A fine-tuned interaction between trimeric autotransporter haemophilus surface fibrils and vitronectin leads to serum resistance and adherence to respiratory epithelial cells

Recruitment of Vitronectin by Bacterial Pathogens: A Comprehensive Overview

The key factor that enables pathogenic bacteria to establish successful infections lies largely in their ability to escape the host's immune response and adhere to host surfaces. Vitronectin (Vn) is a multidomain glycoprotein ubiquitously present in blood and the extracellular matrix of several tissues, where it plays important roles as a regulator of membrane attack complex (MAC) formation and as a mediator of cell adhesion. Vn has emerged as an intriguing target for several microorganisms. Vn binding by bacterial receptors confers protection from lysis resulting from MAC deposition. Furthermore, through its Arg-Gly-Asp (RGD) motif, Vn can bind several host cell integrins. Therefore, Vn recruited to the bacterial cell functions as a molecular bridge between bacteria and host surfaces, where it triggers several host signaling events that could promote bacterial internalization. Each bacterium uses different receptors that recognize specific Vn domains. In this review, we update the current knowledge of Vn receptors of major bacterial pathogens, emphasizing the role they may play in the host upon Vn binding. Focusing on the structural properties of bacterial proteins, we provide details on the residues involved in their interaction with Vn. Furthermore, we discuss the possible involvement of Vn adsorption on biomaterials in promoting bacterial adhesion on abiotic surfaces and infection.

Epidemic Trends and Biofilm Formation Mechanisms of Haemophilus influenzae: Insights into Clinical Implications and Prevention Strategies

Haemophilus influenzae (H. influenzae) is a significant pathogen responsible for causing respiratory tract infections and invasive diseases, leading to a considerable disease burden. The Haemophilus influenzae type b (Hib) conjugate vaccine has notably decreased the incidence of severe infections caused by Hib strains, and other non-typable H. influenzae (NTHi) serotypes have emerged as epidemic strains worldwide. As a result, the global epidemic trends and antibiotic resistance characteristics of H. influenzae have been altered. Researches on the virulence factors of H. influenzae, particularly the mechanisms underlying biofilm formation, and the development of anti-biofilm strategies hold significant clinical value. This article provides a summary of the epidemic trends, typing methods, virulence factors, biofilm formation mechanisms, and prevention strategies of H. influenzae. The increasing prevalence of NTHi strains and antibiotic resistance among H. influenzae, especially the high β-lactamase positivity and the emergence of BLNAR strains have increased clinical difficulties. Understanding its virulence factors, especially the formation mechanism of biofilm, and formulating effective anti-biofilm strategies may help to reduce the clinical impact. Therefore, future research efforts should focus on developing new approaches to prevent and control H. influenzae infections.

Dynamic immune response characteristics of piglets infected with Actinobacillus pleuropneumoniae through omic

Porcine infectious pleuropneumonia is characterized by a high-rate of carriage and mixed infection with other pathogens. The host immune response induced by Actinobacillus pleuropneumoniae (APP) is the basis for elucidating pathogenesis and controlling disease. However, there is currently no comprehensive and dynamic data characterising the host immune response. In this study, piglets were infected with APP and differentially expressed proteins of bronchoalveolar lavage fluid (BALF) and peripheral serum were identified by iTRAQ-LC-MS/MS, and differentially expressed genes of peripheral blood mononuclear cells (PBMC) by RNA-seq. The results of the integrated analysis of serum, BALF and PBMC showed significant metabolism and local immune responses in BALF, the general immune response in PBMC mainly involves cytokines, while that in serum mainly involves biosynthesis, phagosome, and complement and coagulation cascades. Furthermore, immune responses in PBMCs and serum were rapid and maintained compared to the lung where metabolism and cell adhesion activities were enriched. Some innate immunity pathways of the cellular response to ROS, neutrophil mediated immunity, granulocyte activation and leukocyte cell-cell adhesion were identified as central points, connecting multiple signaling pathways to form an integrated large network. At 24 h post-infection, 14 molecules were up regulated in BALF, 10 of which were shared with PBMC, but at 120 h, 20 down-regulated molecules were identified in BALF, 11 of them still up- regulated in PBMC. We conclude that, the immune response in the lung is different from that in blood, but there is a similarity in response in PBMC and serum.

