C3 promotes clearance of Klebsiella pneumoniae by A549 epithelial cells

Inhibition of porcine origin Klebsiella pneumoniae capsular polysaccharide and immune escape by BY3 compounded traditional Chinese medicine residue fermentation broth

Klebsiella pneumoniae (K. pneumoniae) is a gram-negative conditionally pathogenic bacterium that causes disease primarily in immunocompromised individuals. Recently, highly virulent K. pneumoniae strains have caused severe disease in healthy individuals, posing significant challenges to global infection control. Capsular polysaccharide (CPS), a major virulence determinant of K. pneumoniae, protects the bacteria from being killed by the host immune system, suggesting an urgent need for the development of drugs to prevent or treat K. pneumoniae infections. In this study, BY3 compounded traditional Chinese medicine residue (TCMR) was carried out using Lactobacillus rhamnosus as a fermentation strain, and BY3 compounded TCMR fermentation broth (BY3 fermentation broth) was obtained. The transcription of K. pneumoniae CPS-related biosynthesis genes after treatment with BY3 fermentation broth was detected using quantitative real-time polymerase chain reaction. The effects of BY3 fermentation broth on K. pneumoniae serum killing, macrophage phagocytosis, complement deposition and human β-defensin transcription were investigated. The therapeutic effect of BY3 fermentation broth on K. pneumoniae-infected mice was also observed, and the major active components of BY3 fermentation broth were analysed via LC‒MS analysis, network pharmacology, and molecular docking. The results showed that BY3 fermentation broth inhibited K. pneumoniae CPS production and downregulated transcription of CPS-related biosynthesis genes, which weakened bacterial resistance to serum killing and phagocytosis, while promoting bacterial surface complement C3 deposition and human β-defensin expression. BY3 fermentation broth demonstrated safety and therapeutic effects in vivo and in vitro, restoring body weight and visceral indices, significantly reducing the organ bacterial load and serum cytokine levels, and alleviating pathological organ damage in mice. In addition, three natural compounds-oleanolic acid, quercetin, and palmitoleic acid-were identified as the major active components in the BY3 fermentation broth. Therefore, BY3 fermentation broth may be a promising strategy for the prevention or treatment of K. pneumoniae infections.

In Vivo Evolution of a Klebsiella pneumoniae Capsule Defect With wcaJ Mutation Promotes Complement-Mediated Opsonophagocytosis During Recurrent Infection

BACKGROUND

Klebsiella pneumoniae carbapenemase-producing K pneumoniae (KPC-Kp) bloodstream infections are associated with high mortality. We studied clinical bloodstream KPC-Kp isolates to investigate mechanisms of resistance to complement, a key host defense against bloodstream infection.

METHODS

We tested growth of KPC-Kp isolates in human serum. In serial isolates from a single patient, we performed whole genome sequencing and tested for complement resistance and binding by mixing study, direct enzyme-linked immunosorbent assay, flow cytometry, and electron microscopy. We utilized an isogenic deletion mutant in phagocytosis assays and an acute lung infection model.

RESULTS

We found serum resistance in 16 of 59 (27%) KPC-Kp clinical bloodstream isolates. In 5 genetically related bloodstream isolates from a single patient, we noted a loss-of-function mutation in the capsule biosynthesis gene, wcaJ. Disruption of wcaJ was associated with decreased polysaccharide capsule, resistance to complement-mediated killing, and surprisingly, increased binding of complement proteins. Furthermore, an isogenic wcaJ deletion mutant exhibited increased opsonophagocytosis in vitro and impaired in vivo control in the lung after airspace macrophage depletion in mice.

CONCLUSIONS

Loss of function in wcaJ led to increased complement resistance, complement binding, and opsonophagocytosis, which may promote KPC-Kp persistence by enabling coexistence of increased bloodstream fitness and reduced tissue virulence.

The complement cascade in lung injury and disease

BACKGROUND

The complement system is an important arm of immune defense bringing innate and adaptive immunity. Although originally regarded as a major complementary defense mechanism against pathogens, continuously emerging evidence has uncovered a central role of this complex system in several diseases including lung pathologies.

MAIN BODY

Complement factors such as anaphylatoxins C3a and C5a, their receptors C3aR, C5aR and C5aR2 as well as complement inhibitory proteins CD55, CD46 and CD59 have been implicated in pathologies such as the acute respiratory distress syndrome, pneumonia, chronic obstructive pulmonary disease, asthma, interstitial lung diseases, and lung cancer. However, the exact mechanisms by which complement factors induce these diseases remain unclear. Several complement-targeting monoclonal antibodies are reported to treat lung diseases.

CONCLUSIONS

The complement system contributes to the progression of the acute and chronic lung diseases. Better understanding of the underlying mechanisms will provide groundwork to develop new strategy to target complement factors for treatment of lung diseases.

Klebsiella pneumoniae manipulates human macrophages to acquire iron

Background

Klebsiella pneumoniae (KP) is a major cause of hospital-acquired infections, such as pneumonia. Moreover, it is classified as a pathogen of concern due to sprawling anti-microbial resistance. During infection, the gram-negative pathogen is capable of establishing an intracellular niche in macrophages by altering cellular metabolism. One factor critically affecting the host-pathogen interaction is the availability of essential nutrients, like iron, which is required for KP to proliferate but which also modulates anti-microbial immune effector pathways. We hypothesized, that KP manipulates macrophage iron homeostasis to acquire this crucial nutrient for sustained proliferation.

Methods

We applied an in-vitro infection model, in which human macrophage-like PMA-differentiated THP1 cells were infected with KP (strain ATCC 43816). During a 24-h course of infection, we quantified the number of intracellular bacteria via serial plating of cell lysates and evaluated the effects of different stimuli on intracellular bacterial numbers and iron acquisition. Furthermore, we analyzed host and pathogen specific gene and protein expression of key iron metabolism molecules.

Results

Viable bacteria are recovered from macrophage cell lysates during the course of infection, indicative of persistence of bacteria within host cells and inefficient pathogen clearing by macrophages. Strikingly, following KP infection macrophages strongly induce the expression of the main cellular iron importer transferrin-receptor-1 (TFR1). Accordingly, intracellular KP proliferation is further augmented by the addition of iron loaded transferrin. The induction of TFR1 is mediated via the STAT-6-IL-10 axis, and pharmacological inhibition of this pathway reduces macrophage iron uptake, elicits bacterial iron starvation, and decreases bacterial survival.

Conclusion

Our results suggest, that KP manipulates macrophage iron metabolism to acquire iron once confined inside the host cell and enforces intracellular bacterial persistence. This is facilitated by microbial mediated induction of TFR1 via the STAT-6-IL-10 axis. Mechanistic insights into immune metabolism will provide opportunities for the development of novel antimicrobial therapies.

