Interaction of chlamydiae and host cells in vitro

Current Evidence of Maternal Infection With Chlamydia trachomatis and Preeclampsia Risk

Chlamydia trachomatis is the most common bacterial sexually transmitted infection (STI) in the United States. Ascending C. trachomatis can cause pelvic inflammatory disease (PID), potentially leading to subsequent infertility, ectopic pregnancy, and adverse pregnancy outcomes. There is growing evidence implicating infections (e.g., COVID-19, cytomegalovirus) in preeclampsia etiology, a maternal hypertensive disorder and leading cause of maternal morbidity and mortality. However, few studies have investigated the impact of STIs on preeclampsia risk. In this review, we provide an overview of the potential association between C. trachomatis and preeclampsia and identify future research needs through a critical evaluation of epidemiologic, in vitro, and in vivo studies. Unfortunately, current methodological limitations such as lower-quality study designs, selection bias, confounding bias, and variations in chlamydia diagnostic methods inhibit our understanding of the impact of C. trachomatis on preeclampsia. In addition, bench-side approaches such as animal models and in vitro studies have not elucidated the mechanisms linking C. trachomatis to preeclampsia. Understanding the biological pathways that could be disrupted by chlamydia is important as it may ultimately guide the development and use of novel therapeutics to augment standard antibiotic therapy to reduce pathology.

Chlamydia trachomatis invasion: a duet of effectors

Members of the genus Chlamydia require an intracellular niche for growth and replication, thus highlighting the extreme significance of its ability to invade epithelial cells-the favored host cell in vivo. Because epithelial cells are not phagocytic, the uptake of Chlamydia must be driven by the pathogen. To this end, two bacterial proteins, translocated actin-recruiting protein (TarP) and translocated membrane effector A (TmeA), identified in Chlamydia trachomatis are translocated from the infectious chlamydial elementary bodies to the host cell cytosol to facilitate extensive remodeling of the cortical actin network to produce protrusive structures designed for pathogen engulfment. Notably, both effectors act by promoting highly localized actin nucleation at sites of bacterial adhesion. However, they have non-redundant functions, with both required for optimal actin remodeling dynamics and efficient invasion. Finally, these effectors also mediate the latter stages of the invasion process, specifically by modulating host dynamin 2, a large GTPase critical to closure and scission of invaginating vesicles harboring elementary bodies. In summary, TarP and TmeA orchestrate major aspects of C. trachomatis invasion.

Development of an sRNA-mediated conditional knockdown system for Chlamydia trachomatis

We describe a new Chlamydia trachomatis protein depletion method that uses an engineered small RNA (sRNA) to inhibit translation of a target gene. In proof-of-principle experiments, we induced functional knockdown of IncA, a fusion-mediating inclusion membrane protein, as shown with Western blots, loss of IncA staining at the inclusion membrane, and production of multiple chlamydial inclusions within an infected cell. These effects were titratable and reversible. To test for polar effects, we separately targeted the inclusion membrane proteins IncE and IncG, which are expressed from the incDEFG operon. Knockdown of IncE caused loss of IncE and its interacting host protein SNX6 at the inclusion membrane, without affecting IncG protein levels. Similarly, IncG knockdown significantly reduced IncG levels and prevented recruitment of its interacting host protein 14-3-3β, without altering IncE protein levels. These data provide the first genetic evidence that IncE and IncG are necessary for the recruitment of SNX6 and 14-3-3β, respectively, demonstrating the value of this knockdown approach. We also successfully depleted the major chlamydial surface protein, major outer membrane protein (MOMP), which is encoded by a likely essential gene that has not been previously disrupted or knocked down. MOMP knockdown caused severe defects in bacterial morphology and progeny production. Thus, our sRNA-based approach has broad potential as a conditional knockdown method for studying the function of C. trachomatis genes, including essential genes and genes in an operon.IMPORTANCEWe describe a new method to reduce protein levels of a selected gene in the pathogenic bacterium Chlamydia trachomatis. This approach utilizes an engineered small RNA (sRNA) to inhibit translation of the mRNA for a target gene and produced inducible and reversible protein knockdown. Our method successfully knocked down four proteins, including a likely essential gene and individual genes in an operon, without altering protein levels of a neighboring gene. This conditional knockdown method will be useful for studying the function of genes in Chlamydia. It also has the potential to be applied to other obligate intracellular bacteria, including Rickettsia and Coxiella.

The secreted host-cell protein clusterin interacts with PmpD and promotes Chlamydia trachomatis infection

Attachment and uptake into host cells are pivotal steps in the life cycle of the Chlamydiaceae, a family of obligate intracellular pathogens. Chlamydia trachomatis (Ctr) possesses a family of nine polymorphic membrane proteins (Pmps), which have been shown to be crucial for adhesion and internalization. However, the host-cell molecules involved have so far remained unknown. Here, we show that a fragment of Ctr PmpD, which forms high-molecular-weight oligomers in solution and adheres to epithelial cells, also binds to secreted clusterin (sCLU), a chaperone-like protein that is secreted into the extracellular space by the host cell, and forms part of the chaperone- and receptor-mediated extracellular protein degradation (CRED) pathway. Using in vitro assays, we demonstrate that sCLU interacts directly with soluble rPmpD. In infection experiments, depletion of sCLU from the culture medium leads to a significant decrease in Ctr infection. Thus, sCLU is the first host-cell interaction partner identified for a Ctr Pmp and the first case in which sCLU has been shown to be a vital component for the establishment of a bacterial infection.

The acetylase activity of Cdu1 regulates bacterial exit from infected cells by protecting Chlamydia effectors from degradation

Many cellular processes are regulated by ubiquitin-mediated proteasomal degradation. Pathogens can regulate eukaryotic proteolysis through the delivery of proteins with de-ubiquitinating (DUB) activities. The obligate intracellular pathogen Chlamydia trachomatis secretes Cdu1 (ChlaDUB1), a dual deubiquitinase and Lys-acetyltransferase, that promotes Golgi remodeling and survival of infected host cells presumably by regulating the ubiquitination of host and bacterial proteins. Here, we determined that Cdu1's acetylase but not its DUB activity is important to protect Cdu1 from ubiquitin-mediated degradation. We further identified three C. trachomatis proteins on the pathogen-containing vacuole (InaC, IpaM, and CTL0480) that required Cdu1's acetylase activity for protection from degradation and determined that Cdu1 and these Cdu1-protected proteins are required for optimal egress of Chlamydia from host cells. These findings highlight a non-canonical mechanism of pathogen-mediated protection of virulence factors from degradation after their delivery into host cells and the coordinated regulation of secreted effector proteins.

Neisseria gonorrhoeae drives Chlamydia trachomatis into a persistence-like state during in vitro co-infection

Chlamydia trachomatis and Neisseria gonorrhoeae are the most prevalent bacterial sexually transmitted infections (STIs) globally. Despite frequent co-infections in patients, few studies have investigated how mono-infections may differ from co-infections. We hypothesized that a symbiotic relationship between the pathogens could account for the high rates of clinical co-infection. During in vitro co-infection, we observed an unexpected phenotype where the C. trachomatis developmental cycle was impaired by N. gonorrhoeae. C. trachomatis is an obligate intracellular pathogen with a unique biphasic developmental cycle progressing from infectious elementary bodies (EB) to replicative reticulate bodies (RB), and back. After 12 hours of co-infection, we observed fewer EBs than in a mono-infection. Chlamydial genome copy number remained equivalent between mono- and co-infections. This is a hallmark of Chlamydial persistence. Chlamydial persistence alters inclusion morphology but varies depending on the stimulus/stress. We observed larger, but fewer, Chlamydia during co-infection. Tryptophan depletion can induce Chlamydial persistence, but tryptophan supplementation did not reverse the co-infection phenotype. Only viable and actively growing N. gonorrhoeae produced the inhibition phenotype in C. trachomatis. Piliated N. gonorrhoeae had the strongest effect on C. trachomatis, but hyperpiliated or non-piliated N. gonorrhoeae still produced the phenotype. EB development was modestly impaired when N. gonorrhoeae were grown in transwells above the infected monolayer. C. trachomatis serovar L2 was not impaired during co-infection. Chlamydial impairment could be due to cytoskeletal or osmotic stress caused by an as-yet-undefined mechanism. We conclude that N. gonorrhoeae induces a persistence-like state in C. trachomatis that is serovar dependent.

