Chlamydia trachomatis OmcB protein is a surface-exposed glycosaminoglycan-dependent adhesin

Human antibody signatures towards the Chlamydia trachomatis major outer membrane protein after natural infection and vaccination

BACKGROUND

Chlamydia trachomatis (CT) Major Outer Membrane Protein (MOMP) holds a neutralising epitope in the Variable Domain 4 (VD4), and this region's immune dominance during infection is well known. This study aimed to assess the antibody response induced after infection and compare it for specificity and functionality to the response following vaccination with the vaccine CTH522, which contains VD4's from serovars D, E, F, and G.

METHODS

We assessed the antibody epitopes in MOMP by a high density peptide array. Furthermore, the role of the VD4 epitope in neutralisation was explored by competitive inhibition experiments with a fusion protein holding the neutralising VD4 linear epitope. This was done in two independent groups: 1) MOMP seropositive individuals infected with CT (n = 10, from case-control study) and 2) CTH522/CAF®01-vaccinated females (n = 14) from the CHLM-01 clinical trial.

FINDINGS

We identified the major antigenic regions in MOMP as VD4 and the conserved region just before VD3 in individuals infected with CT. The same regions, with the addition of VD1, were identified in vaccine recipients. Overall, the VD4 peptide responses were uniform in vaccinated individuals and led to inhibition of infection in vitro in all tested samples, whereas the VD4 responses were more heterogenous in individuals infected with CT, and only 2 out of 10 samples had VD4-mediated neutralising antibody responses.

INTERPRETATION

These data provide insights into the role of antibodies against MOMP VD4 induced after infection and vaccination, and show that their functionality differs. The induction of functional VD4-specific antibodies in vaccine recipients mimics previous results from animal models.

FUNDING

This work was supported by the European Commission through contract FP7-HEALTH-2011.1.4-4-280873 (ADITEC) and Fonden til Lægevidenskabens Fremme.

Diversity and genetic characterization of Chlamydia isolated from Siamese crocodiles (Crocodylus siamensis)

Chlamydiosis, an infection caused by several Chlamydia species, has been reported in Nile, saltwater, and Siamese crocodiles. Despite its widespread reports in various countries, including Thailand, genetic information on Chlamydia species remains limited. This study presents a whole-genome-based characterization of Siamese crocodile-isolated Chlamydia. The results showed that Siamese crocodile Chlamydia contained a single circular chromosome with a size of 1.22-1.23 Mbp and a plasmid with a size of 7.7-8.0 kbp. A plasmid containing eight coding sequences (CDSs) was grouped in a β lineage. A chromosome sequence had approximately 1,018-1,031 CDSs. Chlamydial factors involving virulence were documented in terms of the presence of cytotoxins and several virulence factors in the chromosomes of Siamese crocodile Chlamydia. The analysis of antimicrobial resistance genes in the Chlamydia genome revealed that the most common resistance genes were associated with aminoglycosides, fluoroquinolones, macrolides, tetracyclines, and cephalosporins, with loose matching (identities between 21.12 % and 74.65 %). Phylogenetic analyses, encompassing the assessments of both whole proteome and nine taxonomic markers, revealed that Siamese crocodile Chlamydia was separated into three lineages (lineages I-III) with high bootstrapping statistic support. Interestingly, isolate 12-01 differed genetically from the others, suggesting that it is a new member of Chlamydia. The study findings indicate that Siamese crocodiles are susceptible hosts to Chlamydia, involving more than one species. This study is the first employing the highest number of whole-genome data on Siamese crocodile Chlamydia and provides better insights into pathogen genetics.

A Machine Learning-Based Analytic Pipeline Applied to Clinical and Serum IgG Immunoproteome Data To Predict Chlamydia trachomatis Genital Tract Ascension and Incident Infection in Women

We developed a reusable and open-source machine learning (ML) pipeline that can provide an analytical framework for rigorous biomarker discovery. We implemented the ML pipeline to determine the predictive potential of clinical and immunoproteome antibody data for outcomes associated with Chlamydia trachomatis (Ct) infection collected from 222 cis-gender females with high Ct exposure. We compared the predictive performance of 4 ML algorithms (naive Bayes, random forest, extreme gradient boosting with linear booster [xgbLinear], and k-nearest neighbors [KNN]), screened from 215 ML methods, in combination with two different feature selection strategies, Boruta and recursive feature elimination. Recursive feature elimination performed better than Boruta in this study. In prediction of Ct ascending infection, naive Bayes yielded a slightly higher median value of are under the receiver operating characteristic curve (AUROC) 0.57 (95% confidence interval [CI], 0.54 to 0.59) than other methods and provided biological interpretability. For prediction of incident infection among women uninfected at enrollment, KNN performed slightly better than other algorithms, with a median AUROC of 0.61 (95% CI, 0.49 to 0.70). In contrast, xgbLinear and random forest had higher predictive performances, with median AUROC of 0.63 (95% CI, 0.58 to 0.67) and 0.62 (95% CI, 0.58 to 0.64), respectively, for women infected at enrollment. Our findings suggest that clinical factors and serum anti-Ct protein IgGs are inadequate biomarkers for ascension or incident Ct infection. Nevertheless, our analysis highlights the utility of a pipeline that searches for biomarkers and evaluates prediction performance and interpretability. IMPORTANCE Biomarker discovery to aid early diagnosis and treatment using machine learning (ML) approaches is a rapidly developing area in host-microbe studies. However, lack of reproducibility and interpretability of ML-driven biomarker analysis hinders selection of robust biomarkers that can be applied in clinical practice. We thus developed a rigorous ML analytical framework and provide recommendations for enhancing reproducibility of biomarkers. We emphasize the importance of robustness in selection of ML methods, evaluation of performance, and interpretability of biomarkers. Our ML pipeline is reusable and open-source and can be used not only to identify host-pathogen interaction biomarkers but also in microbiome studies and ecological and environmental microbiology research.

Plasmid-mediated virulence in Chlamydia

Chlamydia trachomatis infection of ocular conjunctiva can lead to blindness, while infection of the female genital tract can lead to chronic pelvic pain, ectopic pregnancy, and/or infertility. Conjunctival and fallopian tube inflammation and the resulting disease sequelae are attributed to immune responses induced by chlamydial infection at these mucosal sites. The conserved chlamydial plasmid has been implicated in enhancing infection, via improved host cell entry and exit, and accelerating innate inflammatory responses that lead to tissue damage. The chlamydial plasmid encodes eight open reading frames, three of which have been associated with virulence: a secreted protein, Pgp3, and putative transcriptional regulators, Pgp4 and Pgp5. Although Pgp3 is an important plasmid-encoded virulence factor, recent studies suggest that chlamydial plasmid-mediated virulence extends beyond the expression of Pgp3. In this review, we discuss studies of genital, ocular, and gastrointestinal infection with C. trachomatis or C. muridarum that shed light on the role of the plasmid in disease development, and the potential for tissue and species-specific differences in plasmid-mediated pathogenesis. We also review evidence that plasmid-associated inflammation can be independent of bacterial burden. The functions of each of the plasmid-encoded proteins and potential molecular mechanisms for their role(s) in chlamydial virulence are discussed. Although the understanding of plasmid-associated virulence has expanded within the last decade, many questions related to how and to what extent the plasmid influences chlamydial infectivity and inflammation remain unknown, particularly with respect to human infections. Elucidating the answers to these questions could improve our understanding of how chlamydia augment infection and inflammation to cause disease.

