Complement regulator Factor H mediates a two-step uptake of Streptococcus pneumoniae by human cells

Streptococcus pneumoniae invades endothelial host cells via multiple pathways and is killed in a lysosome dependent manner

Streptococcus pneumoniae is one of the major causative agents of pneumonia, sepsis, meningitis and other morbidities. In spite of its heavy disease burden, surprisingly little is known about the mechanisms involved in the switch of life style, from commensal colonizer of the nasopharynx to invasive pathogen. In vitro experiments, and mouse models have shown that S. pneumoniae can be internalized by host cells, which coupled with intracellular vesicle transport through the cells, i.e. transcytosis, is suggested to be the first step of invasive disease. To further dissect the process of S. pneumoniae internalization, we chemically inhibited discrete parts of the cellular uptake system. We show that this invasion of the host cells was facilitated via both clathrin- and caveolae-mediated endocytosis. After internalization we demonstrated that the bulk of the internalized S. pneumoniae was killed in the lysosome. Interestingly, inhibition of the lysosome altered transcytosis dynamics as it resulted in an increase in the transport of the internalized bacteria out of the cells via the basal side. These results show that uptake of S. pneumoniae into host cells occurs via multiple pathways, as opposed to the often proposed view of invasion being dependent on specific, and singular receptor-mediated endocytosis. This indicates that the endothelium not only has a critical role as a physical barrier against S. pneumoniae in the blood stream, but also in degrading S. pneumonia cells that have adhered to, and invaded the endothelial cells.

The interaction between bacterial enolase and plasminogen promotes adherence of Streptococcus pneumoniae to epithelial and endothelial cells

Binding and conversion of the plasma protein plasminogen is an important pathogenesis mechanism of the human pathogen Streptococcus pneumoniae. Once converted into plasmin, the proteolytic activity of this major fibrinolysis component promotes degradation of extracellular matrix and the dissolution of fibrin clots. Here, we present the exploitation of plasminogen-binding as a further pivotal strategy of pneumococci facilitating adherence to eukaryotic cells. Flow cytometric measurements demonstrated the immobilization of plasminogen on host cell surfaces of human alveolar type II pneumocytes (A549), nasopharyngeal epithelium (Detroit 562) and brain-derived endothelial cells (HBMEC). These host-derived cells were employed in cell culture infection analyses followed by confocal microscopy to monitor the plasminogen-mediated adherence. Results of these studies revealed that host cell-bound plasminogen promotes pneumococcal adherence to human epithelial and endothelial cells in dose-dependent manner, whereas pneumococcal internalization was not enhanced. As an opposed effect pneumococcal-bound plasminogen reduced attachment to the epithelial and endothelial cells, and increased the interaction with neutrophil granulocytes. Moreover, the surface-displayed enolase, which serves as major pneumococcal plasminogen receptor, was identified as a key factor for plasminogen-mediated bacterial attachment in infection analyses with S. pneumoniae enolase mutants.

The choline-binding protein PspC of Streptococcus pneumoniae interacts with the C-terminal heparin-binding domain of vitronectin

Adherence of Streptococcus pneumoniae is directly mediated by interactions of adhesins with eukaryotic cellular receptors or indirectly by exploiting matrix and serum proteins as molecular bridges. Pneumococci engage vitronectin, the human adhesive glycoprotein and complement inhibitor, to facilitate attachment to epithelial cells of the mucosal cavity, thereby modulating host cell signaling. In this study, we identified PspC as a vitronectin-binding protein interacting with the C-terminal heparin-binding domain of vitronectin. PspC is a multifunctional surface-exposed choline-binding protein displaying various adhesive properties. Vitronectin binding required the R domains in the mature PspC protein, which are also essential for the interaction with the ectodomain of the polymeric immunoglobulin receptor and secretory IgA. Consequently, secretory IgA competitively inhibited binding of vitronectin to purified PspC and to PspC-expressing pneumococci. In contrast, Factor H, which binds to the N-terminal part of mature PspC molecules, did not interfere with the PspC-vitronectin interaction. Using a series of vitronectin peptides, the C-terminal heparin-binding domain was shown to be essential for the interaction of soluble vitronectin with PspC. Binding experiments with immobilized vitronectin suggested a region N-terminal to the identified heparin-binding domain as an additional binding region for PspC, suggesting that soluble, immobilized, as well as cellularly bound vitronectin possesses different conformations. Finally, vitronectin bound to PspC was functionally active and inhibited the deposition of the terminal complement complex. In conclusion, this study identifies and characterizes (on the molecular level) the interaction between the pneumococcal adhesin PspC and the human glycoprotein vitronectin.