Chlamydial YAP activation in host endocervical epithelial cells mediates pro-fibrotic paracrine stimulation of fibroblasts

IMPORTANCE

Chronic or repeated infection of the female upper genital tract by C. trachomatis can lead to severe fibrotic sequelae, including tubal factor infertility and ectopic pregnancy. However, the molecular mechanisms underlying this effect are unclear. In this report, we define a transcriptional program specific to C. trachomatis infection of the upper genital tract, identifying tissue-specific induction of host YAP-a pro-fibrotic transcriptional cofactor-as a potential driver of infection-mediated fibrotic gene expression. Furthermore, we show that infected endocervical epithelial cells stimulate collagen production by fibroblasts and implicate chlamydial induction of YAP in this effect. Our results define a mechanism by which infection mediates tissue-level fibrotic pathology via paracrine signaling and identify YAP as a potential therapeutic target for the prevention of Chlamydia-associated scarring of the female genital tract.

Genetic markers of Chlamydia pecorum virulence in ruminants support short term host-pathogen evolutionary relationships in the koala, Phascolarctos cinereus

In ruminants infected with Chlamydia pecorum, shorter lengths of coding tandem repeats (CTR) within two genes, the inclusion membrane protein (incA) and Type III secretor protein (ORF663), have been previously associated with pathogenic outcomes. In other chlamydial species, the presence of a chlamydial plasmid has been linked to heightened virulence, and the plasmid is not ubiquitous in C. pecorum across the koala's range. We therefore investigated these three markers: incA, ORF663 and C. pecorum plasmid, as potential indicators of virulence in two koala populations in New South Wales with differing expression of urogenital chlamydiosis; the Liverpool Plains and one across the Southern Highlands and South-west Sydney (SHSWS). We also investigated the diversity of these loci within strains characterised by the national multi-locus sequence typing (MLST) scheme. Although CTR lengths of incA and ORF663 varied across the populations, they occurred only within previously described pathogenic ranges for ruminants. This suggests a relatively short-term host-pathogen co-evolution within koalas and limits the utility of CTR lengths for incA and ORF663 as virulence markers in the species. However, in contrast to reports of evolution of C. pecorum towards lower virulence, as indicated by longer CTR lengths in ruminants and swine, CTR lengths for ORF663 appeared to be diverging towards less common shorter CTR lengths within strains recently introduced to koalas in the Liverpool Plains. We detected the plasmid across 90% and 92% of samples in the Liverpool Plains and SHSWS respectively, limiting its utility as an indicator of virulence. It would be valuable to examine the CTR lengths of these loci across koala populations nationally. Investigation of other hypervariable loci may elucidate the evolutionary trajectory of virulence in C. pecorum induced disease in koalas. Profiling of virulent strains will be important in risk assessments for strain movement to naïve or susceptible populations through translocations and wildlife corridor construction.

Euo is a developmental regulator that represses late genes and activates midcycle genes in Chlamydia trachomatis

IMPORTANCE

In this study, we developed a correlative approach that combined DNA immunoprecipitation-seq and RNA-seq analyses to define the regulon of the Chlamydia trachomatis transcription factor Euo. We confirmed the proposed role of Euo as a transcriptional repressor of late chlamydial genes but also showed that Euo activates transcription of a subset of midcycle genes and autoregulates its own expression via negative feedback. This study validates and expands the role of Euo as an important developmental regulator in C. trachomatis. In addition, this genome-wide correlative approach can be applied to study transcription factors in other pathogenic bacteria.

Plasticity in the cell division processes of obligate intracellular bacteria

Most bacteria divide through a highly conserved process called binary fission, in which there is symmetric growth of daughter cells and the synthesis of peptidoglycan at the mid-cell to enable cytokinesis. During this process, the parental cell replicates its chromosomal DNA and segregates replicated chromosomes into the daughter cells. The mechanisms that regulate binary fission have been extensively studied in several model organisms, including Eschericia coli, Bacillus subtilis, and Caulobacter crescentus. These analyses have revealed that a multi-protein complex called the divisome forms at the mid-cell to enable peptidoglycan synthesis and septation during division. In addition, rod-shaped bacteria form a multi-protein complex called the elongasome that drives sidewall peptidoglycan synthesis necessary for the maintenance of rod shape and the lengthening of the cell prior to division. In adapting to their intracellular niche, the obligate intracellular bacteria discussed here have eliminated one to several of the divisome gene products essential for binary fission in E. coli. In addition, genes that encode components of the elongasome, which were mostly lost as rod-shaped bacteria evolved into coccoid organisms, have been retained during the reductive evolutionary process that some coccoid obligate intracellular bacteria have undergone. Although the precise molecular mechanisms that regulate the division of obligate intracellular bacteria remain undefined, the studies summarized here indicate that obligate intracellular bacteria exhibit remarkable plasticity in their cell division processes.

Ticks and Chlamydia-Related Bacteria in Swiss Zoological Gardens Compared to in Contiguous and Distant Control Areas

Ticks are vectors of numerous agents of medical importance and may be infected by various Chlamydia-related bacteria, such as members of Parachlamydiaceae and Rhabdochlamydiaceae families, which are sharing the same biphasic life cycle with the pathogenic Chlamydia. However, the veterinary importance of ticks and of their internalized pathogens remains poorly studied. Thus, we wondered (i) whether the prevalence of ticks was higher in zoological gardens than in control areas with similar altitude, vegetation, humidity and temperature, and (ii) whether the presence of Chlamydia-related bacteria in ticks may vary according to the environment in which the ticks are collected. A total of 212 Ixodes ricinus ticks were collected, and all were tested for the presence of DNA from any member of the Chlamydiae phylum using a pan-Chlamydiae quantitative PCR (qPCR). We observed a higher prevalence of ticks outside animal enclosures in both zoos, compared to in enclosures. Tick prevalence was also higher outside zoos, compared to in enclosures. With 30% (3/10) of infected ticks, the zoological gardens presented a prevalence of infected ticks that was higher than that in contiguous areas (13.15%, 10/76), and higher than the control distant areas (8.65%, 9/104). In conclusion, zoological gardens in Switzerland appear to contain fewer ticks than areas outside zoological gardens. However, ticks from zoos more often contain Chlamydia-like organisms than ticks from contiguous or distant control areas.

Type III Secretion in Chlamydia

Type III secretion systems (T3SSs) are utilized by Gram-negative pathogens to enhance their pathogenesis. This secretion system is associated with the delivery of effectors through a needle-like structure from the bacterial cytosol directly into a target eukaryotic cell. These effector proteins then manipulate specific eukaryotic cell functions to benefit pathogen survival within the host. The obligate intracellular pathogens of the family Chlamydiaceae have a highly evolutionarily conserved nonflagellar T3SS that is an absolute requirement for their survival and propagation within the host with about one-seventh of the genome dedicated to genes associated with the T3SS apparatus, chaperones, and effectors. Chlamydiae also have a unique biphasic developmental cycle where the organism alternates between an infectious elementary body (EB) and replicative reticulate body (RB). T3SS structures have been visualized on both EBs and RBs. And there are effector proteins that function at each stage of the chlamydial developmental cycle, including entry and egress. This review will discuss the history of the discovery of chlamydial T3SS and the biochemical characterization of components of the T3SS apparatus and associated chaperones in the absence of chlamydial genetic tools. These data will be contextualized into how the T3SS apparatus functions throughout the chlamydial developmental cycle and the utility of heterologous/surrogate models to study chlamydial T3SS. Finally, there will be a targeted discussion on the history of chlamydial effectors and recent advances in the field.

