Evaluation of FlaB1, FlaB2, FlaB3, and Tp0463 of Treponema pallidum for serodiagnosis of syphilis

Dual Immunization with Lipoprotein Tp0663 and Flagellin FlaB3 Offers Augmented Protection against Treponema pallidum in Mice

Syphilis is a significant multistage sexually transmitted infection caused by the bacterium Treponema pallidum. The pathogenesis of this pathogen remains inadequately understood, impeding the progress of syphilis vaccine development. Our prior study has demonstrated the potential of the Tp0663 protein as a viable candidate for a vaccine against T. pallidum. In the present study, we sought to explore the protective response of dual immunization using two different antigenic entities (i.e., flagellin FlaB3 and lipoprotein Tp0663) against T. pallidum in a murine model. Our investigation revealed that FlaB3 + Tp0663 can elicit robust humoral and cellular immune responses. In addition, the FlaB3 + Tp0663 vaccine demonstrated a notable reduction in the Treponemal burden within various anatomical sites of infected mice, including the blood, brain, liver, lymph nodes, spleen, and testicles. It is worth noting that the FlaB3 + Tp0663 vaccine suppressed the dissemination of T. pallidum in C57BL/6 mice. The findings demonstrate that T. pallidum flagellin FlaB3 may augment the immunoprotection of Tp0663. This represents a valuable practical perspective and offers insights into developing a syphilis vaccine.

Treponema pallidum Flagellin FlaB3 Activates Inflammation and Inhibits Autophagy in HMC3 Cells via the TLR4 Pathway

Neurosyphilis, a neurological manifestation of syphilis, is closely related to neuroinflammation. Autophagy, a fundamental cellular mechanism that mediates the degradation of intracellular components, plays a crucial role in immune regulation and inflammation. Microglia, resident immune cells in the brain, are central to these processes. However, the interplay between autophagy and neuroinflammation in the context of neurosyphilis remains poorly understood. In this research, the recombinant Treponema pallidum flagellin, FlaB3, was constructed to treat human microglia clone 3 (HMC3) cells and HMC3 cells in which TLR4 (Toll-like receptor 4) had been knocked down. We discovered that FlaB3 promotes IL-6 and IL-8 secretion through the TLR4 pathway. We also observed that FlaB3 regulates the expression of autophagy-related proteins Beclin1, LC3B, and P62 via the TLR4/PI3K/AKT/mTOR pathway, thereby inhibiting autophagy and autophagic flux in HMC3 cells. Subsequently, we discovered that the concentration of soluble amyloid β1-42 (Aβ1-42) was decreased in the cerebrospinal fluid of neurosyphilis patients. Immunofluorescence analysis further revealed that FlaB3 suppresses the degradation of Aβ by autophagosomes in HMC3 cells. Additionally, treatment with the autophagy activators Rapamycin and LY294002 decreased the levels of IL-6 and IL-8 secretion, indicating that autophagy modulates inflammation in HMC3 cells. In summary, our study demonstrates that FlaB3 promotes inflammation in HMC3 cells by inhibiting autophagy. This inhibition also impedes Aβ degradation, providing new insights into the pathogenesis of neurosyphilis.

Treponema denticola major surface protein (Msp): a key player in periodontal pathogenicity and immune evasion

Treponema denticola, a bacterium that forms a "red complex" with Porphyromonas gingivalis and Tannerella forsythia, is associated with periodontitis, pulpitis, and other oral infections. The major surface protein (Msp) is a surface glycoprotein with a relatively well-established overall domain structure (N-terminal, central and C-terminal regions) and a controversial tertiary structure. As one of the key virulence factors of T. denticola, Msp is associated with adherence, immune response, and pore formation by the microorganism. It also mediates several pathological changes in histocytes, such as cytoskeleton disruption, neutrophil phagocytosis, and phosphoinositide balance interruption. In addition, the Msp of T. denticola is also an ortholog of the Treponema pallidum repeat (Tpr) proteins and Msp or Msp-like proteins that have been detected in other oral treponeme species. This review will discuss the structure, pathogenicity and homologs of Msp produced by T. denticola, illuminate the controversy regarding the structure and membrane topology of native Msp, explore the potential roles of Msp in the mechanism of T. denticola immune escape and provide an overview of the cytotoxicity and adherence ability of Msp. Further understanding of the structure and functions of Msp will offer new insights that will help promote further investigations of the pathogenic mechanisms of T. denticola and other treponemes, leading to more effective prophylactic or therapeutic treatments for relevant diseases.

