Oral Chlamydia vaccination induces transmucosal protection in the airway

Diverse animal models for Chlamydia infections: unraveling pathogenesis through the genital and gastrointestinal tracts

Chlamydia trachomatis is responsible for infections in various mucosal tissues, including the eyes, urogenital, respiratory, and gastrointestinal tracts. Chronic infections can result in severe consequences such as blindness, ectopic pregnancy, and infertility. The underlying mechanisms leading to these diseases involve sustained inflammatory responses, yet thorough comprehension of the underlying mechanisms remains elusive. Chlamydial biologists employ in multiple methods, integrating biochemistry, cell biology, and genetic tools to identify bacterial factors crucial for host cell interactions. While numerous animal models exist to study chlamydial pathogenesis and assess vaccine efficacy, selecting appropriate models for biologically and clinically relevant insights remains a challenge. Genital infection models in animals have been pivotal in unraveling host-microbe dynamics, identifying potential chlamydial virulence factors influencing genital pathogenicity. However, the transferability of this knowledge to human pathogenic mechanisms remains uncertain. Many putative virulence factors lack assessment in optimal animal tissue microenvironments, despite the diverse chlamydial infection models available. Given the propensity of genital Chlamydia to spread to the gastrointestinal tract, investigations into the pathogenicity and immunological impact of gut Chlamydia become imperative. Notably, the gut emerges as a promising site for both chlamydial infection vaccination and pathogenesis. This review elucidates the pathogenesis of Chlamydia infections and delineates unique features of prevalent animal model systems. The primary focus of this review is to consolidate and summarize current animal models utilized in Chlamydia researches, presenting findings, discussions on their contributions, and suggesting potential directions for further studies.

Regulation of chlamydial spreading from the small intestine to the large intestine by IL-22-producing CD4+ T cells

Following an oral inoculation, Chlamydia muridarum descends to the mouse large intestine for long-lasting colonization. However, a mutant C. muridarum that lacks the plasmid-encoded protein pGP3 due to an engineered premature stop codon (designated as CMpGP3S) failed to do so even following an intrajejunal inoculation. This was because a CD4+ T cell-dependent immunity prevented the spread of CMpGP3S from the small intestine to the large intestine. In the current study, we found that mice deficient in IL-22 (IL-22-/-) allowed CMpGP3S to spread from the small intestine to the large intestine on day 3 after intrajejunal inoculation, indicating a critical role of IL-22 in regulating the chlamydial spread. The responsible IL-22 is produced by CD4+ T cells since IL-22-/- mice were rescued to block the CMpGP3S spread by donor CD4+ T cells from C57BL/6J mice. Consistently, CD4+ T cells lacking IL-22 failed to block the spread of CMpGP3S in Rag2-/- mice, while IL-22-competent CD4+ T cells did block. Furthermore, mice deficient in cathelicidin-related antimicrobial peptide (CRAMP) permitted the CMpGP3S spread, but donor CD4+ T cells from CRAMP-/- mice were still sufficient for preventing the CMpGP3S spread in Rag2-/- mice, indicating a critical role of CRAMP in regulating chlamydial spreading, and the responsible CRAMP is not produced by CD4+ T cells. Thus, the IL-22-producing CD4+ T cell-dependent regulation of chlamydial spreading correlated with CRAMP produced by non-CD4+ T cells. These findings provide a platform for further characterizing the subset(s) of CD4+ T cells responsible for regulating bacterial spreading in the intestine.

IL-23 receptor signaling licenses group 3-like innate lymphoid cells to restrict a live-attenuated oral Chlamydia vaccine in the gut

