Lower Levels of Cervicovaginal Tryptophan Are Associated With Natural Clearance of Chlamydia in Women

A Narrative Review on Spontaneous Clearance of Urogenital Chlamydia trachomatis: Host, Microbiome, and Pathogen-Related Factors

ABSTRACT

Chlamydia trachomatis (CT) is the most commonly reported sexually transmitted infection in the United States. Untreated urogenital infection in women can result in adverse sequelae such as pelvic inflammatory disease and infertility. Despite national screening and treatment guidelines, rates continue to rise; because most infections are asymptomatic, the actual prevalence of CT infection is likely significantly higher than reported. Spontaneous clearance of CT in women (in the absence of antibiotic treatment) has been described in multiple epidemiologic studies. Given the serious consequences and high prevalence of CT infection, there is growing interest in understanding this phenomenon and factors that may promote CT clearance in women. Spontaneous CT clearance is likely the result of complex interactions between CT, the host immune system, and the vaginal microbiota (i.e., the communities of bacteria inhabiting the vagina), which has been implicated in CT acquisition. Herein, we briefly review current literature regarding the role of each of these factors in spontaneous CT clearance, identify knowledge gaps, and discuss future directions and possible implications for the development of novel interventions that may protect against CT infection, facilitate clearance, and prevent reproductive sequelae.

Natural Clearance of Chlamydia trachomatis Infection Is Associated With Distinct Differences in Cervicovaginal Metabolites

BACKGROUND

Natural clearance of Chlamydia trachomatis in women occurs in the interval between screening and treatment. In vitro, interferon-γ (IFN-γ)-mediated tryptophan depletion results in C. trachomatis clearance, but whether this mechanism occurs in vivo remains unclear. We previously found that women who naturally cleared C. trachomatis had lower cervicovaginal levels of tryptophan and IFN-γ compared to women with persisting infection, suggesting IFN-γ-independent pathways may promote C. trachomatis clearance.

METHODS

Cervicovaginal lavages from 34 women who did (n = 17) or did not (n = 17) naturally clear C. trachomatis were subjected to untargeted high-performance liquid chromatography mass-spectrometry to identify metabolites and metabolic pathways associated with natural clearance.

RESULTS

In total, 375 positively charged metabolites and 149 negatively charged metabolites were annotated. Compared to women with persisting infection, C. trachomatis natural clearance was associated with increased levels of oligosaccharides trehalose, sucrose, melezitose, and maltotriose, and lower levels of indoline and various amino acids. Metabolites were associated with valine, leucine, and isoleucine biosynthesis pathways.

CONCLUSIONS

The cervicovaginal metabolome in women who did or did not naturally clear C. trachomatis is distinct. In women who cleared C. trachomatis, depletion of various amino acids, especially valine, leucine, and isoleucine, suggests that amino acids other than tryptophan impact C. trachomatis survival in vivo.

Effects of azithromycin and doxycycline on the vaginal microbiota of women with urogenital Chlamydia trachomatis infection: a substudy of the Chlazidoxy randomized controlled trial

OBJECTIVES

Dysbiotic bacterial communities within the vagina are associated with Chlamydia trachomatis infection. We compared the effect of treatment with azithromycin and doxycycline on the vaginal microbiota in a cohort of women with a urogenital C. trachomatis infection randomly assigned to one of these treatments (Chlazidoxy trial).

METHODS

We analysed vaginal samples from 284 women (135 in the azithromycin group and 149 in the doxycycline group) collected at baseline and 6 weeks after treatment initiation. The vaginal microbiota was characterized using 16S rRNA gene sequencing and classified into community state types (CSTs).

RESULTS

At baseline, 75% (212/284) of the women had a high-risk microbiota (CST-III or CST-IV). A cross-sectional comparison 6 weeks after treatment showed that 15 phylotypes were differentially abundant, but this difference was not reflected at the CST (p=0.772) or diversity level (p=0.339). Between baseline and the 6-week visit, α-diversity (p=0.140) and transition probabilities between CSTs were not significantly different between the groups, and no phylotype was differentially abundant.