Virulence Protein Pgp3 Is Insufficient To Mediate Plasmid-Dependent Infectivity of Chlamydia trachomatis

Chlamydia trachomatis is the most common cause of infectious blindness and sexually transmitted bacterial infection globally. C. trachomatis contains a conserved chlamydial plasmid with eight coding sequences. Plasmid-cured Chlamydia strains are attenuated and display reduced infectivity in cell culture and in vivo genital infection of female mice. Mutants that do not express the plasmid-encoded proteins Pgp3, a secreted protein with unknown function, or Pgp4, a putative regulator of pgp3 and other chromosomal loci, display an infectivity defect similar to plasmid-deficient strains. Our objective was to determine the combined and individual contributions of Pgp3 and Pgp4 to this phenotype. Deletion of pgp3 and pgp4 resulted in an infectivity defect detected by competition assay in cell culture and in mice. The pgp3 locus was placed under the control of an anhydrotetracycline-inducible promoter to examine the individual contributions of Pgp3 and Pgp4 to infectivity. Expression of pgp3 was induced 100- to 1,000-fold after anhydrotetracycline administration, regardless of the presence or absence of pgp4. However, secreted Pgp3 was not detected when pgp4 was deleted, confirming a role for Pgp4 in Pgp3 secretion. We discovered that expression of pgp3 or pgp4 alone was insufficient to restore normal infectivity, which required expression of both Pgp3 and Pgp4. These results suggest Pgp3 and Pgp4 are both required for infectivity during C. trachomatis infection. Future studies are required to determine the mechanism by which Pgp3 and Pgp4 influence chlamydial infectivity as well as the potential roles of Pgp4-regulated loci.

Serum antibodies to surface proteins of Chlamydia trachomatis as candidate biomarkers of disease: results from the Baltimore Chlamydia Adolescent/Young Adult Reproductive Management (CHARM) cohort

We previously observed that the nine-member family of autotransported polymorphic membrane proteins (Pmps) of Chlamydia trachomatis is variably expressed in cell culture. Additionally, C. trachomatis-infected patients display variable Pmp-specific serum antibody profiles indirectly suggesting expression of unique Pmp profiles is an adaptive response to host-specific stimuli during infection. Here, we propose that the host response to Pmps and other outer surface proteins may correlate with disease severity. This study tests this hypothesis using an ELISA that measures serum IgG antibodies specific for the nine C. trachomatis Pmp subtypes and four immunodominant antigens (MOMP, OmcB, Hsp60, ClpP) in 265 participants of the Chlamydia Adolescent/Young Adult Reproductive Management (CHARM) cohort. More C. trachomatis-infected females displayed high Pmp-specific antibody levels (cut-off Indexes) than males (35.9%-40.7% of females vs. 24.2%-30.0% of males), with statistical significance for PmpC, F and H (P < 0.05). Differences in Pmp-specific antibody profiles were not observed between C. trachomatis-infected females with a clinical diagnosis of pelvic inflammatory disease (PID) and those without. However, a statistically significant association between high levels of OmcB-specific antibody and a PID diagnosis (P< 0.05) was observed. Using antibody levels as an indirect measure of antigen expression, our results suggest that gender- and/or site-specific (cervix in females vs. urethra in males) stimuli may control pmp expression in infected patients. They also support the possible existence of immune biomarkers of chlamydial infection associated with disease and underline the need for high resolution screening in human serum.

Polymorphic Membrane Protein 17G of Chlamydia psittaci Mediated the Binding and Invasion of Bacteria to Host Cells by Interacting and Activating EGFR of the Host

Chlamydia psittaci (C. psittaci) is an obligate intracellular, gram-negative bacterium, and mainly causes systemic disease in psittacine birds, domestic poultry, and wild fowl. The pathogen is threating to human beings due to closely contacted to employees in poultry industry. The polymorphic membrane proteins (Pmps) enriched in C. psittaci includes six subtypes (A, B/C, D, E/F, G/I and H). Compared to that of the 1 pmpG gene in Chlamydia trachomatis (C. trachomatis), the diverse pmpG gene-coding proteins of C. psittaci remain elusive. In the present study, polymorphic membrane protein 17G (Pmp17G) of C. psittaci mediated adhesion to different host cells. More importantly, expression of Pmp17G in C. trachomatis upregulated infections to host cells. Afterwards, crosstalk between Pmp17G and EGFR was screened and identified by MALDI-MS and Co-IP. Subsequently, EGFR overexpression in CHO-K1 cells and EGFR knockout in HeLa 229 cells were assessed to determine whether Pmp17G directly correlated with EGFR during Chlamydial adhesion. Finally, the EGFR phosphorylation was recognized by Grb2, triggering chlamydial invasion. Based on above evidence, Pmp17G possesses adhesive property that serves as an adhesin and activate intracellular bacterial internalization by recognizing EGFR during C. psittaci infection

Chlamydia abortus OmcB protein is essential for adhesion to host cells

Chlamydia abortus (C. abortus) is one of the most important zoonotic pathogens, causing a number of serious diseases. The adhesion of C. abortus to host cells is the first and crucial step in the process of infection. Outer membrane protein 2 (OmcB) is the second most abundant outer membrane protein. It has been shown to be an important adhesin of Chlamydia trachomatis and Chlamydia pneumoniae. In the present study, the OmcB gene of C. abortus was cloned and expressed in Escherichia coli, and the recombinant OmcB protein with His-tag was used to prepare polyclonal antibodies. Infectivity inhibition assays carried out with C. abortus in the presence of recombinant OmcB showed a considerable reduction (∼50%) in infectivity. Using anti-OmcB serum in infectivity inhibition assays resulted in a 30% reduction in infectivity. Anti-OmcB serum and recombinant OmcB protein in infection inhibition assays showed that OmcB is a surface-exposed protein that functions as an adhesin. The constructed deletion variant of the OmcB motif for infection inhibition assays showed that the first XBBXBX motif of the C. abortus OmcB protein is essential for binding to host cells.

Cross-Reactive Effects of Vaccines: Heterologous Immunity between Tetanus and Chlamydia

Vaccines can have heterologous effects on the immune system, i.e., effects other than triggering an immune response against the disease targeted by the vaccine. We investigated whether monoclonal antibodies (mAbs) specific for tetanus could cross-react with Chlamydia and confer heterologous protection against chlamydial infection. The capability of two tetanus-specific mAbs, namely mAb26 and mAb51, to prevent chlamydial infection has been assessed: (i) in vitro, by performing a neutralization assay using human conjunctival epithelial (HCjE) cells infected with Chlamydia trachomatis serovar B, and (ii) in vivo, by using a guinea pig model of Chlamydiacaviae-induced inclusion conjunctivitis. The mAb26 has been superior in comparison with mAb51 in the prevention of chlamydial infection in HCjE cells. The mAb26 has conferred ≈40% inhibition of the infection, compared to less than 5% inhibition in the presence of the mAb51. In vivo, mAb26 significantly diminished ocular pathology intensity in guinea pigs infected with C. caviae compared to either the mAb51-treated or sham-treated guinea pigs. Our data provide insights that tetanus immunization generates antibodies which induce heterologous chlamydial immunity and promote protection beyond the intended target pathogen.

The occurrence and pathology of chlamydiosis in the male reproductive tract of non-human mammals: A review

Organisms belonging to the Family Chlamydiaceae are responsible for a broad range of diseases in humans, livestock, companion animals and non-domestic species. Infection of the reproductive organs can cause a range of syndromes of which sub- and infertility are the most frequently observed clinical manifestations. While the gross and histological lesions associated with the isolation of Chlamydiaceae from the non-human female reproductive tract are well documented, little attention has been given to the pathological effects of this infection in the male genital system. As such, the occurrence and importance of Chlamydia-associated disease in male non-human mammalian species is less well documented. In order to improve our understanding of the significance of chlamydiosis in domestic, laboratory and wild animals, this review provides an up-to-date summary of Chlamydia-associated male reproductive pathology, whether that infection occurs naturally or experimentally. Although most lesions in males are described as incidental and of minor significance, results of recent studies suggest that infection with Chlamydiaceae can adversely impact male fertility and/or be instrumental in disease transmission. Although in humans, bulls and mice Chlamydia infection has been associated with morphological and functional abnormalities of the spermatozoa, this review will focus on the gross and histological findings linked to the colonisation of the genital system by this pathogen. Advances in our understanding of male reproductive chlamydiosis are necessary for diagnostic and therapeutic strategies, as well as epidemiological and conservation studies.

