Haemophilus influenzae uses the surface protein E to acquire human plasminogen and to evade innate immunity

The emerging role of tranexamic acid and its principal target, plasminogen, in skeletal health

The worldwide burden of skeletal diseases such as osteoporosis, degenerative joint disease and impaired fracture healing is steadily increasing. Tranexamic acid (TXA), a plasminogen inhibitor and anti-fibrinolytic agent, is used to reduce bleeding with high effectiveness and safety in major surgical procedures. With its widespread clinical application, the effects of TXA beyond anti-fibrinolysis have been noticed and prompted renewed interest in its use. Some clinical trials have characterized the effects of TXA on reducing postoperative infection rates and regulating immune responses in patients undergoing surgery. Also, several animal studies suggest potential therapeutic effects of TXA on skeletal diseases such as osteoporosis and fracture healing. Although a direct effect of TXA on the differentiation and function of bone cells in vitro was shown, few mechanisms of action have been reported. Here, we summarize recent findings of the effects of TXA on skeletal diseases and discuss the underlying plasminogen-dependent and -independent mechanisms related to bone metabolism and the immune response. We furthermore discuss potential novel indications for TXA application as a treatment strategy for skeletal diseases.

NADH oxidase of Mycoplasma synoviae is a potential diagnostic antigen, plasminogen/fibronectin binding protein and a putative adhesin

BACKGROUND

Mycoplasma synoviae (MS) is an important pathogen causing respiratory diseases and arthritis in chickens and turkeys, thus, resulting in serious economic losses to the poultry industry. Membrane-associated proteins are thought to play important roles in cytoadherence and pathogenesis. NADH oxidase (NOX) is an oxidoreductase involved in glycolysis, which is thought to be a multifunctional protein and potential virulence factor in some pathogens. However, little is known regarding the NOX of MS (MSNOX). We previously demonstrated that MSNOX was a metabolic enzyme distributed in not only the cytoplasm but also the MS membrane. This study was aimed at exploring NOX's potential as a diagnostic antigen and its role in MS cytoadherence.

RESULTS

Western blots and ELISAs indicated that recombinant MSNOX (rMSNOX) protein reacted with sera positive for various MS isolates, but not MG isolates or other avian pathogens, thus, suggesting that rMSNOX is a potential diagnostic antigen. In addition, rabbit anti-rMSNOX serum showed substantial complement-dependent mycoplasmacidal activity toward various MS isolates and MG R_low. MSNOX protein was found not only in the cytoplasm but also on the membrane of MS through suspension immunofluorescence and immunogold electron microscopy assays. Indirect immunofluorescence assays indicated that rMSNOX adhered to DF-1 cells, and this adherence was inhibited by rabbit anti-rMSNOX, but not anti-MG serum. Furthermore, indirect immunofluorescence and colony counting assays confirmed that the rabbit anti-rMSNOX serum inhibited the adherence of various MS isolates but not MG R_low to DF-1 cells. Moreover, plasminogen (Plg)- and fibronectin (Fn)-binding assays demonstrated that rMSNOX bound Plg and Fn in a dose-dependent manner, thereby further confirming that MSNOX may be a putative adhesin.

CONCLUSION

MSNOX was identified to be a surface immunogenic protein that has good immunoreactivity and specificity in Western blot and ELISA, and therefore, may be used as a potential diagnostic antigen in the future. In addition, rMSNOX adhered to DF-1 cells, an effect inhibited by rabbit anti-rMSNOX, but not anti-MG serum, and anti-rMSNOX serum inhibited the adherence of various MS isolates, but not MG R_low, to DF-1 cells, thus indicating that the inhibition of adherence by anti-MSNOX serum was MS specific. Moreover, rMSNOX adhered to extracellular matrix proteins including Plg and Fn, thus suggesting that NOX may play important roles in MS cytoadherence and pathogenesis. Besides, rabbit anti-rMSNOX serum presented complement-dependent mycoplasmacidal activity toward both MS and MG, indicating the MSNOX may be further studied as a potential protective vaccine candidate.

The Exploration of Complement-Resistance Mechanisms of Pathogenic Gram-Negative Bacteria to Support the Development of Novel Therapeutics

Resistance to antibiotics in Bacteria is one of the biggest threats to human health. After decades of attempting to isolate or design antibiotics with novel mechanisms of action against bacterial pathogens, few approaches have been successful. Antibacterial drug discovery is now moving towards targeting bacterial virulence factors, especially immune evasion factors. Gram-negative bacteria present some of the most significant challenges in terms of antibiotic resistance. However, they are also able to be eliminated by the component of the innate immune system known as the complement system. In response, Gram-negative bacteria have evolved a variety of mechanisms by which they are able to evade complement and cause infection. Complement resistance mechanisms present some of the best novel therapeutic targets for defending against highly antibiotic-resistant pathogenic bacterial infections.

An invasive Haemophilus influenzae serotype b infection in an Anglo-Saxon plague victim

Background

The human pathogen Haemophilus influenzae was the main cause of bacterial meningitis in children and a major cause of worldwide infant mortality before the introduction of a vaccine in the 1980s. Although the occurrence of serotype b (Hib), the most virulent type of H. influenzae, has since decreased, reports of infections with other serotypes and non-typeable strains are on the rise. While non-typeable strains have been studied in-depth, very little is known of the pathogen’s evolutionary history, and no genomes dating prior to 1940 were available.

Results

We describe a Hib genome isolated from a 6-year-old Anglo-Saxon plague victim, from approximately 540 to 550 CE, Edix Hill, England, showing signs of invasive infection on its skeleton. We find that the genome clusters in phylogenetic division II with Hib strain NCTC8468, which also caused invasive disease. While the virulence profile of our genome was distinct, its genomic similarity to NCTC8468 points to mostly clonal evolution of the clade since the 6th century. We also reconstruct a partial Yersinia pestis genome, which is likely identical to a published first plague pandemic genome of Edix Hill.

Conclusions

Our study presents the earliest genomic evidence for H. influenzae, points to the potential presence of larger genomic diversity in the phylogenetic division II serotype b clade in the past, and allows the first insights into the evolutionary history of this major human pathogen. The identification of both plague and Hib opens questions on the effect of plague in immunocompromised individuals already affected by infectious diseases.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-021-02580-z.

Lipoprotein(a), an Opsonin, Enhances the Phagocytosis of Nontypeable Haemophilus influenzae by Macrophages

We recently showed that both nontypeable Haemophilus influenzae (NTHi) and its surface plasminogen- (Plg-) binding proteins interact with lipoprotein(a) (Lp(a)) in a lysine-dependent manner. Because Lp(a) can be taken up by macrophages, we postulated that it serves as an opsonin to enhance phagocytosis of NTHi by macrophages. Based on colony-forming unit (CFU) counts, Lp(a) was found to increase U937 macrophage-mediated phagocytosis of NTHi49247 and NTHi49766 by 34% and 43%, respectively, after 120 min. In contrast, Lp(a) did not enhance phagocytosis of Escherichia coli BL21 or E. coli JM109, which were unable to bind to Lp(a). As with U937 macrophages, Lp(a) was capable of increasing phagocytosis of NTHi49247 by peripheral blood mononuclear cell-derived macrophages. Opsonic phagocytosis by Lp(a) was inhibited by the addition of recombinant kringle IV type 10 (rKIV₁₀), a lysine-binding competitor; moreover, Lp(a) did not increase phagocytosis of NTHi by U937 macrophages that were pretreated with a monoclonal antibody against the scavenger receptor CD36. Taken together, our observation suggests that Lp(a) might serve as a lysine-binding opsonin to assist macrophages in rapid recognition and phagocytosis of NTHi.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Binding of Plasminogen to Streptococcus suis Protein Endopeptidase O Facilitates Evasion of Innate Immunity in Streptococcus suis

