Identification of a chlamydial protease-like activity factor responsible for the degradation of host transcription factors

Pathogenic and Protective Roles of Neutrophils in Chlamydia trachomatis Infection

Chlamydia trachomatis (Ct) is an obligate intracellular pathogen that causes the most commonly diagnosed bacterial sexually transmitted infection (STI) and is a leading cause of preventable blindness globally. Ct infections can generate a strong pro-inflammatory immune response, leading to immune-mediated pathology in infected tissues. Neutrophils play an important role in mediating both pathology and protection during infection. Excessive neutrophil activation, migration, and survival are associated with host tissue damage during Chlamydia infections. In contrast, neutrophils also perform phagocytic killing of Chlamydia in the presence of IFN-γ and anti-Chlamydia antibodies. Neutrophil extracellular traps (NETs) and many neutrophil degranulation products have also demonstrated strong anti-Chlamydia functions. To counteract this neutrophil-mediated protection, Chlamydia has developed several evasion strategies. Various Chlamydia proteins can limit potentially protective neutrophil responses by directly targeting receptors present on the surface of neutrophils or neutrophil degranulation products. In this review, we provide a survey of current knowledge regarding the role of neutrophils in pathogenesis and protection, including the ways that Chlamydia circumvents neutrophil functions, and we propose critical areas for future research.

A TriAdj-Adjuvanted Chlamydia trachomatis CPAF Protein Vaccine Is Highly Immunogenic in Pigs

Chlamydia trachomatis (Ct) infections are the most common sexually transmitted infection (STI). Despite effective antibiotics for Ct, undetected infections or delayed treatment can lead to infertility, ectopic pregnancies, and chronic pelvic pain. Besides humans, chlamydia poses similar health challenges in animals such as C. suis (Cs) in pigs. Based on the similarities between humans and pigs, as well as their chlamydia species, we use pigs as a large biomedical animal model for chlamydia research. In this study, we used the pig model to develop a vaccine candidate against Ct. The vaccine candidate consists of TriAdj-adjuvanted chlamydial-protease-like activity factor (CPAF) protein. We tested two weekly administration options-twice intranasal (IN) followed by twice intramuscular (IM) and twice IM followed by twice IN. We assessed the humoral immune response in both serum using CPAF-specific IgG (including antibody avidity determination) and also in cervical and rectal swabs using CPAF-specific IgG and IgA ELISAs. The systemic T-cell response was analyzed following in vitro CPAF restimulation via IFN-γ and IL-17 ELISpots, as well as intracellular cytokine staining flow cytometry. Our data demonstrate that while the IN/IM vaccination mainly led to non-significant systemic immune responses, the vaccine candidate is highly immunogenic if administered IM/IN. This vaccination strategy induced high serum anti-CPAF IgG levels with strong avidity, as well as high IgA and IgG levels in vaginal and rectal swabs and in uterine horn flushes. In addition, this vaccination strategy prompted a pronounced cellular immune response. Besides inducing IL-17 production, the vaccine candidate induced a strong IFN-γ response with CD4 T cells. In IM/IN-vaccinated pigs, these cells also significantly downregulated their CCR7 expression, a sign of differentiation into peripheral-tissue-homing effector/memory cells. Conclusively, this study demonstrates the strong immunogenicity of the IM/IN-administered TriAdj-adjuvanted Ct CPAF vaccine candidate. Future studies will test the vaccine efficacy of this promising Ct vaccine candidate. In addition, this project demonstrates the suitability of the Cs pre-exposed outbred pig model for Ct vaccine development. Thereby, we aim to open the bottleneck of large animal models to facilitate the progression of Ct vaccine candidates into clinical trials.

Requirement of GrgA for Chlamydia infectious progeny production, optimal growth, and efficient plasmid maintenance

IMPORTANCE

Hallmarks of the developmental cycle of the obligate intracellular pathogenic bacterium Chlamydia are the primary differentiation of the infectious elementary body (EB) into the proliferative reticulate body (RB) and the secondary differentiation of RBs back into EBs. The mechanisms regulating these transitions remain unclear. In this report, we developed an effective novel strategy termed dependence on plasmid-mediated expression (DOPE) that allows for the knockdown of essential genes in Chlamydia. We demonstrate that GrgA, a Chlamydia-specific transcription factor, is essential for the secondary differentiation and optimal growth of RBs. We also show that GrgA, a chromosome-encoded regulatory protein, controls the maintenance of the chlamydial virulence plasmid. Transcriptomic analysis further indicates that GrgA functions as a critical regulator of all three sigma factors that recognize different promoter sets at developmental stages. The DOPE strategy outlined here should provide a valuable tool for future studies examining chlamydial growth, development, and pathogenicity.

Requirement of GrgA for Chlamydia infectious progeny production, optimal growth, and efficient plasmid maintenance

Chlamydia, an obligate intracellular bacterial pathogen, has a unique developmental cycle involving the differentiation of invading elementary bodies (EBs) to noninfectious reticulate bodies (RBs), replication of RBs, and redifferentiation of RBs into progeny EBs. Progression of this cycle is regulated by three sigma factors, which direct the RNA polymerase to their respective target gene promoters. We hypothesized that the Chlamydia-specific transcriptional regulator GrgA, previously shown to activate σ66 and σ28, plays an essential role in chlamydial development and growth. To test this hypothesis, we applied a novel genetic tool known as dependence on plasmid-mediated expression (DOPE) to create Chlamydia trachomatis with conditional GrgA-deficiency. We show that GrgA-deficient C. trachomatis RBs have a growth rate that is approximately half of the normal rate and fail to transition into progeny EBs. In addition, GrgA-deficient C. trachomatis fail to maintain its virulence plasmid. Results of RNA-seq analysis indicate that GrgA promotes RB growth by optimizing tRNA synthesis and expression of nutrient-acquisition genes, while it enables RB-to-EB conversion by facilitating the expression of a histone and outer membrane proteins required for EB morphogenesis. GrgA also regulates numerous other late genes required for host cell exit and subsequent EB invasion into host cells. Importantly, GrgA stimulates the expression of σ54, the third and last sigma factor, and its activator AtoC, and thereby indirectly upregulating the expression of σ54-dependent genes. In conclusion, our work demonstrates that GrgA is a master transcriptional regulator in Chlamydia and plays multiple essential roles in chlamydial pathogenicity.

TargeTron Inactivation of Chlamydia trachomatis gseA Results in a Lipopolysaccharide 3-Deoxy-d-Manno-Oct-2-Ulosonic Acid-Deficient Strain That Is Cytotoxic for Cells

All members of the family Chlamydiaceae have lipopolysaccharides (LPS) that possess a shared carbohydrate trisaccharide antigen, 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo) that is functionally uncharacterized. A single gene, genus-specific epitope (gseA), is responsible for attaching the tri-Kdo to lipid IVA. To investigate the function of Kdo in chlamydial host cell interactions, we made a gseA-null strain (L2ΔgseA) by using TargeTron mutagenesis. Immunofluorescence microscopy and immunoblotting with a Kdo-specific monoclonal antibody demonstrated that L2ΔgseA lacked Kdo. L2ΔgseA reacted by immunoblotting with a monoclonal antibody specific for a conserved LPS glucosamine-PO4 epitope, indicating that core lipid A was retained by the mutant. The mutant strain produced a similar number of inclusions as the parental strain but yielded lower numbers of infectious elementary bodies. Transmission electron microscopy of L2ΔgseA-infected cells showed atypical developmental forms and a reduction in the number of elementary bodies. Immunoblotting of dithiothreitol-treated L2ΔgseA-infected cells lysates revealed a marked reduction in outer membrane OmcB disulfide cross-linking, suggesting that the elementary body outer membrane structure was affected by the lack of Kdo. Notably, lactic acid dehydrogenase release by infected cells demonstrated that L2ΔgseA was significantly more cytotoxic to host cells than the wild type. The cytotoxic phenotype may result from an altered outer membrane biogenesis structure and/or function or, conversely, from a direct pathobiological effect of Kdo on an unknown host cell target. These findings implicate a previously unrecognized role for Kdo in host cell interactions that facilitates postinfection host cell survival.

