MyD88 deficiency markedly worsens tissue inflammation and bacterial clearance in mice infected with Treponema pallidum, the agent of syphilis

Development and utilization of Treponema pallidum expressing green fluorescent protein to study spirochete-host interactions and antibody-mediated clearance: expanding the toolbox for syphilis research

Syphilis is a sexually transmitted infection caused by the highly invasive and immunoevasive spirochetal pathogen Treponema pallidum subsp. pallidum (TPA). Untreated syphilis can lead to infection of multiple organ systems, including the central nervous system. The alarming increase in syphilis cases globally underscores the importance of developing novel strategies to understand the complexities of syphilis pathogenesis. In this study, we took advantage of recent advances in in vitro cultivation and genetic manipulation of syphilis spirochetes to engineer a TPA strain that constitutively expresses green fluorescent protein (GFP). GFP+ TPA grew identically to the Nichols parent strain in vitro and exhibited wild-type infectivity in the rabbit model. We then used the GFP+ strain to visualize TPA interactions with host cells during co-cultivation in vitro, within infected rabbit testes, and following opsonophagocytosis by murine bone marrow-derived macrophages. The development of fluorescent strain also enabled us to develop a flow cytometric-based assay to assess antibody-mediated damage to the spirochete's fragile outer membrane (OM), demonstrating dose-dependent growth inhibition and OM disruption in vitro. Notably, we observed greater OM disruption of GFP+ TPA with sera from immune rabbits infected with the TPA Nichols strain compared to sera generated against the genetically distinct SS14 strain. These latter findings highlight the importance of OM protein-specific antibody responses for clearance of TPA during syphilitic infection. The availability of fluorescent TPA strains paves the way for future studies investigating spirochete-host interactions as well as functional characterization of antibodies-directed treponemal OM proteins, the presumptive targets for protective immunity.

IMPORTANCE

Syphilis, a sexually transmitted infection caused by Treponema pallidum (TPA), remains a pressing threat to global public health. TPA has a remarkable and still poorly understood ability to disseminate rapidly from the site of inoculation and establish persistent infection throughout the body. Recent advances in in vitro cultivation and genetic manipulation of syphilis spirochetes enabled the development of fluorescent TPA. In the study, we generated and characterized an infectious TPA strain that constitutively expresses green fluorescent protein and used this strain to visualize the interaction of TPA with host cells and functionally characterize antibodies directed against treponemal outer membrane proteins. Most notably, we assessed the ability of surface-bound antibodies to inhibit the growth of TPA in vitro and/or disrupt the spirochete's fragile outer membrane. Fluorescent TPA strains provide a powerful new tool for elucidating host-pathogen interactions that enable the syphilis spirochete to establish infection and persistent long-term within its obligate human host.

Immunodominant extracellular loops of Treponema pallidum FadL outer membrane proteins elicit antibodies with opsonic and growth-inhibitory activities

The global resurgence of syphilis has created a potent stimulus for vaccine development. To identify potentially protective antibodies against Treponema pallidum (TPA), we used Pyrococcus furiosus thioredoxin (PfTrx) to display extracellular loops (ECLs) from three TPA outer membrane protein families (outer membrane factors for efflux pumps, eight-stranded β-barrels, and FadLs) to assess their reactivity with immune rabbit serum (IRS). We identified five immunodominant loops from the FadL orthologs TP0856, TP0858 and TP0865 by immunoblotting and ELISA. Rabbits and mice immunized with these five PfTrx constructs produced loop-specific antibodies that promoted opsonophagocytosis of TPA by rabbit peritoneal and murine bone marrow-derived macrophages at levels comparable to IRS and mouse syphilitic serum. Heat-inactivated IRS and loop-specific rabbit and mouse antisera also impaired viability, motility, and cellular attachment of spirochetes during in vitro cultivation. The results support the use of ECL-based vaccines and suggest that loop-specific antibodies promote spirochete clearance via Fc receptor-independent as well as Fc receptor-dependent mechanisms.

