Interactions of fibronectin with Treponema pallidum

Treponema pallidum subsp. pallidum TP0136 protein is heterogeneous among isolates and binds cellular and plasma fibronectin via its NH2-terminal end

Adherence-mediated colonization plays an important role in pathogenesis of microbial infections, particularly those caused by extracellular pathogens responsible for systemic diseases, such as Treponema pallidum subsp. pallidum (T. pallidum), the agent of syphilis. Among T. pallidum adhesins, TP0136 is known to bind fibronectin (Fn), an important constituent of the host extracellular matrix. To deepen our understanding of the TP0136-Fn interaction dynamics, we used two naturally-occurring sequence variants of the TP0136 protein to investigate which region of the protein is responsible for Fn binding, and whether TP0136 would adhere to human cellular Fn in addition to plasma Fn and super Fn as previously reported. Fn binding assays were performed with recombinant proteins representing the two full-length TP0136 variants and their discrete regions. As a complementary approach, we tested inhibition of T. pallidum binding to Fn by recombinant full-length TP0136 proteins and fragments, as well as by anti-TP0136 immune sera. Our results show that TP0136 adheres more efficiently to cellular Fn than to plasma Fn, that the TP0136 NH2-terminal conserved region of the protein is primarily responsible for binding to plasma Fn but that binding sites for cellular Fn are also present in the protein’s central and COOH-terminal regions. Additionally, message quantification studies show that tp0136 is highly transcribed during experimental infection, and that its message level increases in parallel to the host immune pressure on the pathogen, which suggests a possible role for this protein in T. pallidum persistence. In a time where syphilis incidence is high, our data will help in the quest to identify suitable targets for development of a much needed vaccine against this important disease.

The study of Treponema pallidum subsp. pallidum (T. pallidum) proteins that mediate adhesion to host tissue components is pivotal to understand how the syphilis agent establishes infection and is able to invade virtually every organ system following dissemination from the site of entry. This study focuses on T. pallidum TP0136, a known plasma fibronectin (Fn) and super Fn binding protein that is heterogeneous in sequence among T. pallidum isolates. This study shows that TP0136 also mediates attachment to human cellular Fn, that TP0136 conserved NH₂-terminus is primarily responsible for binding to plasma Fn, but that cellular Fn binding sites appears to be scattered throughout the molecule. Message quantification experiments reveal that tp0136 transcription is high during experimental syphilis and increases at the time of bacterial immune clearance, suggesting a role for this antigen in counteracting the host defenses during infection, as reported for other Fn binding proteins in other pathogens. Our data deepen the current knowledge of the function of T. pallidum TP0136 and further support a role for this virulence factor in syphilis pathogenesis.

Fibronectin binding to the Treponema pallidum adhesin protein fragment rTp0483 on functionalized self-assembled monolayers

Past work has shown that Treponema pallidum, the causative agent of syphilis, binds host fibronectin (FN). FN and other host proteins are believed to bind to rare outer membrane proteins (OMPs) of T. pallidum, and it is postulated that this interaction may facilitate cell attachment and mask antigenic targets on the surface. This research seeks to prepare a surface capable of mimicking the FN binding ability of T. pallidum in order to investigate the impact of FN binding with adsorbed Tp0483 on the host response to the surface. By understanding this interaction, it may be possible to develop more effective treatments for infection and possibly mimic the stealth properties of the bacteria. Functionalized self-assembled monolayers (SAMs) on gold were used to investigate rTp0483 and FN adsorption. Using a quartz crystal microbalance (QCM), rTp0483 adsorption and subsequent FN adsorption onto rTp0483 were determined to be higher on negatively charged carboxylate-terminated self-assembled monolayers (-COO(-) SAMs) compared to the other surfaces analyzed. Kinetic analysis of rTp0483 adsorption using surface plasmon resonance (SPR) supported this finding. Kinetic analysis of FN adsorption using SPR revealed a multistep event, where the concentration of immobilized rTp0483 plays a role in FN binding. An examination of relative QCM dissipation energy compared to the shift in frequency showed a correlation between the physical properties of adsorbed rTp0483 and SAM surface chemistry. In addition, AFM images of rTp0483 on selected SAMs illustrated a preference of rTp0483 to bind as aggregates. Adsorption on -COO(-) SAMs was more uniform across the surface, which may help further explain why FN bound more strongly. rTp0483 antibody studies suggested the involvement of amino acids 274-289 and 316-333 in binding between rTp0483 to FN, while a peptide blocking study only showed inhibition of binding with amino acids 316-333. Finally, surface adsorbed rTp0483 with FN bound significantly less anti-RGD and gelatin compared to FN adsorbed directly to -COO(-) SAMs, indicating that one or both binding regions may play a role in binding between rTp0483 and FN.

