Extrusions are phagocytosed and promote Chlamydia survival within macrophages

Chlamydia-containing spheres are a novel and predominant form of egress by the pathogen Chlamydia psittaci

The egress of intracellular bacteria from host cells and cellular tissues is a critical process during the infection cycle. This process is essential for bacteria to spread inside the host and can influence the outcome of an infection. For the obligate intracellular Gram-negative zoonotic bacterium Chlamydia psittaci, little is known about the mechanisms resulting in bacterial egress from the infected epithelium. Here, we describe and characterize Chlamydia-containing spheres (CCSs), a novel and predominant type of non-lytic egress utilized by Chlamydia spp. CCSs are spherical, low-phase contrast structures surrounded by a phosphatidylserine-exposing membrane with specific barrier functions. They contain infectious progeny and morphologically impaired cellular organelles. CCS formation is a sequential process starting with the proteolytic cleavage of a DEVD tetrapeptide-containing substrate that can be detected inside the chlamydial inclusions, followed by an increase in the intracellular calcium concentration of the infected cell. Subsequently, blebbing of the plasma membrane begins, the inclusion membrane destabilizes, and the proteolytic cleavage of a DEVD-containing substrate increases rapidly within the whole infected cell. Finally, infected, blebbing cells detach and leave the monolayer, thereby forming CCS. This sequence of events is unique for chlamydial CCS formation and fundamentally different from previously described Chlamydia egress pathways. Thus, CCS formation represents a major, previously uncharacterized egress pathway for intracellular pathogens that could be linked to Chlamydia biology in general and might influence the infection outcome in vivo.IMPORTANCEHost cell egress is essential for intracellular pathogens to spread within an organism and for host-to-host transmission. Here, we characterize Chlamydia-containing sphere (CCS) formation as a novel and predominant non-lytic egress pathway of the intracellular pathogens Chlamydia psittaci and Chlamydia trachomatis. CCS formation is fundamentally different from extrusion formation, the previously described non-lytic egress pathway of C. trachomatis. CCS formation is a unique sequential process, including proteolytic activity, followed by an increase in intracellular calcium concentration, inclusion membrane destabilization, plasma membrane blebbing, and the final detachment of a whole phosphatidylserine-exposing former host cell. Thus, CCS formation represents an important and previously uncharacterized egress pathway for intracellular pathogens that could possibly be linked to Chlamydia biology, including host tropism, protection from host cell defense mechanisms, or bacterial pathogenicity.

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.

Insights into innate immune cell evasion by Chlamydia trachomatis

Chlamydia trachomatis, is a kind of obligate intracellular pathogen. The removal of C. trachomatis relies primarily on specific cellular immunity. It is currently considered that CD4+ Th1 cytokine responses are the major protective immunity against C. trachomatis infection and reinfection rather than CD8+ T cells. The non-specific immunity (innate immunity) also plays an important role in the infection process. To survive inside the cells, the first process that C. trachomatis faces is the innate immune response. As the "sentry" of the body, mast cells attempt to engulf and remove C. trachomatis. Dendritic cells present antigen of C. trachomatis to the "commanders" (T cells) through MHC-I and MHC-II. IFN-γ produced by activated T cells and natural killer cells (NK) further activates macrophages. They form the body's "combat troops" and produce immunity against C. trachomatis in the tissues and blood. In addition, the role of eosinophils, basophils, innate lymphoid cells (ILCs), natural killer T (NKT) cells, γδT cells and B-1 cells should not be underestimated in the infection of C. trachomatis. The protective role of innate immunity is insufficient, and sexually transmitted diseases (STDs) caused by C. trachomatis infections tend to be insidious and recalcitrant. As a consequence, C. trachomatis has developed a unique evasion mechanism that triggers inflammatory immunopathology and acts as a bridge to protective to pathological adaptive immunity. This review focuses on the recent advances in how C. trachomatis evades various innate immune cells, which contributes to vaccine development and our understanding of the pathophysiologic consequences of C. trachomatis 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.

Fusobacterium nucleatum dissemination by neutrophils

Recent studies uncovered that Fusobacterium nucleatum (Fn), a common, opportunistic bacterium in the oral cavity, is associated with a growing number of systemic diseases, ranging from colon cancer to Alzheimer's disease. However, the pathological mechanisms responsible for this association are still poorly understood. Here, we leverage recent technological advances to study the interactions between Fn and neutrophils. We show that Fn survives within human neutrophils after phagocytosis. Using in vitro microfluidic devices, we determine that human neutrophils can protect and transport Fn over large distances. Moreover, we validate these observations in vivo by showing that neutrophils disseminate Fn using a zebrafish model. Our data support the emerging hypothesis that bacterial dissemination by neutrophils is a mechanistic link between oral and systemic diseases. Furthermore, our results may ultimately lead to therapeutic approaches that target specific host-bacteria interactions, including the dissemination process.

GSDMD-mediated pyroptosis restrains intracellular Chlamydia trachomatis growth in macrophages

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

Intracellular lifestyle of Chlamydia trachomatis and host-pathogen interactions

In recent years, substantial progress has been made in the understanding of the intracellular lifestyle of Chlamydia trachomatis and how the bacteria establish themselves in the human host. As an obligate intracellular pathogenic bacterium with a strongly reduced coding capacity, C. trachomatis depends on the provision of nutrients from the host cell. In this Review, we summarize the current understanding of how C. trachomatis establishes its intracellular replication niche, how its metabolism functions in the host cell, how it can defend itself against the cell autonomous and innate immune response and how it overcomes adverse situations through the transition to a persistent state. In particular, we focus on those processes for which a mechanistic understanding has been achieved.

