Are nontreponemal tests suitable for monitoring syphilis treatment efficacy? Evidence from rabbit infection models

Population Prevalence and Correlates of Syphilis in Rural, Southwestern Uganda

Background

The global incidence of syphilis has increased in recent years. Understanding syphilis epidemiology will inform screening and treatment programs. However, such data are lacking in many communities. We outline a population-based syphilis screening program in a rural community in southwestern Uganda to describe the population prevalence of syphilis.

Methods

In June 2019 we conducted a cross-sectional, population-based study of adults >18 years of age. Two-stage syphilis testing was completed with Treponema pallidum hemagglutination (TPHA) rapid immunochromatographic testing, confirmed by rapid plasma reagin (RPR) in those with positive TPHA (syphilis seroprevalence). We calculate inverse probability of treatment (IPT) weights using logistic regression to estimate the population prevalence of positive TPHA. We included covariates with a univariable α = .10 in multivariable logistic regression models, stratified by sex, to estimate correlates of syphilis seroprevalence.

Results

Among 749 participants who participated in this population-based study, 724 (97%) completed syphilis screening. The median age was 42.9 years (standard deviation, 15.6 years) and 456 of 724 (63%) were women. Based on the IPT-weighted adjusted model, the syphilis population seroprevalence was 10.6% (95% confidence interval, 8.4%-13.4%). Among the 62 of 79 (78%) participants who completed RPR testing, all had titers ≤1:4. Syphilis seroprevalence was associated with less education, human immunodeficiency virus (HIV) infection, and ≥2 sexual partners in the prior month among women and with HIV among men.

Conclusions

We describe a high prevalence of current or former syphilis (10.6%) in a population-based study in rural Uganda. Syphilis screening and surveillance programs in this region require expansion to capture populations not routinely in care.

Expanding Horizons in Syphilis Treatment: Challenges, Advances, and Opportunities for Alternative Antibiotics

PURPOSE OF REVIEW

This review examines the growing need for alternative technologies to address the resurgence of syphilis, particularly its congenital and late-stage manifestations. It explores current treatment paradigms, highlights the limitations of penicillin, and evaluates emerging evidence on new therapies and diagnostics to inform future strategies.

RECENT FINDINGS

Recent breakthroughs in Treponema pallidum culture techniques have enabled antibiotic susceptibility testing, expanding knowledge on both established and emerging treatment options. Alternatives like ceftriaxone, doxycycline, cefixime, and dalbavancin show promise, with other candidates in trials, though evidence is limited beyond early-stage syphilis. Shortened penicillin regimens also challenge historical assumptions about treatment duration. Advanced molecular diagnostics may complement currently limited serologic monitoring to improve evaluations in healthcare and research. While penicillin remains effective, its limitations necessitate alternatives. Emerging antibiotics and improved diagnostics offer opportunities to simplify treatment and enhance care. Future robust trials should validate new treatments, refine dosing strategies, and integrate innovative diagnostics, particularly including underserved and vulnerable populations.

The serological dilemma: rethinking syphilis treatment evaluation

INTRODUCTION

Nontreponemal tests (NTT) constitute a significant method in syphilis detection, playing a crucial role in screening, diagnosing, and monitoring disease activity. However, recent discussions have cast doubt on the traditional belief that NTT are suitable for evaluating treatment efficacy, as inconsistencies between NTT results and syphilis treatment outcomes have been observed with some frequency.

AREAS COVERED

We have delineated the current status of serological methods for evaluating the therapeutic efficacy of syphilis treatment and summarized and discussed the exploration of biomarkers for syphilis therapy. A literature search was conducted in PubMed over all available dates for relevant published papers and conference abstracts with the search terms, 'Syphilis,' 'Treponema pallidum,' 'Serofast,' 'Treatment efficacy evaluation,' 'Nontreponemal,' 'RPR,' 'TRUST,' 'VDRL,' 'Neurosyphilis,' 'Pregnant syphilis,' 'Congenital syphilis,' 'Reinfection,' 'Cardiolipin,' 'Serology.'