Kidney cell culture Macaca fascicularis as a candidate for vaccine development and in vitro model

BACKGROUND

Cell culture is the proliferation of a cell population in vitro by isolating from the original tissue or growing from existing ones. One essential source is the monkey kidney cell cultures which have an essential role in biomedical study. This is due to the significant homology between the human and macaque genomes making these useful for cultivating human viruses, especially enteroviruses, and growing vaccines.

METHODS

This study developed cell cultures derived from the kidney of Macaca fascicularis (Mf) and validated its gene expression.

RESULTS

The primary cultures were successfully subcultured up to six passages, grew as monolayers, and exhibited epithelial-like morphology. The cultured cells remained heterogeneous in phenotype and they expressed CD155 and CD46 as viral receptors, cell morphology (CD24, endosialin, and vWF), proliferation, also apoptosis markers (Ki67 and p53).

CONCLUSIONS

These results indicated that the cell cultures can be used as in vitro model cells for vaccine development and bioactive compound.

Epidemiological Features and Impact of High Glucose Level on Virulence Gene Expression and Serum Resistance of Klebsiella pneumoniae Causing Liver Abscess in Diabetic Patients

Purpose

Klebsiella pneumoniae (K. pneumoniae) is a Gram-negative bacterium that is predominantly associated with liver abscesses in global diabetic patients. High levels of glucose in the surrounding of K. pneumonia increase its pathogenicity including capsular polysaccharide (CPS) and fimbriae. Other important virulent factors include outer membrane protein A (ompA) and regulator mucoid phenotype A (rmpA). The objective of this investigation was to elucidate the effects of high glucose on rmpA and ompA gene expression and serum resistance of K. pneumoniae causing liver abscess.

Patients and Methods

The clinical history of 57 patients suffering from K. pneumoniae-caused liver abscesses (KLA) was acquired and their clinical and laboratory manifestations in the presence or absence of diabetes were analyzed. The antimicrobial susceptibility, serotypes, and virulence genes were tested. Clinical isolates of 3 serotype-K1 hypervirulent K. pneumoniae (hvKP) were used to detect the effect of exogenous high glucose on rmpA, ompA, and clbB genes expression, and bacterial serum resistance.

Results

KLA patients with diabetes showed higher C-reactive protein (CRP) compared to non-diabetic KLA patients. Furthermore, the diabetic group showed increased incidences of sepsis and invasive infections, and their length of hospital stay was also prolonged. Pre-incubation of K. pneumoniae in high glucose (0.5%) concentration up-regulated rmpA, ompA, and clbB genes expression. However, cAMP supplementation, which was inhibited by environmental glucose, reversed the increase of rmpA and ompA in a cAMP-dependent manner. Moreover, hvKP strains incubated in high glucose also exhibited enhanced protection from serum killing.

Conclusion

High glucose levels reflected by poor glycemic control has increased gene expression of rmpA and ompA in hvKP by the cAMP signaling pathway and enhanced its resistance to serum killing, thus providing a new and reasonable explanation for the high incidences of sepsis and invasive infections in KLA patients with diabetes.

The making of hypervirulent Klebsiella pneumoniae

Klebsiella pneumoniae is a notorious bacterium in clinical practice. Virulence, carbapenem-resistance and their convergence among K. pneumoniae are extensively discussed in this article. Hypervirulent K. pneumoniae (HvKP) has spread from the Asian Pacific Rim to the world, inducing various invasive infections, such as pyogenic liver abscess, endophthalmitis, and meningitis. Furthermore, HvKP has acquired more and more drug resistance. Among multidrug-resistant HvKP, hypervirulent carbapenem-resistant K. pneumoniae (Hv-CRKP), and carbapenem-resistant hypervirulent K. pneumoniae (CR-HvKP) are both devastating for their extreme drug resistance and virulence. The hypervirulence of HvKP is primarily attributed to hypercapsule, macromolecular exopolysaccharides, or excessive siderophores, although it has many other factors, for example, lipopolysaccharides, fimbriae, and porins. In contrast with classical determination of HvKP, that is, animal lethality test, molecular determination could be an optional and practical method after improvement. HvKP, including Hv-CRKP and CR-HvKP, has been progressing. R-M and CRISPR-Cas systems may play pivotal roles in such evolutions. Hv-CRKP and CR-HvKP, in particular the former, should be of severe concern due to their being more and more prevalent.

Airway Epithelial Cells Differentially Adapt Their Iron Metabolism to Infection With Klebsiella pneumoniae and Escherichia coli In Vitro

Background

Pneumonia is often elicited by bacteria and can be associated with a severe clinical course, respiratory failure and the need for mechanical ventilation. In the alveolus, type-2-alveolar-epithelial-cells (AECII) contribute to innate immune functions. We hypothesized that AECII actively adapt cellular iron homeostasis to restrict this essential nutrient from invading pathogens - a defense strategy termed 'nutritional immunity', hitherto mainly demonstrated for myeloid cells.

Methods

We established an in-vitro infection model using the human AECII-like cell line A549. We infected cells with Klebsiella pneumoniae (K. pneumoniae) and Escherichia coli (E. coli), two gram-negative bacteria with different modes of infection and frequent causes of hospital-acquired pneumonia. We followed the entry and intracellular growth of these gram-negative bacteria and analyzed differential gene expression and protein levels of key inflammatory and iron metabolism molecules.

Results

Both, K. pneumoniae and E. coli are able to invade A549 cells, whereas only K. pneumoniae is capable of proliferating intracellularly. After peak bacterial burden, the number of intracellular pathogens declines, suggesting that epithelial cells initiate antimicrobial immune effector pathways to combat bacterial proliferation. The extracellular pathogen E. coli induces an iron retention phenotype in A549 cells, mainly characterized by the downregulation of the pivotal iron exporter ferroportin, the upregulation of the iron importer transferrin-receptor-1 and corresponding induction of the iron storage protein ferritin. In contrast, cells infected with the facultative intracellular bacterium K. pneumoniae exhibit an iron export phenotype indicated by ferroportin upregulation. This differential regulation of iron homeostasis and the pathogen-specific inflammatory reaction is likely mediated by oxidative stress.

Conclusion

AECII-derived A549 cells show pathogen-specific innate immune functions and adapt their iron handling in response to infection. The differential regulation of iron transporters depends on the preferential intra- or extracellular localization of the pathogen and likely aims at limiting bacterial iron availability.