Chlamydia: what is on the outside does matter

This review summarises major highlights on the structural biology of the chlamydial envelope. Chlamydiae are obligate intracellular bacteria, characterised by a unique biphasic developmental cycle. Depending on the stage of their lifecycle, they appear in the form of elementary or reticulate bodies. Since these particles have distinctive functions, it is not surprising that their envelope differs in lipid as well as in protein content. Vice versa, by identifying surface proteins, specific characteristics of the particles such as rigidity or immunogenicity may be deduced. Detailed information on the bacterial membranes will increase our understanding on the host-pathogen interactions chlamydiae employ to survive and grow and might lead to new strategies to battle chlamydial infections.

Pathogenic Puppetry: Manipulation of the Host Actin Cytoskeleton by Chlamydia trachomatis

The actin cytoskeleton is crucially important to maintenance of the cellular structure, cell motility, and endocytosis. Accordingly, bacterial pathogens often co-opt the actin-restructuring machinery of host cells to access or create a favorable environment for their own replication. The obligate intracellular organism Chlamydia trachomatis and related species exemplify this dynamic: by inducing actin polymerization at the site of pathogen-host attachment, Chlamydiae induce their own uptake by the typically non-phagocytic epithelium they infect. The interaction of chlamydial adhesins with host surface receptors has been implicated in this effect, as has the activity of the chlamydial effector TarP (translocated actin recruitment protein). Following invasion, C. trachomatis dynamically assembles and maintains an actin-rich cage around the pathogen’s membrane-bound replicative niche, known as the chlamydial inclusion. Through further induction of actin polymerization and modulation of the actin-crosslinking protein myosin II, C. trachomatis promotes egress from the host via extrusion of the inclusion. In this review, we present the experimental findings that can inform our understanding of actin-dependent chlamydial pathogenesis, discuss lingering questions, and identify potential avenues of future study.

Proteomic characterisation of the Chlamydia abortus outer membrane complex (COMC) using combined rapid monolithic column liquid chromatography and fast MS/MS scanning

Data are presented on the identification and partial characterisation of proteins comprising the chlamydial outer membrane complex (COMC) fraction of Chlamydia abortus (C. abortus)—the aetiological agent of ovine enzootic abortion. Inoculation with the COMC fraction is known to be highly effective in protecting sheep against experimental challenge and its constituent proteins are therefore of interest as potential vaccine candidates. Sodium N-lauroylsarcosine (sarkosyl) insoluble COMC proteins resolved by SDS-PAGE were interrogated by mass spectrometry using combined rapid monolithic column liquid chromatography and fast MS/MS scanning. Downstream database mining of processed tandem MS data revealed the presence of 67 proteins in total, including putative membrane associated proteins (n = 36), such as porins, polymorphic membrane proteins (Pmps), chaperonins and hypothetical membrane proteins, in addition to others (n = 22) that appear more likely to have originated from other subcellular compartments. Electrophoretic mobility data combined with detailed amino acid sequence information derived from secondary fragmentation spectra for 8 Pmps enabled peptides originating from protein cleavage fragments to be mapped to corresponding regions of parent precursor molecules yielding preliminary evidence in support of endogenous post-translational processing of outer membrane proteins in C. abortus. The data presented here will facilitate a deeper understanding of the pathogenesis of C. abortus infection and represent an important step towards the elucidation of the mechanisms of immunoprotection against C. abortus infection and the identification of potential target vaccine candidate antigens.

The multiple functions of the numerous Chlamydia trachomatis secreted proteins: the tip of the iceberg

Chlamydia trachomatis serovars are obligate intracellular bacterial pathogens mainly causing ocular and urogenital infections that affect millions of people worldwide and which can lead to blindness or sterility. They reside and multiply intracellularly within a membrane-bound vacuolar compartment, known as inclusion, and are characterized by a developmental cycle involving two morphologically and physiologically distinct chlamydial forms. Completion of the developmental cycle involves the secretion of > 70 C. trachomatis proteins that function in the host cell cytoplasm and nucleus, in the inclusion membrane and lumen, and in the extracellular milieu. These proteins can, for example, interfere with the host cell cytoskeleton, vesicular and non-vesicular transport, metabolism, and immune signalling. Generally, this promotes C. trachomatis invasion into, and escape from, host cells, the acquisition of nutrients by the chlamydiae, and evasion of cell-autonomous, humoral and cellular innate immunity. Here, we present an in-depth review on the current knowledge and outstanding questions about these C. trachomatis secreted proteins.

Chlamydia trachomatis and Chlamydia muridarum spectinomycin resistant vectors and a transcriptional fluorescent reporter to monitor conversion from replicative to infectious bacteria

Chlamydia trachomatis infections are the leading cause of sexually transmitted infections of bacterial origin. Lower genital tract infections are often asymptomatic, and therefore left untreated, leading to ascending infections that have long-term consequences on female reproductive health. Human pathology can be recapitulated in mice with the mouse adapted strain C. muridarum. Eight years into the post-genetic era, significant advances to expand the Chlamydia genetic toolbox have been made to facilitate the study of this important human pathogen. However, the need for additional tools remains, especially for C. muridarum. Here, we describe a new set of spectinomycin resistant E. coli-Chlamydia shuttle vectors, for C. trachomatis and C. muridarum. These versatile vectors allow for expression and localization studies of Chlamydia effectors, such as Inc proteins, and will be instrumental for mutant complementation studies. In addition, we have exploited the differential expression of specific Chlamydia genes during the developmental cycle to engineer an omcA::gfp fluorescent transcriptional reporter. This novel tool allows for monitoring RB to EB conversion at the bacterial level. Spatiotemporal tracking of GFP expression within individual inclusions revealed that RB to EB conversion initiates in bacteria located at the edge of the inclusion and correlates with the time post initiation of bacterial replication and inclusion size. Comparison between primary and secondary inclusions potentially suggests that the environment in which the inclusions develop influences the timing of conversion. Altogether, the Chlamydia genetic tools described here will benefit the field, as we continue to investigate the molecular mechanisms underlying Chlamydia-host interaction and pathogenesis.

Anti-chlamydia IgG and IgA are insufficient to prevent endometrial chlamydia infection in women, and increased anti-chlamydia IgG is associated with enhanced risk for incident infection

PROBLEM

Chlamydia infections in women can ascend to the upper genital tract, and repeated infections are common, placing women at risk for sequelae. The protective role of anti-chlamydia antibodies to surface exposed antigens in ascending and incident infection is unclear.

METHOD OF STUDY

A whole-bacterial ELISA was used to quantify chlamydia-specific IgG and IgA in serum and cervical secretions of 151 high-risk women followed longitudinally. Correlations were determined between antibody and cervical burden, and causal mediation analysis investigated the effect of antibody on ascension. We examined the relationship of antibody to incident infection using the marginal Cox model.

RESULTS

Serum and cervical anti-chlamydia IgG and cervical IgA levels correlated inversely with cervical burden. While lower burden was associated with reduced ascension, causal mediation analysis revealed that the indirect effects of antibody mediated through reductions in bacterial burden were insufficient to prevent ascension. Analysis of women uninfected at enrollment revealed that serum and cervical anti-chlamydia IgG were associated with increased risk of incident infection; hazard ratio increased 3.6-fold (95% CI, 1.3-10.3), and 22.6-fold (95% CI, 3.1-165.2) with each unit of serum and cervical IgG, respectively.

CONCLUSION

Although anti-chlamydia IgG and IgA correlated with reduced cervical chlamydia burden, they failed to prevent ascension and increased levels of anti-chlamydia IgG were associated with increased risk for incident infection.

