The chlamydial inclusion: escape from the endocytic pathway

Chlamydial metabolism revisited: interspecies metabolic variability and developmental stage-specific physiologic activities

Chlamydiae are a group of obligate intracellular bacteria comprising important human and animal pathogens as well as symbionts of ubiquitous protists. They are characterized by a developmental cycle including two main morphologically and physiologically distinct stages, the replicating reticulate body and the infectious nondividing elementary body. In this review, we reconstruct the history of studies that have led to our current perception of chlamydial physiology, focusing on their energy and central carbon metabolism. We then compare the metabolic capabilities of pathogenic and environmental chlamydiae highlighting interspecies variability among the metabolically more flexible environmental strains. We discuss recent findings suggesting that chlamydiae may not live as energy parasites throughout the developmental cycle and that elementary bodies are not metabolically inert but exhibit metabolic activity under appropriate axenic conditions. The observed host-free metabolic activity of elementary bodies may reflect adequate recapitulation of the intracellular environment, but there is evidence that this activity is biologically relevant and required for extracellular survival and maintenance of infectivity. The recent discoveries call for a reconsideration of chlamydial metabolism and future in-depth analyses to better understand how species- and stage-specific differences in chlamydial physiology may affect virulence, tissue tropism, and host adaptation.

Divergent outcomes following transcytosis of IgG targeting intracellular and extracellular chlamydial antigens

Antibodies can have a protective but non-essential role in natural chlamydial infections dependent on antigen specificity and antibody isotype. IgG is the dominant antibody in both male and female reproductive tract mucosal secretions, and is bi-directionally trafficked across epithelia by the neonatal Fc receptor (FcRn). Using pH-polarized epididymal epithelia grown on Transwells, IgG specifically targeted at an extracellular chlamydial antigen; the major outer membrane protein (MOMP), enhanced uptake and translocation of infection at pH 6-6.5 but not at neutral pH. This was dependent on FcRn expression. Conversely, FcRn-mediated transport of IgG targeting the intracellular chlamydial inclusion membrane protein A (IncA), induced aberrant inclusion morphology, recruited autophagic proteins independent of lysosomes and significantly reduced infection. Challenge of female mice with MOMP-specific IgG-opsonized Chlamydia muridarum delayed infection clearance but exacerbated oviduct occlusion. In male mice, MOMP-IgG elicited by immunization afforded no protection against testicular chlamydial infection, whereas the transcytosis of IncA-IgG significantly reduced testicular chlamydial burden. Together these data show that the protective and pathological effects of IgG are dependent on FcRn-mediated transport as well as the specificity of IgG for intracellular or extracellular antigens.

Expression and targeting of secreted proteins from Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular pathogen that replicates in a vacuole termed the inclusion. Many of the interactions of chlamydiae with the host cell are dependent upon bacterial protein synthesis and presumably exposure of these proteins to the cytosol. Because of the dearth of genetic tools for chlamydiae, previous studies examining secreted proteins required the use of heterologous bacterial systems. Recent advances in genetic manipulation of chlamydia now allow for transformation of the bacteria with plasmids. We describe here a shuttle vector system, pBOMB4, that permits expression of recombinant proteins under constitutive or conditional promoter control. We show that the inclusion membrane protein IncD is secreted in a type III-dependent manner from Yersinia pseudotuberculosis and also secreted from C. trachomatis in infected cells where it localizes appropriately to the inclusion membrane. IncD truncated of the first 30 amino acids containing the secretion signal is no longer secreted and is retained by the bacteria. Cytosolic exposure of secreted proteins can be confirmed by using CyaA, GSK, or microinjection assays. A protein predicted to be retained within the bacteria, NrdB is indeed localized to the chlamydia. In addition, we have shown that the chlamydial effector protein, CPAF, which is secreted into the host cell cytosol by a Sec-dependent pathway, also accesses the cytosol when expressed from this system. These assays should prove useful to assess the secretion of other chlamydial proteins that are potentially exposed to the cytosol of the host cell.

Quantitative assessment of cytosolic Salmonella in epithelial cells

Within mammalian cells, Salmonella enterica serovar Typhimurium (S. Typhimurium) inhabits a membrane-bound vacuole known as the Salmonella-containing vacuole (SCV). We have recently shown that wild type S. Typhimurium also colonizes the cytosol of epithelial cells. Here we sought to quantify the contribution of cytosolic Salmonella to the total population over a time course of infection in different epithelial cell lines and under conditions of altered vacuolar escape. We found that the lysosomotropic agent, chloroquine, acts on vacuolar, but not cytosolic, Salmonella. After chloroquine treatment, vacuolar bacteria are not transcriptionally active or replicative and appear degraded. Using a chloroquine resistance assay, in addition to digitonin permeabilization, we found that S. Typhimurium lyses its nascent vacuole in numerous epithelial cell lines, albeit with different frequencies, and hyper-replication in the cytosol is also widespread. At later times post-infection, cytosolic bacteria account for half of the total population in some epithelial cell lines, namely HeLa and Caco-2 C2Bbe1. Both techniques accurately measured increased vacuole lysis in epithelial cells upon treatment with wortmannin. By chloroquine resistance assay, we also determined that Salmonella pathogenicity island-1 (SPI-1), but not SPI-2, the virulence plasmid nor the flagellar apparatus, was required for vacuolar escape and cytosolic replication in epithelial cells. Together, digitonin permeabilization and the chloroquine resistance assay will be useful, complementary tools for deciphering the mechanisms of SCV lysis and Salmonella replication in the epithelial cell cytosol.

Hidden behind autophagy: the unconventional roles of ATG proteins

Macroautophagy (hereafter referred to as autophagy) is an evolutionarily conserved intracellular catabolic transport route that generally allows the lysosomal degradation of cytoplasmic components, including bulk cytosol, protein aggregates, damaged or superfluous organelles and invading microbes. Target structures are sequestered by double‐membrane vesicles called autophagosomes, which are formed through the concerted action of the autophagy (ATG)‐related proteins. Until recently it was assumed that ATG proteins were exclusively involved in autophagy. A growing number of studies, however, have attributed functions to some of them that are distinct from their classical role in autophagosome biogenesis. Autophagy‐independent roles of the ATG proteins include the maintenance of cellular homeostasis and resistance to pathogens. For example, they assist and enhance the turnover of dead cells and microbes upon their phagocytic engulfment, and inhibit murine norovirus replication. Moreover, bone resorption by osteoclasts, innate immune regulation triggered by cytoplasmic DNA and the ER‐associated degradation regulation all have in common the requirement of a subset of ATG proteins. Microorganisms such as coronaviruses, Chlamydia trachomatis or Brucella abortus have even evolved ways to manipulate autophagy‐independent functions of ATG proteins in order to ensure the completion of their intracellular life cycle. Taken together these novel mechanisms add to the repertoire of functions and extend the number of cellular processes involving the ATG proteins.