Immunogenicity of trimeric autotransporter adhesins and their potential as vaccine targets

The current problem of increasing antibiotic resistance and the resurgence of numerous infections indicate the need for novel vaccination strategies more than ever. In vaccine development, the search for and the selection of adequate vaccine antigens is the first important step. In recent years, bacterial outer membrane proteins have become of major interest, as they are the main proteins interacting with the extracellular environment. Trimeric autotransporter adhesins (TAAs) are important virulence factors in many Gram-negative bacteria, are localised on the bacterial surface, and mediate the first adherence to host cells in the course of infection. One example is the Neisseria adhesin A (NadA), which is currently used as a subunit in a licensed vaccine against Neisseria meningitidis. Other TAAs that seem promising vaccine candidates are the Acinetobacter trimeric autotransporter (Ata), the Haemophilus influenzae adhesin (Hia), and TAAs of the genus Bartonella. Here, we review the suitability of various TAAs as vaccine candidates.

Complement evasion by the human respiratory tract pathogens Haemophilus influenzae and Moraxella catarrhalis

All infective bacterial species need to conquer the innate immune system in order to colonize and survive in their hosts. The human respiratory pathogens Haemophilus influenzae and Moraxella catarrhalis are no exceptions and have developed sophisticated mechanisms to evade complement-mediated killing. Both bacterial species carry lipooligosaccharides preventing complement attacks and attract and utilize host complement regulators C4b binding protein and factor H to inhibit the classical and alternative pathways of complement activation, respectively. In addition, the regulator of the terminal pathway of complement activation, vitronectin, is hijacked by both bacteria. An array of different outer membrane proteins (OMP) in H. influenzae and M. catarrhalis simultaneously binds complement regulators, but also plasminogen. Several of the bacterial complement-binding proteins are important adhesins and contain highly conserved regions for interactions with the host. Thus, some of the OMP are viable targets for new therapeutics, including vaccines aimed at preventing respiratory tract diseases such as otitis media in children and exacerbations in patients suffering from chronic obstructive pulmonary disease.

Structural diversity of coiled coils in protein fibers of the bacterial cell envelope

The cell envelope of bacteria shows great diversity in architecture and composition, to a large extent due to its proteome. Proteins localized to the cell envelope, whether integrally embedded in the membrane, membrane-anchored, or peripherally associated as part of a macromolecular complex, often form elongated fibers, in which coiled coils represent a prominent structural element. These coiled-coil segments show a surprising degree of structural variability, despite being shaped by a small number of simple biophysical rules, foremost being their geometry of interaction referred to as 'knobs-into-holes'. Here we will review this diversity, particularly as it has emerged over the last decade.

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.

Assays for Studying the Role of Vitronectin in Bacterial Adhesion and Serum Resistance

Bacteria utilize complement regulators as a means of evading the host immune response. Here, we describe protocols for evaluating the role vitronectin acquisition at the bacterial cell surface plays in resistance to the host immune system. Flow cytometry experiments identified human plasma vitronectin as a ligand for the bacterial receptor outer membrane protein H of Haemophilus influenzae type f. An enzyme-linked immunosorbent assay was employed to characterize the protein-protein interactions between purified recombinant protein H and vitronectin, and binding affinity was assessed using bio-layer interferometry. The biological importance of the binding of vitronectin to protein H at the bacterial cell surface in evasion of the host immune response was confirmed using a serum resistance assay with normal and vitronectin-depleted human serum. The importance of vitronectin in bacterial adherence was analyzed using glass slides with and without vitronectin coating, followed by Gram staining. Finally, bacterial adhesion to human alveolar epithelial cell monolayers was investigated. The protocols described here can be easily adapted to the study of any bacterial species of interest.