The Gram-positive bacterial species Streptococcus suis is an important porcine and human pathogen that causes severe life-threatening diseases associated with high mortality rates. However, the mechanisms by which S. suis evades host innate immunity remain elusive, so identifying novel virulence factors involved in immune evasion is crucial to gain control over this threatening pathogen. Our previous work has shown that S. suis protein endopeptidase O (SsPepO) is a novel fibronectin-binding protein. Here, we identified that recombinant SsPepO binds human plasminogen in a dose-dependent manner. Moreover, the binding of SsPepO and plasminogen, upon the activation of urokinase-type plasminogen activator, generated plasmin, which could cleave complement C3b, thus playing an important role in complement control. Additionally, a SspepO-deficient mutant showed impaired adherence to plasminogen as well as impaired adherence to and invasion of rat brain microvascular endothelial cells compared with the wildtype strain. We further found that the SspepO-deficient mutant was efficiently killed by human serum and blood. We also confirmed that the SspepO-deficient mutant had a lower mortality rate than the wildtype strain in a mouse model. In conclusion, these results indicate that SsPepO is a novel plasminogen-binding protein that contributes to S. suis immune evasion.

Interaction between Borrelia miyamotoi variable major proteins Vlp15/16 and Vlp18 with plasminogen and complement

Borrelia miyamotoi, a relapsing fever spirochete transmitted by Ixodid ticks causes B. miyamotoi disease (BMD). To evade the human host´s immune response, relapsing fever borreliae, including B. miyamotoi, produce distinct variable major proteins. Here, we investigated Vsp1, Vlp15/16, and Vlp18 all of which are currently being evaluated as antigens for the serodiagnosis of BMD. Comparative analyses identified Vlp15/16 but not Vsp1 and Vlp18 as a plasminogen-interacting protein of B. miyamotoi. Furthermore, Vlp15/16 bound plasminogen in a dose-dependent fashion with high affinity. Binding of plasminogen to Vlp15/16 was significantly inhibited by the lysine analog tranexamic acid suggesting that the protein–protein interaction is mediated by lysine residues. By contrast, ionic strength did not have an effect on binding of plasminogen to Vlp15/16. Of relevance, plasminogen bound to the borrelial protein cleaved the chromogenic substrate S-2251 upon conversion by urokinase-type plasminogen activator (uPa), demonstrating it retained its physiological activity. Interestingly, further analyses revealed a complement inhibitory activity of Vlp15/16 and Vlp18 on the alternative pathway by a Factor H-independent mechanism. More importantly, both borrelial proteins protect serum sensitive Borrelia garinii cells from complement-mediated lysis suggesting multiple roles of these two variable major proteins in immune evasion of B. miyamotoi.

Interactions Between Pathogenic Burkholderia and the Complement System: A Review of Potential Immune Evasion Mechanisms

The genus Burkholderia contains over 80 different Gram-negative species including both plant and human pathogens, the latter of which can be classified into one of two groups: the Burkholderia pseudomallei complex (Bpc) or the Burkholderia cepacia complex (Bcc). Bpc pathogens Burkholderia pseudomallei and Burkholderia mallei are highly virulent, and both have considerable potential for use as Tier 1 bioterrorism agents; thus there is great interest in the development of novel vaccines and therapeutics for the prevention and treatment of these infections. While Bcc pathogens Burkholderia cenocepacia, Burkholderia multivorans, and Burkholderia cepacia are not considered bioterror threats, the incredible impact these infections have on the cystic fibrosis community inspires a similar demand for vaccines and therapeutics for the prevention and treatment of these infections as well. Understanding how these pathogens interact with and evade the host immune system will help uncover novel therapeutic targets within these organisms. Given the important role of the complement system in the clearance of bacterial pathogens, this arm of the immune response must be efficiently evaded for successful infection to occur. In this review, we will introduce the Burkholderia species to be discussed, followed by a summary of the complement system and known mechanisms by which pathogens interact with this critical system to evade clearance within the host. We will conclude with a review of literature relating to the interactions between the herein discussed Burkholderia species and the host complement system, with the goal of highlighting areas in this field that warrant further investigation.

Preventing Candida albicans from subverting host plasminogen for invasive infection treatment

Candida albicans is a common fungal pathogen in humans that colonizes the skin and mucosal surfaces of the majority healthy individuals. How C. albicans disseminates into the bloodstream and causes life-threatening systemic infections in immunocompromised patients remains unclear. Plasminogen system activation can degrade a variety of structural proteins in vivo and is involved in several homeostatic processes. Here, for the first time, we characterized that C. albicans could capture and “subvert” host plasminogen to invade host epithelial cell surface barriers through cell-wall localized Eno1 protein. We found that the “subverted” plasminogen system plays an important role in development of invasive infection caused by C. albicans in mice. Base on this finding, we discovered a mouse monoclonal antibody (mAb) 12D9 targeting C. albicans Eno1, with high affinity to the ₂₅₄FYKDGKYDL₂₆₂ motif in α-helices 6, β-sheet 6 (H6S6) loop and direct blocking activity for C. albicans capture host plasminogen. mAb 12D9 could prevent C. albicans from invading human epithelial and endothelial cells, and displayed antifungal activity and synergistic effect with anidulafungin or fluconazole in proof-of-concept in vivo studies, suggesting that blocking the function of cell surface Eno1 was effective for controlling invasive infection caused by Candida spp. In summary, our study provides the evidence of C. albicans invading host by “subverting” plasminogen system, suggesting a potential novel treatment strategy for invasive fungal infections.

Strategies used by Leptospira spirochetes to evade the host complement system

Leptospires are highly invasive spirochetes equipped with efficient strategies for dissemination in the host. The Leptospira genus currently comprises 64 species divided into two major clades: the saprophytes composed of nonpathogenic, free-living organisms, and the pathogens encompassing all the species that cause mild or severe infections in humans and animals. While saprophytes are highly susceptible to the lytic action of the complement system, pathogenic (virulent) strains have evolved virulence strategies that allow efficient colonization of a variety of hosts and target organs, including mechanisms to circumvent hosts' innate and acquired immune responses. Pathogenic Leptospira avoid complement-mediated killing by recruiting host complement regulatory proteins and by targeting complement proteins using own and host-expressed proteases. This review outlines the role of complement in eradicating saprophytic Leptospira and the stratagems adopted by pathogenic Leptospira to maneuver the host complement system for their benefit.

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.

The interplay between host haemostatic systems and Leptospira spp. infections

Hemostasis is a defence mechanism that protects the integrity of the vascular system and is comprised of the coagulation cascade, fibrinolysis, platelet aggregation, and vascular endothelium. Besides the primary function in preserving the vascular integrity, the haemostatic system cooperates with immune and inflammatory processes to eliminate invading pathogens during microbial infections. Under pathological manifestations, hemostasis must therefore interact in a coordinated manner with inflammatory responses and immune reactions. Several pathogens can modulate these host-derived countermeasures by specifically targeting certain haemostatic components for their own benefit. Thus, the ability to modulate host defence systems has to be considered as an essential bacterial virulence mechanism. Complications that bacterial pathogens can induce are therefore often the consequence of evoked host responses. A comprehensive understanding of the molecular mechanisms triggered in infectious processes may help to develop prophylactic methods and novel therapies for the patients suffering from a particular infectious disease. This review aims to provide a critical updated compiling of recent studies on how the pathogenic Leptospira can interact with and manipulate the host haemostatic systems and the consequences for leptospirosis pathogenesis.

