B cells enhance antigen-specific CD4 T cell priming and prevent bacteria dissemination following Chlamydia muridarum genital tract infection

Murine modeling of menstruation identifies immune correlates of protection during Chlamydia muridarum challenge

The menstrual cycle influences the risk of acquiring sexually transmitted infections (STIs), including Chlamydia trachomatis (C. trachomatis), although the underlying immune contributions are poorly defined. A mouse model simulating the immune-mediated process of menstruation could provide valuable insights into tissue-specific determinants of protection against chlamydial infection within the cervicovaginal and uterine mucosae comprising the female reproductive tract (FRT). Here, we used the pseudopregnancy approach in naïve C57Bl/6 mice and performed vaginal challenge with Chlamydia muridarum (C. muridarum) at decidualization, endometrial tissue remodeling, or uterine repair. This strategy identified that the time frame comprising uterine repair correlated with robust infection and greater bacterial burden as compared with mice on hormonal contraception, while challenges during endometrial remodeling were least likely to result in a productive infection. By comparing the infection site at early time points following chlamydial challenge, we found that a greater abundance of innate effector populations and proinflammatory signaling, including IFNγ correlated with protection. FRT immune profiling in uninfected mice over pseudopregnancy or in pig-tailed macaques over the menstrual cycle identified NK cell infiltration into the cervicovaginal tissues and lumen over the course of endometrial remodeling. Notably, NK cell depletion over this time frame reversed protection, with mice now productively infected with C. muridarum following challenge. This study shows that the pseudopregnancy murine menstruation model recapitulates immune changes in the FRT as a result of endometrial remodeling and identifies NK cell localization at the FRT as essential for immune protection against primary C. muridarum infection.

BHLHE40 drives protective polyfunctional CD4 T cell differentiation in the female reproductive tract against Chlamydia

The protein basic helix-loop-helix family member e40 (BHLHE40) is a transcription factor recently emerged as a key regulator of host immunity to infections, autoimmune diseases and cancer. In this study, we investigated the role of Bhlhe40 in protective T cell responses to the intracellular bacterium Chlamydia in the female reproductive tract (FRT). Mice deficient in Bhlhe40 exhibited severe defects in their ability to control Chlamydia muridarum shedding from the FRT. The heightened bacterial burdens in Bhlhe40-/- mice correlated with a marked increase in IL-10-producing T regulatory type 1 (Tr1) cells and decreased polyfunctional CD4 T cells co-producing IFN-γ, IL-17A and GM-CSF. Genetic ablation of IL-10 or functional blockade of IL-10R increased CD4 T cell polyfunctionality and partially rescued the defects in bacterial control in Bhlhe40-/- mice. Using single-cell RNA sequencing coupled with TCR profiling, we detected a significant enrichment of stem-like T cell signatures in Bhlhe40-deficient CD4 T cells, whereas WT CD4 T cells were further down on the differentiation trajectory with distinct effector functions beyond IFN-γ production by Th1 cells. Altogether, we identified Bhlhe40 as a key molecular driver of CD4 T cell differentiation and polyfunctional responses in the FRT against Chlamydia.

Elimination of Chlamydia muridarum from the female reproductive tract is IL-12p40 dependent, but independent of Th1 and Th2 cells

Chlamydia vaccine approaches aspire to induce Th1 cells for optimal protection, despite the fact that there is no direct evidence demonstrating Th1-mediated Chlamydia clearance from the female reproductive tract (FRT). We recently reported that T-bet-deficient mice can resolve primary Chlamydia infection normally, undermining the potentially protective role of Th1 cells in Chlamydia immunity. Here, we show that T-bet-deficient mice develop robust Th17 responses and that mice deficient in Th17 cells exhibit delayed bacterial clearance, demonstrating that Chlamydia-specific Th17 cells represent an underappreciated protective population. Additionally, Th2-deficient mice competently clear cervicovaginal infection. Furthermore, we show that sensing of IFN-γ by non-hematopoietic cells is essential for Chlamydia immunity, yet bacterial clearance in the FRT does not require IFN-γ secretion by CD4 T cells. Despite the fact that Th1 cells are not necessary for Chlamydia clearance, protective immunity to Chlamydia is still dependent on MHC class-II-restricted CD4 T cells and IL-12p40. Together, these data point to IL-12p40-dependent CD4 effector maturation as essential for Chlamydia immunity, and Th17 cells to a lesser extent, yet neither Th1 nor Th2 cell development is critical. Future Chlamydia vaccination efforts will be more effective if they focus on induction of this protective CD4 T cell population.