Temperature Affects the Host Range of Rhabdochlamydia porcellionis

The Rhabdochlamydiaceae family is a recent addition to the Chlamydiae phylum. Its members were discovered in cockroaches and woodlice, but recent metagenomics surveys demonstrated the widespread distribution of this family in the environment. It was, moreover, estimated to be the largest family of the Chlamydiae phylum based on the diversity of its 16S rRNA encoding gene. Unlike most Chlamydia-like organisms, no Rhabdochlamydiaceae member could be cultivated in amoebae, and its host range remains unknown. We tested the permissivity of various mammalian and arthropod cell lines to determine the host range of Rhabdochlamydia porcellionis, the only cultured representative of this family. While growth could initially be obtained only in the Sf9 cell line, lowering the incubation temperature of the mammalian cells from 37°C to 28°C allowed the growth of R. porcellionis. Furthermore, a 6-h exposure to 37°C was sufficient to irreversibly block the replication of R. porcellionis, suggesting that this bacterium either lost or never acquired the ability to grow at 37°C. We next sought to determine if temperature would also affect the infectivity of elementary bodies. Although we could not purify enough bacteria to reach a conclusive result for R. porcellionis, our experiment showed that the elementary bodies of Chlamydia trachomatis and Waddlia chondrophila lose their infectivity faster at 37°C than at room temperature. Our results demonstrate that members of the Chlamydiae phylum adapt to the temperature of their host organism and that this adaptation can in turn restrict their host range. IMPORTANCE The Rhabdochlamydiaceae family is part of the Chlamydiae, a phylum of bacteria that includes obligate intracellular bacteria sharing the same biphasic developmental cycle. This family has been shown to be highly prevalent in the environment, particularly in freshwater and soil, and despite being estimated to be the largest family in the Chlamydiae phylum is only poorly studied. Members of the Rhabdochlamydiaceae have been detected in various arthropods like ticks, spiders, cockroaches, and woodlice, but the full host range of this family is currently unknown. In this study, we showed that R. porcellionis, the only cultured representative of the Rhabdochlamydiaceae family, cannot grow at 37°C and is quickly inactivated at this temperature. A similar temperature sensitivity was also observed for elementary bodies of chlamydial species adapted to mammals. Our work demonstrates that chlamydiae adapt to the temperature of their reservoir, making a jump between species with different body temperatures unlikely.

Genomic analysis of 61 Chlamydia psittaci strains reveals extensive divergence associated with host preference

BACKGROUND

Chlamydia (C.) psittaci, the causative agent of avian chlamydiosis and human psittacosis, is a genetically heterogeneous species. Its broad host range includes parrots and many other birds, but occasionally also humans (via zoonotic transmission), ruminants, horses, swine and rodents. To assess whether there are genetic markers associated with host tropism we comparatively analyzed whole-genome sequences of 61 C. psittaci strains, 47 of which carrying a 7.6-kbp plasmid.

RESULTS

Following clean-up, reassembly and polishing of poorly assembled genomes from public databases, phylogenetic analyses using C. psittaci whole-genome sequence alignment revealed four major clades within this species. Clade 1 represents the most recent lineage comprising 40/61 strains and contains 9/10 of the psittacine strains, including type strain 6BC, and 10/13 of human isolates. Strains from different non-psittacine hosts clustered in Clades 2- 4. We found that clade membership correlates with typing schemes based on SNP types, ompA genotypes, multilocus sequence types as well as plasticity zone (PZ) structure and host preference. Genome analysis also revealed that i) sequence variation in the major outer membrane porin MOMP can result in 3D structural changes of immunogenic domains, ii) past host change of Clade 3 and 4 strains could be associated with loss of MAC/perforin in the PZ, rather than the large cytotoxin, iii) the distinct phylogeny of atypical strains (Clades 3 and 4) is also reflected in their repertoire of inclusion proteins (Inc family) and polymorphic membrane proteins (Pmps).

CONCLUSIONS

Our study identified a number of genomic features that can be correlated with the phylogeny and host preference of C. psittaci strains. Our data show that intra-species genomic divergence is associated with past host change and includes deletions in the plasticity zone, structural variations in immunogenic domains and distinct repertoires of virulence factors.

Chlamydia Infection Remodels Host Cell Mitochondria to Alter Energy Metabolism and Subvert Apoptosis

Chlamydia infection represents an important cause for concern for public health worldwide. Chlamydial infection of the genital tract in females is mostly asymptomatic at the early stage, often manifesting as mucopurulent cervicitis, urethritis, and salpingitis at the later stage; it has been associated with female infertility, spontaneous abortion, ectopic pregnancy, and cervical cancer. As an obligate intracellular bacterium, Chlamydia depends heavily on host cells for nutrient acquisition, energy production, and cell propagation. The current review discusses various strategies utilized by Chlamydia in manipulating the cell metabolism to benefit bacterial propagation and survival through close interaction with the host cell mitochondrial and apoptotic pathway molecules.

Pathogen vacuole membrane contact sites - close encounters of the fifth kind

Vesicular trafficking and membrane fusion are well-characterized, versatile, and sophisticated means of 'long range' intracellular protein and lipid delivery. Membrane contact sites (MCS) have been studied in far less detail, but are crucial for 'short range' (10-30 nm) communication between organelles, as well as between pathogen vacuoles and organelles. MCS are specialized in the non-vesicular trafficking of small molecules such as calcium and lipids. Pivotal MCS components important for lipid transfer are the VAP receptor/tether protein, oxysterol binding proteins (OSBPs), the ceramide transport protein CERT, the phosphoinositide phosphatase Sac1, and the lipid phosphatidylinositol 4-phosphate (PtdIns(4)P). In this review, we discuss how these MCS components are subverted by bacterial pathogens and their secreted effector proteins to promote intracellular survival and replication.

Homologues of the Chlamydia trachomatis and Chlamydia muridarum Inclusion Membrane Protein IncS Are Interchangeable for Early Development but Not for Inclusion Stability in the Late Developmental Cycle

Chlamydia trachomatis is an obligate intracellular bacterium, which undergoes a biphasic developmental cycle inside a vacuole termed the inclusion. Chlamydia-specific effector proteins embedded into the inclusion membrane, the Inc proteins, facilitate inclusion interaction with cellular organelles. A subset of Inc proteins engages with specific host factors at the endoplasmic reticulum (ER)-inclusion membrane contact site (MCS), which is a discrete point of contact between the inclusion membrane and the endoplasmic reticulum (ER). Here, we report that the C. trachomatis Inc protein CTL0402/IncS_Ct is a novel component of the ER-inclusion MCS that specifically interacts with and recruits STIM1, a previously identified host component of the ER-inclusion MCS with an unassigned interacting partner at the inclusion membrane. In comparison, the Chlamydia muridarum IncS homologue (TC0424/IncS_Cm) does not interact with or recruit STIM1 to the inclusion, indicating species specificity. To further investigate IncS function and overcome the recently reported early developmental defect of the incS mutant, we achieved temporal complementation by expressing IncS exclusively during the early stages of the developmental cycle. Additionally, we used allelic exchange to replace the incS_Ct open reading frame with incS_Cm in the C. trachomatis chromosome. Inclusions harboring either of these strains progressed through the developmental cycle but were STIM1 negative and displayed increased inclusion lysis 48 h postinfection. Expression of incS_Ct in trans complemented these phenotypes. Altogether, our results indicate that IncS is necessary and sufficient to recruit STIM1 to C. trachomatis inclusion and that IncS plays an early developmental role conserved in C. trachomatis and C. muridarum and a late role in inclusion stability specific to C. trachomatis. IMPORTANCE Obligate intracellular pathogens strictly rely on the host for replication. Specialized pathogen-encoded effector proteins play a central role in sophisticated mechanisms of host cell manipulation. In Chlamydia, a subset of these effector proteins, the inclusion membrane proteins, are embedded in the membrane of the vacuole in which the bacteria replicate. Chlamydia encodes 50 to 100 putative Inc proteins. Many are conserved among species, including the human and mouse pathogens Chlamydia trachomatis and Chlamydia muridarum, respectively. However, whether the function(s) of Inc proteins is indeed conserved among species is poorly understood. Here, we characterized the function of the Inc protein IncS conserved in C. trachomatis and C. muridarum. Our work reveals that a single effector protein can play multiple functions at various stages of the developmental cycle. However, these functions are not necessarily conserved across species, suggesting a complex evolutionary path among Chlamydia species.