Tryptophan residue of plasmid-encoded Pgp3 is important for Chlamydia muridarum to induce hydrosalpinx in mice

The crucial role of plasmid-encoded protein Pgp3 in Chlamydia pathogenesis has been demonstrated in various animal models. Previous studies have revealed that the Pgp3-deficient C. muridarum mutant fails to induce hydrosalpinx after vaginal inoculation in mice. Structural analysis of C. trachomatis Pgp3 trimer has indicated that Trp234 may play a critical role in trimeric crystal packing interactions and that Tyr197 is involved at predominant cation-binding sites. In this study, we constructed C. muridarum transformants harboring Pgp3, Trp234, or Tyr197 point mutations (Pgp3W234A and Pgp3Y197A). C3H/HeJ mice infected with Pgp3W234A mutant failed to induce severe hydrosalpinx in the oviduct tissue, which largely phenocopied the full-length Pgp3-deficient C. muridarum. The Pgp3Y197A variant induced an intermediate severity of pathology. The attenuated pathogenicity caused by the Pgp3W234A mutant may be due to its decreased survival in the lower genital tracts of mice, reduced ascension to the oviduct, and milder induction of inflammatory cell infiltration in the oviduct tissue. Thus, our results point to an important amino acid residue involved in Pgp3 virulence, providing a potential therapeutic target for chlamydial infection.

GSDMD-mediated pyroptosis restrains intracellular Chlamydia trachomatis growth in macrophages

Pyroptosis, a type of programmed necrosis associated with inflammatory, is a host defense mechanism against microbial infections. Although Chlamydia has been shown to induce pyroptosis, whether pyroptosis directly impacts the growth of Chlamydia has not been demonstrated. In this study, we found that C. trachomatis L2 infection of the mouse macrophage RAW 264.7 cells induced pyroptosis by monitoring the ultrastructural changes under transmission electron microscopy and the release of LDH and IL-1β. More importantly, this C. trachomatis-triggered pyroptosis with activation of caspase-1 and caspase-11 was also accompanied by gasdermin D (GSDMD) activation. Suppression of these two inflammatory caspases inhibited GSDMD activation. Interestingly, the C. trachomatis-triggered pyroptosis significantly inhibited the intracellular growth of C. trachomatis since inactivation of either GSDMD or caspase-1/11 significantly rescued infectious C. trachomatis yields, which suggests pyroptosis response can be utilized as an intrinsic mechanism to restrict C. trachomatis intracellular infection in addition to the well- documented extrinsic mechanisms by recruiting and enhancing inflammatory responses. This study may reveal novel targets for attenuating C. trachomatis infectivity and/or pathogenicity.

In Search of a Mechanistic Link between Chlamydia trachomatis-Induced Cellular Pathophysiology and Oncogenesis

Centrosome duplication and cell cycle progression are essential cellular processes that must be tightly controlled to ensure cellular integrity. Despite their complex regulatory mechanisms, microbial pathogens have evolved sophisticated strategies to co-opt these processes to promote infection. While misregulation of these processes can greatly benefit the pathogen, the consequences to the host cell can be devastating. During infection, the obligate intracellular pathogen Chlamydia trachomatis induces gross cellular abnormalities, including supernumerary centrosomes, multipolar spindles, and defects in cytokinesis. While these observations were made over 15 years ago, identification of the bacterial factors responsible has been elusive due to the genetic intractability of Chlamydia. Recent advances in techniques of genetic manipulation now allows for the direct linking of bacterial virulence factors to manipulation of centrosome duplication and cell cycle progression. In this review, we discuss the impact, both immediate and downstream, of C. trachomatis infection on the host cell cycle regulatory apparatus and centrosome replication. We highlight links between C. trachomatis infection and cervical and ovarian cancers and speculate whether perturbations of the cell cycle and centrosome are sufficient to initiate cellular transformation. We also explore the biological mechanisms employed by Inc proteins and other secreted effector proteins implicated in the perturbation of these host cell pathways. Future work is needed to better understand the nuances of each effector's mechanism and their collective impact on Chlamydia's ability to induce host cellular abnormalities.

A Novel Cleavage Pattern of Complement C5 Induced by Chlamydia trachomatis Infection via the Chlamydial Protease CPAF

Urogenital Chlamydia trachomatis infection is one of the most common bacterial sexually transmitted diseases globally. Untreated C. trachomatis infections can ascend to the upper genital tract and establish a series of severe complications. Previous studies using C3^−/− and C5^−/− mice models demonstrated that C3-independent activation of C5 occurred during C. trachomatis infection. However, the mechanism of how chlamydial infection activates C5 in the absence of C3 has yet to be elucidated. To delineate interactions between C5 and chlamydial infection, cleavage products in a co-incubation system containing purified human C5 and C. trachomatis-HeLa229 cell lysates were analyzed, and a novel cleavage pattern of C5 activation induced by C. trachomatis infection was identified. C5 was cleaved efficiently at the previously unidentified site K970, but was cleaved poorly at site R751. C5b was modified to C5b_Ct, which later formed C5b_Ct-9, which had enhanced lytic ability compared with C5b-9. The chlamydial serine protease CPAF contributed to C3-independent C5 activation during C. trachomatis infection. Nafamostat mesylate, a serine protease inhibitor with a good safety profile, had a strong inhibitory effect on C5 activation induced by chlamydial infection. These discoveries reveal the mechanism of C3-independent C5 activation induced by chlamydial infection, and furthermore provide a potential therapeutic target and drug for preventing tubal fibrosis caused by chlamydial infection.

Bacteria-Mediated Oncogenesis and the Underlying Molecular Intricacies: What We Know So Far

Cancers are known to have multifactorial etiology. Certain bacteria and viruses are proven carcinogens. Lately, there has been in-depth research investigating carcinogenic capabilities of some bacteria. Reports indicate that chronic inflammation and harmful bacterial metabolites to be strong promoters of neoplasticity. Helicobacter pylori-induced gastric adenocarcinoma is the best illustration of the chronic inflammation paradigm of oncogenesis. Chronic inflammation, which produces excessive reactive oxygen species (ROS) is hypothesized to cause cancerous cell proliferation. Other possible bacteria-dependent mechanisms and virulence factors have also been suspected of playing a vital role in the bacteria-induced-cancer(s). Numerous attempts have been made to explore and establish the possible relationship between the two. With the growing concerns on anti-microbial resistance and over-dependence of mankind on antibiotics to treat bacterial infections, it must be deemed critical to understand and identify carcinogenic bacteria, to establish their role in causing cancer.

Modelling Chlamydia and HPV co-infection in patient-derived ectocervix organoids reveals distinct cellular reprogramming

Coinfections with pathogenic microbes continually confront cervical mucosa, yet their implications in pathogenesis remain unclear. Lack of in-vitro models recapitulating cervical epithelium has been a bottleneck to study coinfections. Using patient-derived ectocervical organoids, we systematically modeled individual and coinfection dynamics of Human papillomavirus (HPV)16 E6E7 and Chlamydia, associated with carcinogenesis. The ectocervical stem cells were genetically manipulated to introduce E6E7 oncogenes to mimic HPV16 integration. Organoids from these stem cells develop the characteristics of precancerous lesions while retaining the self-renewal capacity and organize into mature stratified epithelium similar to healthy organoids. HPV16 E6E7 interferes with Chlamydia development and induces persistence. Unique transcriptional and post-translational responses induced by Chlamydia and HPV lead to distinct reprogramming of host cell processes. Strikingly, Chlamydia impedes HPV-induced mechanisms that maintain cellular and genome integrity, including mismatch repair in the stem cells. Together, our study employing organoids demonstrates the hazard of multiple infections and the unique cellular microenvironment they create, potentially contributing to neoplastic progression.

Here, Koster et al., model human papillomavirus and Chlamydia coinfection dynamics in patient-derived ectocervical organoids, and characterize the effects of multiple infections in the cellular microenvironment, potentially contributing to neoplasia.

Phagosome maturation and modulation of macrophage effector function by intracellular pathogens: target for therapeutics

Macrophages are important cells that regulate various innate functions. Macrophages after engulfment of pathogens proceed for phagosome maturation and finally fuse with lysosomes to kill pathogens. Although pathogen degradation is one of the important functions of phagosomes, various immune-effector functions of macrophages are also dependent on the phagosome maturation process. This review discusses signaling processes regulating phagosome maturation as well as various effector functions of macrophages such as apoptosis, antigen presentation, autophagy and inflammasome that are dependent on the phagosome maturation process. It also discusses strategies adopted by various intracellular pathogens to counteract these functions to evade intracellular destruction mechanisms. These studies may give direction for the development of new therapeutics to control various intracellular infections.

