Demonstration of extracellular material at the surface of pathogenic T. pallidum cells

Investigation of the immune escape mechanism of Treponema pallidum

BACKGROUND

Syphilis is a chronic sexually transmitted disease caused by Treponema pallidum subspecies pallidum (T. pallidum), which is a public health problem that seriously affects human health worldwide. T. pallidum is characterized by early transmission and immune escape and is therefore termed an "invisible pathogen".

METHODS

This review systematically summarizes the host's innate and adaptive immune responses to T. pallidum infection as well as the escape mechanisms of T. pallidum.

PURPOSE

To lay the foundation for assessing the pathogenic mechanism and the systematic prevention and treatment of syphilis.

CONCLUSION

The immune escape mechanism of T. pallidum plays an important role in its survival. Exploring the occurrence and development of these mechanisms has laid the foundation for the development of syphilis vaccine.

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.

The importance of different components of normal human serum and lysozyme in the rapid immobilisation of purified Treponema pallidum, Nichols strain

OBJECTIVES

To study the role of different components in normal human serum and the role of lysozyme in rapid immobilisation of Percoll purified T pallidum (Nichols).

MATERIALS AND METHODS

The immobilisation of Percoll purified T pallidum was studied after pre-incubations with different serum fractions (Fr) of normal human serum (Fr 1, containing IgM; Fr 2, containing IgG and a low level of haemolytic complement, and Fr 1 (abs), depleted of IgG). A guinea-pig serum pool was used as a complement source in the immobilisation experiments. The influence was studied of removal of lysozyme from guinea-pig serum on the immobilisation reactions. Further experiments were performed, using a fluorescence technique, to detect C3b depositions on fixed treponemes and treponemes in suspension.

RESULTS

Rapid immobilisation of Percoll-purified treponemes by the NHS serum fractions occurred only after preincubation with Fr 1 and Fr 2 simultaneously. This was largely dependent on the presence of a small amount of haemolytic C in Fr 2. Removal of lysozyme reduced this rapid rate of immobilisation. In fluorescence experiments it was demonstrated that C3b deposition on fixed (that is damaged) treponemes occurred upon their incubation with Fr 2 or the combination of Fr 1 and 2. However, on treponemes in suspension C3b deposition occurred only after incubation with the combination of Fr 1 and 2.

CONCLUSION

The rapid immobilisation of Percoll purified treponemes by serum fractions from normal human serum requires antibodies of the IgM and IgG class, together with complement and lysozyme. Omission of one of these reactants slows immobilisation. Our experiments suggest that the reactants act in sequence: the loss of integrity of the outer membrane by an attack by IgM and C offers the opportunity for lysozyme to hydrolyse the peptidoglycan layer surrounding the cytoplasmic membrane of the treponemes, which then is accessible for attack by antibodies and C.

The influence of different sera on the in vitro immobilisation of Percoll purified Treponema pallidum, Nichols strain

OBJECTIVES

Investigation of sera, especially rabbit serum, in preventing in vitro immobilisation of Percoll purified T. pallidum.

MATERIALS AND METHODS

The immobilisation of Percoll purified T. pallidum (Nichols) was studied after pre-incubations with basal reduced medium (BRM), heat-inactivated serum of seven different species of animals, heat-inactivated normal human serum (NHS) and rabbit sera containing a different level of antitreponemal antibodies. Also increasing percentages of heat-inactivated normal rabbit serum (NRS) were studied.

RESULTS

The rapid immobilisation of purified treponemes by NHS is delayed by pre-incubation with NRS in a dose-dependent manner. The treponemes from 5-day infections were immobilised significantly more slowly than treponemes from 7- and 8-day infections. Compared with NRS, pre-incubations with a high-titred, low-titred and "autologous" serum resulted in significantly more rapid immobilisation of the treponemes. With most other animal sera resistance to immobilisation was slight compared with that produced by NRS. Immunofluorescent studies revealed that the treponemes were covered with a layer of the human third complement factor (C3b), within an hour of incubation. With two sequential pre-incubations, a delay of the immobilisation was only noted in those test mixtures in which NRS had been present in both preincubations.

CONCLUSION

Rabbit serum delays the rapid in vitro immobilisation of Percoll purified treponemes by normal human serum. There was no evidence that this was caused by preventing access of antibodies (in vivo as well as in vitro) to, or preventing the activation of complement on, the treponemal surface. The evidence points to a mechanism in the fluid phase, suggesting participation of a third factor in the immobilisation process, for instance an enzyme, which can be partially inhibited by rabbit serum component(s).