López Font F, A. Zayas Rodríguez F. Persistence in Chlamydia

Chlamydia spp. are important causes of acute and persistent/chronic infections. All Chlamydia spp. display a unique biphasic developmental cycle alternating between an infectious elementary body (EB) and a replicative form, the reticulate body (RB), followed by the multiplication of RBs by binary fission and progressive differentiation back into EBs. During its intracellular life, Chlamydia employs multiple mechanisms to ensure its persistence inside the host. These include evasion of diverse innate immune responses, modulation of host cell structure and endocytosis, inhibition of apoptosis, activation of pro-signaling pathways, and conversion to enlarged, non-replicative but viable “aberrant bodies” (ABs). Early research described several systems for Chlamydial persistence with a significant number of variables that make a direct comparison of results difficult. Now, emerging tools for genetic manipulations in Chlamydia and advances in global microarray, transcriptomics, and proteomics have opened new and exciting opportunities to understand the persistent state of Chlamydia and link the immune and molecular events of persistence with the pathogenesis of recurrent and chronic Chlamydial infections. This chapter reviews our current understanding and advances in the molecular biology of Chlamydia persistence.

Chlamydia trachomatis Cell-to-Cell Spread through Tunneling Nanotubes

Tunneling nanotubes (TNTs) are transient cellular connections that consist of dynamic membrane protrusions. They play an important role in cell-to-cell communication and mediate the intercellular exchanges of molecules and organelles. TNTs can form between different cell types and may contribute to the spread of pathogens by serving as cytoplasmic corridors. We demonstrate that Chlamydia (C.) trachomatis-infected human embryonic kidney (HEK) 293 cells and other cells form TNT-like structures through which reticulate bodies (RBs) pass into uninfected cells. Observed TNTs have a life span of 1 to 5 h and contain microtubules, which are essential for chlamydial transfer. They can bridge distances of up to 50 μm between connecting neighboring cells. Consistent with the biological role for TNTs, we show that C. trachomatis spread also occurs under conditions in which the extracellular route of chlamydial entry into host cells is blocked. Based on our findings, we propose that TNTs play a critical role in the direct, cell-to-cell transmission of chlamydia. IMPORTANCE Intracellular bacterial pathogens often undergo a life cycle in which they parasitize infected host cells in membranous vacuoles. Two pathways have been described by which chlamydia can exit infected host cells: lytic cell destruction or exit via extrusion formation. Whether direct, cell-to-cell contact may also play a role in the spread of infection is unknown. Tunneling nanotubes (TNTs) interconnect the cytoplasm of adjacent cells to mediate efficient communication and the exchange of material between them. We used Chlamydia trachomatis and immortalized cells to analyze whether TNTs mediate bacterial transmission from an infected donor to uninfected acceptor cells. We show that chlamydia-infected cells build TNTs through which the intracellular reticulate bodies (RBs) of the chlamydia can pass into uninfected neighboring cells. Our study contributes to the understanding of the function of TNTs in the cell-to-cell transmission of intracellular pathogens and provides new insights into the strategies by which chlamydia spreads among multicellular tissues.

Streptococcus agalactiae npx Is Required for Survival in Human Placental Macrophages and Full Virulence in a Model of Ascending Vaginal Infection during Pregnancy

Streptococcus agalactiae, also known as group B Streptococcus (GBS), is a Gram-positive encapsulated bacterium that colonizes the gastrointestinal tract of 30 to 50% of humans. GBS causes invasive infection during pregnancy that can lead to chorioamnionitis, funisitis, preterm prelabor rupture of membranes (PPROM), preterm birth, neonatal sepsis, and maternal and fetal demise. Upon infecting the host, GBS encounters sentinel innate immune cells, such as macrophages, within reproductive tissues. Once phagocytosed by macrophages, GBS upregulates the expression of the gene npx, which encodes an NADH peroxidase. GBS mutants with an npx deletion (Δnpx) are exquisitely sensitive to reactive oxygen stress. Furthermore, we have shown that npx is required for GBS survival in both THP-1 and placental macrophages. In an in vivo murine model of ascending GBS vaginal infection during pregnancy, npx is required for invading reproductive tissues and is critical for inducing disease progression, including PPROM and preterm birth. Reproductive tissue cytokine production was also significantly diminished in Δnpx mutant-infected animals compared to that in animals infected with wild-type (WT) GBS. Complementation in trans reversed this phenotype, indicating that npx is critical for GBS survival and the initiation of proinflammatory signaling in the gravid host. IMPORTANCE This study sheds new light on the way that group B Streptococcus (GBS) defends itself against oxidative stress in the infected host. The enzyme encoded by the GBS gene npx is an NADH peroxidase that, our study reveals, provides defense against macrophage-derived reactive oxygen stress and facilitates infections of the uterus during pregnancy. This enzyme could represent a tractable target for future treatment strategies against invasive GBS infections.

