Strain and virulence diversity in the mouse pathogen Chlamydia muridarum

Study Models for Chlamydia trachomatis Infection of the Female Reproductive Tract

Chlamydia trachomatis (Ct) is a leading cause of sexually transmitted infections globally, often resulting in inflammatory disorders, ectopic pregnancies, and infertility. Studying Ct's pathogenesis remains challenging due to its unique life cycle and host-specific interactions, which require diverse experimental models. Animal studies using mouse, guinea pig, pig, and non-human primate models provide valuable insights into immune responses, hormonal influences, and disease progression. However, they face limitations in terms of translational relevance due to physiological differences, as well as ethical concerns. Complementing these, in vitro systems, ranging from simple monolayer to advanced three-dimensional models, exhibit improved physiological relevance by replicating the human tissue architecture. This includes the detailed investigation of epithelial barrier disruptions, epithelium-stroma interactions, and immune responses at a cellular level. Nonetheless, in vitro models fall short in mimicking the intricate tissue structures found in vivo and, therefore, cannot faithfully replicate the host-pathogen interactions or infection dynamics observed in living organisms. This review presents a comprehensive overview of the in vivo and in vitro models employed over the past few decades to investigate Ct and its pathogenesis, addressing their strengths and limitations. Furthermore, we explore emerging technologies, including organ-on-chip and in silico models, as promising tools to overcome the existing challenges and refine our understanding of Ct infections.

Chlamydiosis in Animals

The Chlamydiaceae family consists of Gram-negative, obligate intracellular bacteria that replicate within the cells of a diverse range of hosts. These hosts include domesticated animals such as cats, dogs, and livestock, as well as wildlife like koalas and birds, exotic species such as reptiles and amphibians, and humans. Chlamydial infection can result in various clinical signs, including respiratory diseases, reproductive failures, ocular pathologies, and enteritis, though the infected organism may remain asymptomatic. In recent years, chlamydial nomenclature has undergone several revisions due to the wide range of hosts, the frequent discovery of novel strains, and the reclassification of existing ones. Given this and the clinical significance of these infections, ranging from asymptomatic to fatal, an updated review is essential. This article outlines key characteristics of Chlamydia species and provides an updated overview of their nomenclature, offering a concise reference for future research on chlamydial diseases.

Advances in vaccine development for Chlamydia trachomatis

Chlamydia trachomatis is the most prevalent bacterial sexually transmitted infection globally. Antibiotic treatment is highly effective, but infection is often asymptomatic resulting in most individuals going undetected and untreated. This untreated infection can ascend to the upper female genital tract to cause pelvic inflammatory disease, tubal factor infertility, and ectopic pregnancy. Chlamydia screening and treatment programs have failed to control this epidemic and demonstrate the need for an efficacious vaccine to prevent transmission and disease. Animal models and human epidemiological data reveal that natural immunity can provide partial or short-lived sterilizing immunity. These data further demonstrate the importance of eliciting interferon gamma (IFNγ)-producing cluster of differentiation 4 (CD4) T cells (Th1 and Th1/17 cells) that can likely synergize with antibody-mediated opsonophagocytosis to provide optimal protection. These studies have guided preclinical rational vaccine design for decades and the first Phase 1 clinical trials have recently been completed. Recent advances have led to improvements in vaccine platforms and clinically safe adjuvants that help provide a path forward. This review describes vaccine models, correlates of immunity, antigen and adjuvant selection, and future clinical testing for Chlamydia vaccine development.

Chlamydia suis displays high transformation capacity with complete cloning vector integration into the chromosomal rrn-nqrF plasticity zone

IMPORTANCE

The obligate intracellular Chlamydia genus contains many pathogens with a negative impact on global health and economy. Despite recent progress, there is still a lack of genetic tools limiting our understanding of these complex bacteria. This study provides new insights into genetic manipulation of Chlamydia with the opportunistic porcine pathogen Chlamydia suis, the only chlamydial species naturally harboring an antibiotic resistance gene, originally obtained by horizontal gene transfer. C. suis is transmissible to humans, posing a potential public health concern. We report that C. suis can take up vectors that lack the native plasmid, a requirement for most chlamydial transformation systems described to date. Additionally, we show that C. trachomatis, the most common cause for bacterial sexually transmitted infections and infectious blindness worldwide, can be transformed with C. suis vectors. Finally, the chromosomal region that harbors the resistance gene of C. suis is highly susceptible to complete vector integration.

Neisseria gonorrhoeae Coinfection during Chlamydia muridarum Genital Latency Does Not Modulate Murine Vaginal Bacterial Shedding

Chlamydia trachomatis and Neisseria gonorrhoeae are the most frequently reported agents of bacterial sexually transmitted disease worldwide. Nonetheless, C. trachomatis/N. gonorrhoeae coinfection remains understudied. C. trachomatis/N. gonorrhoeae coinfections are more common than expected by chance, suggesting C. trachomatis/N. gonorrhoeae interaction, and N. gonorrhoeae infection may reactivate genital chlamydial shedding in women with latent (quiescent) chlamydial infection. We hypothesized that N. gonorrhoeae would reactivate latent genital Chlamydia muridarum infection in mice. Two groups of C. muridarum-infected mice were allowed to transition into genital latency. One group was then vaginally inoculated with N. gonorrhoeae; a third group received N. gonorrhoeae alone. C. muridarum and N. gonorrhoeae vaginal shedding was measured over time in the coinfected and singly infected groups. Viable C. muridarum was absent from vaginal swabs but detected in rectal swabs, confirming C. muridarum genital latency and consistent with the intestinal tract as a C. muridarum reservoir. C. muridarum inclusions were observed in large intestinal, but not genital, tissues during latency. Oviduct dilation was associated with C. muridarum infection, as expected. Contradicting our hypothesis, N. gonorrhoeae coinfection did not reactivate latent C. muridarum vaginal shedding. In addition, latent C. muridarum infection did not modulate recovery of vaginal viable N. gonorrhoeae. Evidence for N. gonorrhoeae-dependent increased C. muridarum infectivity has thus not been demonstrated in murine coinfection, and the ability of C. muridarum coinfection to potentiate N. gonorrhoeae infectivity may depend on actively replicating vaginal C. muridarum. The proportion of mice with increased vaginal neutrophils (PMNs) was higher in N. gonorrhoeae-infected than in C. muridarum-infected mice, as expected, while that of C. muridarum/N. gonorrhoeae-coinfected mice was intermediate to the singly infected groups, suggesting latent C. muridarum murine infection may limit PMN response to subsequent N. gonorrhoeae infection. IMPORTANCE Our work builds upon the limited understanding of C. muridarum/N. gonorrhoeae coinfection. Previously, N. gonorrhoeae infection of mice with acute (actively replicating) vaginal C. muridarum infection was shown to increase recovery of viable vaginal N. gonorrhoeae and vaginal PMNs, with no effect on C. muridarum vaginal shedding (R. A. Vonck et al., Infect Immun 79:1566-1577, 2011). It has also been shown that chlamydial infection of human and murine PMNs prevents normal PMN responses, including the response to N. gonorrhoeae (K. Rajeeve et al., Nat Microbiol 3:824-835, 2018). Our findings show no effect of latent genital C. muridarum infection on the recovery of viable N. gonorrhoeae, in contrast to the previously reported effect of acute C. muridarum infection, and suggesting that acute versus latent C. muridarum infection may have distinct effects on PMN function in mice. Together, these studies to date provide evidence that Chlamydia/N. gonorrhoeae synergistic interactions may depend on the presence of replicating Chlamydia in the genital tract, while chlamydial effects on vaginal PMNs may extend beyond acute infection.