Resurgence of syphilis: focusing on emerging clinical strategies and preclinical models

Syphilis, a sexually transmitted disease (STD) caused by Treponema pallidum (T. pallidum), has had a worldwide resurgence in recent years and remains a public health threat. As such, there has been a great deal of research into clinical strategies for the disease, including diagnostic biomarkers and possible strategies for treatment and prevention. Although serological testing remains the predominant laboratory diagnostic method for syphilis, it is worth noting that investigations pertaining to the DNA of T. pallidum, non-coding RNAs (ncRNAs), chemokines, and metabolites in peripheral blood, cerebrospinal fluid, and other bodily fluids have the potential to offer novel perspectives on the diagnosis of syphilis. In addition, the global spread of antibiotic resistance, such as macrolides and tetracyclines, has posed significant challenges for the treatment of syphilis. Fortunately, there is still no evidence of penicillin resistance. Hence, penicillin is the recommended course of treatment for syphilis, whereas doxycycline, tetracycline, ceftriaxone, and amoxicillin are viable alternative options. In recent years, efforts to discover a vaccine for syphilis have been reignited with better knowledge of the repertoire of T. pallidum outer membrane proteins (OMPs), which are the most probable syphilis vaccine candidates. However, research on therapeutic interventions and vaccine development for human subjects is limited due to practical and ethical considerations. Thus, the preclinical model is ideal for conducting research, and it plays an important role in clinical transformation. Different preclinical models have recently emerged, such as in vitro culture and mouse models, which will lay a solid foundation for clinical treatment and prevention of syphilis. This review aims to provide a comprehensive summary of the most recent syphilis tactics, including detection, drug resistance treatments, vaccine development, and preclinical models in clinical practice.

Advancements in the development of nucleic acid vaccines for syphilis prevention and control

Syphilis, a chronic systemic sexually transmitted disease, is caused by the bacterium Treponema pallidum (T. pallidum). Currently, syphilis remains a widespread infectious disease with significant disease burden in many countries. Despite the absence of identified penicillin-resistant strains, challenges in syphilis treatment persist due to penicillin allergies, supply issues, and the emergence of macrolide-resistant strains. Vaccines represent the most cost-effective strategy to prevent and control the syphilis epidemic. In light of the ongoing global coronavirus disease 2019 (COVID-19) pandemic, nucleic acid vaccines have gained prominence in the field of vaccine research and development, owing to their superior efficiency compared to traditional vaccines. This review summarizes the current state of the syphilis epidemic and the preliminary findings in T. pallidum nucleic acid vaccine research, discusses the challenges associated with the development of T. pallidum nucleic acid vaccines, and proposes strategies and measures for future T. pallidum vaccine development.

Tp0684, Tp0750, and Tp0792 Recombinant Proteins as Antigens for the Serodiagnosis of Syphilis

The incidence of syphilis has increased alarmingly over the years. Its diagnosis continues to be a challenge, leading to the search for new alternative and effective methods. The objective of this study was to select and evaluate three Treponema pallidum recombinant proteins for potential use in syphilis serodiagnosis. Bioinformatics analysis was performed with three T. pallidum antigens (Tp0684, Tp0750, and Tp0792) to assess their physical, antigenic, and structural characteristics. The antigens were chemically synthesized, recombinant plasmids were expressed in Escherichia coli BL21 Star™ (DE3), and the recombinant proteins were purified by nickel affinity chromatography. The antigenicity of the recombinant proteins was evaluated by western blotting and enzyme-linked immunosorbent assay (ELISA), using the sera from patients with primary and latent syphilis. In silico analysis indicated the antigenic potential once the exposed B cell epitopes were detected in the evaluated proteins. Sera from patients with primary and latent syphilis specifically recognized rTp0684, rTp0750, and rTp0792 recombinant antigens. Moreover, the rTp0684-ELISA receiver operating characteristic (ROC) analysis showed an area under the ROC curve of 0.99, indicating high diagnostic efficacy with 97.62% specificity and 95% sensitivity. In conclusion, rTp0684 showed better potential as an antigen for the development of syphilis serodiagnosis. Thus, bioinformatic analysis can be an important tool to guide the selection of antigens for serological diagnosis.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12088-022-01017-w.