The Streptococcus agalactiae cell wall-anchored protein PbsP mediates adhesion to and invasion of epithelial cells by exploiting the host vitronectin/αv integrin axis

Binding of microbial pathogens to host vitronectin (Vtn) is a common theme in the pathogenesis of invasive infections. In this study, we characterized the role of Vtn in the invasion of mucosal epithelial cells by Streptococcus agalactiae (i.e. group B streptococcus or GBS), a frequent human pathogen. Moreover, we identified PbsP, a previously described plasminogen-binding protein of GBS, as a dual adhesin that can also interact with human Vtn through its streptococcal surface repeat (SSURE) domains. Deletion of the pbsP gene decreases both bacterial adhesion to Vtn-coated inert surfaces and the ability of GBS to interact with epithelial cells. Bacterial adherence to and invasion of epithelial cells were either inhibited or enhanced by cell pretreatment with, respectively, anti-Vtn antibodies or Vtn, confirming the role of Vtn as a GBS ligand on host cells. Finally, antibodies directed against the integrin α_v subunit inhibited Vtn-dependent cell invasion by GBS. Collectively, these results indicate that Vtn acts as a bridge between the SSURE domains of PbsP on the GBS surface and host integrins to promote bacterial invasion of epithelial cells. Therefore, inhibition of interactions between PbsP and extracellular matrix components could represent a viable strategy to prevent colonization and invasive disease by GBS.

Bacterial Outer Membrane Vesicles Induce Vitronectin Release Into the Bronchoalveolar Space Conferring Protection From Complement-Mediated Killing

Pathogens causing pneumonia utilize the complement regulator vitronectin to evade complement-mediated killing. Although vitronectin is associated with several chronic lung diseases, the role of bronchoalveolar vitronectin in pneumonia has not been studied. This study sought to reveal the involvement of vitronectin in the bronchoalveolar space during pneumonia, to assess the effect of outer membrane vesicles and endotoxin on vitronectin release, and to determine whether bacterial pathogens utilize pulmonary vitronectin for evasion. Vitronectin was analyzed in cell-free bronchoalveolar lavage fluid harvested from patients with pneumonia (n = 8) and from healthy volunteers after subsegmental endotoxin instillation (n = 13). Vitronectin binding by Pseudomonas aeruginosa and Haemophilus influenzae was analyzed, and subsequent complement evasion was assessed by serum challenge. The effects of outer membrane vesicles on vitronectin production in mouse lungs and human type II alveolar epithelial cells (A549) were determined. We detected increased vitronectin concentrations in lavage fluid during pneumonia (p = 0.0063) and after bronchial endotoxin challenge (p = 0.016). The capture of vitronectin by bacteria significantly reduced complement-mediated lysis. Following challenge with vesicles, vitronectin was detected in mouse bronchoalveolar space, and mouse alveolar epithelial cells in vivo as well as A549 cells in vitro contained increased levels of vitronectin. Taken together, outer membrane vesicles and endotoxin from Gram-negative bacteria induce vitronectin, which is released into the bronchoalveolar space, and used for evasion of complement-mediated clearance.

Bioinformatic analysis of meningococcal Msf and Opc to inform vaccine antigen design

Neisseria meningitidis is an antigenically and genetically variable Gram-negative bacterium and a causative agent of meningococcal meningitis and septicaemia. Meningococci encode many outer membrane proteins, including Opa, Opc, Msf, fHbp and NadA, identified as being involved in colonisation of the host and evasion of the immune response. Although vaccines are available for the prevention of some types of meningococcal disease, none currently offer universal protection. We have used sequences within the Neisseria PubMLST database to determine the variability of msf and opc in 6,500 isolates. In-silico analysis revealed that although opc is highly conserved, it is not present in all isolates, with most isolates in clonal complex ST-11 lacking a functional opc. In comparison, msf is found in all meningococcal isolates, and displays diversity in the N-terminal domain. We identified 20 distinct Msf sequence variants (Msf SV), associated with differences in number of residues within the putative Vn binding motifs. Moreover, we showed distinct correlations with certain Msf SVs and isolates associated with either hyperinvasive lineages or those clonal complexes associated with a carriage state. We have demonstrated differences in Vn binding between three Msf SVs and generated a cross reactive Msf polyclonal antibody. Our study has highlighted the importance of using large datasets to inform vaccine development and provide further information on the antigenic diversity exhibited by N. meningitidis.