IFNγ and Antibody Synergize To Enhance Protective Immunity against Chlamydia Dissemination and Female Reproductive Tract Reinfections

CD4 T cell-dependent IFNγ production and antibody are the two best known effectors for protective immunity against Chlamydia female reproductive tract (FRT) infection. Nevertheless, mice lacking either IFNγ or B cells can clear the vast majority of Chlamydia from the FRT, while suffering from varying degrees of disseminated infection. In this study, we investigated whether IFNγ and B cells play complementary roles in host defense against Chlamydia and evaluated their relative contributions in systemic and mucosal tissues. Using mice deficient in both IFNγ and B cells (IFNγ^-/- x μMT), we showed that mice lacking both effectors were highly susceptible to lethal systemic bacterial dissemination following Chlamydia muridarum intravaginal infection. Passive transfer of immune convalescent serum, but not recombinant IFNγ, reduced bacterial burden in both systemic and mucosal tissues in IFNγ^-/- x μMT mice. Notably, over the course of primary infection, we observed a reduction of bacterial shedding of more than 2 orders of magnitude in IFNγ^-/- x μMT mice following both C. muridarum and C. trachomatis FRT infections. In contrast, no protective immunity against C. muridarum reinfection was detected in the absence of IFNγ and B cells. Together, our results suggest that IFNγ and B cells synergize to combat systemic Chlamydia dissemination, while additional IFNγ and B cell-independent mechanisms exist for host resistance to Chlamydia in the lower FRT.

The bacterial effector GarD shields Chlamydia trachomatis inclusions from RNF213-mediated ubiquitylation and destruction

Chlamydia trachomatis is the leading cause of sexually transmitted bacterial infections and a major threat to women's reproductive health in particular. This obligate intracellular pathogen resides and replicates within a cellular compartment termed an inclusion, where it is sheltered by unknown mechanisms from gamma-interferon (IFNγ)-induced cell-autonomous host immunity. Through a genetic screen, we uncovered the Chlamydia inclusion membrane protein gamma resistance determinant (GarD) as a bacterial factor protecting inclusions from cell-autonomous immunity. In IFNγ-primed human cells, inclusions formed by garD loss-of-function mutants become decorated with linear ubiquitin and are eliminated. Leveraging cellular genome-wide association data, we identified the ubiquitin E3 ligase RNF213 as a candidate anti-Chlamydia protein. We demonstrate that IFNγ-inducible RNF213 facilitates the ubiquitylation and destruction of GarD-deficient inclusions. Furthermore, we show that GarD operates as a cis-acting stealth factor barring RNF213 from targeting inclusions, thus functionally defining GarD as an RNF213 antagonist essential for chlamydial growth during IFNγ-stimulated immunity.

Reemergence of the Murine Bacterial Pathogen Chlamydia muridarum in Research Mouse Colonies

Chlamydia muridarum (Cm) was detected in 2 colonies of mice with lymphoplasmacytic pulmonary infiltrates by using PCR and immunohistochemistry. This discovery was unexpected, as Cm infection had not been reported in laboratory mice since the 1940s. A Cm specific PCR assay was developed and testing implemented for the resident colonies of 8 vivaria from 3 academic institutions, 58 incoming mouse shipments from 39 academic institutions, and mice received from 55 commercial breeding colonies (4 vendors). To estimate Cm's global prevalence in research colonies, a database containing 11,387 metagenomic fecal microbiota samples from 120 institutions and a cohort of 900 diagnostic samples from 96 institutions were examined. Results indicate significant prevalence among academic institutions, with Cm detected in 63% of soiled bedding sentinels from 3 institutions; 33% of incoming mouse shipments from 39 academic institutions; 14% of 120 institutions submitting microbiota samples; and 16% of the diagnostic sample cohort. All samples from commercial breeding colonies were negative. In addition, naïve NOD. Cg-Prkdc^scidIl2rg^tm1Wjl/SzJ (NSG) mice exposed to Cm-shedding mice and/or their soiled bedding developed clinical disease at 21 to 28 d after exposure. These mice had a moderate-to-severe histiocytic and neutro- philic bronchointerstitial pneumonia, with their respiratory epithelium demonstrating inclusions, chlamydial major outer membrane protein immunostaining, and hybridization with a Cm reference sequence (GenBank accession no. U68436). Cm was isolated from lungs, cecum, and feces of a Cm-infected NSG mouse by using HeLa 229 cells. The considerable prevalence of Cm is likely due to widespread global interinstitutional distribution of unique mouse strains and failure to recognize that some of these mice were from enzootically infected colonies. Given that experimental Cm colonization of mice results in a robust immune response and, on occasion, pathology, natural infection may confound experimental results. Therefore, Cm should be excluded and eradicated from enzootically infected mouse colonies.

Th1 cells are dispensable for primary clearance of Chlamydia from the female reproductive tract of mice

Protective immune responses to Chlamydia infection within the female reproductive tract (FRT) are incompletely understood. MHC class II-restricted CD4 Th1 responses are believed to be vital for bacterial clearance due to their capacity to secrete IFN-γ, but an essential requirement for T-bet-expressing Th1 cells has yet to be demonstrated in the mouse model of Chlamydia infection. Here, we investigated the role of T-bet and IFN-γ in primary clearance of Chlamydia after FRT infection. Surprisingly, IFN-γ producing CD4 T cells from the FRT expressed low levels of T-bet throughout infection, suggesting that classical T-bet-expressing Th1 cells are inefficiently generated and therefore unlikely to participate in bacteria clearance. Furthermore, mice deficient in T-bet expression or with a CD4-specific T-bet deficiency cleared FRT infection similarly to wild-type controls. T-bet-deficient mice displayed significant skewing of FRT CD4 T cells towards Th17 responses, demonstrating that compensatory effector pathways are generated in the absence of Th1 cells. In marked contrast, IFN-γ-, and IFN-γR-deficient mice were able to reduce FRT bacterial burdens, but suffered systemic bacterial dissemination and 100% mortality. Together, these data demonstrate that IFN-γ signaling is essential to protect mice from fatal systemic disease, but that classical T-bet-expressing Th1 cells are non-essential for primary clearance within the FRT. Exploring the protective contribution of Th1 cells versus other CD4 effector lineages could provide important information for the generation of new Chlamydia vaccines.

The production of IFN-γ by CD4 Th1 cells is thought to be critical for the clearance of Chlamydia from the female reproductive tract (FRT), but this has not been formally tested. Here we demonstrate that T-bet+ Th1 cells are not essential for effective Chlamydia clearance. Furthermore, the impact of IFN-γ deficiency or depletion is largely observed as a failure to control bacterial dissemination, rather than clearance from the FRT. Together, these data suggest that different immunological mechanisms are responsible for restraining systemic spread of bacteria versus FRT control. Defining alternative non-Th1 CD4 effector mechanisms that are responsible for controlling Chlamydia replication within the FRT could be foundational for future vaccine development.

Chlamydia muridarum Can Invade the Central Nervous System via the Olfactory and Trigeminal Nerves and Infect Peripheral Nerve Glial Cells

Chlamydia pneumoniae can infect the brain and has been linked to late-onset dementia. Chlamydia muridarum, which infects mice, is often used to model human chlamydial infections. While it has been suggested to be also important for modelling brain infection, nervous system infection by C. muridarum has not been reported in the literature. C. pneumoniae has been shown to infect the olfactory bulb in mice after intranasal inoculation, and has therefore been suggested to invade the brain via the olfactory nerve; however, nerve infection has not been shown to date. Another path by which certain bacteria can reach the brain is via the trigeminal nerve, but it remains unknown whether Chlamydia species can infect this nerve. Other bacteria that can invade the brain via the olfactory and/or trigeminal nerve can do so rapidly, however, whether Chlamydia spp. can reach the brain earlier than one-week post inoculation remains unknown. In the current study, we showed that C. muridarum can within 48 h invade the brain via the olfactory nerve, in addition to infecting the trigeminal nerve. We also cultured the glial cells of the olfactory and trigeminal nerves and showed that C. muridarum readily infected the cells, constituting a possible cellular mechanism explaining how the bacteria can invade the nerves without being eliminated by glial immune functions. Further, we demonstrated that olfactory and trigeminal glia differed in their responses to C. muridarum, with olfactory glia showing less infection and stronger immune response than trigeminal glia.