Streptococcus agalactiae cadD alleviates metal stress and promotes intracellular survival in macrophages and ascending infection during pregnancy

Perinatal infection with Streptococcus agalactiae, or Group B Streptococcus (GBS), is associated with preterm birth, neonatal sepsis, and stillbirth. Here, we study the interactions of GBS with macrophages, essential sentinel immune cells that defend the gravid reproductive tract. Transcriptional analyses of GBS-macrophage co-cultures reveal enhanced expression of a gene encoding a putative metal resistance determinant, cadD. Deletion of cadD reduces GBS survival in macrophages, metal efflux, and resistance to metal toxicity. In a mouse model of ascending infection during pregnancy, the ΔcadD strain displays attenuated bacterial burden, inflammation, and cytokine production in gestational tissues. Furthermore, depletion of host macrophages alters cytokine expression and decreases GBS invasion in a cadD-dependent fashion. Our results indicate that GBS cadD plays an important role in metal detoxification, which promotes immune evasion and bacterial proliferation in the pregnant host.

The Type III Secretion Effector CteG Mediates Host Cell Lytic Exit of Chlamydia trachomatis

Chlamydia trachomatis is an obligate intracellular bacterium causing ocular and urogenital infections in humans that are a significant burden worldwide. The completion of its characteristic infectious cycle relies on the manipulation of several host cell processes by numerous chlamydial type III secretion effector proteins. We previously identified the C. trachomatis CteG effector and showed it localizes at the host cell plasma membrane at late stages of infection. Here, we showed that, from 48 h post-infection, mammalian cells infected by wild-type C. trachomatis contained more infectious chlamydiae in the culture supernatant than cells infected by a CteG-deficient strain. This phenotype was CteG-dependent as it could be complemented in cells infected by the CteG-deficient strain carrying a plasmid encoding CteG. Furthermore, we detected a CteG-dependent defect on host cell cytotoxicity, indicating that CteG mediates chlamydial lytic exit. Previous studies showed that Pgp4, a global regulator of transcription encoded in the C. trachomatis virulence plasmid, also mediates chlamydial lytic exit. However, by using C. trachomatis strains encoding or lacking Pgp4, we showed that production and localization of CteG are not regulated by Pgp4. A C. trachomatis strain lacking both CteG and Pgp4 was as defective in promoting host cell cytotoxicity as mutant strains lacking only CteG or Pgp4. Furthermore, CteG overproduction in a plasmid suppressed the host cell cytotoxic defect of CteG- and Pgp4-deficient chlamydiae. Overall, we revealed the first chlamydial type III secretion effector involved in host cell lytic exit. Our data indicates that CteG and Pgp4 participate in a single cascade of events, but involving multiple layers of regulation, leading to lysis of host cells and release of the infectious chlamydiae.

A targeted approach to investigating immune genes of an iconic Australian marsupial

Disease is a contributing factor to the decline of wildlife populations across the globe. Koalas, iconic yet declining Australian marsupials, are predominantly impacted by two pathogens, Chlamydia and koala retrovirus. Chlamydia is an obligate intracellular bacterium and one of the most widespread sexually transmitted infections in humans worldwide. In koalas, Chlamydia infections can present as asymptomatic or can cause a range of ocular and urogenital disease signs, such as conjunctivitis, cystitis and infertility. In this study, we looked at differences in response to Chlamydia in two northern populations of koalas using a targeted gene sequencing of 1209 immune genes in addition to genome‐wide reduced representation data. We identified two MHC Class I genes associated with Chlamydia disease progression as well as 25 single nucleotide polymorphisms across 17 genes that were associated with resolution of Chlamydia infection. These genes are involved in the innate immune response (TLR5) and defence (TLR5, IFNγ, SERPINE1, STAT2 and STX4). This study deepens our understanding of the role that genetics plays in disease progression in koalas and leads into future work that will use whole genome resequencing of a larger sample set to investigate in greater detail regions identified in this study. Elucidation of the role of host genetics in disease progression and resolution in koalas will directly contribute to better design of Chlamydia vaccines and management of koala populations which have recently been listed as “endangered.”

Ascension of Chlamydia is moderated by uterine peristalsis and the neutrophil response to infection

Chlamydia trachomatis is a common sexually transmitted infection that is associated with a range of serious reproductive tract sequelae including in women Pelvic Inflammatory Disease (PID), tubal factor infertility, and ectopic pregnancy. Ascension of the pathogen beyond the cervix and into the upper reproductive tract is thought to be necessary for these pathologies. However, Chlamydia trachomatis does not encode a mechanism for movement on its genome, and so the processes that facilitate ascension have not been elucidated. Here, we evaluate the factors that may influence chlamydial ascension in women. We constructed a mathematical model based on a set of stochastic dynamics to elucidate the moderating factors that might influence ascension of infections in the first month of an infection. In the simulations conducted from the stochastic model, 36% of infections ascended, but only 9% had more than 1000 bacteria ascend. The results of the simulations indicated that infectious load and the peristaltic contractions moderate ascension and are inter-related in impact. Smaller initial loads were much more likely to ascend. Ascension was found to be dependent on the neutrophil response. Overall, our results indicate that infectious load, menstrual cycle timing, and the neutrophil response are critical factors in chlamydial ascension in women.