The adhesins of non-typeable Haemophilus influenzae

INTRODUCTION

Nontypeable Haemophilus influenzae (NTHi) is an opportunistic pathogen of the respiratory tract and the greatest contributor to invasive Haemophilus disease. Additionally, in children, NTHi is responsible for the majority of otitis media (OM) which can lead to chronic infection and hearing loss. In adults, NTHi infection in the lungs is responsible for the onset of acute exacerbations in chronic obstructive pulmonary disease (COPD). Unfortunately, there is currently no vaccine available to protect against NTHi infections. Areas covered: NTHi uses an arsenal of adhesins to colonise the respiratory epithelium. The adhesins also have secondary roles that aid in the virulence of NTHi, including mechanisms that avoid immune clearance, adjust pore size to avoid antimicrobial destruction, form micro-colonies and invoke phase variation for protein mediation. Bacterial adhesins can also be ideal antigens for subunit vaccine design due to surface exposure and immunogenic capabilities. Expert commentary: The host-pathogen interactions of the NTHi adhesins are not fully investigated. The relationship between adhesins and the extracellular matrix (ECM) play a part in the success of NTHi colonisation and virulence by immune evasion, migration and biofilm development. Further research into these immunogenic proteins would further our understanding and enable a basis for better combatting NTHi disease.

RNA gene profile variation in peripheral blood mononuclear cells from rhesus macaques immunized with Hib conjugate vaccine, Hib capsular polysaccharide and TT carrier protein

Background

The Haemophilus influenzae type b (Hib) conjugate vaccine has been widely used in children to prevent invasive Hib disease because of its strong immunogenicity and antibody response induction relative to the capsular polysaccharide (CPS) antigen. The data from vaccine studies suggest that the conjugate vaccine contains carrier proteins that enhance and/or regulate the antigen’s immunogenicity, but the mechanism of this enhancement remains unclear.

Methods

To explore the immunological role of the conjugate vaccine, we compared the immune responses and gene profiles of rhesus macaques after immunization with CPS, carrier protein tetanus toxoid (TT) or conjugate vaccine.

Results

A distinct immune response was induced by the Hib conjugate vaccine but not by CPS or carrier protein TT. The genes that were dynamically regulated in conjunction with the macaque immune responses to the conjugate vaccine were investigated.

Conclusions

We propose that these genes are involved in the induction of specific immunity that is characterized by the appearance and maintenance of antibodies against Hib.

Electronic supplementary material

The online version of this article (10.1186/s12865-018-0240-5) contains supplementary material, which is available to authorized users.

The extended leader peptide of Haemophilus parasuis trimeric autotransporters conditions their protein expression in Escherichia coli

Trimeric autotransporters are surface-exposed proteins of Gram-negative bacteria belonging to the type V secretion system. They are involved in virulence and are targets for vaccine and diagnostic tool development, so optimal systems for their expression and purification are required. In the present study, the impact of the extended leader peptide of the Haemophilus parasuis virulence-associated trimeric autotransporters (VtaA) in its production as recombinant proteins in Escherichia coli was evaluated. The 13 genes encoding the VtaA1 to VtaA13 passenger domains of the strain Nagasaki were cloned in the pASK-IBA33plus plasmid and expressed in E. coli. Recombinant protein production was higher for truncated forms in which the entire leader peptide was deleted, and the recombinant protein accumulated in the cytoplasm of the cells. The yield of protein production of the different VtaAs was size dependent, and reached maximal amount at 2-4 h post -induction. The optimization of these conditions allowed to scale-up the production to obtain enough recombinant protein to immunize large animals.