The role of an enzymatically inactive CPAF mutant vaccination in Chlamydia muridarum genital tract infection

Chlamydia trachomatis urogenital tract infection causes pelvic inflammatory disease and infertility, increases the risk of co-infection with HPV and HIV. Chlamydial vaccination is considered the most promising approach to prevent and control its infection. Among various chlamydial vaccine candidates, chlamydial protease-like activity factor (CPAF) have been reported to provide robust protective immunity against genital chlamydial infection in mice with reduced vaginal shedding and oviduct pathology. However, CPAF is a serine protease which has enzymatical activity to degrade a large number of substrates. In order to increase the safety of CPAF vaccine, in this study, we used a mutant CPAF that is deficient in enzymatical activity to determine whether proteolytic activity of CPAF affect its vaccine efficacy. The wild type or mutant CPAF immunization causes a significant lower chlamydial shedding from the vaginal and resolve the infection as early as day 20, compared to day 28 in adjuvant control mice. More important, reduced upper reproductive tract pathology were also observed in these two groups. The mutant or wild type CPAF immunization induced not only robust splenic IFN-γ and serum IgG2a but also sIgA secretion in the vaginal fluids. Furthermore, neutralization of chlamydia with immune sera did not provide protection against oviduct pathology. However, adoptive transfer of CD4⁺ splenocytes isolated from the mutant or wild type CPAF immunized mice resulted in a significant and comparable reduced oviduct pathology. Our results indicate mutant CPAF vaccination is as same efficacy as wild type, and the protection relies on CD4⁺ T cells, which will further promote the development of CPAF as clinical chlamydial vaccine.

GlgA plays an important role in the induction of hydrosalpinx by Chlamydia muridarum

While glycogen synthase A deficiency can reduce the growth and proliferation of Chlamydia muridarum, the effect of glycogen synthase A on the pathogenic process of C. muridarum remains unclear. To characterize the effect of glycogen synthase A deficiency on the pathogenicity of C. muridarum in the genital tract, BALB/c mice were intravaginally inoculated with wild-type, plasmid-free and glycogen synthase A-deficient C. muridarum, and the genital tract tissue was isolated to assess the severity of hydrosalpinx and the levels of oviduct dilatation at day 60 after infection. The glycogen storage capacity and in vitro infection ability of different C. muridarum strains were analyzed by periodic acid-Schiff staining and quantification of progeny elementary body(EB) formation. The tissue homogenate was used to determine the recovery of different C. muridarum strains. The results show that glycogen synthase A-deficient C. muridarum induced reduction of hydrosalpinx and attenuated the extent of oviduct dilatation in mice, and exhibited reduced growth and proliferation in the mouse lower genital tract. In addition, glycogen synthase A point mutations at different sites reduced the glycogen storage capacity and in vitro infectivity of C. muridarum to different degrees. Glycogen synthase A deficiency also reduced the host inflammatory reaction and ascending infection of C. muridarum.

Chlamydia trachomatis Infection Impairs MHC-I Intracellular Trafficking and Antigen Cross-Presentation by Dendritic Cells

During cross-presentation, exogenous antigens (i.e. intracellular pathogens or tumor cells) are internalized and processed within the endocytic system and also by the proteasome in the cytosol. Then, antigenic peptides are associated with Major Histocompatibility Complex (MHC) class I molecules and these complexes transit to the plasma membrane in order to trigger cytotoxic immune responses through the activation of CD8+ T lymphocytes. Dendritic cells (DCs) are particularly adapted to achieve efficient antigen cross-presentation and their endocytic network displays important roles during this process, including a sophisticated MHC-I transport dependent on recycling compartments. In this study, we show that C. trachomatis, an obligate intracellular pathogen that exhibits multiple strategies to evade the immune system, is able to induce productive infections in the murine DC line JAWS-II. Our results show that when C. trachomatis infects these cells, the bacteria-containing vacuole strongly recruits host cell recycling vesicles, but no other endosomal compartments. Furthermore, we found that chlamydial infection causes significant alterations of MHC-I trafficking in JAWS-II DCs: reduced levels of MHC-I expression at the cell surface, disruption of the perinuclear MHC-I intracellular pool, and impairment of MHC-I endocytic recycling to the plasma membrane. We observed that all these modifications lead to a hampered cross-presentation ability of soluble and particulate antigens by JAWS-II DCs and primary bone marrow-derived DCs. In summary, our findings provide substantial evidence that C. trachomatis hijacks the DC endocytic recycling system, causing detrimental changes on MHC-I intracellular transport, which are relevant for competent antigen cross-presentation.

Eutopic endometrium from women with endometriosis and chlamydial endometritis share immunological cell types and DNA repair imbalance: A transcriptome meta-analytical perspective

The aim of this study was to identify the key similarities between the eutopic endometrium of women with endometriosis and chlamydia-induced endometritis taking into account tissue microenvironment heterogeneity, transcript gene profile, and enriched pathways. A meta-analysis of whole transcriptome microarrays was performed using publicly available data, including samples containing both glandular and stromal endometrial components. Control samples were obtained from women without any reported pathological condition. Only samples obtained during the proliferative menstrual phase were included. Cellular tissue heterogeneity was predicted using a method that integrates gene set enrichment and deconvolution approaches. The batch effect was estimated by principal variant component analysis and removed using an empirical Bayes method. Differentially expressed genes were identified using an adjusted p-value < 0.05 and fold change = 1.5. The protein-protein interaction network was built using the STRING database and interaction score over 400. The Molecular Signatures Database was used to analyse the functional enrichment analysis. Both conditions showed similarities in cell types in the microenvironment, particularly CD4⁺ and CD8⁺ Tem cells, NKT cells, Th2 cells, basophils, and eosinophils. With regards to the regulation of cellular senescence and DNA integrity/damage checkpoint, which are commonly enriched pathways, 21 genes were down-regulated and directly related to DNA repair. Compared to the endometriosis samples, some chlamydial endometritis samples presented a lack of enriched immune pathways. Our results suggest that both conditions show similar distributions of microenvironment cell types, the downregulation of genes involved in DNA repair and cell cycle control, and pathways involved in immune response evasion.

An endometrial organoid model of interactions between Chlamydia and epithelial and immune cells

Our understanding of how the obligate intracellular bacterial pathogen Chlamydia trachomatis reprograms the function of infected cells in the upper genital tract is largely based on observations made in cell culture with transformed epithelial cell lines. Here, we describe a primary organoid system derived from endometrial tissue to recapitulate epithelial cell diversity, polarity and ensuing responses to Chlamydia infection. Using high-resolution and time-lapse microscopy, we catalog the infection process in organoids from invasion to egress, including the reorganization of the cytoskeleton and positioning of intracellular organelles. We show this model is amenable to screening C. trachomatis mutants for defects in the fusion of pathogenic vacuoles, the recruitment of intracellular organelles and inhibition of cell death. Moreover, we reconstructed a primary immune cell response by co-culturing infected organoids with neutrophils, and determined that effectors like CPAF (also known as CT858) and TepP (also known as CT875) limit the recruitment of neutrophils to infected organoids. Collectively, our model can be applied to study the cell biology of Chlamydia infections in three-dimensional structures that better reflect the diversity of cell types and polarity encountered by Chlamydia in their animal hosts.

Human Fallopian Tube Epithelial Cell Culture Model To Study Host Responses to Chlamydia trachomatis Infection

Chlamydia trachomatis infection of the human fallopian tubes can lead to damaging inflammation and scarring, ultimately resulting in infertility. To study the human cellular responses to chlamydial infection, researchers have frequently used transformed cell lines that can have limited translational relevance. We developed a primary human fallopian tube epithelial cell model based on a method previously established for culture of primary human bronchial epithelial cells. After protease digestion and physical dissociation of excised fallopian tubes, epithelial cell precursors were expanded in growth factor-containing medium. Expanded cells were cryopreserved to generate a biobank of cells from multiple donors and cultured at an air-liquid interface. Culture conditions stimulated cellular differentiation into polarized mucin-secreting and multiciliated cells, recapitulating the architecture of human fallopian tube epithelium. The polarized and differentiated cells were infected with a clinical isolate of C. trachomatis, and inclusions containing chlamydial developmental forms were visualized by fluorescence and electron microscopy. Apical secretions from infected cells contained increased amounts of proteins associated with chlamydial growth and replication, including transferrin receptor protein 1, the amino acid transporters SLC3A2 and SLC1A5, and the T-cell chemoattractants CXCL10, CXCL11, and RANTES. Flow cytometry revealed that chlamydial infection induced cell surface expression of T-cell homing and activation proteins, including ICAM-1, VCAM-1, HLA class I and II, and interferon gamma receptor. This human fallopian tube epithelial cell culture model is an important tool with translational potential for studying cellular responses to Chlamydia and other sexually transmitted pathogens.