TargeTron inactivation of plasmid-regulated Chlamydia trachomatis CT084 results in a nonlytic phenotype

Chlamydia trachomatis is an obligate intracellular bacterium that causes blinding trachoma and sexually transmitted disease. The chlamydial plasmid is a critical virulence factor in the pathogenesis of these diseases. Plasmid gene protein 4 (Pgp4) plays a major role in chlamydial virulence by regulating the expression of both chromosomal genes and Pgp3. Despite the importance of Pgp4 in mediating lytic exit from host cells the pathogenic mechanism by which it functions is unknown. CT084 is a highly conserved chromosomal gene with homology to phospholipase D. We showed CT084 expression is regulated by Pgp4 and expressed late in the chlamydial developmental cycle. To investigate the function of CT084 in chlamydial lytic exit from infected cells, we made a CT084 null strain (ct084::bla) by using Targetron. The ct084::bla strain grew normally in vitro compared to wild-type strain; however, the strain did not lyse infected cells and produced significantly less and smaller plaques. Collectively, our finding shows Pgp4-regulated CT084-mediated chlamydia lytic exit from infected host cells.

The hormonal environment and estrogen receptor signaling alters Chlamydia muridarum infection in vivo

Genital Chlamydia is the most common bacterial sexually transmitted infection in the United States and worldwide. Previous studies indicate that the progression of chlamydial infection is influenced by various factors, including the female sex hormones estrogen and progesterone. Sex hormone levels naturally fluctuate in women throughout their menstrual cycle. Varying concentrations of estrogen and progesterone may impact the progression of chlamydial infection and the host's immune response to Chlamydia. Estrogen signals through estrogen receptors (ERs), ERα and ERβ. These receptors are similar in structure and function, but are differentially expressed in tissues throughout the body, including the genital tract and on cells of the immune system. In this study, we used ovariectomized (OVT) BALB/c mice to investigate the impact of long-term administration of physiologically relevant concentrations of estrogen (E2), progesterone (P4), or a combination of E2/P4 on the progression of and immune response to C. muridarum infection. Additionally, we used ERα and ERβ knockout C57/BL6 mice to determine the how ERs affect chlamydial infection and the resulting immune response. Estrogen exposure prevented C. muridarum infection in vaginally infected OVT mice exposed to E2 alone or in combination with P4, while OVT or Sham mice exposed to hormone free, P4 or depo-medroxyprogesterone acetate shed similar amounts of chlamydiae. The hormonal environment also altered T cell recruitment and IFNϵ production the genital tracts of infected OVT and Sham mice on day 10 post infection. The absence of ERα, but not ERβ, in ER knockout mouse strains significantly changed the timing of C. muridarum infection. ERαKO mice shed significantly more chlamydiae at day 3 post infection and resolved the infection faster than WT or ERβKO animals. At day 9 post infection, flow cytometry showed that ERαKO mice had more T cells present and targeted RNA sequencing revealed increased expression of CD4 and FOXP3, suggesting that ERαKO mice had increased numbers of regulatory T cells compared to ERβKO and WT mice. Mock and chlamydia-infected ERαKO mice also expressed more IFNϵ early during infection. Overall, the data from these studies indicate that sex hormones and their receptors, particularly ERα and ERβ, differentially affect C. muridarum infection in murine models of infection.

Chlamydia trachomatis suppresses host cell store-operated Ca2+ entry and inhibits NFAT/calcineurin signaling

The obligate intracellular bacterium, Chlamydia trachomatis, replicates within a parasitophorous vacuole termed an inclusion. During development, host proteins critical for regulating intracellular calcium (Ca²⁺) homeostasis interact with the inclusion membrane. The inclusion membrane protein, MrcA, interacts with the inositol-trisphosphate receptor (IP₃R), an ER cationic channel that conducts Ca²⁺. Stromal interaction molecule 1 (STIM1), an ER transmembrane protein important for regulating store-operated Ca²⁺ entry (SOCE), localizes to the inclusion membrane via an uncharacterized interaction. We therefore examined Ca²⁺ mobilization in C. trachomatis infected cells. Utilizing a variety of Ca²⁺ indicators to assess changes in cytosolic Ca²⁺ concentration, we demonstrate that C. trachomatis impairs host cell SOCE. Ca²⁺ regulates many cellular signaling pathways. We find that the SOCE-dependent NFAT/calcineurin signaling pathway is impaired in C. trachomatis infected HeLa cells and likely has major implications on host cell physiology as it relates to C. trachomatis pathogenesis.

Chlamydia trachomatis RsbU Phosphatase Activity Is Inhibited by the Enolase Product, Phosphoenolpyruvate

The intracellular pathogen Chlamydia temporally regulates the expression of its genes, but the upstream signals that control transcription are not known. The best-studied regulatory pathway is a partner-switching mechanism that involves an anti-sigma factor, RsbW, which inhibits transcription by binding and sequestering the sigma subunit of RNA polymerase. RsbW is itself regulated by an anti-anti-sigma factor, RsbV, whose phosphorylation state is controlled by the phosphatase RsbU. In this study, we showed that Chlamydia trachomatis RsbU requires manganese or magnesium as a cofactor and dephosphorylates RsbV1 and RsbV2, which are the two chlamydial paralogs of RsbV. The gene for RsbU is adjacent to the enolase gene in a number of Chlamydia genomes, and we showed that eno and rsbU are cotranscribed from the same operon. In other bacteria, there is no known functional connection between the Rsb pathway and enolase, which is an enzyme in the glycolytic pathway. We found, however, that Chlamydia RsbU phosphatase activity was inhibited by phosphoenolpyruvate (PEP), the product of the enolase reaction, but not by 2-phosphoglycerate (2PGA), which is the substrate. These findings suggest that the enolase reaction and, more generally, glucose metabolism, may provide an upstream signal that regulates transcription in Chlamydia through the RsbW pathway. IMPORTANCE The RsbW pathway is a phosphorelay that regulates gene expression in Chlamydia, but its upstream signal has not been identified. We showed that RsbU, a phosphatase in this pathway, is inhibited by phosphoenolpyruvate, which is the product of the enolase reaction. As enolase is an enzyme in the glycolytic pathway, these results reveal an unrecognized link between glucose metabolism and gene regulation in chlamydiae. Moreover, as these intracellular bacteria acquire glucose from the infected host cell, our findings suggest that glucose availability may be an external signal that controls chlamydial gene expression.

New insights into Chlamydia pathogenesis: Role of leukemia inhibitory factor

Chlamydia trachomatis (Ct) is the leading cause of bacterial sexually transmitted infections worldwide. Since the symptoms of Ct infection are often subtle or absent, most people are unaware of their infection until they are tested or develop severe complications such as infertility. It is believed that the primary culprit of Ct-associated tissue damage is unresolved chronic inflammation, resulting in aberrant production of cytokines, chemokines, and growth factors, as well as dysregulated tissue influx of innate and adaptive immune cells. A member of the IL-6 cytokine family, leukemia inhibitory factor (LIF), is one of the cytokines induced by Ct infection but its role in Ct pathogenesis is unclear. In this article, we review the biology of LIF and LIF receptor (LIFR)-mediated signaling pathways, summarize the physiological role of LIF in the reproductive system, and discuss the impact of LIF in chronic inflammatory conditions and its implication in Ct pathogenesis. Under normal circumstances, LIF is produced to maintain epithelial homeostasis and tissue repair, including the aftermath of Ct infection. However, LIF/LIFR-mediated signaling – particularly prolonged strong signaling – can gradually transform the microenvironment of the fallopian tube by altering the fate of epithelial cells and the cellular composition of epithelium. This harmful transformation of epithelium may be a key process that leads to an enhanced risk of infertility, ectopic pregnancy and cancer following Ct infection.

The inclusion membrane protein IncS is critical for initiation of the Chlamydia intracellular developmental cycle

All Chlamydia species are obligate intracellular bacteria that undergo a unique biphasic developmental cycle strictly in the lumen of a membrane bound compartment, the inclusion. Chlamydia specific Type III secreted effectors, known as inclusion membrane proteins (Inc), are embedded into the inclusion membrane. Progression through the developmental cycle, in particular early events of conversion from infectious (EB) to replicative (RB) bacteria, is important for intracellular replication, but poorly understood. Here, we identified the inclusion membrane protein IncS as a critical factor for Chlamydia development. We show that a C. trachomatis conditional mutant is impaired in transition from EB to RB in human cells, and C. muridarum mutant bacteria fail to develop in a mouse model of Chlamydia infection. Thus, IncS represents a promising target for therapeutic intervention of the leading cause of sexually transmitted infections of bacterial origin.