Host Organelle Hijackers: a similar modus operandi for Toxoplasma gondii and Chlamydia trachomatis: co-infection model as a tool to investigate pathogenesis

The bacterium Chlamydia trachomatis and the protozoan parasite Toxoplasma gondii are the causative agents of chlamydiosis and toxoplasmosis in humans, respectively. Both microorganisms are obligate intracellular pathogens and notorious for extensively modifying the cytoskeletal architecture and the endomembrane system of their host cells to establish productive infections. This review highlights the similar tactics developed by these two pathogens to manipulate their host cell despite their genetic unrelatedness. Using an in vitro cell culture model whereby single fibroblasts are infected by C. trachomatis and T. gondii simultaneously, thus setting up an intracellular competition, we demonstrate that the solutions to the problem of intracellular survival deployed by the parasite and the bacterium may represent an example of convergent evolution, driven by the necessity to acquire nutrients in a hostile environment.

Novel HLA-B27-restricted epitopes from Chlamydia trachomatis generated upon endogenous processing of bacterial proteins suggest a role of molecular mimicry in reactive arthritis

Reactive arthritis (ReA) is an HLA-B27-associated spondyloarthropathy that is triggered by diverse bacteria, including Chlamydia trachomatis, a frequent intracellular parasite. HLA-B27-restricted T-cell responses are elicited against this bacterium in ReA patients, but their pathogenetic significance, autoimmune potential, and relevant epitopes are unknown. High resolution and sensitivity mass spectrometry was used to identify HLA-B27 ligands endogenously processed and presented by HLA-B27 from three chlamydial proteins for which T-cell epitopes were predicted. Fusion protein constructs of ClpC, Na(+)-translocating NADH-quinone reductase subunit A, and DNA primase were expressed in HLA-B27(+) cells, and their HLA-B27-bound peptidomes were searched for endogenous bacterial ligands. A non-predicted peptide, distinct from the predicted T-cell epitope, was identified from ClpC. A peptide recognized by T-cells in vitro, NQRA(330-338), was detected from the reductase subunit. This is the second HLA-B27-restricted T-cell epitope from C. trachomatis with relevance in ReA demonstrated to be processed and presented in live cells. A novel peptide from the DNA primase, DNAP(211-223), was also found. This was a larger variant of a known epitope and was highly homologous to a self-derived natural ligand of HLA-B27. All three bacterial peptides showed high homology with human sequences containing the binding motif of HLA-B27. Molecular dynamics simulations further showed a striking conformational similarity between DNAP(211-223) and its homologous and much more flexible human-derived HLA-B27 ligand. The results suggest that molecular mimicry between HLA-B27-restricted bacterial and self-derived epitopes is frequent and may play a role in ReA.

Mutations in hemG mediate resistance to salicylidene acylhydrazides, demonstrating a novel link between protoporphyrinogen oxidase (HemG) and Chlamydia trachomatis infectivity

Salicylidene acylhydrazides (SAHs) inhibit the type III secretion system (T3S) of Yersinia and other Gram-negative bacteria. In addition, SAHs restrict the growth and development of Chlamydia species. However, since the inhibition of Chlamydia growth by SAH is suppressed by the addition of excess iron and since SAHs have an iron-chelating capacity, their role as specific T3S inhibitors is unclear. We investigated here whether SAHs exhibit a function on C. trachomatis that goes beyond iron chelation. We found that the iron-saturated SAH INP0341 (IS-INP0341) specifically affects C. trachomatis infectivity with reduced generation of infectious elementary body (EB) progeny. Selection and isolation of spontaneous SAH-resistant mutant strains revealed that mutations in hemG suppressed the reduced infectivity caused by IS-INP0341 treatment. Structural modeling of C. trachomatis HemG predicts that the acquired mutations are located in the active site of the enzyme, suggesting that IS-INP0341 inhibits this domain of HemG and that protoporphyrinogen oxidase (HemG) and heme metabolism are important for C. trachomatis infectivity.

Perforin-2 restricts growth of Chlamydia trachomatis in macrophages

Chlamydia trachomatis is a Gram-negative obligate intracellular bacterium that preferentially infects epithelial cells. Professional phagocytes provide C. trachomatis only a limited ability to survive and are proficient killers of chlamydiae. We present evidence herein that identifies a novel host defense protein, perforin-2, that plays a significant role in the eradication of C. trachomatis during the infection of macrophages. Knockdown of perforin-2 in macrophages did not alter the invasion of host cells but did result in chlamydial growth that closely mirrored that detected in HeLa cells. C trachomatis L2, serovar B, and serovar D and C. muridarum were all equally susceptible to perforin-2-mediated killing. Interestingly, induction of perforin-2 expression in epithelial cells is blocked during productive chlamydial growth, thereby protecting chlamydiae from bactericidal attack. Ectopic expression of perforin-2 in HeLa cells, however, does result in killing. Overall, our data implicate a new innate resistance protein in the control of chlamydial infection and may help explain why the macrophage environment is hostile to chlamydial growth.

Role for chlamydial inclusion membrane proteins in inclusion membrane structure and biogenesis

The chlamydial inclusion membrane is extensively modified by the insertion of type III secreted effector proteins. These inclusion membrane proteins (Incs) are exposed to the cytosol and share a common structural feature of a long, bi-lobed hydrophobic domain but little or no primary amino acid sequence similarity. Based upon secondary structural predictions, over 50 putative inclusion membrane proteins have been identified in Chlamydia trachomatis. Only a limited number of biological functions have been defined and these are not shared between chlamydial species. Here we have ectopically expressed several C. trachomatis Incs in HeLa cells and find that they induce the formation of morphologically distinct membranous vesicular compartments. Formation of these vesicles requires the bi-lobed hydrophobic domain as a minimum. No markers for various cellular organelles were observed in association with these vesicles. Lipid probes were incorporated by the Inc-induced vesicles although the lipids incorporated were dependent upon the specific Inc expressed. Co-expression of Inc pairs indicated that some colocalized in the same vesicle, others partially overlapped, and others did not associate at all. Overall, it appears that Incs may have an intrinsic ability to induce membrane formation and that individual Incs can induce membranous structures with unique properties.