Signalling or binding: the role of the platelet-activating factor receptor in invasive pneumococcal disease

Streptococcus pneumoniae (the pneumococcus) is an opportunistic human pathogen, which causes serious invasive disease such as pneumonia, bacteraemia and meningitis. The interaction of the bacteria with host receptors precedes the development of invasive disease. One host receptor implicated in pneumococcal adhesion to, invasion of and ultimately translocation of cell layers is the platelet-activating factor receptor (PAFR). PAFR is a G-protein coupled receptor which binds PAF, a potent phospholipid activator involved in many leucocyte functions, platelet aggregation and inflammation. PAFR has been proposed to bind S. pneumoniae and as such facilitate adhesion to, uptake by and transcytosis of endothelial cells leading to invasive disease. However, there is a shortage of biochemical data supporting direct interaction between PAFR and the bacteria, in addition to conflicting data on its role in development of invasive pneumococcal disease (IPD). In this review, we will discuss current literature on PAFR and S. pneumoniae and other pathogens,including data concerning human PAFR genetic variation related to IPD clinical aspects, to shed light on the importance of PAFR in IPD. Clarification of the role of this receptor in IPD development has the potential to enable the development of novel therapeutic strategies for treating pneumococcal disease by interfering with the PAFR.

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.

Streptococcus pneumoniae capsular serotype invasiveness correlates with the degree of factor H binding and opsonization with C3b/iC3b

Different capsular serotypes of Streptococcus pneumoniae vary markedly in their ability to cause invasive infection, but the reasons why are not known. As immunity to S. pneumoniae infection is highly complement dependent, variations in sensitivity to complement between S. pneumoniae capsular serotypes could affect invasiveness. We have used 20 capsule-switched variants of strain TIGR4 to investigate whether differences in the binding of the alternative pathway inhibitor factor H (FH) could be one mechanism causing variations in complement resistance and invasive potential between capsular serotypes. Flow cytometry assays were used to assess complement factor binding and complement-dependent neutrophil association for the TIGR4 capsule-switched strains. FH binding varied with the serotype and inversely correlated with the results of factor B binding, C3b/iC3b deposition, and neutrophil association. Differences between strains in FH binding were lost when assays were repeated with pspC mutant strains, and loss of PspC also reduced differences in C3b/iC3b deposition between strains. Median FH binding was high in capsule-switched mutant strains expressing more invasive serotypes, and a principal component analysis demonstrated a strong correlation between serotype invasiveness, high FH binding, and resistance to complement and neutrophil association. Further data obtained with 33 clinical strains also demonstrated that FH binding negatively correlated with C3b/iC3b deposition and that median FH binding was high in strains expressing more invasive serotypes. These data suggest that variations in complement resistance between S. pneumoniae strains and the association of a serotype with invasiveness could be related to capsular serotype effects on FH binding.