Eukaryotic Cell Permeabilisation to Identify New Putative Chlamydial Type III Secretion System Effectors Secreted within Host Cell Cytoplasm

Chlamydia trachomatis and Waddlia chondrophila are strict intracellular bacteria belonging to the Chlamydiales order. C. trachomatis is the most frequent bacterial cause of genital and ocular infections whereas W. chondrophila is an opportunistic pathogen associated with adverse pregnancy outcomes and respiratory infections. Being strictly intracellular, these bacteria are engaged in a complex interplay with their hosts to modulate their environment and create optimal conditions for completing their life cycle. For this purpose, they possess several secretion pathways and, in particular, a Type III Secretion System (T3SS) devoted to the delivery of effector proteins in the host cell cytosol. Identifying these effectors is a crucial step in understanding the molecular basis of bacterial pathogenesis. Following incubation of infected cells with perfringolysin O, a pore-forming toxin that binds cholesterol present in plasma membranes, we analysed by mass spectrometry the protein content of the host cell cytoplasm. We identified 13 putative effectors secreted by C. trachomatis and 19 secreted by W. chondrophila. Using Y. enterocolitica as a heterologous expression and secretion system, we confirmed that four of these identified proteins are secreted by the T3SS. Two W. chondrophila T3SS effectors (hypothetical proteins Wcw_0499 and Wcw_1706) were further characterised and demonstrated to be early/mid-cycle effectors. In addition, Wcw_1706 is associated with a tetratricopeptide domain-containing protein homologous to C. trachomatis class II chaperone. Furthermore, we identified a novel C. trachomatis effector, CT460 that localises in the eukaryotic nucleus when ectopically expressed in 293 T cells.

Chlamydia trachomatis Whole-Proteome Microarray Analysis of The Netherlands Chlamydia Cohort Study

Chlamydia trachomatis (Ct) whole-proteome microarrays were utilized to identify antibody patterns associated with infection; pelvic inflammatory disease (PID), tubal factor infertility, chronic pelvic pain (CPP) and ectopic pregnancy in a subsample of the Netherlands Chlamydia cohort study. Serum pools were analyzed on whole-proteome arrays. The 121 most reactive antigens identified during whole-proteome arrays were selected for further analysis with minimized microarrays that allowed for single sera analysis. From the 232 single sera; 145 (62.5%) serum samples were reactive for at least one antigen. To discriminate between positive and negative serum samples; we created a panel of in total 18 antigens which identified 96% of all microarray positive samples. Antigens CT_858; CT_813 and CT_142 were most reactive. Comparison of antibody reactivity's among women with and without Ct related sequelae revealed that the reactivity of CT_813 and CT_142 was less common among women with PID compared to women without (29.0% versus 58.6%, p = 0.005 and 25.8% versus 50.6%, p = 0.017 respectively). CT_858 was less common among CPP cases compared to controls (33.3% versus 58.6; p = 0.028). Using a whole-proteome array to select antigens for minimized arrays allows for the identification of novel informative antigens as general infection markers or disease associated antigens.

Navigating to the most promising directions amid complex fields of vaccine development: a chlamydial case study

Background: Vaccine-development research is proliferating making it difficult to determine the most promising vaccine candidates. Exemplary of this problem is vaccine development against Chlamydia, a pathogen of global public health and financial importance.Methods: We systematically extracted data from studies that included chlamydial load or host immune parameter measurements, estimating 4,453 standardized effect sizes between control and chlamydial immunization experimental groups.Results: Chlamydial immunization studies most often used (78%) laboratory mouse models. Depending on chlamydial species, single and multiple recombinant protein, viral and bacterial vectors, dendritic transfer, and dead whole pathogen were most effective at reducing chlamydial load. Immunization-driven decrease in chlamydial load was associated with increases in IFNg, IgA, IgG1, and IgG2a. Using data from individual studies, the magnitude of IgA and IgG2a increase was correlated with chlamydial load reduction. IFNg also showed this pattern for C. trachomatis, but not for C. muridarum. We also reveal the chlamydial vaccine development field to be highly bias toward studies showing these effects, limiting lessons learned from negative results.Conclusions: Most murine immunizations against Chlamydia reduced chlamydial load and increased host immune parameters. These methods are novel for vaccine development and are critical in identifying trends where large quantities of literature exist.

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.

Insights into the Autoproteolytic Processing and Catalytic Mechanism of the Chlamydia trachomatis Virulence-Associated Protease CPAF

CPAF (chlamydial protease-like activity factor) is a Chlamydia trachomatis protease that is translocated into the host cytosol during infection. CPAF activity results in dampened host inflammation signaling, cytoskeletal remodeling, and suppressed neutrophil activation. Although CPAF is an emerging antivirulence target, its catalytic mechanism has been unexplored to date. Steady state kinetic parameters were obtained for recombinant CPAF with vimentin-derived peptide substrates using a high-performance liquid chromatography-based discontinuous assay (k_cat = 45 ± 0.6 s^-1; k_cat/K_m = 0.37 ± 0.02 μM^-1 s^-1) or a new fluorescence-based continuous assay (k_cat = 23 ± 0.7 s^-1; k_cat/K_m = 0.29 ± 0.03 μM^-1 s^-1). Residues H105, S499, E558, and newly identified D103 were found to be indispensable for autoproteolytic processing by mutagenesis, while participation of C500 was ruled out despite its proximity to the S499 nucleophile. Pre-steady state kinetics indicated a burst kinetic profile, with fast acylation (k_acyl = 110 ± 2 s^-1) followed by slower, partially rate-limiting deacylation (k_deacyl = 57 ± 1 s^-1). Both k_cat- and k_cat/K_m-pH profiles showed single acidic limb ionizations with pK_a values of 6.2 ± 0.1 and 6.5 ± 0.1, respectively. A forward solvent deuterium kinetic isotope effect of 2.6 ± 0.1 was observed for ^D₂Ok_cat^app, but a unity effect was found for ^D₂Ok_cat/K_m^app. The k_cat proton inventory was linear, indicating transfer of a single proton in the rate-determining transition state, most likely from H105. Collectively, these data provide support for the classification of CPAF as a serine protease and provide a mechanistic foundation for the future design of inhibitors.

Detection of Chlamydia Developmental Forms and Secreted Effectors by Expansion Microscopy

Expansion microscopy (ExM) is a novel tool to improve the resolution of fluorescence-based microscopy that has not yet been used to visualize intracellular pathogens. Here we show the expansion of the intracellular pathogen Chlamydia trachomatis, enabling to differentiate its two distinct forms, catabolic active reticulate bodies (RB) and infectious elementary bodies (EB), on a conventional confocal microscope. We show that ExM enables the possibility to precisely locate chlamydial effector proteins, such as CPAF or Cdu1, within and outside of the chlamydial inclusion. Thus, we claim that ExM offers the possibility to address a broad range of questions and may be useful for further research on various intracellular pathogens.

Distinct Roles of Chromosome- versus Plasmid-Encoded Genital Tract Virulence Factors in Promoting Chlamydia muridarum Colonization in the Gastrointestinal Tract

The genital pathogen Chlamydia is known to colonize the gastrointestinal tract. Orally delivered Chlamydia muridarum can reach the colon and maintain a long-lasting colonization there. However, C. muridarum with mutations in chromosomal genes tc0237 and tc0668 (designated a chromosomal mutant) or deficient in plasmid-encoded pGP3 (designated a plasmid mutant) is unable to do so. We now report that the chromosomal mutant is still able to reach the colon while the plasmid mutant fails to do so following an oral delivery, suggesting that lack of colon colonization by different mutants may involve distinct mechanisms. Consistently, a direct intracolonic delivery selectively restored the ability of the plasmid mutant, but not the chromosomal mutant, to colonize the colon. The chromosomal mutant was rescued only in the colon of mice deficient in gamma interferon (IFN-γ). Thus, the chromosomal mutant's deficiency in colonizing colonic mucosal tissue is likely due to its increased susceptibility to IFN-γ-mediated immunity. Furthermore, IFN-γ deficiency was sufficient for rescuing colon colonization of an orally delivered chromosomal mutant but not plasmid mutant while mice deficient in gastric acid production rescued the plasmid mutant but not the chromosomal mutant. Both mutants are attenuated in inducing genital tract pathology. Thus, we propose that chlamydial chromosomal-gene-encoded genital tract virulence factors may be essential for Chlamydia to maintain long-lasting colonization in the colon while the plasmid may enable Chlamydia to reach the colon by promoting evasion of gastric barriers.