Chlamydial intracellular survival strategies

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

A novel co-infection model with Toxoplasma and Chlamydia trachomatis highlights the importance of host cell manipulation for nutrient scavenging

Toxoplasma and Chlamydia trachomatis are obligate intracellular pathogens that have evolved analogous strategies to replicate within mammalian cells. Both pathogens are known to extensively remodel the cytoskeleton, and to recruit endocytic and exocytic organelles to their respective vacuoles. However, how important these activities are for infectivity by either pathogen remains elusive. Here, we have developed a novel co-infection system to gain insights into the developmental cycles of Toxoplasma and C. trachomatis by infecting human cells with both pathogens, and examining their respective ability to replicate and scavenge nutrients. We hypothesize that the common strategies used by Toxoplasma and Chlamydia to achieve development results in direct competition of the two pathogens for the same pool of nutrients. We show that a single human cell can harbour Chlamydia and Toxoplasma. In co-infected cells, Toxoplasma is able to divert the content of host organelles, such as cholesterol. Consequently, the infectious cycle of Toxoplasma progresses unimpeded. In contrast, Chlamydia's ability to scavenge selected nutrients is diminished, and the bacterium shifts to a stress-induced persistent growth. Parasite killing engenders an ordered return to normal chlamydial development. We demonstrate that C. trachomatis enters a stress-induced persistence phenotype as a direct result from being barred from its normal nutrient supplies as addition of excess nutrients, e.g. amino acids, leads to substantial recovery of Chlamydia growth and infectivity. Co-infection of C. trachomatis with slow growing strains of Toxoplasma or a mutant impaired in nutrient acquisition does not restrict chlamydial development. Conversely, Toxoplasma growth is halted in cells infected with the highly virulent Chlamydia psittaci. This study illustrates the key role that cellular remodelling plays in the exploitation of host intracellular resources by Toxoplasma and Chlamydia. It further highlights the delicate balance between success and failure of infection by intracellular pathogens in a co-infection system at the cellular level.

A C. trachomatis cloning vector and the generation of C. trachomatis strains expressing fluorescent proteins under the control of a C. trachomatis promoter

Here we describe a versatile cloning vector for conducting genetic experiments in C. trachomatis. We successfully expressed various fluorescent proteins (i.e. GFP, mCherry and CFP) from C. trachomatis regulatory elements (i.e. the promoter and terminator of the incDEFG operon) and showed that the transformed strains produced wild type amounts of infectious particles and recapitulated major features of the C. trachomatis developmental cycle. C. trachomatis strains expressing fluorescent proteins are valuable tools for studying the C. trachomatis developmental cycle. For instance, we show the feasibility of investigating the dynamics of inclusion fusion and interaction with host proteins and organelles by time-lapse video microscopy.

Protochlamydia induces apoptosis of human HEp-2 cells through mitochondrial dysfunction mediated by chlamydial protease-like activity factor

Obligate amoebal endosymbiotic bacterium Protochlamydia with ancestral pathogenic chlamydial features evolved to survive within protist hosts, such as Acanthamoba, 0.7–1.4 billion years ago, but not within vertebrates including humans. This observation raises the possibility that interactions between Protochlamydia and human cells may result in a novel cytopathic effect, leading to new insights into host-parasite relationships. Previously, we reported that Protochlamydia induces apoptosis of the immortalized human cell line, HEp-2. In this study, we attempted to elucidate the molecular mechanism underlying this apoptosis. We first confirmed that, upon stimulation with the bacteria, poly (ADP-ribose) polymerase (PARP) was cleaved at an early stage in HEp-2 cells, which was dependent on the amount of bacteria. A pan-caspase inhibitor and both caspase-3 and -9 inhibitors similarly inhibited the apoptosis of HEp-2 cells. A decrease of the mitochondrial membrane potential was also confirmed. Furthermore, lactacystin, an inhibitor of chlamydial protease-like activity factor (CPAF), blocked the apoptosis. Cytochalasin D also inhibited the apoptosis, which was dependent on the drug concentration, indicating that bacterial entry into cells was required to induce apoptosis. Interestingly, Yersinia type III inhibitors (ME0052, ME0053, and ME0054) did not have any effect on the apoptosis. We also confirmed that the Protochlamydia used in this study possessed a homologue of the cpaf gene and that two critical residues, histidine-101 and serine-499 of C. trachomatis CPAF in the active center, were conserved. Thus, our results indicate that after entry, Protochlamydia-secreted CPAF induces mitochondrial dysfunction with a decrease of the membrane potential, followed by caspase-9, caspase-3 and PARP cleavages for apoptosis. More interestingly, because C. trachomatis infection can block the apoptosis, our finding implies unique features of CPAF between pathogenic and primitive chlamydiae.

Oligomerization of rab/effector complexes in the regulation of vesicle trafficking

Rabs comprise the largest member of the Ras superfamily of small GTPases with over 60 proteins in mammals and 11 proteins in yeast. Like all small GTPases, Rabs oscillate between an inactive GDP-bound conformation and an active GTP-bound state that is tethered to lipid membranes via a C-terminal prenylation site on conserved cysteine residues. In their active state, Rabs regulate various aspects of membrane trafficking, including vesicle formation, transport, docking, and fusion. The critical element of biological activity is the recruitment of cytosolic effector proteins to specific endomembranes by active Rabs. The importance of Rabs in cellular processes is apparent from their links to genetic disorders, immunodeficiency, cancer, and pathogen invasion. During the last decade, numerous structures of complexes have shed light on the molecular basis for Rab/effector specificity and their topological organization on subcellular membranes. Here, I review the known structures of Rab/effector complexes and their modes of oligomerization. This is followed by a brief discussion on the thermodynamics of effector recruitment, which has not been documented sufficiently in previous reviews. A summary of diseases associated with Rab/effector trafficking pathways concludes this chapter.

Rab11-family of interacting protein 2 associates with chlamydial inclusions through its Rab-binding domain and promotes bacterial multiplication

Chlamydia trachomatis, an obligate intracellular pathogen, survives within host cells in a special compartment named 'inclusion' and takes advantage of host vesicular transport pathways for its growth and replication. Rab GTPases are key regulatory proteins of intracellular trafficking. Several Rabs, among them Rab11 and Rab14, are implicated in chlamydial development. FIP2, a member of the Rab11-Family of Interacting Proteins, presents at the C-terminus a Rab-binding domain that interacts with both Rab11 and Rab14. In this study, we determined and characterized the recruitment of endogenous and GFP-tagged FIP2 to the chlamydial inclusions. The recruitment of FIP2 is specific since other members of the Rab11-Family of Interacting Proteins do not associate with the chlamydial inclusions. The Rab-binding domain of FIP2 is essential for its association. Our results indicate that FIP2 binds to Rab11 at the chlamydial inclusion membrane through its Rab-binding domain. The presence of FIP2 at the chlamydial inclusion favours the recruitment of Rab14. Furthermore, our results show that FIP2 promotes inclusion development and bacterial replication. In agreement, the silencing of FIP2 decreases the bacterial progeny. C. trachomatis likely recruits FIP2 to hijack host intracellular trafficking to redirect vesicles full of nutrients towards the inclusion.