EXPERT OPINION

Currently, the assessment of treatment efficacy through serological methods is limited to the use of the NTT. Therefore, it is imperative for clinicians to gain a deeper understanding of the significance and limitations of the NTT, as well as a focus on exploring more potential indicators for evaluating treatment efficacy.

Treponema pallidum protein Tp0136 promotes angiogenesis to facilitate the dissemination of Treponema pallidum

The incompletely eliminated Treponema pallidum (T. pallidum) during primary syphilis chancre infection can result in the progression of secondary, tertiary, or latent syphilis in individuals, suggesting that T. pallidum has successfully evaded the immune response and spread to distant sites. The mechanism underlying the dissemination of T. pallidum is unclear. Here, a syphilitic rabbit model dorsal-injected with recombinant Tp0136 protein or Tp0136 antibody subcutaneously was used to demonstrate the role of Tp0136 protein in promoting the dissemination of T. pallidum to the testis and angiogenesis in vivo; vascular endothelial cell line HMEC-1 was employed to display that Tp0136 protein enhances the angiogenesis. Furthermore, the three-dimensional microfluidic angiogenesis system showed that the angiogenesis would heighten vascular permeability. Then transcriptome sequencing analysis, in conjunction with cell-level validation, elucidated the critical role of the PI3K-AKT signaling pathway in the promotion of angiogenesis by Tp0136 protein, resulting in heightened permeability. These findings elucidate the strategy employed by T. pallidum in evading immune clearance.

Platelet-derived major histocompatibility complex class I coating on Treponema pallidum attenuates natural killer cell lethality

The evasive tactics of Treponema pallidum pose a major challenge in combating and eradicating syphilis. Natural killer (NK) cells mediate important effector functions in the control of pathogenic infection, preferentially eliminating targets with low or no expression of major histocompatibility complex (MHC) class I. To clarify T. pallidum's mechanisms in evading NK-mediated immunosurveillance, experiments were performed to explore the cross-talk relations among T. pallidum, NK cells, and platelets. T. pallidum adhered to, activated, and promoted particle secretion of platelets. After preincubation with T. pallidum, platelets expressed and secreted high levels of MHC class I, subsequently transferring them to the surface of T. pallidum, potentially inducing an immune phenotype characterized by the "pseudo-expression" of MHC class I on the surface of T. pallidum (hereafter referred to a "pseudo-expression" of MHC class I). The polA mRNA assay showed that platelet-preincubated T. pallidum group exhibited a significantly higher copy number of polA transcript than the T. pallidum group. The survival rate of T. pallidum mirrored that of polA mRNA, indicating that preincubation of T. pallidum with platelets attenuated NK cell lethality. Platelets pseudo-expressed the MHC class I ligand on the T. pallidum surface, facilitating binding to killer cell immunoglobulin-like receptors with two immunoglobulin domains and long cytoplasmic tail 3 (KIR2DL3) on NK cells and initiating dephosphorylation of Vav1 and phosphorylation of Crk, ultimately attenuating NK cell lethality. Our findings elucidate the mechanism by which platelets transfer MHC class I to the T. pallidum surface to evade NK cell immune clearance.

Evaluating the clinical utility of semi-quantitative luciferase immunosorbent assay using Treponema pallidum antigens in syphilis diagnosis and treatment monitoring