Design and Characterization of Protein E-PilA, a Candidate Fusion Antigen for Nontypeable Haemophilus influenzae Vaccine

Nontypeable Haemophilus influenzae (NTHi) is a pathogen known for being a frequent cause of acute otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease. In the present study, a vaccine antigen based on the fusion of two known NTHi adhesive proteins, protein E (PE) and a pilin subunit (PilA), was developed. The quality of the combined antigen was investigated through functional, biophysical, and structural analyses.

Nontypeable Haemophilus influenzae (NTHi) is a pathogen known for being a frequent cause of acute otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease. In the present study, a vaccine antigen based on the fusion of two known NTHi adhesive proteins, protein E (PE) and a pilin subunit (PilA), was developed. The quality of the combined antigen was investigated through functional, biophysical, and structural analyses. It was shown that the PE and PilA individual structures are not modified in the PE-PilA fusion and that PE-PilA assembles as a dimer in solution, reflecting PE dimerization. PE-PilA was found to bind vitronectin by enzyme-linked immunosorbent assay, as isolated PE does. Disulfide bridges were conserved and homogeneous, which was determined by peptide mapping and top-down analysis of PE, PilA, and PE-PilA molecules. Finally, the PE-PilA crystal showed a PE entity with a three-dimensional (3D) structure similar to that of the recently published isolated PE, while the structure of the PilA entity was similar to that of a 3D model elaborated from two other type 4 pilin subunits. Taken together, our observations suggest that the two tethered proteins behave independently within the chimeric molecule and display structures similar to those of the respective isolated antigens, which are important characteristics for eliciting optimal antibody-mediated immunity. PE and PilA can thus be further developed as a single fusion protein in a vaccine perspective, in the knowledge that tethering the two antigens does not perceptibly compromise the structural attributes offered by the individual antigens.

A Protein E-PilA Fusion Protein Shows Vaccine Potential against Nontypeable Haemophilus influenzae in Mice and Chinchillas

PE-PilA is a fusion protein composed of immunologically relevant parts of protein E (PE) and the majority subunit of the type IV pilus (PilA), two major antigens of nontypeable Haemophilus influenzae (NTHi). Here we report on the preclinical evaluation of PE-PilA as a vaccine antigen. The immunogenic potential of the PE and PilA within the fusion was compared with that of isolated PE and PilA antigens.

PE-PilA is a fusion protein composed of immunologically relevant parts of protein E (PE) and the majority subunit of the type IV pilus (PilA), two major antigens of nontypeable Haemophilus influenzae (NTHi). Here we report on the preclinical evaluation of PE-PilA as a vaccine antigen. The immunogenic potential of the PE and PilA within the fusion was compared with that of isolated PE and PilA antigens. When injected intramuscularly into mice, the immunogenicity of PE within the fusion was equivalent to that of isolated PE, except when it was formulated with alum. In contrast, in our murine models PilA was consistently found to be more immunogenic as a subentity of the PE-PilA fusion protein than when it was injected as an isolated antigen. Following immunization with PE-PilA, anti-PE antibodies demonstrated the same capacity to inhibit the binding of PE to vitronectin as those induced after PE immunization. Likewise, PE-PilA-induced anti-PilA antibodies inhibited the formation of NTHi biofilms and disrupted established biofilms in vitro. These experiments support the immunogenic equivalence between fused PE-PilA and isolated PE and PilA. Further, the potential of PE-PilA immunization against NTHi-induced disease was evaluated. After intranasal NTHi challenge, colonization of the murine nasopharynx significantly dropped in animals formerly immunized with PE-PilA, and in chinchillas, signs of otitis media were significantly reduced in animals that had received anti-PE-PilA antibodies. Taken together, our data support the use of PE-PilA as an NTHi vaccine antigen.

A complex interplay between the extracellular matrix and the innate immune response to microbial pathogens

The role of the host extracellular matrix (ECM) in infection tends to be neglected. However, the complex interactions between invading pathogens, host tissues and immune cells occur in the context of the ECM. On the pathogen side, a variety of surface and secreted molecules, including microbial surface components recognizing adhesive matrix molecules and tissue-degrading enzymes, are employed that interact with different ECM proteins to effectively establish an infection at specific sites. Microbial pathogens can also hijack or misuse host proteolytic systems to modify the ECM, evade immune responses or process biologically active molecules such as cell surface receptors and cytokines that direct cell behaviour and immune defence. On the host side, the ECM composition and three-dimensional ultrastructure undergo significant modifications, which have a profound impact on the specific signals that the ECM conveys to immune cells at the forefront of infection. Unexpectedly, activated immune cells participate in the remodelling of the local ECM by synthesizing ECM glycoproteins, proteoglycans and collagen molecules. The close interplay between the ECM and the innate immune response to microbial pathogens ultimately affects the outcome of infection. This review explores and discusses recent data that implicate an active role for the ECM in the immune response to infection, encompassing antimicrobial activities, microbial recognition, macrophage activation, phagocytosis, leucocyte population balance, and transcriptional and post-transcriptional regulation of inflammatory networks, and may foster novel antimicrobial approaches.

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.

The Complement Binding and Inhibitory Protein CbiA of Borrelia miyamotoi Degrades Extracellular Matrix Components by Interacting with Plasmin(ogen)

The emerging relapsing fever spirochete Borrelia (B.) miyamotoi is transmitted by ixodid ticks and causes the so-called hard tick-borne relapsing fever or B. miyamotoi disease (BMD). More recently, we identified a surface-exposed molecule, CbiA exhibiting complement binding and inhibitory capacity and rendering spirochetes resistant to complement-mediated lysis. To gain deeper insight into the molecular principles of B. miyamotoi-host interaction, we examined CbiA as a plasmin(ogen) receptor that enables B. miyamotoi to interact with the serine protease plasmin(ogen). Recombinant CbiA was able to bind plasminogen in a dose-dependent fashion. Moreover, lysine residues appear to play a crucial role in the protein-protein interaction as binding of plasminogen was inhibited by the lysine analog tranexamic acid as well as increasing ionic strength. Of relevance, plasminogen bound to CbiA can be converted by urokinase-type plasminogen activator (uPa) to active plasmin which cleaved both, the chromogenic substrate S-2251 and its physiologic substrate fibrinogen. Concerning the involvement of specific amino acids in the interaction with plasminogen, lysine residues located at the C-terminus are frequently involved in the binding as reported for various other plasminogen-interacting proteins of Lyme disease spirochetes. Lysine residues located within the C-terminal domain were substituted with alanine to generate single, double, triple, and quadruple point mutants. However, binding of plasminogen to the mutated CbiA proteins was not affected, suggesting that lysine residues distant from the C-terminus might be involved in the interaction.