Host cell death during infection with Chlamydia: a double-edged sword

The phylum Chlamydiae constitutes a group of obligate intracellular bacteria that infect a remarkably diverse range of host species. Some representatives are significant pathogens of clinical or veterinary importance. For instance, Chlamydia trachomatis is the leading infectious cause of blindness and the most common bacterial agent of sexually transmitted diseases. Chlamydiae are exceptionally dependent on their eukaryotic host cells as a consequence of their developmental biology. At the same time, host cell death is an integral part of the chlamydial infection cycle. It is therefore not surprising that the bacteria have evolved exquisite and versatile strategies to modulate host cell survival and death programs to their advantage. The recent introduction of tools for genetic modification of Chlamydia spp., in combination with our increasing awareness of the complexity of regulated cell death in eukaryotic cells, and in particular of its connections to cell-intrinsic immunity, has revived the interest in this virulence trait. However, recent advances also challenged long-standing assumptions and highlighted major knowledge gaps. This review summarizes current knowledge in the field and discusses possible directions for future research, which could lead us to a deeper understanding of Chlamydia's virulence strategies and may even inspire novel therapeutic approaches.

Global prevalence of chlamydial infections in birds: A systematic review and meta-analysis

Chamydia species are bacterial pathogens that can infect a wide range of animal hosts including humans. In birds, Chlamydia psittaci is a leading cause of chlamydial infections (avian chlamydiosis) and is a zoonotic pathogen causing human psittacosis. The objectives of this study were to estimate the global prevalence of chlamydial infections in birds, to assess heterogeneity of the prevalence for some particular characteristics (continents, bird orders, specimen types, and diagnostic techniques), and to determine the trend of the prevalence over time. The relevant citations on the prevalence of chlamydial infections in birds were retrieved from PubMed, Scopus, and Web of Science. Titles and abstracts of the retrieved citations were screened for possible eligibility. Then, full-texts of eligible articles were assessed for data extraction. A random effects model was used for estimating the global prevalence of chlamydial infections in birds and for all other meta-analyses. Subgroup meta-analysis was used to assess heterogeneity of the prevalence for the characteristics mentioned above. Meta-regression analysis and cumulative meta-analysis were used to determine the trend of the prevalence over time. The quality of each included study was also evaluated. Of 579 citations, 74 studies (a total of 39,225 bird samples from 26 countries, five continents) were included in meta-analysis. Almost all included studies reported birds with C. psittaci infections. The global prevalence of chlamydial infections in birds was 19.5 % (95 % CI, 16.3 %-23.1 %). No significant differences of the prevalence were observed among continents, the prevalence ranged from 16.5 % (95 % CI, 9.8 %-26.5 %) in South America to 21.7 % (95 % CI, 12.1 %-35.9 %) in North America. No significant differences of the prevalence were observed among bird orders, the prevalence ranged from 13.4 % (95 % CI, 7.0 %-23.9 %) in Passeriformes to 32.0 % (95 % CI, 20.6 %-46.1 %) in Galliformes. No significant differences of the prevalence were observed between specimen types; the prevalence was 18.4 % (95 % CI, 15.2 %-22.2 %) for live specimens and 26.3 % (95 % CI, 17.1 %-38.1 %) for carcass specimens. The prevalence was significantly higher in non-PCR techniques (29.5 %, [95 % CI, 22.4 %-37.8 %]) than in PCR techniques (15.3 %, [95 % CI, 12.1 %-19.1 %]). The cumulative evidence showed that the prevalence of chlamydial infections in birds has been relatively stable around 20 % since 2012. This study indicates that the global prevalence of chlamydial infections in birds is quite high and all bird orders are potential sources for human psittacosis.

Chlamydia trachomatis: Cell biology, immunology and vaccination

Chlamydia trachomatis is the causative agent of a highly prevalent sexually transmitted bacterial disease and is associated with a number of severe disease complications. Current therapy options are successful at treating disease, but patients are left without protective immunity and do not benefit the majority asymptomatic patients who do not seek treatment. As such, there is a clear need for a broad acting, protective vaccine that can prevent transmission and protect against symptomatic disease presentation. There are three key elements that underlie successful vaccine development: 1) Chlamydia biology and immune-evasion adaptations, 2) the correlates of protection that prevent disease in natural and experimental infection, 3) reflection upon the evidence provided by previous vaccine attempts. In this review, we give an overview of the unique intra-cellular biology of C. trachomatis and give insight into the dynamic combination of adaptations that allow Chlamydia to subvert host immunity and survive within the cell. We explore the current understanding of chlamydial immunity in animal models and in humans and characterise the key immune correlates of protection against infection. We discuss in detail the specific immune interactions involved in protection, with relevance placed on the CD4+ T lymphocyte and B lymphocyte responses that are key to pathogen clearance. Finally, we provide a timeline of C. trachomatis vaccine research to date and evaluate the successes and failures in development so far. With insight from these three key elements of research, we suggest potential solutions for chlamydial vaccine development and promising avenues for further exploration.

Hematogenous dissemination of Chlamydia muridarum from the urethra in macrophages causes testicular infection and sperm DNA damage†

The incidence of Chlamydia infection, in both females and males, is increasing worldwide. Male infections have been associated clinically with urethritis, epididymitis, and orchitis, believed to be caused by ascending infection, although the impact of infection on male fertility remains controversial. Using a mouse model of male chlamydial infection, we show that all the major testicular cell populations, germ cells, Sertoli cells, Leydig cells, and testicular macrophages can be productively infected. Furthermore, sperm isolated from vas deferens of infected mice also had increased levels of DNA damage as early as 4 weeks post-infection. Bilateral vasectomy, prior to infection, did not affect the chlamydial load recovered from testes at 2, 4, and 8 weeks post-infection, and Chlamydia-infected macrophages were detectable in blood and the testes as soon as 3 days post-infection. Partial depletion of macrophages with clodronate liposomes significantly reduced the testicular chlamydial burden, consistent with a hematogenous route of infection, with Chlamydia transported to the testes in infected macrophages. These data suggest that macrophages serve as Trojan horses, transporting Chlamydia from the penile urethra to the testes within 3 days of infection, bypassing the entire male reproductive tract. In the testes, infected macrophages likely transfer infection to Leydig, Sertoli, and germ cells, causing sperm DNA damage and impaired spermatogenesis.