An IFNγ-susceptible mutant of Chlamydia muridarum is attenuated in pathogenicity in the genital tract and was recently licensed as an intracellular Oral vaccine vector or intrOv. Oral delivery of intrOv induces transmucosal protection in the genital tract, but intrOv itself is cleared from the gut (without shedding any infectious particles externally) by IFNγ from group 3-like innate lymphoid cells (ILC3s). We further characterized the intrOv interactions with ILC3s in the current study, since the interactions may impact both the safety and efficacy of intrOv as an oral Chlamydia vaccine. Intracolonic inoculation with intrOv induced IFNγ that in return inhibited intrOv. The intrOv-IFNγ interactions were dependent on RORγt, a signature transcriptional factor of ILC3s. Consistently, the transfer of oral intrOv-induced ILC3s from RORγt-GFP reporter mice to IFNγ-deficient mice rescued the inhibition of intrOv. Thus, IFNγ produced by intrOv-induced ILC3s is likely responsible for inhibiting intrOv, which is further supported by the observation that oral intrOv did induce significant levels of IFNγ-producing LC3s (IFNγ+ILC3s). Interestingly, IL-23 receptor knockout (IL-23R-/-) mice no longer inhibited intrOv, which was accompanied by reduced colonic IFNγ. Transfer of oral intrOv-induced ILC3s rescued the IL-23R-/- mice to inhibit intrOv, validating the dependence of ILC3s on IL-23R signaling for inhibiting intrOv. Clearly, intrOv induces intestinal IFNγ+ILC3s for its own inhibition in the gut, which is facilitated by IL-23R signaling. These findings have provided a mechanism for ensuring the safety of intrOv as an oral Chlamydia vaccine and a platform for investigating how oral intrOv induces transmucosal protection in the genital tract.

Preclinical screen for protection efficacy of chlamydial antigens that are immunogenic in humans

To search for subunit vaccine candidates, immunogenic chlamydial antigens identified in humans were evaluated for protection against both infection and pathology in a mouse genital tract infection model under three different immunization regimens. The intramuscular immunization regimen was first used to evaluate 106 chlamydial antigens, which revealed that two antigens significantly reduced while 11 increased genital chlamydial burden. The two infection-reducing antigens failed to prevent pathology and 23 additional antigens even exacerbated pathology. Thus, intranasal mucosal immunization was tested next since intranasal inoculation with live Chlamydia muridarum prevented both genital infection and pathology. Two of the 29 chlamydial antigens evaluated were found to prevent genital infection but not pathology and three exacerbate pathology. To further improve protection efficacy, a combinational regimen (intranasal priming + intramuscular boosting + a third intraperitoneal/subcutaneous boost) was tested. This regimen identified four infection-reducing antigens, but only one of them prevented pathology. Unfortunately, this protective antigen was not advanced further due to its amino acid sequence homology with several human molecules. Two pathology-exacerbating antigens were also found. Nevertheless, intranasal mucosal priming with viable C. muridarum in control groups consistently prevented both genital infection and pathology regardless of the subsequent boosters. Thus, screening 140 different chlamydial antigens with 21 repeated multiple times in 17 experiments failed to identify a subunit vaccine candidate but demonstrated the superiority of viable chlamydial organisms in inducing immunity against both genital infection and pathology, laying the foundation for developing a live-attenuated Chlamydia vaccine.

Induction of Transmucosal Protection by Oral Vaccination with an Attenuated Chlamydia

Chlamydia muridarum has been used to study chlamydial pathogenesis because it induces mice to develop hydrosalpinx, a pathology observed in C. trachomatis-infected women. We identified a C. muridarum mutant that is no longer able to induce hydrosalpinx. In the current study, we evaluated the mutant as an attenuated vaccine. Following an intravaginal immunization with the mutant, mice were protected from hydrosalpinx induced by wild-type C. muridarum. However, the mutant itself productively colonized the mouse genital tract and produced infectious organisms in vaginal swabs. Nevertheless, the mutant failed to produce infectious shedding in the rectal swabs following an oral inoculation. Importantly, mice orally inoculated with the mutant mounted transmucosal immunity against challenge infection of wild-type C. muridarum in the genital tract. The protection was detected as early as day 3 following the genital challenge infection and the orally immunized mice were protected from any significant pathology in the upper genital tract. However, the same orally immunized mice failed to prevent the colonization of wild-type C. muridarum in the gastrointestinal tract. The transmucosal immunity induced by the oral mutant was further validated in the airway. The orally vaccinated mice were protected from both lung infection and systemic toxicity caused by intranasally inoculated wild-type C. muridarum although the same mice still permitted the gastrointestinal colonization by the wild-type C. muridarum. These observations suggest that the mutant C. muridarum may be developed into an intracellular oral vaccine vector (or IntrOv) for selectively inducing transmucosal immunity in extra-gut tissues.