Chlamydia pecorum-Induced Arthritis in Experimentally and Naturally Infected Sheep

Chlamydia pecorum is an obligate intracellular pathogen with a wide host range including livestock such as sheep, cattle, goats, and pigs as well as wildlife species such as koalas. Chlamydial polyarthritis is an economically important disease resulting in swollen joints, lameness, stiffness, and weight loss in young sheep. In the present study, tissues from sheep experimentally or naturally infected with Chlamydia pecorum were assessed by histopathology and immunohistochemistry. Carpal, hock, and stifle joints as well as spleen, liver, kidney, lymph nodes, lung, and brain of 35 sheep from different inoculation groups were available. Two different C. pecorum strains (IPA and E58), different routes of administration (intraarticular or intravenous), UVA-irradiated IPA strain, and corresponding noninfected control groups were investigated. Similar investigations on tissues from 5 naturally infected sheep were performed. The most obvious inflammatory lesions were observed in synovial tissues and, notably, in the renal pelvis from the experimentally infected group and naturally infected animals. This resulted in chronic or chronic-active arthritis and pyelitis. Intralesional chlamydial inclusions could be demonstrated by immunohistochemistry in both tissues. Immunohistochemical evaluation of the presence and distribution of macrophages, T and B cells in synovial tissues revealed macrophages as the most prevalent inflammatory cell population. Previous observations indicated that C. pecorum isolates can infect circulating monocytes. Together with the finding of the histological lesions in synovial tissues and internal organs alongside the presence of C. pecorum DNA, these observations suggest chlamydial arthritis in lambs is the result of hematogeneous spread of C. pecorum.

Comprehensive genome analysis and comparisons of the swine pathogen, Chlamydia suis reveals unique ORFs and candidate host-specificity factors

Chlamydia suis, a ubiquitous swine pathogen, has the potential for zoonotic transmission to humans and often encodes for resistance to the primary treatment antibiotic, tetracycline. Because of this emerging threat, comparative genomics for swine isolate R19 with inter- and intra-species genomes was performed. A 1.094 Mb genome was determined through de novo assembly of Illumina high throughput sequencing reads. Annotation and subsystem analyses were conducted, revealing 986 putative genes (Chls_###) that are predominantly orthologs to other known Chlamydia genes. Subsequent comparative genomics revealed a high level of genomic synteny and overall sequence identity with other Chlamydia while 92 unique C. suis open reading frames were annotated. Direct comparison of Chlamydia-specific gene families that included the plasticity zone, inclusion membrane proteins, polymorphic membrane proteins and the major outer membrane protein, demonstrated high gene content identity with C. trachomatis and C. muridarum. These comparisons also identified diverse components that potentially could contribute to host-specificity. This study constitutes the first genome-wide comparative analysis for C. suis, generating a fully annotated reference genome. These studies will enable focused efforts on factors that provide key species specificity and adaptation to cognate hosts that are attributed to chlamydial infections, including humans.

A primary study on genes with selected mutations by in vitro passage of Chlamydia muridarum strains

OBJECTIVE

This study is to investigate the functions of newly discovered genes in Chlamydia muridarum (C. muridarum) strains with single gene differences.

METHODS

Using whole genome sequencing and plaque formation assays, C. muridarum parental and passaging strains were established, and the isogenic clones expressing certain genotypes were isolated. Strains with single gene differences were obtained. Based on prediction, the valuable strains with single gene differences of tc0412, tc0668 or tc0237 were subjected to the in vitro and in vivo experiments for biological characterization and virulence analysis.

RESULTS

Insertional -472840T mutation of the tc0412 gene (T28T/B3 type) matching with the nonmutant tc0668 gene and tc0237 gene with point mutations G797659T (Q117E) might slow the growth of Chlamydia due to the lack of a plasmid. The nonmutant tc0668 in the strain might induce a high incidence of hydrosalpinx in mice, while tc0668 with a G797659T point mutation was significantly attenuated. Compared with the nonmutant tc0237, the strains containing mutant tc0237 were characterized by reduced centrifugation dependence during infection.

CONCLUSION

The identification and characterization of these genes might contribute to the comprehensive understanding of the pathogenic mechanism of Chlamydia.

The Role of MicroRNA-155 in Chlamydia muridarum Infected lungs

Our laboratory has investigated the role of an evolutionarily conserved RNA species called microRNAs (miRs) in regulation of anti-chlamydial protective immunity. MiRs including miR-155 expressed in specific immune effector cells are critical for antigen specific protective immunity and IFN-γ production. Using miR-155 deficient mice, and a murine pulmonary model for chlamydial infection, we report here 1) the effect of host miR-155 on bacterial burden, and 2) identify probable immune genes regulated by miR-155.

Chlamydiaceae: Diseases in Primary Hosts and Zoonosis

Bacteria of the Chlamydiaceae family are a type of Gram-negative microorganism typified by their obligate intracellular lifestyle. The majority of the members in the Chlamydiaceae family are known pathogenic organisms that primarily infect the host mucosal surfaces in both humans and animals. For instance, Chlamydia trachomatis is a well-known etiological agent for ocular and genital sexually transmitted diseases, while C. pneumoniae has been implicated in community-acquired pneumonia in humans. Other chlamydial species such as C. abortus, C. caviae, C. felis, C. muridarum, C. pecorum, and C. psittaci are important pathogens that are associated with high morbidities in animals. Importantly, some of these animal pathogens have been recognized as zoonotic agents that pose a significant infectious threat to human health through cross-over transmission. The current review provides a succinct recapitulation of the characteristics as well as transmission for the previously established members of the Chlamydiaceae family and a number of other recently described chlamydial organisms.

A Synthetic, Small, Sulfated Agent Is a Promising Inhibitor of Chlamydia spp. Infection in vivo

Chlamydia is the most frequently reported sexually transmitted bacteria causing 2.9 million infections annually in the United States. Diagnosis, treatment, and sequelae of chlamydial disease cost billions of dollars each year in the United States alone. Considering that a heparin sulfate-like cell surface receptor is involved in Chlamydia infections, we reasoned that sulfated and sulfonated mimics of heparin sulfate would be useful in topical prophylactic prevention of Chlamydia. In this study, we tested a small, synthetic sulfated agent sulfated pentagalloyl glucoside (SPGG) and three synthetic sulfonated polymers PSS and SPS with average molecular weight in the range of 11 to 1000 kDa for inhibition against Chlamydia. Infection of HeLa cells with C. muridarum or C. trachomatis in the presence of increasing concentrations of SPGG or sulfonated polymers were quantified by immunofluorescence of Chlamydia inclusions. To determine whether in vitro pre-treatment of SPGG inhibits infection of C. muridarum, HeLa monolayers were incubated with SPGG-containing media, and then infected with Chlamydia. Our in vitro results show that SPGG pre-treatment inhibits Chlamydia infection in a dose-dependent manner. In addition, we further determined if SPGG treatment has an inhibitory effect during infection, therefore cell monolayers were infected with C. muridarum in the concurrent presence of SPGG. Our results show that SPGG inhibits C. muridarum infection with an IC50 at 10 μg/ml levels. We also tested the inhibitory effect of synthetic polymers PSS and SPS against Chlamydia and found inhibition of C. muridarum and C. trachomatis infections with IC50 ranging from 0.3 to 0.8 μg/ml. SPGG, PSS, and SPS inhibit formation of Chlamydia inclusions in a concentration-dependent manner. For evaluation of in vivo efficacy of the most effective agent in blocking C. muridarum, SPGG, we intravaginally pre-treated mice with SPGG before infection with C. muridarum. Cervical swabs were collected post-infection to quantify Chlamydia inclusions in vitro. Our in vivo data show that the SPGG-treated group has a statistically significant reduction of infection compared to the no-treatment control. Overall, our results show that SPGG could serve as a promising topical inhibitor for preventing Chlamydia infection.