Role of TRAIL-R in Primary and Secondary Genital and Respiratory Chlamydia muridarum Infections in Mice

The tumor necrosis factor (TNF)-related apoptosis-inducing ligand receptor (TRAIL-R) suppresses inflammation and could therefore affect the course of Chlamydia infections and their long-term sequelae. Wild-type (WT) and TRAIL-R-/- C57BL/6 mice were inoculated vaginally with Chlamydia muridarum; the course of the infection was followed with vaginal cultures and the presence of hydrosalpinx determined. To evaluate the role of TRAIL-R following a secondary infection, the mice were vaginally reinfected. WT and TRAIL-R-/- male mice were also infected and reinfected in the respiratory tract, and the course of the diseases and the infections were followed. Following the primary and secondary vaginal infection, no significant differences in vaginal shedding or hydrosalpinx formation were observed between the WT and TRAIL-R-/- mice. The WT and TRAIL-R-/- mice mounted antibody responses in serum and vaginal washes that were not significantly different. After the primary and secondary intranasal infections of the male mice, changes in body weight were determined, and no significant differences were observed between the WT and TRAIL-R-/- mice. Ten days after the primary and the secondary infections, the weight of the lungs and number of C. muridarum inclusion forming units (IFU) were determined. The lungs of the WT mice weighed less compared with the TRAIL-R-/- mice following a primary infection but not after a secondary infection. No differences in the number of C. muridarum IFU in the lungs were observed between the two groups of mice. In conclusion, despite playing a role in inflammation cell-signaling pathways in vitro, TRAIL-R does not appear to play a major role in the susceptibility, clinical outcomes, or long-term sequelae of C. muridarum infections in vivo. IMPORTANCE TNF-related apoptosis-inducing ligand receptor (TRAIL-R) is involved in suppressing inflammatory responses. Bacterial pathogens such as Chlamydia spp. elicit inflammatory responses in humans following genital, ocular, and respiratory infections. The inflammatory responses are important to control the spread of Chlamydia. However, in certain instances, these inflammatory responses can produce long-term sequelae, including fibrosis. Fibrosis, or scarring, in the genital tract, eye, and respiratory system results in functional deficiencies, including infertility, blindness, and chronic obstructive lung disease, respectively. The goal of this study was to determine if mice deficient in TRAIL-R infected in the genital and respiratory tracts with Chlamydia spp. suffer more or less severe infections, infertility, or lung diseases than wild-type mice. Our results show no differences between the immune responses, infection severity, and long-term sequelae between TRAIL-R knockout and wild-type animals following a genital or a respiratory infection with Chlamydia.

Chlamydia trachomatis vaccine development - a view on the current challenges and how to move forward

INTRODUCTION

Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the world. A licensed vaccine is not yet available, but the first vaccines have entered clinical trials.

AREAS COVERED

: We describe the progress that has been made in our understanding of the type of immunity that a protective vaccine should induce, and the challenges that vaccine developers face. We also focus on the clinical development of a chlamydia vaccine. The first chlamydia vaccine candidate has now been tested in a clinical phase-I trial, and another phase-I trial is currently running. We discuss what it will take to continue this development and what future trial setups could look like.

EXPERT OPINION

The chlamydia field is coming of age and the first phase I clinical trial of a C. trachomatis vaccine has been successfully completed. We expect and hope that this will motivate various stakeholders to support further development of chlamydia vaccines in humans.

A Reverse Genetic Approach for Studying sRNAs in Chlamydia trachomatis

sRNAs are noncoding transcripts that play critical roles in posttranscriptional regulation in prokaryotes. In the intracellular bacterium Chlamydia, sRNAs have been identified, but functional studies have been limited to an E. coli heterologous system. We have developed an inducible sRNA overexpression system in Chlamydia trachomatis and used it to screen putative sRNAs for effects on the Chlamydia developmental cycle, which involves conversion between replicating (RB) and infectious (EB) chlamydial forms. Overexpression of 4 of 13 C. trachomatis sRNAs decreased production of infectious EBs. We performed detailed characterization of CtrR3 and CtrR7, the two sRNAs that caused the largest progeny defects in our screen. By quantifying chlamydial number and infectious progeny, and by visualizing chlamydial forms using electron microscopy, we showed that overexpression of CtrR3 prevented RB-to-EB conversion, whereas CtrR7 overexpression blocked bacterial replication. We also describe a workflow that allowed us to identify the mRNA targets of CtrR3 in Chlamydia. We first used MS2 aptamer affinity purification coupled with RNA sequencing as an unbiased approach to isolate interacting mRNAs. We then prioritized candidates based on sequence complementarity to the CtrR3 target recognition sequence, which we had identified with bioinformatic and mutational analyses. Finally, we tested putative targets with translational fusion assays in E. coli and C. trachomatis. Using this integrated approach, we provide experimental evidence that YtgB and CTL0389 are mRNA targets of CtrR3 in Chlamydia. These findings demonstrate how our C. trachomatis sRNA overexpression system can be used to investigate the functions and mRNA targets of chlamydial sRNAs.

Differential Effects of Small Molecule Inhibitors on the Intracellular Chlamydia Infection

Chlamydia are obligate intracellular bacteria that reside within a membrane-bound compartment called the chlamydial inclusion inside a eukaryotic host cell. These pathogens have a complex biphasic developmental cycle, which involves conversion between a replicating, but noninfectious, reticulate body (RB) and an infectious elementary body (EB). Small molecule inhibitors have been reported to have deleterious effects on the intracellular Chlamydia infection, but these studies have typically been limited in terms of assays and time points of analysis. We compared published and novel inhibitors and showed that they can differentially alter inclusion size, chlamydial number and infectious EB production, and that these effects can vary over the course of the intracellular infection. Our results provide the justification for analysis with multiple assays performed either at the end of the infection or over a time course. We also show that this approach has the potential to identify the particular step in the developmental cycle that is impacted by the inhibitor. We furthermore propose that the magnitude of inhibitor-induced progeny defects are best quantified and compared by using a new value called maximal progeny production (Progeny_max). As a demonstration of the validity of this systematic approach, we applied it to inhibitors of Akt and AMPK, which are host kinases involved in lipid synthesis and cholesterol trafficking pathways. Both inhibitors reduced EB production, but Akt disruption primarily decreased RB-to-EB conversion while AMPK inhibition paradoxically enhanced RB replication.

The Chlamydia trachomatis Inclusion Membrane Protein CTL0390 Mediates Host Cell Exit via Lysis through STING Activation

The obligate intracellular bacterium Chlamydia trachomatis is the causative agent of the most frequently reported bacterial sexually transmitted disease. Upon internalization into host cells, C. trachomatis remains within a membrane-bound compartment known as an inclusion, where it undergoes its developmental cycle. After completion of this cycle, bacteria exit the host cell. One mechanism of exit is lysis, whereby the inclusion and host cell rupture to release bacteria; however, the mechanism of lysis is not well characterized. A subset of C. trachomatis effectors, known as inclusion membrane proteins (Inc), are embedded within the inclusion membrane to facilitate host cell manipulation. The functions of many Inc proteins are unknown. We sought to characterize the Inc protein CTL0390. We determined that CTL0390 is expressed throughout the developmental cycle and that its C-terminal tail is exposed to the cytosol. To investigate the function of CTL0390, we generated a ctl0390 mutant complemented with ctl0390 on a plasmid. Loss of CTL0390 did not affect infectious progeny production but resulted in a reduction in lysis. Overexpression of CTL0390 induced premature lysis and host nuclear condensation, the latter of which could be reduced upon inhibition of the cGAS-STING DNA sensing pathway. Infection with the clt0390 mutant led to reduced Golgi translocation of STING, and chemical and genetic approaches to inactivate STING revealed that STING plays a role in lysis in a CTL0390-dependent manner. Together, these results reveal a role for CTL0390 in bacterial exit via lysis at late stages of the Chlamydia developmental cycle and through STING activation.