CD46 Genetic Variability and HIV-1 Infection Susceptibility

CD46 is the main receptor for complement protein C3 and plays an important role in adaptive immune responses. CD46 genetic variants are associated with susceptibility to several infectious and autoimmune diseases. Additionally, CD46 function can be subverted by HIV-1 to evade attack by complement, a strategy shared by viruses of other families. We sought to determine the association between CD46 gene variants and HIV-1 acquired through intravenous drug use (IDU) and sexual routes (n = 823). Study subjects were of European ancestry and were HIV-1 infected (n = 438) or exposed but seronegative (n = 387). Genotyping of the rs2796265 SNP located in the CD46 gene region was done by allele-specific real-time PCR. A meta-analysis merging IDU and sexual cohorts indicates that the minor genotype (CC) was associated with increased resistance to HIV-1 infection OR = 0.2, 95% CI (0.07–0.61), p = 0.004. The HIV-1-protective genotype is correlated with reduced CD46 expression and alterations in the ratio of CD46 mRNA splicing isoforms.

The Mutation in wbaP cps Gene Cluster Selected by Phage-Borne Depolymerase Abolishes Capsule Production and Diminishes the Virulence of Klebsiella pneumoniae

Klebsiella pneumoniae is considered one of the most critical multidrug-resistant pathogens and urgently requires new therapeutic strategies. Capsular polysaccharides (CPS), lipopolysaccharides (LPS), and exopolysaccharides (EPS) are the major virulence factors protecting K. pneumoniae against the immune response and thus may be targeted by phage-based therapeutics such as polysaccharides-degrading enzymes. Since the emergence of resistance to antibacterials is generally considered undesirable, in this study, the genetic and phenotypic characteristics of resistance to the phage-borne CPS-degrading depolymerase and its effect on K. pneumoniae virulence were investigated. The K63 serotype targeting depolymerase (KP36gp50) derived from Klebsiella siphovirus KP36 was used as the selective agent during the treatment of K. pneumoniae 486 biofilm. Genome-driven examination combined with the surface polysaccharide structural analysis of resistant mutant showed the point mutation and frameshift in the wbaP gene located within the cps gene cluster, resulting in the loss of the capsule. The sharp decline in the yield of CPS was accompanied by the production of a larger amount of smooth LPS. The modification of the surface polysaccharide layers did not affect bacterial fitness nor the insensitivity to serum complement; however, it made bacteria more prone to phagocytosis combined with the higher adherence and internalization to human lung epithelial cells. In that context, it was showed that the emerging resistance to the antivirulence agent (phage-borne capsule depolymerase) results in beneficial consequences, i.e., the sensitization to the innate immune response.

Membrane cofactor protein (MCP; CD46): deficiency states and pathogen connections

Membrane cofactor protein (MCP; CD46), a ubiquitously expressed complement regulatory protein, serves as a cofactor for serine protease factor I to cleave and inactivate C3b and C4b deposited on host cells. However, CD46 also plays roles in human reproduction, autophagy, modulating T cell activation and effector functions and is a member of the newly identified intracellular complement system (complosome). CD46 also is a receptor for 11 pathogens ('pathogen magnet'). While CD46 deficiencies contribute to inflammatory disorders, its overexpression in cancers and role as a receptor for some adenoviruses has led to its targeting by oncolytic agents and adenoviral-based therapeutic vectors, including coronavirus disease of 2019 (COVID-19) vaccines. This review focuses on recent advances in identifying disease-causing CD46 variants and its pathogen connections.

More than a Pore: Nonlytic Antimicrobial Functions of Complement and Bacterial Strategies for Evasion

The complement system is an evolutionarily ancient defense mechanism against foreign substances. Consisting of three proteolytic activation pathways, complement converges on a common effector cascade terminating in the formation of a lytic pore on the target surface. The classical and lectin pathways are initiated by pattern recognition molecules binding to specific ligands, while the alternative pathway is constitutively active at low levels in circulation. Complement-mediated killing is essential for defense against many Gram-negative bacterial pathogens, and genetic deficiencies in complement can render individuals highly susceptible to infection, for example, invasive meningococcal disease. In contrast, Gram-positive bacteria are inherently resistant to the direct bactericidal activity of complement due to their thick layer of cell wall peptidoglycan. However, complement also serves diverse roles in immune defense against all bacteria by flagging them for opsonization and killing by professional phagocytes, synergizing with neutrophils, modulating inflammatory responses, regulating T cell development, and cross talk with coagulation cascades. In this review, we discuss newly appreciated roles for complement beyond direct membrane lysis, incorporate nonlytic roles of complement into immunological paradigms of host-pathogen interactions, and identify bacterial strategies for complement evasion.

Klebsiella pneumoniae: Prevalence, Reservoirs, Antimicrobial Resistance, Pathogenicity, and Infection: A Hitherto Unrecognized Zoonotic Bacterium

Klebsiella pneumoniae is considered an opportunistic pathogen, constituting an ongoing health concern for immunocompromised patients, the elderly, and neonates. Reports on the isolation of K. pneumoniae from other sources are increasing, many of which express multidrug-resistant (MDR) phenotypes. Three phylogroups were identified based on nucleotide differences. Niche environments, including plants, animals, and humans appear to be colonized by different phylogroups, among which KpI (K. pneumoniae) is commonly associated with human infection. Infections with K. pneumoniae can be transmitted through contaminated food or water and can be associated with community-acquired infections or between persons and animals involved in hospital-acquired infections. Increasing reports are describing detections along the food chain, suggesting the possibility exists that this could be a hitherto unexplored reservoir for this opportunistic bacterial pathogen. Expression of MDR phenotypes elaborated by these bacteria is due to the nature of various plasmids carrying antimicrobial resistance (AMR)-encoding genes, and is a challenge to animal, environmental, and human health alike. Raman spectroscopy has the potential to provide for the rapid identification and screening of antimicrobial susceptibility of Klebsiella isolates. Moreover, hypervirulent isolates linked with extraintestinal infections express phenotypes that may support their niche adaptation. In this review, the prevalence, reservoirs, AMR, Raman spectroscopy detection, and pathogenicity of K. pneumoniae are summarized and various extraintestinal infection pathways are further narrated to extend our understanding of its adaptation and survival ability in reservoirs, and associated disease risks.

