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

The guinea pig as a model of infectious diseases

The words 'guinea pig' are synonymous with scientific experimentation, but much less is known about this species than many other laboratory animals. This animal model has been used for approximately 200 y and was the first to be used in the study of infectious diseases such as tuberculosis and diphtheria. Today the guinea pig is used as a model for a number of infectious bacterial diseases, including pulmonary, sexually transmitted, ocular and aural, gastrointestinal, and other infections that threaten the lives of humans. Most studies on the immune response to these diseases, with potential therapies and vaccines, have been conducted in animal models (for example, mouse) that may have less similarity to humans because of the large number of immunologic reagents available for these other species. This review presents some of the diseases for which the guinea pig is regarded as the premier model to study infections because of its similarity to humans with regard to symptoms and immune response. Furthermore, for diseases in which guinea pigs share parallel pathogenesis of disease with humans, they are potentially the best animal model for designing treatments and vaccines. Future studies of immune regulation of these diseases, novel therapies, and preventative measures require the development of new immunologic reagents designed specifically for the guinea pig.

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.

Fibronectin fragments modulate monocyte VLA-5 expression and monocyte migration

To identify the mechanisms that cause monocyte localization in infarcted myocardium, we studied the impact of ischemia-reperfusion injury on the surface expression and function of the monocyte fibronectin (FN) receptor VLA-5 (alpha(5)beta(1) integrin, CD49e/CD29). Myocardial infarction was associated with the release of FN fragments into cardiac extracellular fluids. Incubating monocytes with postreperfusion cardiac lymph that contained these FN fragments selectively reduced expression of VLA-5, an effect suppressed by specific immunoadsorption of the fragments. Treating monocytes with purified, 120-kDa cell-binding FN fragments (FN120) likewise decreased VLA-5 expression, and did so by inducing a serine proteinase-dependent proteolysis of this beta(1) integrin. We postulated that changes in VLA-5 expression, which were induced by interactions with cell-binding FN fragments, may alter monocyte migration into tissue FN, a prominent component of the cardiac extracellular matrix. Support for this hypothesis came from experiments showing that FN120 treatment significantly reduced both spontaneous and MCP-1-induced monocyte migration on an FN-impregnated collagen matrix. In vivo, it is likely that contact with cell-binding FN fragments also modulates VLA-5/FN adhesive interactions, and this causes monocytes to accumulate at sites where the fragment concentration is sufficient to ensure proteolytic degradation of VLA-5.

Epitope mapping of B-cell determinants on the 15-kilodalton lipoprotein of Treponema pallidum (Tpp15) with synthetic peptides

The antigenicity of the 15-kDa lipoprotein of Treponema pallidum (Tpp15 or TpN15) was comprehensively evaluated in epitope-scanning studies with overlapping deca- and octapeptides and polygonal rabbit and human infant immunoglobulins (Igs) and antisera. This approach enabled us to identify potentially important regions and to determine the optimal dilutions of Igs or antisera for use in further studies. IgM and IgG from both species were capable of recognizing multiple, continuous epitopes. A total of 13 peptides, principally clustered in the central regions of the protein, were recognized by all syphilitic sera and Ig fractions. On the basis of window analyses, frequency profiles, and alanine substitution studies, five heptapeptides were selected for mimetic studies. Two of these five immunodominant, continuous epitopes initially appeared to be species specific; however, antisera elicited against mimetics of all five epitopes were polyspecific, recognizing similar motifs on several other treponemal proteins, including those of avirulent organisms. The only mimetic which yielded positive reactions with infant IgM and syphilitic sera in the absence of cross-reactions with rabbit antisera to avirulent treponemes was the variant of the VMYASSG motif. These findings are relevant to the development of simple, inexpensive assays for the serodiagnosis of active syphilis.