Life inside and out: making and breaking protein disulfide bonds in Chlamydia

Disulphide bonds are widely used among all domains of life to provide structural stability to proteins and to regulate enzyme activity. Chlamydia spp. are obligate intracellular bacteria that are especially dependent on the formation and degradation of protein disulphide bonds. Members of the genus Chlamydia have a unique biphasic developmental cycle alternating between two distinct cell types; the extracellular infectious elementary body (EB) and the intracellular replicating reticulate body. The proteins in the envelope of the EB are heavily cross-linked with disulphides and this is known to be critical for this infectious phase. In this review, we provide a comprehensive summary of what is known about the redox state of chlamydial envelope proteins throughout the developmental cycle. We focus especially on the factors responsible for degradation and formation of disulphide bonds in Chlamydia and how this system compares with redox regulation in other organisms. Focussing on the unique biology of Chlamydia enables us to provide important insights into how specialized suites of disulphide bond (Dsb) proteins cater for specific bacterial environments and lifecycles.

Performance of Chlamydia trachomatis OmcB Enzyme-Linked Immunosorbent Assay in Serodiagnosis of Chlamydia trachomatis Infection in Women

Chlamydia trachomatis serological assays with improved sensitivity over commercially available assays are needed to evaluate the burden of C. trachomatis infection and the effectiveness of prevention efforts. We evaluated the performance of a C. trachomatis outer membrane complex protein B (OmcB) enzyme-linked immunosorbent assay (ELISA) in the detection of anti-C. trachomatis antibody responses in C. trachomatis-infected women. OmcB ELISA was less sensitive than our C. trachomatis elementary body (EB) ELISA, but it was highly specific. The magnitude of the antibody response was higher in African-Americans and those with prior C. trachomatis infection. Unlike EB ELISA, the IgG1 response to C. trachomatis OmcB was short-lived and was not maintained by repeat C. trachomatis infection.

Male genital tract immune response against Chlamydia trachomatis infection

Chlamydia trachomatis is the most commonly reported agent of sexually transmitted bacterial infections worldwide. This pathogen frequently leads to persistent, long-term, subclinical infections, which in turn may cause severe pathology in susceptible hosts. This is in part due to the strategies that Chlamydia trachomatis uses to survive within epithelial cells and to evade the host immune response, such as subverting intracellular trafficking, interfering signaling pathways and preventing apoptosis. Innate immune receptors such as toll-like receptors expressed on epithelial and immune cells in the genital tract mediate the recognition of chlamydial molecular patterns. After bacterial recognition, a subset of pro-inflammatory cytokines and chemokines are continuously released by epithelial cells. The innate immune response is followed by the initiation of the adaptive response against Chlamydia trachomatis, which in turn may result in T helper 1-mediated protection or in T helper 2-mediated immunopathology. Understanding the molecular mechanisms developed by Chlamydia trachomatis to avoid killing and host immune response would be crucial for designing new therapeutic approaches and developing protective vaccines. In this review, we focus on chlamydial survival strategies and the elicited immune responses in male genital tract infections.

Effects of iota-carrageenan on ocular Chlamydia trachomatis infection in vitro and in vivo

Ocular chlamydial infections with the ocular serovars A, B, Ba, and C of Chlamydia trachomatis represent the world's leading cause of infectious blindness. Carrageenans are naturally occurring, sulfated polysaccharides generally considered safe for food and topical applications. Carrageenans can inhibit infection caused by a variety of viruses and bacteria. To investigate whether iota-carrageenan (I-C) isolated from the red alga Chondrus crispus could prevent ocular chlamydial infection, we assessed if targeted treatment of the conjunctival mucosa with I-C affects chlamydial attachment, entry, and replication in the host cell. Immortalized human conjunctival epithelial cells were treated with I-C prior to C. trachomatis infection and analyzed by flow cytometry and immunofluorescence microscopy. In vivo effects were evaluated in an ocular guinea pig inclusion conjunctivitis model. Ocular pathology was graded daily, and chlamydial clearance was investigated. Our study showed that I-C reduces the infectivity of C. trachomatis in vitro. In vivo results showed a slight reduced ocular pathology and significantly less shedding of infectious elementary bodies by infected animals. Our results indicate that I-C could be a promising agent to reduce the transmission of ocular chlamydial infection and opens perspectives to develop prophylactic approaches to block C. trachomatis entry into the host cell.

The novel chlamydial adhesin CPn0473 mediates the lipid raft-dependent uptake of Chlamydia pneumoniae

Chlamydiae are Gram‐negative, obligate intracellular pathogens that pose a serious threat to public health worldwide. Chlamydial surface molecules are essential for host cell invasion. The first interaction with the host cell is thereby accomplished by the Outer membrane complex protein B (OmcB) binding to heparan sulfate moieties on the host cell surface, followed by the interaction of the chlamydial polymorphic membrane proteins (Pmps) with host cell receptors. Specifically, the interaction of the Pmp21 adhesin and invasin with its human interaction partner, the epidermal growth factor receptor, results in receptor activation, down‐stream signalling and finally internalization of the bacteria. Blocking both, the OmcB and Pmp21 adhesion pathways, did not completely abolish infection, suggesting the presence of additional factors relevant for host cell invasion. Here, we show that the novel surface protein CPn0473 of Chlamydia pneumoniae contributes to the binding and invasion of infectious chlamydial particles. CPn0473 is expressed late in the infection cycle and located on the infectious chlamydial cell surface. Soluble recombinant CPn0473 as well as rCPn0473‐coupled fluorescent latex beads adhere to human epithelial HEp‐2 cells. Interestingly, in classical infection blocking experiments pretreatment of HEp‐2 cells with rCPn0473 does not attenuate adhesion but promotes dose‐dependently internalization by C. pneumoniae suggesting an unusual mode of action for this adhesin. This CPn0473‐dependent promotion of infection by C. pneumoniae depends on two different domains within the protein and requires intact lipid rafts. Thus, inhibition of the interaction of CPn0473 with the host cell could provide a way to reduce the virulence of C. pneumoniae.

Chlamydia trachomatis: Protective Adaptive Responses and Prospects for a Vaccine

Chlamydia trachomatis is the most common cause of sexually transmitted bacterial infection globally. These infections translate to a significant public health burden, particularly women's healthcare costs due to serious disease sequelae such as pelvic inflammatory disease (PID), tubal factor infertility, chronic pelvic pain, and ectopic pregnancy. There is no evidence that natural immunity can provide complete, long-term protection necessary to prevent chronic pathology, making human vaccine development critical. Vaccine design will require careful consideration of protective versus pathological host-response mechanisms in concert with elucidation of optimal antigens and adjuvants. Evidence suggests that a Th1 response, facilitated by IFN-γ-producing CD4 T cells, will be instrumental in generating long-term, sterilizing immunity. Although the role of antibodies is not completely understood, they have exhibited a protective effect by enhancing chlamydial clearance. Future work will require investigation of broadly neutralizing antibodies and antibody-augmented cellular immunity to successfully design a vaccine that potently elicits both arms of the immune response. Sterilizing immunity is the ultimate goal. However, vaccine-induced partial immunity that prevents upper genital tract infection and inflammation would be cost-effective compared to current screening and treatment strategies. In this chapter, we examine evidence from animal and human studies demonstrating protective adaptive immune responses to Chlamydia and discuss future challenges and prospects for vaccine development.

