized C. neoformans cells totally abrogated the killing activity of neutrophils. We monitored how exposure of neutrophils to C. neoformans cells would interfere with any further killing activity of the conditioned medium and found that pre-incubation with live but not “heat-killed” fungal cells significantly inhibits further killing activity of the medium. We then studied whether activation of SMS at the site of C. neoformans infection is dependent on T and NK cells. Using matrix-assisted laser desorption–ionization tissue imaging in infected lung we found that similar to previous observations in the isogenic wild-type CBA/J mice, SM 16:0 levels are significantly elevated at the site of infection in mice lacking T and NK cells, but only at early time points. This study highlights that C. neoformans may negatively regulate the killing activity of neutrophils and that SMS activation in neutrophils appears to be partially independent of T and/or NK cells.

Glutamate utilization promotes meningococcal survival in vivo through avoidance of the neutrophil oxidative burst

Polymorphonuclear neutrophil leucocytes (PMNs) are a critical part of innate immune defence against bacterial pathogens, and only a limited subset of microbes can escape killing by these phagocytic cells. Here we show that Neisseria meningitidis, a leading cause of septicaemia and meningitis, can avoid killing by PMNs and this is dependent on the ability of the bacterium to acquire L-glutamate through its GltT uptake system. We demonstrate that the uptake of available L-glutamate promotes N. meningitidis evasion of PMN reactive oxygen species produced by the oxidative burst. In the meningococcus, L-glutamate is converted to glutathione, a key molecule for maintaining intracellular redox potential, which protects the bacterium from reactive oxygen species such as hydrogen peroxide. We show that this mechanism contributes to the ability of N. meningitidis to cause bacteraemia, a critical step in the disease process during infections caused by this important human pathogen.

Coevolution of morphology and virulence in Candida species

Many of the major human fungal pathogens are known to undergo morphological changes, which in certain cases are associated with virulence. Although there has been an intense research focus on morphology in fungi, very little is known about how morphology evolved in conjunction with a variety of other virulence properties. However, several recent important discoveries, primarily in Candida species, are beginning to shed light on this important area and answer many longstanding questions. In this minireview, we first provide a description of the major fungal morphologies, as well as the roles of morphology and morphology-associated gene expression in virulence. Next, focusing largely on Candida species, we examine the evolutionary relationships among specific morphological forms. Finally, drawing on recent findings, we begin to address the question of how specific morphological changes came to be associated with virulence of Candida species during evolution.

Morphological plasticity promotes resistance to phagocyte killing of uropathogenic Escherichia coli

Uropathogenic Escherichia coli proceed through a complex intracellular developmental pathway that includes multiple morphological changes. During intracellular growth within Toll-like receptor 4-activated superficial bladder epithelial cells, a subpopulation of uropathogenic E. coli initiates SulA-mediated filamentation. In this study, we directly investigated the role of bacterial morphology in the survival of uropathogenic E. coli from killing by phagocytes. We initially determined that both polymorphonuclear neutrophils and macrophages are recruited to murine bladder epithelium at times coincident with extracellular bacillary and filamentous uropathogenic E. coli. We further determined that bacillary uropathogenic E. coli were preferentially destroyed when mixed uropathogenic E. coli populations were challenged with cultured murine macrophages in vitro. Consistent with studies using elliptical-shaped polymers, the initial point of contact between the phagocyte and filamentous uropathogenic E. coli influenced the efficacy of internalization. These findings demonstrate that filamentous morphology provides a selective advantage for uropathogenic E. coli evasion of killing by phagocytes and defines a mechanism for the essential role for SulA during bacterial cystitis. Thus, morphological plasticity can be viewed as a distinct class of mechanism used by bacterial pathogens to subvert host immunity.

The pathogenesis of sepsis

Sepsis is a serious clinical condition that represents a patient's response to a severe infection and has a very high mortality rate. Normal immune and physiologic responses eradicate pathogens, and the pathophysiology of sepsis is due to the inappropriate regulation of these normal reactions. In an ideal scenario, the first pathogen contact with the inflammatory system should eliminate the microbe and quickly return the host to homeostasis. The septic response may accelerate due to continued activation of neutrophils and macrophages/monocytes. Upregulation of lymphocyte costimulatory molecules and rapid lymphocyte apoptosis, delayed apoptosis of neutrophils, and enhanced necrosis of cells/tissues also contribute to the pathogenesis of sepsis. The coagulation system is closely tied to the inflammatory response, with cross talk between the two systems driving the dysregulated response. Biomarkers may be used to help diagnose patients with sepsis, and they may also help to identify patients who would benefit from immunomodulatory therapies.

Phagosome dynamics during phagocytosis by neutrophils

The neutrophil is a key player in immunity, and its activities are essential for the resolution of infections. Neutrophil-pathogen interactions usually trigger a large arsenal of antimicrobial measures that leads to the highly efficient killing of pathogens. In neutrophils, the phagocytic process, including the formation and maturation of the phagosome, is in many respects very different from that in other phagocytes. Although the complex mechanisms that coordinate the membrane traffic, oxidative burst, and release of granule contents required for the microbicidal activities of neutrophils are not completely understood, it is evident that they are unique and differ from those in macrophages. Neutrophils exhibit more rapid rates of phagocytosis and higher intensity of oxidative respiratory response than do macrophages. The phagosome maturation pathway in macrophages, which is linked to the endocytic pathway, is replaced in neutrophils by the rapid delivery of preformed granules to nonacidic phagosomes. This review describes the plasticity and dynamics of the phagocytic process with a special focus on neutrophil phagosome maturation.

Use of a Dictyostelium model for isolation of genetic loci associated with phagocytosis and virulence in Klebsiella pneumoniae

Phagocytosis resistance is an important virulence factor in Klebsiella pneumoniae. Dictyostelium has been used to study the interaction between phagocytes and bacteria because of its similarity to mammalian macrophages. In this study, we used a Dictyostelium model to investigate genes for resistance to phagocytosis in NTUH-K2044, a strain of K. pneumoniae causing pyogenic liver abscess that is highly resistant to phagocytosis. A total of 2,500 transposon mutants were screened by plaque assay, and 29 of them permitted phagocytosis by Dictyostelium. In the 29 mutants, six loci were identified; three were capsular synthesis genes. Of the other three, one was related to carnitine metabolism, one encoded a subunit of protease (clpX), and one encoded a lipopolysaccharide O-antigen transporter (wzm). Deletion and complementation of these genes showed that only ΔclpX and Δwzm mutants became susceptible to Dictyostelium phagocytosis, and their complementation restored the phagocytosis resistance phenotype. These two mutants were also susceptible to phagocytosis by human neutrophils and revealed attenuated virulence in a mouse model, implying that they play important roles in the pathogenesis of K. pneumoniae. Furthermore, we demonstrated that clpP, which exists in an operon with clpX, was also involved in resistance to phagocytosis. The transcriptional profile of ΔclpX was examined by microarray analysis and revealed a 3-fold lower level of expression of capsular synthesis genes. Therefore, we have identified genes involved in resistance to phagocytosis in K. pneumoniae using Dictyostelium, and this model is useful to explore genes associated with resistance to phagocytosis in heavily encapsulated bacteria.