Bifunctional role of the Treponema pallidum extracellular matrix binding adhesin Tp0751

Treponema pallidum, the causative agent of syphilis, is a highly invasive pathogenic spirochete capable of attaching to host cells, invading the tissue barrier, and undergoing rapid widespread dissemination via the circulatory system. The T. pallidum adhesin Tp0751 was previously shown to bind laminin, the most abundant component of the basement membrane, suggesting a role for this adhesin in host tissue colonization and bacterial dissemination. We hypothesized that similar to that of other invasive pathogens, the interaction of T. pallidum with host coagulation proteins, such as fibrinogen, may also be crucial for dissemination via the circulatory system. To test this prediction, we used enzyme-linked immunosorbent assay (ELISA) methodology to demonstrate specific binding of soluble recombinant Tp0751 to human fibrinogen. Click-chemistry-based palmitoylation profiling of heterologously expressed Tp0751 confirmed the presence of a lipid attachment site within this adhesin. Analysis of the Tp0751 primary sequence revealed the presence of a C-terminal putative HEXXH metalloprotease motif, and in vitro degradation assays confirmed that recombinant Tp0751 purified from both insect and Escherichia coli expression systems degrades human fibrinogen and laminin. The proteolytic activity of Tp0751 was abolished by the presence of the metalloprotease inhibitor 1,10-phenanthroline. Further, inductively coupled plasma-mass spectrometry showed that Tp0751 binds zinc and calcium. Collectively, these results indicate that Tp0751 is a zinc-dependent, membrane-associated protease that exhibits metalloprotease-like characteristics. However, site-directed mutagenesis of the HEXXH motif to HQXXH did not abolish the proteolytic activity of Tp0751, indicating that further mutagenesis studies are required to elucidate the critical active site residues associated with this protein. This study represents the first published description of a T. pallidum protease capable of degrading host components and thus provides novel insight into the mechanism of T. pallidum dissemination.

Biological basis for syphilis

Syphilis is a chronic sexually transmitted disease caused by Treponema pallidum subsp. pallidum. Clinical manifestations separate the disease into stages; late stages of disease are now uncommon compared to the preantibiotic era. T. pallidum has an unusually small genome and lacks genes that encode many metabolic functions and classical virulence factors. The organism is extremely sensitive to environmental conditions and has not been continuously cultivated in vitro. Nonetheless, T. pallidum is highly infectious and survives for decades in the untreated host. Early syphilis lesions result from the host's immune response to the treponemes. Bacterial clearance and resolution of early lesions results from a delayed hypersensitivity response, although some organisms escape to cause persistent infection. One factor contributing to T. pallidum's chronicity is the paucity of integral outer membrane proteins, rendering intact organisms virtually invisible to the immune system. Antigenic variation of TprK, a putative surface-exposed protein, is likely to contribute to immune evasion. T. pallidum remains exquisitely sensitive to penicillin, but macrolide resistance has recently been identified in a number of geographic regions. The development of a syphilis vaccine, thus far elusive, would have a significant positive impact on global health.

Treponema pallidum fibronectin-binding proteins

Putative adhesins were predicted by computer analysis of the Treponema pallidum genome. Two treponemal proteins, Tp0155 and Tp0483, demonstrated specific attachment to fibronectin, blocked bacterial adherence to fibronectin-coated slides, and supported attachment of fibronectin-producing mammalian cells. These results suggest Tp0155 and Tp0483 are fibronectin-binding proteins mediating T. pallidum-host interactions.

Polypeptides of Treponema pallidum: progress toward understanding their structural, functional, and immunologic roles. Treponema Pallidum Polypeptide Research Group

