Receptors for fibrinogen and aggregated beta 2-microglobulin detected in strains of group B streptococci

FbsC, a novel fibrinogen-binding protein, promotes Streptococcus agalactiae-host cell interactions

Streptococcus agalactiae (group B Streptococcus or GBS) is a common cause of invasive infections in newborn infants and adults. The ability of GBS to bind human fibrinogen is of crucial importance in promoting colonization and invasion of host barriers. We characterized here a novel fibrinogen-binding protein of GBS, designated FbsC (Gbs0791), which is encoded by the prototype GBS strain NEM316. FbsC, which bears two bacterial immunoglobulin-like tandem repeat domains and a C-terminal cell wall-anchoring motif (LPXTG), was found to be covalently linked to the cell wall by the housekeeping sortase A. Studies using recombinant FbsC indicated that it binds fibrinogen in a dose-dependent and saturable manner, and with moderate affinity. Expression of FbsC was detected in all clinical GBS isolates, except those belonging to the hypervirulent lineage ST17. Deletion of fbsC decreases NEM316 abilities to adhere to and invade human epithelial and endothelial cells, and to form biofilm in vitro. Notably, bacterial adhesion to fibrinogen and fibrinogen binding to bacterial cells were abolished following fbsC deletion in NEM316. Moreover, the virulence of the fbsC deletion mutant and its ability to colonize the brain were impaired in murine models of infection. Finally, immunization with recombinant FbsC significantly protected mice from lethal GBS challenge. In conclusion, FbsC is a novel fibrinogen-binding protein expressed by most GBS isolates that functions as a virulence factor by promoting invasion of epithelial and endothelial barriers. In addition, the protein has significant immunoprotective activity and may be a useful component of an anti-GBS vaccine.

FbsA-driven fibrinogen polymerization: a bacterial "deceiving strategy"

We show that FbsA, a cell wall protein of the bacterium Streptococcus agalactiae, promotes large-scale aggregation of human plasma fibrinogen, leading to the formation of a semiflexible polymerlike network. This extensive aggregation process takes place not only in solution, but also on FbsA-functionalized colloidal particles, and leads to the formation of a thick layer on the bacterial cell wall itself, which becomes an efficient mask against phagocytosis.

Adherence to and invasion of human brain microvascular endothelial cells are promoted by fibrinogen-binding protein FbsA of Streptococcus agalactiae

Streptococcus agalactiae is a frequent cause of bacterial sepsis and meningitis in neonates. During the course of infection, S. agalactiae colonizes and invades a number of host compartments, thereby interacting with different host tissues. Deletion of the fbsA gene, encoding the fibrinogen protein FbsA, significantly impaired the adherence and invasion of human brain microvascular endothelial cells (HBMEC) by S. agalactiae. The adherence and invasiveness of an fbsA deletion mutant were restored by reintroducing the fbsA gene on an expression vector. Heterologous expression of fbsA in Lactococcus lactis enabled this bacterium to adhere to but not to invade HBMEC, suggesting that FbsA is a streptococcal adhesin. Finally, host cell adherence and invasion were significantly blocked in competition experiments with either purified FbsA fusion protein or a monoclonal antibody directed against the fibrinogen-binding epitope of FbsA. The S. agalactiae fbsA mutant induced a release of the neutrophil chemoattractant interleukin-8 (IL-8) equal to that induced by the wild type. Taken together, our studies demonstrate that FbsA promotes the adherence of S. agalactiae to HBMEC but that FbsA neither mediates the bacterial invasion into host cells nor plays a role in IL-8 release for HBMEC.

The novel fibrinogen-binding protein FbsB promotes Streptococcus agalactiae invasion into epithelial cells

Streptococcus agalactiae is a major cause of bacterial sepsis and meningitis in human newborns. The interaction of S. agalactiae with host proteins and the entry into host cells thereby represent important virulence traits of these bacteria. The present report describes the identification of the fbsB gene, encoding a novel fibrinogen-binding protein that plays a crucial role in the invasion of S. agalactiae into human cells. In Western blots and enzyme-linked immunosorbent assay (ELISA) experiments, the FbsB protein was demonstrated to interact with soluble and immobilized fibrinogen. Binding studies showed the N-terminal 388 residues of FbsB and the Aalpha-subunit of human fibrinogen to recognize each other. By reverse transcription (RT)-PCR, the fbsB gene was shown to be cotranscribed with the gbs0851 gene in S. agalactiae. Deletion of the fbsB gene in the genome of S. agalactiae did not influence the binding of the bacteria to fibrinogen, suggesting that FbsB does not participate in the attachment of S. agalactiae to fibrinogen. In tissue culture experiments, however, the fbsB deletion mutant was severely impaired in its invasion into lung epithelial cells. Bacterial invasion could be reestablished by introducing the fbsB gene on a shuttle plasmid into the fbsB deletion mutant. Furthermore, treatment of lung epithelial cells with FbsB fusion protein blocked S. agalactiae invasion of epithelial cells in a dose-dependent fashion. These results suggest an important role of the FbsB protein in the overall process of host cell entry by S. agalactiae.