Rapid in vitro immobilisation of purified Treponema pallidum (Nichols strain), and protection by extraction fluids from rabbit testes

The use of Percoll-purified treponemes in an assay similar to the Treponema pallidum Immobilisation test demonstrated that immobilisation of purified treponemes by seronegative normal human serum proceeded at a much higher rate than that of unpurified treponemes. This suggests that the removal of the testicular extract makes the treponemes more vulnerable to this action. A preincubation of the purified treponemes with the testicular extract from infected or uninfected testes delayed their rate of immobilisation to that demonstrated by the unpurified treponemes. This showed that substances produced during the infection are probably not responsible for the delay in immobilisation. Discrimination between the classical and the alternative pathway of complement activation, studied by the ethylene glycol-bis (beta-aminoethyl ether) N,N,N',N'-tetraacetic acid (EGTA) method, showed that the classical pathway was responsible for the rapid immobilisation of the purified treponemes. However, the slow immobilisation in the EGTA-serum samples suggested a minor role of the alternative pathway in the immobilisation of the purified treponemes. Since the testicular extracts exerted an anti-complement activity, it needs to be investigated whether the protection offered to the purified treponemes by the testicular extracts is based on their deteriorating effect on the classical complement pathway or is due to a re-establishment of the protective cover around the treponemes.

The inaccessibility of the outer membrane of adherent Treponema pallidum (Nichols strain) to anti-treponemal antibodies, a possible role of serum proteins

Fresh and aged adherent T pallidum were used to study the accessibility of their outer membrane to antibodies by means of an indirect immunofluorescent technique. The integrity of the outer membrane was demonstrated by the non-reactivity with a monoclonal antibody directed against the axial filaments. Using the sera from patients with sero-positive primary and secondary syphilis no binding of IgG and IgM antibodies was observed. However, IgG and IgM antibody fractions isolated from the sera of patients with secondary syphilis, gave with the fresh fibroblast-adhering treponemes a mean of 14.5% IgG- and of 43.2% IgM positive treponemes. These means were 32.1% and 87.3% respectively for aged treponemes. Lower percentages were observed when fibronectin adhering treponemes were used. This demonstrates the inability of the outer membrane to bind antibodies in a majority of the fresh treponemes. This is partly lost on in vitro aging. Absence of IgG- and IgM fluorescence was also observed when sequential incubations with the antibody fractions and control sera were used. This was accompanied by the deposition of the third complement factor (C3) around the treponemes. Incubations of IgG- or IgM pre-coated adherent treponemes with heat-inactivated control sera or a C3 deficient serum did not result in the deposition of C3, and partially restored the detection of human antibodies. The most likely explanation for the absence of fluorescence is that antibodies become buried in an extra-cellular layer of serum proteins. The deposition of C3 from control sera alone most probably points to the classical pathway of complement activation and suggests that antibodies of rabbit origin constitute a part of the extracellular layer of treponemes.

Light and electron microscopy of rabbit testes infected with Treponema pallidum (Nichols strain): nature of deposited mucopolysaccharides and localisation of treponemes

The mucopolysaccharide nature of the material deposited in rabbit testes infected with Treponema pallidum was confirmed by histochemical staining with alcian blue. Differential staining of mucopolysaccharides showed the presence of sulphated mucopolysaccharides as an almost constant feature, whereas in little more than half of the orchitic testes studied variable deposits of hyaluronic acid were seen. The treponemes were almost exclusively present in the areas rich in mucopolysaccharide. A combination staining with the Warthin-Starry method and alcian blue showed treponemes in close association with pre-existing fibrils and cells contained in these fibrils. The latter findings were confirmed by electron microscopy, and the fibroblasts to which treponemes adhered displayed the characteristics of activated cells. The close parallel between the histopathological changes observed here and their descriptions in published reports shows that our specific strain still behaves the same as the original Nichols pathogenic strain of T pallidum.

