Function and Protective Capacity of Treponema pallidum subsp. pallidum Glycerophosphodiester Phosphodiesterase

Infectious syphilis, caused by the spirochete bacterium Treponema pallidum subsp. pallidum, remains a public health concern worldwide. The immune-response evasion mechanisms employed by T. pallidum are poorly understood, and prior attempts to identify immunoprotective antigens for subsequent vaccine design have been unsuccessful. Previous investigations conducted in our laboratory identified the T. pallidum glycerophosphodiester phosphodiesterase as a potential immunoprotective antigen by using a differential immunologic expression library screen. In studies reported here, heterologous expression of the T. pallidum glycerophosphodiester phosphodiesterase in Escherichia coli yielded a full-length, enzymatically active protein. Characterization of the recombinant molecule showed it to be bifunctional, in that it exhibited specific binding to human immunoglobulin A (IgA), IgD, and IgG in addition to possessing enzymatic activity. IgG fractionation studies revealed specific binding of the recombinant enzyme to the Fc fragment of human IgG, a characteristic that may play a role in enabling the syphilis spirochete to evade the host immune response. In further investigations, immunization with the recombinant enzyme significantly protected rabbits from subsequent T. pallidum challenge, altering lesion development at the sites of challenge. In all cases, animals immunized with the recombinant molecule developed atypical pale, flat, slightly indurated, and nonulcerative reactions at the challenge sites that resolved before lesions appeared in the control animals. Although protection in the immunized rabbits was incomplete, as demonstrated by the presence of T. pallidum in the rabbit infectivity test, glycerophosphodiester phosphodiesterase nevertheless represents a significantly immunoprotective T. pallidum antigen and thus may be useful for inclusion in an antigen cocktail vaccine for syphilis.     

Mucopolysaccharidase of Treponema pallidum

Treponema pallidum (Nichols strain) exhibited mucopolysaccharidase activity. Acidic mucopolysaccharides were broken down more rapidly by viable treponemes than by heat-inactivated treponemes or membrane filtrates of treponemal suspensions. Ouchterlony immunodiffusion demonstrated the occurrence of antibodies to the hyaluronidase-like enzyme within syphilitic sera. After intratesticular inoculation of 2 x 10(7) to 6 x 10(7) treponemes, these anti-mucopolysaccharidase antibodies were detected between 9 and 35 days postinoculation. In addition, acidic mucopolysaccharides were present in the serum of infected animals 9 and 16 days postinoculation. Immune serum that contained antibodies to the mucopolysaccharidase restricted treponemal breakdown of acidic mucopolysaccharides. It has been previously demonstrated that immune rabbit serum contains a factor that blocks attachment of T. pallidum (Nichols strain) to cultured mammalian cells. This factor was effectively absorbed by prior incubation with bovine hyaluronidase. It is postulated that T. pallidum attaches to acidic mucopolysaccharides on the surface of cultured cells through the mucopolysaccharidase enzyme at the surface of the organisms. These findings are discussed in terms of the histopathogenesis of T. pallidum with applications to the healing immune response.     

Comparative Behavior of Virulent Strains of Treponema pallidum and Treponema pertenue in Gradient Cultures of Various Mammalian Cells

Two strains of virulent Treponema pallidum and two of virulent T. pertenue were investigated for their ability to attach to and survive in gradient cultures of five different mammalian cells under aerobic conditions. The strains of T. pallidum studied were the high-rabbit-passage Nichols and the low-rabbit-passage KKJ. The former was known to readily attach to cottontail rabbit epithelial cells (Sf1Ep) and to survive in the virulent state for up to 21 days. We therefore compared attachment of the other virulent treponemes with that of T. pallidum (Nichols). The KKJ strain of T. pallidum behaved in a fashion similar to T. pallidum (Nichols) in all of the cultures. Both strains exhibited preferential attachment to cells of Sf1Ep and those derived from the ear of a nude athymic (nu/nu) mouse. In these cultures, we observed a consistent three- to fivefold increase in attached treponemes up to 12 days after initial inoculation. The strains of T. pertenue were the human-derived Gauthier and cynocephalus-derived FB. These two strains of T. pertenue also attached to cells of all five types of cultures, but in smaller numbers than were seen with T. pallidum and equally to all of the cultures. Neither preferential attachment to Sf1Ep and nude mouse ear cells nor increased attachment with time was seen.     

Fatty Acid Requirements of the Kazan 5 and Reiter Strains of Treponema pallidum

The fatty acid requirements of two avirulent treponemes were investigated by using a “lipid-poor” albumin-thioglycolate medium. The Kazan 5 and the Reiter stains of Treponema pallidum required a pair of fatty acids for growth. One member of the pair was saturated and the other was unsaturated. The saturated fatty acids could contain either an odd or even number of carbon atoms, but a chain length of at least 14 carbon atoms was necessary. Unsaturated fatty acids with one, two, or three double bonds were satisfactory if they contained 15 or more carbon atoms. The pair of fatty acids could be replaced with a single 18-carbon monounsaturated fatty acid if it was in the trans configuration rather than the naturally occurring cis form.     