To assess the clinical applicability of a semi-quantitative luciferase immunosorbent assay (LISA) for detecting antibodies against Treponema pallidum antigens TP0171 (TP15), TP0435 (TP17), and TP0574 (TP47) in diagnosing and monitoring syphilis. LISA for detection of anti-TP15, TP17, and TP47 antibodies were developed and evaluated for syphilis diagnosis using 261 serum samples (161 syphilis, 100 non-syphilis). Ninety serial serum samples from 6 syphilis rabbit models (3 treated, 3 untreated) and 110 paired serum samples from 55 syphilis patients were used to assess treatment effects by utilizing TRUST as a reference. Compared to TPPA, LISA-TP15, LISA-TP17, and LISA-TP47 showed a sensitivity of 91.9%, 96.9%, and 98.8%, specificity of 99%, 99%, and 98%, and AUC of 0.971, 0.992, and 0.995, respectively, in diagnosing syphilis. Strong correlations (rs = 0.89-0.93) with TPPA were observed. In serial serum samples from rabbit models, significant differences in the relative light unit (RLU) were observed between the treatment and control group for LISA-TP17 (days 31-51) and LISA-TP47 (day 41). In paired serum samples from syphilis patients, TRUST titres and the RLU of LISA-TP15, LISA-TP17, and LISA-TP47 decreased post-treatment (P < .001). When TRUST titres decreased by 0, 2, 4, or ≥8-folds, the RLU decreased by 17.53%, 31.34%, 48.62%, and 72.79% for LISA-TP15; 8.84%, 17.00%, 28.37%, and 50.57% for LISA-TP17; 22.25%, 29.79%, 51.75%, and 70.28% for LISA-TP47, respectively. Semi-quantitative LISA performs well for syphilis diagnosis while LISA-TP17 is more effective for monitoring syphilis treatment in rabbit models and clinical patients.

Development of toluidine red particle agglutination-based turbidimetric immunoassay for anticardiolipin antibody detection in syphilis

INTRODUCTION

Serological tests of non-treponemal and treponemal types are the most frequently used for syphilis diagnosis. Nontreponemal tests are used to monitor disease activity. Toluidine red unheated serum test (TRUST), as one of nontreponemal tests, is generally applicable to hospitals at different levels. However, accurate judgment of TRUST results is inseparable from an experienced and accurate operator. To reduce current shortcomings of manual TRUST method, we attempted to convert the manual TRUST test into automatic TRUST test, that is, to determine the degree of aggregation of toluidine red particles by detecting the absorbance value of serum after reaction with toluidine red particles.

METHODS

50 μL of serum sample and 80 μL toluidine red particles were added to 96-well plate. Then, the 96-well plate was placed on a microplate reader at medium grade for 8 min to mix. Then, plasma reagin reacted with toluidine red particles and promoted the aggregation of toluidine red particles to form a large clot, which eventually caused a decrease in the absorbance at 540 nm.

RESULTS

The results showed that the specificity of the automatic TRUST test was 100%, the sensitivity was 87%. And this method showed 93.5% correlation with manual TRUST test. The developed method is simple and involves less subjectivity in reading results, opening new avenues for syphilis diagnostic testing.

CONCLUSION

Turbidimetric immunoassay can avoid the shortcomings of subjective interpretation, time-consuming and manual operation of manual TRUST method, and is more suitable for large-scale screening in health examination.

A 4-fold or greater decrease in TPPA titers may indicate effective BPG treatment in primary syphilis

BACKGROUND

In the majority of clinical environments, the treponema pallidum particle agglutination (TPPA) test is known for its higher specificity compared to the rapid plasma reagin (RPR) test and is commonly employed for the diagnosis of syphilis, but their use for serological monitoring after syphilis therapy is controversial.

OBJECTIVES

We aim to evaluate whether the TPPA titers is suitable for monitoring syphilis treatment efficacy.

METHODS

At first, 232 patients with primary syphilis were recruited. Serological testing was performed at baseline (initial visit) and at 6 months (±1 month) after benzathine penicillin G (BPG) treatment. Second, New Zealand white male rabbits were infected with Treponema pallidum (T. pallidum) to evaluate the changes in TPPA titers after BPG therapy. Finally, we compared the TPPA titers in the culture supernatant of rabbit splenocytes stimulated with T. pallidum with or without BPG.

RESULTS

After 6 months of treatment, 150 (64.7%) of 232 primary syphilis patients achieved serological cure, and 82 (35.3%) had adverse outcomes. Among 110 patients with TPPA titers decreased by more than fourfold, 109 of them were serological cure patients (≥4-fold decrease in RPR titers) (P ＜ 0.0001). In the rabbit model of syphilis, the TPPA titers was significantly decreased in the treatment subgroup (P = 0.016) and remained constant (±2-fold) or increased (≥4-fold) in the nontreatment subgroup. In addition, T. pallidum resulted in a positive TPPA titers in the culture supernatant of splenocytes (median titers was 1: 80), while BPG could directly reduce the TPPA titers in the culture supernatant (median titers was 1: 40) (P = 0.032).