Actin recruitment to the Chlamydia inclusion is spatiotemporally regulated by a mechanism that requires host and bacterial factors

The ability to exit host cells at the end of their developmental growth is a critical step for the intracellular bacterium Chlamydia. One exit strategy, extrusion, is mediated by host signaling pathways involved with actin polymerization. Here, we show that actin is recruited to the chlamydial inclusion as a late event, occurring after 20 hours post-infection (hpi) and only within a subpopulation of cells. This event increases significantly in prevalence and extent from 20 to 68 hpi, and actin coats strongly correlated with extrusions. In contrast to what has been reported for other intracellular pathogens, actin nucleation on Chlamydia inclusions did not 'flash', but rather exhibited moderate depolymerization dynamics. By using small molecule agents to selectively disrupt host signaling pathways involved with actin nucleation, modulate actin polymerization dynamics and also to disable the synthesis and secretion of chlamydial proteins, we further show that host and bacterial proteins are required for actin coat formation. Transient disruption of either host or bacterial signaling pathways resulted in rapid loss of coats in all infected cells and a reduction in extrusion formation. Inhibition of Chlamydia type III secretion also resulted in rapid loss of actin association on inclusions, thus implicating chlamydial effector proteins(s) as being central factors for engaging with host actin nucleating factors, such as formins. In conclusion, our data illuminate the host and bacterial driven process by which a dense actin matrix is dynamically nucleated and maintained on the Chlamydia inclusion. This late stage event is not ubiquitous for all infected cells in a population, and escalates in prevalence and extent throughout the developmental cycle of Chlamydia, culminating with their exit from the host cell by extrusion. The initiation of actin recruitment by Chlamydia appears to be novel, and may serve as an upstream determinant of the extrusion mechanism.

Reorganization of the host cytoskeleton by the intracellular pathogen Chlamydia trachomatis

Chlamydiae are obligate intracellular pathogens that cause a wide range of human diseases. Chlamydia resides in a membrane bound vacuole ("inclusion") that expands to accommodate replicating bacteria. We recently reported that Chlamydia remodels and recruit two major cytoskeletal components of the host cell- F-actin and Intermediate filaments-to form a dynamic scaffold that provides structural stability to the inclusion. As the inclusion expands, a secreted chlamydial protease progressively modifies the intermediate filaments scaffold, presumably to increase the inclusion's flexibility and accommodate the increased bacterial load. This represents a unique mechanism employed by an intracellular pathogen to support its intracellular niche and may be linked to immune evasion by this pathogen. Here, we discuss the potential consequences of Chlamydia-mediated alteration of host cytoskeletal dynamics on the pathogenesis of chlamydial infections.

Isolation of Chlamydia trachomatis and membrane vesicles derived from host and bacteria

The study of intracellular bacteria and nanometer-size membrane vesicles within infected host cells poses an important challenge as it is difficult to identify each distinct population in the context of the complex populations generated from active host-pathogen interactions. Here, suspension cultures of L929 cells infected with the prevalent obligate intracellular bacterium Chlamydia trachomatis strain F/Cal-IC-13 are utilized for the large scale preparation and isolation of natural membrane vesicles and bacterial forms. Cell lysis with nitrogen cavitation in combination with differential centrifugation, OptiPrep™ density gradient separation, and immunoenrichment using anti-chlamydial lipopolysaccharide antibodies and MagnaBind beads allows for the isolation of both productive and persistent bacterial forms, as well as membrane vesicles derived from the host and pathogen. We have evaluated these populations by electron microscopy and Western blot analysis for identification of biomarkers. In addition, purified persistent forms of C. trachomatis induced by ampicillin display adenosine-5'-triphosphate (ATP) transport activity, suggesting that ampicillin-induced persistent C. trachomatis organisms, at least in part, rely upon host ATP as an energy source. Importantly, several chlamydial cytotoxic and/or secreted proteins are demonstrated to be associated with these vesicles, supporting the idea that membrane vesicles are generated by Chlamydia as a means of carrying and delivering virulence factors necessary for pathogenesis. The ability to produce large-scale infections and generate distinct bacteria and host-derived populations for biochemical analysis, while reducing the burdens of time and cost have implications in all areas of chlamydiology. These protocols can be applied to other strains of C. trachomatis or other intracellular bacteria.

Distinct intensity of host-pathogen interactions in Chlamydia psittaci- and Chlamydia abortus-infected chicken embryos

Factors and mechanisms determining the differences in virulence and host specificity between the zoonotic agents Chlamydia psittaci and Chlamydia abortus are still largely unknown. In the present study, two strains were compared for their invasiveness, virulence, and capability of eliciting an immune response in chicken embryos. On breeding day 10, embryonated chicken eggs were inoculated with 5 × 10(4) inclusion-forming units. As shown by immunohistochemistry and quantitative real-time PCR, C. psittaci displayed a significantly better capability of disseminating in the chorioallantoic membrane (CAM) and internal organs than C. abortus. The higher infectious potential of C. psittaci in birds was underlined by significantly higher mRNA expression rates of essential chlamydial genes, such as incA, groEL (in CAM, liver, and spleen), cpaf, and ftsW (in CAM). Although the immune responses to both pathogens were similar, C. psittaci elicited higher macrophage numbers and a stronger expression of a subset of immune-related proteins. The data imply that invasiveness of Chlamydia spp. and propagation in the host are not solely dependent on the level of host immune response but, even to a greater extent, on the expression of bacterial factors related to virulence. The fact that C. psittaci has coped far better than C. abortus with the avian embryo's response by upregulating essential genes may be a key to understanding the mechanisms underlying host adaptation and etiopathology.

Interferon-inducible effector mechanisms in cell-autonomous immunity

Interferons (IFNs) induce the expression of hundreds of genes as part of an elaborate antimicrobial programme designed to combat infection in all nucleated cells - a process termed cell-autonomous immunity. As described in this Review, recent genomic and subgenomic analyses have begun to assign functional properties to novel IFN-inducible effector proteins that restrict bacteria, protozoa and viruses in different subcellular compartments and at different stages of the pathogen life cycle. Several newly described host defence factors also participate in canonical oxidative and autophagic pathways by spatially coordinating their activities to enhance microbial killing. Together, these IFN-induced effector networks help to confer vertebrate host resistance to a vast and complex microbial world.

Engineered phage-based therapeutic materials inhibit Chlamydia trachomatis intracellular infection

Developing materials that are effective against sexually transmitted pathogens such as Chlamydia trachomatis (Ct) and HIV-1 is challenging both in terms of material selection and improving bio-membrane and cellular permeability at desired mucosal sites. Here, we engineered the prokaryotic bacterial virus (M13 phage) carrying two functional peptides, integrin binding peptide (RGD) and a segment of the polymorphic membrane protein D (PmpD) from Ct, as a phage-based material that can ameliorate Ct infection. Ct is a globally prevalent human pathogen for which there are no effective vaccines or microbicides. We show that engineered phage stably express both RGD motifs and Ct peptides and traffic intracellularly and into the lumen of the inclusion in which the organism resides within the host cell. Engineered phage were able to significantly reduce Ct infection in both HeLa and primary endocervical cells compared with Ct infection alone. Polyclonal antibodies raised against PmpD and co-incubated with constructs prior to infection did not alter the course of infection, indicating that PmpD is responsible for the observed decrease in Ct infection. Our results suggest that phage-based design approaches to vector delivery that overcome mucosal cellular barriers may be effective in preventing Ct and other sexually transmitted pathogens.