Surface Mucopolysaccharides of Treponema pallidum

The viscous mucoid fluid that accumulates within syphilitic lesions may be due to breakdown of host tissue during infection, or may be synthesized by Treponema pallidum. Experiments were performed to investigate the acidic mucopolysaccharides that occur at the surface of T. pallidum (Nichols strain). These mucopolysaccharides were demonstrated by reaction with acidified bovine serum albumin and by agglutination with wheat germ agglutinin and soybean agglutinin. The polycations ruthenium red and toluidine blue influenced treponemal survival. Concentrations of both compounds at 200 mug/ml inhibited survival, whereas concentrations at 0.1mug/ml enhanced survival. The mucopolysaccharide concentration within the mucoid fluid that accumulates during intratesticular infection was determined by reaction with acidified bovine serum albumin; it ranged from 10,000 mug/ml to less than 8 mug/ml. The addition of this mucoid fluid to treponemal suspensions resulted in differing effects on T. pallidum survival. Some preparations were inhibitory, and others were stimulatory. Commercial preparations of hyaluronic acid and chondroitin sulfate at 400, 200, 100, and 50 mug/ml were detrimental to treponemal survival. The organisms exhibited pronounced clumping in the presence of the higher concentrations of hyaluronic acid. These clumps of treponemes were comprised of mucopolysaccharides as shown by acidified bovine serum albumin and toluidine blue reactions and by hyaluronidase degradation. Results are discussed in terms of the derivation and potential role of acidic mucopolysaccharides at the surface of T. pallidum.     

Oxygen Uptake by Treponema pallidum

Virulent Treponema pallidum has been shown to consume O(2) at a rate similar to that of the known aerobic spirochaete, Leptospira. Such O(2) uptake is cyanide sensitive, indicating a functioning cytochrome oxidase. Inhibition of O(2) uptake by azide, chlorpromazine, and amytal further suggests a functioning electron transport system for the oxidation of nicotinamide adenine dinucleotide (reduced) to O(2). Evidence is consistent with the probability that this terminal electron-transport system is coupled to oxidative phosphorylation. The potential significance of these findings is discussed.     

Monoclonal antibody analysis of specific antigenic similarities among pathogenic Treponema pallidum subspecies

Murine monoclonal antibodies directed against a 47,000-dalton immunodominant surface-exposed antigen of Treponema pallidum subsp. pallidum (Nichols) were isolated. These monoclonal antibodies cross-reacted with analogous 47,000-dalton antigens of two other virulent treponemes, T. pallidum subsp. pertenue and T. pallidum subsp. endemicum (Bosnia A), as determined by radioimmunoassay and immunoblot analyses. Immunoelectron microscopy confirmed that the 47,000-dalton antigen of T. pallidum subsp. pallidum was a surface-associated cellular component. Surface binding assays and immunoelectron microscopic studies also suggested that the analogous 47,000-dalton antigenic component of T. pallidum subsp. pertenue may not have been oriented toward the bacterial surface in the same way as the T. pallidum subsp. pallidum antigen or that the relevant antigenic determinant(s) may not have been exposed to the outer surface in the same way. The significance of this antigen relative to its apparent conservation among pathogenic treponemes and its possible diagnostic and vaccinogenic potentials are discussed.     

Surface Characterization of Virulent Treponema pallidum

Characterization of the surface of Treponema pallidum was accomplished by [¹²⁵I]lactoperoxidase-catalyzed iodination of intact organisms and sensitive radioimmunoprecipitation and gel electrophoresis technology. At least 11 outer membrane proteins with molecular weights ranging from 89,000 (89K) to 20K were identified, and all elicited high titers of antibody in experimentally infected rabbits. Proteins of 89.5K, 29.5K, and 25.5K previously implicated as ligands involved in attachment (J. B. Baseman and E. C. Hayes, J. Exp. Med. 151:573-586, 1980) were found to reside on the treponemal surface. Low levels of the 89.5K treponemal protein were released by high salt concentrations, whereas the remaining comigrating material was neither radioiodinated nor released with selective detergents. Other lower-molecular-weight (60K, 45K, and 30K) surface proteins were extracted with octyl glucoside detergent, suggesting their hydrophobic interaction with the external membrane. The molecular organization of surface proteins was studied by employing the cross-linker dithiobis(succinimidyl)-propionate, and data suggested the presence of a highly fluid envelope resulting in random collisions by the surface proteins. The biological function of the treponemal outer envelope proteins was evaluated using, as the indicator system, adherence of T. pallidum to monolayer cultures of eucaryotic cells. Trypsin treatment of motile, freshly harvested organisms decreased the extent of surface parasitism to normal rabbit testicular cells, reinforcing the idea of the proteinaceous nature and role of treponemal ligands for attachment. Other data supported functional and antigenic relatedness among the implicated ligands. Finally, brief periodate treatment of human epithelial (HEp-2) and normal rat testicular cells as well as casein-elicited rabbit peritoneal macrophages significantly reduced the extent of treponemal parasitism, suggesting a role of specific host membrane molecules as mediators of attachment.     