Chlamydial polymorphic membrane proteins: regulation, function and potential vaccine candidates

Pmps (Polymorphic Membrane Proteins) are a group of membrane bound surface exposed chlamydial proteins that have been characterized as autotransporter adhesins and are important in the initial phase of chlamydial infection. These proteins all contain conserved GGA (I, L, V) and FxxN tetrapeptide motifs in the N-terminal portion of each protein. All chlamydial species express Pmps. Even in the chlamydia-related bacteria Waddlia chondrophila, a Pmp-like adhesin has been identified, demonstrating the importance of Pmps in Chlamydiales biology. Chlamydial species vary in the number of pmp genes and their differentially regulated expression during the infectious cycle or in response to stress. Studies have also demonstrated that Pmps are able to induce innate immune functional responses in infected cells, including production of IL-8, IL-6 and MCP-1, by activating the transcription factor NF-κB. Human serum studies have indicated that although anti-Pmp specific antibodies are produced in response to a chlamydial infection, the response is variable depending on the Pmp protein. In C. trachomatis, PmpB, PmpC, PmpD and PmpI were the proteins eliciting the strongest immune response among adolescents with and without pelvic inflammatory disease (PID). In contrast, PmpA and PmpE elicited the weakest antibody response. Interestingly, there seems to be a gender bias for Pmp recognition with a stronger anti-Pmp reactivity in male patients. Furthermore, anti-PmpA antibodies might contribute to adverse pregnancy outcomes, at least among women with PID. In vitro studies indicated that dendritic cells infected with C. muridarum were able to present PmpG and PmpF on their MHC class II receptors and T cells were able to recognize the MHC class-II bound peptides. In addition, vaccination with PmpEFGH and Major Outer Membrane Protein (MOMP) significantly protected mice against a genital tract C. muridarum infection, suggesting that Pmps may be an important component of a multi-subunit chlamydial vaccine. Thus, Pmps might be important not only for the pathogenesis of chlamydial infection, but also as potential candidate vaccine proteins.

Chlamydia trachomatis In Vivo to In Vitro Transition Reveals Mechanisms of Phase Variation and Down-Regulation of Virulence Factors

Research on the obligate intracellular bacterium Chlamydia trachomatis demands culture in cell-lines, but the adaptive process behind the in vivo to in vitro transition is not understood. We assessed the genomic and transcriptomic dynamics underlying C. trachomatis in vitro adaptation of strains representing the three disease groups (ocular, epithelial-genital and lymphogranuloma venereum) propagated in epithelial cells over multiple passages. We found genetic features potentially underlying phase variation mechanisms mediating the regulation of a lipid A biosynthesis enzyme (CT533/LpxC), and the functionality of the cytotoxin (CT166) through an ON/OFF mechanism. We detected inactivating mutations in CT713/porB, a scenario suggesting metabolic adaptation to the available carbon source. CT135 was inactivated in a tropism-specific manner, with CT135-negative clones emerging for all epithelial-genital populations (but not for LGV and ocular populations) and rapidly increasing in frequency (~23% mutants per 10 passages). RNA-sequencing analyses revealed that a deletion event involving CT135 impacted the expression of multiple virulence factors, namely effectors known to play a role in the C. trachomatis host-cell invasion or subversion (e.g., CT456/Tarp, CT694, CT875/TepP and CT868/ChlaDub1). This reflects a scenario of attenuation of C. trachomatis virulence in vitro, which may take place independently or in a cumulative fashion with the also observed down-regulation of plasmid-related virulence factors. This issue may be relevant on behalf of the recent advances in Chlamydia mutagenesis and transformation where culture propagation for selecting mutants/transformants is mandatory. Finally, there was an increase in the growth rate for all strains, reflecting gradual fitness enhancement over time. In general, these data shed light on the adaptive process underlying the C. trachomatis in vivo to in vitro transition, and indicates that it would be prudent to restrict culture propagation to minimal passages and check the status of the CT135 genotype in order to avoid the selection of CT135-negative mutants, likely originating less virulent strains.

All subtypes of the Pmp adhesin family are implicated in chlamydial virulence and show species-specific function

The bacterial pathogens Chlamydia trachomatis and C. pneumoniae are obligate intracellular parasites, cause a number of serious diseases, and can infect various cell types in humans. Chlamydial infections are probably initiated by binding of the bacterial outer membrane protein OmcB to host cell glycosaminoglycans (GAGs). Here, we show that all nine members of the polymorphic membrane protein (Pmp) family of C. trachomatis mediate adhesion to human epithelial and endothelial cells. Importantly, exposure of infectious particles to soluble recombinant Pmps blocks subsequent infection, thus implicating an important function of the entire protein family in the infection process. Analogous experiments with pairs of recombinant Pmps or a combination of Pmp and OmcB revealed that all Pmps probably act in an adhesion pathway that is distinct from the OmcB-GAG pathway. Finally, we provide evidence that the Pmps of C. trachomatis and C. pneumoniae exhibit species and tissue specificity. These findings argue for the involvement of C. trachomatis Pmps in the initial phase of infection and suggest that they may interact with a receptor other than the epidermal growth factor receptor recently identified for their counterparts in C. pneumoniae.

Modulation of host signaling and cellular responses by Chlamydia

Modulation of host cell signaling and cellular functions is key to intracellular survival of pathogenic bacteria. Intracellular growth has several advantages e.g. escape from the humoral immune response and access to a stable nutrient rich environment. Growth in such a preferred niche comes at the price of an ongoing competition between the bacteria and the host as well as other microbes that compete for the very same host resources. This requires specialization and constant evolution of dedicated systems for adhesion, invasion and accommodation. Interestingly, obligate intracellular bacteria of the order Chlamydiales have evolved an impressive degree of control over several important host cell functions. In this review we summarize how Chlamydia controls its host cell with a special focus on signal transduction and cellular modulation.

Characterization of the interaction between the chlamydial adhesin OmcB and the human host cell

In a previous study, we reported that the OmcB protein from Chlamydia pneumoniae mediates adhesion of the infectious elementary body to human HEp-2 cells by interacting with heparin/heparan sulfate-like glycosaminoglycans (GAGs) via basic amino acids located in the first of a pair of XBBXBX heparin-binding motifs (K. Moelleken and J. H. Hegemann, Mol. Microbiol. 67:403-419, 2008). In the present study, we show that the basic amino acid at position 57 (arginine) in the first XBBXBX motif, the basic amino acid at position 61 (arginine) in the second motif, and another amino acid (lysine 69) C terminal to it play key roles in the interaction. In addition, we show that discrimination between heparin-dependent and -independent adhesion by C. trachomatis OmcBs is entirely dependent on three variable amino acids in the so-called variable domain C terminal to the conserved XBBXBX motif. Here, the predicted conformational change in the secondary structure induced by the proline at position 66 seems to be crucial for heparin recognition. Finally, we performed neutralization experiments using different anti-heparan sulfate antibodies to gain insight into the nature of the GAGs recognized by OmcB. The results suggest that C. trachomatis serovar L2 OmcB interacts with 6-O-sulfated domains of heparan sulfate, while C. pneumoniae OmcB apparently interacts with domains of heparan sulfate harboring a diverse subset of O-sulfations.

Chlamydial intracellular survival strategies

Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen and the causative agent of blinding trachoma. Although Chlamydia is protected from humoral immune responses by residing within remodeled intracellular vacuoles, it still must contend with multilayered intracellular innate immune defenses deployed by its host while scavenging for nutrients. Here we provide an overview of Chlamydia biology and highlight recent findings detailing how this vacuole-bound pathogen manipulates host-cellular functions to invade host cells and maintain a replicative niche.

Induction of protective immunity against Chlamydia muridarum intravaginal infection with the chlamydial immunodominant antigen macrophage infectivity potentiator

We previously reported that 5 Chlamydia muridarum antigens reacted with antisera from >90% mice urogenitally infected with C. muridarum and they are TC0660 (ABC transporter or ArtJ), TC0727 (outer membrane complex protein B or OmcB), TC0828 (macrophage infectivity potentiator or MIP), TC0726 (inclusion membrane protein or Inc) & TC0268 (hypothetical protein or HP). The orthologs of these antigens in Chlamydia trachomatis were also highly reactive with antisera from women urogenitally infected with C. trachomatis. In the current study, we evaluated these C. muridarum antigens for their ability to induce protection against a C. muridarum intravaginal challenge infection in mice. We found that only MIP induced the most pronounced protection against C. muridarum infection. The protection correlated well with robust C. muridarum MIP-specific antibody and Th1-dominant T cell responses. The MIP-immunized mice displayed significantly reduced live organism shedding from the lower genital tract and highly attenuated inflammatory pathologies in the upper genital tissues. These results demonstrate that MIP, an immunodominant antigen identified by both human and mouse antisera, may be considered a component of a multi-subunit chlamydial vaccine for inducing protective immunity.