Pulmonary Innate Immune Response Determines the Outcome of Inflammation During Pneumonia and Sepsis-Associated Acute Lung Injury

The lung is a primary organ for gas exchange in mammals that represents the largest epithelial surface in direct contact with the external environment. It also serves as a crucial immune organ, which harbors both innate and adaptive immune cells to induce a potent immune response. Due to its direct contact with the outer environment, the lung serves as a primary target organ for many airborne pathogens, toxicants (aerosols), and allergens causing pneumonia, acute respiratory distress syndrome (ARDS), and acute lung injury or inflammation (ALI). The current review describes the immunological mechanisms responsible for bacterial pneumonia and sepsis-induced ALI. It highlights the immunological differences for the severity of bacterial sepsis-induced ALI as compared to the pneumonia-associated ALI. The immune-based differences between the Gram-positive and Gram-negative bacteria-induced pneumonia show different mechanisms to induce ALI. The role of pulmonary epithelial cells (PECs), alveolar macrophages (AMs), innate lymphoid cells (ILCs), and different pattern-recognition receptors (PRRs, including Toll-like receptors (TLRs) and inflammasome proteins) in neutrophil infiltration and ALI induction have been described during pneumonia and sepsis-induced ALI. Also, the resolution of inflammation is frequently observed during ALI associated with pneumonia, whereas sepsis-associated ALI lacks it. Hence, the review mainly describes the different immune mechanisms responsible for pneumonia and sepsis-induced ALI. The differences in immune response depending on the causal pathogen (Gram-positive or Gram-negative bacteria) associated pneumonia or sepsis-induced ALI should be taken in mind specific immune-based therapeutics.

Targeting the Sugary Armor of Klebsiella Species

The emergence of multidrug-resistant strains of Gram-negative Klebsiella species is an urgent global threat. The World Health Organization has listed Klebsiella pneumoniae as one of the global priority pathogens in critical need of next-generation antibiotics. Compared to other Gram-negative pathogens, K. pneumoniae accumulates a greater diversity of antimicrobial-resistant genes at a higher frequency. The evolution of a hypervirulent phenotype of K. pneumoniae is yet another concern. It has a broad ecological distribution affecting humans, agricultural animals, plants, and aquatic animals. Extracellular polysaccharides of Klebsiella, such as lipopolysaccharides, capsular polysaccharides, and exopolysaccharides, play crucial roles in conferring resistance against the host immune response, as well as in colonization, surface adhesion, and for protection against antibiotics and bacteriophages. These extracellular polysaccharides are major virulent determinants and are highly divergent with respect to their antigenic properties. Wzx/Wzy-, ABC-, and synthase-dependent proteinaceous nano-machineries are involved in the biosynthesis, transport, and cell surface expression of these sugar molecules. Although the proteins involved in the biosynthesis and surface expression of these sugar molecules represent potential drug targets, variation in the amino acid sequences of some of these proteins, in combination with diversity in their sugar composition, poses a major challenge to the design of a universal drug for Klebsiella infections. This review discusses the challenges in universal Klebsiella vaccine and drug development from the perspective of antigen sugar compositions and the proteins involved in extracellular antigen transport.

Complement mediated Klebsiella pneumoniae capsule changes

The Gram-negative bacterium Klebsiella pneumoniae is an opportunistic pathogen, which can cause life-threatening infections such as sepsis. Worldwide, emerging multidrug resistant K. pneumoniae infections are challenging to treat, hence leading to increased mortality. Therefore, understanding the interactions between K. pneumoniae and the immune system is important to develop new treatment options. We characterized ten clinical K. pneumoniae isolates obtained from blood of bacteremia patients. The interaction of the isolates with human serum was investigated to elucidate how K. pneumoniae escapes the host immune system, and how complement activation by K. pneumoniae changed the capsule structure. All K. pneumoniae isolates activated the alternative complement pathway despite serum resistance of seven isolates. One serum sensitive isolate activated two or all three pathways, and this isolate was lysed and had numerous membrane attack complexes in the outer membrane. However, we also found deposition of complement components in the capsule of serum resistant isolates resulting in morphological capsule changes and capsule shedding. These bacteria did not lyse, and no membrane attack complex was observed despite deposition of C5b-9 within the capsule, indicating that the capsule of serum resistant K. pneumoniae isolates is a defense mechanism against complement-mediated lysis.

Hypervirulent Klebsiella pneumoniae

Hypervirulent K. pneumoniae (hvKp) is an evolving pathotype that is more virulent than classical K. pneumoniae (cKp). hvKp usually infects individuals from the community, who are often healthy. Infections are more common in the Asian Pacific Rim but are occurring globally. hvKp infection frequently presents at multiple sites or subsequently metastatically spreads, often requiring source control. hvKp has an increased ability to cause central nervous system infection and endophthalmitis, which require rapid recognition and site-specific treatment. The genetic factors that confer hvKp's hypervirulent phenotype are present on a large virulence plasmid and perhaps integrative conjugal elements. Increased capsule production and aerobactin production are established hvKp-specific virulence factors. Similar to cKp, hvKp strains are becoming increasingly resistant to antimicrobials via acquisition of mobile elements carrying resistance determinants, and new hvKp strains emerge when extensively drug-resistant cKp strains acquire hvKp-specific virulence determinants, resulting in nosocomial infection. Presently, clinical laboratories are unable to differentiate cKp from hvKp, but recently, several biomarkers and quantitative siderophore production have been shown to accurately predict hvKp strains, which could lead to the development of a diagnostic test for use by clinical laboratories for optimal patient care and for use in epidemiologic surveillance and research studies.

Microbial uptake by the respiratory epithelium: outcomes for host and pathogen

Intracellular occupancy of the respiratory epithelium is a useful pathogenic strategy facilitating microbial replication and evasion of professional phagocytes or circulating antimicrobial drugs. A less appreciated but growing body of evidence indicates that the airway epithelium also plays a crucial role in host defence against inhaled pathogens, by promoting ingestion and quelling of microorganisms, processes that become subverted to favour pathogen activities and promote respiratory disease. To achieve a deeper understanding of beneficial and deleterious activities of respiratory epithelia during antimicrobial defence, we have comprehensively surveyed all current knowledge on airway epithelial uptake of bacterial and fungal pathogens. We find that microbial uptake by airway epithelial cells (AECs) is a common feature of respiratory host-microbe interactions whose stepwise execution, and impacts upon the host, vary by pathogen. Amidst the diversity of underlying mechanisms and disease outcomes, we identify four key infection scenarios and use best-characterised host-pathogen interactions as prototypical examples of each. The emergent view is one in which effi-ciency of AEC-mediated pathogen clearance correlates directly with severity of disease outcome, therefore highlighting an important unmet need to broaden our understanding of the antimicrobial properties of respiratory epithelia and associated drivers of pathogen entry and intracellular fate.