Isolation of the outer membranes from Treponema pallidum and Treponema vincentii

The outer membranes from Treponema pallidum subsp. pallidum and Treponema vincentii were isolated by a novel method. Purified outer membranes from T. pallidum and T. vincentii following sucrose gradient centrifugation banded at 7 and 31% (wt/wt) sucrose, respectively. Freeze fracture electron microscopy of purified membrane vesicles from T. pallidum and T. vincentii revealed an extremely low density of protein particles; the particle density of T. pallidum was approximately six times less than that of T. vincentii. The great majority of T. vincentii lipopolysaccharide was found in the outer membrane preparation. The T. vincentii outer membrane also contained proteins of 55 and 65 kDa. 125I-penicillin V labeling demonstrated that t. pallidum penicillin-binding proteins were found exclusively with the protoplasmic cylinders and were not detectable with purified outer membrane material, indicating the absence of inner membrane contamination. Isolated T. pallidum outer membrane was devoid of the 19-kDa 4D protein and the normally abundant 47-kDa lipoprotein known to be associated with the cytoplasmic membrane; only trace amounts of the periplasmic endoflagella were detected. Proteins associated with the T. pallidum outer membrane were identified by one- and two-dimensional electrophoretic analysis using gold staining and immunoblotting. Small amounts of strongly antigenic 17- and 45-kDa proteins were detected and shown to correspond to previously identified lipoproteins which are found principally with the cytoplasmic membrane. Less antigenic proteins of 65, 31 (acidic pI), 31 (basic pI), and 28 kDa were identified. Compared with whole-organism preparations, the 65- and the more basic 31-kDa proteins were found to be highly enriched in the outer membrane preparation, indicating that they may represent the T. pallidum rare outer membrane proteins. Reconstitution of solubilized T. pallidum outer membrane into lipid bilayer membranes revealed porin activity with two estimated channel diameters of 0.35 and 0.68 nm based on the measured single-channel conductances in 1 M KCl of 0.40 and 0.76 nS, respectively.

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.

Role of complement component C1q in phagocytosis of Listeria monocytogenes by murine macrophage-like cell lines

Listeria monocytogenes is a facultative intracellular pathogen of a great variety of cells. Among them, macrophages constitute the major effector cells of listerial immunity during the course of an infection. Although the molecular bases of L. monocytogenes attachment and entry to phagocytes are not completely understood, it has been demonstrated that C3b significantly increases L. monocytogenes uptake by macrophages via complement receptor type 3. The first component of complement, C1q, is present in organic fluids at a relatively high concentration, and C1q receptor sites in macrophages are also abundant. In the present report, results of studies on the role of C1q in the internalization and infectivity of L. monocytogenes by macrophages are presented. L. monocytogenes uptake is enhanced by prior treatment of bacteria with normal sera. Heated serum or C1q-deficient serum abrogates this enhancement. Purified C1q specifically restored uptake. This effect was blocked by the addition of F(ab')2 anti-C1q antibody but not by an irrelevant matched antibody. Direct binding of C1q to L. monocytogenes was specific, saturable, and dose dependent with both fluorescent and radiolabeled C1q. N-Acetyl-D-alanyl-L-isoglutamine, diaminopimelic acid, and L-rhamnose caused a significant dose-dependent inhibition of C1q binding to bacteria, suggesting that these molecules, at least, are involved in the attachment of C1q to L. monocytogenes cell wall. When C1q binding structures on macrophage-like cells were blocked with saturating concentrations of C1q, the uptake of C1q-opsonized bacteria was less than in untreated cells. These experiments demonstrate that, in addition to other reported mechanisms, L. monocytogenes binds C1q, which mediates enhanced uptake by macrophages through C1q binding structures.

Evidence that autologous idiotypic regulation of anti-arginine-glycine-aspartic acid autoantibodies may influence development and progression of syphilitic lesions in infected rabbits

The 83-kDa receptor protein of Treponema pallidum (TpN83) recognizes and binds fibronectin (Fn) at the amino acid sequence RGD site. By using experimentally infected animals, we have demonstrated that immunoglobulin G antibodies to this antigen and autoantibodies to the RGD site of Fn are putative components of immune complexes. This, and other findings, led us to initially hypothesize that anti-idiotypes (anti-Id) of an anti-TpN83 response are autoantibodies to RGD. Alternatively, we reasoned that if anti-Fn autoantibodies played a role in the pathogenesis of syphilis, then down-regulation of such a response by the Id network might directly affect progression of the disease. To test the hypothesis, rabbits were immunized with either affinity-purified TpN83 antigen or the synthetic Fn-7 peptide, KYGRGDS, and subsequently challenged with T. pallidum. Compared with results obtained with unimmunized, control rabbits, accelerated lesion development was noted in the rabbits immunized with TpN83. Pronounced, though unexpected, differences with respect to lesion development and progression were noted in the animals immunized with Fn-7 and then challenged intravenously; a minimal number of lesions appeared with a delayed onset. These lesions, like the localized chancres seen following intradermal challenge, were smaller and minimally ulcerated, and they healed rapidly. The Fn-7-immunized rabbits all differed from the controls in that anti-Id to anti-RGD F(ab')2 were demonstrable within 4 weeks following infection; decreases in anti-Fn autoantibody levels were associated with concomitant increases in anti-Id levels. Immunoglobulin Gs (anti-Id) from these animals following elution from anti-RGD F(ab')2 immunoaffinity columns also reacted with affinity-purified TpN83 antigen in immunoassays. These results suggest that down-regulation of autoreactive clones by manipulation of the idiotypic network in experimental syphilis warrants further investigation.