Chlamydia trachomatis outer membrane complex protein B (OmcB) is processed by the protease CPAF

We previously reported that the Chlamydia trachomatis outer membrane complex protein B (OmcB) was partially processed in Chlamydia-infected cells. We have now confirmed that the OmcB processing occurred inside live cells during chlamydial infection and was not due to proteolysis during sample harvesting. OmcB processing was preceded by the generation of active CPAF, a serine protease known to be able to cross the inner membrane via a Sec-dependent pathway, suggesting that active CPAF is available for processing OmcB in the periplasm. In a cell-free system, CPAF activity is both necessary and sufficient for processing OmcB. Both depletion of CPAF from Chlamydia-infected cell lysates with a CPAF-specific antibody and blocking CPAF activity with a CPAF-specific inhibitory peptide removed the OmcB processing ability of the lysates. A highly purified wild-type CPAF but not a catalytic residue-substituted mutant CPAF was sufficient for processing OmcB. Most importantly, in chlamydial culture, inhibition of CPAF with a specific inhibitory peptide blocked OmcB processing and reduced the recovery of infectious organisms. Thus, we have identified OmcB as a novel authentic target for the putative chlamydial virulence factor CPAF, which should facilitate our understanding of the roles of CPAF in chlamydial biology and pathogenesis.

Chlamydial biology and its associated virulence blockers

Chlamydiales are obligate intracellular parasites of eukaryotic cells. They can be distinguished from other Gram-negative bacteria through their characteristic developmental cycle, in addition to special biochemical and physical adaptations to subvert the eukaryotic host cell. The host spectrum includes humans and other mammals, fish, birds, reptiles, insects and even amoeba, causing a plethora of diseases. The first part of this review focuses on the specific chlamydial infection biology and metabolism. As resistance to classical antibiotics is emerging among Chlamydiae as well, the second part elaborates on specific compounds and tools to block chlamydial virulence traits, such as adhesion and internalization, Type III secretion and modulation of gene expression.

Attachment of Chlamydia trachomatis L2 to host cells requires sulfation

Chlamydia trachomatis is a pathogen responsible for a prevalent sexually transmitted disease. It is also the most common cause of infectious blindness in the developing world. We performed a loss-of-function genetic screen in human haploid cells to identify host factors important in C. trachomatis L2 infection. We identified and confirmed B3GAT3, B4GALT7, and SLC35B2, which encode glucuronosyltransferase I, galactosyltransferase I, and the 3'-phosphoadenosine 5'-phosphosulfate transporter 1, respectively, as important in facilitating Chlamydia infection. Knockout of any of these three genes inhibits Chlamydia attachment. In complementation studies, we found that the introduction of functional copies of these three genes into the null clones restored full susceptibility to Chlamydia infection. The degree of attachment of Chlamydia strongly correlates with the level of sulfation of the host cell, not simply with the amount of heparan sulfate. Thus, other, as-yet unidentified sulfated macromolecules must contribute to infection. These results demonstrate the utility of screens in haploid cells to study interactions of human cells with bacteria. Furthermore, the human null clones generated can be used to investigate the role of heparan sulfate and sulfation in other settings not limited to infectious disease.

Novel molecular markers of Chlamydia pecorum genetic diversity in the koala (Phascolarctos cinereus)

Background

Chlamydia pecorum is an obligate intracellular bacterium and the causative agent of reproductive and ocular disease in several animal hosts including koalas, sheep, cattle and goats. C. pecorum strains detected in koalas are genetically diverse, raising interesting questions about the origin and transmission of this species within koala hosts. While the ompA gene remains the most widely-used target in C. pecorum typing studies, it is generally recognised that surface protein encoding genes are not suited for phylogenetic analysis and it is becoming increasingly apparent that the ompA gene locus is not congruent with the phylogeny of the C. pecorum genome. Using the recently sequenced C. pecorum genome sequence (E58), we analysed 10 genes, including ompA, to evaluate the use of ompA as a molecular marker in the study of koala C. pecorum genetic diversity.

Results

Three genes (incA, ORF663, tarP) were found to contain sufficient nucleotide diversity and discriminatory power for detailed analysis and were used, with ompA, to genotype 24 C. pecorum PCR-positive koala samples from four populations. The most robust representation of the phylogeny of these samples was achieved through concatenation of all four gene sequences, enabling the recreation of a "true" phylogenetic signal. OmpA and incA were of limited value as fine-detailed genetic markers as they were unable to confer accurate phylogenetic distinctions between samples. On the other hand, the tarP and ORF663 genes were identified as useful "neutral" and "contingency" markers respectively, to represent the broad evolutionary history and intra-species genetic diversity of koala C. pecorum. Furthermore, the concatenation of ompA, incA and ORF663 sequences highlighted the monophyletic nature of koala C. pecorum infections by demonstrating a single evolutionary trajectory for koala hosts that is distinct from that seen in non-koala hosts.

Conclusions

While the continued use of ompA as a fine-detailed molecular marker for epidemiological analysis appears justified, the tarP and ORF663 genes also appear to be valuable markers of phylogenetic or biogeographic divisions at the C. pecorum intra-species level. This research has significant implications for future typing studies to understand the phylogeny, genetic diversity, and epidemiology of C. pecorum infections in the koala and other animal species.

A Chlamydia trachomatis OmcB C-terminal fragment is released into the host cell cytoplasm and is immunogenic in humans

The Chlamydia trachomatis outer membrane complex protein B (OmcB) is an antigen with diagnostic and vaccine relevance. To further characterize OmcB, we generated antibodies against OmcB C-terminal (OmcBc) and N-terminal (OmcBn) fragments. Surprisingly, the anti-OmcBc antibody detected dominant signals in the host cell cytosol, while the anti-OmcBn antibody exclusively labeled intrainclusion signals in C. trachomatis-infected cells permeabilized with saponin. Western blot analyses revealed that OmcB was partially processed into OmcBc and OmcBn fragments. The processed OmcBc was released into host cell cytosol, while the OmcBn and remaining full-length OmcB were retained within the chlamydial inclusions. The organism-associated OmcB epitopes became detectable only after the C. trachomatis-infected cells were permeabilized with strong detergents such as SDS. However, the harsh permeabilization conditions also led to the leakage of the already secreted OmcBc and chlamydia-secreted protease (CPAF) out of the host cells. The OmcBc processing and release occurred in all biovars of C. trachomatis. Moreover, the released OmcBc but not the retained OmcBn was highly immunogenic in C. trachomatis-infected women, which is consistent with the concept that exposure of chlamydial proteins to host cell cytosol is accompanied by increased immunogenicity. These observations have provided important information for further exploring/optimizing OmcB as a target for the development of diagnosis methods and vaccines.