Klebsiella pneumoniae infection biology: living to counteract host defences

Klebsiella species cause a wide range of diseases including pneumonia, urinary tract infections (UTIs), bloodstream infections and sepsis. These infections are particularly a problem among neonates, elderly and immunocompromised individuals. Klebsiella is also responsible for a significant number of community-acquired infections. A defining feature of these infections is their morbidity and mortality, and the Klebsiella strains associated with them are considered hypervirulent. The increasing isolation of multidrug-resistant strains has significantly narrowed, or in some settings completely removed, the therapeutic options for the treatment of Klebsiella infections. Not surprisingly, this pathogen has then been singled out as an 'urgent threat to human health' by several organisations. This review summarises the tremendous progress that has been made to uncover the sophisticated immune evasion strategies of K. pneumoniae. The co-evolution of Klebsiella in response to the challenge of an activated immune has made Klebsiella a formidable pathogen exploiting stealth strategies and actively suppressing innate immune defences to overcome host responses to survive in the tissues. A better understanding of Klebsiella immune evasion strategies in the context of the host-pathogen interactions is pivotal to develop new therapeutics, which can be based on antagonising the anti-immune strategies of this pathogen.

Complement Susceptibility in Relation to Genome Sequence of Recent Klebsiella pneumoniae Isolates from Thai Hospitals

The capacity to resist the bactericidal action of complement (C') is a strong but poorly understood virulence trait in Klebsiella spp. Killing requires activation of one or more C' pathways, assembly of C5b-9 membrane attack complexes (MACs) on the surface of the outer membrane (OM), and penetration of MACs into the target bilayer. We interrogated whole-genome sequences of 164 Klebsiella isolates from three tertiary hospitals in Thailand for genes encoding surface-located macromolecules considered to play a role in determination of C' resistance. Most isolates (154/164) were identified as Klebsiella pneumoniae, and the collection conformed to previously established population structures and antibiotic resistance patterns. The distribution of sequence types (STs) and capsular (K) types were also typical of global populations. The majority (64%) of isolates were resistant to C', and the remainder were either rapidly or slowly killed. All isolates carried genes encoding capsular polysaccharides (K antigens), which have been strongly linked to C' resistance. In contrast to previous reports, there were no differences in the amount of capsule produced by C'-resistant isolates compared to C'-susceptible isolates, nor was there any correlation between serum reactivity and the presence of hypermucoviscous capsules. Similarly, there were no correlations between the presence of genes specifying lipopolysaccharide O-side chains or major OM proteins. Some virulence factors were found more frequently in C'-resistant isolates but were considered to reflect clonal ST expansion. Thus, no single gene accounts for the C' resistance of the isolates sequenced in this study.IMPORTANCE Multidrug-resistant Klebsiella pneumoniae is responsible for an increasing proportion of nosocomial infections, and emerging hypervirulent K. pneumoniae clones now cause severe community-acquired infections in otherwise healthy individuals. These bacteria are adept at circumventing immune defenses, and most survive and grow in serum; their capacity to avoid C'-mediated destruction is correlated with their invasive potential. Killing of Gram-negative bacteria occurs following activation of the C' cascades and stable deposition of C5b-9 MACs onto the OM. For Klebsiella, studies with mutants and conjugants have invoked capsules, lipopolysaccharide O-side chains, and OM proteins as determinants of C' resistance, although the precise roles of the macromolecules are unclear. In this study, we sequenced 164 Klebsiella isolates with different C' susceptibilities to identify genes involved in resistance. We conclude that no single OM constituent can account for resistance, which is likely to depend on biophysical properties of the target bilayer.

Rhinoscleroma pathogenesis: The type K3 capsule of Klebsiella rhinoscleromatis is a virulence factor not involved in Mikulicz cells formation

Rhinoscleroma is a human specific chronic granulomatous infection of the nose and upper airways caused by the Gram-negative bacterium Klebsiella pneumoniae subsp. rhinoscleromatis. Although considered a rare disease, it is endemic in low-income countries where hygienic conditions are poor. A hallmark of this pathology is the appearance of atypical foamy monocytes called Mikulicz cells. However, the pathogenesis of rhinoscleroma remains poorly investigated. Capsule polysaccharide (CPS) is a prominent virulence factor in bacteria. All K. rhinoscleromatis strains are of K3 serotype, suggesting that CPS can be an important driver of rhinoscleroma disease. In this study, we describe the creation of the first mutant of K. rhinoscleromatis, inactivated in its capsule export machinery. Using a murine model recapitulating the formation of Mikulicz cells in lungs, we observed that a K. rhinoscleromatis CPS mutant (KR cps-) is strongly attenuated and that mice infected with a high dose of KR cps- are still able to induce Mikulicz cells formation, unlike a K. pneumoniae capsule mutant, and to partially recapitulate the characteristic strong production of IL-10. Altogether, the results of this study show that CPS is a virulence factor of K. rhinoscleromatis not involved in the specific appearance of Mikulicz cells.

Inactivation of the Thymidylate Synthase thyA in Non-typeable Haemophilus influenzae Modulates Antibiotic Resistance and Has a Strong Impact on Its Interplay with the Host Airways

Antibacterial treatment with cotrimoxazol (TxS), a combination of trimethoprim and sulfamethoxazole, generates resistance by, among others, acquisition of thymidine auxotrophy associated with mutations in the thymidylate synthase gene thyA, which can modify the biology of infection. The opportunistic pathogen non-typeable Haemophilus influenzae (NTHi) is frequently encountered in the lower airways of chronic obstructive pulmonary disease (COPD) patients, and associated with acute exacerbation of COPD symptoms. Increasing resistance of NTHi to TxS limits its suitability as initial antibacterial against COPD exacerbation, although its relationship with thymidine auxotrophy is unknown. In this study, the analysis of 2,542 NTHi isolates recovered at Bellvitge University Hospital (Spain) in the period 2010–2014 revealed 119 strains forming slow-growing colonies on the thymidine low concentration medium Mueller Hinton Fastidious, including one strain isolated from a COPD patient undergoing TxS therapy that was a reversible thymidine auxotroph. To assess the impact of thymidine auxotrophy in the NTHi-host interplay during respiratory infection, thyA mutants were generated in both the clinical isolate NTHi375 and the reference strain RdKW20. Inactivation of the thyA gene increased TxS resistance, but also promoted morphological changes consistent with elongation and impaired bacterial division, which altered H. influenzae self-aggregation, phosphorylcholine level, C3b deposition, and airway epithelial infection patterns. Availability of external thymidine contributed to overcome such auxotrophy and TxS effect, potentially facilitated by the nucleoside transporter nupC. Although, thyA inactivation resulted in bacterial attenuation in a lung infection mouse model, it also rendered a lower clearance upon a TxS challenge in vivo. Thus, our results show that thymidine auxotrophy modulates both the NTHi host airway interplay and antibiotic resistance, which should be considered at the clinical setting for the consequences of TxS administration.

