Rabbit model of disseminated syphilis: immunoblot and immunohistologic evidence for a role of specific immune complexes in lesion pathogenesis

The immunopathobiology of syphilis: the manifestations and course of syphilis are determined by the level of delayed-type hypersensitivity

Syphilis has plagued mankind for centuries and is currently resurgent in the Western hemisphere. Although there has been a significant reduction of tertiary disease and recognition of facilitative interactions with human immunodeficiency virus infection, the natural history of syphilis has remained largely unchanged; thus, new strategies are required to more effectively combat this pathogen. The immunopathologic features of experimental syphilis in the rabbit; the course, stages, and pathology of human syphilis; and a comparison of human syphilis with leprosy suggest that the clinical course of syphilis and its tissue manifestations are determined by the balance between delayed-type hypersensitivity (DTH) and humoral immunity to the causative agent, Treponema pallidum. A strong DTH response is associated with clearance of the infecting organisms in a well-developed chancre, whereas a cytotoxic T-cell response or strong humoral antibody response is associated with prolonged infection and progression to tertiary disease. Many of the protean symptoms/appearances of secondary and tertiary human syphilis are manifestations of immune reactions that fail to clear the organism, due to a lack of recruitment and, more importantly, activation of macrophages by sensitized CD4 T cells. The Bacillus Calmette-Guerin vaccination can enhance DTH and has been shown to produce a low, but measurable, beneficial effect in the prevention of leprosy, a disease that shows a disease spectrum with characteristics in common with syphilis. In the prevention of syphilis, a potential vaccine protective against syphilis should be designed to augment the DTH response.

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.

Livedo vasculitis: vasculitis or thrombotic vasculopathy?

Elevated fibrinopeptide A levels, suggestive of a thrombogenic state, were observed in six patients with livedo vasculitis. Serological studies demonstrated normal complement levels, and circulating immune complexes were not identified by standard assays. Morphological studies showed no evidence of immune complex deposition or a neutrophilic vascular reaction. These results support classification of this disorder as a thrombogenic vasculopathy rather than as a small vessel vasculitis. Further investigations into the thrombotic abnormalities underlying this entity are warranted.

Contribution of rabbit leukocyte defensins to the host response in experimental syphilis

In the companion paper (L. A. Borenstein, M. E. Selsted, R. I. Lehrer, and J. N. Miller, Infect. Immun. 59:1359-1367, 1991), we report that rabbit alveolar macrophage and neutrophil derived defensins possess antimicrobial activity against Treponema pallidum subsp. pallidum, the etiologic agent of syphilis. In this study, antisera specific for NP-1 and NP-2 (defensins present in certain macrophages and polymorphonuclear leukocytes) and NP-5 (a defensin produced only in neutrophils) were used to detect these peptides by immunoperoxidase staining in testicular lesions from infected rabbits. Profound amounts of cell-free and cell-associated defensins were detected in the tunica albuginea and interstitial spaces during the first 24 h of infection. The presence of defensins was transient and almost undetectable by day 4. Interstitial defensins were detected again at day 10 and increased through day 16, at which time lesion healing was evident by hematoxylin and eosin staining. The appearance and increase in detectable defensins between days 10 and 16 of infection correlated with a reduction in numbers and disappearance of T. pallidum, as demonstrated by using silver staining. The extent and pattern of immunostaining for NP-1 and NP-2 corresponded with immunostaining for NP-5 and identified neutrophils as the cellular source of the defensins. These findings indicate that defensins may contribute to the control of local T. pallidum infection and suggest a role for acute inflammatory processes in the resolution of early experimental syphilis.

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.

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.