Outer Envelope of Virulent Treponema pallidum

Ultrathin sections of virulent Treponema pallidum (Nichols strain) were examined with the electron microscope, and the presence of an outer cell envelope was documented.     

Lipids of Treponema pallidum Kazan 5

The lipid composition of Treponema pallidum Kazan 5 cultivated in a lipid-defined medium was investigated. Lipids comprised 18 to 20% of the dry weight of the treponeme. Glycolipid and phospholipids accounted for 90 to 95% of the total lipids and free fatty acids made up the remaining 5 to 10%. The major polar lipids were the glycolipid, 1-(O-alpha-d-galactopyranosyl)-2,3-diglyceride (45 to 55%), and phosphatidylcholine (30 to 40%). Phosphatidylethanolamine (5 to 10%), an unidentified compound (1 to 2%), and occasional trace amounts of diphosphatidylglycerol (cardiolipin) were also found. The monogalactosyldiglyceride was also a major component (50%) of the lipids of the Reiter, Noguchi, and Nichols strains of T. pallidum. The fatty acid composition of Kazan 5 usually consisted of saturated and unsaturated fatty acids ranging from 14 to 18 carbons depending upon the fatty acids added to the culture medium. When the cells were cultivated on elaidic acid (trans-9-octadecenoic acid), their lipids contained only elaidic acid.     

Molecular differentiation of Treponema pallidum subspecies

Treponema pallidum includes three subspecies of antigenically highly related treponemes. These organisms cause clinically distinct diseases and cannot be distinguished by any existing test. In this report, genetic signatures are identified in two tpr genes which, in combination with the previously published signature in the 5' flanking region of the tpp15 gene, can differentiate the T. pallidum subspecies, as well as a simian treponeme.     

Antibody responses elicited against the Treponema pallidum repeat proteins differ during infection with different isolates of Treponema pallidum subsp. pallidum

Variation in the expression of the different Tpr proteins in the syphilis spirochete, Treponema pallidum subsp. pallidum, may have important implications in its ability to evade host immune detection and cause persistent infection. In the present study we examined the pattern of antibody responsiveness to different Tpr members during infection with three isolates of T. pallidum. There was variability in the specificities and temporal patterns of reactivity of the antibodies elicited against the individual Tpr proteins, suggesting that isolates may express different repertoires of Tpr proteins during infection.     

Glucose incorporation by Treponema pallidum.

Treponema pallidum incorporated glucose into trichloroacetic acid-precipitable material. The amount of incorporation was proportional to the number of treponemes and was estimated to equal 3% of the glucose oxidized.     

Pyruvate oxidation by Treponema pallidum.

Cell-free extracts of Treponema pallidum catalyzed the decarboxylation of pyruvate. This activity was suppressed at low O2 tensions and appeared to be coenzyme A independent. Pyruvate decarboxylation was inorganic phosphate dependent, and evidence suggested that acetyl phosphate was a product. Oxygen was consumed, and data indicated that H2O2 was produced. These results indicated that the overall oxidation of pyruvate was: pyruvate + O2 + inorganic phosphate leads to CO2 + acetyl phosphate + H2O2. Phosphotransacetylase and acetate kinase activities were also observed in the cell-free extracts and could catalyze formation of acetyl coenzyme A and adenosine 5'-triphosphate, respectively, from acetyl phosphate.     

Penicillin-binding proteins and peptidoglycan of Treponema pallidum subsp. pallidum.

Penicillin-binding proteins (PBPs) of Treponema pallidum subsp. pallidum (T. pallidum) were characterized by using [3H]penicillin G and a conjugate consisting of ampicillin and 125I-labeled Bolton-Hunter reagent. Both antibiotics specifically radiolabeled proteins with molecular masses of 94, 80, 63, and 58 kilodaltons (kDa); 125I-labeled Bolton-Hunter reagent-ampicillin also radiolabeled several polypeptides with lower molecular masses. The 94- and 58-kDa proteins demonstrated the highest binding affinities for [3H]penicillin G and were radiolabeled at concentrations of 8 and 40 nM, respectively. Radiolabeling of PBPs was detectable after 1 min of incubation in 1 microM [3H]penicillin G and was nearly maximal within 10 min. The rapidity of penicillin binding contrasted with the observation that only 40% of virulent treponemes became immobilized during prolonged incubation in vitro with a much higher concentration (1 mM) of unlabeled penicillin. Two lines of evidence indicated that most, if not all, of the PBPs are integral cytoplasmic membrane proteins: (i) preincubation of organisms in 0.1% Triton X-100 solubilized nearly all of the outer membranes but did not affect radiolabeling of PBPs, and (ii) except for the 80-kDa protein, the PBPs partitioned into the detergent phase following extraction with the nonionic detergent Triton X-114. The presence of peptidoglycan in T. pallidum was confirmed by the detection of muramic acid in the sodium dodecyl sulfate-insoluble, proteinase K-resistant residue obtained from Triton X-114-extracted organisms.     