Complement resistance mechanisms of Klebsiella pneumoniae

The current emergence of antibiotic-resistant bacteria causes major problems in hospitals worldwide. To survive within the host, bacterial pathogens exploit several escape mechanisms to prevent detection and killing by the immune system. As a major player in immune defense, the complement system recognizes and destroys bacteria via different effector mechanisms. The complement system can label bacteria for phagocytosis or directly kill Gram-negative bacteria via insertion of a pore-forming complex in the bacterial membrane. The multi-drug resistant pathogen Klebsiella pneumoniae exploits several mechanisms to resist complement. In this review, we present an overview of strategies used by K. pneumoniae to prevent recognition and killing by the complement system. Understanding these complement evasion strategies is crucial for the development of innovative strategies to combat K. pneumoniae.

Resistance of Klebsiella pneumoniae to the innate immune system of African green monkeys

In recent years, an emergent Klebsiella pneumoniae hypermucoviscosity (HMV) phenotype has been associated with increased invasiveness and pathogenicity in primates. In this project, bacteria recovered from infected African green monkeys (AGM) (Chlorocebus aethiops sabaeus) were screened for HMV phenotype, and were compared to non-HMV isolates in in vitro, serum, and oxidative-mediated killing assays. Complement-mediated killing was assessed utilizing freshly collected serum from healthy AGM. Oxidative-mediated killing was investigated utilizing sodium hypochlorite and hydrogen peroxide. Compared to non-HMV isolates, HMV isolates were more resistant to serum-mediated and oxidative killing (p<0.05). Phagocytosis resistance was evaluated using AGM peripheral blood monocytes (PBMC), and results indicated that non-HMV isolates associated with the AGM PBMC to a greater extent than HMV isolates (p<0.001). Measurement of lactate dehydrogenase release showed that HMV isolates were more cytotoxic to AGM PBMC than non-HMV isolates (p<0.001). Thus, the hypermucoid phenotype appears to be an important virulence factor that promotes evasion of innate immune defenses.

Overexpression of MexCD-OprJ reduces Pseudomonas aeruginosa virulence by increasing its susceptibility to complement-mediated killing

We evaluated the resistance to complement-mediated killing of a collection of isogenic Pseudomonas aeruginosa strains expressing different antimicrobial resistance phenotypes. Only the nfxB mutant demonstrated increased susceptibility to complement compared with that for the wild-type strain. This increment was due to the overexpression of MexCD-OprJ, which led to increased C3 opsonization and a reduced ability to infect the lungs of mice. Our results show that the acquisition of antibiotic resistance may alter the interplay of P. aeruginosa with the host immune system.

Role of bacterial surface structures on the interaction of Klebsiella pneumoniae with phagocytes

Phagocytosis is a key process of the immune system. The human pathogen Klebsiella pneumoniae is a well known example of a pathogen highly resistant to phagocytosis. A wealth of evidence demonstrates that the capsule polysaccharide (CPS) plays a crucial role in resistance to phagocytosis. The amoeba Dictyostelium discoideum shares with mammalian macrophages the ability to phagocytose and kill bacteria. The fact that K. pneumoniae is ubiquitous in nature and, therefore, should avoid predation by amoebae, poses the question whether K. pneumoniae employs similar means to counteract amoebae and mammalian phagocytes. Here we developed an assay to evaluate K. pneumoniae-D. discoideum interaction. The richness of the growth medium affected the threshold at which the cps mutant was permissive for Dictyostelium and only at lower nutrient concentrations the cps mutant was susceptible to predation by amoebae. Given the critical role of bacterial surface elements on host-pathogen interactions, we explored the possible contribution of the lipopolysaccharide (LPS) and outer membrane proteins (OMPs) to combat phagoyctosis by D. discoideum. We uncover that, in addition to the CPS, the LPS O-polysaccharide and the first core sugar participate in Klebsiella resistance to predation by D. discoideum. K. pneumoniae LPS lipid A decorations are also necessary to avoid predation by amoebae although PagP-dependent palmitoylation plays a more important role than the lipid A modification with aminoarabinose. Mutants lacking OMPs OmpA or OmpK36 were also permissive for D. discoideium growth. Except the LPS O-polysaccharide mutants, all mutants were more susceptible to phagocytosis by mouse alveolar macrophages. Finally, we found a correlation between virulence, using the pneumonia mouse model, and resistance to phagocytosis. Altogether, this work reveals novel K. pneumoniae determinants involved in resistance to phagocytosis and supports the notion that Dictyostelium amoebae might be useful as host model to measure K. pneumoniae virulence and not only phagocytosis.

Serum heat inactivation affects protein corona composition and nanoparticle uptake

Nanoparticles are of an appropriate size to interact with cells, and are likely to use a range of cellular machinery for internalisation and trafficking to various sub-cellular compartments. It is now understood that once in contact with biological fluids, the nanoparticle surface gets covered by a highly specific layer of proteins, forming the nanoparticle protein corona. This protein layer is stable for times longer than the typical time scale of nanoparticle import, and thus can impact on particle uptake and trafficking inside the cells. In this work, the effect of the corona composition on nanoparticle uptake has been investigated, by studying the impact of serum heat inactivation and complement depletion on the load of nanoparticles accumulated inside the cell. For the same material and nanoparticle size, cellular uptake was found to be significantly different when the nanoparticles were dispersed in medium where the serum was heat inactivated or not heat inactivated, even for non-specialized cells, suggesting that different sera can lead to different nanoparticle doses. The fact that uptake was correlated with the amount of protein bound into the nanoparticle corona suggests the need for commonly agreed dispersion protocols for in vitro nanoparticle-cell studies.

Analysis of the proteome of human airway epithelial secretions

BACKGROUND

Airway surface liquid, often referred to as mucus, is a thin layer of fluid covering the luminal surface that plays an important defensive role against foreign particles and chemicals entering the lungs. Airway mucus contains various macromolecules, the most abundant being mucin glycoproteins, which contribute to its defensive function. Airway epithelial cells cultured in vitro secrete mucins and nonmucin proteins from their apical surface that mimics mucus production in vivo. The current study was undertaken to identify the polypeptide constituents of human airway epithelial cell secretions to gain a better understanding of the protein composition of respiratory mucus.

RESULTS

Fifty-five proteins were identified in the high molecular weight fraction of apical secretions collected from in vitro cultures of well-differentiated primary human airway epithelial cells and isolated under physiological conditions. Among these were MUC1, MUC4, MUC5B, and MUC16 mucins. By proteomic analysis, the nonmucin proteins could be classified as inflammatory, anti-inflammatory, anti-oxidative, and/or anti-microbial.

CONCLUSIONS

Because the majority of the nonmucin proteins possess molecular weights less than that selected for analysis, it is theoretically possible that they may associate with the high molecular weight and negatively charged mucins to form a highly ordered structural organization that is likely to be important for maintaining the proper defensive function of airway mucus.