T Cell-Independent Gamma Interferon and B Cells Cooperate To Prevent Mortality Associated with Disseminated Chlamydia muridarum Genital Tract Infection

CD4 T cells and antibody are required for optimal acquired immunity to Chlamydia muridarum genital tract infection, and T cell-mediated gamma interferon (IFN-γ) production is necessary to clear infection in the absence of humoral immunity. However, the role of T cell-independent immune responses during primary infection remains unclear. We investigated this question by inoculating wild-type and immune-deficient mice with C. muridarum CM001, a clonal isolate capable of enhanced extragenital replication. Genital inoculation of wild-type mice resulted in transient dissemination to the lungs and spleen that then was rapidly cleared from these organs. However, CM001 genital infection proved lethal for STAT1-/- and IFNG-/- mice, in which IFN-γ signaling was absent, and for Rag1-/- mice, which lacked T and B cells and in which innate IFN-γ signaling was retained. In contrast, B cell-deficient muMT mice, which can generate a Th1 response, and T cell-deficient mice with intact B cell and innate IFN-γ signaling survived. These data collectively indicate that IFN-γ prevents lethal CM001 dissemination in the absence of T cells and suggests a B cell corequirement. Adoptive transfer of convalescent-phase immune serum but not naive IgM to Rag1-/- mice infected with CM001 significantly increased the survival time, while transfer of naive B cells completely rescued Rag1-/- mice from CM001 lethality. Protection was associated with a significant reduction in the lung chlamydial burden of genitally infected mice. These data reveal an important cooperation between T cell-independent B cell responses and innate IFN-γ in chlamydial host defense and suggest that interactions between T cell-independent antibody and IFN-γ are essential for limiting extragenital dissemination.

Elimination of Mycoplasma contamination in Chlamydia stocks as a result of in vivo passage or plaque isolation

OBJECTIVE

This study aims to eliminate Mycoplasma spp. contamination from laboratory stocks of Chlamydia spp. by in vivo passage or by plaque assay.

RESULTS

We have described two methods of eliminating Mycoplasma contamination from Chlamydia laboratory stocks. We conclude that Mycoplasma species commonly contaminating chlamydial stocks do not survive passage in mice. Chlamydia may also be derived Mycoplasma-free by plaque assay.

The Chlamydia muridarum plasmid revisited : new insights into growth kinetics

Background: Research in chlamydial genetics is challenging because of its obligate intracellular developmental cycle. In vivo systems exist that allow studies of different aspects of basic biology of chlamydiae, the murine Chlamydia muridarum model is one of great importance and thus an essential research tool. C. muridarum carries a plasmid that has a role in virulence.  Our aim was to compare and contrast the C. muridarum plasmid-free phenotype with that of a chromosomally isogenic plasmid-bearing strain, through the inclusion phase of the developmental cycle.

Methods: We measured infectivity for plasmid bearing and plasmid-cured C. muridarum by inclusion forming assays in McCoy cells and in parallel bacterial chromosome replication by quantitative PCR, throughout the developmental cycle. In addition to these studies, we have carefully monitored chlamydial inclusion formation by confocal microscopy and transmission electron microscopy. A new E.coli/chlamydial shuttle vector (pNigg::GFP) was constructed using standard cloning technology and used to transform C. muridarum for further phenotypic studies.

Results: We have advanced the definition of the chlamydial phenotype away from the simple static observation of mature inclusions and redefined the C. muridarum plasmid-based phenotype on growth profile and inclusion morphology. Our observations on the growth properties of plasmid-cured C. muridarum challenge the established interpretations, especially with regard to inclusion growth kinetics. Introduction of the shuttle plasmid pNigg::GFP into plasmid-cured C. muridarum restored the wild-type plasmid-bearing phenotype and confirmed that loss of the plasmid was the sole cause for the changes in growth and chromosomal replication.

Conclusions: Accurate growth curves and sampling at multiple time points throughout the developmental cycle is necessary to define plasmid phenotypes.  There are subtle but important (previously unnoticed) differences in the overall growth profile of plasmid-bearing and plasmid-free C. muridarum.  We have proven that the differences described are solely due to the plasmid pNigg.

Nonpathogenic Colonization with Chlamydia in the Gastrointestinal Tract as Oral Vaccination for Inducing Transmucosal Protection

Chlamydia has been detected in the gastrointestinal tracts of humans and animals. We now report that gastrointestinal Chlamydia muridarum is able to induce robust transmucosal protection in mice. C. muridarum colonization in the gastrointestinal tract correlated with both a shortened course of C. muridarum genital tract infection and stronger protection against subsequent genital tract challenge infection. Mice preinoculated intragastrically with C. muridarum became highly resistant to subsequent C. muridarum infection in the genital tract, resulting in prevention of pathology in the upper genital tract. The transmucosal protection in the genital tract was rapidly induced, durable, and dependent on major histocompatibility complex (MHC) class II antigen presentation but not MHC class I antigen presentation. Although a deficiency in CD4⁺ T cells only partially reduced the transmucosal protection, depletion of CD4⁺ T cells from B cell-deficient mice completely abolished the protection, suggesting a synergistic role of both CD4⁺ T and B cells in the gastrointestinal C. muridarum-induced transmucosal immunity. However, the same protective immunity did not significantly affect C. muridarum colonization in the gastrointestinal tract. The long-lasting colonization with C. muridarum was restricted to the gastrointestinal tract and was nonpathogenic to either gastrointestinal or extragastrointestinal tissues. Furthermore, gastrointestinal C. muridarum did not alter the gut microbiota or the development of gut mucosal resident memory T cell responses to a nonchlamydial infection. Thus, Chlamydia may be developed into a safe and orally deliverable replicating vaccine for inducing transmucosal protection.

Host and pathogen interface: microRNAs are modulators of disease outcome

Chlamydiae are a group of intracellular bacterium that infect a range of hosts and are responsible for the most common sexual transmitted infections, which could be the result of a plethora of factors leading to varied pathological outcomes. This review aims to show that Chlamydia possibly manipulates host defenses through microRNAs interaction.

The effect of infectious dose on humoral and cellular immune responses in Chlamydophila caviae primary ocular infection

Following infection, the balance between protective immunity and immunopathology often depends on the initial infectious load. Several studies have investigated the effect of infectious dose; however, the mechanism by which infectious dose affects disease outcomes and the development of a protective immune response is not known. The aim of this study was to investigate how the infectious dose modulates the local and systemic humoral and the cellular immune responses during primary ocular chlamydial infection in the guinea pig animal model. Guinea pigs were infected by ocular instillation of a Chlamydophila caviae-containing eye solution in the conjunctival sac in three different doses: 1×10², 1×10⁴, and 1×10⁶ inclusion forming units (IFUs). Ocular pathology, chlamydial clearance, local and systemic C. caviae-specific humoral and cellular immune responses were assessed. All inocula of C. caviae significantly enhanced the local production of C. caviae-specific IgA in tears, but only guinea pigs infected with the higher doses showed significant changes in C. caviae-specific IgA levels in vaginal washes and serum. On complete resolution of infection, the low dose of C. caviae did not alter the ratio of CD4⁺ and CD8⁺ cells within guinea pigs’ submandibular lymph node (SMLN) lymphocytes while the higher doses increased the percentages of CD4⁺ and CD8⁺ cells within the SMLN lymphocytes. A significant negative correlation between pathology intensity and the percentage of CD4⁺ and CD8⁺ cells within SMLN lymphocyte pool at selected time points post-infection was recorded for both 1×10⁴, and 1×10⁶ IFU infected guinea pigs. The relevance of the observed dose-dependent differences on the immune response should be further investigated in repeated ocular chlamydial infections.

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.