Regulation of chlamydial colonization by IFNγ delivered via distinct cells

The mouse-adapted pathogen Chlamydia muridarum (CM) induces pathology in the mouse genital tract but fails to do so in the gastrointestinal tract. CM is cleared from both the genital tract and small intestine by IFNγ delivered by antigen-specific CD4+ T cells but persists for a long period in the large intestine. The long-lasting colonization of CM in the large intestine is regulated by IFNγ delivered by group 3 innate lymphoid cells (ILC3s). Interestingly, the ILC3-delivered IFNγ can inhibit the human pathogen Chlamydia trachomatis (CT) in the mouse endometrium. Thus, IFNγ produced/delivered by different cells may selectively restrict chlamydial colonization in different tissues. Revealing the underlying mechanisms of chlamydial interactions with IFNγ produced by different cells may yield new insights into both chlamydial pathogenicity and mucosal immunity.

Induction of transmucosal protection by oral vaccination with an attenuated Chlamydia

Chlamydia muridarum has been used to study chlamydial pathogenesis since it induces mice to develop hydrosalpinx, a pathology observed in C. trachomatis -infected women. We identified a C. muridarum mutant that is no longer able to induce hydrosalpinx. In the current study, we evaluated the mutant as an attenuated vaccine. Following an intravaginal immunization with the mutant, mice were protected from hydrosalpinx induced by wild type C. muridarum . However, the mutant itself productively colonized the mouse genital tract and produced infectious organisms in vaginal swabs. Nevertheless, the mutant failed to produce infectious shedding in the rectal swabs following an oral inoculation. Importantly, mice orally inoculated with the mutant mounted transmucosal immunity against challenge infection of wild type C. muridarum in the genital tract. The protection was detected as early as day 3 following the challenge infection and the immunized mice were protected from any significant pathology in the upper genital tract. However, the same orally immunized mice failed to prevent the colonization of wild type C. muridarum in the gastrointestinal tract. The transmucosal immunity induced by the oral mutant was further validated in the airway. The orally vaccinated mice were protected from both lung infection and systemic toxicity caused by intranasally inoculated wild type C. muridarum although the same mice still permitted the gastrointestinal colonization by the wild type C. muridarum . These observations suggest that the mutant C. muridarum may be developed into an intr acellular o ral v accine vector (or IntrOv) for selectively inducing transmucosal immunity in extra-gut tissues.

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

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

Chlamydia trachomatis: quest for an eye-opening vaccine breakthrough

INTRODUCTION

Chlamydia trachomatis, commonly referred to as chlamydia (a bacterium), is a common sexually transmitted infection, and if attended to early, it can be treatable. However, if left untreated it can lead to serious consequences. C. trachomatis infects both females and males although its occurrence in females is more common, and it can spread to the eyes causing disease and in some case blindness.

AREA COVERED

With ongoing attempts in the most impoverished regions of the country, trachoma will be eradicated as a blinding disease by the year 2022. A prophylactic vaccine candidate with established safety and efficacy is a cogent tool to achieve this goal. This manuscript covers the vaccine development programs for chlamydial infection.

EXPERT OPINION

Currently, the Surgery Antibiotics Facial Environmental (SAFE) program is being implemented in endemic countries in order to reduce transmission and control of the disease. Vaccines have been shown over the years to protect against infectious diseases. Charge variant-based adjuvant can also be used for the successful delivery of chlamydial specific antigen for efficient vaccine delivery through nano delivery platform. Thus, a vaccine against C. trachomatis would be of great public health benefit.

Chlamydia Deficient in Plasmid-Encoded Glycoprotein 3 (pGP3) as an Attenuated Live Oral Vaccine

Despite the extensive efforts, there is still a lack of a licensed vaccine against Chlamydia trachomatis in humans. The mouse genital tract infection with Chlamydia muridarum has been used to both investigate chlamydial pathogenic mechanisms and evaluate vaccine candidates due to the C. muridarum's ability to induce mouse hydrosalpinx. C. muridarum mutants lacking the entire plasmid or deficient in only the plasmid-encoded pGP3 are highly attenuated in inducing hydrosalpinx. We now report that intravaginal immunization with these mutants as live attenuated vaccines protected mice from hydrosalpinx induced by wild type C. muridarum. However, these mutants still productively infected the mouse genital tract. Further, the mutant-infected mice were only partially protected against the subsequent infection with wild type C. muridarum. Thus, these mutants as vaccines are neither safe nor effective when they are delivered via the genital tract. Interestingly, these mutants were highly deficient in colonizing the gastrointestinal tract. Particularly, the pGP3-deficient mutant failed to shed live organisms from mice following an oral inoculation, suggesting that the pGP3-deficient mutant may be developed into a safe oral vaccine. Indeed, oral inoculation with the pGP3-deficient mutant induced robust transmucosal immunity against both the infection and pathogenicity of wild type C. muridarum in the genital tract. Thus, we have demonstrated that the plasmid-encoded virulence factor pGP3 may be targeted for developing an attenuated live oral vaccine.