CONCLUSIONS

A 4-fold or greater decrease in TPPA titers may indicate effective treatment in primary syphilis. Combining TPPA titers with RPR titers results may potentially aid in the early diagnosis of syphilis.

Changes in rapid plasma reagin titers in patients with syphilis before and after treatment: A retrospective cohort study in an HIV/AIDS referral hospital in Tokyo

INTRODUCTION

Although the rapid plasma reagin (RPR) test is used to determine treatment efficacy for syphilis, animal studies show that it decreases gradually after an initial increase even without treatment. Pre-treatment changes in RPR titer in humans and its relationship with post-treatment changes in RPR titer are not well known.

METHODS

We retrospectively analyzed the clinical records of syphilitic patients who underwent automated RPR (Mediace) testing twice before treatment (i.e., at diagnosis and treatment initiation) within 1-3 months at an HIV/AIDS referral hospital in Japan between 2006 and 2018. The RPR values were expressed as the ratio to the value at treatment initiation. The mean monthly relative change in the RPR after treatment was calculated on the log2 scale for each patient and analyzed by multivariable linear regression.

RESULTS

Sixty-eight patients were identified. The median age was 45 (interquartile range [IQR], 38-50), 98.5% (67/68) were men, and 97.1% (66/68) had HIV. The median RPR titer ratio at treatment initiation/diagnosis was 0.87 (IQR, 0.48-1.30). The RPR titer decreased more than twofold in 26.5% (18/68) and more than fourfold in 10.3% (7/68) before treatment. In the multivariable analysis, higher age (predicted monthly RPR relative change on the log2 scale 0.23/10 years [95% confidence interval [CI], 0.090-0.37]), history of syphilis (0.36 [95% CI, 0.07-0.65]), and a lower ratio of RPR at treatment initiation/diagnosis (-0.52/every 10-fold increase [95% CI, -0.81 to -0.22]) were associated with a slower RPR decrease after treatment.

CONCLUSIONS

In a mostly HIV patient population, RPR titer can show more than four-fold spontaneous increase or decrease within 1-3 months. Pre-treatment spontaneous decrease of RPR titer was associated with a slower decrease in post-treatment RPR titer.

Longitudinal Variations in the tprK Gene of Treponema pallidum in an Amoy Strain-Infected Rabbit Model

Heterogeneous tprK sequences have been hypothesized to be an important factor for persistent infection of Treponema pallidum subsp. pallidum (T. pallidum) in humans. Previous research has only explored tprK diversity using a rabbit model infected with almost clonal isolates, which is inconsistent with the fact that infected human isolates contain multiple heterogeneous tprK sequences. Here, we used the T. pallidum Amoy strain with heterogeneous tprK sequences to establish a rabbit infection model and explore longitudinal variations in the tprK gene under normal infection, immunosuppression treatment, and benzathine penicillin G (BPG) treatment using next-generation sequencing. The diversity of the tprK gene was high in all three groups but was highest in the control group and lowest in the BPG group. Interestingly, the overall diversity of tprK in all three groups decreased during infection, exhibiting a "more to less" trend, indicating that survival selection may be an important factor affecting tprK variation in the later infection stage. BPG treatment appeared to reduce the diversity of tprK but increased the frequency of predominant sequence changes, which might facilitate the escape of T. pallidum from the host immune clearance. Furthermore, the original predominant V region sequence did not disappear with disease progression but retained a relatively high proportion within the population, suggesting a new direction for tprK-related vaccine research. This study provides insights into longitudinal variations within the highly heterogeneous tprK gene sequences of T. pallidum and will contribute to further exploration of the pathogenesis of syphilis. IMPORTANCE The tprK variations are an important factor in persistent T. pallidum infection. A nearly clonal isolate has been used previously to investigate the mechanism of tprK gene variations; however, clinical T. pallidum isolates in infected humans exhibit multiple heterogeneous tprK sequences. Here, we use next-generation sequencing to explore longitudinal variations in the tprK gene under normal infection and immunosuppression and benzathine penicillin G treatment in a rabbit model infected with the Amoy strain with heterogeneous tprK sequences. The overall diversity of tprK in all three groups was high and decreased during infection, exhibiting a "more to less" trend. Benzathine penicillin G treatment reduced the diversity of tprK but increased the frequency of predominant sequence changes. Moreover, the original predominant V region sequence did not disappear as the disease progressed but remained at a relatively high proportion within the population. The research results give us a new understanding about tprK variation.