Global Prevalence of Chlamydial Infections in Reptiles: A Systematic Review and Meta-Analysis

Background and Objectives: Chlamydia spp. are potential zoonotic pathogens that can infect a wide range of animal hosts. In reptiles, Chlamydia can cause hepatitis, pneumonitis, and conjunctivitis and it can cause high mortality in young animals. The objectives of this study were to estimate the pooled prevalence of chlamydial infections in reptiles and to assess the trend of these infections over time. Materials and Methods: The study followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Relevant studies were retrieved from PubMed, Scopus, and Web of Science. The retrieved studies were screened for eligibility. Then, important data were extracted from the included studies. A random effects model was used for all analyses. Subgroup analysis was used to assess heterogeneity for orders of reptiles, continents where the studies were conducted, and types of specimens. Cumulative meta-analysis and meta-regression were used to determine the trend of the prevalence over time. The quality of each included study was evaluated. Results: Of 106 studies (with a total of 2607 samples), 20 met the inclusion criteria and were included in the meta-analysis. The pooled prevalence of chlamydial infections in reptiles was 23.5% (95% confidence interval [CI]: 15.4-34.0). The trend of chlamydial infections increased from 1990 to 2008; thereafter, it was almost stable at slightly over 20%. The most commonly reported Chlamydia spp. were Chlamydia psittaci, Chlamydia pneumoniae, Chlamydia pecorum, and Chlamydia caviae. Among reptiles, the prevalence of chlamydial infections was highest in crocodiles (57.3% [95% CI: 32.5-78.9]). Among continents, the prevalence of chlamydial infections was highest in Australia (68.6% [95% CI: 36.8-89.1]). Conclusions: Based on the included studies, the prevalence of chlamydial infections in reptiles was high, especially in crocodiles. Because C. psittaci and C. pneumoniae are commonly found in reptiles and are well-known zoonotic pathogens, they should be of concern for human health.

An Ancient Molecular Arms Race: Chlamydia vs. Membrane Attack Complex/Perforin (MACPF) Domain Proteins

Dynamic interactions that govern the balance between host and pathogen determine the outcome of infection and are shaped by evolutionary pressures. Eukaryotic hosts have evolved elaborate and formidable defense mechanisms that provide the basis for innate and adaptive immunity. Proteins containing a membrane attack complex/Perforin (MACPF) domain represent an important class of immune effectors. These pore-forming proteins induce cell killing by targeting microbial or host membranes. Intracellular bacteria can be shielded from MACPF-mediated killing, and Chlamydia spp. represent a successful paradigm of obligate intracellular parasitism. Ancestors of present-day Chlamydia likely originated at evolutionary times that correlated with or preceded many host defense pathways. We discuss the current knowledge regarding how chlamydiae interact with the MACPF proteins Complement C9, Perforin-1, and Perforin-2. Current evidence indicates a degree of resistance by Chlamydia to MACPF effector mechanisms. In fact, chlamydiae have acquired and adapted their own MACPF-domain protein to facilitate infection.

A platform for studying the transfer of Chlamydia pneumoniae infection between respiratory epithelium and phagocytes

The obligate intracellular bacterium, Chlamydia pneumoniae, has been identified as a risk factor for several chronic inflammatory diseases in addition to respiratory tract infections. The dissemination of C. pneumoniae from respiratory tract to secondary sites of infection occurs via infected monocyte / macrophage line cells, in which C. pneumoniae can persist as an antibiotic-refractory phenotype. To allow more detailed studies on the epithelium-monocyte/macrophage transition of the infection, new in vitro bioassays are needed. To this end, a coculture system with human continuous cell lines was established. Respiratory epithelial HL cells were infected with C. pneumoniae and THP-1 monocytes were added into the cultures at 67 h post infection. After a 5 h coculture, THP-1 cells were collected with a biotinylated HLA antibody and streptavidin-coated magnetic beads and C. pneumoniae genome copy numbers in THP-1 determined by quantitative PCR. The assay was optimized for cell densities, incubation time, THP-1 separation technique and buffer composition, and its robustness was demonstrated by a Z' value of 0.6. The mitogen-activated protein kinase (MAPK) inhibitors: SP600125 (JNK inhibitor), SB203580 (p38 inhibitor) and FR180204 (ERK inhibitor) suppressed the transfer of C. pneumoniae from HL to THP-1 cells, making them suitable positive controls for the assay. Based on analysis of separate steps of the process, the MAPK inhibitors suppress the bacterial entry to THP-1 cells. The transfer of C. pneumoniae from epithelium to phagocytes represents a crucial step in the establishment of persistent infections by this pathogen, and the presented methods enables future studies to block this process by therapeutic means.