Chlamydia overcomes multiple gastrointestinal barriers to achieve long-lasting colonization

Chlamydia trachomatis (CT) is frequently detected in the human gastrointestinal (GI) tract despite its leading role in sexually transmitted bacterial infections in the genital tract. Chlamydia muridarum (CM), a model pathogen for investigating CT pathogenesis in the genital tract, can also colonize the mouse GI tract for long periods. Genital-tract mutants of CM no longer colonize the GI tract. The mutants lacking plasmid functions are more defective in colonizing the upper GI tract while certain chromosomal gene-deficient mutants are more defective in the lower GI tract, suggesting that Chlamydia may use the plasmid for promoting its spread to the large intestine while using the chromosome-encoded factors for maintaining its colonization in the large intestine. The plasmid-encoded Pgp3 is critical for Chlamydia to resist the acid barrier in the stomach and to overcome a CD4⁺ T cell barrier in the small intestine. On reaching the large intestine, Pgp3 is no longer required. Instead, the chromosome-encoded open reading frames TC0237/TC0668 become essential for Chlamydia to evade the group 3-like innate lymphoid cell-secreted interferon (IFN)γ in the large intestine. These findings are important for exploring the medical significance of chlamydial colonization in the gut and for understanding the mechanisms of chlamydial pathogenicity in the genital tract.

Regulation of Chlamydia spreading from the small intestine to the large intestine via an immunological barrier

The obligate intracellular bacterium Chlamydia is a genital tract pathogen that can also colonize the gastrointestinal tract for long periods. The long-lasting colonization is dependent on chlamydial spreading from the small intestine to the large intestine. We previously reported that a mutant Chlamydia was able to activate an intestinal barrier for blocking its own spreading to the large intestine. In the current study, we used the mutant Chlamydia colonization model to confirm the intestinal barrier function and further to determine the immunological basis of the barrier with gene-deficient mice. Recombination activating gene 1^-/- mice failed to block the mutant Chlamydia spreading, while mice deficient in toll-like receptors, myeloid differentiation primary response 88 or stimulator of interferon genes still blocked the spreading, suggesting that the intestinal barrier function is dependent on lymphocytes that express antigen receptors. Mice deficient in CD4, but not CD8 nor μ chain failed to prevent the chlamydial spreading, indicating a protective role of CD4⁺ cells in the intestinal barrier. Consistently, adoptive transfer of CD4⁺ T cells reconstituted the intestinal barrier in CD4^-/- mice. More importantly, CD4⁺ but not CD8⁺ T cells nor B cells restored the intestinal barrier function in recombination activating gene 1^-/- mice. Thus, CD4⁺ T cells are necessary and sufficient for maintaining the intestinal barrier function, indicating that the spread of an intracellular bacterium from the small intestine to the large intestine is regulated by an immunological barrier. This study has also laid a foundation for further illuminating the mechanisms by which a CD4⁺ T cell-dependent intestinal barrier regulates bacterial spreading in the gut.

Adoptive Transfer of Group 3-Like Innate Lymphoid Cells Restores Mouse Colon Resistance to Colonization of a Gamma Interferon-Susceptible Chlamydia muridarum Mutant