Assessment of recombinant antigens Tp0100 and Tp1016 of Treponema pallidum for serological diagnosis of syphilis

Objective

To discover novel serodiagnostic candidates for the serological diagnosis of syphilis.

Methods

Two recombinant Treponema pallidum proteins Tp0100 and Tp1016 were expressed, purified, and identified by Western Blotting. A total of 600 clinical serum samples were tested with the Tp0100‐based ELISA, the Tp1016‐based ELISA, and the commercial LICA Syphilis TP kit (ChIVD, Beijing, China). The sensitivities were determined by testing 340 samples from individuals with clinically diagnosed primary, secondary, latent, and tertiary syphilis. The specificities were determined by screening 260 samples from healthy controls and individuals with potentially cross‐reactive infections, including leptospirosis, Lyme disease, hepatitis B, tuberculosis, rheumatoid arthritis, systemic lupus erythematosus. Kappa (κ) values were applied to compare the agreement between clinical syphilis diagnosis and the Tp0100‐based ELISA, the Tp1016‐based ELISA, or the LICA Syphilis TP test.

Results

Using clinical syphilis diagnosis as the gold standard, Tp0100 exhibited an overall sensitivity of 95.6% and specificity of 98.1% for testing IgG antibody while Tp1016 demonstrated only an overall sensitivity of 75.0% and specificity of 79.6%. In contrast, the LICA Syphilis TP test revealed an overall sensitivity of 97.6% and specificity of 96.2%. In addition, the overall percent agreement and corresponding κ values were 96.7% (95% CI 95.6%–97.8%) and 0.93 for the Tp0100‐based ELISA, 77.0% (95% CI 74.3%–79.7%) and 0.54 for the Tp1016‐based ELISA, and 97.0% (95% CI 96.0%–98.0%) and 0.94 for the LICA Syphilis TP test, respectively.

Conclusion

The recombinant T. pallidum protein Tp0100 shows promise as a novel diagnostic antigen in the serological tests for syphilis.

The N-terminal D1 domain of Treponema pallidum flagellin binding to TLR5 is required but not sufficient in activation of TLR5

Syphilis is a chronic bacterial infection caused by Treponema pallidum (T pallidum) and the pathogenesis that T pallidum infection induces immunopathological damages in skin and other tissues remains unclear. We have previously reported that recombinant flagellins of T pallidum can elicit IL‐6 and IL‐8 transcriptions via TLR5 pathway. To identify the domains which induced the pro‐inflammatory activity and the importance of the interactions between TLR5 and domains, homology‐based modelling and comparative structural analyses revealed that Tpflagellins can combine with TLR5 directly. Deletion mutations showed that the ND1 domain binding to TLR5 is required but not sufficient in TLR5 activation. Moreover, site‐directed mutagenesis analysis indicated that the arginine residue (Tpflagellins R89) of the ND1 domain and its adjacent residues (Tpflagellins L93 and E113) constitute a hot spot that elicits IL‐6, IL‐8 transcriptions and TLR5 activation, and affects the binding of Tpflagellins to TLR5. Taken together, these results give insight into the pathogenesis of T pallidum and may contribute to the future design of Tpflagellins‐based therapeutics and syphilis vaccine.

Immunogenicity and protective efficacy against Treponema pallidum in New Zealand rabbits immunized with plasmid DNA encoding flagellin