Advances and Obstacles in the Genetic Dissection of Chlamydial Virulence

Obligate intracellular pathogens in the family Chlamydiaceae infect taxonomically diverse eukaryotes ranging from amoebae to mammals. However, many fundamental aspects of chlamydial cell biology and pathogenesis remain poorly understood. Genetic dissection of chlamydial biology has historically been hampered by a lack of genetic tools. Exploitation of the ability of chlamydia to recombine genomic material by lateral gene transfer (LGT) ushered in a new era in chlamydia research. With methods to map mutations in place, genetic screens were able to assign functions and phenotypes to specific chlamydial genes. Development of an approach for stable transformation of chlamydia also provided a mechanism for gene delivery and platforms for disrupting chromosomal genes. Here, we explore how these and other tools have been used to test hypotheses concerning the functions of known chlamydial virulence factors and discover the functions of completely uncharacterized genes. Refinement and extension of the existing genetic tools to additional Chlamydia spp. will substantially advance understanding of the biology and pathogenesis of this important group of pathogens.

MicroRNAs Modulate Pathogenesis Resulting from Chlamydial Infection in Mice

Not all women infected with chlamydiae develop upper genital tract disease, but the reason(s) for this remains undefined. Host genetics and hormonal changes associated with the menstrual cycle are possible explanations for variable infection outcomes. It is also possible that disease severity depends on the virulence of the chlamydial inoculum. It is likely that the inoculum contains multiple genetic variants, differing in virulence. If the virulent variants dominate, then the individual is more likely to develop severe disease. Based on our previous studies, we hypothesized that the relative degree of virulence of a chlamydial population dictates the microRNA (miRNA) expression profile of the host, which, in turn, through regulation of the host inflammatory response, determines disease severity. Thus, we infected C57BL/6 mice with two populations of Chlamydia muridarum, each comprised of multiple genetic variants and differing in virulence: an attenuated strain (Nigg_A) and a virulent strain (Nigg_V). Nigg_A and Nigg_V elicited upper tract pathology in 54% and 91% of mice, respectively. miRNA expression analysis in Nigg_V-infected mice showed significant downregulation of miRNAs involved in dampening fibrosis (miR-200b, miR-200b-5p, and 200b-3p miR-200a-3p) and in transcriptional regulation of cytokine responses (miR-148a-3p, miR-152-3p, miR-132, and miR-212) and upregulation of profibrotic miRNAs (miR-142, and miR-147). Downregulated miRNAs were associated with increased expression of interleukin 8 (IL-8), CXCL2, IL-1β, tumor necrosis factor alpha (TNF-α), and IL-6. Infection with Nigg_V but not Nigg_A led to decreased expression of Dicer and Ago 2, suggesting that Nigg_V interaction with host cells inhibits expression of the miRNA biogenesis machinery, leading to increased cytokine expression and pathology.

Chlamydial Pre-Infection Protects from Subsequent Herpes Simplex Virus-2 Challenge in a Murine Vaginal Super-Infection Model

Chlamydia trachomatis and Herpes Simplex Virus-2 (HSV-2) genital tract co-infections have been reported in humans and studied in vitro but the clinical consequences are unknown. Limited epidemiologic evidence suggests that these co-infections could be more severe than single infections of either pathogen, but the host-pathogen interactions during co-infection remain uncharacterized. To determine whether disease progression and/or pathogen shedding differs between singly-infected and super-infected animals, we developed an in vivo super-infection model in which female BALB/c mice were vaginally infected with Chlamydia muridarum (Cm) followed later by HSV-2. Pre-infection with Chlamydia 3 or 9 days prior to HSV-2 super-infection conferred significant protection from HSV-2-induced neurologic disease and significantly reduced viral recovery compared to HSV-2 singly-infected controls. Neither protection from mortality nor reduced viral recovery were observed when mice were i) super-infected with HSV-2 on day 27 post Cm; ii) infected with UV-irradiated Cm and super-infected with HSV-2; or iii) azithromycin-treated prior to HSV-2 super-infection. Therefore, protection from HSV-2-induced disease requires active infection with viable chlamydiae and is not observed after chlamydial shedding ceases, either naturally or due to antibiotic treatment. Thus, Chlamydia-induced protection is transient and requires the continued presence of chlamydiae or their components. These data demonstrate that chlamydial pre-infection can alter progression of subsequent HSV-2 infection, with implications for HSV-2 transmission from co-infected humans.

The Chromosome-Encoded Hypothetical Protein TC0668 Is an Upper Genital Tract Pathogenicity Factor of Chlamydia muridarum

We previously associated a missense mutation of the tc0668 gene of serial in vitro-passaged Chlamydia muridarum, a murine model of human urogenital C. trachomatis, with severely attenuated disease development in the upper genital tract of female mice. Since these mutants also contained a TC0237 Q117E missense mutation that enhances their in vitro infectivity, an effort was made here to isolate and characterize a tc0668 single mutant to determine its individual contribution to urogenital pathogenicity. Detailed genetic analysis of C. muridarum passages revealed a truncated variant with a G216* nonsense mutation of the 408-amino-acid TC0668 protein that does not produce a detectable product. Intracellular growth and infectivity of C. muridarum in vitro remain unaffected in the absence of TC0668. Intravaginal inoculation of the TC0668 null mutant into C3H/HeJ mice results in a typical course of lower genital tract infection but, unlike a pathogenic isogenic control, is unable to elicit significant chronic inflammation of the oviduct and fails to induce hydrosalpinx. Thus, TC0668 is demonstrated as an important chromosome-encoded urogenital pathogenicity factor of C. muridarum and the first with these characteristics to be discovered for a Chlamydia pathogen.

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

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

In vivo whole animal body imaging reveals colonization of Chlamydia muridarum to the lower genital tract at early stages of infection

PURPOSE

The leading cause of sexually transmitted bacterial infection is Chlamydia trachomatis. The aim of this study is to investigate the early events in colonization of this bacterium within the murine genital tract.

PROCEDURES

An in vivo animal body imaging technology was used to track fluorophore labeled C. muridarum elementary bodies (EBs) inoculated intravaginally in C57BL/6 mice during the first 24 h of infection.

RESULTS

Ascension of viable EBs was observed (1) to be localized to the lower regions of the murine genital tract within the first 24 h post challenge and (2) was dose independent during this early exposure period. Molecular detection revealed enhanced bacterial load in lower regions of the genital tract with increasing bacterial load in the upper region beginning 12 h post inoculation.

CONCLUSION

This study provides additional insight into chlamydial colonization in the murine genital tract during the first 12-24 h following inoculation.

Chlamydia trachomatis virulence factor CT135 is stable in vivo but highly polymorphic in vitro

Chlamydia trachomatis is an important human pathogen causing both ocular and sexually transmitted disease. Recently, we identified CT135 as an important virulence determinant in a mouse infection model. Results from CEL 1 digestion assays and sequencing analyses indicated that CT135 was much more polymorphic in high in vitro passage reference serovars than it was in clinical strains that had undergone limited passaging. Herein, we used targeted next-generation sequencing of the CT134-135 locus, from reference strains and clinical isolates, enabling accurate discovery of single nucleotide polymorphisms and other population genetic variations. Our results indicate that CT134 is stable in all C. trachomatis serovars examined. In contrast, CT135 is highly polymorphic in high-passaged reference ocular and non-LGV genital serovars, with the majority of the mutations resulting in gene disruption. In low-passaged ocular clinical isolates, CT135 was frequently disrupted, whereas in genital clinical isolates CT135 was intact in almost all instances. When a serovar K isolate, with an intact CT134 and CT135, was subjected to serial passage in vitro CT134 remained invariable, while numerous gene interrupting mutations rapidly accumulated in CT135. Collectively, our data indicate that, for genital serovars, CT135 is under strong positive selection in vivo, and negative selection in vitro.