The crystal structure of PD1, a Haemophilus surface fibril domain

The Haemophilus surface fibril (Hsf) is an unusually large trimeric autotransporter adhesin (TAA) expressed by the most virulent strains of H. influenzae. Hsf is known to mediate adhesion between pathogen and host, allowing the establishment of potentially deadly diseases such as epiglottitis, meningitis and pneumonia. While recent research has suggested that this TAA might adopt a novel `hairpin-like' architecture, the characterization of Hsf has been limited to in silico modelling and electron micrographs, with no high-resolution structural data available. Here, the crystal structure of Hsf putative domain 1 (PD1) is reported at 3.3 Å resolution. The structure corrects the previous domain annotation by revealing the presence of an unexpected N-terminal TrpRing domain. PD1 represents the first Hsf domain to be solved, and thus paves the way for further research on the `hairpin-like' hypothesis.

Hijacking Complement Regulatory Proteins for Bacterial Immune Evasion

The human complement system plays an important role in the defense against invading pathogens, inflammation and homeostasis. Invading microbes, such as bacteria, directly activate the complement system resulting in the formation of chemoattractants and in effective labeling of the bacteria for phagocytosis. In addition, formation of the membrane attack complex is responsible for direct killing of Gram-negative bacteria. In turn, bacteria have evolved several ways to evade complement activation on their surface in order to be able to colonize and invade the human host. One important mechanism of bacterial escape is attraction of complement regulatory proteins to the microbial surface. These molecules are present in the human body for tight regulation of the complement system to prevent damage to host self-surfaces. Therefore, recruitment of complement regulatory proteins to the bacterial surface results in decreased complement activation on the microbial surface which favors bacterial survival. This review will discuss recent advances in understanding the binding of complement regulatory proteins to the bacterial surface at the molecular level. This includes, new insights that have become available concerning specific conserved motives on complement regulatory proteins that are favorable for microbial binding. Finally, complement evasion molecules are of high importance for vaccine development due to their dominant role in bacterial survival, high immunogenicity and homology as well as their presence on the bacterial surface. Here, the use of complement evasion molecules for vaccine development will be discussed.

Vitronectin Binds to a Specific Stretch within the Head Region of Yersinia Adhesin A and Thereby Modulates Yersinia enterocolitica Host Interaction

Complement resistance is an important virulence trait of Yersinia enterocolitica (Ye). The predominant virulence factor expressed by Ye is Yersinia adhesin A (YadA), which enables bacterial attachment to host cells and extracellular matrix and additionally allows the acquisition of soluble serum factors. The serum glycoprotein vitronectin (Vn) acts as an inhibitory regulator of the terminal complement complex by inhibiting the lytic pore formation. Here, we show YadA-mediated direct interaction of Ye with Vn and investigated the role of this Vn binding during mouse infection in vivo. Using different Yersinia strains, we identified a short stretch in the YadA head domain of Ye O:9 E40, similar to the 'uptake region' of Y. pseudotuberculosis YPIII YadA, as crucial for efficient Vn binding. Using recombinant fragments of Vn, we found the C-terminal part of Vn, including heparin-binding domain 3, to be responsible for binding to YadA. Moreover, we found that Vn bound to the bacterial surface is still functionally active and thus inhibits C5b-9 formation. In a mouse infection model, we demonstrate that Vn reduces complement-mediated killing of Ye O:9 E40 and, thus, improved bacterial survival. Taken together, these findings show that YadA-mediated Vn binding influences Ye pathogenesis.

Host-pathogen interactions of nontypeable Haemophilus influenzae: from commensal to pathogen

Nontypeable Haemophilus influenzae (NTHi) is a commensal microbe often isolated from the upper and lower respiratory tract. This bacterial species can cause sinusitis, acute otitis media in preschool children, exacerbations in patients suffering from chronic obstructive pulmonary disease, as well as conjunctivitis and bacteremia. Since the introduction of a vaccine against H. influenzae serotype b in the 1990s, the burden of H. influenzae-related infections has been increasingly dominated by NTHi. Understanding the ability of NTHi to cause infection is currently an expanding area of study. NTHi is able to exert differential binding to the host tissue through the use of a broad range of adhesins. NTHi survival in the host is multifaceted, that is, using virulence factors involved in complement resistance, biofilm, modified immunoglobulin responses, and, finally, formation and utilization of host proteins as a secondary strategy of increasing the adhesive ability.