The Chlamydia trachomatis Early Effector Tarp Outcompetes Fascin in Forming F-Actin Bundles In Vivo

The intracellular pathogen Chlamydia trachomatis secretes multiple early effectors into the host cell to promote invasion. A key early effector during host cell entry, Tarp (translocated actin-recruiting phosphoprotein) is comprised of multiple protein domains known to have roles in cell signaling, G-actin nucleation and F-actin bundle formation. In vitro, the actin bundles generated by Tarp are uncharacteristically flexible, however, in vivo, the biological significance of Tarp-mediated actin bundles remains unknown. We hypothesize that Tarp's ability to generate unique actin bundles, in part, facilitates chlamydial entry into epithelial cells. To study the in vivo interaction between Tarp and F-actin, we transgenically expressed Tarp in Drosophila melanogaster tissues. Tarp expressed in Drosophila is phosphorylated and forms F-actin-enriched aggregates in tissues. To gain insight into the significance of Tarp actin bundles in vivo, we utilized the well-characterized model system of mechanosensory bristle development in Drosophila melanogaster. Tarp expression in wild type flies produced curved bristles, indicating a perturbation in F-actin dynamics during bristle development. Two F-actin bundlers, Singed/Fascin and Forked/Espin, are important for normal bristle shape. Surprisingly, Tarp expression in the bristles displaced Singed/Fascin away from F-actin bundles. Tarp's competitive behavior against Fascin during F-actin bundling was confirmed in vitro. Loss of either singed or forked in flies leads to highly deformed bristles. Strikingly, Tarp partially rescued the loss of singed, reducing the severity of the bristle morphology defect. This work provides in vivo confirmation of Tarp's F-actin bundling activity and further uncovers a competitive behavior against the host bundler Singed/Fascin during bundle assembly. Also, we demonstrate the utility of Drosophila melanogaster as an in vivo cell biological platform to study bacterial effector function.

AAUS guideline for chlamydial urethritis

Urogenital chlamydial infection is the most common sexually transmitted infection. Many cases of chlamydial infection are reported worldwide every year. Genital chlamydial infection in women can also cause obstetric issues, including infertility and miscarriage. For that purpose, appropriate care should be conducted with the latest knowledge. Only few guidelines come from Asian countries. The Asian Association of Urinary Tract Infection and Sexually Transmitted Infection (AAUS) belonging to the Urological Association of Asia (UAA) had developed the guidelines regarding chlamydial urethritis. We have collected the feedback and updated the guidelines which is now submitted for consideration of publication. In addition to the levels of evidence, the recommendation grades were defined using the modified GRADE methodology. Herein, we present the new edition of the UAA-AAUS guidelines for chlamydial urethritis.

Idiosyncratic Biogenesis of Intracellular Pathogens-Containing Vacuoles

While most bacterial species taken up by macrophages are degraded through processing of the bacteria-containing vacuole through the endosomal-lysosomal degradation pathway, intravacuolar pathogens have evolved to evade degradation through the endosomal-lysosomal pathway. All intra-vacuolar pathogens possess specialized secretion systems (T3SS-T7SS) that inject effector proteins into the host cell cytosol to modulate myriad of host cell processes and remodel their vacuoles into proliferative niches. Although intravacuolar pathogens utilize similar secretion systems to interfere with their vacuole biogenesis, each pathogen has evolved a unique toolbox of protein effectors injected into the host cell to interact with, and modulate, distinct host cell targets. Thus, intravacuolar pathogens have evolved clear idiosyncrasies in their interference with their vacuole biogenesis to generate a unique intravacuolar niche suitable for their own proliferation. While there has been a quantum leap in our knowledge of modulation of phagosome biogenesis by intravacuolar pathogens, the detailed biochemical and cellular processes affected remain to be deciphered. Here we discuss how the intravacuolar bacterial pathogens Salmonella, Chlamydia, Mycobacteria, Legionella, Brucella, Coxiella, and Anaplasma utilize their unique set of effectors injected into the host cell to interfere with endocytic, exocytic, and ER-to-Golgi vesicle traffic. However, Coxiella is the main exception for a bacterial pathogen that proliferates within the hydrolytic lysosomal compartment, but its T4SS is essential for adaptation and proliferation within the lysosomal-like vacuole.

Keeping the host alive - lessons from obligate intracellular bacterial pathogens

Mammals have evolved sophisticated host cell death signaling pathways as an important immune mechanism to recognize and eliminate cell intruders before they establish their replicative niche. However, intracellular bacterial pathogens that have co-evolved with their host have developed a multitude of tactics to counteract this defense strategy to facilitate their survival and replication. This requires manipulation of pro-death and pro-survival host signaling pathways during infection. Obligate intracellular bacterial pathogens are organisms that absolutely require an eukaryotic host to survive and replicate, and therefore they have developed virulence factors to prevent diverse forms of host cell death and conserve their replicative niche. This review encapsulates our current understanding of these host-pathogen interactions by exploring the most relevant findings of Anaplasma spp., Chlamydia spp., Rickettsia spp. and Coxiella burnetii modulating host cell death pathways. A detailed comprehension of the molecular mechanisms through which these obligate intracellular pathogens manipulate regulated host cell death will not only increase the current understanding of these difficult-to-study pathogens but also provide insights into new tools to study regulated cell death and the development of new therapeutic approaches to control infection.

Inhibition of Chlamydial Infection by CRISPR/Cas9-SAM Mediated Enhancement of Human Peptidoglycan Recognition Proteins Gene Expression in HeLa Cells

The global problem of emerging resistance of microorganisms to antibiotics makes the search for new natural substances with antibacterial properties relevant. Such substances include peptidoglycan recognition proteins (PGLYRP), which are the components of the innate immunity of many organisms, including humans. These proteins have a unique mechanism of action that allows them to evade the resistance of bacteria to them, as well as to be active against both Gram-positive and Gram-negative bacteria. However, the use of antimicrobial recombinant proteins is not always advisable due to the complexity of local delivery of the proteins and their stability; in this regard it seems appropriate to activate the components of the innate immunity. The aim of this study was to increase the expression level of native peptidoglycan recognition protein genes in HeLa cells using genome-editing technology with synergistic activation mediators (CRISPR/Cas9-SAM) and evaluate antichlamydial effect of PGLYRP. We demonstrated activation of the chlamydial two-component gene system (ctcB-ctcC), which played a key role in the mechanism of action of the peptidoglycan recognition proteins. We generated the HeLa cell line transduced with lentiviruses encoding CRISPR/Cas9-SAM activation system with increased PGLYRP gene expression. It was shown that activation of the own peptidoglycan recognition proteins gene expression in the cell line caused inhibition of the chlamydial infection development. The proposed approach makes it possible to use the capabilities of innate immunity to combat infectious diseases caused by Gram-positive and Gram-negative bacteria.

The Small Molecule H89 Inhibits Chlamydia Inclusion Growth and Production of Infectious Progeny

Chlamydia is an obligate intracellular bacterium and the most common reportable cause of human infection in the United States. This pathogen proliferates inside a eukaryotic host cell, where it resides within a membrane-bound compartment called the chlamydial inclusion. It has an unusual developmental cycle, marked by conversion between a replicating form, the reticulate body (RB), and an infectious form, the elementary body (EB). We found that the small molecule H89 slowed inclusion growth and decreased overall RB replication by 2-fold but caused a 25-fold reduction in infectious EBs. This disproportionate effect on EB production was mainly due to a defect in RB-to-EB conversion and not to the induction of chlamydial persistence, which is an altered growth state. Although H89 is a known inhibitor of specific protein kinases and vesicular transport to and from the Golgi apparatus, it did not cause these anti-chlamydial effects by blocking protein kinase A or C or by inhibiting protein or lipid transport. Thus, H89 is a novel anti-chlamydial compound that has a unique combination of effects on an intracellular Chlamydia infection.

A Chlamydial Plasmid-Dependent Secretion System for the Delivery of Virulence Factors to the Host Cytosol

Chlamydia are obligate intracellular Gram-negative bacteria distinguished by a unique developmental biology confined within a parasitophorous vacuole termed an inclusion. The chlamydial plasmid is a central virulence factor in the pathogenesis of infection. Plasmid gene protein 4 (Pgp4) regulates the expression of plasmid gene protein 3 (Pgp3) and chromosomal glycogen synthase (GlgA), virulence factors secreted from the inclusion to the host cytosol by an unknown mechanism. Here, we identified a plasmid-dependent secretion system for the cytosolic delivery of Pgp3 and GlgA. The secretion system consisted of a segregated population of globular structures originating from midcycle reticulate bodies. Globular structures contained the Pgp4-regulated proteins CT143, CT144, and CT050 in addition to Pgp3 and GlgA. Genetic replacement of Pgp4 with Pgp3 or GlgA negated the formation of globular structures, resulting in retention of Pgp3 and GlgA in chlamydial organisms. The generation of globular structures and secretion of virulence factors occurred independently of type 2 and type 3 secretion systems. Globular structures were enriched with lipopolysaccharide but lacked detectable major outer membrane protein and heat shock protein 60, implicating them as outer membrane vesicles. Thus, we have discovered a novel chlamydial plasmid-dependent secretion system that transports virulence factor cargo from the chlamydial inclusion to the host cytosol.