Mutational Analysis of the Chlamydia muridarum Plasticity Zone

Pathogenically diverse Chlamydia spp. can have surprisingly similar genomes. Chlamydia trachomatis isolates that cause trachoma, sexually transmitted genital tract infections (chlamydia), and invasive lymphogranuloma venereum (LGV) and the murine strain Chlamydia muridarum share 99% of their gene content. A region of high genomic diversity between Chlamydia spp. termed the plasticity zone (PZ) may encode niche-specific virulence determinants that dictate pathogenic diversity. We hypothesized that PZ genes might mediate the greater virulence and gamma interferon (IFN-γ) resistance of C. muridarum compared to C. trachomatis in the murine genital tract. To test this hypothesis, we isolated and characterized a series of C. muridarum PZ nonsense mutants. Strains with nonsense mutations in chlamydial cytotoxins, guaBA-add, and a phospholipase D homolog developed normally in cell culture. Two of the cytotoxin mutants were less cytotoxic than the wild type, suggesting that the cytotoxins may be functional. However, none of the PZ nonsense mutants exhibited increased IFN-γ sensitivity in cell culture or were profoundly attenuated in a murine genital tract infection model. Our results suggest that C. muridarum PZ genes are transcribed--and some may produce functional proteins--but are dispensable for infection of the murine genital tract.

In vitro passage selects for Chlamydia muridarum with enhanced infectivity in cultured cells but attenuated pathogenicity in mouse upper genital tract

Although modern Chlamydia muridarum has been passaged for decades, there are no reports on the consequences of serial passage with strong selection pressure on its fitness. In order to explore the potential for Pasteurian selection to induce genomic and phenotypic perturbations to C. muridarum, a starter population was passaged in cultured cells for 28 generations without standard infection assistance. The resultant population, designated CMG28, displays markedly reduced in vitro dependence on centrifugation for infection and low incidence and severity of upper genital tract pathology following intravaginal inoculation into mice compared to the parental C. muridarum population, CMG0. Deep sequencing of CMG0 and CMG28 revealed novel protein variants in the hypothetical genes TC0237 (Q117E) and TC0668 (G322R). In vitro attachment assays of isogenic plaque clone pairs with mutations in either TC0237 and TC0668 or only TC0237 reveal that TC0237(Q117E) is solely responsible for enhanced adherence to host cells. Paradoxically, double mutants, but not TC0237(Q117E) single mutants, display severely attenuated in vivo pathogenicity. These findings implicate TC0237 and TC0668 as novel genetic factors involved in chlamydial attachment and pathogenicity, respectively, and show that serial passage under selection pressure remains an effective tool for studying Chlamydia pathogenicity.

Genomic variant representation in a Chlamydia population is dynamic and adaptive with dependence on in vitro and in vivo passage

We have previously shown that Chlamydia muridarum has multiple genomic variants that concomitantly vary in their in vitro and in vivo phenotype. Herein, we used real-time polymerase chain reaction-based genotyping assays to query plaque-cloned isolates of C. muridarum for the frequency of eight selected polymorphisms. These strains had no history of passage in vivo since their original isolation from laboratory mice. There was significant variance in the frequency of two of the eight polymorphisms assessed with the remaining exhibiting a low rate of variance. To determine if any of these polymorphisms were more favorable for in vivo conditions, we blindly passaged non-clonal C. muridarum three times at 7-day intervals through the urogenital tract of mice. Seven of the eight polymorphisms varied in frequency following in vivo passage and four of these varied between C. muridarum strains. Selected isolates displayed variable growth rates and cytopathic effect in vitro. We conclude that multiple genotypic variants are present within the existing known C. muridarum strains and that the frequency of these variants changes upon introduction into the mouse host. These findings lend support to the concept that genotypic proportional representation in a chlamydial population is dynamic and adaptive.

Evidence for horizontal gene transfer between Chlamydophila pneumoniae and Chlamydia phage

Chlamydia-infecting bacteriophages, members of the Microviridae family, specifically the Gokushovirinae subfamily, are small (4.5–5 kb) single-stranded circles with 8–10 open-reading frames similar to E. coli phage ϕX174. Using sequence information found in GenBank, we examined related genes in Chlamydophila pneumoniae and Chlamydia-infecting bacteriophages. The 5 completely sequenced C. pneumoniae strains contain a gene orthologous to a phage gene annotated as the putative replication initiation protein (PRIP, also called VP4), which is not found in any other members of the Chlamydiaceae family sequenced to date. The C. pneumoniae strain infecting koalas, LPCoLN, in addition contains another region orthologous to phage sequences derived from the minor capsid protein gene, VP3. Phylogenetically, the phage PRIP sequences are more diverse than the bacterial PRIP sequences; nevertheless, the bacterial sequences and the phage sequences each cluster together in their own clade. Finally, we found evidence for another Microviridae phage-related gene, the major capsid protein gene, VP1 in a number of other bacterial species and 2 eukaryotes, the woodland strawberry and a nematode. Thus, we find considerable evidence for DNA sequences related to genes found in bacteriophages of the Microviridae family not only in a variety of prokaryotic but also eukaryotic species.

Benzylidene acylhydrazides inhibit chlamydial growth in a type III secretion- and iron chelation-independent manner

Chlamydiae are widespread Gram-negative pathogens of humans and animals. Salicylidene acylhydrazides, developed as inhibitors of type III secretion system (T3SS) in Yersinia spp., have an inhibitory effect on chlamydial infection. However, these inhibitors also have the capacity to chelate iron, and it is possible that their antichlamydial effects are caused by iron starvation. Therefore, we have explored the modification of salicylidene acylhydrazides with the goal to uncouple the antichlamydial effect from iron starvation. We discovered that benzylidene acylhydrazides, which cannot chelate iron, inhibit chlamydial growth. Biochemical and genetic analyses suggest that the derivative compounds inhibit chlamydiae through a T3SS-independent mechanism. Four single nucleotide polymorphisms were identified in a Chlamydia muridarum variant resistant to benzylidene acylhydrazides, but it may be necessary to segregate the mutations to differentiate their roles in the resistance phenotype. Benzylidene acylhydrazides are well tolerated by host cells and probiotic vaginal Lactobacillus species and are therefore of potential therapeutic value.

Early microRNA expression profile as a prognostic biomarker for the development of pelvic inflammatory disease in a mouse model of chlamydial genital infection

It is not currently possible to predict the probability of whether a woman with a chlamydial genital infection will develop pelvic inflammatory disease (PID). To determine if specific biomarkers may be associated with distinct chlamydial pathotypes, we utilized two Chlamydia muridarum variants (C. muridarum Var001 [CmVar001] and CmVar004) that differ in their abilities to elicit upper genital tract pathology in a mouse model. CmVar004 has a lower growth rate in vitro and induces pathology in only 20% of C57BL/6 mouse oviducts versus 83.3% of oviducts in CmVar001-infected mice. To determine if chemokine and cytokine production within 24 h of infection is associated with the outcome of pathology, levels of 15 chemokines and cytokines were measured. CmVar004 infection induced significantly lower levels of CXCL1, CXCL2, tumor necrosis factor alpha (TNF-α), and CCL2 in comparison to CmVar001 infection with similar rRNA (rs16) levels for Chlamydiae. A combination of microRNA (miRNA) sequencing and quantitative real-time PCR (qRT-PCR) analysis of 134 inflammation-related miRNAs was performed 24 h postinfection to determine if the chemokine/cytokine responses would also be reflected in miRNA expression profiles. Interestingly, 12 miRNAs (miR-135a-5p, miR298-5p, miR142-3p, miR223-3p, miR299a-3p, miR147-3p, miR105, miR325-3p, miR132-3p, miR142-5p, miR155-5p, and miR-410-3p) were overexpressed during CmVar004 infection compared to CmVar001 infection, inversely correlating with the respective chemokine/cytokine responses. To our knowledge, this is the first report demonstrating that early biomarkers elicited in the host can differentiate between two pathological variants of chlamydiae and be predictive of upper tract disease.