Development and utilization of Treponema pallidum expressing green fluorescent protein to study spirochete-host interactions and antibody-mediated clearance: expanding the toolbox for syphilis research

Syphilis is a sexually transmitted infection caused by the highly invasive and immunoevasive spirochetal pathogen Treponema pallidum subsp. pallidum (TPA). Untreated syphilis can lead to infection of multiple organ systems, including the central nervous system. The alarming increase in syphilis cases globally underscores the importance of developing novel strategies to understand the complexities of syphilis pathogenesis. In this study, we took advantage of recent advances in in vitro cultivation and genetic manipulation of syphilis spirochetes to engineer a TPA strain that constitutively expresses green fluorescent protein (GFP). GFP+ TPA grew identically to the Nichols parent strain in vitro and exhibited wild-type infectivity in the rabbit model. We then used the GFP+ strain to visualize TPA interactions with host cells during co-cultivation in vitro, within infected rabbit testes, and following opsonophagocytosis by murine bone marrow-derived macrophages. Development of fluorescent strain also enabled us to develop a flow cytometric-based assay to assess antibody-mediated damage to the spirochete's fragile outer membrane (OM), demonstrating dose-dependent growth inhibition and OM disruption in vitro. Notably, we observed greater OM disruption of GFP+ TPA with sera from immune rabbits infected with the TPA Nichols strain compared to sera generated against the genetically distinct SS14 strain. These latter findings highlight the importance of OM protein-specific antibody responses for clearance of TPA during syphilitic infection. The availability of fluorescent TPA strains paves the way for future studies investigating spirochete-host interactions as well as functional characterization of antibodies directed treponemal OM proteins, the presumptive targets for protective immunity.

Immunodominant extracellular loops of Treponema pallidum FadL outer membrane proteins elicit antibodies with opsonic and growth-inhibitory activities

The global resurgence of syphilis has created a potent stimulus for vaccine development. To identify potentially protective antibodies (Abs) against Treponema pallidum (TPA), we used Pyrococcus furiosus thioredoxin (PfTrx) to display extracellular loops (ECLs) from three TPA outer membrane protein families (outer membrane factors for efflux pumps, eight-stranded β-barrels, and FadLs) to assess their reactivity with immune rabbit serum (IRS). Five ECLs from the FadL orthologs TP0856, TP0858 and TP0865 were immunodominant. Rabbits and mice immunized with these five PfTrx constructs produced ECL-specific Abs that promoted opsonophagocytosis of TPA by rabbit peritoneal and murine bone marrow-derived macrophages at levels comparable to IRS and mouse syphilitic serum. ECL-specific rabbit and mouse Abs also impaired viability, motility, and cellular attachment of spirochetes during in vitro cultivation. The results support the use of ECL-based vaccines and suggest that ECL-specific Abs promote spirochete clearance via Fc receptor-independent as well as Fc receptor-dependent mechanisms.

Serofast status in syphilis: Pathogenesis to therapeutics

Syphilis, a sexually transmitted infection caused by Treponema pallidum, has been experiencing a rise in prevalence in recent years. "Syphilis serofast" describes a unique serological reaction in patients with syphilis whose clinical symptoms have resolved following consistent anti-syphilitic therapy, but the non-Treponema pallidum antigen serologic test is still positive. Syphilis serofast is a risk factor for syphilis recurrence, neurosyphilis, and multisystem involvement. Considering the current lack of comprehensive knowledge about the epidemiological characteristics, pathogenesis, and therapies of syphilis serofast, we conducted an online search of research relating to syphilis serofast over the last twenty years. Previous research has shown that the pathogenesis of syphilis serofast is mainly related to clinical factors, immune factors, syphilis subtypes, and T.pallidum membrane protein repeat gene antigen. There are two distinct viewpoints on the treatment of serofast: no excessive treatment and active treatment. In addition, serofast patients also showed two clinical outcomes: syphilis recurrence and persistent serofast status. This article systematically reviews the related factors, treatment, and clinical outcomes of syphilis serofast, provides a theoretical basis for its research, diagnosis, and treatment, and helps clinicians develop a follow-up treatment management plan for syphilis serofast.