Chlamydia muridarum Induces Pathology in the Female Upper Genital Tract via Distinct Mechanisms

Sexually transmitted infection with Chlamydia trachomatis may lead to fibrotic blockage in women's upper genital tracts, resulting in tubal infertility. Intravaginal inoculation with C. muridarum readily induces fibrotic blockage or hydrosalpinx in mice and is used for investigating C. trachomatis pathogenicity. Using this model in combination with an antibody depletion approach, we confirmed CD4⁺ T cell-mediated protective immunity and a CD8⁺ T cell-dependent pathogenic mechanism during chlamydial infection in C57BL/6J mice. However, when mice genetically deficient in CD8⁺ T cells were evaluated, we found, surprisingly, that these mice were still able to develop robust hydrosalpinx following C. muridarum infection, both contradicting the observation made in C57BL/6J mice and suggesting a pathogenic mechanism that is independent of CD8⁺ T cells. We further found that depletion of CD4⁺ T cells from CD8⁺ T cell-deficient mice significantly reduced chlamydial induction of hydrosalpinx, indicating that CD4⁺ T cells became pathogenic in mice genetically deficient in CD8⁺ T cells. Since depletion of CD4⁺ T cells both promoted chlamydial infection and reduced chlamydial pathogenicity in CD8⁺ T cell-deficient mice, we propose that in the absence of CD8⁺ T cells, some CD4⁺ T cells may remain protective (as in C57BL/6J mice), while others may directly contribute to chlamydial pathogenicity. Thus, chlamydial pathogenicity can be mediated by distinct host mechanisms, depending upon host genetics and infection conditions. The CD8⁺ T cell-deficient mouse model may be useful for further investigating the mechanisms by which CD4⁺ T cells promote chlamydial pathogenicity.

Antigen-Specific CD4+ T Cell-Derived Gamma Interferon Is Both Necessary and Sufficient for Clearing Chlamydia from the Small Intestine but Not the Large Intestine

The genital tract pathogen Chlamydia trachomatis is frequently detected in the gastrointestinal tract, but the host immunity that regulates chlamydial colonization in the gut remains unclear. In a Chlamydia muridarum-C57 mouse model, chlamydial organisms are cleared from the genital tract in ∼4 weeks, but the genital organisms can spread to the gastrointestinal tract. We found that the gastrointestinal chlamydial organisms were cleared from the small intestine by day 28, paralleling their infection course in the genital tract, but persisted in the large intestine for long periods. Mice deficient in α/β T cells or CD4+ T cells but not CD8+ T cells showed chlamydial persistence in the small intestine, indicating a critical role for CD4+ T cells in clearing Chlamydia from the small intestine. The CD4+ T cell-dependent clearance is likely mediated by gamma interferon (IFN-γ), since mice deficient in IFN-γ but not interleukin 22 (IL-22) signaling pathways rescued chlamydial colonization in the small intestine. Furthermore, exogenous IFN-γ was sufficient for clearing Chlamydia from the small intestine but not the large intestine. Mice deficient in developing Chlamydia-specific Th1 immunity showed chlamydial persistence in the small intestine. Finally, IFN-γ-producing CD4+ but not CD8+ T cells from immunized donor mice were sufficient for eliminating Chlamydia from the small intestine but not the large intestine of recipient mice. Thus, we have demonstrated a critical role for Th1 immunity in clearing Chlamydia from the small intestine but not the large intestine, indicating that chlamydial colonization in different regions of the gastrointestinal tract is regulated by distinct immune mechanisms.

Transcriptional Expression of the ompA, cpaf, tarp, and tox Genes of Chlamydia trachomatis Clinical Isolates at Different Stages of the Developmental Cycle

The transcriptional gene expression patterns of Chlamydia trachomatis have mainly been studied using reference strains propagated in cultured cells. Here, using five low-passage-number C. trachomatis clinical isolates that originated from asymptomatic or symptomatic female patients, the in vitro expression of the ompA, cpaf, tarp, and tox genes was studied with reverse transcriptase real-time PCR during the chlamydial developmental cycle. We observed dissimilarities in the gene expression patterns between the low-passage-number clinical isolates and the reference strains. The expression of ompA and the peak of the tox expression were observed earlier in the reference strains than in most of the clinical isolates. The expression of cpaf was high in the reference strains compared with the clinical isolates at the mid-phase (6–24 hours post infection) of the developmental cycle. All of the strains had a rather similar tarp expression profile. Four out of five clinical isolates exhibited slower growth kinetics compared with the reference strains. The use of low-passage-number C. trachomatis clinical isolates instead of reference strains in the studies might better reflect the situation in human infection.

The Structural Integrity of Plasmid-Encoded Pgp3 Is Essential for Induction of Hydrosalpinx by Chlamydia muridarum

Pgp3 consists of globular N- and C-terminal domains connected by a triple-helical coiled-coil middle domain. We demonstrated previously that Pgp3 is required for induction of hydrosalpinx by Chlamydia muridarum. We constructed C. muridarum transformants harboring deletion of the Pgp3 N-terminus (pgp3Δn), C-terminus (pgp3Δc), or middle domain (pgp3Δm). C3H/HeJ and CBA/J mice infected with pgp3Δn or pgp3Δm failed to induce hydrosalpinx in oviduct tissue. However, the pgp3Δc transformant induced mild hydrosalpinx in 20% of C3H/HeJ mice (severity score 0.2 ± 0.6) and in 40% of CBA/J mice (severity score 0.8 ± 1.3). The attenuated pathogenicity of the transformants harboring Pgp3 domain deletions was correlated with impaired in vitro growth and significantly reduced infectivity in the mouse lower genital tract. Moreover, the oviduct tissue of C3H/HeJ and CBA/J mice infected with the Pgp3-domain-deficient transformants displayed less inflammatory cell infiltration. Thus, the structural integrity of plasmid-encoded Pgp3 is essential for induction of hydrosalpinx by C. muridarum.

Infection of HeLa cells with Chlamydia trachomatis inhibits protein synthesis and causes multiple changes to host cell pathways

The obligate intracellular bacterium Chlamydia trachomatis replicates in a cytosolic vacuole in human epithelial cells. Infection of human cells with C. trachomatis causes substantial changes to many host cell-signalling pathways, but the molecular basis of such influence is not well understood. Studies of gene transcription of the infected cell have shown altered transcription of many host cell genes, indicating a transcriptional response of the host cell to the infection. We here describe that infection of HeLa cells with C. trachomatis as well as infection of murine cells with Chlamydia muridarum substantially inhibits protein synthesis of the infected host cell. This inhibition was accompanied by changes to the ribosomal profile of the infected cell indicative of a block of translation initiation, most likely as part of a stress response. The Chlamydia protease-like activity factor (CPAF) also reduced protein synthesis in uninfected cells, although CPAF-deficient C. trachomatis showed no defect in this respect. Analysis of polysomal mRNA as a proxy of actively transcribed mRNA identified a number of biological processes differentially affected by chlamydial infection. Mapping of differentially regulated genes onto a protein interaction network identified nodes of up- and down-regulated networks during chlamydial infection. Proteomic analysis of protein synthesis further suggested translational regulation of host cell functions by chlamydial infection. These results demonstrate reprogramming of the host cell during chlamydial infection through the alteration of protein synthesis.