CONCLUSION

In women with urogenital C. trachomatis infection, the vaginal microbiota does not seem to be affected by azithromycin or doxycycline 6 weeks after treatment. Because the vaginal microbiota remains susceptible to C. trachomatis infection (with CST-III or CST-IV) following antibiotic treatment, women remain at risk of reinfection, which could originate from unprotected sexual intercourse or untreated anorectal C. trachomatis infection. This last consideration advocates for the use of doxycycline instead of azithromycin because of its higher anorectal microbiological cure rate.

The role of tryptophan in Chlamydia trachomatis persistence

Chlamydia trachomatis (C. trachomatis) is the most common etiological agent of bacterial sexually transmitted infections (STIs) and a worldwide public health issue. The natural course with C. trachomatis infection varies widely between individuals. Some infections clear spontaneously, others can last for several months or some individuals can become reinfected, leading to severe pathological damage. Importantly, the underlying mechanisms of C. trachomatis infection are not fully understood. C. trachomatis has the ability to adapt to immune response and persist within host epithelial cells. Indoleamine-2,3-dioxygenase (IDO) induced by interferon-gamma (IFN-γ) degrades the intracellular tryptophan pool, to which C. trachomatis can respond by converting to a non-replicating but viable state. C. trachomatis expresses and encodes for the tryptophan synthase (TS) genes (trpA and trpB) and tryptophan repressor gene (trpR). Multiple genes interact to regulate tryptophan synthesis from exogenous indole, and persistent C. trachomatis can recover its infectivity by converting indole into tryptophan. In this review, we discuss the characteristics of chlamydial infections, biosynthesis and regulation of tryptophan, the relationship between tryptophan and C. trachomatis, and finally, the links between the tryptophan/IFN-γ axis and C. trachomatis persistence.

Association of Bacterial Vaginosis With Higher Vaginal Indole Levels

ABSTRACT

We adapted a simple hydroxylamine-based indole assay to detect indole from stored vaginal swab specimens from women with and without bacterial vaginosis (BV). Women with BV had significantly higher vaginal indole levels compared with women without BV (6451.5 vs 5632.4 μM; P = 0.01), suggesting that indole-producing bacteria are a component of BV.

Problems with Understanding Chlamydia trachomatis Immunology

The pay off for understanding Chlamydia. trachomatis (CT) immunology is the development of a vaccine. Two lines of research have contributed to our current understanding of CT immunology. The first is the Grayston model of type specific immunity and genus specific pathology which was elaborated by Caldwell and Morrison as the major outer membrane protein (MOMP) and heat shock protein 60 (HSP60) paradigm. The second is the murine model of C. muridarum (CM) infection which established the essential role of major histocompatibility complex class II and CD4 T cells in immunity. However neither approach has yielded a vaccine. I review these two lines of research and conclude with six problem areas in human CT immunology whose resolution may result in a vaccine.(122 words)

Differences in the Genital Microbiota in Women Who Naturally Clear Chlamydia trachomatis Infection Compared to Women Who Do Not Clear; A Pilot Study

In vitro studies indicate IFNγ is central to Chlamydia trachomatis (Ct) eradication, but its function may be compromised by anaerobes typically associated with bacterial vaginosis (BV), a frequent co-morbidity in women with Ct. Here we investigated the associations between natural clearance of cervical Ct infection, the vaginal microbiome, and the requirements for IFNγ by evaluating the vaginal microbial and cytokine composition of Ct treatment visit samples from women who cleared Ct infection in the interim between their Ct screening and Ct treatment visit. The pilot cohort was young, predominantly African American, and characterized by a high rate of BV that was treated with metronidazole at the Ct screening visit. The rate of natural Ct clearance was 23.6% by the Ct treatment visit (median 9 days). 16S rRNA gene sequencing revealed that metronidazole-treated women who had a Lactobacillus spp.-dominant vaginal microbiota (CST 2 or 3) at the Ct treatment visit, were more prevalent in the Ct clearing population than the non-clearing population (86% v. 50%). L. iners (CST2) was the major Lactobacillus spp. present in Ct clearers, and 33% still remained anaerobe-dominant (CST1). Vaginal IFNγ levels were not significantly different in Ct clearers and non-clearers and were several logs lower than that required for killing Ct in vitro. An expanded panel of IFNγ-induced and proinflammatory cytokines and chemokines also did not reveal differences between Ct clearers and non-clearers, but, rather, suggested signatures better associated with specific CSTs. Taken together, these findings suggest that BV-associated bacteria may impede Ct clearance, but a Lactobacillus spp.-dominant microbiome is not an absolute requirement to clear. Further, IFNγ may be required at lower concentrations than in vitro modeling indicates, suggesting it may act together with other factors in vivo. Data also revealed that the vaginal bacteria-driven inflammation add complexity to the genital cytokine milieu, but changes in this microbiota may contribute to, or provide cytokine biomarkers, for a shift to Ct clearance.