Conservation of the 15-kilodalton lipoprotein among Treponema pallidum subspecies and strains and other pathogenic treponemes: genetic and antigenic analyses.

The 15-kDa lipoprotein of Treponema pallidum is a major immunogen during natural syphilis infection in humans and experimental infection in other hosts. The humoral and cellular immune responses to this molecule appear late in infection as resistance to reinfection is developing. One therefore might hypothesize that this antigen is important for protective immunity. This possibility is explored by using both genetic and antigenic approaches. Limited or no cross-protection has been demonstrated between the T. pallidum subspecies and strains or between Treponema species. We therefore hypothesized that if the 15-kDa antigen was of major importance in protective immunity, it might be a likely site of antigenic diversity. To explore this possibility, the sequences of the open reading frames of the 15-kDa gene have been determined for Treponema pallidum subsp. pallidum (Nichols and Bal-3 strains), T. pallidum subsp. pertenue (Gauthier strain), T. pallidum subsp. endemicum (Bosnia strain), Treponema paraluiscuniculi (Cuniculi A, H, and K strains), and a little-characterized simian isolate of Treponema sp. (Fribourg-Blanc strain). No significant differences in DNA sequences of the genes for the coding region of the 15-kDa antigen were found among the different species and subspecies studied. In addition, all organisms showed expression of the 15-kDa antigen as determined by monoclonal antibody staining. The role of the 15-kDa antigen in protection against homologous infection with T. pallidum subsp. pallidum Nichols was examined in rabbits immunized with a purified recombinant 15-kDa fusion protein. No alteration in chancre development was observed in immunized, compared to unimmunized, rabbits, and the antisera induced by the immunization failed to enhance phagocytosis of T. pallidum subsp. pallidum by macrophages in vitro. These results do not support a major role for this antigen in protection against syphilis infection.     

Molecular basis of immunological cross-reactivity between Treponema pallidum and Treponema pertenue

Protein antigens of Treponema pallidum, Nichols strain, and Treponema pertenue, Gauthier strain, were identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting techniques. Treponemal proteins were solubilized in 1% sodium dodecyl sulfate, electrophoresed on 12.5% polyacrylamide gels, and either stained with Coomassie brilliant blue or electrophoretically transferred to nitrocellulose paper. These antigen blots were incubated with sera from rabbits infected with either T. pallidum or T. pertenue and 125I-labeled staphylococcal protein A and exposed to X-ray film for visualization of antigenic molecules. Protein profiles of each organism separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and stained with Coomassie brilliant blue showed no distinguishable differences. Antigenic profiles as determined by Western blots were similar with two exceptions. A 39,500-dalton band was present on T. pertenue but absent from T. pallidum, and a 19,000-dalton band was present on T. pallidum but absent from T. pertenue (although two additional antigenic bands at 21,000 and 18,000 daltons were seen on T. pertenue). Because these differences were detected by using antisera raised against either T. pallidum or T. pertenue, these molecules must contain some antigenic determinants in common despite their differences in molecular weight.     

Phagocytosis of opsonized Treponema pallidum subsp. pallidum proceeds slowly.

Macrophages were found to phagocytize Treponema pallidum subsp. pallidum attached to polycarbonate filters. This environment simulated the in vivo interaction of surface-adherent treponemes with macrophages. The phagocytosis of T. pallidum subsp. pallidum was found to proceed slowly. Heat-killed T. pallidum subsp. pallidum were susceptible to opsonization with 2% immune serum, whereas live treponemes were resistant to this concentration of antibody. High concentrations of immune serum were found to increase phagocytosis of the spirochetes. Live T. pallidum subsp. pallidum had bound limited quantities of immunoglobulin G in vivo, and only opsonization with 20% immune serum resulted in a detectable increase in surface-bound immunoglobulin in vitro. Kinetic studies suggested a steady rate of phagocytosis that is considerably slower than with other bacteria. Scanning electron microscopy studies of the phagocytizing macrophages showed that the treponemes were detached from the membrane filters and scooped onto the ruffled portion of the macrophage surface. This lengthy physical process, along with the lack of a dramatic increase in ingestion after opsonization, may account for the slow rate of phagocytosis.