Plasmid DNA encoding flagellin FlaB3 was used as a vaccination candidate for the evaluation of immunogenicity and protection against Treponema pallidum subsp. pallidum dissemination. First, intramuscular injection of the flagellin encoded by the plasmid DNA into New Zealand rabbits elicited both humoral and cellular immune responses. Total IgG production increased in response to flagellin. In addition, serum IFN-γ secretion and CD8+ cells were substantially greater in the rabbits immunized with the plasmid encoding flagellin FlaB3 than those in the rabbits immunized with recombinant flagellin. The flagellin encoded by the plasmid DNA induced significant upregulation of serum IL-6 and IL-8 compared to that of the control rabbits. Subsequently, intradermal challenge of the vaccinated New Zealand rabbits with 1 × 10⁷T. pallidum resulted in a significant reduction of the bacterial organ burden in the blood, liver, spleen, and testicles in the flagellin plasmid DNA-vaccinated rabbits. Furthermore, the histopathological analysis demonstrated that the rabbits immunized with the plasmid DNA-encoded flagellin (FlaB3) showed better immune protection. These findings provide evidence that plasmid DNA-encoded flagellin (FlaB3) may be useful as a potential immunization route for future development of a vaccine to inhibit T. pallidum dissemination in related animals.

Performance of novel infection phase-dependent antigens in syphilis serodiagnosis and treatment efficacy determination

OBJECTIVE

The objective of this study was to screen new antigens for syphilis serodiagnosis.

METHODS

First, we determined whether the Treponema pallidum proteins Tp0971, Tp0768 and Tp0462 were infection phase-dependent antigens by observing serum reactivity differences in New Zealand rabbits infected with activated or inactivated T. pallidum. A non-infection phase-dependent antigen, the Tp92 membrane protein, was used as the negative control. Next, Tp0971-, Tp0768- and Tp0462-based ELISA was performed on 2138 human serum samples and compared with the T. pallidum passive particle agglutination assay (TPPA) and LiZhu™ Tp-ELISA. In addition, another 60 paired serum samples from patients at follow-up were analysed to evaluate the relationships between titre changes and differences in the A450 nm values of the Tp0971, Tp0768, Tp0462 and Tp92 antibodies measured by ELISA.

RESULTS

Compared with Tp92 (negative control), Tp0971, Tp0768 and Tp0462 were determined to be infection phase-dependent antigens. Compared with those of the TPPA, the sensitivities of Tp0971-, Tp0768- and Tp0462-based ELISA were 96.4%, 96.9% and 93.0%, respectively, and the specificities were 97.7%, 95.4% and 98.9%, respectively, resulting in consistencies of 97.1%, 96.2% and 95.9%, respectively. Compared with those of the LiZhu™ Tp-ELISA, the consistencies of Tp0971-, Tp0768- and Tp0462-based ELISA were 95.1%, 94.2% and 94.0%, respectively, with kappa values of 0.902, 0.884 and 0.880, respectively. Tp0971, Tp0768 and Tp0462 demonstrated high sensitivities and specificities, as well as high conformity to the TPPA and LiZhu™ Tp-ELISA. Moreover, a significantly positive Spearman rank correlation coefficient (0.82,*P < 0.05) was found between the difference in the A450 nm values of the Tp0971 antibody and the RPR titre change.

CONCLUSION

The infection phase-dependent antigens Tp0971, Tp0768 and Tp0462 are promising for syphilis diagnosis, and Tp0971 may be utilized to monitor curative effects during syphilis treatment.

Screening and identification of immunoactive FlaB protein fragments of Treponema pallidum for the serodiagnosis of syphilis

Flagellin is a classical pathogen-associated molecular pattern that can evoke a robust immune response. We have demonstrated previously that three full-length flagellins of Treponema pallidum, namely FlaB1, FlaB2 and FlaB3, did have diagnostic value in the serodiagnosis of syphilis. Here, we selected and constructed three recombinant fragments of each complete FlaB, both the conserved N-terminal and the C-terminal region, and the middle variable part, with the goal of exploring fragments unique to Treponema pallidum for use as antigen targets in a fragment-based serological test. The diagnostic performance of fragments was evaluated using different panels of serum specimens (= 332) by indirect IgG enzyme-linked immunosorbent assay. The data showed that all the conserved fragments exhibited excellent sensitivities (91.1-95.0%) but poor specificities (64.1-78.4%), while the three middle regions demonstrated higher sensitivities and specificities for detecting IgG antibody, with 92.7% and 96.1% for FlaB1M ('B1M'), 91.6% and 94.8% for B2M, and 95.0% and 100% for B3M, respectively. In comparison, the sensitivity and specificity of Architect Syphilis TP was found to be 95.5% and 94.8%, respectively. These findings revealed that the middle portion of each FlaB had epitopes specific for Treponema pallidum and identified B3M as a promising candidate antigen for the serodiagnosis of syphilis.