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).

Interaction of Mycobacterium avium complex with human macrophages: roles of membrane receptors and serum proteins

Invasive, disease-associated members of the Mycobacterium avium complex are facultative intracellular pathogens of mammalian macrophages. The mechanism(s) by which M. avium is ingested by mononuclear phagocytes is unknown. We examined the role of membrane receptors on macrophages as well as the role of opsonic components of the serum (complement, fibronectin, C-reactive protein and fibrinogen in the attachment and ingestion of M. avium by human monocyte-derived macrophages. Preincubation of serum with antibodies against C3 and fibronectin, in contrast to preincubation of serum with antibodies against complement-reactive protein and fibrinogen, significantly reduced the binding of M. avium to macrophages in concentration-dependent manner (57 to 93% and 35 to 61% inhibition by anti-C3 and anti-fibronectin antibody, respectively, in a concentration range of 25 to 100 micrograms/ml). We also observed that incubation of macrophages with OKM1 anti-complement receptor type 3 (CR3) antibody in the presence of serum decreased the binding of M. avium to macrophages by 86% +/- 6%, while the same antibody inhibited binding by 63% +/- 7% in the absence of serum. In contrast, incubation of macrophages with anti-LFA-1, anti-p 150.95, anti-CR1, or anti-Mac-1 had no effect on the ability of M. avium to bind to the cell. In addition, incubation of macrophages with alpha-methyl-D-mannoside was also associated with decreased attachment of M. avium to macrophages. These results provide evidence for the role of three macrophage receptors (CR3, fibronectin, and mannosyl-fucosyl receptors) in the uptake of M. avium by human macrophages.

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.

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.

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.

Demonstration and immunochemical characterization of natural, autologous anti-idiotypic antibodies throughout the course of experimental syphilis

Immunoglobulin Gs (IgGs) from serial bleeds obtained 1 week to 5 months after infection of rabbits with Treponema pallidum were examined for autologous anti-idiotypic (auto-anti-Id) antibodies. The capacities of IgGs to bind 125I-IgG F(ab')2 anti-T. pallidum prepared from earlier bleeds were determined by using radioimmunoassays. Results obtained in these assays suggested that auto-anti-Id IgGs were demonstrable between 60 and 180 days postinfection. Following reinfection, 90 days after the initial infection, auto-anti-Id IgGs peaked 6 to 8 weeks later, even though they were first noted as early as 2 weeks postreinfection. In the reinfection model the auto-anti-Id continued to persist throughout the remaining 3 months of the study. As auto-anti-Id antibodies to anti-fibronectin antibodies were demonstrable also in reinfected animals, our findings raise questions concerning the possible role of the anti-Id in modulating the Id expression of both T. pallidum-specific and autoimmune humoral responses.

Complement component C1q enhances invasion of human mononuclear phagocytes and fibroblasts by Trypanosoma cruzi trypomastigotes

Internalization and infectivity of Trypanosoma cruzi trypomastigotes by macrophages is enhanced by prior treatment of parasites with normal human serum. Heating serum or removing C1q from serum abrogates the enhancement, but augmentation of attachment and infectivity is restored by addition of purified C1q to either serum source. Although both noninfective epimastigotes (Epi) and vertebrate-stage tissue culture trypomastigotes (TCT) bind C1q in saturable fashion at 4 degrees C, internalization by monocytes and macrophages of TCT but not Epi-bearing C1q is enhanced in comparison to untreated parasites. Adherence of human monocytes and macrophages to surfaces coated with C1q also induces a marked enhancement of the internalization of native TCT. C1q enhances attachment of both Epi and TCT to human foreskin fibroblasts, but only when C1q is on the parasite and not when the fibroblasts are plated on C1q-coated surfaces. Only TCT coated with C1q show enhanced invasion into fibroblasts. Although trypomastigotes produce an inhibitor of the complement cascade which limits C3 deposition during incubation in normal human serum, C1q binds to the parasite and enhances entry of trypomastigotes into target cells.