Chlamydial Infection From Outside to Inside

Chlamydia are obligate intracellular bacteria, characterized by a unique biphasic developmental cycle. Specific interactions with the host cell are crucial for the bacteria’s survival and amplification because of the reduced chlamydial genome. At the start of infection, pathogen-host interactions are set in place in order for Chlamydia to enter the host cell and reach the nutrient-rich peri-Golgi region. Once intracellular localization is established, interactions with organelles and pathways of the host cell enable the necessary hijacking of host-derived nutrients. Detailed information on the aforementioned processes will increase our understanding on the intracellular pathogenesis of chlamydiae and hence might lead to new strategies to battle chlamydial infection. This review summarizes how chlamydiae generate their intracellular niche in the host cell, acquire host-derived nutrients in order to enable their growth and finally exit the host cell in order to infect new cells. Moreover, the evolution in the development of molecular genetic tools, necessary for studying the chlamydial infection biology in more depth, is discussed in great detail.

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

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

The Chlamydia trachomatis Extrusion Exit Mechanism Is Regulated by Host Abscission Proteins

The cellular exit strategies of intracellular pathogens have a direct impact on microbial dissemination, transmission, and engagement of immune responses of the host. Chlamydia exit their host via a budding mechanism called extrusion, which offers protective benefits to Chlamydia as they navigate their extracellular environment. Many intracellular pathogens co-opt cellular abscission machinery to facilitate cell exit, which is utilized to perform scission of two newly formed daughter cells following mitosis. Similar to viral budding exit strategies, we hypothesize that an abscission-like mechanism is required to physically sever the chlamydial extrusion from the host cell, co-opting the membrane fission activities of the endosomal sorting complex required for transport (ESCRT) family of proteins that are necessary for cellular scission events, including abscission. To test this, C. trachomatis L2-infected HeLa cells were depleted of key abscission machinery proteins charged multivesicle body protein 4b (CHMP4B), ALIX, centrosome protein 55 (CEP55), or vacuolar protein sorting-associated protein 4A (VPS4A), using RNA interference (RNAi). Over 50% reduction in extrusion formation was achieved by depletion of CHMP4B, VPS4A, and ALIX, but no effect on extrusion was observed with CEP55 depletion. These results demonstrate a role for abscission machinery in C. trachomatis extrusion from the host cell, with ALIX, VPS4A and CHMP4B playing key functional roles in optimal extrusion release.

Immunobiology of monocytes and macrophages during Chlamydia trachomatis infection

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

Genetic Inactivation of Chlamydia trachomatis Inclusion Membrane Protein CT228 Alters MYPT1 Recruitment, Extrusion Production, and Longevity of Infection

Chlamydia trachomatis is an obligate intracellular pathogen with global health and economic impact. Upon infection, C. trachomatis resides within a protective niche, the inclusion, wherein it replicates and usurps host cell machinery and resources. The inclusion membrane is the key host-pathogen interface that governs specific protein-protein interactions to manipulate host signaling pathways. At the conclusion of the infection cycle, C. trachomatis exits the host cell via lysis or extrusion. Extrusion depends on the phosphorylation state of myosin light chain 2 (MLC2); the extent of phosphorylation is determined by the ongoing opposing activities of myosin phosphatase (MYPT1) and myosin kinase (MLCK). Previously, it was shown that MYPT1 is recruited to the inclusion and interacts with CT228 for regulation of host cell egress. In this study, we generated a targeted chromosomal mutation of CT228 (L2-ΔCT228) using the TargeTron system and demonstrate a loss of MYPT1 recruitment and increase in extrusion production in vitro. Mutation of CT228 did not affect chlamydial growth in cell culture or recruitment of MLC2. Moreover, we document a delay in clearance of L2-ΔCT228 during murine intravaginal infection as well as a reduction in systemic humoral response, relative to L2-wild type. Taken together, the data suggest that loss of MYPT1 recruitment (as a result of CT228 disruption) regulates the degree of host cell exit via extrusion and affects the longevity of infection in vivo.

Chlamydia trachomatis fails to protect its growth niche against pro-apoptotic insults

Chlamydia trachomatis is an obligate intracellular bacterial agent responsible for ocular infections and sexually transmitted diseases. It has been postulated that Chlamydia inhibits apoptosis in host cells to maintain an intact replicative niche until sufficient infectious progeny can be generated. Here we report that, while cells infected with C. trachomatis are protected from apoptosis at early and mid-stages of infection, they remain susceptible to the induction of other cell death modalities. By monitoring the fate of infected cells by time-lapse video microscopy and by analyzing host plasma membrane integrity and the activity of caspases, we determined that C. trachomatis-infected cells exposed to pro-apoptotic stimuli predominately died by a mechanism resembling necrosis. This necrotic death of infected cells occurred with kinetics similar to the induction of apoptosis in uninfected cells, indicating that C. trachomatis fails to considerably prolong the lifespan of its host cell when exposed to pro-apoptotic insults. Inhibitors of bacterial protein synthesis partially blocked necrotic death of infected cells, suggesting that the switch from apoptosis to necrosis relies on an active contribution of the bacteria. Tumor necrosis factor alpha (TNF-α)-mediated induction of necrosis in cells infected with C. trachomatis was not dependent on canonical regulators of necroptosis, such as RIPK1, RIPK3, or MLKL, yet was blocked by inhibition or depletion of CASP8. These results suggest that alternative signaling pathways regulate necrotic death in the context of C. trachomatis infections. Finally, consistent with the inability of C. trachomatis to preserve host cell viability, necrosis resulting from pro-apoptotic conditions significantly impaired production of infectious progeny. Taken together, our findings suggest that Chlamydia’s anti-apoptotic activities are not sufficient to protect the pathogen’s replicative niche.