Conserved Patterns of Microbial Immune Escape: Pathogenic Microbes of Diverse Origin Target the Human Terminal Complement Inhibitor Vitronectin via a Single Common Motif

Pathogenicity of many microbes relies on their capacity to resist innate immunity, and to survive and persist in an immunocompetent human host microbes have developed highly efficient and sophisticated complement evasion strategies. Here we show that different human pathogens including Gram-negative and Gram-positive bacteria, as well as the fungal pathogen Candida albicans, acquire the human terminal complement regulator vitronectin to their surface. By using truncated vitronectin fragments we found that all analyzed microbial pathogens (n = 13) bound human vitronectin via the same C-terminal heparin-binding domain (amino acids 352-374). This specific interaction leaves the terminal complement complex (TCC) regulatory region of vitronectin accessible, allowing inhibition of C5b-7 membrane insertion and C9 polymerization. Vitronectin complexed with the various microbes and corresponding proteins was thus functionally active and inhibited complement-mediated C5b-9 deposition. Taken together, diverse microbial pathogens expressing different structurally unrelated vitronectin-binding molecules interact with host vitronectin via the same conserved region to allow versatile control of the host innate immune response.

Nontypeable Haemophilus influenzae exploits the interaction between protein-E and vitronectin for the adherence and invasion to bronchial epithelial cells

Background

Nontypeable Haemophilus influenzae (NTHi) is one of the most common Gram-negative pathogens in otitis media and exacerbation of chronic obstructive pulmonary disease. NTHi has been reported to invade bronchial epithelial cells. This penetration enables NTHi to evade the host immune system and antibiotics, and it seems to be related to the intractable features of these diseases. However, the precise mechanism of the invasion has been unknown. We hypothesized that protein-E, an outer membrane protein of NTHi, plays a role in this penetration into bronchial epithelial cells.

Results

We utilized two NTHi strains. NTHi efficiently attached to plate-bound vitronectin (254–309 / field at 1,000× magnification) and this attachment was blocked by pretreatment with protein-E peptide (PE^84–108). The blockade of adhesion was dependent on the concentration of PE^84–108. NTHi strains invaded bronchial epithelial cells and the intracellular bacteria were localized in early endosomes. Furthermore, intracellular invasion of NTHi was also blocked by PE^84–108, but not by Arg-Gly-Asp (RGD) peptide. Pretreatment with PE^84–108 significantly prevented cells from being invaded by both NTHi strains, which was confirmed by fluorescent microscope observation. In addition, pretreatment with PE^84–108 significantly reduced percentages of CFU after gentamicin treatment of cells per input CFU.

Conclusions

These results suggest that NTHi does not directly bind to the cell surface, but binds to host vitronectin that is bound to the cell surface, via bacterial protein-E. Bacterial protein-E and host vitronectin play a role in the attachment to bronchial epithelial cells and is also involved in the subsequent intracellular invasion of NTHi. A novel vaccine or treatment strategy targeting the protein-E-vitronectin axis may prevent respiratory intracellular infection of NTHi and may lead to better clinical outcomes.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-015-0600-8) contains supplementary material, which is available to authorized users.

Haemophilus influenzae Type f Hijacks Vitronectin Using Protein H To Resist Host Innate Immunity and Adhere to Pulmonary Epithelial Cells