Activation of the classical and alternative pathways of complement by Treponema pallidum subsp. pallidum and Treponema vincentii

Both in vivo and in vitro studies have indicated that complement plays an important role in the syphilitic immune responses. Few quantitative data are available concerning activation of the classical pathway by Treponema pallidum subsp. pallidum, and no information is available on treponemal activation of the alternative pathway. Activation of both pathways was compared by using T. pallidum subsp. pallidum and the nonpathogen T. vincentii. With rabbit and human sources of complement, both organisms rapidly activated the classical pathway, as shown by hemolysis of sensitized sheep erythrocytes and by the generation of soluble C4a. With human sources of complement, both organisms also activated the alternative pathway, as shown by hemolysis of rabbit erythrocytes and by the generation of soluble C3a in the presence of magnesium ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA). During incubation, organisms remained actively mobile and did not lyse, indicating that activation was a function of complement reactivity with the intact outer treponemal surface. In addition, freshly harvested T. pallidum subsp. pallidum immediately activated both pathways of complement; preincubation of organisms did not enhance complement reactivity. T. vincentii was a more potent activator of this pathway. T. pallidum subsp. pallidum contained almost four times as much surface sialic acid as T. vincentii did. When sialic acid was enzymatically removed from T. pallidum subsp. pallidum, enhanced activation of the alternative pathway was detected. It is proposed that T. pallidum subsp. pallidum retards complement-mediated damage by the alternative pathway through surface-associated sialic acid. This may be an important virulence determinant that enables these organisms to readily disseminate through the bloodstream to infect other tissues.

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.

Immunoglobulin G subclasses of fluorescent anti-Treponema pallidum antibodies: evidence for sequential development of specific anti-T. pallidum immunoglobulin G responses in patients with early syphilis

The development of immunoglobulin G (IgG) subclass-specific anti-Treponema pallidum antibodies during the course of syphilis in humans was studied with sera from 50 untreated male patients. The patients were divided into five diagnosis groups. In the fluorescent treponemal antibody test, which delineates the presence of cross-reacting antibodies, as well as specific antitreponema antibodies, IgG1, IgG2, and IgG3 subclass antibodies were already present during the seronegative primary stage. Specific antibodies, which were detected by the fluorescent treponemal antibody absorption test, were first present during the serotype-variable primary stage. These antibodies were almost exclusively of the IgG1 and IgG3 subclasses. In later stages, antibodies of other subclasses were detectable. Titration of IgG1 antitreponema antibodies in three electrophoretically different IgG fractions revealed an asymmetric distribution in these fractions during primary syphilis. The antibodies were largely confined to the most basic fraction during primary syphilis. A sudden change in the distribution was noted between the end of the primary stage and the secondary stage; an even distribution of IgG1 antitreponema antibodies existed in the late latent stage. These findings confirm and extend previous results from our laboratory. The development of antibodies detected by both tests is discussed in terms of a sequential stimulation of the immune system due to the presence of an extracellular layer covering the treponemas or, alternatively, in terms of a suppression of the immune response during early syphilis.

Characterization of the proteoglycans synthesized by rabbit testis in response to infection by Treponema pallidum

Organ cultures of syphilitic and normal rabbit testes were incubated with 35S-sulfate for labeling of proteoglycans. Syphilitic rabbit testes synthesized three macromolecular fractions (I, II, and III) which were not detected in extracts of normal uninfected tissue. The three fractions comprised a larger (approximately 10(6) mol wt) chondroitin sulfate/dermatan sulfate proteoglycan (Fraction I), a smaller (approximately 10(5) mol wt) chondroitin sulfate/dermatan sulfate proteoglycan (Fraction II), and a putative sulfated glycoprotein of Mr 40 kd (Fraction III). The glycosaminoglycan chains of both proteoglycans eluted with a Kav of 0.45 on Sepharose CL-6B, consistent with a molecular weight of 25,000. The smaller proteoglycan was not a cleavage product of the larger species. Erythromycin had no significant effect on the synthesis of any of the three macromolecules. In contrast, the synthesis of both proteoglycans was totally inhibited by a 2-hour preincubation with cycloheximide, which suggests that the constitutive "pools" of the two core proteins were small. The putative sulfated 40-kd glycoprotein was insensitive to a 2-hour preincubation with cycloheximide.