A Promising Tool in Serological Diagnosis: Current Research Progress of Antigenic Epitopes in Infectious Diseases

Infectious diseases, caused by various pathogens in the clinic, threaten the safety of human life, are harmful to physical and mental health, and also increase economic burdens on society. Infections are a complex mechanism of interaction between pathogenic microorganisms and their host. Identification of the causative agent of the infection is vital for the diagnosis and treatment of diseases. Etiological laboratory diagnostic tests are therefore essential to identify pathogens. However, due to its rapidity and automation, the serological diagnostic test is among the methods of great significance for the diagnosis of infections with the basis of detecting antigens or antibodies in body fluids clinically. Epitopes, as a special chemical group that determines the specificity of antigens and the basic unit of inducing immune responses, play an important role in the study of immune responses. Identifying the epitopes of a pathogen may contribute to the development of a vaccine to prevent disease, the diagnosis of the corresponding disease, and the determination of different stages of the disease. Moreover, both the preparation of neutralizing antibodies based on useful epitopes and the assembly of several associated epitopes can be used in the treatment of disease. Epitopes can be divided into B cell epitopes and T cell epitopes; B cell epitopes stimulate the body to produce antibodies and are therefore commonly used as targets for the design of serological diagnostic experiments. Meanwhile, epitopes can fall into two possible categories: linear and conformational. This article reviews the role of B cell epitopes in the clinical diagnosis of infectious diseases.

Point-of-Care Diagnostics for Diagnosis of Active Syphilis Infection: Needs, Challenges and the Way Forward

Syphilis, a curable sexually transmitted infection, has re-emerged as a global public health threat with an estimated 5.6 million new cases every year. Pregnant women and men who have sex with men are key target populations for syphilis control and prevention programs. Frequent syphilis testing for timely and accurate diagnosis of active infections for appropriate clinical management is a key strategy to effectively prevent disease transmission. However, there are persistent challenges in the diagnostic landscape and service delivery/testing models that hinder global syphilis control efforts. In this commentary, we summarise the current trends and challenges in diagnosis of active syphilis infection and identify the data gaps and key areas for research and development of novel point-of-care diagnostics which could help to overcome the present technological, individual and structural barriers in access to syphilis testing. We present expert opinion on future research which will be required to accelerate the validation and implementation of new point-of-care diagnostics in real-world settings.

The whole process of macrophage-Treponema pallidum interactions: Opsonic phagocytosis, nonopsonic phagocytosis and active invasion

Despite the acknowledged central role of opsonophagocytosis in the process of syphilis, the interaction between Treponema pallidum and human macrophages during nonopsonophagocytosis and active invasion remains controversial. To investigate whether nonopsonic phagocytosis and active invasion, similar to opsonic phagocytosis, also participate in the process of macrophage-T. pallidum interactions, monocyte-derived macrophages were used to study the interactions of T. pallidum and macrophages in the presence of nonsyphytic or syphilitic serum and in the absence of serum in vitro using indirect immunofluorescence and flow cytometry to quantitate treponeme-macrophage interactions. The results showed that macrophages phagocytose T. pallidum under both nonopsonizing conditions (no serum or normal human serum (NHS)) and in the presence of opsonizing serum (secondary syphilitic serum (SSS)) in a time-dependent manner. The percentages of spirochete-positive macrophages in the SSS group were higher than those in the NHS and no-serum groups. Blocking FcγR or inactivating complement caused a significant decrease in the percentage of spirochete-positive macrophages in the SSS group but did not cause a decrease in the percentages of spirochete-positive macrophages in the NHS and no-serum groups. In addition, after inhibiting macrophage phagocytosis, approximately 30% of macrophages internalized spirochetes, verifying that T. pallidum actively penetrated macrophages rather than was ingested by them. This study provides evidence that opsonic phagocytosis, nonopsonic phagocytosis and active invasion are all active during T. pallidum-macrophage interactions and reveals a process of treponeme-macrophage interactions in T. pallidum pathogenesis.