IMPORTANCE

It is apparent that an infecting chlamydial population consists of multiple genetic variants with differing capabilities of eliciting a pathological response; thus, it may be possible to identify biomarkers specific for a given virulence pathotype. miRNAs are known to regulate genes that in turn regulate signaling pathways involved in disease pathogenesis. Importantly, miRNAs are stable and can reflect a tissue response and therefore have the potential to be biomarkers of disease severity. Currently, with respect to chlamydial infections, there is no way to predict whether an infected patient is more or less likely to develop PID. However, data presented in this study indicate that the expression of a specific miRNA profile associated with a virulent variant early in the infection course may be predictive of an increased risk of pelvic inflammatory disease, allowing more aggressive treatment before significant pathology develops.

Chlamydia genomics: providing novel insights into chlamydial biology

Chlamydiaceae are obligate intracellular pathogens that have successfully evolved to colonize a diverse range of hosts. There are currently 11 described species of Chlamydia, most of which have a significant impact on the health of humans or animals. Expanding chlamydial genome sequence information has revolutionized our understanding of chlamydial biology, including aspects of their unique lifecycle, host-pathogen interactions, and genetic differences between Chlamydia strains associated with different host and tissue tropisms. This review summarizes the major highlights of chlamydial genomics and reflects on the considerable impact these have had on understanding the biology of chlamydial pathogens and the changing nature of genomics tools in the 'post-genomics' era.

Plasmid CDS5 influences infectivity and virulence in a mouse model of Chlamydia trachomatis urogenital infection

The native plasmid of both Chlamydia muridarum and Chlamydia trachomatis has been shown to control virulence and infectivity in mice and in lower primates. We recently described the development of a plasmid-based genetic transformation protocol for Chlamydia trachomatis that for the first time provides a platform for the molecular dissection of the function of the chlamydial plasmid and its individual genes or coding sequences (CDS). In the present study, we transformed a plasmid-free lymphogranuloma venereum isolate of C. trachomatis, serovar L2, with either the original shuttle vector (pGFP::SW2) or a derivative of pGFP::SW2 carrying a deletion of the plasmid CDS5 gene (pCDS5KO). Female mice were inoculated with these strains either intravaginally or transcervically. We found that transformation of the plasmid-free isolate with the intact pGFP::SW2 vector significantly enhanced infectivity and induction of host inflammatory responses compared to the plasmid-free parental isolate. Transformation with pCDS5KO resulted in infection courses and inflammatory responses not significantly different from those observed in mice infected with the plasmid-free isolate. These results indicate a critical role of plasmid CDS5 in in vivo fitness and in induction of inflammatory responses. To our knowledge, these are the first in vivo observations ascribing infectivity and virulence to a specific plasmid gene.

Innate immunity is sufficient for the clearance of Chlamydia trachomatis from the female mouse genital tract

Chlamydia muridarum and Chlamydia trachomatis, mouse and human strains, respectively, have been used to study immunity in a murine model of female genital tract infection. Despite evidence that unique genes of these otherwise genomically similar strains could play a role in innate immune evasion in their respective mouse and human hosts, there have been no animal model findings to directly support this conclusion. Here, we infected C57BL/6 and adaptive immune-deficient Rag1(-/-) female mice with these strains and evaluated their ability to spontaneously resolve genital infection. Predictably, C57BL/6 mice spontaneously cleared infection caused by both chlamydial strains. In contrast, Rag1(-/-) mice which lack mature T and B cell immunity but maintain functional innate immune effectors were incapable of resolving C. muridarum infection but spontaneously cleared C. trachomatis infection. This distinct dichotomy in adaptive and innate immune-mediated clearance between mouse and human strains has important cautionary implications for the study of natural immunity and vaccine development in the mouse model.

Use of the plaque assay for testing the antibiotic susceptibility of intracellular bacteria

The plaque assay was first described for titration of bacterial inoculums and clonal isolation, and was later adapted for testing antibiotics susceptibility and to study virulence factors and motility of bacteria. Over time, the sensitivity and reproducibility of the technique has been improved. Usually, the number of plaques is counted; however, the recent development of informatics tools has stimulated interest in the quantification of plaque size. Owing to this new approach, the plaque assay has been used to characterize the host cell response when infected cells are treated with antimicrobial agents. It was found that statins prevented cell lesions following rickettsial infection; in other studies, some antibiotics were found to cause apoptosis of host cells, suggesting a toxic activity. Here, we present an overview of the plaque assay as it has been used to investigate intracellular bacteria.

TRAIL-R1 is a negative regulator of pro-inflammatory responses and modulates long-term sequelae resulting from Chlamydia trachomatis infections in humans

The immune system eliminates Chlamydia trachomatis infection through inflammation. However, uncontrolled inflammation can enhance pathology. In mice, TNF-related apoptosis-inducing ligand receptor (TRAIL-R), known for its effects on apoptosis, also regulates inflammation. In humans, the four homologues of TRAIL-R had never been investigated for effects on inflammation. Here, we examined whether TRAIL-R regulates inflammation during chlamydial infection. We examined TRAIL-R1 single nucleotide polymorphisms (SNPs) in an Ecuadorian cohort with and without C. trachomatis infections. There was a highly significant association for the TRAIL+626 homozygous mutant GG for infection vs no infection in this population. To confirm the results observed in the human population, primary lung fibroblasts and bone marrow-derived macrophages (BMDMs) were isolated from wildtype (WT) and TRAIL-R-deficient mice, and TRAIL-R1 levels in human cervical epithelial cells were depleted by RNA interference. Infection of BMDMs and primary lung fibroblasts with C. trachomatis strain L₂, or the murine pathogen C. muridarum, led to higher levels of MIP2 mRNA expression or IL-1β secretion from TRAIL-R-deficient cells than WT cells. Similarly, depletion of TRAIL-R1 expression in human epithelial cells resulted in a higher level of IL-8 mRNA expression and protein secretion during C. trachomatis infection. We conclude that human TRAIL-R1 SNPs and murine TRAIL-R modulate the innate immune response against chlamydial infection. This is the first evidence that human TRAIL-R1 is a negative regulator of inflammation and plays a role in modulating Chlamydia pathogenesis.

Plasmid deficiency in urogenital isolates of Chlamydia trachomatis reduces infectivity and virulence in a mouse model

We hypothesized that the plasmid of urogenital isolates of Chlamydia trachomatis would modulate infectivity and virulence in a mouse model. To test this hypothesis, we infected female mice in the respiratory or urogenital tract with graded doses of a human urogenital isolate of C. trachomatis, serovar F, possessing the cognate plasmid. For comparison, we inoculated mice with a plasmid‐free serovar F isolate. Following urogenital inoculation, the plasmid‐free isolate displayed significantly reduced infectivity compared with the wild‐type strain with the latter yielding a 17‐fold lower infectious dose to yield 50% infection. When inoculated via the respiratory tract, the plasmid‐free isolate exhibited reduced infectivity and virulence (as measured by weight change) when compared to the wild‐type isolate. Further, differences in infectivity, but not in virulence were observed in a C. trachomatis, serovar E isolate with a deletion within the plasmid coding sequence 1 when compared to a serovar E isolate with no mutations in the plasmid. We conclude that plasmid loss reduces virulence and infectivity in this mouse model. These findings further support a role for the chlamydial plasmid in infectivity and virulence in vivo.