Poly(ADP-Ribose) Polymerases in Host-Pathogen Interactions, Inflammation, and Immunity

The literature review presented here details recent research involving members of the poly(ADP-ribose) polymerase (PARP) family of proteins. Among the 17 recognized members of the family, the human enzyme PARP1 is the most extensively studied, resulting in a number of known biological and metabolic roles. This review is focused on the roles played by PARP enzymes in host-pathogen interactions and in diseases with an associated inflammatory response. In mammalian cells, several PARPs have specific roles in the antiviral response; this is perhaps best illustrated by PARP13, also termed the zinc finger antiviral protein (ZAP). Plant stress responses and immunity are also regulated by poly(ADP-ribosyl)ation. PARPs promote inflammatory responses by stimulating proinflammatory signal transduction pathways that lead to the expression of cytokines and cell adhesion molecules. Hence, PARP inhibitors show promise in the treatment of inflammatory disorders and conditions with an inflammatory component, such as diabetes, arthritis, and stroke. These functions are correlated with the biophysical characteristics of PARP family enzymes. This work is important in providing a comprehensive understanding of the molecular basis of pathogenesis and host responses, as well as in the identification of inhibitors. This is important because the identification of inhibitors has been shown to be effective in arresting the progression of disease.

Immunobiology of monocytes and macrophages during Chlamydia trachomatis infection

Infections caused by the intracellular bacterium Chlamydia trachomatis are a global health burden affecting more than 100 million people annually causing damaging long-lasting infections. In this review, we will present and discuss important aspects of the interaction between C. trachomatis and monocytes/macrophages.

CPAF, HSP60 and MOMP antigens elicit pro-inflammatory cytokines production in the peripheral blood mononuclear cells from genital Chlamydia trachomatis-infected patients

BACKGROUND

Persistent inflammation caused by Chlamydia trachomatis in the female genital compartment represents one of the major causes of pelvic inflammatory disease (PID), ectopic pregnancy and infertility in females. Here, we examined the pro-inflammatory cytokine response following stimulation with three different types of C. trachomatis antigens, viz. chlamydial protease-like factor (CPAF), heat shock protein 60 (HSP60) and major outer membrane protein (MOMP).

METHODS

A total of 19 patients with genital C. trachomatis infection and 10 age-matched healthy controls were recruited for the study. Peripheral blood mononuclear cells (PBMCs) isolated from genital C. trachomatis-infected females were cultured in the presence of CPAF, HSP60 and MOMP antigens, and cytokines were measured by ELISA assay.

RESULTS

We reported that pro-inflammatory cytokines (TNF-α, IL-1β and IL-6) were robustly secreted following antigenic exposure. Notably, CPAP and MOMP were more potent in triggering IL-1β, as compared to HSP60. Elevated levels of the proinflammatory cytokines were also noted in the samples infected with plasmid-bearing C. trachomatis as compared to those infected with plasmid-free strains.

CONCLUSIONS

Our study highlights distinct ability of chlamydial antigens in triggering pro-inflammatory response in the host immune cells.

Pathways of host cell exit by intracellular pathogens

Host cell exit is a critical step in the life-cycle of intracellular pathogens, intimately linked to barrier penetration, tissue dissemination, inflammation, and pathogen transmission. Like cell invasion and intracellular survival, host cell exit represents a well-regulated program that has evolved during host-pathogen co-evolution and that relies on the dynamic and intricate interplay between multiple host and microbial factors. Three distinct pathways of host cell exit have been identified that are employed by three different taxa of intracellular pathogens, bacteria, fungi and protozoa, namely (i) the initiation of programmed cell death, (ii) the active breaching of host cellderived membranes, and (iii) the induced membrane-dependent exit without host cell lysis. Strikingly, an increasing number of studies show that the majority of intracellular pathogens utilize more than one of these strategies, dependent on life-cycle stage, environmental factors and/or host cell type. This review summarizes the diverse exit strategies of intracellular-living bacterial, fungal and protozoan pathogens and discusses the convergently evolved commonalities as well as system-specific variations thereof. Key microbial molecules involved in host cell exit are highlighted and discussed as potential targets for future interventional approaches.

Peptidase Inhibitor 15 (PI15) Regulates Chlamydial CPAF Activity

Obligate intracellular pathogenic Chlamydia trachomatis express several serine proteases whose roles in chlamydial development and pathogenicity are not completely understood. The chlamydial protease CPAF is expressed during the replicative phase of the chlamydial developmental cycle and is secreted into the lumen of the Chlamydia-containing vacuole called inclusion. How the secreted protease is activated in the inclusion lumen is currently not fully understood. We have identified human serine peptidase inhibitor PI15 as a potential host factor involved in the regulation of CPAF activation. Silencing expression as well as over expression of PI15 affected normal development of Chlamydia. PI15 was transported into the chlamydial inclusion lumen where it co-localized with CPAF aggregates. We show that PI15 binds to the CPAF zymogen and potentially induces CPAF protease activity at low concentrations. However, at high concentrations PI15 inhibits CPAF activity possibly by blocking its protease domain. Our findings shed light on a new aspect of chlamydial host co-evolution which involves the recruitment of host cell proteins into the inclusion to control the activation of bacterial proteases like CPAF that are important for the normal development of Chlamydia.

Plasmid Negative Regulation of CPAF Expression Is Pgp4 Independent and Restricted to Invasive Chlamydia trachomatis Biovars

Chlamydia trachomatis is an obligate intracellular bacterial pathogen that causes blinding trachoma and sexually transmitted disease. C. trachomatis isolates are classified into 2 biovars—lymphogranuloma venereum (LGV) and trachoma—which are distinguished biologically by their natural host cell infection tropism. LGV biovars infect macrophages and are invasive, whereas trachoma biovars infect oculo-urogenital epithelial cells and are noninvasive. The C. trachomatis plasmid is an important virulence factor in the pathogenesis of these infections. Central to its pathogenic role is the transcriptional regulatory function of the plasmid protein Pgp4, which regulates the expression of plasmid and chromosomal virulence genes. As many gene regulatory functions are post-transcriptional, we employed a comparative proteomic study of cells infected with plasmid-cured C. trachomatis serovars A and D (trachoma biovar), a L2 serovar (LGV biovar), and the L2 serovar transformed with a plasmid containing a nonsense mutation in pgp4 to more completely elucidate the effects of the plasmid on chlamydial infection biology. Our results show that the Pgp4-dependent elevations in the levels of Pgp3 and a conserved core set of chromosomally encoded proteins are remarkably similar for serovars within both C. trachomatis biovars. Conversely, we found a plasmid-dependent, Pgp4-independent, negative regulation in the expression of the chlamydial protease-like activity factor (CPAF) for the L2 serovar but not the A and D serovars. The molecular mechanism of plasmid-dependent negative regulation of CPAF expression in the LGV serovar is not understood but is likely important to understanding its macrophage infection tropism and invasive infection nature.

The Chlamydia trachomatis plasmid is an important virulence factor in the pathogenesis of chlamydial infection. It is known that plasmid protein 4 (Pgp4) functions in the transcriptional regulation of the plasmid virulence protein 3 (Pgp3) and multiple chromosomal loci of unknown function. Since many gene regulatory functions can be post-transcriptional, we undertook a comparative proteomic analysis to better understand the plasmid’s role in chlamydial and host protein expression. We report that Pgp4 is a potent and specific master positive regulator of a common core of plasmid and chromosomal virulence genes shared by multiple C. trachomatis serovars. Notably, we show that the plasmid is a negative regulator of the expression of the chlamydial virulence factor CPAF. The plasmid regulation of CPAF is independent of Pgp4 and restricted to a C. trachomatis macrophage-tropic strain. These findings are important because they define a previously unknown role for the plasmid in the pathophysiology of invasive chlamydial infection.

Development of a Chlamydia suis-specific antibody enzyme-linked immunosorbent assay based on the use of a B-cell epitope of the polymorphic membrane protein C

Chlamydia suis infections lead to economic loss in the pork industry. Chlamydia suis infections could be successfully treated with tetracyclines until the appearance of a tetracycline resistant phenotype, which was acquired via horizontal gene transfer of the tet(C) gene. Given the importance of C. suis as a swine pathogen and as a recently emerged tetracycline resistant pathogen with zoonotic potential, our aim was to develop a sensitive C. suis-specific antibody ELISA based on the polymorphic membrane proteins (Pmps). Chlamydia Pmps are important virulence factors and candidate antigens for serodiagnosis. We identified nine Pmps (PmpA to I) in C. suis strain MD56 using a recently developed Hidden-Markov model. PmpC was the most promising candidate for the development of a C. suis-specific antibody ELISA as the protein was absent in C. abortus, C. pecorum and C. psittaci which also infect pigs and as the protein contained C. suis-specific amino acid regions, absent in C. trachomatis PmpC. We identified an immunodominant B-cell epitope in C. suis PmpC using experimental porcine sera. The sensitivity and specificity of the PmpC ELISA was compared to the complement fixation test (CFT) and to a recombinant MOMP ELISA using experimental sera. The PmpC ELISA detected all positive control sera and was in contrast to CFT and the rMOMP ELISA 100% C. suis specific as positive control sera against other Chlamydia species did not react in the PmpC ELISA. The test was successfully validated using slaughterhouse sera and sera from clinically affected pigs. The PmpC ELISA could assist in diminishing the spread of C. suis infections in the pork industry.