Enolase of Streptococcus pneumoniae binds human complement inhibitor C4b-binding protein and contributes to complement evasion

Streptococcus pneumoniae (pneumococcus) is a pathogen that causes severe local and life-threatening invasive diseases, which are associated with high mortality rates. Pneumococci have evolved several strategies to evade the host immune system, including complement to disseminate and to survive in various host niches. Thus, pneumococci bind complement inhibitors such as C4b-binding protein (C4BP) and factor H via pneumococcal surface protein C, thereby inhibiting the classical and alternative complement pathways. In this study, we identified the pneumococcal glycolytic enzyme enolase, a nonclassical cell surface and plasminogen-binding protein, as an additional pneumococcal C4BP-binding protein. Furthermore, we demonstrated that human, but not mouse, C4BP bound pneumococci. Recombinant enolase bound in a dose-dependent manner C4BP purified from plasma, and the interaction was reduced by increasing ionic strength. Enolase recruited C4BP and plasminogen, but not factor H, from human serum. Moreover, C4BP and plasminogen bound to different domains of enolase as they did not compete for the interaction with enolase. In direct binding assays with recombinant C4BP mutants lacking individual domains, two binding sites for enolase were identified on the complement control protein (CCP) domain 1/CCP2 and CCP8 of the C4BP α-chains. C4BP bound to the enolase retained its cofactor activity as determined by C4b degradation. Furthermore, in the presence of exogenously added enolase, an increased C4BP binding to and subsequently decreased C3b deposition on pneumococci was observed. Taken together, pneumococci specifically interact with human C4BP via enolase, which represents an additional mechanism of human complement control by this versatile pathogen.

Pneumococcal surface proteins: when the whole is greater than the sum of its parts

Surface-exposed proteins of pathogenic bacteria are considered as potential virulence factors through their direct contribution to host-pathogen interactions. Four families of surface proteins decorate the cell surface of the human pathogen Streptococcus pneumoniae. Besides lipoproteins and LPXTG proteins, also present in other gram-positive bacteria, the pneumococcus presents the choline-binding protein (CBP) family and the non-classical surface proteins (NCSPs). The CBPs present specific structural features that allow their anchorage to the cell envelope through non-covalent interaction with choline residues of lipoteichoic acid and teichoic acid. NCSP is an umbrella term for less characterized proteins displaying moonlighting functions on the pneumococcal surface that lack a leader peptide and membrane-anchor motif. Considering the unceasing evolution of microbial species under the selective pressure of antibiotic use, detailed understanding of the interaction between pathogen and the host cells is required for the development of novel therapeutic strategies to combat pneumococcal infections. This article reviews recent progress in the investigation of the three-dimensional structures of surface-exposed pneumococcal proteins. The modular nature of some of them produces a great versatility and sophistication of the virulence functions that, in most cases, cannot be deduced by the structural analysis of the isolated modules.

Impact of pneumococcal microbial surface components recognizing adhesive matrix molecules on colonization

Microorganisms have evolved elaborate strategies to adhere to host cells and to evade the host complement and immune attack, ensuring survival in various host niches and dissemination into sterile parts of the human body. Streptococcus pneumoniae (the pneumococcus) is not only a commensal of the human respiratory tract but also the etiological agent of severe and life-threatening diseases. Pneumococcal attachment to mucosal surfaces is a highly dynamic process requiring the contact of pneumococcal surface-exposed proteins with soluble or immobilized host factors. These avid interactions may trigger proteolytic cascades or result in engagement of cell surface receptors and intracellularly associated signaling machineries for subsequent uptake of pneumococci into host cells. In the present review, the intimate communication of S. pneumoniae molecules recognizing adhesive matrix molecules (microbial surface components recognizing adhesive matrix molecules) with their host counterparts and their individual role in pneumococcal colonization is discussed.

Acquisition of complement inhibitor serine protease factor I and its cofactors C4b-binding protein and factor H by Prevotella intermedia

Infection with the Gram-negative pathogen Prevotella intermedia gives rise to periodontitis and a growing number of studies implies an association of P. intermedia with rheumatoid arthritis. The serine protease Factor I (FI) is the central inhibitor of complement degrading complement components C3b and C4b in the presence of cofactors such as C4b-binding protein (C4BP) and Factor H (FH). Yet, the significance of complement inhibitor acquisition in P. intermedia infection and FI binding by Gram-negative pathogens has not been addressed. Here we show that P. intermedia isolates bound purified FI as well as FI directly from heat-inactivated human serum. FI bound to bacteria retained its serine protease activity as shown in degradation experiments with ¹²⁵I-labeled C4b. Since FI requires cofactors for its activity we also investigated the binding of purified cofactors C4BP and FH and found acquisition of both proteins, which retained their activity in FI mediated degradation of C3b and C4b. We propose that FI binding by P. intermedia represents a new mechanism contributing to complement evasion by a Gram-negative bacterial pathogen associated with chronic diseases.