Immunohaemostasis: a new view on haemostasis during sepsis

Host infection by a micro-organism triggers systemic inflammation, innate immunity and complement pathways, but also haemostasis activation. The role of thrombin and fibrin generation in host defence is now recognised, and thrombin has become a partner for survival, while it was seen only as one of the "principal suspects" of multiple organ failure and death during septic shock. This review is first focused on pathophysiology. The role of contact activation system, polyphosphates and neutrophil extracellular traps has emerged, offering new potential therapeutic targets. Interestingly, newly recognised host defence peptides (HDPs), derived from thrombin and other "coagulation" factors, are potent inhibitors of bacterial growth. Inhibition of thrombin generation could promote bacterial growth, while HDPs could become novel therapeutic agents against pathogens when resistance to conventional therapies grows. In a second part, we focused on sepsis-induced coagulopathy diagnostic challenge and stratification from "adaptive" haemostasis to "noxious" disseminated intravascular coagulation (DIC) either thrombotic or haemorrhagic. Besides usual coagulation tests, we discussed cellular haemostasis assessment including neutrophil, platelet and endothelial cell activation. Then, we examined therapeutic opportunities to prevent or to reduce "excess" thrombin generation, while preserving "adaptive" haemostasis. The fail of international randomised trials involving anticoagulants during septic shock may modify the hypothesis considering the end of haemostasis as a target to improve survival. On the one hand, patients at low risk of mortality may not be treated to preserve "immunothrombosis" as a defence when, on the other hand, patients at high risk with patent excess thrombin and fibrin generation could benefit from available (antithrombin, soluble thrombomodulin) or ongoing (FXI and FXII inhibitors) therapies. We propose to better assess coagulation response during infection by an improved knowledge of pathophysiology and systematic testing including determination of DIC scores. This is one of the clues to allocate the right treatment for the right patient at the right moment.

Recombinant truncated E protein as a new vaccine candidate against nontypeable H. influenzae: Its expression and immunogenic evaluation

Protein E (PE) is a conserved entity observed in both nontypeable Haemophilus influenzae (NTHi) and encapsulated H. influenzae. This is a small surface lipoprotein, consisting of only 160 amino acids, involved in the adhesion of H. influenzae to various types of epithelial cells. A 384-bp-long fragment from NTHi PE was cloned into the prokaryotic expression vector pBAD-gIIIA. The recombinant protein was expressed with arabinose and then purified by affinity purification on an Ni-NTA agarose matrix. BALB/c mice were immunized by subcutaneous injection with purified recombinant truncated PE mixed with an alum adjuvant. Serum antibody response and the functional activity of antibodies were evaluated by enzyme-linked immunosorbent assay (ELISA) and serum bactericidal assay (SBA), respectively. Colony PCR, double digestion, and sequencing were used to verify successful cloning of truncated PE. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and western blot analyses indicated the presence of a ∼15-kDa recombinant protein. Serum IgG, IgG1, and IgG2a levels were significantly higher in the group immunized by recombinant truncated PE mixed with an alum adjuvant, compared to the non-vaccinated control group. Development of a strong bactericidal effect against NTHi was observed in the serum samples from immunized animals. Our findings suggest that recombinant truncated PE is a potential vaccine candidate for NTHi.

Insights on persistent airway infection by non-typeable Haemophilus influenzae in chronic obstructive pulmonary disease

Non-typeable Haemophilus influenzae (NTHi) is the most common bacterial cause of infection of the lower airways in adults with chronic obstructive pulmonary disease (COPD). Infection of the COPD airways causes acute exacerbations, resulting in substantial morbidity and mortality. NTHi has evolved multiple mechanisms to establish infection in the hostile environment of the COPD airways, allowing the pathogen to persist in the airways for months to years. Persistent infection of the COPD airways contributes to chronic airway inflammation that increases symptoms and accelerates the progressive loss of pulmonary function, which is a hallmark of the disease. Persistence mechanisms of NTHi include the expression of multiple redundant adhesins that mediate binding to host cellular and extracellular matrix components. NTHi evades host immune recognition and clearance by invading host epithelial cells, forming biofilms, altering gene expression and displaying surface antigenic variation. NTHi also binds host serum factors that confer serum resistance. Here we discuss the burden of COPD and the role of NTHi infections in the course of the disease. We provide an overview of NTHi mechanisms of persistence that allow the pathogen to establish a niche in the hostile COPD airways.

Developing a vaccine to prevent otitis media caused by nontypeable Haemophilus influenzae

Nontypeable Haemophilus influenzae (NTHi) is a predominant organism of the upper respiratory nasopharyngeal microbiota. Its disease spectrum includes otitis media, sinusitis, non-bacteremic pneumonia and invasive infections. Protein-based vaccines to prevent NTHi infections are needed to alleviate these infections in children and vulnerable populations such as the elderly and those with chronic obstructive pulmonary disease (COPD). One NTHi protein is included in a pneumococcal conjugate vaccine and has been shown to provide efficacy. Our lab has been interested in understanding the immunogenicity of NTHi vaccine candidates P6, protein D and OMP26 for preventing acute otitis media in young children. We expect that continued investigation and progress in the development of an efficacious protein based vaccine against NTHi infections is achievable in the near future.

Plasmin cleaves fibrinogen and the human complement proteins C3b and C5 in the presence of Leptospira interrogans proteins: A new role of LigA and LigB in invasion and complement immune evasion

Plasminogen is a single-chain glycoprotein found in human plasma as the inactive precursor of plasmin. When converted to proteolytically active plasmin, plasmin(ogen) regulates both complement and coagulation cascades, thus representing an important target for pathogenic microorganisms. Leptospira interrogans binds plasminogen, which is converted to active plasmin. Leptospiral immunoglobulin-like (Lig) proteins are surface exposed molecules that interact with extracellular matrix components and complement regulators, including proteins of the FH family and C4BP. In this work, we demonstrate that these multifunctional molecules also bind plasminogen through both N- and C-terminal domains. These interactions are dependent on lysine residues and are affected by ionic strength. Competition assays suggest that plasminogen does not share binding sites with C4BP or FH on Lig proteins at physiological molar ratios. Plasminogen bound to Lig proteins is converted to proteolytic active plasmin in the presence of urokinase-type plasminogen activator (uPA). Lig-bound plasmin is able to cleave the physiological substrates fibrinogen and the complement proteins C3b and C5. Taken together, our data point to a new role of LigA and LigB in leptospiral invasion and complement immune evasion. Plasmin(ogen) acquisition by these versatile proteins may contribute to Leptospira infection, favoring bacterial survival and dissemination inside the host.

Fructose-1,6-bisphosphate aldolase of Neisseria meningitidis binds human plasminogen via its C-terminal lysine residue

Neisseria meningitidis is a leading cause of fatal sepsis and meningitis worldwide. As for commensal species of human neisseriae, N. meningitidis inhabits the human nasopharynx and asymptomatic colonization is ubiquitous. Only rarely does the organism invade and survive in the bloodstream leading to disease. Moonlighting proteins perform two or more autonomous, often dissimilar, functions using a single polypeptide chain. They have been increasingly reported on the surface of both prokaryotic and eukaryotic organisms and shown to interact with a variety of host ligands. In some organisms moonlighting proteins perform virulence‐related functions, and they may play a role in the pathogenesis of N. meningitidis. Fructose‐1,6‐bisphosphate aldolase (FBA) was previously shown to be surface‐exposed in meningococci and involved in adhesion to host cells. In this study, FBA was shown to be present on the surface of both pathogenic and commensal neisseriae, and surface localization and anchoring was demonstrated to be independent of aldolase activity. Importantly, meningococcal FBA was found to bind to human glu‐plasminogen in a dose‐dependent manner. Site‐directed mutagenesis demonstrated that the C‐terminal lysine residue of FBA was required for this interaction, whereas subterminal lysine residues were not involved.