The Genital Tract Virulence Factor pGP3 Is Essential for Chlamydia muridarum Colonization in the Gastrointestinal Tract

The cryptic plasmid is essential for Chlamydia muridarum dissemination from the genital tract to the gastrointestinal (GI) tract. Following intravaginal inoculation, a C. muridarum strain deficient in plasmid-encoded pGP3 or pGP4 but not pGP5, pGP7, or pGP8 failed to spread to the mouse gastrointestinal tract, although mice infected with these strains developed productive genital tract infections. pGP3- or pGP4-deficient strains also failed to colonize the gastrointestinal tract when delivered intragastrically. pGP4 regulates pGP3, while pGP3 does not affect pGP4 expression, indicating that pGP3 is critical for C. muridarum colonization of the gastrointestinal tract. Mutants deficient in GlgA, a chromosome-encoded protein regulated by pGP4, also consistently colonized the mouse gastrointestinal tract. Interestingly, C. muridarum colonization of the gastrointestinal tract positively correlated with pathogenicity in the upper genital tract. pGP3-deficient C. muridarum strains did not induce hydrosalpinx or spread to the GI tract even when delivered to the oviduct by intrabursal inoculation. Thus, the current study not only has revealed that pGP3 is a novel chlamydial colonization factor in the gastrointestinal tract but also has laid a foundation for investigating the significance of gastrointestinal Chlamydia.

IL-6-mediated signaling pathways limit Chlamydia muridarum infection and exacerbate its pathogenicity in the mouse genital tract

Chlamydia muridarum induction of mouse hydrosalpinx, depending on both tubal infection and inflammation, has been used for investigating Chlamydia trachomatis pathogenesis. We now report that IL-6 both inhibits C. muridarum infection and exacerbates pathogenicity in the mouse genital tract. When intravaginally inoculated with a high dose of C. muridarum, IL-6-deficient mice developed more extensive genital tract infection with severe hydrosalpinx, suggesting that IL-6 is required for controlling the high dose infection but not essential for C. muridarum-induced pathology. However, at a low dose, IL-6-deficient mice still developed more extensive infection in the genital tract but no longer with significant pathology, suggesting that IL-6 is required for both controlling the low dose infection and exacerbating the low dose infection-induced pathology. The lack of hydrosalpinx in IL-6-deficient mice correlated with significantly reduced inflammatory infiltration in the oviduct tissue and decreased spleen CD4+ and CD8+ T cells that produce TNFα. Thus, IL-6-dependent pathways are important for both limiting chlamydial colonization in the genital tract mucosal tissues regardless of the infection doses and exacerbating chlamydial pathogenicity in the upper genital tract when IL-6-independent pathogenic mechanisms are not yet activated with a low infection dose.

National Institute of Allergy and Infectious Diseases workshop report: "Chlamydia vaccines: The way forward"

Chlamydia trachomatis (Ct), an intracellular pathogen, is the most common bacterial sexually transmitted infection. In addition to acute cervicitis and urethritis, Ct can lead to serious sequelae of significant public health burden including pelvic inflammatory disease (PID) and infertility. Ct control efforts have not resulted in desired outcomes such as reduced incidence and reinfection, and this highlights the need for the development of an effective Ct vaccine. To this end, NIAID organized a workshop to consider the current status of Ct vaccine research and address critical questions in Ct vaccine design and clinical testing. Topics included the goal(s) of a vaccine and the feasibility of achieving these goals, animal models of infection including mouse and nonhuman primate (NHP) models, and correlates of protection to guide vaccine design. Decades of research have provided both whole cell-based and subunit vaccine candidates for development. At least one is currently in clinical development and efforts now need to be directed toward further development of the most attractive candidates. Overall, the discussions and presentations from the workshop highlighted optimism about the current status of Ct vaccine research and detailed the remaining gaps and questions needed to move vaccines forward.

Chlamydia trachomatis: the Persistent Pathogen

Chlamydia trachomatis is an obligate intracellular bacterium whose only natural host is humans. Although presenting as asymptomatic in most women, genital tract chlamydial infections are a leading cause of pelvic inflammatory disease, tubal factor infertility, and ectopic pregnancy. C. trachomatis has evolved successful mechanisms to avoid destruction by autophagy and the host immune system and persist within host epithelial cells. The intracellular form of this organism, the reticulate body, can enter into a persistent nonreplicative but viable state under unfavorable conditions. The infectious form of the organism, the elementary body, is again generated when the immune attack subsides. In its persistent form, C. trachomatis ceases to produce its major structural and membrane components, but synthesis of its 60-kDa heat shock protein (hsp60) is greatly upregulated and released from the cell. The immune response to hsp60, perhaps exacerbated by repeated cycles of productive infection and persistence, may promote damage to fallopian tube epithelial cells, scar formation, and tubal occlusion. The chlamydial and human hsp60 proteins are very similar, and hsp60 is one of the first proteins produced by newly formed embryos. Thus, the development of immunity to epitopes in the chlamydial hsp60 that are also present in the corresponding human hsp60 may increase susceptibility to pregnancy failure in infected women. Delineation of host factors that increase the likelihood that C. trachomatis will avoid immune destruction and survive within host epithelial cells and utilization of this knowledge to design individualized preventative and treatment protocols are needed to more effectively combat infections by this persistent pathogen.

Crossing the border - Solute entry into the chlamydial inclusion

Chlamydiales comprise important human and animal pathogens as well as endosymbionts of amoebae. Generally, these obligate intracellular living bacteria are characterized by a biphasic developmental cycle, a reduced genome and a restricted metabolic capacity. Because of their metabolic impairment, Chlamydiales essentially rely on the uptake of diverse metabolites from their hosts. Chlamydiales thrive in a special compartment, the inclusion, and hence are surrounded by an additional membrane. Solutes might enter the inclusion through pores and open channels or by redirection of host vesicles, which fuse with the inclusion membrane and release their internal cargo. Recent investigations shed new light on the chlamydia-host interaction and identified an additional way for nutrient uptake into the inclusion. Proteome studies and targeting analyses identified chlamydial and host solute carriers in inclusions of Chlamydia trachomatis infected cells. These transporters are involved in the provision of UDP-glucose and biotin, and probably deliver further metabolites to the inclusion. By the controlled recruitment of specific solute carriers to the inclusion, the chlamydial resident thus can actively manipulate the metabolite availability and composition in the inclusion. This review summarizes recent findings and new ideas on carrier mediated solute uptake into the chlamydial inclusion in the context of the bacterial and host metabolism.

Molecular Genetic Analysis of Chlamydia Species

Species of Chlamydia are the etiologic agent of endemic blinding trachoma, the leading cause of bacterial sexually transmitted diseases, significant respiratory pathogens, and a zoonotic threat. Their dependence on an intracellular growth niche and their peculiar developmental cycle are major challenges to elucidating their biology and virulence traits. The last decade has seen tremendous advances in our ability to perform a molecular genetic analysis of Chlamydia species. Major achievements include the generation of large collections of mutant strains, now available for forward- and reverse-genetic applications, and the introduction of a system for plasmid-based transformation enabling complementation of mutations; expression of foreign, modified, or reporter genes; and even targeted gene disruptions. This review summarizes the current status of the molecular genetic toolbox for Chlamydia species and highlights new insights into their biology and new challenges in the nascent field of Chlamydia genetics.