Lack of effective anti-apoptotic activities restricts growth of Parachlamydiaceae in insect cells

The fundamental role of programmed cell death in host defense is highlighted by the multitude of anti-apoptotic strategies evolved by various microbes, including the well-known obligate intracellular bacterial pathogens Chlamydia trachomatis and Chlamydia (Chlamydophila) pneumoniae. As inhibition of apoptosis is assumed to be essential for a successful infection of humans by these chlamydiae, we analyzed the anti-apoptotic capacity of close relatives that occur as symbionts of amoebae and might represent emerging pathogens. While Simkania negevensis was able to efficiently replicate within insect cells, which served as model for metazoan-derived host cells, the Parachlamydiaceae (Parachlamydia acanthamoebae and Protochlamydia amoebophila) displayed limited intracellular growth, yet these bacteria induced typical features of apoptotic cell death, including formation of apoptotic bodies, nuclear condensation, internucleosomal DNA fragmentation, and effector caspase activity. Induction of apoptosis was dependent on bacterial activity, but not bacterial de novo protein synthesis, and was detectable already at very early stages of infection. Experimental inhibition of host cell death greatly enhanced parachlamydial replication, suggesting that lack of potent anti-apoptotic activities in Parachlamydiaceae may represent an important factor compromising their ability to successfully infect non-protozoan hosts. These findings highlight the importance of the evolution of anti-apoptotic traits for the success of chlamydiae as pathogens of humans and animals.

Flotillin-1 (Reggie-2) contributes to Chlamydia pneumoniae growth and is associated with bacterial inclusion

Chlamydiae are obligate intracellular pathogens replicating only inside the eukaryotic host. Here, we studied the effect of human flotillin-1 protein on Chlamydia pneumoniae growth in human line (HL) and A549 epithelial cell lines. RNA interference was applied to disrupt flotillin-1-mediated endocytosis. Host-associated bacteria were detected by quantitative PCR, and C. pneumoniae growth was evaluated by inclusion counts. C. pneumoniae attachment to host cells was unaffected, but bacterial intracellular growth was attenuated in the flotillin-1-silenced cells. By using confocal microscopy, we detected flotillin-1 colocalized with the inclusion membrane protein A (IncA) in the C. pneumoniae inclusion membranes. In addition, flotillin-1 was associated with IncA in detergent-resistant membrane microdomains (DRMs) in biochemical fractioning. These results suggest that flotillin-1 localizes to the C. pneumoniae inclusion membrane and plays an important role for intracellular growth of C. pneumoniae.

Chlamydia pneumoniae entry into epithelial cells by clathrin-independent endocytosis

A gram-negative obligate intracellular bacterium, Chlamydia pneumoniae, is a common respiratory pathogen. Here, we examined the invasion and attachment of C. pneumoniae K6 into nonphagocytic HL epithelial cell line by manipulating host plasma membranes by using cholesterol-depleting methyl-beta-cyclodextrin (MβCD) and cholesterol-loading MβCD complexed cholesterol (chol-MβCD). The invasion was attenuated by MβCD-treatment while chol-MβCD augmented the attachment and invasion. In addition, the invasion was inhibited by cholesterol sequestering reagents, nystatin and filipin. Furthermore, exposure of host cells to sphingomyelinase inhibited the invasion. RNA interference was used to assay the role of clathrin and human scavenger receptor B, type I (SR-BI) in the entry of C. pneumoniae into A549 lung epithelial adenocarcinoma cells. In contrast to Chlamydia trachomatis L2, the entry of C. pneumoniae was found to be independent of clathrin. In addition, the entry was found to be SR-BI-independent, but interestingly, the chlamydial growth was attenuated in the SR-BI-silenced cells. These findings suggest that the attachment and invasion of C. pneumoniae into nonphagocytic epithelial cells is dependent on the formation of cholesterol- and sphingomyelin-rich plasma membrane microdomains, and the entry is a clathrin-independent process. In addition, our data indicate that SR-BI supports the growth of C. pneumoniae in epithelial cells.

Emerging role of lipid droplets in host/pathogen interactions

Lipid droplets (LDs) are highly dynamic cell organelles involved in energy homeostasis and membrane trafficking. Here, we review how select pathogens interact with LDs. Several RNA viruses use host LDs at different steps of their life cycle. Some intracellular bacteria and parasites usurp host LDs or encode their own lipid biosynthesis machinery, thus allowing production of LDs independently of their host. Although many mechanistic details of host/pathogen LD interactions are unknown, a picture emerges in which the unique cellular architecture and energy stored in LDs are important in the replication of diverse pathogens.

Chlamydia trachomatis infection control programs: lessons learned and implications for vaccine development

Chlamydia trachomatis control efforts that enhance detection and treatment of infected women may paradoxically increase susceptibility of the population to infection. Conversely, these surveillance programs lower incidences of adverse sequelae elicited by genital tract infection (e.g., pelvic inflammatory disease and ectopic pregnancy), suggesting enhanced identification and eradication of C. trachomatis simultaneously reduces pathogen-induced upper genital tract damage and abrogates formation of protective immune responses. In this paper, we detail findings from C. trachomatis infection control programs that increase our understanding of chlamydial immunoepidemiology and discuss their implications for prophylactic vaccine design.

Chlamydia trachomatis inclusions induce asymmetric cleavage furrow formation and ingression failure in host cells

Chlamydia trachomatis infection has been suggested to induce host genome duplication and is linked to increased risks of cervical cancer. We describe here the mechanism by which Chlamydia causes a cleavage furrow defect that consistently results in the formation of multinucleated host cells, a phenomenon linked to tumorigenesis. Host signaling proteins essential for cleavage furrow initiation, ingression, and stabilization are displaced from one of the prospective furrowing cortices after Chlamydia infection. This protein displacement leads to the formation of a unique asymmetrical, unilateral cleavage furrow in infected human cells. The asymmetrical distribution of signaling proteins is caused by the physical presence of the Chlamydia inclusion at the cell equator. By using ingested latex beads, we demonstrate that the presence of a large vacuole at the cell equator is sufficient to cause furrow ingression failure and can lead to multinucleation. Interestingly, internalized latex beads of similar size do not localize to the cell equator as efficiently as Chlamydia inclusions; moreover, inhibition of bacterial protein synthesis with antibiotic reduces the frequency at which Chlamydia localizes to the cell equator. Together, these results suggest that Chlamydia effectors are involved in strategic positioning of the inclusion during cell division.