The Chlamydia trachomatis Tarp effector targets the Hippo pathway

Chlamydia trachomatis injects bacterial effector proteins into human epithelial cells to facilitate the establishment of new infections. The chlamydial type III secreted effector translocated actin recruiting phosphoprotein (Tarp) has been shown to nucleate and bundle actin filaments. It is also believed to initiate new signaling pathways via an N-terminal phosphorylation domain. A comprehensive understanding of the host pathways that are controlled by Tarp to aid in the establishment of a successful infection remains incomplete. To gain further insight into the cell signaling regulated by Tarp, we generated transgenic fruit flies engineered to express the N-terminal domain of Tarp. As many signaling pathways are conserved between flies and mammals, we hypothesized that expression of the Tarp N-domain in the fruit fly might disrupt key pathways, resulting in developmental defects. Tarp N-domain expression in the fruit fly resulted in a mechanosensory bristle duplication phenotype similar to a previously characterized fly phenotype found to be a consequence of defects in the Hippo pathway. Tarp-dependent disruption of the Hippo pathway was confirmed in a C. trachomatis tissue culture infection model. The capability of Tarp to alter Hippo pathway signaling in infected epithelial cells is a previously unrecognized pathway commandeered by chlamydia and likely contributes to the establishment of chlamydia's intracellular niche.

Insights Into Host Cell Cytokines in Chlamydia Infection

Chlamydial infection causes a number of clinically relevant diseases and induces significant morbidity in humans. Immune and inflammatory responses contribute to both the clearance of Chlamydia infection and pathology in host tissues. Chlamydia infection stimulates host cells to produce a large number of cytokines that trigger and regulate host immune responses against Chlamydia. However, inappropriate responses can occur with excessive production of cytokines, resulting in overreactive inflammatory responses and alterations in host or Chlamydia metabolism. As a result, Chlamydia persists and causes wound healing delays, leading to more severe tissue damage and triggering long-lasting fibrotic sequelae. Here, we summarize the roles of cytokines in Chlamydia infection and pathogenesis, thus advancing our understanding chlamydial infection biology and the pathogenic mechanisms involved.

The growing repertoire of genetic tools for dissecting chlamydial pathogenesis

Multiple species of obligate intracellular bacteria in the genus Chlamydia are important veterinary and/or human pathogens. These pathogens all share similar biphasic developmental cycles and transition between intracellular vegetative reticulate bodies and infectious elementary forms, but vary substantially in their host preferences and pathogenic potential. A lack of tools for genetic engineering of these organisms has long been an impediment to the study of their biology and pathogenesis. However, the refinement of approaches developed in C. trachomatis over the last 10 years, and adaptation of some of these approaches to other Chlamydia spp. in just the last few years, has opened exciting new possibilities for studying this ubiquitous group of important pathogens.

Hiding in Plain Sight: Formation and Function of Stress Granules During Microbial Infection of Mammalian Cells

Stress granule (SG) formation is a host cell response to stress-induced translational repression. SGs assemble with RNA-binding proteins and translationally silent mRNA. SGs have been demonstrated to be both inhibitory to viruses, as well as being subverted for viral roles. In contrast, the function of SGs during non-viral microbial infections remains largely unexplored. A handful of microbial infections have been shown to result in host SG assembly. Nevertheless, a large body of evidence suggests SG formation in hosts is a widespread response to microbial infection. Diverse stresses caused by microbes and their products can activate the integrated stress response in order to inhibit translation initiation through phosphorylation of the eukaryotic translation initiation factor 2α (eIF2α). This translational response in other contexts results in SG assembly, suggesting that SG assembly can be a general phenomenon during microbial infection. This review explores evidence for host SG formation in response to bacterial, fungal, and protozoan infection and potential functions of SGs in the host and for adaptations of the pathogen.

Chlamydia trachomatis vaccines for genital infections: where are we and how far is there to go?

INTRODUCTION

Chlamydia trachomatis is the most common sexually transmitted bacterial pathogen in the world. Antibiotic treatment does not prevent against reinfection and a vaccine is not yet available.

AREAS COVERED

We focus the review on the progress made of our understanding of the immunological responses required for a vaccine to elicit protection, and on the antigens, adjuvants, routes of immunization and delivery systems that have been tested in animal models. PubMed and Google Scholar were used to search publication on these topics for the last 5 years and recent Reviews were examined.

EXPERT OPINION

The first Phase 1 clinical trial of a C. trachomatis vaccine to protect against genital infections was successfully completed. We expect that, in the next five years, additional vaccine clinical trials will be implemented.

A 2-pyridone amide inhibitor of transcriptional activity in Chlamydia trachomatis

Chlamydia trachomatis is a strict intracellular bacterium that causes sexually transmitted infections and eye infections that can lead to life-long sequelae. Treatment options are limited to broad-spectrum antibiotics that disturb the commensal flora and contribute to selection of antibiotic-resistant bacteria. Hence, development of novel drugs that specifically target C. trachomatis would be beneficial. 2-pyridone amides are potent and specific inhibitors of Chlamydia infectivity. The first generation compound KSK120, inhibits the developmental cycle of Chlamydia resulting in reduced infectivity of progeny bacteria. Here, we show that the improved, highly potent second-generation 2-pyridone amide KSK213 allowed normal growth and development of C. trachomatis and the effect was only observable upon re-infection of new cells. Progeny elementary bodies (EBs) produced in the presence of KSK213 were unable to activate transcription of essential genes in early development and did not differentiate into the replicative form, the reticulate body (RB). The effect was specific to C. trachomatis since KSK213 was inactive in the closely related animal pathogen C. muridarum and in C. caviae The molecular target of KSK213 may thus be different in C. trachomatis or non-essential in C. muridarum and C. caviae Resistance to KSK213 was mediated by a combination of amino acid substitutions in both DEAD/DEAH RNA helicase and RNAse III, which may indicate inhibition of the transcriptional machinery as the mode of action. 2-pyridone amides provide a novel antibacterial strategy and starting points for development of highly specific drugs for C. trachomatis infections.

Stochastic Chlamydia Dynamics and Optimal Spread

Chlamydia trachomatis is an important bacterial pathogen that has an unusual developmental switch from a dividing form (reticulate body or RB) to an infectious form (elementary body or EB). RBs replicate by binary fission within an infected host cell, but there is a delay before RBs convert into EBs for spread to a new host cell. We developed stochastic optimal control models of the Chlamydia developmental cycle to examine factors that control the number of EBs produced. These factors included the probability and timing of conversion, and the duration of the developmental cycle before the host cell lyses. Our mathematical analysis shows that the observed delay in RB-to-EB conversion is important for maximizing EB production by the end of the intracellular infection.

Generation of a novel affibody molecule targeting Chlamydia trachomatis MOMP

Abstract

Chlamydia trachomatis (C. trachomatis) is the leading cause of preventable blindness worldwide and the most prevalent cause of bacterial sexually transmitted diseases. At present, there is no available vaccine, and recurrences after antibiotics treatment are substantial problems. Major outer membrane protein (MOMP) accounts for 60% of the outer mass of C. trachomatis, functioning as trimeric porin, and it is highly antigenic. Therefore, MOMP is the most promising candidate for vaccine developing and target therapy of Chlamydia. Affibody, a new class of affinity ligands derived from the Z-domain in the binding region of Staphylococcus aureus protein A, has been the focus of researchers as a viable alternative to antibodies. In this study, the MOMP-targeted affibody molecule (Z_MOMP:461) was screened by phage-displayed peptide library. Further, the affinity and specificity were characterized by surface plasmon resonance (SPR) and Western blot. Immunofluorescence assay (IFA) indicated that the MOMP-binding affibody could recognize native MOMP in HeLa229 cells infected C. trachomatis. Immunoprecipitation assay confirmed further that Z_MOMP:461 molecule specifically recognizes the epitope on relaxed trimer MOMP. Our findings provide strong evidence that affibody molecule (Z_MOMP:461) serves as substitute for MOMP antibody for biological applications and has a great potential for delivering drugs for target therapy.