Immunisation with the glycolytic enzyme enolase inhibits dissemination of Treponema pallidum in C57BL/6 mice

Treponema pallidum (T. pallidum), an obligate extracellular bacterium, is the causative agent of sexually transmitted bacterial diseases. In this study, the glycolytic enzyme enolase (Tp Eno) of T. pallidum were injected intramuscularly into C57BL/6 mice, resulting in higher levels of specific anti-Tp Eno antibodies and Tp Eno-specific splenocyte proliferation than those in the mice immunized with recombinant protein Tp Eno. Cytokine (IL-4, IL-6, IL-10, IFN-γ, and TNF-α) analysis of splenocytes showed that the Tp Eno could slightly trigger the Th1-biased immune response. Furthermore, immunization of mice with Tp Eno elicited a significant production of IFN-γ by CD4+ T-cells in the spleen. Subsequently, mice were inoculated intradermally (between the scapulae), intraperitoneally, intrarectally and via the corpora cavernosa with 2.5 × 106 organisms per site (1 × 107 total organisms). The bacterial organ burden detected in the blood, spleen, liver, testes or brain of immunized mice suggested that Tp Eno enhances protective immunity to inhibit T. pallidum colonization in distal tissues. Therefore, Tp Eno vaccination enhances Tp Eno-specific immunogenicity and provides protection against T. pallidum dissemination.

Use of Epivolve phage display to generate a monoclonal antibody with opsonic activity directed against a subdominant epitope on extracellular loop 4 of Treponema pallidum BamA (TP0326)

Introduction

Syphilis, a sexually transmitted infection caused by the spirochete Treponema pallidum (Tp), is resurging globally. Tp's repertoire of outer membrane proteins (OMPs) includes BamA (β-barrel assembly machinery subunit A/TP0326), a bipartite protein consisting of a 16-stranded β-barrel with nine extracellular loops (ECLs) and five periplasmic POTRA (polypeptide transport-associated) domains. BamA ECL4 antisera promotes internalization of Tp by rabbit peritoneal macrophages.

Methods

Three overlapping BamA ECL4 peptides and a two-stage, phage display strategy, termed "Epivolve" (for epitope evolution) were employed to generate single-chain variable fragments (scFvs). Additionally, antisera generated by immunizing mice and rabbits with BamA ECL4 displayed by a Pyrococcus furiosus thioredoxin scaffold (PfTrxBamA/ECL4). MAbs and antisera reactivities were evaluated by immunoblotting and ELISA. A comparison of murine and rabbit opsonophagocytosis assays was conducted to evaluate the functional ability of the Abs (e.g., opsonization) and validate the mouse assay. Sera from Tp-infected mice (MSS) and rabbits (IRS) were evaluated for ECL4-specific Abs using PfTrxBamA/ECL4 and overlapping ECL4 peptides in immunoblotting and ELISA assays.

Results

Each of the five mAbs demonstrated reactivity by immunoblotting and ELISA to nanogram amounts of PfTrxBamA/ECL4. One mAb, containing a unique amino acid sequence in both the light and heavy chains, showed activity in the murine opsonophagocytosis assay. Mice and rabbits hyperimmunized with PfTrxBamA/ECL4 produced opsonic antisera that strongly recognized the ECL presented in a heterologous scaffold and overlapping ECL4 peptides, including S2. In contrast, Abs generated during Tp infection of mice and rabbits poorly recognized the peptides, indicating that S2 contains a subdominant epitope.

Discussion

Epivolve produced mAbs target subdominant opsonic epitopes in BamA ECL4, a top syphilis vaccine candidate. The murine opsonophagocytosis assay can serve as an alternative model to investigate the opsonic potential of vaccinogens. Detailed characterization of BamA ECL4-specific Abs provided a means to dissect Ab responses elicited by Tp infection.