The incidence of invasive Haemophilus influenzae type b (Hib) disease has significantly decreased since the introduction of an efficient vaccine against Hib. However, in contrast to Hib, infections caused by H. influenzae serotype f (Hif) are emerging. We recently did a whole genome sequencing of an invasive Hif isolate, and reported that Hif interacts with factor H by expressing protein H (PH). In this study, upon screening with various human complement regulators, we revealed that PH is also a receptor for vitronectin (Vn), an abundant plasma protein that regulates the terminal pathway of the human complement system in addition to being a component of the extracellular matrix. Bacterial Vn binding was significantly reduced when the lph gene encoding PH was deleted in an invasive Hif isolate. The dissociation constant (KD) of the interaction between recombinant PH and Vn was 2.2 μM, as revealed by Biolayer interferometry. We found that PH has different regions for simultaneous interaction with both Vn and factor H, and that it recognized the C-terminal part of Vn (aa 352-362). Importantly, PH-dependent Vn binding resulted in better survival of the wild-type Hif or PH-expressing Escherichia coli when exposed to human serum. Finally, we observed that PH mediated an increased bacterial adherence to alveolar epithelial cells in the presence of Vn. In conclusion, our study reveals that PH most likely plays an important role in Hif pathogenesis by increasing serum resistance and adhesion to the airways.

Haemophilus influenzae P4 Interacts With Extracellular Matrix Proteins Promoting Adhesion and Serum Resistance

Interaction with the extracellular matrix (ECM) is one of the successful colonization strategies employed by nontypeable Haemophilus influenzae (NTHi). Here we identified Haemophilus lipoprotein e (P4) as a receptor for ECM proteins. Purified recombinant P4 displayed a high binding affinity for laminin (Kd = 9.26 nM) and fibronectin (Kd = 10.19 nM), but slightly less to vitronectin (Kd = 16.51 nM). A P4-deficient NTHi mutant showed a significantly decreased binding to these ECM components. Vitronectin acquisition conferred serum resistance to both P4-expressing NTHi and Escherichia coli transformants. P4-mediated bacterial adherence to pharynx, type II alveolar, and bronchial epithelial cells was mainly attributed to fibronectin. Importantly, a significantly reduced bacterial infection was observed in the middle ear of the Junbo mouse model when NTHi was devoid of P4. In conclusion, our data provide new insight into the role of P4 as an important factor for Haemophilus colonization and subsequent respiratory tract infection.

Moraxella catarrhalis Binds Plasminogen To Evade Host Innate Immunity

Several bacterial species recruit the complement regulators C4b-binding protein, factor H, and vitronectin, resulting in resistance against the bactericidal activity of human serum. It was recently demonstrated that bacteria also bind plasminogen, which is converted to plasmin that degrades C3b and C5. In this study, we found that a series of clinical isolates (n = 58) of the respiratory pathogen Moraxella catarrhalis, which is commonly isolated from preschool children and adults with chronic obstructive pulmonary disease (COPD), significantly binds human plasminogen. Ubiquitous surface protein A2 (UspA2) and hybrid UspA2 (UspA2H) were identified as the plasminogen-binding factors in the outer membrane proteome of Moraxella. Furthermore, expression of a series of truncated recombinant UspA2 and UspA2H proteins followed by a detailed analysis of protein-protein interactions suggested that the N-terminal head domains bound to the kringle domains of plasminogen. The binding affinity constant (KD) values of full-length UspA2(30-539) (amino acids 30 to 539 of UspA2) and full-length UspA2H(50-720) for immobilized plasminogen were 4.8 × 10(-8) M and 3.13 × 10(-8) M, respectively, as measured by biolayer interferometry. Plasminogen bound to intact M. catarrhalis or to recombinant UspA2/UspA2H was readily accessible for a urokinase plasminogen activator that converted the zymogen into active plasmin, as verified by the specific substrate S-2251 and a degradation assay with fibrinogen. Importantly, plasmin bound at the bacterial surface also degraded C3b and C5, which consequently may contribute to reduced bacterial killing. Our findings suggest that binding of plasminogen to M. catarrhalis may lead to increased virulence and, hence, more efficient colonization of the host.

Identification of outer membrane Porin D as a vitronectin-binding factor in cystic fibrosis clinical isolates of Pseudomonas aeruginosa

BACKGROUND

Pseudomonas aeruginosa is a pathogen that frequently colonizes patients with cystic fibrosis (CF) or chronic obstructive pulmonary disease (COPD). Several pathogens are known to bind vitronectin to increase their virulence. Vitronectin has been shown to enhance P. aeruginosa adhesion to host epithelial cells.