Role for the SRC family kinase Fyn in sphingolipid acquisition by chlamydiae

The bacterial obligate intracellular pathogen Chlamydia trachomatis replicates within a membrane-bound vacuole termed the inclusion. From within this protective environment, chlamydiae usurp numerous functions of the host cell to promote chlamydial survival and replication. Here we utilized a small interfering RNA (siRNA)-based screening protocol designed to identify host proteins involved in the trafficking of sphingomyelin to the chlamydial inclusion. Twenty-six host proteins whose deficiency significantly decreased sphingomyelin trafficking to the inclusion and 16 proteins whose deficiency significantly increased sphingomyelin trafficking to the inclusion were identified. The reduced sphingomyelin trafficking caused by downregulation of the Src family tyrosine kinase Fyn was confirmed in more-detailed analyses. Fyn silencing did not alter sphingomyelin synthesis or trafficking in the absence of chlamydial infection but reduced the amount of sphingomyelin trafficked to the inclusion in infected cells, as determined by two independent quantitative assays. Additionally, inhibition of Src family kinases resulted in increased cellular retention of sphingomyelin and significantly decreased incorporation into elementary bodies of both C. trachomatis and Chlamydophila caviae.

Chlamydia trachomatis Slc1 is a type III secretion chaperone that enhances the translocation of its invasion effector substrate TARP

Bacterial type III secretion system (T3SS) chaperones pilot substrates to the export apparatus in a secretion-competent state, and are consequently central to the translocation of effectors into target cells. Chlamydia trachomatis is a genetically intractable obligate intracellular pathogen that utilizes T3SS effectors to trigger its entry into mammalian cells. The only well-characterized T3SS effector is TARP (translocated actin recruitment protein), but its chaperone is unknown. Here we exploited a known structural signature to screen for putative type III secretion chaperones encoded within the C. trachomatis genome. Using bacterial two-hybrid, co-precipitation, cross-linking and size exclusion chromatography we show that Slc1 (SycE-like chaperone 1; CT043) specifically interacts with a 200-amino-acid residue N-terminal region of TARP (TARP¹⁻²⁰⁰). Slc1 formed homodimers in vitro, as shown in cross-linking and gel filtration experiments. Biochemical analysis of an isolated Slc1-TARP¹⁻²⁰⁰ complex was consistent with a characteristic 2:1 chaperone-effector stoichiometry. Furthermore, Slc1 was co-immunoprecipitated with TARP from C. trachomatis elementary bodies. Also, coexpression of Slc1 specifically enhanced host cell translocation of TARP by a heterologous Yersinia enterocolitica T3SS. Taken together, we propose Slc1 as a chaperone of the C. trachomatis T3SS effector TARP.

Development of a transformation system for Chlamydia trachomatis: restoration of glycogen biosynthesis by acquisition of a plasmid shuttle vector

Chlamydia trachomatis remains one of the few major human pathogens for which there is no transformation system. C. trachomatis has a unique obligate intracellular developmental cycle. The extracellular infectious elementary body (EB) is an infectious, electron-dense structure that, following host cell infection, differentiates into a non-infectious replicative form known as a reticulate body (RB). Host cells infected by C. trachomatis that are treated with penicillin are not lysed because this antibiotic prevents the maturation of RBs into EBs. Instead the RBs fail to divide although DNA replication continues. We have exploited these observations to develop a transformation protocol based on expression of β-lactamase that utilizes rescue from the penicillin-induced phenotype. We constructed a vector which carries both the chlamydial endogenous plasmid and an E.coli plasmid origin of replication so that it can shuttle between these two bacterial recipients. The vector, when introduced into C. trachomatis L2 under selection conditions, cures the endogenous chlamydial plasmid. We have shown that foreign promoters operate in vivo in C. trachomatis and that active β-lactamase and chloramphenicol acetyl transferase are expressed. To demonstrate the technology we have isolated chlamydial transformants that express the green fluorescent protein (GFP). As proof of principle, we have shown that manipulation of chlamydial biochemistry is possible by transformation of a plasmid-free C. trachomatis recipient strain. The acquisition of the plasmid restores the ability of the plasmid-free C. trachomatis to synthesise and accumulate glycogen within inclusions. These findings pave the way for a comprehensive genetic study on chlamydial gene function that has hitherto not been possible. Application of this technology avoids the use of therapeutic antibiotics and therefore the procedures do not require high level containment and will allow the analysis of genome function by complementation.

Golgi-associated Rab14, a new regulator for Chlamydia trachomatis infection outcome

Chlamydia trachomatis is the causing agent of the most frequent bacterial sexually-transmitted diseases worldwide and is an underlying cause of chronic pelvic inflammatory diseases and cervical cancer. It is an obligate intracellular bacterium that establishes a close relationship with the Golgi complex and parasites the biosynthetic machinery of host cells. In a recent study, we have demonstrated that Rab14, a newly-described Golgi-associated Rab, is involved in the delivery of sphingolipids to the growing bacteria-containing vacuole. The interference with Rab14-controlled trafficking pathways delays chlamydial inclusion enlargement, decreases bacterial lipid uptake, negatively impact on bacterial differentiation, and reduces bacterial progeny and infectivity. C. trachomatis manipulation of host trafficking pathways for the acquisition of endogenously-biosynthesized nutrients arises as one of the characteristics of this highly evolved pathogen. The development of therapeutic strategies targeted to interfere with bacterium-host cell interaction is a new challenge for pharmacological approaches to control chlamydial infections.

Dynamic modification of microtubule-dependent transport by effector proteins of intracellular Salmonella enterica

Intracellular Salmonella enterica translocate effector proteins that modify microtubule-dependent transport processes of the host cell and modulate the biogenesis of the Salmonella-containing vacuole (SCV). One functional consequence is the induction of tubular aggregates of endosomal membranes, termed Salmonella-induced filaments or SIFs, and further tubular membrane compartments have recently been described. SIFs are unique, highly dynamic compartments that form by modification of vesicular transport on microtubules. The molecular mechanism of the interference of intracellular Salmonella with host cell vesicular transport is still elusive, but recent studies demonstrate the complexity of pathogenic activities and the intricacy of manipulating host cell functions.

The lipid transfer protein CERT interacts with the Chlamydia inclusion protein IncD and participates to ER-Chlamydia inclusion membrane contact sites

Bacterial pathogens that reside in membrane bound compartment manipulate the host cell machinery to establish and maintain their intracellular niche. The hijacking of inter-organelle vesicular trafficking through the targeting of small GTPases or SNARE proteins has been well established. Here, we show that intracellular pathogens also establish direct membrane contact sites with organelles and exploit non-vesicular transport machinery. We identified the ER-to-Golgi ceramide transfer protein CERT as a host cell factor specifically recruited to the inclusion, a membrane-bound compartment harboring the obligate intracellular pathogen Chlamydia trachomatis. We further showed that CERT recruitment to the inclusion correlated with the recruitment of VAPA/B-positive tubules in close proximity of the inclusion membrane, suggesting that ER-Inclusion membrane contact sites are formed upon C. trachomatis infection. Moreover, we identified the C. trachomatis effector protein IncD as a specific binding partner for CERT. Finally we showed that depletion of either CERT or the VAP proteins impaired bacterial development. We propose that the presence of IncD, CERT, VAPA/B, and potentially additional host and/or bacterial factors, at points of contact between the ER and the inclusion membrane provides a specialized metabolic and/or signaling microenvironment favorable to bacterial development.