Electron microscopy of Treponema pallidum (Nichols) cultivated in tissue cultures of Sf1Ep cells

The in vitro interaction between Treponema pallidum and Sf1Ep cells during treponemal replication was investigated by using transmission electron microscopy. The Sf1Ep cells grown on Teflon-treated cover slips after 12 days of cocultivation were fixed in situ, overlaid with agar, embedded, and vertically sectioned. Large numbers of treponemes were found extracellularly not only at the upper cell surfaces but also in the narrow spaces between the cells and between the cells and the cover slips. These narrow spaces supported treponemal growth and survival, as did those at the upper cell surfaces. Although few in number, organisms were also seen in cell vacuoles either surrounded by a membrane or free in the cytoplasm. Some extracellular treponemes attached to host cells by body spirals or the terminal end and formed electron-dense layers at attachment sites. Some treponemes were often surrounded with amorphous, extracellular material which appeared to "connect" them to host cell surface. After 12 days of cocultivation, host cells showed excessive vacuolation and appeared to be damaged. This did not seem to be due to treponemal infection alone, because cells from uninfected cultures also showed similar vacuolation.

Survival of treponemes after treatment: comments, clinical conclusions, and recommendations

Treponemes may persist after treatment that has been accepted as effective; the reasons for this are discussed. Nevertheless, the epidemic of syphilis after the second world war was not followed by an epidemic of late syphilis, and the results of treatment with penicillin are excellent. Neurological signs may progress in some treated patients, and the standard doses of soluble penicillin and any dose of benzathine penicillin (even with added probenecid by mouth) cannot be relied on to achieve treponemicidal concentrations in the cerebrospinal fluid (CSF). There are no large scale studies of CSF findings after treatment of early syphilis with benzathine penicillin. Standard dosage, such as procaine penicillin G 600 000 international units (IU) by intramuscular injection for 10 days, is the treatment of choice for the patient suffering from uncomplicated early syphilis; this should be preferred to benzathine penicillin, which should only be used when standard treatment as above cannot be given. Treponemicidal concentrations of penicillin should be achieved in the CSF of patients suffering from neurosyphilis by schedules of probenecid by mouth and procaine penicillin by single daily intramuscular injections; treatment should last for 17 to 21 days. Benzathine penicillin should not be used for the treatment of patients suffering from neurosyphilis or from the iritis of late syphilis including that accompanying interstitial keratitis. Treatment for interstitial keratitis should initially be as for neurosyphilis, but in recurrent cases it may have to be prolonged to eradicate Treponema pallidum that is dividing slowly. Doxycycline 200 mg by mouth daily for 21 days provides a supervisable outpatient schedule for patients allergic to penicillin. Cephaloridine (and probably cefuroxime and the new cephalosporins) may be useful for patients who are allergic to penicillin but have not developed anaphylactic allergy. If erythromycin is used for treating syphilis in pregnant women who are allergic to penicillin, then the newborn babies should be treated with penicillin.

Mucopolysaccharides in suspensions of Treponema pallidum extracted from infected rabbit testes

The amount and nature of mucopolysaccharides present in extraction fluids routinely obtained in the isolation procedure of Treponema pallidum from infected rabbit testes was investigated. The mean quantity of mucopolysaccharides extracted from both testes of groups of 10 rabbits was 3.09 mg after infection for seven days and 26.88 mg after infection for 12 days, while from the testes of uninfected rabbits a mean of 0.42 mg was obtained. On electrophoresis the isolated mucopolysaccharides showed only one single band with the migration characteristics of hyaluronic acid. This band disappeared completely after pretreatment with hyaluronidase from bovine testes, which showed that during infection with T pallidum increasing amounts of hyaluronic acid accumulate. They can, at least in part, be extracted by a gentle extraction procedure, suggesting that this material binds loosely. The amount of hyaluronic acid isolated 12 days after infection showed positive correlations with the wet weight of testes as well as the number of treponemes isolated; seven days after infection such correlations were not present.

Binding of glycosaminoglycans to the surface of Treponema pallidum and subsequent effects on complement interactions between antigen and antibody

Acidified bovine serum albumin (acid BSA) reacts with glycosaminoglycans to form a precipitate. This reaction was adapted to Treponema pallidum to show glycosaminoglycans associated with the surface of the micro-organism. As testicular infection progressed from days 4 to 18, treponemes showed increasing amounts of these surface components. High speed centrifuging effectively removed the glycosaminoglycans, thus indicating that they were loosely bound. The subsequent addition of commercial preparations of hyaluronic acid or chondroitin sulphate resulted in their immediate adherence to the surface of the pathogens T pallidum and T pertenue, but not to the non-pathogens T vincenti, T denticola, or T phagedenis. The amount adhering to the treponemal surface varied depending on the concentration added. Intradermal inoculation showed that the virulence of T pallidum was not altered by the glycosaminoglycans associated with its surface. The coating of treponemes with hyaluronic acid or chondroitin sulphate did not interfere with neutralising antibodies or antibodies found by radioimmunoassay using whole organisms. In contrast, hyaluronic acid or chondroitin sulphate on the treponemal surface did interfere with immobilising antibodies. Results are discussed in terms of the potential role of the treponemal glycosaminoglycans in the infectious process.