Chlamydia muridarum with Mutations in Chromosomal Genes tc0237 and/or tc0668 Is Deficient in Colonizing the Mouse Gastrointestinal Tract

Chlamydiae colonize the gastrointestinal tracts of both animals and humans. However, their medical significance remains unknown. We have previously shown that wild-type Chlamydia muridarum spreads to and establishes stable colonization of the gastrointestinal tract following intravaginal inoculation. In the present study, we found that C. muridarum with mutations in chromosomal genes tc0237 and/or tc0668 was defective in spreading to the mouse gastrointestinal tract, which correlated with its attenuated pathogenicity in the upper genital tract. This correlation was more consistent than that of chlamydial pathogenicity with ascending infection in the genital tract, since attenuated C. muridarum spread significantly less to the gastrointestinal tract but maintained robust ascending infection of the upper genital tract. Transcervical inoculation further confirmed the correlation between C. muridarum spreading to the gastrointestinal tract and its pathogenicity in the upper genital tract. Finally, defective spreading of C. muridarum mutants was due to their inability to colonize the gastrointestinal tract since intragastric inoculation did not rescue the mutants' colonization. Thus, promoting C. muridarum colonization of the gastrointestinal tract may represent a primary function of the TC0237 and TC0668 proteins. Correlation of chlamydial colonization of the gastrointestinal tract with chlamydial pathogenicity in the upper genital tract suggests a potential role for gastrointestinal chlamydiae in genital tract pathogenicity.

Hypothetical protein Cpn0423 triggers NOD2 activation and contributes to Chlamydia pneumoniae-mediated inflammation

BACKGROUND

Chlamydia pneumoniae (C. pneumoniae) is pathogenic to humans, by causing pulmonary inflammation or bronchitis in both adolescents and young adults. However, the molecular signals linking C. pneumoniae components to inflammation remain elusive. This study was to investigate the effect of Chlamydia-specific Cpn0423 of C. pneumoniae on C. pneumoniae-mediated inflammation.

RESULTS

Cpn0423 was detected outside of C. pneumoniae inclusions, which induced production of several cytokines including macrophage inflammatory protein-2 (MIP-2) and interleukins (ILs). Production of the Cpn0423-induced cytokines was markedly reduced in cells pretreated with NOD2-siRNA, but not with negative control oligonucleotides. Mice treated with Cpn0423 through intranasal administration exhibited pulmonary inflammation as evidenced by infiltration of inflammatory cells, increased inflammatory scores in the lung histology, recruitment of neutrophils and increased cytokines levels in the BALF.

CONCLUSION

Cpn0423 could be sensed by NOD2, which was identified as an essential element in a pathway contributing to the development of C. pneumoniae -mediated inflammation.

Engineering of obligate intracellular bacteria: progress, challenges and paradigms

It is estimated that approximately one billion people are at risk of infection with obligate intracellular bacteria, but little is known about the underlying mechanisms that govern their life cycles. The difficulty in studying Chlamydia spp., Coxiella spp., Rickettsia spp., Anaplasma spp., Ehrlichia spp. and Orientia spp. is, in part, due to their genetic intractability. Recently, genetic tools have been developed; however, optimizing the genomic manipulation of obligate intracellular bacteria remains challenging. In this Review, we describe the progress in, as well as the constraints that hinder, the systematic development of a genetic toolbox for obligate intracellular bacteria. We highlight how the use of genetically manipulated pathogens has facilitated a better understanding of microbial pathogenesis and immunity, and how the engineering of obligate intracellular bacteria could enable the discovery of novel signalling circuits in host-pathogen interactions.

Update on Chlamydia trachomatis Vaccinology

Attempts to produce a vaccine to protect against Chlamydia trachomatis-induced trachoma were initiated more than 100 years ago and continued for several decades. Using whole organisms, protective responses were obtained. However, upon exposure to C. trachomatis, disease exacerbation developed in some immunized individuals, precluding the implementation of the vaccine. Evidence of the role of C. trachomatis as a sexually transmitted pathogen started to emerge in the 1960s, and it soon became evident that it can cause acute infections and long-term sequelae in women, men, and newborns. The main focus of this minireview is to summarize recent findings and discuss formulations, including antigens, adjuvants, routes, and delivery systems for immunization, primarily explored in the female mouse model, with the goal of implementing a vaccine against C. trachomatis genital infections.

Culture-independent genomics of a novel chlamydial pathogen of fish provides new insight into host-specific adaptations utilized by these intracellular bacteria

Several Chlamydiales families are associated with epitheliocystis, a common condition of the fish gill epithelium. These families share common ancestors with the Chlamydiaceae and environmental Chlamydiae. Due to the lack of culture systems, little is known about the biology of these chlamydial fish pathogens. We investigated epitheliocystis in cultured Orange-spotted grouper (Epinephelus coioides) from North Queensland, Australia. Basophilic inclusions were present in the gills of 22/31 fish and the presence of the chlamydial pathogen in the cysts was confirmed by in situ hybridization. Giant grouper (Epinephelus lanceolatus) cultured in the same systems were epitheliocystis free. 16S rRNA gene sequencing revealed a novel member of the Candidatus Parilichlamydiaceae: Ca. Similichlamydia epinephelii. Using metagenomic approaches, we obtained an estimated 68% of the chlamydial genome, revealing that this novel chlamydial pathogen shares a number of key pathogenic hallmarks with the Chlamydiaceae, including an intact Type III Secretion system and several chlamydial virulence factors. This provides additional evidence that these pathogenic mechanisms were acquired early in the evolution of this unique bacterial phylum. The identification and genomic characterization of Ca. S. epinephelii provides new opportunities to study the biology of distantly-related chlamydial pathogens while shining a new light on the evolution of pathogenicity of the Chlamydiaceae.

Chlamydial Lipoproteins Stimulate Toll-Like Receptors 1/2 Mediated Inflammatory Responses through MyD88-Dependent Pathway

Chlamydiae are very important pathogens which could cause several types of diseases in human, but little is known about its pathogenic mechanism. In order to elucidate host inflammatory response and the signal pathway induced by Chlamydial lipoproteins, the predicted lipoproteins of Chlamydia trachomatis were tested for their ability to induce the release of proinflammatory cytokines by mouse macrophages or human TLR (Toll-Like Receptor) expressing cell lines. The results showed that recombinant proteins of C. trachomatis D381, D541, D067, and D775 displayed a strong ability to induce the release of IL-8 in TLR expressing cell line. The signal pathways involved TLR1/2 and TLR2/CD14 but not TLR4. Moreover, except D067, the proinflammatory cytokine induction by D381, D541, and D775 required the thioacylation site (cysteine) for lipid modification and the induction was through MyD88-mediated pathway. Our data supported that lipoproteins played a vital role in pathogenesis of C. trachomatis-induced inflammatory responses via TLR pathway. It was the first study to characterize other chlamydial lipoproteins after identifying the role of MIP (D541) on pathogenesis of Chlamydial diseases.

Chlamydial Protease-Like Activity Factor and Type III Secreted Effectors Cooperate in Inhibition of p65 Nuclear Translocation

The chlamydial protease-like activity factor (CPAF) is hypothesized to be an important secreted virulence factor; however, challenges in denaturing its proteolytic activity have hampered attempts to identify its legitimate targets. Here, we use a genetic and proteomic approach to identify authentic CPAF targets. Human epithelial cells infected with CPAF-sufficient and CPAF-deficient chlamydiae were lysed using known CPAF-denaturing conditions. Their protein profiles were analyzed using isobaric mass tags and liquid chromatography-tandem mass spectrometry. Comparative analysis of CPAF-sufficient and CPAF-deficient infections identified a limited number of CPAF host and chlamydial protein targets. Host targets were primarily interferon-stimulated gene products, whereas chlamydial targets were type III secreted proteins. We provide evidence supporting a cooperative role for CPAF and type III secreted effectors in blocking NF-κB p65 nuclear translocation, resulting in decreased beta interferon and proinflammatory cytokine synthesis. Genetic complementation of null organisms with CPAF restored p65 nuclear translocation inhibition and proteolysis of chlamydial type III secreted effector proteins (T3SEs). We propose that CPAF and T3SEs cooperate in the inhibition of host innate immunity.

Chlamydia trachomatis is an important human pathogen responsible for over 100 million infections each year worldwide. Its success as an intracellular pathogen revolves around its ability to evade host immunity. The chlamydial protease-like activity factor (CPAF) is a conserved serine protease secreted into the host cytosol of infected cells that is thought to play an important role in immune evasion. Currently, CPAF’s authentic in situ target(s) and mechanism of action in immune evasion are poorly characterized. Using a CPAF-deficient strain and high-throughput proteomics, we report novel CPAF host and chlamydial targets. Host targets were primarily interferon-stimulated genes, whereas chlamydial targets were exclusively type III secreted proteins. We propose a novel mechanism for CPAF and type III secreted proteins in the evasion of host innate immune responses. These findings provide new insights into CPAF’s function as a virulence factor and a better understanding of how chlamydiae evade host immunity.