CipA of Acinetobacter baumannii Is a Novel Plasminogen Binding and Complement Inhibitory Protein

Acinetobacter baumannii is an emerging opportunistic pathogen, responsible for up to 10% of gram-negative, nosocomial infections. The global increase of multidrug-resistant and pan-resistant Acinetobacter isolates presents clinicians with formidable challenges. To establish a persistent infection,A. baumannii must overcome the detrimental effects of complement as the first line of defense against invading microorganisms. However, the immune evasion principles underlying serum resistance inA. baumannii remain elusive. Here, we identified a novel plasminogen-binding protein, termed CipA. Bound plasminogen, upon conversion to active plasmin, degraded fibrinogen and complement C3b and contributed to serum resistance. Furthermore, CipA directly inhibited the alternative pathway of complement in vitro, irrespective of its ability to bind plasminogen. A CipA-deficient mutant was efficiently killed by human serum and showed a defect in the penetration of endothelial monolayers, demonstrating that CipA is a novel multifunctional protein that contributes to the pathogenesis ofA. baumannii.

Leptospira interrogans Lsa23 protein recruits plasminogen, factor H and C4BP from normal human serum and mediates C3b and C4b degradation

It has been reported that pathogenic Leptospira are resistant to normal human serum (NHS) due to their ability to evade the complement immune system by interacting with factor H (FH) and C4b-binding protein (C4BP) regulators. Moreover, plasmin generation on the leptospiral surface diminishes C3b and IgG deposition, decreasing opsonophagocytosis by immune competent cells. We have previously reported that Lsa23 (LIC11360) is a multipurpose protein capable of binding purified extracellular matrix molecules, FH, C4BP and plasminogen (PLG)/plasmin in the presence of PLG activators. In this work, we provide further evidence that Lsa23 is located at the bacterial surface by using immunofluorescence microscopy. We show that Lsa23 has the ability to acquire FH, C4BP and PLG from NHS, and use these interactions to evade innate immunity. The binding with the complement regulators FH and C4BP preserves factor I (FI) activity, leading to C3b and C4b degradation products, respectively. C3b and C4b alpha-chain cleavage was also observed when Lsa23 bound to PLG generating plasmin, an effect blocked by the protease inhibitor aprotinin. Lsa23 also inhibited lytic activity by NHS mediated by both classical and alternative complement pathways. Thus, Lsa23 has the ability to block both pathways of the complement system, and may help pathogenic Leptospira to escape complement-mediated clearance in human hosts. Indeed, NHS treated with Lsa23 confers a partial serum resistance phenotype to Leptospira biflexa, whereas blocking this protein with anti-Lsa23 renders pathogenic L. interrogans more susceptible to complement-mediated killing. Thus, Lsa23 is a multifunctional protein involved in many pathways, featuring C4b cleavage by plasmin, knowledge that may help in the development of preventive approaches to intervene with human complement escape by this versatile pathogen.

Bacterial pathogens activate plasminogen to breach tissue barriers and escape from innate immunity

Both coagulation and fibrinolysis are tightly connected with the innate immune system. Infection and inflammation cause profound alterations in the otherwise well-controlled balance between coagulation and fibrinolysis. Many pathogenic bacteria directly exploit the host's hemostatic system to increase their virulence. Here, we review the capacity of bacteria to activate plasminogen. The resulting proteolytic activity allows them to breach tissue barriers and evade innate immune defense, thus promoting bacterial spreading. Yersinia pestis, streptococci of group A, C and G and Staphylococcus aureus produce a specific bacterial plasminogen activator. Moreover, surface plasminogen receptors play an established role in pneumococcal, borrelial and group B streptococcal infections. This review summarizes the mechanisms of bacterial activation of host plasminogen and the role of the fibrinolytic system in infections caused by these pathogens.

Translation Elongation Factor Tuf of Acinetobacter baumannii Is a Plasminogen-Binding Protein

Acinetobacter baumannii is an important nosocomial pathogen, causing a variety of opportunistic infections of the skin, soft tissues and wounds, urinary tract infections, secondary meningitis, pneumonia and bacteremia. Over 63% of A. baumannii infections occurring in the United States are caused by multidrug resistant isolates, and pan-resistant isolates have begun to emerge that are resistant to all clinically relevant antibiotics. The complement system represents the first line of defense against invading pathogens. However, many A. baumannii isolates, especially those causing severe bacteremia are resistant to complement-mediated killing, though the underlying mechanisms remain poorly understood. Here we show for the first time that A. baumannii binds host-derived plasminogen and we identify the translation elongation factor Tuf as a moonlighting plasminogen-binding protein that is exposed on the outer surface of A. baumannii. Binding of plasminogen to Tuf is at least partly dependent on lysine residues and ionic interactions. Plasminogen, once bound to Tuf can be converted to active plasmin and proteolytically degrade fibrinogen as well as the key complement component C3b. Thus, Tuf acts as a multifunctional protein that may contribute to virulence of A. baumannii by aiding in dissemination and evasion of the complement system.

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.

A Novel Interaction between Complement Inhibitor C4b-binding Protein and Plasminogen That Enhances Plasminogen Activation

The complement, coagulation, and fibrinolytic systems are crucial for the maintenance of tissue homeostasis. To date numerous interactions and cross-talks have been identified between these cascades. In line with this, here we propose a novel, hitherto unknown interaction between the complement inhibitor C4b-binding protein (C4BP) and plasminogen of the fibrinolytic pathway. Binding of C4BP to Streptococcus pneumoniae is a known virulence mechanism of this pathogen and it was increased in the presence of plasminogen. Interestingly, the acute phase variant of C4BP lacking the β-chain and protein S binds plasminogen much stronger than the main isoform containing the β-chain and protein S. Indeed, the complement control protein (CCP) 8 domain of C4BP, which would otherwise be sterically hindered by the β-chain, primarily mediates this interaction. Moreover, the lysine-binding sites in plasminogen kringle domains facilitate the C4BP-plasminogen interaction. Furthermore, C4BP readily forms complexes with plasminogen in fluid phase and such complexes are present in human serum and plasma. Importantly, whereas the presence of plasminogen did not affect the factor I cofactor activity of C4BP, the activation of plasminogen by urokinase-type plasminogen activator to active plasmin was significantly augmented in the presence of C4BP. Taken together, our data demonstrate a novel interaction between two proteins of the complement and fibrinolytic system. Most complexes might be formed during the acute phase of inflammation and have an effect on the homeostasis at the site of injury or acute inflammation.

Subunits of the Pyruvate Dehydrogenase Cluster of Mycoplasma pneumoniae Are Surface-Displayed Proteins that Bind and Activate Human Plasminogen

The dual role of glycolytic enzymes in cytosol-located metabolic processes and in cell surface-mediated functions with an influence on virulence is described for various micro-organisms. Cell wall-less bacteria of the class Mollicutes including the common human pathogen Mycoplasma pneumoniae possess a reduced genome limiting the repertoire of virulence factors and metabolic pathways. After the initial contact of bacteria with cells of the respiratory epithelium via a specialized complex of adhesins and release of cell-damaging factors, surface-displayed glycolytic enzymes may facilitate the further interaction between host and microbe. In this study, we described detection of the four subunits of pyruvate dehydrogenase complex (PDHA-D) among the cytosolic and membrane-associated proteins of M. pneumoniae. Subunits of PDH were cloned, expressed and purified to produce specific polyclonal guinea pig antisera. Using colony blotting, fractionation of total proteins and immunofluorescence experiments, the surface localization of PDHA-C was demonstrated. All recombinant PDH subunits are able to bind to HeLa cells and human plasminogen. These interactions can be specifically blocked by the corresponding polyclonal antisera. In addition, an influence of ionic interactions on PDHC-binding to plasminogen as well as of lysine residues on the association of PDHA-D with plasminogen was confirmed. The PDHB subunit was shown to activate plasminogen and the PDHB-plasminogen complex induces degradation of human fibrinogen. Hence, our data indicate that the surface-associated PDH subunits might play a role in the pathogenesis of M. pneumoniae infections by interaction with human plasminogen.