Characterization of Treponema pallidum Dissemination in C57BL/6 Mice

The spirochetal pathogen Treponema pallidum causes 5 million new cases of venereal syphilis worldwide each year. One major obstacle to syphilis prevention and treatment is the lack of suitable experimental animal models to study its pathogenesis. Accordingly, in this study, we further evaluated the responses of mice to Treponema pallidum. Quantitative polymerase chain reaction showed that Treponema pallidum could colonize the heart, liver, spleen, kidneys, and testicles of C57BL/6 mice, and the organism may be able to rapidly penetrate the blood-brain barrier in mice by 24 h after infection. In subsequent rabbit infectivity tests, we observed evident signs of the microorganism in the mouse lymph node suspension. After infection, bacterial loads were higher in the tissues than in the blood of C57BL/6 mice. Moreover, a significant Th1 immune response was recorded by cytokine assays. Flow cytometric analysis suggested an obvious increase in the proportion of CD3+ T and CD4+ T cells in the spleen cells in the infected mice. Thus, improving our understanding of the response of C57BL/6 mice for Treponema pallidum will help to comprehensive elucidate the pathogenic mechanisms of this bacterium and lay the foundation for the development of a new research model of Treponema pallidum.

MiR-223-3p inhibits rTp17-induced inflammasome activation and pyroptosis by targeting NLRP3

The incidence of syphilis caused by Treponema pallidum subsp pallidum (T pallidum) infection is accompanied by inflammatory injuries of vascular endothelial cells. Studies have revealed that T pallidum infection could induce inflammasome activation and pyroptosis in macrophages. MicroRNA‐223‐3p (miR‐223‐3p) was reported to be a negative regulator in inflammatory diseases. The present study aimed to explore whether miR‐223‐3p regulates T pallidum‐induced inflammasome activation and pyroptosis in vascular endothelial cells, and determine the mechanisms which underlie this process. MiR‐223‐3p levels in syphilis and control samples were determined. The biological function of miR‐223‐3p in the NLRP3 inflammasome and pyroptosis was evaluated in T pallidum‐infected human umbilical vein endothelial cells (HUVECs). We observed a dramatic decrease in miR‐223‐3p levels in syphilis patients (n = 20) when compared to healthy controls (n = 20). Moreover, miR‐223‐3p showed a notable inhibitory effect on recombinant Tp17 (rTP17)‐induced caspase‐1 activation, resulting in decrease in IL‐1β production and pyroptosis, which was accompanied by the release of lactate dehydrogenase (LDH) in HUVECs. Additionally, the dual‐luciferase assay confirmed that NLRP3 is a direct target of miR‐223‐3p. Moreover, NLRP3 overexpression or knockdown largely blocked the effects of miR‐223‐3p on T pallidum‐induced inflammasome activation and pyroptosis in HUVECs. Most importantly, a notable negative correlation was observed between miR‐223‐3p and NLRP3, caspase‐1, and IL‐1β, respectively, in the serum of syphilis patients and healthy controls. Taken together, our results reveal that miR‐223‐3p targets NLRP3 to suppress inflammasome activation and pyroptosis in T pallidum‐infected endothelial cells, implying that miR‐223‐3p could be a potential target for syphilis patients.

Syphilis Reactivates Latent Epstein-Barr Virus Reservoir via Toll-Like Receptor 2 and B-Cell Receptor Activation

Latent Epstein-Barr virus (EBV) infection can clinically reactivate in immunosuppressed individuals causing lymphoproliferative disease and rarely hepatitis. In this study, we provide in vivo and in vitro evidence that Treponema pallidum infection can cause EBV reactivation with hepatitis in an immunocompetent patient. We report the diagnostic challenges and immunological findings of coinciding syphilis and EBV-associated hepatitis. Using an in vitro EBV-reactivation assay, we demonstrate that T pallidum reactivates latent EBV in a Toll-like receptor (TLR)2/B-cell receptor signaling-dependent manner. Epstein-Barr virus-associated reactivation or lymphoproliferation should be considered in infections with pathogens that activate TLR2.