Key points

• We screened a novel affibody molecule Z_MOMP:461 targeting Chlamydia trachomatis MOMP.

• Z_MOMP:461 recognizes the recombinant and native MOMP with high affinity and specificity.

• Z_MOMP:461 could be internalized into live target cells.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-021-11128-x.

Eukaryotic SNARE VAMP3 Dynamically Interacts with Multiple Chlamydial Inclusion Membrane Proteins

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs.

Chlamydia trachomatis, an obligate intracellular pathogen, undergoes a biphasic developmental cycle within a membrane-bound vacuole called the chlamydial inclusion. To facilitate interactions with the host cell, Chlamydia modifies the inclusion membrane with type III secreted proteins, called Incs. As with all chlamydial proteins, Incs are temporally expressed, modifying the chlamydial inclusion during the early and mid-developmental cycle. VAMP3 and VAMP4 are eukaryotic SNARE proteins that mediate membrane fusion and are recruited to the inclusion to facilitate inclusion expansion. Their recruitment requires de novo chlamydial protein synthesis during the mid-developmental cycle. Thus, we hypothesize that VAMP3 and VAMP4 are recruited by Incs. In chlamydia-infected cells, identifying Inc binding partners for SNARE proteins specifically has been elusive. To date, most studies examining chlamydial Inc and eukaryotic proteins have benefitted from stable interacting partners or a robust interaction at a specific time postinfection. While these types of interactions are the predominant class that have been identified, they are likely the exception to chlamydia-host interactions. Therefore, we applied two separate but complementary experimental systems to identify candidate chlamydial Inc binding partners for VAMPs. Based on these results, we created transformed strains of C. trachomatis serovar L2 to inducibly express a candidate Inc-FLAG protein. In chlamydia-infected cells, we found that five Incs temporally and transiently interact with VAMP3. Further, loss of incA or ct813 expression altered VAMP3 localization to the inclusion. For the first time, our studies demonstrate the transient nature of certain host protein-Inc interactions that contribute to the chlamydial developmental cycle.

Competing Substrates for the Bifunctional Diaminopimelic Acid Epimerase/Glutamate Racemase Modulate Peptidoglycan Synthesis in Chlamydia trachomatis

The Chlamydia trachomatis genome encodes multiple bifunctional enzymes, such as DapF, which is capable of both diaminopimelic acid (DAP) epimerase and glutamate racemase activity. Our previous work demonstrated the bifunctional activity of chlamydial DapF in vitro and in a heterologous system (Escherichia coli). In the present study, we employed a substrate competition strategy to demonstrate DapF _Ct function in vivo in C. trachomatis We reasoned that, because DapF _Ct utilizes a shared substrate-binding site for both racemase and epimerase activities, only one activity can occur at a time. Therefore, an excess of one substrate relative to another must determine which activity is favored. We show that the addition of excess l-glutamate or meso-DAP (mDAP) to C. trachomatis resulted in 90% reduction in bacterial titers, compared to untreated controls. Excess l-glutamate reduced in vivo synthesis of mDAP by C. trachomatis to undetectable levels, thus confirming that excess racemase substrate led to inhibition of DapF _Ct DAP epimerase activity. We previously showed that expression of dapF_Ct in a murI (racemase) ΔdapF (epimerase) double mutant of E. coli rescues the d-glutamate auxotrophic defect. Addition of excess mDAP inhibited growth of this strain, but overexpression of dapF_Ct allowed the mutant to overcome growth inhibition. These results confirm that DapF _Ct is the primary target of these mDAP and l-glutamate treatments. Our findings demonstrate that suppression of either the glutamate racemase or epimerase activity of DapF compromises the growth of C. trachomatis Thus, a substrate competition strategy can be a useful tool for in vivo validation of an essential bifunctional enzyme.

Hijacking and Use of Host Kinases by Chlamydiae

Chlamydia species are causative agents of sexually transmitted infections, blinding trachoma, and animal infections with zoonotic potential. Being an obligate intracellular pathogen, Chlamydia relies on the host cell for its survival and development, subverting various host cell processes throughout the infection cycle. A key subset of host proteins utilized by Chlamydia include an assortment of host kinase signaling networks which are vital for many chlamydial processes including entry, nutrient acquisition, and suppression of host cell apoptosis. In this review, we summarize the recent advancements in our understanding of host kinase subversion by Chlamydia.

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.

Transcriptional Landscape of Waddlia chondrophila Aberrant Bodies Induced by Iron Starvation

Chronic infections caused by obligate intracellular bacteria belonging to the Chlamydiales order are related to the formation of persistent developmental forms called aberrant bodies (ABs), which undergo DNA replication without cell division. These enlarged bacteria develop and persist upon exposure to different stressful conditions such as β-lactam antibiotics, iron deprivation and interferon-γ. However, the mechanisms behind ABs biogenesis remain uncharted. Using an RNA-sequencing approach, we compared the transcriptional profile of ABs induced by iron starvation to untreated bacteria in the Chlamydia-related species Waddliachondrophila, a potential agent of abortion in ruminants and miscarriage in humans. Consistent with the growth arrest observed following iron depletion, our results indicate a significant reduction in the expression of genes related to energy production, carbohydrate and amino acid metabolism and cell wall/envelope biogenesis, compared to untreated, actively replicating bacteria. Conversely, three putative toxin-antitoxin modules were among the most up-regulated genes upon iron starvation, suggesting that their activation might be involved in growth arrest in adverse conditions, an uncommon feature in obligate intracellular bacteria. Our work represents the first complete transcriptomic profile of a Chlamydia-related species in stressful conditions and sets the grounds for further investigations on the mechanisms underlying chlamydial persistence.

Building peptidoglycan inside eukaryotic cells: A view from symbiotic and pathogenic bacteria

The peptidoglycan (PG), as the exoskeleton of most prokaryotes, maintains a defined shape and ensures cell integrity against the high internal turgor pressure. These important roles have attracted researchers to target PG metabolism in order to control bacterial infections. Most studies, however, have been performed in bacteria grown under laboratory conditions, leading to only a partial view on how the PG is synthetized in natural environments. As a case in point, PG metabolism and its regulation remain poorly understood in symbiotic and pathogenic bacteria living inside eukaryotic cells. This review focuses on the PG metabolism of intracellular bacteria, emphasizing the necessity of more in vivo studies involving the analysis of enzymes produced in the intracellular niche and the isolation of PG from bacteria residing within eukaryotic cells. The review also points to persistent infections caused by some intracellular bacterial pathogens and the extent at which the PG could contribute to establish such physiological state. Based on recent evidences, I speculate on the idea that certain structural features of the PG may facilitate attenuation of intracellular growth. Lastly, I discuss recent findings in endosymbionts supporting a cooperation between host and bacterial enzymes to assemble a mature PG.

Host and Bacterial Glycolysis during Chlamydia trachomatis Infection

The obligate intracellular pathogen Chlamydia trachomatis is the leading cause of noncongenital blindness and causative agent of the most common sexually transmitted infection of bacterial origin. With a reduced genome, C. trachomatis is dependent on its host for survival, in part due to a need for the host cell to compensate for incomplete bacterial metabolic pathways. However, relatively little is known regarding how C. trachomatis is able to hijack host cell metabolism. In this study, we show that two host glycolytic enzymes, aldolase A and pyruvate kinase, as well as lactate dehydrogenase, are enriched at the C. trachomatis inclusion membrane during infection. Inclusion localization was not species specific, since a similar phenotype was observed with C. muridarum Time course experiments showed that the number of positive inclusions increased throughout the developmental cycle. In addition, these host enzymes colocalized to the same inclusion, and their localization did not appear to be dependent on sustained bacterial protein synthesis or on intact host actin, vesicular trafficking, or microtubules. Depletion of the host glycolytic enzyme aldolase A resulted in decreased inclusion size and infectious progeny production, indicating a role for host glycolysis in bacterial growth. Finally, quantitative PCR analysis showed that expression of C. trachomatis glycolytic enzymes inversely correlated with host enzyme localization at the inclusion. We discuss potential mechanisms leading to inclusion localization of host glycolytic enzymes and how it could benefit the bacteria. Altogether, our findings provide further insight into the intricate relationship between host and bacterial metabolism during Chlamydia infection.