Treponema pallidum subsp. pallidum, the spirochete that causes syphilis, is unusual in a number of respects, including its small genome size, inability to grow under standard in vitro culture conditions, microaerophilism, apparent paucity of outer membrane proteins, structurally complex periplasmic flagella, and ability to evade the host immune responses and cause disease over a period of years to decades. Many of these attributes are related ultimately to its protein content. Our knowledge of the activities, structure, and immunogenicity of its proteins has been expanded by the application of recombinant DNA, hybridoma, and structural fractionation techniques. The purpose of this monograph is to summarize and correlate this new information by using two-dimensional gel electrophoresis, monoclonal antibody reactivity, sequence data, and other properties as the bases of polypeptide identification. The protein profiles of the T. pallidum subspecies causing syphilis, yaws, and endemic syphilis are virtually indistinguishable but differ considerably from those of other treponemal species. Among the most abundant polypeptides are a group of lipoproteins of unknown function that appear to be important in the immune response during syphilitic infection. The periplasmic flagella of T. pallidum and other spirochetes are unique with regard to their protein content and ultrastructure, as well as their periplasmic location. They are composed of three core proteins (homologous to the other members of the eubacterial flagellin family) and a single, unrelated sheath protein; the functional significance of this arrangement is not understood at present. Although the bacterium contains the chaperonins GroEL and DnaK, these proteins are not under the control of the heat shock regulon as they are in most organisms. Studies of the immunogenicity of T. pallidum proteins indicate that many may be useful for immunodiagnosis and immunoprotection. Future goals in T. pallidum polypeptide research include continued elucidation of their structural locations and functional activities, identification and characterization of the low-abundance outer membrane proteins, further study of the immunoprotective and immunodiagnostic potential of T. pallidum proteins, and clarification of the roles of treponemal proteins in pathogenesis.

Immunoglobulin class and subclass restriction of autoimmune responses in secondary syphilis

The immunoglobulin (Ig) class and IgG subclasses of autoantibodies to commercial VDRL antigen, creatine kinase (CK), and fibronectin (Fn) in the sera of patients with various stages of syphilis were quantified using solid-phase radioimmunoassays (RIA) and ELISA. Sera from patients with active secondary syphilis, initially positive for anti-Fn and anti-CK autoantibodies by RIA, were re-evaluated by ELISA using monoclonal antibodies (MoAb) for detection of human Ig class and subclass responses. Results of these assays revealed that anti-Fn and anti-CK responses were not only IgG in nature, but dramatically skewed to IgG1 and IgG3 subclasses. While the restricted, co-expression of these isotypes seemingly paralleled anti-treponemal activity, inverse relationships actually existed between the subclass responses to Fn and those to Treponema pallidum. In contrast, anti-VDRL were predominantly IgM in 17 of 22 patients. Of those sera exhibiting detectable anti-VDRL IgG activity, responses appeared to be restricted to IgG1. These results suggest that different control mechanisms may be responsible for regulation of the various autoantibody responses expressed during syphilitic infection.

Kinetics of pathogen-specific humoral response in Treponema pallidum-infected young and old inbred strain 2 guinea pigs

The kinetics of the humoral response to pathogen-specific polypeptides was examined in Treponema pallidum-infected young (3-5 months old) and old (10-20 months old) inbred strain-2 guinea pigs. Sera collected before and at various times after infection were pooled and examined by immunoblotting and two serologic tests (ELISA and FTA) before and after sequential adsorption with CNBr-activated sepharose coupled to normal rabbit proteins and antigens from five nonpathogenic treponemal species. Prior to adsorption the kinetics of the humoral response to T. pallidum antigens did not seem to differ significantly between the two groups. After adsorption, however, a delay in the appearance of detectable antibodies and a milder response to various pathogen-specific polypeptides was observed in the older group. After adsorption, a sharp drop in ELISA-TP, ELISA-TR and FTA titres occurred in both groups. Six months post-infection, between 9 and 10 pathogen-specific polypeptides (97, 57, 47, 45, 43, 39, 37, 33, 17 and 15 kD) were recognized by both groups. The effect of age and levels of natural treponemal antibodies on the clinical symptoms of primary lesions and humoral response to pathogen-specific polypeptides is discussed.

Factors that inhibit adherence of Treponema pallidum (Nichols strain) to a human fibroblastic cell line: development in serum of patients with syphilis

Serum samples from 25 patients at five different stages of syphilis were investigated for their ability to inhibit the adherence of pathogenic Nichols treponemes to cultured human fibroblasts. Serum taken from patients at the end of the primary stage showed an appreciable inhibition of treponemal adherence, and maximum inhibition of adherence was produced by serum from patients with secondary syphilis. Some freshly harvested treponemal suspensions were resistant to the adherence inhibition factors in serum from patients with syphilis; after incubation in vitro for 24 hours this resistance was lost. In vitro incubation almost doubled the number of adherent treponemes/fibroblast. These phenomena are discussed in terms of loss and reconstruction of the treponemal outer envelope. This leads to the suggestion that adherence occurs not only at the tips of the treponemes, but that surface components are also implicated, possibly as an initial contact mechanism. The composition of the outer envelope may in this way determine localisation versus dissemination of the treponemes.