Investigating Human Dendritic Cell Immune Responses to Borrelia burgdorferi

Dendritic cells (DCs) are professional antigen-presenting cells that recognize and phagocytose pathogens, and help to orchestrate adaptive immune responses to combat them. DCs are abundant in the skin where Borrelia burgdorferi first enters the body during a tick bite, and are thus critical in determining the initial stages of the innate and adaptive immune responses against Borrelia. Here, we describe two methods to study the response of DCs to Borrelia; an in vitro approach using monocyte-derived DCs (moDCs) and an ex vivo approach using a human skin model.

Syphilis

Treponema pallidum subspecies pallidum (T. pallidum) causes syphilis via sexual exposure or via vertical transmission during pregnancy. T. pallidum is renowned for its invasiveness and immune-evasiveness; its clinical manifestations result from local inflammatory responses to replicating spirochaetes and often imitate those of other diseases. The spirochaete has a long latent period during which individuals have no signs or symptoms but can remain infectious. Despite the availability of simple diagnostic tests and the effectiveness of treatment with a single dose of long-acting penicillin, syphilis is re-emerging as a global public health problem, particularly among men who have sex with men (MSM) in high-income and middle-income countries. Syphilis also causes several hundred thousand stillbirths and neonatal deaths every year in developing nations. Although several low-income countries have achieved WHO targets for the elimination of congenital syphilis, an alarming increase in the prevalence of syphilis in HIV-infected MSM serves as a strong reminder of the tenacity of T. pallidum as a pathogen. Strong advocacy and community involvement are needed to ensure that syphilis is given a high priority on the global health agenda. More investment is needed in research on the interaction between HIV and syphilis in MSM as well as into improved diagnostics, a better test of cure, intensified public health measures and, ultimately, a vaccine.

Treponema pallidum, the syphilis spirochete: making a living as a stealth pathogen

The past two decades have seen a worldwide resurgence in infections caused by Treponema pallidum subsp. pallidum, the syphilis spirochete. The well-recognized capacity of the syphilis spirochete for early dissemination and immune evasion has earned it the designation 'the stealth pathogen'. Despite the many hurdles to studying syphilis pathogenesis, most notably the inability to culture and to genetically manipulate T. pallidum, in recent years, considerable progress has been made in elucidating the structural, physiological, and regulatory facets of T. pallidum pathogenicity. In this Review, we integrate this eclectic body of information to garner fresh insights into the highly successful parasitic lifestyles of the syphilis spirochete and related pathogenic treponemes.

The role of primary infection of Schwann cells in the aetiology of infective inflammatory neuropathies

Numerous different pathogens are responsible for infective peripheral neuropathies and this is generally the result of the indirect effects of pathogen infection, namely anti pathogen antibodies cross reacting with epitopes on peripheral nerve, auto reactive T cells attacking myelin, circulating immune complexes and complement fixation. Primary infection of Schwann cells (SC) associated with peripheral nerve inflammation is rare requiring pathogens to cross the Blood Peripheral Nerve Barrier (BPNB) evade anti-pathogen innate immune pathways and invade the SC. Spirochetes Borrelia bourgdorferi and Trepomema pallidum are highly invasive, express surface lipo proteins, but despite this SC are rarely infected. However, Trypanosoma cruzi (Chaga's disease) and Mycobacterium leprae. Leprosy are two important causes of peripheral nerve infection and both demonstrate primary infection of SC. This is due to two novel strategies; T. cruzi express a trans-silalidase that mimics host neurotrophic factors and infects SC via tyrosine kinase receptors. M. leprae demonstrates multi receptor SC tropism and subsequent infection promotes nuclear reprogramming and dedifferentiation of host SC into progenitor stem like cells (pSLC) that are vulnerable to M. leprae infection. These two novel pathogen evasion strategies, involving stem cells and receptor mimicry, provide potential therapeutic targets relevant to the prevention of peripheral nerve inflammation by inhibiting primary SC infection.