Antigenic evidence for host origin of exudative fluids in lesions of Treponema pallidum-infected rabbits

Mucoid fluid accumulating within syphilitic lesions has been considered to be of Treponema pallidum origin. To test this assumption, we examined testicular exudative fluids from T. pallidum-infected rabbits for the presence of T. pallidum antigens by various sensitive immunochemical methods, including Western blot analysis. Antigenic analysis of these fluids revealed host components but not treponemal antigens. Prolonged immunization of rabbits, guinea pigs, and a goat with this material in complete Freund adjuvant elicited low titers (fluorescent-treponemal-antibody test titer, less than or equal to 10) of antitreponemal antibodies in the rabbits and guinea pigs but not in the goat. The data suggest that these mucoid fluids are of host origin. The presence of mucopolysaccharides in these fluids may be related to the infective process. The possible mechanism by which mucopolysaccharides protect T. pallidum from immune mechanisms and its potential relationship to the pathogenesis of the disease are discussed.

Polyanions in syphilis: evidence that glycoproteins and macromolecules resembling glycosaminoglycans are synthesised by host tissues in response to infection with Treponema pallidum

We investigated by means of radiolabelled precursors the source and nature of the polyanionic macromolecules present in rabbit tissues during active syphilis infection. Previous studies indicated that Treponema pallidum itself does not synthesise glycosaminoglycans, at least in vitro. In replicate experiments on unilaterally infected rabbits, tissue from the orchitic testis incorporated two to three times more 35S-sulphate and 3H-glucosamine (on a wet weight basis) than tissue from the non-orchitic contralateral testis. Incorporation of 35S-sulphate was independent of the number of viable T pallidum organisms present in the infested tissue, which suggested that incorporation represented biosynthesis by the host and not the treponeme. Testes from syphilitic rabbits two days after treatment with high doses (100 mg/kg) of penicillin incorporated less 35S-sulphate than untreated infected testes, but more than normal uninfected rabbit testes. This suggests that active syphilitic infection was necessary for maximum biosynthesis of the macromolecule(s) by host tissue. Hydrodynamic profiles showed incorporation of radiolabelled precursors into two distinct fractions of different sizes, which may represent a proteoglycan and a sulphated glycoprotein. Alcian blue staining of syphilitic testes at or after peak orchitis showed focal deposition of newly synthesised polyanionic components during peak orchitis and a more generalised fibrosis in testes after peak orchitis.

Studies of rabbit testes infected with Treponema pallidum. I Immunopathology

Rabbit testes were injected with suspensions of Treponema pallidum, washed T pallidum, heat killed T pallidum, or Reiter treponemes. The testes were removed three to 24 days after injection and examined for the number of treponemes, the presence of treponemal antibodies, histopathological changes, and presence of T and B cells. In animals infected with T pallidum a substantial number (10(6)-10(7)/ml) of organisms were still present at day 24 in spite of early local production of antibodies and increasing infiltration with plasma cells, T lymphocytes, and macrophages. In animals infected with washed T pallidum a lower degree of inflammation was observed than in those infected with unmodified T pallidum, and the treponemal antibodies were detected simultaneously in samples of testicular fluid and serum. In the groups injected with heat killed T pallidum and Reiter treponemes no macroscopical or microscopical changes were detected, although in the group injected with heat killed T pallidum treponemal antibodies were detected in the testicular fluid on day 24.

Murine monoclonal antibodies specific for virulent Treponema pallidum (Nichols)