The association of Chlamydia trachomatis and Mycoplasma genitalium infection with the vaginal metabolome

Chlamydia trachomatis (CT) and Mycoplasma genitalium (MG) are two highly prevalent bacterial sexually transmitted infections (STIs) with a significant rate of co-infection in some populations. Vaginal metabolites are influenced by resident vaginal microbiota, affect susceptibility to sexually transmitted infections (STIs), and may impact local inflammation and patient symptoms. Examining the vaginal metabolome in the context of CT mono (CT+) and CT/MG co-infection (CT+/MG+) may identify biomarkers for infection or provide new insights into disease etiology and pathogenesis. Yet, the vaginal metabolome in the setting of CT infection is understudied and the composition of the vaginal metabolome in CT/MG co-infected women is unknown. Therefore, in this analysis, we used an untargeted metabolomic approach combined with 16S rRNA gene amplicon sequencing to characterize the vaginal microbiota and metabolomes of CT+, CT+/MG+, and uninfected women. We found that CT+ and CT+/MG+ women had distinct vaginal metabolomic profiles as compared to uninfected women both before and after adjustment for the vaginal microbiota. This study provides important foundational data documenting differences in the vaginal metabolome between CT+, CT+/MG+ and uninfected women. These data may guide future mechanistic studies that seek to provide insight into the pathogenesis of CT and CT/MG infections.

Ammonia generation by tryptophan synthase drives a key genetic difference between genital and ocular Chlamydia trachomatis isolates

A striking difference between genital and ocular clinical isolates of Chlamydia trachomatis is that only the former express a functional tryptophan synthase and therefore can synthesize tryptophan by indole salvage. Ocular isolates uniformly cannot use indole due to inactivating mutations within tryptophan synthase, indicating a selection against maintaining this enzyme in the ocular environment. Here, we demonstrate that this selection occurs in two steps. First, specific indole derivatives, produced by the human gut microbiome and present in serum, rapidly induce expression of C. trachomatis tryptophan synthase, even under conditions of tryptophan sufficiency. We demonstrate that these indole derivatives function by acting as de-repressors of C. trachomatis TrpR. Second, trp operon de-repression is profoundly deleterious when infected cells are in an indole-deficient environment, because in the absence of indole, tryptophan synthase deaminates serine to pyruvate and ammonia. We have used biochemical and genetic approaches to demonstrate that expression of wild-type tryptophan synthase is required for the bactericidal production of ammonia. Pertinently, although these indole derivatives de-repress the trpRBA operon of C. trachomatis strains with trpA or trpB mutations, no ammonia is produced, and no deleterious effects are observed. Our studies demonstrate that tryptophan synthase can catalyze the ammonia-generating β-elimination reaction within any live bacterium. Our results also likely explain previous observations demonstrating that the same indole derivatives inhibit the growth of other pathogenic bacterial species, and why high serum levels of these indole derivatives are favorable for the prognosis of diseased conditions associated with bacterial dysbiosis.

The genital tract and rectal microbiomes: their role in HIV susceptibility and prevention in women

INTRODUCTION

Young women in sub-Saharan Africa are disproportionately affected by HIV, accounting for 25% of all new infections in 2017. Several behavioural and biological factors are known to impact a young woman's vulnerability for acquiring HIV. One key, but lesser understood, biological factor impacting vulnerability is the vaginal microbiome. This review describes the vaginal microbiome and examines its alterations, its influence on HIV acquisition as well as the efficacy of HIV prevention technologies, the role of the rectal microbiome in HIV acquisition, advances in technologies to study the microbiome and some future research directions.