Impact of Active Metabolism on Chlamydia trachomatis Elementary Body Transcript Profile and Infectivity

Bacteria of the genus Chlamydia include the significant human pathogens Chlamydia trachomatis and C. pneumoniae All chlamydiae are obligate intracellular parasites that depend on infection of a host cell and transition through a biphasic developmental cycle. Following host cell invasion by the infectious elementary body (EB), the pathogen transitions to the replicative but noninfectious reticulate body (RB). Differentiation of the RB back to the EB is essential to generate infectious progeny. While the EB form has historically been regarded as metabolically inert, maintenance of infectivity during incubation with specific nutrients has revealed active maintenance of the infectious phenotype. Using transcriptome sequencing, we show that the transcriptome of extracellular EBs incubated under metabolically stimulating conditions does not cluster with germinating EBs but rather with the transcriptome of EBs isolated directly from infected cells. In addition, the transcriptional profile of the extracellular metabolizing EBs more closely resembled that of EB production than germination. Maintenance of infectivity of extracellular EBs was achieved by metabolizing chemically diverse compounds, including glucose 6-phosphate, ATP, and amino acids, all of which can be found in extracellular environments, including mucosal secretions. We further show that the EB cell type actively maintains infectivity in the inclusion after terminal differentiation. Overall, these findings contribute to the emerging understanding that the EB cell form is actively maintained through metabolic processes after terminal differentiation to facilitate prolonged infectivity within the inclusion and under host cell free conditions, for example, following deposition at mucosal surfaces.IMPORTANCE Chlamydiae are obligate intracellular Gram-negative bacteria that are responsible for a wide range of diseases in both animal and human hosts. According to the Centers for Disease Control and Prevention, C. trachomatis is the most frequently reported sexually transmitted infection in the United States, costing the American health care system nearly $2.4 billion annually. Every year, there are over 4 million new cases of Chlamydia infections in the United States and an estimated 100 million cases worldwide. To cause disease, Chlamydia must successfully complete its complex biphasic developmental cycle, alternating between an infectious cell form (EB) specialized for initiating entry into target cells and a replicative form (RB) specialized for creating and maintaining the intracellular replication niche. The EB cell form has historically been considered metabolically quiescent, a passive entity simply waiting for contact with a host cell to initiate the next round of infection. Recent studies and data presented here demonstrate that the EB maintains its infectious phenotype by actively metabolizing a variety of nutrients. Therefore, the EB appears to have an active role in chlamydial biology, possibly within multiple environments, such as mucosal surfaces, fomites, and inside the host cell after formation.

Male genital tract immune response against Chlamydia trachomatis infection

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

Chlamydia trachomatis inclusion membrane protein MrcA interacts with the inositol 1,4,5-trisphosphate receptor type 3 (ITPR3) to regulate extrusion formation

Chlamydia trachomatis is an obligate intracellular bacterium that replicates within a vacuole termed an inclusion. At the end of their intracellular developmental cycle, chlamydiae are released either by lysis of the host cell or extrusion of the intact inclusion. The inclusion membrane is extensively modified by the insertion of type III secreted inclusion membrane proteins, Incs, which contribute to inclusion membrane structure and facilitate host-pathogen interactions. An interaction was identified between the inclusion membrane protein, MrcA, and the Ca2+ channel inositol-1,4,5-trisphosphate receptor, type 3 (ITPR3). ITPR3 was recruited and localized to active Src-family-kinase rich microdomains on the inclusion membrane as was the Ca2+ sensor, STIM1. Disruption of MrcA by directed mutagenesis resulted in loss of ITPR3 recruitment and simultaneous reduction of chlamydial release by extrusion. Complementation of MrcA restored ITPR3 recruitment and extrusion. Inhibition of extrusion was also observed following siRNA depletion of host ITPR3 or STIM1. Chlamydial extrusion was also inhibited by the calcium chelator BAPTA-AM. Each of these treatments resulted in a concomitant reduction in phosphorylation of the myosin regulatory light chain (MLC2) and a loss of myosin motor activity at the end of the developmental cycle which is consistent with the reduced extrusion formation. These studies suggest that Ca2+ signaling pathways play an important role in regulation of release mechanisms by C. trachomatis.

Impact of Gentamicin Concentration and Exposure Time on Intracellular Yersinia pestis

The study of intracellular bacterial pathogens in cell culture hinges on inhibiting extracellular growth of the bacteria in cell culture media. Aminoglycosides, like gentamicin, were originally thought to poorly penetrate eukaryotic cells, and thus, while inhibiting extracellular bacteria, these antibiotics had limited effect on inhibiting the growth of intracellular bacteria. This property led to the development of the antibiotic protection assay to study intracellular pathogens in vitro. More recent studies have demonstrated that aminoglycosides slowly penetrate eukaryotic cells and can even reach intracellular concentrations that inhibit intracellular bacteria. Therefore, important considerations, such as antibiotic concentration, incubation time, and cell type need to be made when designing the antibiotic protection assay to avoid potential false positive/negative observations. Yersinia pestis, which causes the human disease known as the plague, is a facultative intracellular pathogen that can infect and replicate in macrophages. Y. pestis is sensitive to gentamicin and this antibiotic is often employed in the antibiotic protection assay to study the Y. pestis intracellular life cycle. However, a large variety of gentamicin concentrations and incubation periods have been reported in the Y. pestis literature without a clear characterization of the potential influences that variations in the gentamicin protection assay could have on intracellular growth of this pathogen. This raised concerns that variations in the gentamicin protection assay could influence phenotypes and reproducibility of data. To provide a better understanding of the potential consequences that variations in the gentamicin protection assay could have on Y. pestis, we systematically examined the impact of multiple variables of the gentamicin protection assay on Y. pestis intracellular survival in macrophages. We found that prolonged incubation periods with low concentrations of gentamicin, or short incubation periods with higher concentrations of the antibiotic, have a dramatic impact on intracellular growth. Furthermore, the degree of sensitivity of intracellular Y. pestis to gentamicin was also cell type dependent. These data highlight the importance to empirically establish cell type specific gentamicin protection assays to avoid potential artificial data in Y. pestis intracellular studies.