Murine anti-Treponema pallidum (Nichols) lymphocyte hybridoma cell lines secreting monoclonal antibodies against a variety of treponemal antigens have been generated. Hybridomas isolated were of three major types: those that were directed specifically against T. pallidum antigens, those that were directed against treponemal group antigens (as evidenced by their cross-reactivity with T. phagedenis biotype Reiter antigens), and those that cross-reacted with both treponemal as well as rabbit host testicular tissue antigens. The majority (31 of 39 clones) of these anti-T. pallidum hybridomas, which produced monoclonal antibodies of mouse isotypes immunoglobulin G1 (IgG1), IgG2a, IgG2b, IgG3 or IgM, were directed specifically against T. pallidum and not other treponemal or rabbit antigens tested by radioimmunoassay. Four of these T. pallidum-specific hybridomas secreted monoclonal antibodies with greater binding affinity for "aged" rather than freshly isolated intact T. pallidum cells, suggesting a possible specificity for "unmasked" surface antigens of T. pallidum. Six anti-T. pallidum hybridomas produced complement-fixing monoclonal antibodies (IgG2a, IgG2b, or IgM) that were capable of immobilizing virulent treponemes in the T. pallidum immobilization (TPI) test; these may represent biologically active monoclonal antibodies against treponemal surface antigens. Three other hybridomas secreted monoclonal antibodies which bound to both T. pallidum and T. phagedenis biotype Reiter antigens, thus demonstrating a possible specificity for treponemal group antigens. Five hybridoma cell lines were also isolated which produced IgM monoclonal antibodies that cross-reacted with all treponemal and rabbit host testicular tissue antigens employed in the radioimmunoassays. This report describes the construction and characteristics of these hybridoma cell lines. The potential applications of the anti-T. pallidum monoclonal antibodies are discussed.

Surface-associated antigens of Treponema pallidum concealed by an inert outer layer

Soluble antigens of Treponema pallidum were examined by two dimensional immunoelectrophoresis against antisera from infected or artificially immunized rabbits. Concentrated suspensions of intact cells did not release antigens after storage at 4 degrees, incubation at 37 degrees, or vortex mixing. Antigens were released after disintegration of treponemes by ultrasonic vibration, or by treatment with non-ionic or anionic detergents. An antigenic component of sonicated treponemes, present in both the non-pathogenic, cultivable Reiter treponeme and T. Pallidum, was identified as axial filament. The combination of antibody with unfixed whole organisms was monitored by an indirect fluorescent antibody method, and whereas antibody did not combine with intact organisms, detergent-treated organisms were highly reactive. Immune electron microscopy showed that whereas in intact treponemes, axial filaments were unable to combine with antibody, detergent treatment allowed access to axial filaments by antibody. In intact treponemes the axial filaments are thought to be located beneath the outer membrane, which may thus comprise the postulated antigenically inert outer layer.

Influence of testicular fluid infected with Treponema pallidum on intradermal lesions

A viscous mucoid fluid occasionally accumulates after intratesticular inoculation of rabbits with Treponema pallidum. Experiments were performed to assess the effects of this testicular fluid on the development of syphilitic lesions. Intramuscular injections of this fluid altered host defences as indicated by shorter incubation periods, by reactivation of healing lesions, and by the presence of lesions at a time when solid immunity should have developed.

Immunoglobulin-bearing lymphoid cells in primary syphilis. Quantitative and elution studies

The delay in antibody production in response to infection with Treponema pallidum may be caused by a block in the differentiation of antigen-stimulated B (Bursa-dependent) lymphoid cells towards plasma cells. This hypothesis was tested by a study to detect clonal expansion of immunoglobulin-bearing B lymphoid cells by in-vitro immunofluorescence tests in patients with primary syphilis. In addition, antibodies eluted from circulating lymphoid cells were investigated for treponemal binding by the enzyme-linked immunosorbent assay, the T pallidum immobilisation test, and the immunoglobulin class-specific FTA-ABS test. Results indicated that the number of IgG-bearing lymphoid cells were increased in patients with primary syphilis. However, in only a few cases could antitreponemal antibodies be eluted from isolated lymphoid cells. For this reason, the original hypothesis was rejected.

Surface-associated host proteins on virulent Treponema pallidum

A surface coat of host serum proteins was detected on virulent Treponema pallidum by sodium dodecyl sulfate-gel electrophoresis. The loosely associated serum proteins could be removed by repeated washings in a protein-free medium. Washed T. pallidum retained the ability to readsorb numerous host proteins from rabbit serum as well as iodinated rabbit or human albumin. In addition, various avidly associated host serum proteins including albumin, alpha(2)-macroglobulin, transferrin, ceruloplasmin, immunoglobulin G, immunoglobulin M, and C3 were identified on the outer envelope of washed treponemes by an immunoadsorbent technique with protein A-bearing staphylococcus. Hyaluronidase treatment did not remove the avidly associated host proteins from the surface of washed treponemes, whereas trypsin treatment resulted in decreased levels of agglutination. Electrophoretic patterns of trypsin-treated treponemes showed that treponemal proteins as well as adsorbed host proteins were released concurrently by protease digestion. Reacquisition studies involving alpha(2)-macroglobulin and transferrin suggested the presence of noncompetitive binding sites for serum proteins on the treponemal outer envelope. Finally, differences among the T. pallidum preparations from individual rabbits with respect to incorporation of [(35)S]methionine, extent of agglutination with antisera, and length of time required for removal of avidly associated host proteins by trypsin treatment indicated biological variability among the treponemal populations.