Plasminogen is a complement inhibitor

Plasminogen is a 92-kDa single chain glycoprotein that circulates in plasma as a zymogen and when converted to proteolytically active plasmin dissolves preformed fibrin clots and extracellular matrix components. Here, we characterize the role of plasmin(ogen) in the complement cascade. Plasminogen binds the central complement protein C3, the C3 cleavage products C3b and C3d, and C5. Plasminogen binds to C3, C3b, C3d, and C5 via lysine residues, and the interaction is ionic strength-dependent. Plasminogen and Factor H bind C3b; however, the two proteins bind to different sites and do not compete for binding. Plasminogen affects complement action in multiple ways. Plasminogen enhanced Factor I-mediated C3b degradation in the presence of the cofactor Factor H. Plasminogen when activated to plasmin inhibited complement as demonstrated by hemolytic assays using either rabbit or sheep erythrocytes. Similarly, plasmin either in the fluid phase or attached to surfaces inhibited complement that was activated via the alternative and classical pathways and cleaved C3b to fragments of 68, 40, 30, and 17 kDa. The C3b fragments generated by plasmin differ in size from those generated by the complement protease Factor I, suggesting that plasmin-mediated C3b cleavage fragments lack effector function. Plasmin also cleaved C5 to products of 65, 50, 30, and 25 kDa. Thus, plasmin(ogen) regulates both complement and coagulation, the two central cascade systems of a vertebrate organism. This complement-inhibitory activity of plasmin provides a new explanation why pathogenic microbes utilize plasmin(ogen) for immune evasion and tissue penetration.

Factor h: a complement regulator in health and disease, and a mediator of cellular interactions

Complement is an essential part of innate immunity as it participates in host defense against infections, disposal of cellular debris and apoptotic cells, inflammatory processes and modulation of adaptive immune responses. Several soluble and membrane-bound regulators protect the host from the potentially deleterious effects of uncontrolled and misdirected complement activation. Factor H is a major soluble regulator of the alternative complement pathway, but it can also bind to host cells and tissues, protecting them from complement attack. Interactions of factor H with various endogenous ligands, such as pentraxins, extracellular matrix proteins and DNA are important in limiting local complement-mediated inflammation. Impaired regulatory as well as ligand and cell recognition functions of factor H, caused by mutations or autoantibodies, are associated with the kidney diseases: atypical hemolytic uremic syndrome and dense deposit disease and the eye disorder: age-related macular degeneration. In addition, factor H binds to receptors on host cells and is involved in adhesion, phagocytosis and modulation of cell activation. In this review we discuss current concepts on the physiological and pathophysiological roles of factor H in light of new data and recent developments in our understanding of the versatile roles of factor H as an inhibitor of complement activation and inflammation, as well as a mediator of cellular interactions. A detailed knowledge of the functions of factor H in health and disease is expected to unravel novel therapeutic intervention possibilities and to facilitate the development or improvement of therapies.

Heterologous expression of pneumococcal virulence factor PspC on the surface of Lactococcus lactis confers adhesive properties