Control of cytoskeletal dynamics during cellular responses to pore forming toxins

Following damage by pore forming toxins (PFTs) host cells engage repair processes and display profound cytoskeletal remodeling and concomitant plasma membrane (PM) blebbing. We have recently demonstrated that host cells utilize similar mechanisms to control cytoskeletal dynamics in response to PFTs and during cell migration. This involves assembly of cortical actomyosin bundles, reorganisation of the endoplasmic reticulum (ER) network, and the interaction between the ER chaperone Gp96 and the molecular motor Non-muscle Myosin Heavy Chain IIA (NMHCIIA). Consequently, Gp96 regulates actomyosin activity, PM blebbing and cell migration, and protects PM integrity against PFTs. In addition, we observed that PFTs increase association of Gp96 and ER vacuoles with the cell surface or within PM blebs loosely attached to the cell body. Similarly, gut epithelial cells damaged by PFTs in vivo were shown to release microvilli structures or directly purge cytoplasmic content. Cytoplasmic purging involves profound cytoskeletal remodeling and ER vacuolation, suggesting that our observations recapitulate recovery processes in vivo. Here, we discuss our findings in light of the current understanding of PM repair mechanisms and in vivo recovery responses to PFTs.

Chlamydia trachomatis Cellular Exit Alters Interactions with Host Dendritic Cells

The strategies utilized by pathogens to exit host cells are an area of pathogenesis which has received surprisingly little attention, considering the necessity of this step for infections to propagate. Even less is known about how exit through these pathways affects downstream host-pathogen interactions and the generation of an immune response. Chlamydia trachomatis exits host epithelial cells through two equally active mechanisms: lysis and extrusion. Studies have characterized the outcome of interactions between host innate immune cells, such as dendritic cells and macrophages, and free, extracellular Chlamydia bacteria, such as those resulting from lysis. Exit via extrusion generates a distinct, host-membrane-bound compartment of Chlamydia separate from the original infected cell. In this study, we assessed the effect of containment within extrusions upon the interaction between Chlamydia and host dendritic cells. Extrusion dramatically affected the outcome of Chlamydia-dendritic cell interactions for both the bacterium and the host cell. Dendritic cells rapidly underwent apoptosis in response to engulfment of an extrusion, while uptake of an equivalent dose of free Chlamydia had no such effect. Containment within an extrusion also prolonged bacterial survival within dendritic cells and altered the initial innate immune signaling by the dendritic cell.

Laser-mediated rupture of chlamydial inclusions triggers pathogen egress and host cell necrosis

Remarkably little is known about how intracellular pathogens exit the host cell in order to infect new hosts. Pathogenic chlamydiae egress by first rupturing their replicative niche (the inclusion) before rapidly lysing the host cell. Here we apply a laser ablation strategy to specifically disrupt the chlamydial inclusion, thereby uncoupling inclusion rupture from the subsequent cell lysis and allowing us to dissect the molecular events involved in each step. Pharmacological inhibition of host cell calpains inhibits inclusion rupture, but not subsequent cell lysis. Further, we demonstrate that inclusion rupture triggers a rapid necrotic cell death pathway independent of BAK, BAX, RIP1 and caspases. Both processes work sequentially to efficiently liberate the pathogen from the host cytoplasm, promoting secondary infection. These results reconcile the pathogen's known capacity to promote host cell survival and induce cell death.

Comparison of Murine Cervicovaginal Infection by Chlamydial Strains: Identification of Extrusions Shed In vivo

Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infections (STIs) and preventable blindness. Untreated, asymptomatic infection as well as frequent re-infection are common and may drive pelvic inflammatory disease, ectopic pregnancy, and infertility. In vivo models of chlamydial infection continue to be instrumental in progress toward a vaccine and further elucidating the pathogenesis of this intracellular bacterium, however significant gaps in our understanding remain. Chlamydial host cell exit occurs via two mechanisms, lysis and extrusion, although the latter has yet to be reported in vivo and its biological role is unclear. The objective of this study was to investigate whether chlamydial extrusions are shed in vivo following infection with multiple strains of Chlamydia. We utilized an established C3H/HeJ murine cervicovaginal infection model with C. trachomatis serovars D and L2 and the Chlamydia muridarum strain MoPn to monitor the (i) time course of infection and mode of host cell exit, (ii) mucosal and systemic immune response to infection, and (iii) gross and histopathology following clearance of active infection. The key finding herein is the first identification of chlamydial extrusions shed from host cells in an in vivo model. Extrusions, a recently appreciated mode of host cell exit and potential means of dissemination, had been previously observed solely in vitro. The results of this study demonstrate that chlamydial extrusions exist in vivo and thus warrant further investigation to determine their role in chlamydial pathogenesis.