Klebsiella pneumoniae capsule polysaccharide impedes the expression of beta-defensins by airway epithelial cells

Human beta-defensins (hBDs) contribute to the protection of the respiratory tract against pathogens. It is reasonable to postulate that pathogens have developed countermeasures to resist them. Klebsiella pneumoniae capsule polysaccharide (CPS), but not the lipopolysaccharide O antigen, mediated resistance against hBD1 and hBD2. hBD3 was the most potent hBD against Klebsiella. We investigated the possibility that as a strategy for survival in the lung, K. pneumoniae may not activate the expression of hBDs. Infection of A549 and normal human bronchial cells with 52145-Deltawca(K2), a CPS mutant, increased the expression of hBD2 and hBD3. Neither the wild type nor the lipopolysaccharide O antigen mutant increased the expression of hBDs. In vivo, 52145-Deltawca(K2) induced higher levels of mBD4 and mBD14, possible mouse orthologues of hBD2 and hBD3, respectively, than the wild type. 52145-Deltawca(K2)-dependent upregulation of hBD2 occurred via NF-kappaB and mitogen-activated protein kinases (MAPKs) p44/42, Jun N-terminal protein kinase (JNK)-dependent pathways. The increase in hBD3 expression was dependent on the MAPK JNK. 52145-Deltawca(K2) engaged Toll-like receptors 2 and 4 (TLR2 and TLR4) to activate hBD2, whereas hBD3 expression was dependent on NOD1. K. pneumoniae induced the expression of CYLD and MKP-1, which act as negative regulators for 52145-Deltawca(K2)-induced expression of hBDs. Bacterial engagement of pattern recognition receptors induced CYLD and MKP-1, which may initiate the attenuation of proinflammatory pathways. The results of this study indicate that K. pneumoniae CPS not only protects the pathogen from the bactericidal action of defensins but also impedes their expression. These features of K. pneumoniae CPS may facilitate pathogen survival in the hostile environment of the lung.

Porphyromonas gingivalis mediates the shedding and proteolysis of complement regulatory protein CD46 expressed by oral epithelial cells

INTRODUCTION

Human cells express membrane-bound complement regulatory proteins to prevent complement-mediated autologous tissue damage. In this study, we hypothesized that Porphyromonas gingivalis, the major etiological agent of chronic periodontitis, causes the shedding or proteolysis of the complement regulatory protein CD46 expressed by oral epithelial cells.

METHODS

Oral epithelial cells were treated with a culture of P. gingivalis before measurement of membrane-bound and shed CD46 by enzyme-linked immunosorbent assay (ELISA). The effect of soluble recombinant CD46 on secretion of interleukin-8 (IL-8) by epithelial cells was evaluated by ELISA. The susceptibility of soluble recombinant CD46 to proteolytic degradation by cells and purified Lys-gingipain of P. gingivalis was investigated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis/western immunoblotting analysis.

RESULTS

Oral epithelial cells treated with a culture of P. gingivalis showed a lower reactivity with antibodies directed to CD46. ELISA revealed that such a treatment resulted in increased amounts of CD46 in the conditioned media suggesting that P. gingivalis caused the shedding of membrane-anchored CD46. Stimulation of epithelial cells with soluble recombinant CD46 induced IL-8 secretion in a dose-dependent manner. Whole cells and purified Lys-gingipain of P. gingivalis degraded recombinant CD46 in a dose-dependent manner.

CONCLUSION

This study showed the ability of P. gingivalis to induce the shedding/ proteolysis of CD46 from the surface of oral epithelial cells. This may render host cells susceptible to the complement system and contribute to tissue damage and the inflammatory process in periodontitis.

Klebsiella pneumoniae triggers a cytotoxic effect on airway epithelial cells

BACKGROUND

Klebsiella pneumoniae is a capsulated Gram negative bacterial pathogen and a frequent cause of nosocomial infections. Despite its clinical relevance, little is known about the features of the interaction between K. pneumoniae and lung epithelial cells on a cellular level, neither about the role of capsule polysaccharide, one of its best characterised virulence factors, in this interaction.

RESULTS

The interaction between Klebsiella pneumoniae and cultured airway epithelial cells was analysed. K. pneumoniae infection triggered cytotoxicity, evident by cell rounding and detachment from the substrate. This effect required the presence of live bacteria and of capsule polysaccharide, since it was observed with isolates expressing different amounts of capsule and/or different serotypes but not with non-capsulated bacteria. Cytotoxicity was analysed by lactate dehydrogenase and formazan measurements, ethidium bromide uptake and analysis of DNA integrity, obtaining consistent and complementary results. Moreover, cytotoxicity of non-capsulated strains was restored by addition of purified capsule during infection. While a non-capsulated strain was avirulent in a mouse infection model, capsulated K. pneumoniae isolates displayed different degrees of virulence.

CONCLUSION

Our observations allocate a novel role to K. pneumoniae capsule in promotion of cytotoxicity. Although this effect is likely to be associated with virulence, strains expressing different capsule levels were not equally virulent. This fact suggests the existence of other bacterial requirements for virulence, together with capsule polysaccharide.

Klebsiella pneumoniae increases the levels of Toll-like receptors 2 and 4 in human airway epithelial cells

Airway epithelial cells act as the first barrier against pathogens. These cells recognize conserved structural motifs expressed by microbial pathogens via Toll-like receptors (TLRs) expressed on the surface. In contrast to the level of expression in lymphoid cells, the level of expression of TLR2 and TLR4 in airway epithelial cells is low under physiological conditions. Here we explored whether Klebsiella pneumoniae upregulates the expression of TLRs in human airway epithelial cells. We found that the expression of TLR2 and TLR4 by A549 cells and human primary airway cells was upregulated upon infection with K. pneumoniae. The increased expression of TLRs resulted in enhancement of the cellular response upon stimulation with Pam3CSK4 and lipopolysaccharide, which are TLR2 and TLR4 agonists, respectively. Klebsiella-dependent upregulation of TLR expression occurred via a positive IkappaBalpha-dependent NF-kappaBeta pathway and via negative p38 and p44/42 mitogen-activated protein kinase-dependent pathways. We showed that Klebsiella-induced TLR2 and TLR4 upregulation was dependent on TLR activation. An isogenic capsule polysaccharide (CPS) mutant did not increase TLR2 and TLR4 expression. Purified CPS upregulated TLR2 and TLR4 expression, and polymyxin B did not abrogate CPS-induced TLR upregulation. Although no proteins were detected in the CPS preparation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and colloidal gold staining, we could not rule out the possibility that traces of protein in our CPS preparation could have been responsible, at least in part, for the TLR upregulation.