Lactococcus lactis is a non-pathogenic bacterium that is used in the food industry but is also used as a heterologous host to reveal protein functions of pathogenic bacteria. The adhesin PspC from Streptococcus pneumoniae is a choline-binding protein that is non-covalently anchored to the bacterial cell wall. To assess the exclusive impact of pneumococcal surface protein C (PspC) on the interplay with its host we generated recombinant L. lactis producing a nisin-inducible and covalently anchored variant of PspC on the lactococcal cell surface. A translational fusion of the 5'-end of pspC3.4 with the 3'-end of hic (pspC11.4) was designed to decorate the surface of L. lactis with a chimeric PspC. The PspC3.4 part comprises the first 281 aa residues of PspC3.4, while the Hic sequence consists of the proline-rich and sortase-anchored domain. The results demonstrated that PspC is sufficient for adhesion and subsequent invasion of host epithelial cells expressing the human polymeric Ig receptor (hpIgR). Moreover, invasion via hpIgR was even more pronounced when the chimeric PspC was produced by lactococci compared with pneumococci. This study shows also for the first time that PspC plays no significant role during phagocytosis by macrophages. In contrast, recruitment of Factor H via the PspC chimer has a dramatic effect on phagocytosis of recombinant but not wild-type lactococci, as Factor H interacts specifically with the amino-terminal part of PspC and mediates the contact with phagocytes. Furthermore, L. lactis expressing PspC increased intracellular calcium levels in pIgR-expressing epithelial cells, thus resembling the effect of pneumococci, which induced release of Ca(2+) from intracellular stores via the PspC-pIgR mechanism. In conclusion, expression of the chimeric PspC confers adhesive properties to L. lactis and indicates the potential of L. lactis as a suitable host to study the impact of individual bacterial factors on their capacity to interfere with the host and manipulate eukaryotic epithelial cells.

Sequence TTKF ↓ QE defines the site of proteolytic cleavage in Mhp683 protein, a novel glycosaminoglycan and cilium adhesin of Mycoplasma hyopneumoniae

Mycoplasma hyopneumoniae colonizes the ciliated respiratory epithelium of swine, disrupting mucociliary function and inducing chronic inflammation. P97 and P102 family members are major surface proteins of M. hyopneumoniae and play key roles in colonizing cilia via interactions with glycosaminoglycans and mucin. The p102 paralog, mhp683, and homologs in strains from different geographic origins encode a 135-kDa pre-protein (P135) that is cleaved into three fragments identified here as P45(683), P48(683), and P50(683). A peptide sequence (TTKF↓QE) was identified surrounding both cleavage sites in Mhp683. N-terminal sequences of P48(683) and P50(683), determined by Edman degradation and mass spectrometry, confirmed cleavage after the phenylalanine residue. A similar proteolytic cleavage site was identified by mass spectrometry in another paralog of the P97/P102 family. Trypsin digestion and surface biotinylation studies showed that P45(683), P48(683), and P50(683) reside on the M. hyopneumoniae cell surface. Binding assays of recombinant proteins F1(683)-F5(683), spanning Mhp683, showed saturable and dose-dependent binding to biotinylated heparin that was inhibited by unlabeled heparin, fucoidan, and mucin. F1(683)-F5(683) also bound porcine epithelial cilia, and antisera to F2(683) and F5(683) significantly inhibited cilium binding by M. hyopneumoniae cells. These data suggest that P45(683), P48(683), and P50(683) each display cilium- and proteoglycan-binding sites. Mhp683 is the first characterized glycosaminoglycan-binding member of the P102 family.

Characterization of a new mouse model of empyema and the mechanisms of pleural invasion by Streptococcus pneumoniae

Although empyema affects more than 65,000 people each year in the United States and in the United Kingdom, there are limited data on the pathogenesis of pleural infection. We investigated the pathogenesis of empyema using animal and cell culture models of Streptococcus pneumoniae infection. The pathological processes during the development of empyema associated with murine pneumonia due to S. pneumoniae (strain D39) were investigated. Lungs were examined using histology, and pleural fluid and blood bacterial colony-forming units, cytokine levels, and cellular infiltrate were determined over time. Bacterial migration across mesothelial monolayers was investigated using cell culture techniques, flow cytometry, and confocal microscopy. After intranasal inoculation with 10(7) S. pneumoniae D39 strain, mice developed pneumonia associated with rapid bacterial invasion of the pleural space; raised intrapleural IL-8, VEGF, MCP-1, and TNF-α levels; and caused significant intrapleural neutrophilia followed by the development of fibrinous pleural adhesions. Bacterial clearance from the pleural space was poor, and in vitro assays demonstrated that S. pneumoniae crossed mesothelial layers by translocation through cells rather than by a paracellular route. This study describes key events during the development of S. pneumoniae empyema using a novel murine model of pneumonia-associated empyema that closely mimics human disease. The model allows for future assessment of molecular mechanisms involved in the development of empyema and evaluation of potential new therapies. The data suggest that transmigration of bacteria through mesothelial cells could be important in empyema development. Furthermore, upon entry the pleural cavity offers a protected compartment for the bacteria.