DISCUSSION

Although the composition of each woman's vaginal microbiome is unique, a microbiome dominated by Lactobacillus species is generally associated with a "healthy" vagina. Disturbances in the vaginal microbiota, characterized by a shift from a low-diversity, Lactobacillus-dominant state to a high-diversity non-Lactobacillus-dominant state, have been shown to be associated with a range of adverse reproductive health outcomes, including increasing the risk of genital inflammation and HIV acquisition. Gardnerella vaginalis and Prevotella bivia have been shown to contribute to both HIV risk and genital inflammation. In addition to impacting HIV risk, the composition of the vaginal microbiome affects the vaginal concentrations of some antiretroviral drugs, particularly those administered intravaginally, and thereby their efficacy as pre-exposure prophylaxis (PrEP) for HIV prevention. Although the role of rectal microbiota in HIV acquisition in women is less well understood, the composition of this compartment's microbiome, particularly the presence of species of bacteria from the Prevotellaceae family likely contribute to HIV acquisition. Advances in technologies have facilitated the study of the genital microbiome's structure and function. While next-generation sequencing advanced knowledge of the diversity and complexity of the vaginal microbiome, the emerging field of metaproteomics, which provides important information on vaginal bacterial community structure, diversity and function, is further shedding light on functionality of the vaginal microbiome and its relationship with bacterial vaginosis (BV), as well as antiretroviral PrEP efficacy.

CONCLUSIONS

A better understanding of the composition, structure and function of the microbiome is needed to identify opportunities to alter the vaginal microbiome and prevent BV and reduce the risk of HIV acquisition.

Spontaneous clearance of urogenital, anorectal and oropharyngeal Chlamydia trachomatis and Neisseria gonorrhoeae in women, MSM and heterosexual men visiting the STI clinic: a prospective cohort study

OBJECTIVE

Chlamydia trachomatis (CT) and Neisseria gonorrhoeae (NG) infections can clear without treatment. Despite high prevalence of anorectal infections in men who have sex with men (MSM) and women, studies on anorectal clearance are scarce. Moreover it is unknown whether bacterial load affects urogenital/anorectal CT clearance. In this prospective cohort study, CT and NG clearance is assessed at three anatomical sites of men and women.

METHODS

CT-positive and NG-positive MSM, heterosexual men and women ≥18 years of age visiting our STI clinic between 2011 and 2013 underwent a repeat test when returning for treatment (n=482). The primary outcome was clearance, defined as a positive nucleic acid amplification test (NAAT) at screening-consultation, followed by a negative NAAT at treatment-consultation. Sociodemographics, sexual risk behaviour and CT bacterial load (inhouse quantitative PCR) were tested as determinants for clearance using multivariable logistic regression for CT and Fisher's exact test for NG.

RESULTS

CT clearance was 9.1% (10/110) for urine, 6.8% (20/292) for vaginal swabs, 12.7% (8/63) for anorectal swabs (ie, 4.0% [1/25] in MSM and 18.4% [7/38] in women) and 57.1% (4/7) for oropharyngeal swabs. For NG this was 33.3% (2/6), 28.6% (2/7), 20.0% (2/10) and 27.3% (6/22), respectively. The number of days between tests (median 10, IQR 7-14) was not associated with clearance. Lower bacterial load at screening was the only predictor for CT clearance (urine mean 1.2 vs 2.6 log CT/mL, p=0.001; vaginal swabs mean 2.1 vs 5.2 log CT/mL p<0.0001; anorectal swabs mean 2.0 vs 3.7 log CT/mL, p=0.002). None of the tested determinants were associated with NG clearance.

CONCLUSIONS

This study reports the largest number of anorectal infections tested for CT and NG clearance to date. Clearance in all sample types was substantial: between 7% and 57% for CT, and between 20% and 33% for NG (notwithstanding low absolute numbers). CT clearance was associated with a lower load at screening. However, not all individuals with low bacterial CT load cleared the infection, hampering STI guideline change.