Interaction of spirochetes with the host fibrinolytic system and potential roles in pathogenesis

The pathogenic spirochetes Borrelia burgdorferi, B. hermsii, B. recurrentis, Treponema denticola and Leptospira spp. are the etiologic agents of Lyme disease, relapsing fever, periodontitis and leptospirosis, respectively. Lyme borreliosis is a multi-systemic disorder and the most prevalent tick-borne disease in the northern hemisphere. Tick-borne relapsing fever is persistent in endemic areas worldwide, representing a significant burden in some African regions. Periodontal disease, a chronic inflammatory disorder that often leads to tooth loss, is caused by several potential pathogens found in the oral cavity including T. denticola. Leptospirosis is considered the most widespread zoonosis, and the predominant human disease in tropical, undeveloped regions. What these diseases have in common is that they are a significant burden to healthcare costs in the absence of prophylactic measures. This review addresses the interaction of these spirochetes with the fibrinolytic system, plasminogen (Plg) binding to the surface of bacteria and the generation of plasmin (Pla) on their surface. The consequences on host-pathogen interactions when the spirochetes are endowed with this proteolytic activity are discussed on the basis of the results reported in the literature. Spirochetes equipped with Pla activity have been shown to degrade extracellular matrix (ECM) components, in addition to digesting fibrin, facilitating bacterial invasion and dissemination. Pla generation triggers the induction of matrix metalloproteases (MMPs) in a cascade of events that enhances the proteolytic capacity of the spirochetes. These activities in concert with the interference exerted by the Plg/Pla on the complement system - helping the bacteria to evade the immune system - should illuminate our understanding of the mechanisms involved in host infection.

The transcriptome of a complete episode of acute otitis media

BACKGROUND

Otitis media is the most common disease of childhood, and represents an important health challenge to the 10-15% of children who experience chronic/recurrent middle ear infections. The middle ear undergoes extensive modifications during otitis media, potentially involving changes in the expression of many genes. Expression profiling offers an opportunity to discover novel genes and pathways involved in this common childhood disease. The middle ears of 320 WBxB6 F1 hybrid mice were inoculated with non-typeable Haemophilus influenzae (NTHi) or PBS (sham control). Two independent samples were generated for each time point and condition, from initiation of infection to resolution. RNA was profiled on Affymetrix mouse 430 2.0 whole-genome microarrays.

RESULTS

Approximately 8% of the sampled transcripts defined the signature of acute NTHi-induced otitis media across time. Hierarchical clustering of signal intensities revealed several temporal gene clusters. Network and pathway enrichment analysis of these clusters identified sets of genes involved in activation of the innate immune response, negative regulation of immune response, changes in epithelial and stromal cell markers, and the recruitment/function of neutrophils and macrophages. We also identified key transcriptional regulators related to events in otitis media, which likely determine the expression of these gene clusters. A list of otitis media susceptibility genes, derived from genome-wide association and candidate gene studies, was significantly enriched during the early induction phase and the middle re-modeling phase of otitis but not in the resolution phase. Our results further indicate that positive versus negative regulation of inflammatory processes occur with highly similar kinetics during otitis media, underscoring the importance of anti-inflammatory responses in controlling pathogenesis.

CONCLUSIONS

The results characterize the global gene response during otitis media and identify key signaling and transcription factor networks that control the defense of the middle ear against infection. These networks deserve further attention, as dysregulated immune defense and inflammatory responses may contribute to recurrent or chronic otitis in children.

Vaccines for Nontypeable Haemophilus influenzae: the Future Is Now

Infections due to nontypeable Haemophilus influenzae result in enormous global morbidity in two clinical settings: otitis media in children and respiratory tract infections in adults with chronic obstructive pulmonary disease (COPD). Recurrent otitis media affects up to 20% of children and results in hearing loss, delays in speech and language development and, in developing countries, chronic suppurative otitis media. Infections in people with COPD result in clinic and emergency room visits, hospital admissions, and respiratory failure. An effective vaccine would prevent morbidity, help control health care costs, and reduce antibiotic use, a major contributor to the global crisis in bacterial antibiotic resistance. The widespread use of the pneumococcal conjugate vaccines is causing a relative increase in H. influenzae otitis media. The partial protection against H. influenzae otitis media induced by the pneumococcal H. influenzae protein D conjugate vaccine represents a proof of principle of the feasibility of a vaccine for nontypeable H. influenzae. An ideal vaccine antigen should be conserved among strains, have abundant epitopes on the bacterial surface, be immunogenic, and induce protective immune responses. Several surface proteins of H. influenzae have been identified as potential vaccine candidates and are in various stages of development. With continued research, progress toward a broadly effective vaccine to prevent infections caused by nontypeable H. influenzae is expected over the next several years.

Analysis of Paracoccidioides secreted proteins reveals fructose 1,6-bisphosphate aldolase as a plasminogen-binding protein

Background

Despite being important thermal dimorphic fungi causing Paracoccidioidomycosis, the pathogenic mechanisms that underlie the genus Paracoccidioides remain largely unknown. Microbial pathogens express molecules that can interact with human plasminogen, a protein from blood plasma, which presents fibrinolytic activity when activated into plasmin. Additionally, plasmin exhibits the ability of degrading extracellular matrix components, favoring the pathogen spread to deeper tissues. Previous work from our group demonstrated that Paracoccidioides presents enolase, as a protein able to bind and activate plasminogen, increasing the fibrinolytic activity of the pathogen, and the potential for adhesion and invasion of the fungus to host cells. By using proteomic analysis, we aimed to identify other proteins of Paracoccidioides with the ability of binding to plasminogen.

Results

In the present study, we employed proteomic analysis of the secretome, in order to identify plasminogen-binding proteins of Paracoccidioides, Pb01. Fifteen proteins were present in the fungal secretome, presenting the ability to bind to plasminogen. Those proteins are probable targets of the fungus interaction with the host; thus, they could contribute to the invasiveness of the fungus. For validation tests, we selected the protein fructose 1,6-bisphosphate aldolase (FBA), described in other pathogens as a plasminogen-binding protein. The protein FBA at the fungus surface and the recombinant FBA (rFBA) bound human plasminogen and promoted its conversion to plasmin, potentially increasing the fibrinolytic capacity of the fungus, as demonstrated in fibrin degradation assays. The addition of rFBA or anti-rFBA antibodies was capable of reducing the interaction between macrophages and Paracoccidioides, possibly by blocking the binding sites for FBA. These data reveal the possible participation of the FBA in the processes of cell adhesion and tissue invasion/dissemination of Paracoccidioides.

Conclusions

These data indicate that Paracoccidioides is a pathogen that has several plasminogen-binding proteins that likely play important roles in pathogen-host interaction. In this context, FBA is a protein that might be involved somehow in the processes of invasion and spread of the fungus during infection.

Bacteria under stress by complement and coagulation

The complement and coagulation systems are two related protein cascades in plasma that serve important roles in host defense and hemostasis, respectively. Complement activation on bacteria supports cellular immune responses and leads to direct killing of bacteria via assembly of the Membrane Attack Complex (MAC). Recent studies have indicated that the coagulation system also contributes to mammalian innate defense since coagulation factors can entrap bacteria inside clots and generate small antibacterial peptides. In this review, we will provide detailed insights into the molecular interplay between these protein cascades and bacteria. We take a closer look at how these pathways are activated on bacterial surfaces and discuss the mechanisms by which they directly cause stress to bacterial cells. The poorly understood mechanism for bacterial killing by the MAC will be reevaluated in light of recent structural insights. Finally, we highlight the strategies used by pathogenic bacteria to modulate these protein networks. Overall, these insights will contribute to a better understanding of the host defense roles of complement and coagulation against bacteria.