METHODS

We screened clinical isolates from the airways of CF patients and from the bloodstream of patients with bacteremia for binding of vitronectin. Two-dimensional SDS-PAGE and a proteomic approach were used to identify vitronectin-receptors in P. aeruginosa.

RESULTS

P. aeruginosa from the airways of CF patients (n=27) bound more vitronectin than bacteremic isolates (n=15, p=0.025). Porin D (OprD) was identified as a vitronectin-binding protein. A P. aeruginosa oprD transposon insertion mutant had a decreased binding to soluble and immobilized vitronectin (p≤0.001).

CONCLUSIONS

P. aeruginosa isolates obtained from CF patients significantly bound vitronectin. Porin D was defined as a novel P. aeruginosa vitronectin-receptor, and we postulate that the Porin D-dependent interaction with vitronectin may be important for colonization.

New findings on the function and potential applications of the trimeric autotransporter adhesin

Trimeric autotransporter adhesins (TAAs) are located on the surface of many pathogenic Gram-negative bacteria. TAAs belong to the autotransporter protein family and consist of three identical monomers. These obligate homotrimeric proteins are secreted through the bacterial type Vc secretion system and share a common molecular organization that each monomer consists of a N-terminal "passenger" domain and a C-terminal translocation domain. TAAs are important virulence factors that are involved in bacterial life cycle and participate in mediating infection, invasion, dissemination and evasion of host immune responses. TAAs have also proved to be useful for many applications, such as vaccines and disease biomarkers. We here mainly focused on new findings on bio-function and application of TAAs in addition to their common structure and secretion mechanisms.

Haemophilus influenzae surface fibril (Hsf) is a unique twisted hairpin-like trimeric autotransporter

The Haemophilus surface fibril (Hsf) is an extraordinary large (2413 amino acids) trimeric autotransporter, present in all encapsulated Haemophilus influenzae. It contributes to virulence by directly functioning as an adhesin. Furthermore, Hsf recruits the host factor vitronectin thereby inhibiting the host innate immune response resulting in enhanced survival in serum. Here we observed by electron microscopy that Hsf appears as an 100 nm long fibril at the bacterial surface albeit the length is approximately 200 nm according to a bioinformatics based model. To unveil this discrepancy, we denaturated Hsf at the surface of Hib by using guanidine hydrochloride (GuHCl). Partial denaturation induced in the presence of GuHCl unfolded the Hsf molecules, and resulted in an increased length of fibres in comparison to the native trimeric form. Importantly, our findings were also verified by E. coli expressing Hsf at its surface. In addition, a set of Hsf-specific peptide antibodies also indicated that the N-terminal of Hsf is located near the C-terminal at the base of the fibril. Taken together, our results demonstrated that Hsf is not a straight molecule but is folded and doubled over. This is the first report that provides the unique structural features of the trimeric autotransporter Hsf.

Importance of (antibody-dependent) complement-mediated serum killing in protection against Bordetella pertussis

Pertussis is a highly contagious respiratory disease that is caused by Bordetella pertussis. Despite being vaccine preventable, pertussis rates have been rising steadily over the last decades, even in areas with high vaccine uptake. Recently, experiments with infant baboons indicated that although vaccination with acellular pertussis vaccines prevented disease, no apparent effect was observed on infection and transmission. One explanation may be that current acellular pertussis vaccines do not induce high levels of opsonophagocytic and/or bactericidal activity, implying that engineering of vaccines that promote bacterial killing may improve efficacy. Here, we discuss the importance of complement-mediated killing in vaccine-induced protection against B. pertussis. We first examine how B. pertussis may have evolved different complement evasion strategies. Second, we explore the benefits of opsonophagocytic and/or bactericidal killing in vaccine-induced protection and discuss whether or not inclusion of new opsonophagocytic or bactericidal target antigens in pertussis vaccines may benefit efficacy.