High expression of IDO1 and TGF-β1 during recurrence and post infection clearance with Chlamydia trachomatis, are independent of host IFN-γ response

Background

Chlamydia trachomatis infections in women continue to be a major public health concern due to their high prevalence and consequent reproductive morbidities. While antibiotics are usually efficient to clear the Chlamydia, repeat infections are common and may contribute to pathological outcomes. Interferon-gamma (IFN-γ)-mediated immunity has been suggested to be protective against reinfection, and represent an important anti-chlamydial agent, primarily via the induction of indoleamine-2,3 dioxygenase 1 (IDO1) enzyme. IDO1 catalyzes the degradation of tryptophan, which can eliminate C. trachomatis infection in vitro. Here, we sought to measure IDO1 expression levels and related immune markers during different C. trachomatis infection statuses (repeated vs single infection vs post antibiotic treatment), in vitro and in vivo.

Methods

In this study, we measured the expression levels of IDO1 and immune regulatory markers, transforming growth factor β1 (TGF-β1) and forkhead box P3 (FoxP3), in vaginal swab samples of C. trachomatis-infected women, with either single or repeated infection. In addition, we used an in vitro co-culture model of endometrial carcinoma cell-line and peripheral blood mononuclear cells (PBMCs) to measure the same immune markers.

Results

We found that in women with repeated C. trachomatis infections vaginal IDO1 and TGF-β1 expression levels were significantly increased. Whereas, women who cleared their infection post antibiotic treatment, had increased levels of IDO1 and TGF-β1, as well as FoxP3. Similarly, using the in vitro model, we found significant upregulation of IDO1 and TGF-β1 levels in the co-culture infected with C. trachomatis. Furthermore, we found that in PBMCs infected with C. trachomatis there was a significant upregulation in IDO1 levels, which was independent of IFN-γ. In fact, C. trachomatis infection in PBMCs failed to induce IFN-γ levels in comparison to the uninfected culture.

Conclusions

Our data provide evidence for a regulatory immune response comprised of IDO1, TGF-β1 and FoxP3 in women post antibiotic treatment. In this study, we demonstrated a significant increase in IDO1 expression levels in response to C. trachomatis infection, both in vivo and in vitro, without elevated IFN-γ levels. This study implicates IDO1 and TGF-β1 as part of the immune response to repeated C. trachomatis infections, independently of IFN-γ.

Electronic supplementary material

The online version of this article (10.1186/s12879-019-3843-4) contains supplementary material, which is available to authorized users.

Alzheimer's disease might depend on enabling pathogens which do not necessarily cross the blood-brain barrier

The development of Alzheimer's Disease (AD) might reflect, in its acquired aspects, a cooperative pathogenesis whereby infectious enablers which do not necessarily cross the blood-brain barrier augment the invasive properties of a less virulent organism, thus enabling it to infect the brain. An example interaction is described which involves Chlamydia species, Human papillomavirus (HPV), microbiota, and yeast, where yeast is a pathogen of low virulence which crosses the blood-brain barrier. The cooperative pathogenesis begins at the mucosal epithelium. Infection by Chlamydia, HPV, or dysbiosis of commensal bacteria disrupts the integrity of the mucosal epithelium, thereby allowing colonizing yeast to penetrate the epithelial barrier and enter into the bloodstream. Chlamydia and enabling commensals promote insulin resistance, which provides yeast with glucose and also sets the stage for accumulation of amyloid beta protein (ABP). Meanwhile, HPV-induced and hyperglycemia-induced immunological changes enable the spread of newly invasive yeast to the brain, where the release of inflammatory cytokines in response to yeast promotes production of ABP. Chlamydia also cross reacts with Candida species, which may stimulate further brain inflammation in response to Candida and may augment production of ABP thereby The yeast's less virulent origins, coupled with immune modulation by enablers, might explain why AD as a model of infectious encephalitis is always slow and insidious rather than occasionally febrile, accompanied by seizures, or marked by signs of meningeal inflammation.