Haemophilus influenzae stores and distributes hemin by using protein E

The human pathogen Haemophilus influenzae causes mainly respiratory tract infections such as acute otitis media in children and exacerbations in patients with chronic obstructive pulmonary disease. We recently revealed the crystal structure of H. influenzeae protein E (PE), a multifunctional adhesin that is involved in direct interactions with lung epithelial cells and host proteins. Based upon the PE structure we here suggest a hypothetical binding pocket that is compatible in size with a hemin molecule. An H. influenzae mutant devoid of PE bound significantly less hemin in comparison to the PE-expressing wild type counterpart. In addition, E. coli expressing PE at the surface resulted in a hemin-binding phenotype. An interaction between hemin and recombinant soluble PE was also demonstrated by native-PAGE and UV-visible spectrophotometry. Surface plasmon resonance revealed an affinity (Kd) of 1.6 × 10(-6)M for the hemin-PE interaction. Importantly, hemin that was bound to PE at the H. influenzae surface, was donated to co-cultured luciferase-expressing H. influenzae that were starved of hemin. When hemin is bound to PE it thus may serve as a storage pool for H. influenzae. To our knowledge this is the first report showing that H. influenzae can share hemin via a surface-located outer membrane protein.

Interaction of Leptospira elongation factor Tu with plasminogen and complement factor H: a metabolic leptospiral protein with moonlighting activities

The elongation factor Tu (EF-Tu), an abundant bacterial protein involved in protein synthesis, has been shown to display moonlighting activities. Known to perform more than one function at different times or in different places, it is found in several subcellular locations in a single organism, and may serve as a virulence factor in a range of important human pathogens. Here we demonstrate that Leptospira EF-Tu is surface-exposed and performs additional roles as a cell-surface receptor for host plasma proteins. It binds plasminogen in a dose-dependent manner, and lysine residues are critical for this interaction. Bound plasminogen is converted to active plasmin, which, in turn, is able to cleave the natural substrates C3b and fibrinogen. Leptospira EF-Tu also acquires the complement regulator Factor H (FH). FH bound to immobilized EF-Tu displays cofactor activity, mediating C3b degradation by Factor I (FI). In this manner, EF-Tu may contribute to leptospiral tissue invasion and complement inactivation. To our knowledge, this is the first description of a leptospiral protein exhibiting moonlighting activities.

Plasmin(ogen) acquisition by group A Streptococcus protects against C3b-mediated neutrophil killing

The globally significant human pathogen group A Streptococcus (GAS) sequesters the host protease plasmin to the cell surface during invasive disease initiation. Recent evidence has shown that localized plasmin activity prevents opsonization of several bacterial species by key components of the innate immune system in vitro. Here we demonstrate that plasmin at the GAS cell surface resulted in degradation of complement factor C3b, and that plasminogen acquisition is associated with a decrease in C3b opsonization and neutrophil-mediated killing in vitro. Furthermore, the ability to acquire cell surface plasmin(ogen) correlates directly with a decrease in C3b opsonization, neutrophil phagocytosis, and increased bacterial survival in a humanized plasminogen mouse model of infection. These findings demonstrate that localized plasmin(ogen) plays an important role in facilitating GAS escape from the host innate immune response and increases bacterial virulence in the early stages of infection.

BBA70 of Borrelia burgdorferi is a novel plasminogen-binding protein

The Lyme disease spirochete Borrelia burgdorferi lacks endogenous, surface-exposed proteases. In order to efficiently disseminate throughout the host and penetrate tissue barriers, borreliae rely on recruitment of host proteases, such as plasmin(ogen). Here we report the identification of a novel plasminogen-binding protein, BBA70. Binding of plasminogen is dose-dependent and is affected by ionic strength. The BBA70-plasminogen interaction is mediated by lysine residues, primarily located in a putative C-terminal α-helix of BBA70. These lysine residues appear to interact with the lysine-binding sites in plasminogen kringle domain 4 because a deletion mutant of plasminogen lacking that domain was unable to bind to BBA70. Bound to BBA70, plasminogen activated by urokinase-type plasminogen activator was able to degrade both a synthetic chromogenic substrate and the natural substrate fibrinogen. Furthermore, BBA70-bound plasmin was able to degrade the central complement proteins C3b and C5 and inhibited the bacteriolytic effects of complement. Consistent with these functional activities, BBA70 is located on the borrelial outer surface. Additionally, serological evidence demonstrated that BBA70 is produced during mammalian infection. Taken together, recruitment and activation of plasminogen could play a beneficial role in dissemination of B. burgdorferi in the human host and may possibly aid the spirochete in escaping the defense mechanisms of innate immunity.

Elucidating novel serum biomarkers associated with pulmonary tuberculosis treatment

In an unbiased approach to biomarker discovery, we applied a highly multiplexed proteomic technology (SOMAscan, SomaLogic, Inc, Boulder, CO) to understand changes in proteins from paired serum samples at enrollment and after 8 weeks of TB treatment from 39 patients with pulmonary TB from Kampala, Uganda enrolled in the Center for Disease Control and Prevention's Tuberculosis Trials Consortium (TBTC) Study 29. This work represents the first large-scale proteomic analysis employing modified DNA aptamers in a study of active tuberculosis (TB). We identified multiple proteins that exhibit significant expression differences during the intensive phase of TB therapy. There was enrichment for proteins in conserved networks of biological processes and function including antimicrobial defense, tissue healing and remodeling, acute phase response, pattern recognition, protease/anti-proteases, complement and coagulation cascade, apoptosis, immunity and inflammation pathways. Members of cytokine pathways such as interferon-gamma, while present, were not as highly represented as might have been predicted. The top proteins that changed between baseline and 8 weeks of therapy were TSP4, TIMP-2, SEPR, MRC-2, Antithrombin III, SAA, CRP, NPS-PLA2, LEAP-1, and LBP. The novel proteins elucidated in this work may provide new insights for understanding TB disease, its treatment and subsequent healing processes that occur in response to effective therapy.

Haemophilus influenzae acquires vitronectin via the ubiquitous Protein F to subvert host innate immunity

Acquisition of the complement inhibitor vitronectin (Vn) is important for the respiratory tract pathogen nontypeable Haemophilus influenzae (NTHi) to escape complement-mediated killing. NTHi actively recruits Vn, and we previously showed that this interaction involves Protein E (PE). Here we describe a second Vn-binding protein, a 30 kDa Yersinia YfeA homologue designated as Protein F (PF). An isogenic NTHi 3655Δhpf mutant devoid of PF displayed a reduced binding of Vn, and was consequently more sensitive to killing by human serum compared with the wild type. Surface expression of PF on Escherichia coli conferred binding of Vn that resulted in a serum resistant phenotype. Molecular analyses revealed that the N-terminal of PF (Lys23-Glu48) bound to the C-terminal of Vn (Phe352-Ser374) without disrupting the inhibitory role of Vn on the membrane attack complex. The PF-Vn complex actively delayed C9 deposition on PF-expressing bacteria. Comparative studies of binding affinity and multiple mutants demonstrated that both PE and PF contribute individually to NTHi serum survival. PF was highly conserved and ubiquitously expressed in a series of randomly selected NTHi clinical isolates (n = 18). In conclusion, the multifaceted binding of Vn is beneficial for NTHi survival in serum and may contribute to successful colonization and consequently infection.