The obligate intracellular bacterium Chlamydia muridarum can colonize the mouse colon for a long period, but a gamma interferon (IFN-γ)-susceptible mutant clone fails to do so. Nevertheless, the mutant's colonization is rescued in mice deficient in interleukin-7 receptor (IL-7R) (lacking both lymphocytes and innate lymphoid cells [ILCs]) or IFN-γ but not in mice lacking recombination-activated gene 1 (Rag1^-/- mice) (lacking adaptive immunity lymphocytes), indicating a critical role of ILC-derived IFN-γ in regulating chlamydial colonization. In the current study, we have used an adoptive transfer approach for further characterizing the responsible ILCs. First, intestinal ILCs isolated from Rag1^-/- mice were able to rescue IL-7R-deficient mice to restrict the colonization of the IFN-γ-susceptible Chlamydia muridarum mutant. Second, the responsible ILCs were localized to the intestinal lamina propria since ILCs from the lamina propria but not the intraepithelial compartment conferred the restriction. Third, lamina propria ILCs enriched for RORγt expression but not those negative for RORγt rescued the IL-7R-deficient mice to restrict mutant colonization, indicating a critical role of group 3-like ILCs (ILC3s) since RORγt is a signature transcriptional factor of ILC3s. Fourth, a portion of the ILC3s expressed IFN-γ, thus defined as ex-ILC3s, and the transfer of the ex-ILC3s conferred colon resistance to mutant Chlamydia muridarum colonization in IFN-γ-deficient mice. Finally, genetically labeled RORγt-positive (RORγt⁺) ILCs were able to inhibit mutant colonization. Thus, we have demonstrated that ILC3s are sufficient for regulating chlamydial colonization, laying a foundation for further revealing the mechanisms by which an obligate intracellular bacterium activates colonic ILC3s.

Chlamydia-Specific IgA Secretion in the Female Reproductive Tract Induced via Per-Oral Immunization Confers Protection against Primary Chlamydia Challenge

Chlamydia trachomatis is an obligate intracellular pathogen that causes sexually transmitted disease. In women, chlamydial infections may cause pelvic inflammatory disease (PID), ectopic pregnancy, and infertility. The role of antibodies in protection against a primary Chlamydia infection is unclear and was a focus of this work. Using the C. muridarum mouse infection model, we show that intestinal mucosa is infected via intranasal (i.n.) or per-oral (p.o.) Chlamydia inoculation and that unlike the female reproductive tract (FRT) mucosa, it halts systemic Chlamydia dissemination. Moreover, p.o. immunization or infection with Chlamydia confers protection against per-vaginal (p.v.) challenge, resulting in significantly decreased bacterial burden in the FRT, accelerated Chlamydia clearance, and reduced hydrosalpinx pathology. In contrast, subcutaneous (s.c.) immunization conferred no protection against the p.v. challenge. Both p.o. and s.c. immunizations induced Chlamydia-specific serum IgA. However, IgA was found only in the vaginal washes and fecal extracts of p.o.-immunized animals. Following a p.v. challenge, unimmunized control and s.c.-s.c.-immunized animals developed Chlamydia-specific intestinal IgA yet failed to develop IgA in the FRT, indicating that IgA response in the FRT relies on the FRT to gastrointestinal tract (GIT) antigen transport. Vaginal secretions of p.o.-immunized animals neutralize Chlamydia in vivo, resulting in significantly lower Chlamydia burden in the FRT and Chlamydia transport to the GIT. We also show that infection of the GIT is not necessary for induction of protective immunity in the FRT, a finding that is important for the development of p.o. subunit vaccines to target Chlamydia and possibly other sexually transmitted pathogens.

Evasion of Innate Lymphoid Cell-Regulated Gamma Interferon Responses by Chlamydia muridarum To Achieve Long-Lasting Colonization in Mouse Colon

Revealing the mechanisms by which bacteria establish long-lasting colonization in the gastrointestinal tract is an area of intensive investigation. The obligate intracellular bacterium Chlamydia is known to colonize mouse colon for long periods. A colonization-deficient mutant strain of this intracellular bacterium is able to regain long-lasting colonization in gamma interferon (IFN-γ) knockout mice following intracolon inoculation. We now report that mice deficient in conventional T lymphocytes or recombination-activating gene (Rag) failed to show rescue of mutant colonization. Nevertheless, antibody depletion of IFN-γ or genetic deletion of interleukin 2 (IL-2) receptor common gamma chain in Rag-deficient mice did rescue mutant colonization. These observations suggest that colonic IFN-γ, responsible for inhibiting the intracellular bacterial mutant, is produced by innate lymphoid cells (ILCs). Consistently, depletion of NK1.1⁺ cells in Rag-deficient mice both prevented IFN-γ production and rescued mutant colonization. Furthermore, mice deficient in transcriptional factor RORγt, but not chemokine receptor CCR6, showed full rescue of the long-lasting colonization of the mutant, indicating a role for group 3-like ILCs. However, the inhibitory function of the responsible group 3-like ILCs was not dependent on the natural killer cell receptor (NCR1), since NCR1-deficient mice still inhibited mutant colonization. Consistently, mice deficient in the transcriptional factor T-bet only delayed the clearance of the bacterial mutant without fully rescuing the long-lasting colonization of the mutant. Thus, we have demonstrated that the obligate intracellular bacterium Chlamydia maintains its long-lasting colonization in the colon by evading IFN-γ from group 3-like ILCs.