Antibody-independent interactions of fibronectin, C1q, and human neutrophils with Treponema pallidum

Although recent evidence suggests that fibronectin may be involved in the attachment of treponemes to mammalian cells, its possible role in promoting phagocytosis of Treponema pallidum has not been investigated. In the present study, we examined the antibody-independent interactions of fibronectin, C1q, and human polymorphonuclear leukocytes with T. pallidum. Binding of [125I]fibronectin was specific and saturable with an affinity constant of approximately 2 X 10(7) M-1. The number of binding sites per treponeme at 37 degrees C, irrespective of the mammalian source of fibronectin, was between 2,500 and 7,500, with a mean of approximately 4,700. Binding of [125I]C1q to T. pallidum, in the absence of antibodies to the organism, also was saturable and specific. Pretreatment of treponemes with C1q enhanced binding of soluble [125I]fibronectin two- to threefold and also increased attachment of 125I-surface-labeled treponemes to fibronectin-coated surfaces. Treatment of 125I-labeled T. pallidum with fibronectin alone, or together with C1q, however, did not enhance surface phagocytosis by neutrophils.

Antiserum to the 33,000-dalton periplasmic-flagellum protein of "Treponema phagedenis" reacts with other treponemes and Spirochaeta aurantia

"Treponema phagedenis" periplasmic flagella (PF) have two major protein bands at molecular weights of 33,000 and 39,800 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (R. J. Limberger and N. W. Charon, J. Bacteriol. 166:105-112, 1986). By use of Western blotting and a polyclonal antiserum directed toward the 33,000-molecular-weight PF protein, cell lysates of 12 species of spirochetes were surveyed for reactivity. Eight species of Treponema as well as Spirochaeta aurantia were positive. The results suggest that epitopes residing on the 33,000-molecular-weight PF protein of "T. phagedenis" are evolutionarily well conserved among the spirochetes.

Binding of Streptococcus pyogenes to soluble and insoluble fibronectin

The interaction of soluble and insoluble fibronectin with Streptococcus pyogenes was investigated. Soluble fibronectin bound to S. pyogenes in a dose-dependent and irreversible manner. Lipoteichoic acid competitively inhibited the binding of fibronectin to S. pyogenes but had little effect on the binding of fibronectin to staphylococci or pneumococci. The phase of growth of the streptococci had a slight effect on binding of fibronectin, with optimal binding occurring in the late log phase. S. pyogenes cells bound to fibronectin immobilized on microtiter plates in a dose-dependent and saturable manner. Both soluble fibronectin and lipoteichoic acid inhibited the binding of streptococci to immobilized fibronectin, suggesting that streptococci interact with soluble and insoluble fibronectin in a similar manner. Antibodies to fibronectin blocked the attachment of streptococci to immobilized fibronectin, whereas normal serum had no effect. Adherence of streptococci to buccal epithelial cells was inhibited by antibodies to fibronectin, but not by normal sera or by antibodies to buccal epithelial cells. The data suggest that lipoteichoic acid on the surface of S. pyogenes binds to fibronectin exposed on the host cell and that such binding mediates the attachment of streptococci to host cells.

Enhanced levels of attachment of fibronectin-primed Treponema pallidum to extracellular matrix

Freshly extracted Treponema pallidum organisms treated with exogenous human fibronectin (Fn) (Fn-primed treponemes) showed a 6- to 15-fold increased level of attachment to Fn-coated cover slips and to extracellular matrix (ECM) when compared with unprimed treponemes. Treponemes primed with collagen or laminin showed no similar enhanced binding to immobilized Fn or ECM. Preexposure of immobilized Fn and ECM to anti-Fn serum but not to anti-collagen or anti-laminin serum prevented treponemal adherence. Also, the presence of proteoglycanlike molecules such as dextran sulfate or heparan sulfate inhibited Fn-primed treponemal attachment to Fn or ECM. In contrast Fn-primed treponemes did not exhibit elevated levels of attachment to eucaryotic cell monolayers. To understand the increased tropism of Fn-primed T. pallidum organisms for Fn and ECM-like surfaces, we radiolabeled freshly extracted treponemes with [35S]methionine and examined them for the presence of surface immunoreactive Fn. Magnetic protAspheres and glass beads coated with monospecific anti-Fn serum bound only 20 to 30% of radiolabeled treponemes. Nonadherent treponemes failed to bind to gelatin-agarose, further confirming the absence of surface Fn or Fn-like material. Fn-free organisms, however, did attach to Fn-coated cover slips and to cell monolayers like treponemes of the original population. Incubation of Fn-free treponemes with human Fn resulted in almost total binding of organisms to anti-Fn antibody on glass beads and also produced increased attachment to Fn-coated cover slips and ECM. These results suggest that enhanced interactions between T. pallidum and the host are dependent on the presence of Fn on syphilis spirochetes and the specific location and orientation of Fn in vivo.