A fibrinogen receptor from group B Streptococcus interacts with fibrinogen by repetitive units with novel ligand binding sites

Group B Streptococcus (GBS) is a frequent cause of bacterial sepsis and meningitis in neonates. During the course of infection, GBS colonizes and invades a number of host compartments, thereby interacting with different host proteins. In the present report, we describe the isolation of the fbsA gene, which encodes a fibrinogen receptor from GBS. The deduced FbsA protein is characterized by repetitive units, each 16 amino acids in length. Sequencing of the fbsA gene from five different GBS strains revealed significant variation in the number of repeat-encoding units. The deletion of the fbsA gene in the genome of GBS 6313 completely abolished fibrinogen binding, suggesting that FbsA is the major fibrinogen receptor in this strain. Growth of the fbsA deletion mutant in human blood was significantly impaired, indicating that FbsA protects GBS from opsonophagocytosis. In Western blot experiments with truncated FbsA -proteins, the repeat region of FbsA was identified as mediating fibrinogen binding. Using synthetic peptides, even a single repeat unit of FbsA was demonstrated to bind to fibrinogen. Spot membrane analysis and competitive binding experiments with peptides carrying single amino acid substitutions allowed the prediction of a fibrinogen-binding motif with the consensus sequence G-N/S/T-V-L-A/E/M/Q-R-R-X-K/R/W-A/D/E/N/Q-A/F/I/L/V/Y-X-X-K/R-X-X.

Binding of oral streptococci to human fibrinogen

The interaction of oral streptococci with human fibrinogen was investigated. Streptococcus gordonii was chosen as a representative species to study the binding to fibrinogen. S. gordonii DL1 adhered to immobilized fibrinogen and bovine serum albumin. Binding to immobilized fibrinogen was saturable, concentration and temperature dependent. The binding of S. gordonii DL1 to fibrinogen was inhibited by anti-fibrinogen antibody. Heating of the bacteria for 1 h at 95 degrees C resulted in 90% inhibition of the binding. Trypsin treatment of the bacteria resulted in decreased binding. Neither lipoteichoic acid nor culturing of the bacteria in a sucrose-supplemented medium had any effect on the binding. S. gordonii, Streptococcus sanguinis, Streptococcus mitis, and Streptococcus oralis bound to the immobilized fibrinogen, but mutans streptococci did not. None of the oral streptococci tested bound to the fibrinogen in fluid phase. These results suggest that the binding of S. gordonii DL1 to immobilized fibrinogen is mediated through a specific protein adhesin-receptor interaction, and fibrinogen acts as a cryptitope.

Specific binding sites for monomeric and aggregated beta 2-microglobulin on surface of groups A, B, C, and G streptococci

Human beta 2-microglobulin (beta 2-m) was isolated from urine samples of patients with tubular dysfunctions and aggregated with glutaraldehyde. Four aggregates with molecular weights of 800,000, 480,000, 260,000, and 60,000 were separated by filtration on Sephacryl S-300. The aggregates and monomeric beta 2-m (11,800 MW) were subsequently labeled with 125I and tested for binding to streptococci. Group A streptococci bound only aggregated beta 2-m with a mean binding of 44.5%. Most of the group G streptococci, on the other hand, bound only monomeric beta 2-m with a mean binding of 58%. Among group B streptococci the serotypes with protein antigens interacted mainly with monomeric beta 2-m and those without protein antigens preferentially with aggregated beta 2-m. Nontypable group B streptococcal serotypes did not bind monomeric or aggregated beta 2-m. Of the streptococci belonging to group C, S. equisimilis reacted with monomeric beta 2-m and S. dysgalactiae with aggregated beta 2-m. S. equi did not interact with monomeric beta 2-m or aggregated beta 2-m. Bindings of monomeric beta 2-m and aggregated beta 2-m were saturable and could be inhibited by the respective unlabeled forms of beta 2-m. Fibrinogen, fibronectin, alpha 2-macroglobulin, haptoglobin, or immunoglobulin G did not inhibit the binding of either form of beta 2-m. The binding sites for monomeric beta 2-m were more susceptible to trypsin than those for aggregated beta 2-m. Treatment of streptococci with pronase destroyed their binding activities for monomeric and aggregated beta 2-m. Both monomeric beta 2-m and aggregated beta 2-m binding sites were sensitive to heat. The Scatchard plots of monomeric beta 2-m and aggregated beta 2-m were linear with Kd of 1.29 X 10(-9) M and 1.9 X 10(-9) M respectively. The number of binding sites per bacterium were estimated to be 81,000 for monomeric beta 2-m and 1,210 for aggregated beta 2-m.