Infectivity acts as in vivo selection for maintenance of the chlamydial cryptic plasmid

Chlamydia trachomatis contains a conserved ∼7.5-kb plasmid. Loss of the plasmid results in reduced glycogen accumulation, failure to activate TLR2, and reduced infectivity. We hypothesized that reduced infectivity functions as a means of selection for plasmid maintenance. We directly examined the biological significance of the reduced infectivity associated with plasmid deficiency by determining the relative fitness of plasmid-deficient CM972 versus that of wild-type C. muridarum Nigg in mixed inocula in vitro and in vivo. C. muridarum Nigg rapidly out-competed its plasmid-cured derivative CM972 in vitro but was not competitive with CM3.1, a derivative of CM972 that has reverted to a normal infectivity phenotype. C. muridarum Nigg also effectively competed with CM972 during lower and upper genital tract infection in the mouse, demonstrating that strong selective pressure for plasmid maintenance occurs during infection. The severity of oviduct inflammation and dilatation resulting from these mixed infections correlated directly with the amount of C. muridarum Nigg in the initial inoculum, confirming the role of the plasmid in virulence. Genetic characterization of CM972 and CM3.1 revealed no additional mutations (other than loss of the plasmid) to account for the reduced infectivity of CM972 and detected a single base substitution in TC_0236 in CM3.1 that may be responsible for its restored infectivity. These data demonstrate that a chlamydial strain that differs genetically from its wild-type parent only with respect to the lack of the chlamydial plasmid is unable to compete in vitro and in vivo, likely explaining the rarity of plasmid-deficient isolates in nature.

Protection against Chlamydia promoted by a subunit vaccine (CTH1) compared with a primary intranasal infection in a mouse genital challenge model

BACKGROUND

The chlamydial proteins CT443 (OmcB) and CT521 (rl16) have previously been identified as human B and/or T cell targets during a chlamydial infection in humans. Here we compare the protective effector mechanism promoted by a fusion protein composed of CT521 and CT443 (CTH1) with a primary intranasal Chlamydia muridarum infection known to provide high levels of protection against a genital chlamydial challenge.

METHODOLOGY/PRINCIPAL FINDINGS

The fusion protein CTH1, adjuvanted with a strong Th1 inducing cationic adjuvant (CAF01), significantly reduced the bacterial shedding compared to a control group in both a C. trachomatis Serovar D and C. muridarum challenge model. The CTH1/CAF01 vaccine was found to induce polyfunctional T cells consisting of TNFalpha/IL-2 and TNFalpha/IL-2/IFN-gamma positive cells and high titers of CTH1 specific IgG2a and IgG1. By depletion experiments the protection in the C. muridarum challenge model was demonstrated to be mediated solely by CD4(+) T cells. In comparison, an intranasal infection with C. muridarum induced a T cell response that consisted predominantly of TNFalpha/IFN-gamma co-expressing effector CD4(+) T cells and an antibody response consisting of C. muridarum specific IgG1, IgG2a but also IgA. This response was associated with a high level of protection against challenge-a protection that was only partially dependent on CD4(+) T cells. Furthermore, whereas the antibody response induced by intranasal infection was strongly reactive against the native antigens displayed in the chlamydial elementary body, only low levels of antibodies against this preparation were found after CTH1/CAF01 immunization.

CONCLUSIONS/SIGNIFICANCE

Our data demonstrate that CTH1 vaccination promotes a CD4(+) T cell dependent protective response but compared with intranasal C. muridarum infection lacks a CD4 independent protective mechanism for complete protection.

Identification of Chlamydia trachomatis outer membrane complex proteins by differential proteomics

The extracellular chlamydial infectious particle, or elementary body (EB), is enveloped by an intra- and intermolecular cysteine cross-linked protein shell called the chlamydial outer membrane complex (COMC). A few abundant proteins, including the major outer membrane protein and cysteine-rich proteins (OmcA and OmcB), constitute the overwhelming majority of COMC proteins. The identification of less-abundant COMC proteins has been complicated by limitations of proteomic methodologies and the contamination of COMC fractions with abundant EB proteins. Here, we used parallel liquid chromatography-mass spectrometry/mass spectrometry (LC-MS/MS) analyses of Chlamydia trachomatis serovar L2 434/Bu EB, COMC, and Sarkosyl-soluble EB fractions to identify proteins enriched or depleted from COMC. All well-described COMC proteins were specifically enriched in the COMC fraction. In contrast, multiple COMC-associated proteins found in previous studies were strongly enriched in the Sarkosyl-soluble fraction, suggesting that these proteins are not COMC components or are not stably associated with COMC. Importantly, we also identified novel proteins enriched in COMC. The list of COMC proteins identified in this study has provided reliable information for further understanding chlamydial protein secretion systems and modeling COMC and EB structures.

Chlamydia vaccine candidates and tools for chlamydial antigen discovery

The failure of the inactivated Chlamydia-based vaccine trials in the 1960s has led researchers studying Chlamydia to take cautious and rational approaches to develop safe and effective chlamydial vaccines. Subsequent research efforts focused on three areas. The first is the analysis of the immunobiology of chlamydial infection in animal models, with supporting clinical studies, to identify the immune correlates of both protective immunity and pathological responses. Second, recent radical improvements in genomics, proteomics and associated technologies have assisted in the implementation of creative approaches to search for suitable vaccine candidates. Third, progress in the analysis of host response and adjuvanticity regulating both innate and adaptive immunity at the mucosal site of infection has led to progress in the design of optimal delivery and adjuvant systems for enhancing protective immunity. Considerable progress has been made in the first two areas but research efforts to better define the factors that regulate immunity at mucosal sites of infection and to develop strategies to boost protective immunity via immunomodulation, effective delivery systems and potent adjuvants, have remained elusive. In this article, we will summarize progress in these areas with a focus on chlamydial vaccine antigen discovery, and discuss future directions towards the development of a safe and effective chlamydial vaccine.

New insights into Chlamydia intracellular survival mechanisms

Chlamydia sp. are responsible for a wide range of diseases of significant clinical and public health importance. In this review, we highlight how recent cellular and functional genomic approaches have significantly increased our knowledge of the pathogenic mechanisms used by these genetically intractable bacteria. As the extensive repertoire of chlamydial proteins that are translocated into the mammalian host is identified and characterized, a molecular understanding of how Chlamydiae co-opt host cellular functions and block innate immune pathways is beginning to emerge.

Attachment and entry of Chlamydia have distinct requirements for host protein disulfide isomerase

Chlamydia is an obligate intracellular pathogen that causes a wide range of diseases in humans. Attachment and entry are key processes in infectivity and subsequent pathogenesis of Chlamydia, yet the mechanisms governing these interactions are unknown. It was recently shown that a cell line, CHO6, that is resistant to attachment, and thus infectivity, of multiple Chlamydia species has a defect in protein disulfide isomerase (PDI) N–terminal signal sequence processing. Ectopic expression of PDI in CHO6 cells led to restoration of Chlamydia attachment and infectivity; however, the mechanism leading to this recovery was not ascertained. To advance our understanding of the role of PDI in Chlamydia infection, we used RNA interference to establish that cellular PDI is essential for bacterial attachment to cells, making PDI the only host protein identified as necessary for attachment of multiple species of Chlamydia. Genetic complementation and PDI-specific inhibitors were used to determine that cell surface PDI enzymatic activity is required for bacterial entry into cells, but enzymatic function was not required for bacterial attachment. We further determined that it is a PDI-mediated reduction at the cell surface that triggers bacterial uptake. While PDI is necessary for Chlamydia attachment to cells, the bacteria do not appear to utilize plasma membrane–associated PDI as a receptor, suggesting that Chlamydia binds a cell surface protein that requires structural association with PDI. Our findings demonstrate that PDI has two essential and independent roles in the process of chlamydial infectivity: it is structurally required for chlamydial attachment, and the thiol-mediated oxido-reductive function of PDI is necessary for entry.

Chlamydia is a large burden on global health. It is the most common cause of infectious blindness, and the CDC (Centers for Disease Control and Prevention) estimates that in the United States alone there are more than 2 million people with sexually transmitted Chlamydia infections. Chlamydia is an obligate intracellular bacteria; thus, attachment and subsequent invasion of cells are key steps in Chlamydia pathogenesis. While strides have been made in understanding the molecular mechanism of Chlamydia infection, fundamental aspects of this process still remain elusive. We have identified a host protein, protein disulfide isomerase (PDI), that is essential for Chlamydia attachment as well as for entry into cells. Cell-surface PDI-mediated disulfide reduction is required for Chlamydia entry into cells, whereas bacterial attachment is independent of PDI enzymatic activity. Although PDI is necessary for Chlamydia attachment, the bacteria apparently does not bind directly to cell-associated PDI, suggesting that Chlamydia attaches to a host protein(s) associated with PDI. This study advances our understanding of Chlamydia pathogenesis by the characterization of a host factor essential for independent stages of bacterial attachment and entry.