Laboratory Diagnostic Tools for Syphilis: Current Status and Future Prospects

With the increasing number of patients infected with syphilis in the past 20 years, early diagnosis and early treatment are essential to decline syphilis prevalence. Owing to its diverse manifestations, which may occur in other infections, the disease often makes clinicians confused. Therefore, a sensitive method for detecting T. pallidum is fundamental for the prompt diagnosis of syphilis. Morphological observation, immunohistochemical assay, rabbit infectivity test, serologic tests, and nucleic acid amplification assays have been applied to the diagnosis of syphilis. Morphological observation, including dark-field microscopy, silver-staining, and direct fluorescent antibody staining for T. pallidum, can be used as a direct detection method for chancre specimens in primary syphilis. Immunohistochemistry is a highly sensitive and specific assay, especially in the lesion biopsies from secondary syphilis. Rabbit infectivity test is considered as a sensitive and reliable method for detecting T. pallidum in clinical samples and used as a historical standard for the diagnosis of syphilis. Serologic tests for syphilis are widely adopted using non-treponemal or treponemal tests by either the traditional or reverse algorithm and remain the gold standard in the diagnosis of syphilis patients. In addition, nucleic acid amplification assay is capable of detecting T. pallidum DNA in the samples from patients with syphilis. Notably, PCR is probably a promising method but remains to be further improved. All of the methods mentioned above play important roles in various stages of syphilis. This review aims to provide a summary of the performance characteristics of detection methods for syphilis.

Successful isolation of Treponema pallidum strains from patients' cryopreserved ulcer exudate using the rabbit model

Clinical isolates of Treponema pallidum subspecies pallidum (T. pallidum) would facilitate study of prevalent strains. We describe the first successful rabbit propagation of T. pallidum from cryopreserved ulcer specimens. Fresh ulcer exudates were collected and cryopreserved with consent from syphilis-diagnosed patients (N = 8). Each of eight age-matched adult male rabbits were later inoculated with a thawed specimen, with two rabbits receiving 1.3 ml intratesticularly (IT), and six receiving 0.6 ml intravenously (IV) and IT. Monitoring of serology, blood PCR and orchitis showed that T. pallidum grew in 2/8 rabbits that were inoculated IV and IT with either a penile primary lesion specimen (CDC-SF003) or a perianal secondary lesion specimen (CDC-SF007). Rabbit CDC-SF003 was seroreactive by T. pallidum Particle Agglutination (TP-PA) and Rapid Plasma Reagin (RPR) testing, PCR+, and showed orchitis by week 6. Euthanasia was performed in week 7, with treponemal growth in the testes confirmed and quantified by qPCR and darkfield microscopy (DF). Serial passage of the extract in a second age-matched rabbit also yielded treponemes. Similarly, rabbit CDC-SF007 showed negligible orchitis, but was seroreactive and PCR+ by week 4 and euthanized in week 6 to yield T. pallidum, which was further propagated by second passage. Using the 4-component molecular typing system for syphilis, 3 propagated strains (CDC-SF003, CDC-SF007, CDC-SF008) were typed as 14d9f, 14d9g, and 14d10c, respectively. All 3 isolates including strain CDC-SF011, which was not successfully propagated, had the A2058G mutation associated with azithromycin resistance. Our results show that immediate cryopreservation of syphilitic ulcer exudate can maintain T. pallidum viability for rabbit propagation.