SNARE motif: a common motif used by pathogens to manipulate membrane fusion

To penetrate host cells through their membranes, pathogens use a variety of molecular components in which the presence of heptad repeat motifs seems to be a prevailing element. Heptad repeats are characterized by a pattern of seven, generally hydrophobic, residues. In order to initiate membrane fusion, viruses use glycoproteins-containing heptad repeats. These proteins are structurally and functionally similar to the SNARE proteins known to be involved in eukaryotic membrane fusion. SNAREs also display a heptad repeat motif called the "SNARE motif". As bacterial genomes are being sequenced, microorganisms also appear to be carrying membrane proteins resembling eukaryotic SNAREs. This category of SNARE-like proteins might share similar functions and could be used by microorganisms to either promote or block membrane fusion. Such a recurrence across pathogenic organisms suggests that this architectural motif was evolutionarily selected because it most effectively ensures the survival of pathogens within the eukaryotic environment.

Design and synthesis of 2-arylbenzimidazoles and evaluation of their inhibitory effect against Chlamydia pneumoniae

Chlamydia pneumoniae is an intracellular bacterium that responds poorly to antibiotic treatment. Insufficient antibiotic usage leads to chronic infection, which is linked to disease processes of asthma, atherosclerosis, and Alzheimer's disease. The Chlamydia research lacks genetic tools exploited by other antimicrobial research, and thus other approaches to drug discovery must be applied. A set of 2-arylbenzimidazoles was designed based on our earlier findings, and 33 derivatives were synthesized. Derivatives were assayed against C. pneumoniae strain CWL-029 in an acute infection model using TR-FIA method at a concentration of 10 μM, and the effects of the derivatives on the host cell viability were evaluated at the same concentration. Fourteen compounds showed at least 80% inhibition, with only minor changes in host cell viability. Nine most potential compounds were evaluated using immunofluorescence microscopy on two different strains of C. pneumoniae CWL-029 and CV-6. The N-[3-(1H-benzimidazol-2-yl)phenyl]-3-methylbenzamide (42) had minimal inhibitory concentration (MIC) of 10 μM against CWL-029 and 6.3 μM against the clinical strain CV-6. This study shows the high antichlamydial potential of 2-arylbenzimidazoles, which also seem to have good characteristics for lead compounds.

Native properdin binds to Chlamydia pneumoniae and promotes complement activation

Activation of complement represents one means of natural resistance to infection from a wide variety of potential pathogens. Recently, properdin, a positive regulator of the alternative pathway of complement, has been shown to bind to surfaces and promote complement activation. Here we studied whether properdin-mediated complement activation occurs on the surface of Chlamydia pneumoniae, an obligate intracellular Gram-negative bacterium that causes 10 to 20% of community-acquired pneumonia. We have determined for the first time that the physiological P₂, P₃, and P₄ forms of human properdin bind to the surface of Chlamydia pneumoniae directly. The binding of these physiological forms accelerates complement activation on the Chlamydia pneumoniae surface, as measured by C3b and C9 deposition. Finally, properdin-depleted serum could not control Chlamydia pneumoniae infection of HEp-2 cells compared with normal human serum. However, after addition of native properdin, the properdin-depleted serum recovered the ability to control the infection. Altogether, our data suggest that properdin is a pattern recognition molecule that plays a role in resistance to Chlamydia infection.

The trans-Golgi SNARE syntaxin 6 is recruited to the chlamydial inclusion membrane

Chlamydia trachomatis is an obligate intracellular pathogen that replicates within a parasitophorous vacuole termed an inclusion. The chlamydial inclusion is isolated from the endocytic pathway but fusogenic with Golgi-derived exocytic vesicles containing sphingomyelin and cholesterol. Sphingolipids are incorporated into the chlamydial cell wall and are considered essential for chlamydial development and viability. The mechanisms by which chlamydiae obtain eukaryotic lipids are poorly understood but require chlamydial protein synthesis and presumably modification of the inclusion membrane to initiate this interaction. A polarized cell model of chlamydial infection has demonstrated that chlamydiae preferentially intercept basolaterally directed, sphingomyelin-containing exocytic vesicles. Here we examine the localization and potential function of trans-Golgi and/or basolaterally associated soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins in chlamydia-infected cells. The trans-Golgi SNARE protein syntaxin 6 is recruited to the chlamydial inclusion in a manner that requires chlamydial protein synthesis and is conserved among all chlamydial species examined. The localization of syntaxin 6 to the chlamydial inclusion requires a tyrosine motif or plasma membrane retrieval signal (YGRL). Thus in addition to expression of at least two inclusion membrane proteins that contain SNARE-like motifs, chlamydiae also actively recruit eukaryotic SNARE-family proteins.

Chlamydia trachomatis intercepts Golgi-derived sphingolipids through a Rab14-mediated transport required for bacterial development and replication

Chlamydia trachomatis are obligate intracellular bacteria that survive and replicate in a bacterial-modified phagosome called inclusion. As other intracellular parasites, these bacteria subvert the phagocytic pathway to avoid degradation in phagolysosomes and exploit trafficking pathways to acquire both energy and nutrients essential for their survival. Rabs are host proteins that control intracellular vesicular trafficking. Rab14, a Golgi-related Rab, controls Golgi to endosomes transport. Since Chlamydia establish a close relationship with the Golgi apparatus, the recruitment and participation of Rab14 on inclusion development and bacteria growth were analyzed. Time course analysis revealed that Rab14 associated with inclusions by 10 h post infection and was maintained throughout the entire developmental cycle. The recruitment was bacterial protein synthesis-dependent but independent of microtubules and Golgi integrity. Overexpression of Rab14 dominant negative mutants delayed inclusion enlargement, and impaired bacteria replication as determined by IFU. Silencing of Rab14 by siRNA also decreased bacteria multiplication and infectivity. By electron microscopy, aberrant bacteria were observed in cells overexpressing the cytosolic negative Rab14 mutant. Our results showed that Rab14 facilitates the delivery of sphingolipids required for bacterial development and replication from the Golgi to chlamydial inclusions. Novel anti-chlamydial therapies could be developed based on the knowledge of how bacteria subvert host vesicular transport events through Rabs manipulation.

The potential for the noninvasive delivery of polymeric nanocarriers using propellant-based inhalers in the treatment of Chlamydial respiratory infections

A novel strategy for pulmonary delivery of polymeric nanocarriers (NCs) pressurized-metered dose inhalers (pMDIs) is reported in this work. Core-shell particles consisting of a water soluble, hydrofluoroalkane(HFA)-philic biodegradable copolymer of chitosan and poly(lactic acid), and a core of poly(d,l-lactide-co-glycolide) (PLGA) NCs were prepared by a modified emulsification-diffusion methodology. Dispersions of the core-shell particles in HFA propellant revealed enhanced physical stability compared to polymeric NCs alone, and more importantly, excellent aerosol characteristics as determined by inertial impaction studies. Confocal microscopy revealed that the polymeric NCs from such core-shell particles are capable not only to be taken up by Calu-3 (airway epithelial) cells that have been infected with Chlamydia pneumoniae, an intracellular pathogen, but are also internalized within chlamydial inclusions. Our results suggest that the proposed methodology can be used as a general platform for the delivery of polymeric NCs to the respiratory tract using the inexpensive pMDIs, and that such an approach may be used to target and deliver drugs to treat chlamydial-related infections.