Endosulfatases SULF1 and SULF2 limit Chlamydia muridarum infection

The first step in attachment of Chlamydia to host cells is thought to involve reversible binding to host heparan sulfate proteoglycans (HSPGs), polymers of variably sulfated repeating disaccharide units coupled to diverse protein backbones. However, the key determinants of HSPG structure that are involved in Chlamydia binding are incompletely defined. A previous genome-wide Drosophila RNAi screen suggested that the level of HSPG 6-O sulfation rather than the identity of the proteoglycan backbone maybe a critical determinant for binding. Here, we tested in mammalian cells whether SULF1 or SULF2, human endosulfatases, which remove 6-O sulfates from HSPGs, modulate Chlamydia infection. Ectopic expression of SULF1 or SULF2 in HeLa cells, which decreases cell surface HSPG sulfation, diminished C. muridarum binding and decreased vacuole formation. ShRNA depletion of endogenous SULF2 in a cell line that primarily expresses SULF2 augmented binding and increased vacuole formation. C. muridarum infection of diverse cell lines resulted indownregulation of SULF2 mRNA. In a murine model of acute pneumonia, mice genetically deficient in both endosulfatases or in SULF2 alone demonstrated increased susceptibility to C. muridarum lung infection. Collectively, these studies demonstrate that the level of HSPG 6-O sulfation is a critical determinant of C. muridarum infection in vivo and that 6-O endosulfatases are previously unappreciated modulators of microbial pathogenesis.

Effect of long-term laboratory propagation on Chlamydia trachomatis genome dynamics

It is assumed that bacterial strains maintained in the laboratory for long time shape their genome in a different fashion from the nature-circulating strains. Here, we analyzed the impact of long-term in vitro propagation on the genome of the obligate intracellular pathogen Chlamydia trachomatis. We fully-sequenced the genome of a historical prototype strain (L2/434/Bu) and a clinical isolate (E/CS88), before and after one-year of serial in vitro passaging (up to 3500 bacterial generations). We observed a slow adaptation of C. trachomatis to the in vitro environment, which was essentially governed by four mutations for L2/434/Bu and solely one mutation for E/CS88, corresponding to estimated mutation rates from 3.84 × 10(-10) to 1.10 × 10(-9) mutations per base pair per generation. In a speculative basis, the mutations likely conferred selective advantage as: (i) mathematical modeling showed that selective advantage is mandatory for frequency increase of a mutated clone; (ii) transversions and non-synonymous mutations were overrepresented; (iii) two non-synonymous mutations affected the genes CTL0084 and CTL0610, encoding a putative transferase and a protein likely implicated in transcription regulation respectively, which are families known to be highly prone to undergone laboratory-derived advantageous mutations in other bacteria; and (iv) the mutation for E/CS88 is located likely in the regulatory region of a virulence gene (CT115/incD) believed to play a role in subverting the host cell machinery. Nevertheless, we found no significant differences in the growth rate, plasmid load, and attachment/entry rate, between strains before and after their long-term laboratory propagation. Of note, from the mixture of clones in E/CS88 initial population, an inactivating mutation in the virulence gene CT135 evolved to 100% prevalence, unequivocally indicating that this gene is superfluous for C. trachomatis survival in vitro. Globally, C. trachomatis revealed a slow in vitro adaptation that only modestly modifies the in vivo-derived genomic evolutionary landscape.

Genomic features beyond Chlamydia trachomatis phenotypes: what do we think we know?

The obligate intracellular pathogen Chlamydia trachomatis is the causative agent of the blinding trachoma and the world's leading cause of bacterial sexually transmitted infections. Despite aggressive antibacterial control measures, C. trachomatis infections have been increasing, constituting a serious public health concern due to its morbidity and socioeconomic burden. Still, very little is known about the molecular basis underlying the phenotypic disparities observed among C. trachomatis serovars in terms of tissue tropism (ocular conjunctiva, epithelial-genitalia and lymph nodes), virulence (disease outcomes) and ecological success. This is in part due to the inexistence of straightforward tools to genetically manipulate Chlamydiae and host cell-free growth systems, hampering the elucidation of the biological role of loci. The recent release of tenths of full-genome C. trachomatis sequences depict a strains clustering scenario reflecting the organ/cell-type that they preferentially infect. However, the high degree of genomic conservation implies that few genetic features are involved in phenotypic dissimilarities. The purpose of this review is to gather the most relevant data dispersed throughout the literature concerning the genotypic evidences that support niche-specific phenotypes. This review focus on chromosomal dynamics phenomena like recombination and point-mutations, essentially involving outer and inclusion membrane proteins, type III secretion effectors, and hypothetical proteins with unknown function. The scrutiny of C. trachomatis loci involved in tissue tropism, pathogenesis and ecological success is crucial for the development of disease-specific prophylaxis.

Differences in infectivity and induction of infertility: a comparative study of Chlamydia trachomatis strains in the murine model

Chlamydia trachomatis, although commonly asymptomatic in women, can result in chronic sequelae, such as pelvic inflammatory disease, ectopic pregnancy and infertility. However, a clear relationship has not been determined between specific serovars and the ability to lead to upper genital tract infection or infertility. Thus, in order to investigate differences in pathogenicity, C3H/HeN mice were infected in the ovarian bursa with the C. trachomatis strains D (UW-3/Cx), F (N.I.1), F (IC-Cal-3) and E (Bour). Differences both in the amount of vaginal shedding as well as subsequent fertility rates were observed between D (UW-3/Cx) and F (N.I.1) compared to F (IC-Cal-3) and E (Bour). Approximately 50% of the mice infected with the D (UW-3/Cx) and F (N.I.1) strains had vaginal shedding for up to 3-4 weeks after infection and fertility rates of less than 25%. Furthermore, mice inoculated with D (UW-3/Cx) and F (N.I.1) showed infertility even in the absence of medroxy progesterone acetate (MPA) treatment. In contrast, both MPA and non-MPA treated mice infected with F (IC-Cal-3) or E (Bour) did not show vaginal shedding and had fertility rates between 45 and 88%. Mutations in the CT135 open reading frame have been associated with virulence. However, no nucleotide differences were found among the four isolates for CT135. This murine model of infection with C. trachomatis may help with the understanding of disease pathology in humans and ultimately vaccine development.

Hypothesis: Chlamydia trachomatis infection of the female genital tract is controlled by Type 2 immunity

Chlamydia trachomatis is an obligate intracellular bacterium sexually transmitted to more than 90 million individuals each year. As this level of infectivity implies, C. trachomatis is a successful human parasite; a success facilitated by its ability to cause asymptomatic infection. Host defense against C. trachomatis in the female genital tract is not well defined, but current dogma suggests infection is controlled largely by T(H)1 immunity. Conversely, it is well established that T(H)2 immunity controls allergens, helminths, and other extracellular pathogens that cause repetitive or persistent T cell stimulation but do not induce the exuberant inflammation that drives T(H)1 and T(H)17 immunity. As C. trachomatis persists in female genital tract epithelial cells but does not elicit over tissue inflammation, we now posit that defense is maintained by Type 2 immune responses that control bacterial growth but minimize immunopathological damage to vital reproductive tract anatomy. Evaluation of this hypothesis may uncover novel mechanisms by which Type 2 immunity can control growth of C. trachomatis and other intracellular pathogens, while confirmation that T(H)2 immunity was selected by evolution to control C. trachomatis infection in the female genital tract will transform current research, now focused on developing vaccines that elicit strong, and therefore potentially tissue destructive, Chlamydia-specific T(H)1 immunity.

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.

CD4+ T cells are necessary and sufficient to confer protection against Chlamydia trachomatis infection in the murine upper genital tract

Chlamydia trachomatis infection is the most common bacterial sexually transmitted disease in the United States. Chlamydia infections that ascend to the upper genital tract can persist, trigger inflammation, and result in serious sequelae such as infertility. However, mouse models in which the vaginal vault is inoculated with C. trachomatis do not recapitulate the course of human disease. These intravaginal infections of the mouse do not ascend efficiently to the upper genital tract, do not cause persistent infection, do not induce significant inflammation, and do not induce significant CD4⁺ T cell infiltration. In this article, we describe a noninvasive transcervical infection model in which we bypass the cervix and directly inoculate C. trachomatis into the uterus. We show that direct C. trachomatis infection of the murine upper genital tract stimulates a robust Chlamydia-specific CD4⁺ T cell response that is both necessary and sufficient to clear infection and provide protection against reinfection.