Variations in binding of mammalian fibrinogens to streptococci groups A, B, C, E, G and to Staphylococcus aureus

Twenty-eight beta-hemolytic streptococci of groups A, B, E, G and Streptococcus equisimilis as well as four Staphylococcus aureus strains were tested for their ability to bind fibrinogen preparations from different animal species: homo, baboon, rabbit, rat, guinea-pig, dog, horse, pig, cow and sheep. The patterns of binding indicated differences in the structures of the bacterial fibrinogen receptors. There were higher binding levels in streptococci groups A, G, and S. equisimilis than in representative group B and E strains. Considerable differences in the binding capacity were found within streptococci groups A and E. Group C and group G strains showed rather similar patterns and could be further divided into high-level and low-level binding strains. There is no correlation between binding levels of different animal fibrinogen preparations and the strains isolated from corresponding animals. Recent studies by others have shown that resistance to phagocytosis is mediated by fibrinogen-binding in streptococci group A. The existence of similar fibrinogen-binding structures in several streptococcal species indicates an important role with a definite survival value. It also suggests that M or T protein analogues are present in streptococci groups C, G and E.

On the interaction between beta 2-microglobulin and group A streptococci

beta 2-microglobulin (beta 2m) was found to interact with many group A streptococcal strains. The interaction appeared to require multipoint attachment, since monomeric beta 2m in solution showed no binding, whereas both beta 2m monomers bound to liposomes, and beta 2m in aggregates showed affinity for the bacteria. Aggregated HLA antigens (-A, -B and -C) and aggregated beta 2m exhibited the same binding patterns when tested in binding experiments with various group A streptococcal strains. Furthermore, beta 2m aggregates in excess completely blocked the binding of aggregated HLA antigens, thereby demonstrating that beta 2m is able to interact with streptococcal surface structures also when it is part of the HLA antigen complex. M protein-positive group A streptococcal strains bound significantly more beta 2m than M protein-negative variants of these strains. Purified M 12 protein partly inhibited the binding of radiolabelled beta 2m aggregates to whole streptococci, and in gel filtration and affinity chromatography experiments, the M 12 protein interacted with beta 2m. These various data suggest that the interaction between beta 2m and group A streptococci could be mediated by M protein. Lipoteichoic acid (LTA) is a constituent of the streptococcal cell wall that has been reported to form complexes with M protein at the bacterial cell surface. However, LTA did not influence the interaction between beta 2m and streptococci, suggesting that the binding of beta 2m to streptococcal M protein represents a pure protein-protein interaction. In vivo such an interaction could be established between infecting streptococci and host cells. Among 45 strains of different M types large differences in beta 2m binding were recorded, whereas among 60 strains of the classical nephritogenic M types 12 and 49, all were highly beta 2m-reactive, which points towards a role for beta 2m in streptococcal pathogenicity.

Guanidine extraction enhances the binding of human fibrinogen to group-B streptococci

Treatment of group-B streptococci with guanidine chloride significantly enhanced their capacity to bind 125I-labelled fibrinogen. The increase in binding activity was almost proportional in the range of 0.05 M to 6 M guanidine chloride. Repeated extractions with guanidine chloride further increased the capacity of B streptococci to bind the labelled fibrinogen. On the other hand, urea, even at concentrations up to 8 M, did not alter the fibrinogen binding. The enhancement was most pronounced with B streptococci of serotypes III and III-R. Cultivation of these streptococci in Baker and Kasper medium, which is known to stimulate formation of microcapsules, reduced 125I-fibrinogen binding. Subsequent treatment with guanidine increased their binding activity, presumably by facilitating the accessibility of the binding sites.

Binding of human fibrinogen and its polypeptide chains to group B streptococci

Of 51 group-B streptococcal cultures, 20 bound 125I-labelled human fibrinogen. In contrast to the streptococci of groups A, C and G, the group-B streptococci interacted more with the alpha, beta-chain of fibrinogen than with whole fibrinogen. None of the streptococcal cultures reacted with gamma-chain. The fibrinogen-negative group-B streptococci still bound the alpha, beta-chain. The binding of 125I-labelled, alpha, beta-chain could be inhibited by unlabelled fibrinogen with fibrinogen-positive, but not with fibrinogen-negative streptococci of group B.

Variations in the binding of mammalian fibrinogens to streptococci of different animal origin

Binding of 125I-labelled fibrinogen from humans, bovines, equines, canines and ovines by streptococci of serological groups A, B, C and G was determined quantitatively. All 59 randomly selected streptococcal cultures generally bound more human fibrinogen than the other fibrinogens. Only Sc. dysgalactiae had a higher affinity for bovine fibrinogen. In addition, Sc. dysgalactiae bound distinctly more equine, canine and ovine fibrinogen than the other streptococci. Some cultures of Sc. equi and Sc. zooepidemicus had high binding activities for equine fibrinogen. Low binding capacities were exhibited by B-streptococci, particularly with fibrinogens from horses and dogs.

Absorption of human alpha 2-macroglobulin with selected strains of streptococci

A new interaction was observed between human alpha 2-macroglobulin (alpha 2M) and selected strains of streptococci. The streptococci bound alpha 2M. After its elution from the bacteria, alpha 2M was demonstrated by immunoelectrophoresis against anti-human alpha 2M. The binding of alpha 2M to one streptococcal strain of serological group C and two of group G was confirmed by radial immunodiffusion. Treating the streptococci with trypsin reduced their reactivity with alpha 2M. On the other hand, prolonged treatment with 6 M guanidine-HCl had no effect on the alpha 2M binding. Binding sites for alpha 2M were most likely protein structures on the streptococcal surface.