Surface associated antigens of Dirofilaria immitis adult worms activate the host fibrinolytic system

Cardiopulmonary dirofilariosis (Dirofilaria immitis) is characterized by apparent contradictory events, like the long-term survival of adult worms in the circulatory system of the infected hosts and the development of life-threatening events like thromboembolisms and others. Thus parasite mechanisms, like the activation of fibrinolytic system, are key to the survival of both the worms and the host. The aim of this study was to investigate the interaction between D. immitis adult worms surface-associated antigens (DiSAA) and the fibrinolytic system of the host. We demonstrate that DiSAA extract is able to bind plasminogen and generate plasmin, with the latter occurring in a tissue plasminogen activator (t-PA) dependent manner. Additionally, 11 plasminogen-binding proteins from DiSAA extract were identified by proteomics and mass spectrometry (MS) (actin-5C, actin-1, enolase, fructose-bisphosphate aldolase, GAPDH, MSP domain protein, MSP 2, beta-galactosidase-binding-lectin, galectin, immunoglobulin I-set domain-containing protein and cyclophilin Ovcyp-2). Because in a previous work we have shown the positive interaction between the excretory/secretory antigens of D. immitis (DiES) and the host fibrinolytic system and many of the molecules identified here are shared by both antigens, we hypothesize that DiSAA cooperate in host fibrinolytic system activation promoting the fibrin clot lysis.

Streptococcus pneumoniae endopeptidase O (PepO) is a multifunctional plasminogen- and fibronectin-binding protein, facilitating evasion of innate immunity and invasion of host cells

Streptococcus pneumoniae infections remain a major cause of morbidity and mortality worldwide. Therefore a detailed understanding and characterization of the mechanism of host cell colonization and dissemination is critical to gain control over this versatile pathogen. Here we identified a novel 72-kDa pneumococcal protein endopeptidase O (PepO), as a plasminogen- and fibronectin-binding protein. Using a collection of clinical isolates, representing different serotypes, we found PepO to be ubiquitously present both at the gene and protein level. In addition, PepO protein was secreted in a growth phase-dependent manner to the culture supernatants of the pneumococcal isolates. Recombinant PepO bound human plasminogen and fibronectin in a dose-dependent manner and plasminogen did not compete with fibronectin for binding PepO. PepO bound plasminogen via lysine residues and the interaction was influenced by ionic strength. Moreover, upon activation of PepO-bound plasminogen by urokinase-type plasminogen activator, generated plasmin cleaved complement protein C3b thus assisting in complement control. Furthermore, direct binding assays demonstrated the interaction of PepO with epithelial and endothelial cells that in turn blocked pneumococcal adherence. Moreover, a pepO-mutant strain showed impaired adherence to and invasion of host cells compared with their isogenic wild-type strains. Taken together, the results demonstrated that PepO is a ubiquitously expressed plasminogen- and fibronectin-binding protein, which plays role in pneumococcal invasion of host cells and aids in immune evasion.

The unique structure of Haemophilus influenzae protein E reveals multiple binding sites for host factors

Haemophilus influenzae protein E (PE) is a multifunctional adhesin involved in direct interactions with lung epithelial cells and host proteins, including plasminogen and the extracellular matrix proteins vitronectin and laminin. We recently crystallized PE and successfully collected X-ray diffraction data at 1.8 Å. Here, we solved the structure of a recombinant version of PE and analyzed different functional regions. It is a dimer in solution and in the asymmetric unit of the crystals. The dimer has a structure that resembles a flattened β-barrel. It is, however, not a true β-barrel, as there are differences in both the hydrogen-bonding pattern and the shape. Each monomer consisted of a 6-stranded antiparallel β-sheet with a rigid α-helix at the C terminus tethered to the concave side of the sheet by a disulfide bridge. The laminin/plasminogen binding region (residues 41 to 68) is exposed, while the vitronectin binding region (residues 84 to 108) is partially accessible in the dimer. The dimerized PE explains the simultaneous interaction with laminin and vitronectin. In addition, we found this unique adhesin to be present in many bacterial genera of the family Pasteurellaceae and also orthologues in other, unrelated species (Enterobacter cloacae and Listeria monocytogenes). Peptides corresponding to the surface-exposed regions PE 24 to 37, PE 74 to 89, and PE 134 to 156 were immunogenic in the mouse. Importantly, these peptide-based antibodies also recognized PE at the bacterial surface. Taken together, our detailed structure of PE explains how this important virulence factor of H. influenzae simultaneously interacts with host vitronectin, laminin, or plasminogen, promoting bacterial pathogenesis.

Glycerol-3-phosphate dehydrogenase 2 is a novel factor H-, factor H-like protein 1-, and plasminogen-binding surface protein of Candida albicans

Candida albicans uses human complement regulators such as factor H and factor H-like protein 1 (FHL-1) for immune evasion. To define the whole panel of fungal complement-evasion molecules, C. albicans cell extract was absorbed to a factor H-coupled matrix. One 52-kDa protein was eluted and identified by mass spectrometry as glycerol-3-phosphate dehydrogenase 2 (Gpd2). Consequently, Gpd2 was recombinantly expressed and purified. Recombinant Gpd2 binds factor Hand and FHL-1, mainly via short consensus repeat 7; and binds plasminogen, via lysine residues. The 3 human complement regulators, when attached to candida Gpd2, became functionally active, and the attached host proteins assist in inactivation of the complement cascade or cleave fibrinogen in the extracellular matrix component fibrinogen. Polyclonal Gpd2 antiserum was generated and localized Gpd2 at the surface of C. albicans. In addition, candida Gpd2 bound to human nonphagocytic cells but not to phagocytic U937 cells. Thus, candida Gpd2 is a novel fungal immune evasion protein that binds several human complement regulators and, in addition, binds human cells.

Bacterial plasminogen receptors: mediators of a multifaceted relationship

Multiple species of bacteria are able to sequester the host zymogen plasminogen to the cell surface. Once localised to the bacterial surface, plasminogen can act as a cofactor in adhesion, or, following activation to plasmin, provide a source of potent proteolytic activity. Numerous bacterial plasminogen receptors have been identified, and the mechanisms by which they interact with plasminogen are diverse. Here we provide an overview of bacterial plasminogen receptors and discuss the diverse role bacterial plasminogen acquisition plays in the relationship between bacteria and the host.

Staphylococcus aureus proteins Sbi and Efb recruit human plasmin to degrade complement C3 and C3b

Upon host infection, the human pathogenic microbe Staphylococcus aureus (S. aureus) immediately faces innate immune reactions such as the activated complement system. Here, a novel innate immune evasion strategy of S. aureus is described. The staphylococcal proteins surface immunoglobulin-binding protein (Sbi) and extracellular fibrinogen-binding protein (Efb) bind C3/C3b simultaneously with plasminogen. Bound plasminogen is converted by bacterial activator staphylokinase or by host-specific urokinase-type plasminogen activator to plasmin, which in turn leads to degradation of complement C3 and C3b. Efb and to a lesser extend Sbi enhance plasmin cleavage of C3/C3b, an effect which is explained by a conformational change in C3/C3b induced by Sbi and Efb. Furthermore, bound plasmin also degrades C3a, which exerts anaphylatoxic and antimicrobial activities. Thus, S. aureus Sbi and Efb comprise platforms to recruit plasmin(ogen) together with C3 and its activation product C3b for efficient degradation of these complement components in the local microbial environment and to protect S. aureus from host innate immune reactions.