Serological evaluation of antigen Tp0693 for diagnosis of syphilis

The aim of the present study was to evaluate the diagnostic value of the Treponema pallidum (Tp) antigen Tp0693 for syphilis. ELISA was used to examine the serum levels of Tp0693. The sample-to-cutoff ratio (S/CO) value was used to generate a receiver operating characteristic (ROC) curve. A correlation analysis was performed to compare the detection efficacy of Tp0693-specific ELISA, Treponema pallidum Particle Agglutination (TPPA), Tolulized Red Unheated Serum test (TRUST) and LiZhu™ Tp-ELISA. The area under the ROC curve was 0.99, indicating good diagnostic efficacy. When the diagnostic specificity reached 100%, the diagnostic sensitivity was up to 93.5%. Tp0693-specific ELISA results were not correlated with those of TPPA, TRUST and LiZhu™ Tp-ELISA (correlation coefficient, 0.122, 0.114 and 0.025, respectively). The latent syphilis rate was highest (12%, 9/75) for all syphilis specimens with a S/CO in the grey area. In conclusion, for syphilis specimens with a S/CO in the grey area, TPPA should be used for further confirmation of the diagnosis. Tp0693 may be used as a diagnostic antigen for syphilis; however, further study regarding its potential use is required.

Treponema pallidum flagellins stimulate MMP-9 and MMP-13 expression via TLR5 and MAPK/NF-κB signaling pathways in human epidermal keratinocytes

Syphilis is a chronic disease caused by Treponema pallidum and the pathogenesis is still unclear. T. pallidum infection induced inflammatory responses are involved in the immunopathological damage in skin and other tissues. Flagellin, the monomeric subunit of bacterial flagella, is a classic pathogen associated molecular patterns (PAMPs) that interacts to TLR5 and induces inflammatory responses. Keratinocytes, as immune sentinels recognize the PAMPs via TLRs, play an important role in skin innate immune response. Matrix metalloproteinases (MMPs) expressed by keratinocytes are involved in skin inflammatory responses and promoting pathogens invasion. In this study, we demonstrate that FlaB1, FlaB2 and FlaB3, the flagellins of T. pallidum, induced MMP-9 and MMP-13 production in human immortalized keratinocytes cell line HaCaT. Silencing of TLR5, but not TLR2 and TLR4 attenuated MMP-9 and MMP-13 expressions induced by T. pallidum flagellins. MMP-9 and MMP-13 expressions were also be abrogated by transfection with a dominant negative (DN) plasmid of MyD88. We also found that treatment of HaCaT cells with FlaB1, FlaB2 and FlaB3 activate the MAPK and NF-κB signaling pathways. Inhibited of ERK, JNK, p38 and NF-κB suppressed MMP-9 expression induced by the FlaB1. MMP-13 expression was found to be suppressed by pretreatment with inhibitors of ERK, JNK and NF-κB, but not p38. These findings demonstrate that T. pallidum flagellins (FlaB1, FlaB2 or FlaB3) can stimulate MMP-9 and MMP-13 expression through TLR5 and MAPK/NF-κB signaling pathways in human epidermal keratinocytes, which could contribute to the pathogenesis of T. pallidum infection.

Novel Treponema pallidum Recombinant Antigens for Syphilis Diagnostics: Current Status and Future Prospects

The recombinant protein technology considerably promoted the development of rapid and accurate treponema-specific laboratory diagnostics of syphilis infection. For the last ten years, the immunodominant recombinant inner membrane lipoproteins are proved to be sensitive and specific antigens for syphilis screening. However, the development of an enlarged T. pallidum antigen panel for diagnostics of early and late syphilis and differentiation of syphilis stages or cured syphilis remains as actual goal of multidisciplinary expertise. Current review revealed novel recombinant antigens: surface-exposed proteins, adhesins, and periplasmic and flagellar proteins, which are promising candidates for the improved syphilis serological diagnostics. The opportunities and limitations of diagnostic usage of these antigens are discussed and the criteria for selection of optimal antigens panel summarized.