Suppression of lymphocyte response to concanavalin A by mucopolysaccharide material from Treponema pallidum-infected rabbits

The testicular fluid and serum from rabbits infected intratesticularly with Treponema pallidum inhibited the mitogenic response of normal rabbit peripheral blood lymphocytes to concanavalin A. Mucopolysaccharide material present in the testicular fluid and serum was associated with the lymphocyte-inhibitory activity. Degradation of the mucopolysaccharide material with hyaluronidase resulted in the loss of the inhibitory activity of testicular fluid and serum of T. pallidu-infected rabbits.

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.

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.

Mucopolysaccharide material resulting from the interaction of Treponema pallidum (Nichols strain) with cultured mammalian cells

During incubation of Treponema pallidum (Nichols strain) with cultured mammlian cells derived from normal rabbit testes (NRT), an amorphous material accumulated at the surface of the cultured cells. This material was randomly distributed on all tissue cells within the culture chambers. The amount of amorphous material was dependent on the treponemal inocula. With 3 x 10(8) organisms per ml, this material was readily apparent within 2 days; with 4 x 10(7) organisms per ml, this material was detectable within 4 to 5 days; with lower inocula, the accumulation of amorphous material was far less apparent. Deposition of this surface-associated material required attachment of treponemes to the cultured cells, and the amount deposited was related to the number of treponemes attached per cell. This amorphous material was not detected when NRT cells were incubated with preparations of T. pallidum that were heat or air inactivated. In addition, the accumulaton of amorphous material was not due to a soluble component from host testicular tissue or to a soluble component developing during treponemal infection. This was demonstrated by the inability of membrane filtered preparations of T. pallidum to induce the deposition of amorphous material at the surface of the cultured cells. The nature of this material appeared to be acidic mucopolysaccharide as indicated by its metachromatic staining properties, its stainability with ruthenium red, and its partial degradation by bovine and streptomyces hyaluronidase. This amorphous material that accumulated in vitro at the surface of cultured cells may be similar to the mucoid material that accumulates in vivo during syphilitic infection.

Cell-mediated immunity during syphilis

Evidence is presented which reinforces the complexity of the host-parasite interaction during the course of syphilis. Infection with Treponema pallidum evokes a complicated antibody response and an assortment of cell-mediated immune reactions in the host. It appears that humoral immunity plays a minor role towards the complete elimination of syphilitic infection while the cellular limb of the immune response may be an important host defence mechanism. Information now available indicates that a state of anergy, or immunosuppression, exists in the early stages of human and experimental rabbit syphilis based upon negative skin reactions to T. pallidum antigen(s), the abnormal histological appearance of lymphoid organs, and impaired in vitro lymphocyte reactivity. It is also evident that in the later stages of the disease cellular immunity becomes activated as delayed type skin reactions can normally be elicited in tertiary syphilitics and lymphocyte behaviour in cell culture appears normal. Several mechanisms have been invoked to explain the delay in an effective immune response against syphilitic infection and the duration of the disease: (1) a capsule-like substance on the outer surface of virulant T. pallidum may act as a barrier against treponemicidal antibody; (2) this material and other biological properties of virulent treponemes could enable spirochaetes to escape being engulfed by macrophages and other phagocytic cells; (3) antigenic competition among different treponemal antigens causing partial tolerance; (4) T. pallidum infection may bring about the elaboration of immunosuppressive substances of host or treponemal origin which inhibit the proper function of lymphocytes, macrophages, and other cell types.