Uncivil engineers: Chlamydia, Salmonella and Shigella alter cytoskeleton architecture to invade epithelial cells

The obligate intracellular bacterial pathogen Chlamydia trachomatis is a major cause of blindness and sexually transmitted diseases. Like the enteric pathogens Salmonella and Shigella, Chlamydia injects effector proteins into epithelial cells to initiate extensive remodeling of the actin cytoskeleton at the bacterial attachment site, which culminates in the engulfment of the bacterium by plasma membrane extensions. Numerous Salmonella and Shigella effectors promote this remodeling by activating Rho GTPases and tyrosine kinase signaling cascades and by directly manipulating actin dynamics. Recent studies indicate that similar host-cell alterations occur during Chlamydia invasion, but few effectors are known. The identification of additional Chlamydia effectors and the elucidation of their modes of function are critical steps towards an understanding of how this clinically important pathogen breaches epithelial surfaces and causes infection.

Generation of genetic diversity in microsporidia via sexual reproduction and horizontal gene transfer

Microsporidia are obligate intracellular pathogens mainly infecting both vertebrate and invertebrate hosts. The group comprises approximately 150 genera with 1,200 species. Due to sequence divergence phylogenic reconstructions that are solely based on DNA sequence have been unprecise for these pathogens. Our previous study identified that three microsporidian genomes contained a putative sex-related locus similar to that of zygomycetes. In a comparison of genome architecture of the microsporidia to other fungi, Rhizopus oryzae, a zygomycete fungus, shared more common gene clusters with Encephalitozoon cuniculi, a microsporidian. This provides evidence supporting the hypothesis that microsporidia and zygomycete fungi may share a more recent common ancestor than other fungal lineages. Genetic recombination is an important outcome of sexual development. We describe genetic markers which will enable tests of whether sex occurs within E. cuniculi populations by analyzing tandem repeat DNA regions in three different isolates. Taken together, the phylogenetic relationship of microsporidia to fungi and the presence of a sex-related locus in their genomes suggest the microsporidia may have an extant sexual cycle. In addition, we describe recently reported evidence of horizontal gene transfer from Chlamydia to the E. cuniculi genome and show that these two obligate intracellular pathogens can infect the same host cells.

Inflammation and fibrosis during Chlamydia pneumoniae infection is regulated by IL-1 and the NLRP3/ASC inflammasome

Chlamydia pneumoniae is a common respiratory pathogen associated with atypical pneumonia, and it has been suggested as a trigger or promoter of several chronic inflammatory conditions, such as asthma and atherosclerosis. The beta form of IL-1 (IL-1beta) is a proinflammatory cytokine released by many cell types and is an important mediator of inflammation during infection. IL-1beta production is a tightly controlled process that includes regulation at multiple levels and typically requires two distinct signals for activation and release. In this study, we investigated the ability of C. pneumoniae to induce IL-1beta secretion. We found that C. pneumoniae was unique among the other Chlamydia species tested in its ability to potently induce secretion of mature IL-1beta from unprimed bone marrow-derived macrophages during a productive infection. TLR2 was required for induction of pro-IL-1beta, whereas the NLRP3/ASC was required for caspase-1 activation and pro-IL-1beta cleavage to produce mature IL-1beta. Caspase-1 cleavage was independent of endogenous ATP release, but required potassium flux, lysosomal acidification, and cathepsin B release. We further investigated the role of IL-1 in host defense against C. pneumoniae-induced pneumonia using mice deficient in the type I IL-1R. Although the IL-1R(-/-) mice developed an inflammatory infiltrate, the number of infiltrating neutrophils was lower, whereas there was evidence of increased infiltrating fibroblasts and mesenchymal cells and more lung fibrosis. We conclude that C. pneumoniae directly activates the NLRP3/ASC inflammasome, leading to the release of biologically active IL-1beta, and that concurrent IL-1 signaling is required for optimal host defense against acute bacterial pneumonia.

Multiple host proteins that function in phosphatidylinositol-4-phosphate metabolism are recruited to the chlamydial inclusion

Chlamydiae replicate within a nonacidified vacuole, termed an inclusion. As obligate intracellular bacteria, chlamydiae actively modify their vacuole to exploit host signaling and trafficking pathways. Recently, we demonstrated that several Rab GTPases are actively targeted to the inclusion. To define the biological roles of inclusion localized Rab GTPases, we have begun to identify inclusion-localized Rab effectors. Here we demonstrate that oculocerebrorenal syndrome of Lowe protein 1 (OCRL1), a Golgi complex-localized phosphatidylinositol (PI)-5-phosphatase that binds to multiple Rab GTPases, localizes to chlamydial inclusions. By examining the intracellular localization of green fluorescent protein (GFP) fusion proteins that bind to unique phosphoinositide species, we also demonstrate that phosphatidylinositol-4-phosphate (PI4P), the product of OCRL1, is present at the inclusion membrane. Furthermore, two additional host proteins, Arf1, which together with PI4P mediates the recruitment of PI4P-binding proteins to the Golgi complex, and PI4KII alpha, a major producer of Golgi complex-localized PI4P, also localize to chlamydial inclusions. Depletion of OCRL1, Arf1, or PI4KII alpha by small interfering RNA (siRNA) decreases inclusion formation and the production of infectious progeny. Infectivity is further decreased in cells simultaneously depleted for all three host proteins, suggesting partially overlapping functions in infected cells. Collectively, these data demonstrate that Chlamydia species create a unique replication-competent vacuolar environment by modulating both the Rab GTPase and the PI composition of the chlamydial inclusion.

Chlamydia vaccine candidates and tools for chlamydial antigen discovery

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

Modulation of membrane traffic between endoplasmic reticulum, ERGIC and Golgi to generate compartments for the replication of bacteria and viruses

Several bacteria and viruses remodel cellular membranes to form compartments specialised for replication. Bacteria replicate within inclusions which recruit membrane vesicles from the secretory pathway to provide nutrients for microbial growth and division. Viruses generate densely packed membrane vesicles called viroplasm which provide a platform to recruit host and viral proteins necessary for replication. This review describes examples where both intracellular bacteria (Salmonella, Chlamydia and Legionella) and viruses (picornaviruses and hepatitis C) recruit membrane vesicles to sites of replication by modulating proteins that control the secretory pathway. In many cases this involves modulation of Rab and Arf GTPases.