Immune escape of the human facultative pathogenic yeast Candida albicans: the many faces of the Candida Pra1 protein

Infectious diseases caused by human pathogenic fungi represent a major and global health problem. Based on the limited efficacy of existing drugs and the increasing resistance to antifungal compounds, new strategies are urgently needed to fight such fungal infections. The medically important pathogen Candida albicans can exist as an opportunistic yeast, but can also cause severe diseases, septicaemia, and death. In order to establish new strategies to fight Candida infections and to interfere with survival of the pathogen, it is highly relevant to understand the molecular and immunology interactions between the pathogen C. albicans and the human host. This immune cross talk has moved into the focus of infectious disease research. In this review, we summarize the diverse and multiple levels of the immune cross talk between the fungal pathogen C. albicans and the human host. In particular, we define how one single fungal protein Pra1 (pH-regulated antigen 1) interferes and controls host immune attack at multiple levels and thus contributes to fungal immune escape. Candida Pra1 represents a promising candidate for immune interference.

The pH-regulated antigen 1 of Candida albicans binds the human complement inhibitor C4b-binding protein and mediates fungal complement evasion

Candida albicans binds and utilizes human complement inhibitors, such as C4b-binding protein (C4BP), Factor H, and FHL-1 for immune evasion. Here, we identify Candida pH-regulated antigen 1 (Pra1) as the first fungal C4BP-binding protein. Recombinant Pra1 binds C4BP, as shown by ELISA and isothermal titration calorimetry, and the Pra1-C4BP interaction is ionic in nature. The Pra1 binding domains within C4BP were localized to the complement control protein domain 4 (CCP4), CCP7, and CCP8. C4BP bound to Pra1 maintains complement-inhibitory activity. C4BP and Factor H bind simultaneously to Candida Pra1 and do not compete for binding at physiological levels. A Pra1-overexpressing C. albicans strain, which had about 2-fold Pra1 levels at the surface acquired also about 2-fold C4BP to the surface, compared with the wild type strain CAI4. A Pra1 knock-out strain showed ∼22% reduced C4BP binding. C4BP captured by C. albicans from human serum inhibits C4b and C3b surface deposition and also maintains cofactor activity. In summary, Candida Pra1 represents the first fungal C4BP-binding surface protein. Pra1, via binding to C4BP, mediates human complement control, thereby favoring the immune and complement evasion of C. albicans.

Cellular interactions by LPxTG-anchored pneumococcal adhesins and their streptococcal homologues

In this review we focus on three important families of LPxTG-anchored adhesins in the human pathogen Streptococcus pneumoniae, but also their homologues in related streptococci. We discuss the contribution of these streptococcal adhesins to host tropism, pathogenesis and their interactions with different host cell types. The first surface structures discussed are the heteropolymeric pili that have been found in important streptococcal pathogens such as S. pneumoniae, S. pyogenes, S. agalactiae and E. faecalis/faecium. Major and minor pilus subunit proteins are covalently joined and finally attached to the cell wall through the action of specific sortases. The role of pili and individual pilin subunits in adhesion and pathogenesis and their structure and assembly in different streptococcal species are being covered. Furthermore, we address recent findings regarding a family of large glycosylated serine-rich repeat (SRR) proteins that act as fibrillar adhesins for which homologues have been found in several streptococcal species including pneumococci. In the pneumococcal genome both pili and its giant SRR protein are encoded by accessory genes present in particular clonal lineages for which epidemiological information is available. Finally, we briefly discuss the role played by the pneumococcal neuraminidase NanA in adhesion and pathogenesis.