Anatomy and chemistry of spirochetes

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Unsustained multiplication of treponema pallidum (nichols virulent strain) in vitro in the presence of oxygen

Treponema pallidum (Nichols virulent strain) was incubated with or without oxygen using a modified medium supplemented with reduced glutathione and a variety of nutrients (PRNF10-B). Two- to fourfold increases in treponemal numbers were observed in cultures without mammalian cells within 96 h of incubation under 5 to 6% oxygen. Treponemal motility and multiplication were maintained more satisfactorily in cultures that were diluted and transferred daily, using an equal volume of fresh medium. Treponemes incubated without oxygen did not significantly increase in number. Virulent microorganisms were detected for at least 96 h in the cell-free system. In the presence of 3 to 4% oxygen, two- to fivefold increases in treponemal numbers were observed in the supernatant fluids of cultures containing human prepuce cells after 48 to 120 h at 35 degrees C. Without oxygen, treponemal numbers rarely approached a threefold increase. Virulent treponemes were detected by the rabbit skin lesion test after at least 120 h in vitro. Regardless of the system of incubation, increases in treponemal numbers could not be sustained for longer than 120 h, and treponemal virulence decreased as a function of time in vitro.

Selective in vitro response of thymus-derived lymphocytes from Treponema pallidum-infected rabbits

The blastogenic response of nylon wool-separated peripheral-blood lymphocytes from Treponema pallidum-infected rabbits was tested in vitro with mitogens and T. pallidum antigens. The mitogenic response of the enriched T-cell population to concanavalin A and phytohemagglutinin was depressed during the first 3 to 4 weeks of infection, similar to the pattern observed with unfractionated cells. Shortly thereafter, levels of blastogenesis returned to values of uninfected cultures. Enhanced blast transformation was seen immediately when purified T-cells from infected rabbits were exposed in vitro to T. pallidum antigens. Although these relatively high levels of blastogenesis were maintained for the duration of the experiment, cultures of unfractionated lymphocytes from infected rabbits did not exhibit an increased blastogenic response to the same antigen preparation until 3 to 4 weeks after infection. Autologous serum from infected rabbits decreased the lymphocyte response to T. pallidum antigen. The stimulatory effects of anti-immunoglobulin G and lipopolysaccharide on nylon wool-fractionated or unfractionated lymphocytes from both infected and control rabbits were similar throughout the course of infection. During the first 6 weeks of experimental disease, there was a 25 to 31% increase in the number of lymphocytes circulating in the peripheral blood of T. pallidum-infected rabbits.

Parasitism by virulent Treponema pallidum of host cell surfaces

The interaction between virulent Treponema pallidum extracted from infected rabbit testes and animal cells in culture was examined. The extent of treponemal attachment to monolayers of normal rabbit testicular and HEp-2 cells was dependent upon the incubation temperature and retained motility of the spirochetes. The specific orientation of treponemes to host cell surfaces was demonstrated by dark-field microscopic examination of wet-mount preparations and scanning and transmission electron microscopy. Once attached, T. pallidum organisms remained actively motile yet anchored in place by their terminal tapered structures. After several hours of co-incubation, maximal attachment was attained, and the degree of parasitism seemed regulated not only by available surface sites on individual host cells but also by the proposed membrane response of parasitized cells to continued exposure to treponemes. The avirulent strain, Treponema phagedenis biotype Reiter, did not adhere to monolayer cultures. Characterization of host cell determinants that permitted surface colonization by T. pallidum was attempted. Also, properties of virulent treponemes that enabled surface parasitism were monitored by measuring the effects of enzymes, detergents, and metabolic inhibitors on the host-parasite interaction. Results reinforced the specific nature of the treponemal attachment mechanism. Furthermore, the ability of convalescent rabbit sera to reduce attachment of treponemes to host cells suggested that surface structures on T. pallidum could be masked or inactivated by host components, thus providing a potentially effective research approach for investigating the pathogenesis of syphilis and screening appropriate vaccine candidates.

Scanning electron microscopy of Treponema pallidum (Nichols strain) attached to cultured mammalian cells

This paper describes the attachment of Treponema pallidum (Nichols strain) to cultured mammalian cells as a visualized by scanning electron microscopy. Treponemes were incubated for 3 hr with cultured cells derived from normal rabbit testes or human skin epithelium, then fixed, processed with critical-point drying, and examined with a Cambridge Mark 2A scanning electron microscope. Large numbers of treponemes became attached to the cultured cells without altering the morphological integrity of the cultured cells. Attachment appeared to involve a very close physical proximity of treponemes to the cultured cells; at the site of attachment, no changes such as swelling or indentation of the cultured cell surface were observed. The addition of ruthenium red to the fixatives produced a treponemal-associated surface precipitate. This material, which is probably mucopolysaccharide and/or phospholipid, may be important in protecting the organisms against host defense mechanisms; in addition, it may be involved in the serological unresponsiveness of freshly prepared suspensions of T. pallidum.