Evaluation of an in silico predicted specific and immunogenic antigen from the OmcB protein for the serodiagnosis of Chlamydia trachomatis infections

Background

The OmcB protein is one of the most immunogenic proteins in C. trachomatis and C. pneumoniae infections. This protein is highly conserved leading to serum cross reactivity between the various chlamydial species. Since previous studies based on recombinant proteins failed to identify a species specific immune response against the OmcB protein, this study evaluated an in silico predicted specific and immunogenic antigen from the OmcB protein for the serodiagnosis of C. trachomatis infections.

Results

Using the ClustalW and Antigenic programs, we have selected two predicted specific and immunogenic regions in the OmcB protein: the N-terminal (Nt) region containing three epitopes and the C-terminal (Ct) region containing two epitopes with high scores. These regions were cloned into the PinPoint Xa-1 and pGEX-6P-1 expression vectors, incorporating a biotin purification tag and a glutathione-S-transferase tag, respectively. These regions were then expressed in E. coli. Only the pGEX-6P-1 has been found suitable for serological studies as its tag showed less cross reactivity with human sera and was retained for the evaluation of the selected antigens. Only the Ct region of the protein has been found to be well expressed in E. coli and was evaluated for its ability to be recognized by human sera. 384 sera were tested for the presence of IgG antibodies to C. trachomatis by our in house microimmunofluorescence (MIF) and the developed ELISA test. Using the MIF as the reference method, the developed OmcB Ct ELISA has a high specificity (94.3%) but a low sensitivity (23.9). Our results indicate that the use of the sequence alignment tool might be useful for identifying specific regions in an immunodominant antigen. However, the two epitopes, located in the selected Ct region, of the 24 predicted in the full length OmcB protein account for approximately 25% of the serological response detected by MIF, which limits the use of the developed ELISA test when screening C. trachomatis infections.

Conclusion

The developed ELISA test might be used as a confirmatory test to assess the specificity of serological results found by MIF.

Differential glycosaminoglycan binding of Chlamydia trachomatis OmcB protein from serovars E and LGV

We recently showed that OmcB protein from Chlamydia trachomatis serovar LGV1 functions as an adhesin. In this study, we produced Escherichia coli expressing OmcB from serovar E and compared this OmcB to OmcB from serovar LGV1. Infectivity inhibition assays carried out with serovars LGV1 and E of C. trachomatis in the presence of recombinant OmcB showed considerable (approximately 60%) inhibition of infectivity. In the presence of heparan sulphate, there was significant inhibition (68%) of adherence of E. coli expressing OmcB from serovar LGV1 only. In a further experiment, recombinant OmcB from serovar LGV1 showed minimal binding to glycosaminoglycan (GAG)-deficient cells, whilst to the same cells, recombinant OmcB from serovar E showed binding equal to that to the wild-type cells. Our experiments strongly suggest that OmcB from serovar E, in contrast to that from serovar LGV1, is not binding to host cells through a GAG-dependent mechanism.

Susceptibility of Chlamydia trachomatis to the excipient hydroxyethyl cellulose: pH and concentration dependence of antimicrobial activity

Hydroxyethyl cellulose (HEC) is used as a neutral excipient in microbicides used against sexually transmitted pathogens. However, HEC inhibits the infection of cervical epithelial cells by Chlamydia trachomatis at pH 5 in a concentration-dependent manner. At pH 7, infection is inversely dependent on the concentration of HEC, possibly due to pH-dependent calcium sequestration.

Chlamydia pneumoniae GroEL1 protein is cell surface associated and required for infection of HEp-2 cells

Chlamydia pneumoniae is an important obligate intracellular pathogen that replicates within an inclusion in the eukaryotic cell. The initial event of a chlamydial infection is the adherence to and subsequent uptake of the infectious elementary bodies (EBs) by the human cell. These processes require yet-unidentified bacterial and eukaryotic surface proteins. The GroEL1 protein, which exhibits a very strong antigenicity and in vitro can activate various eukaryotic cells, is a potential pathogenicity factor. We localized the protein during the infection process and found it in the inclusion but outside the chlamydial particles. GroEL1 was also localized on the surface of EBs, and the protein could be washed off the EBs. Latex beads coated with recombinantly produced GroEL1 (rGroEL1) bound in a dose-dependent manner to HEp-2 cells. Likewise, GroEL1, when expressed and displayed on the yeast cell surface, mediated adhesion to HEp-2 cells. Interestingly, the homologous GroEL2 and GroEL3 proteins showed no adhesive properties. Incubation of primary umbilical vein endothelial cells with soluble GroEL1 and GroEL1-coated latex beads activated the translocation of the general transcription factor NF-kappaB into the nucleus. Finally, preincubation of HEp-2 cells with rGroEL1 significantly reduced subsequent infection with C. pneumoniae, although adhesion of infectious bacteria to eukaryotic cells was not affected. Taken together, these data support a role for extracellular GroEL1 in the establishment of the chlamydial infection.

Genome-wide analysis of Chlamydophila pneumoniae gene expression at the late stage of infection

Chlamydophila pneumoniae, an obligate intracellular eubacterium, changes its form from a vegetative reticulate body into an infectious elementary body during the late stage of its infection cycle. Comprehension of the molecular events in the morphological change is important to understand the switching mechanism between acute and chronic infection, which is deemed to relate to the pathogenesis of atherosclerosis. Herein, we have attempted to screen genes expressed in the late stage with a genome-wide DNA microarray, resulting in nomination of 17 genes as the late-stage genes. Fourteen of the 17 genes and six other genes predicted as late-stage genes were confirmed to be up-regulated in the late stage with a quantitative reverse transcriptase–polymerase chain reaction. These 20 late-stage genes were classified into two groups by clustering analysis: ‘drastically induced’ and ‘moderately induced’ genes. Out of eight drastically induced genes, four contain σ²⁸ promoter-like sequences and the other four contain an upstream common sequence. It suggests that besides σ²⁸, there are certain up-regulatory mechanisms at the late stage, which may be involved in the chlamydial morphological change and thus pathogenesis.

The Chlamydia outer membrane protein OmcB is required for adhesion and exhibits biovar-specific differences in glycosaminoglycan binding

Chlamydia pneumoniae, an obligate intracellular human pathogen, causes a number of respiratory diseases. We explored the role of the conserved OmcB protein in C. pneumoniae infections, using yeast display technology. (i) Yeast cells presenting OmcB were found to adhere to human epithelial cells. (ii) Pre-incubation of OmcB yeast cells with heparin, but not other glycosaminoglycans (GAGs), abrogated adhesion. (iii) Pre-treatment of the target cells with heparinase inhibited adherence, and GAG-deficient CHO cell lines failed to bind OmcB yeast. (iv) A heparin-binding motif present near the N-terminus of OmcB is required for host cell binding. (v) Pre-treatment of chlamydial elementary bodies (EBs) with anti-OmcB antibody or pre-incubation of target cells with recombinant OmcB protein reduced infectivity upon challenge with C. pneumoniae. (vi) Adhesion of fluorescently labelled EBs to epithelial or endothelial cells was abrogated by prior addition of heparin or OmcB protein. Thus, C. pneumoniae OmcB is an adhesin that binds heparan sulphate-like GAGs. OmcB from Chlamydia trachomatis serovar L1 also adheres to human cells in a heparin-dependent way, unlike its counterpart from serovar E. We show that a single position in the OmcB sequence determines heparin dependence/independence, and variations there may reflect differences between the two serovars in cell tropism and disease pattern.