Simultaneous Intramuscular And Intranasal Administration Of Chitosan Nanoparticles-Adjuvanted Chlamydia Vaccine Elicits Elevated Protective Responses In The Lung

Background

Chlamydia psittaci is a zoonotic bacteria closely associated with psittacosis/ornithosis. Vaccination has been recognized as the best way to inhibit the spread of C. psittaci due to the majority ignored of infections. The optimal Chlamydia vaccine was obstructed by the defect of single immunization route and the lack of availability of nontoxic and valid adjuvants.

Methods

In this study, we developed a novel immunization strategy, simultaneous (SIM) intramuscular (IM) and intranasal (IN) administration of a C. psittaci antigens (Ags) adjuvanted with chitosan nanoparticles (CNPs). And SIM-CNPs-Ags were used to determine the different types of immune response and the protective role in vivo.

Results

CNPs-Ags with zeta-potential values of 13.12 mV and of 276.1 nm showed excellent stability and optimal size for crossing the mucosal barrier with high 71.7% encapsulation efficiency. SIM-CPN-Ags mediated stronger humoral and mucosal responses by producing meaningfully high levels of IgG and secretory IgA (sIgA) antibodies. The SIM route also led to Ags-specific T-cell responses and increased IFN-γ, IL-2, TNF-α and IL-17A in the splenocyte supernatants. Following respiratory infection with C. psittaci, we found that SIM immunization remarkably reduced bacterial load and the degree of inflammation in the infected lungs and made for a lower level of IFN-γ, TNF-α and IL-6. Furthermore, SIM vaccination with CNPs-Ags had obviously inhibited C. psittaci disseminating to various organs in vivo.

Conclusion

SIM immunization with CNPs-adjuvanted C. psittaci Ags may present a novel strategy for the development of a vaccine against the C. psittaci infection.

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

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

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

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

The Plasmid-Encoded pGP3 Promotes Chlamydia Evasion of Acidic Barriers in Both Stomach and Vagina

Although Chlamydia trachomatis is a human genital tract pathogen, chlamydial organisms have frequently been detected in both vaginal and rectal swab samples of animals and humans. The plasmid-encoded pGP3, a genital tract virulence factor, is essential for Chlamydia muridarum to colonize the mouse gastrointestinal tract. However, intracolon inoculation to bypass the gastric barrier rescued the colonization ability of a pGP3-deficient C. muridarum mutant, suggesting that pGP3 is required for C. muridarum to reach but not to colonize the large intestine. The pGP3-deficient mutant was rapidly cleared in the stomach and was 100-fold more susceptible to gastric killing. In mice genetically deficient in gastrin, a key regulator for gastric acid production, or pharmacologically treated with a proton pump inhibitor, the ability of pGP3-deficient C. muridarum to colonize the gastrointestinal tract was rescued. The pGP3-dependent resistance was further recapitulated in vitro with treatments with HCl, pepsin, or sarkosyl. In the genital tract, deficiency in pGP3 significantly reduced C. muridarum survival in the mouse vagina and increased C. muridarum susceptibility to vaginal killing by ∼8 times. The pGP3-deficient C. muridarum was more susceptible to lactic acid killing, and the pGP3 deficiency also significantly increased C. trachomatis susceptibility to lactic acid. The above-described observations together suggest that Chlamydia may have acquired the plasmid-encoded pGP3 to overcome the gastric barrier during its adaptation to the gastrointestinal tract and the pGP3-dependent resistance may enable chlamydial evasion of the female lower genital tract barrier during sexual transmission.