Modification of lipooligosaccharide with phosphoethanolamine by LptA in Neisseria meningitidis enhances meningococcal adhesion to human endothelial and epithelial cells

The lipooligosaccharide (LOS) of Neisseria meningitidis can be decorated with phosphoethanolamine (PEA) at the 4' position of lipid A and at the O-3 and O-6 positions of the inner core of the heptose II residue. The biological role of PEA modification in N. meningitidis remains unclear. During the course of our studies to elucidate the pathogenicity of the ST-2032 (invasive) meningococcal clonal group, disruption of lptA, the gene that encodes the PEA transferase for 4' lipid A, led to a approximately 10-fold decrease in N. meningitidis adhesion to four kinds of human endothelial and epithelial cell lines at an multiplicity of infection of 5,000. Complementation of the lptA gene in a Delta lptA mutant restored wild-type adherence. By matrix-assisted laser desorption ionization-time-of-flight mass spectrometry analysis, PEA was lost from the lipid A of the Delta lptA mutant compared to that of the wild-type strain. The effect of LptA on meningococcal adhesion was independent of other adhesins such as pili, Opc, Opa, and PilC but was inhibited by the presence of capsule. These results indicate that modification of LOS with PEA by LptA enhances meningococcal adhesion to human endothelial and epithelial cells in unencapsulated N. meningitidis.

The classical complement pathway plays a critical role in the opsonisation of uropathogenic Escherichia coli

Urinary tract infection due to uropathogenic Escherichia coli is a common clinical problem. The innate immune system and the uroepithelium are critical in defence against infection. The complement system is both part of the innate immune system and influences the interaction between epithelium and pathogen. We have therefore investigated the mechanism by which uropathogenic E. coli activate complement and the potential for this to occur during clinical infection. The classical pathway is responsible for bacterial opsonisation when complement proteins are present at low concentrations. At higher concentrations the alternative pathway predominates but still requires the classical pathway for its initiation. In contrast the mannose binding lectin pathway is not involved. Early classical pathway components are present in the urine during infection and actively contribute to bacterial opsonisation. The classical pathway could be initiated by anti-E. coli antibodies of IgG or IgM subclasses that are present in urine during infection. Additionally immunoglobulin-independent mechanisms, such as direct C1q binding to bacteria, may be involved. In conclusion, uropathogenic E. coli are readily opsonised by complement in a classical pathway dependent manner. This can occur within the urinary tract during the development of clinical infection.

Membrane cofactor protein (MCP, CD46) binding to clinical isolates of Streptococcus pyogenes: binding to M type 18 strains is independent of Emm or Enn proteins

The complement regulatory protein CD46 (MCP, membrane cofactor protein) is used as a cell receptor by a number of bacterial and viral pathogens, including Streptococcus pyogenes (Group A Streptococci). The highly variable M (Emm) proteins are virulence factors of S. pyogenes, and Emm proteins of serotypes 5, 6 or 22 are able of binding to CD46, thus mediating the binding of Streptococci to human cells. In this work, using a soluble construction encompassing the extracellular domain of human CD46, we have analyzed its binding to clinical isolates of S. pyogenes, including isolates of the M types 1, 3 and 18 that are frequently found in invasive infections or rheumatic fever. Our data show a strong binding of CD46 to bacteria of M types 1, 3, 8, 18, 24, 28, 29, 31 and 78; weak binding to M6 and M29 and no binding to M types 11, 12, M27 or M30. Surprisingly, CD46 bound to isogenic mutants of one clinical M18 isolate lacking the Emm protein or Emm and the Emm-related protein Enn, regardless of having capsule or not. In addition, these isogenic mutants bound to keratinocytes in a CD46-dependent manner, confirming the role of CD46 as one of the cell receptors for Group A Streptococci. Furthermore, CD46 did not bind to a recombinant Emm 18 construct, confirming that Emm is not involved in CD46 binding to M18 bacteria. Emm-dependent and -independent CD46 binding of clinical isolates of Streptococci confirms the importance of CD46 as a cell target that might confer pathogens some biological advantages over the host.

Complement in lung disease

Complement proteins play an integral role in both innate and adaptive immune responses of the host. Complement activation leads to the formation of bioactive molecules including the anaphylatoxins, C3a and C5a, and the lytic membrane attack complex (C5b-9). These molecules trigger a series of events that culminate in the recruitment of phagocytic cells, release of cytokines/chemokines and reactive oxygen species, enhanced expression of adhesion molecules and apoptosis at the site of inflammation. Several animal models provide evidence that this series of events forms the basis for the pathophysiology found in many lung diseases, such as asthma and acute respiratory distress syndrome. Clinical data further confirm these findings. This review briefly discusses recent data from such studies.

CD46 (Membrane Cofactor Protein) Acts as a Human Epithelial Cell Receptor for Internalization of Opsonized UropathogenicEscherichia coli

Escherichia coli is a common urinary pathogen whose uptake into epithelial cells is mediated by attachment through type 1 fimbriae. In this study, we show by using using human urinary tract epithelial cells that maximal internalization of E. coli is achieved only when bacteria are opsonized with complement. The concentrations of complement proteins in the urine rise sufficiently during infection to allow bacterial opsonization. The complement regulatory protein, CD46 (membrane cofactor protein), acts in cohort with fimbrial adhesion to promote the uptake of pathogenic E. coli. This uptake is inhibited by RNA interference to lower the expression of CD46 and by soluble CD46 that will competitively inhibit opsonized bacteria binding to cell surface CD46. We propose that efficient internalization of uropathogenic E. coli by the human urinary tract depends on cooperation between fimbrial-mediated adhesion and C3 receptor (CD46)-ligand interaction. Complement receptor-ligand interaction could pose a new target for interrupting the cycle of reinfection due to intracellular bacteria.

Magnetic resonance imaging of Klebsiella pneumoniae-induced pneumonia in mice

In vivo imaging of small animals is a rapidly developing field. However, the potential of global imaging of infectious processes in animal models remains poorly explored. We used magnetic resonance imaging (MRI) to follow the development and regression of inflammatory lesions caused by infection by Klebsiella pneumoniae in mouse lungs. A virulent strain caused an intense inflammation within 2 days in the whole lungs, while an avirulent strain did not show significant changes. Mice infected with the virulent strain and subsequently treated with antibiotics presented a severe inflammation localized mainly in the left lung that disappeared after a week. The lesions observed by MRI correlated with the damage seen by histological analysis and a 3D representation of the tissue allowed better visualization of the development and healing of inflammatory lesions. MRI thus represents a powerful technique to study in vivo the interactions between a pathogen and its host in real time.