BHLHE40 drives protective polyfunctional CD4 T cell differentiation in the female reproductive tract against Chlamydia

The protein basic helix-loop-helix family member e40 (BHLHE40) is a transcription factor recently emerged as a key regulator of host immunity to infections, autoimmune diseases and cancer. In this study, we investigated the role of Bhlhe40 in protective T cell responses to the intracellular bacterium Chlamydia in the female reproductive tract (FRT). Mice deficient in Bhlhe40 exhibited severe defects in their ability to control Chlamydia muridarum shedding from the FRT. The heightened bacterial burdens in Bhlhe40-/- mice correlated with a marked increase in IL-10-producing T regulatory type 1 (Tr1) cells and decreased polyfunctional CD4 T cells co-producing IFN-γ, IL-17A and GM-CSF. Genetic ablation of IL-10 or functional blockade of IL-10R increased CD4 T cell polyfunctionality and partially rescued the defects in bacterial control in Bhlhe40-/- mice. Using single-cell RNA sequencing coupled with TCR profiling, we detected a significant enrichment of stem-like T cell signatures in Bhlhe40-deficient CD4 T cells, whereas WT CD4 T cells were further down on the differentiation trajectory with distinct effector functions beyond IFN-γ production by Th1 cells. Altogether, we identified Bhlhe40 as a key molecular driver of CD4 T cell differentiation and polyfunctional responses in the FRT against Chlamydia.

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 IFNγ-PDL1 Pathway Enhances CD8T-DCT Interaction to Promote Hypertension

BACKGROUND

Renal T cells contribute importantly to hypertension, but the underlying mechanism is incompletely understood. We reported that CD8Ts directly stimulate distal convoluted tubule cells (DCTs) to increase NCC (sodium chloride co-transporter) expression and salt reabsorption. However, the mechanistic basis of this pathogenic pathway that promotes hypertension remains to be elucidated.

METHODS

We used mouse models of DOCA+salt (DOCA) treatment and adoptive transfer of CD8+ T cells (CD8T) from hypertensive animals to normotensive animals in in vivo studies. Co-culture of mouse DCTs and CD8Ts was used as in vitro model to test the effect of CD8T activation in promoting NCC-mediated sodium retention and to identify critical molecular players contributing to the CD8T-DCT interaction. Interferon (IFNγ)-KO mice and mice receiving renal tubule-specific knockdown of PDL1 were used to verify in vitro findings. Blood pressure was continuously monitored via radio-biotelemetry, and kidney samples were saved at experimental end points for analysis.

RESULTS

We identified critical molecular players and demonstrated their roles in augmenting the CD8T-DCT interaction leading to salt-sensitive hypertension. We found that activated CD8Ts exhibit enhanced interaction with DCTs via IFN-γ-induced upregulation of MHC-I and PDL1 in DCTs, thereby stimulating higher expression of NCC in DCTs to cause excessive salt retention and progressive elevation of blood pressure. Eliminating IFN-γ or renal tubule-specific knockdown of PDL1 prevented T cell homing into the kidney, thereby attenuating hypertension in 2 different mouse models.

CONCLUSIONS

Our results identified the role of activated CD8Ts in contributing to increased sodium retention in DCTS through the IFNγ-PDL1 pathway. These findings provide a new mechanism for T cell involvement in the pathogenesis of hypertension and reveal novel therapeutic targets.

The Role of IL-17 During Infections in the Female Reproductive Tract

Interleukin-17 (IL-17A) is a cytokine involved in a complex array of both protective and detrimental processes. Although early biological studies focused on the pro-inflammatory function of IL-17 in the context of autoimmune and inflammatory disorders, it has become increasingly evident that the roles of IL-17 are far more nuanced. Recent work has demonstrated that the functions of IL-17 are highly context- and tissue-dependent, and there is a fine balance between the pathogenic and protective functions of IL-17. This is especially evident in mucosal tissues such as the female reproductive tract, where IL-17 has been shown to play an important role in the immune response generated during fungal, bacterial and viral infections associated with protection, but also with inflammation. In this review, we discuss the evolving landscape of IL-17 biology within the context of the vaginal mucosa, focusing on key findings that highlight the importance of this cytokine in genital mucosal immunity.

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.

Human genital antibody-mediated inhibition of Chlamydia trachomatis infection and evidence for ompA genotype-specific neutralization

The endocervix, the primary site of Chlamydia trachomatis (Ct) infection in women, has a unique repertoire of locally synthesized IgG and secretory IgA (SIgA) with contributions from serum IgG. Here, we assessed the ability of genital and serum-derived IgG and IgA from women with a recent positive Ct test to neutralize Ct elementary bodies (EBs) and inhibit inclusion formation in vitro in human endocervical epithelial cells. We also determined if neutralization was influenced by the major outer membrane protein (MOMP) of the infecting strain, as indicated by ompA gene sequencing and genotyping. At equivalent low concentrations of Ct EB (D/UW-3/Cx + E/UW-5/Cx)-specific antibody, genital-derived IgG and IgA and serum IgA, but not serum IgG, significantly inhibited inclusion formation, with genital IgA being most effective, followed by genital IgG, then serum IgA. The well-characterized Ct genotype D strain, D/UW-3/Cx, was neutralized by serum-derived IgG from patients infected with genotype D strains, genital IgG from patients infected with genotype D or E strains, and by genital IgA from patients infected with genotype D, E, or F strains. Additionally, inhibition of D/UW-3/Cx infection by whole serum, rather than purified immunoglobulin, was associated with levels of serum EB-specific IgG rather than the genotype of infecting strain. In contrast, a Ct genotype Ia clinical isolate, Ia/LSU-56/Cx, was neutralized by whole serum in a genotype and genogroup-specific manner, and inhibition also correlated with EB-specific IgG concentrations in serum. Taken together, these data suggest that (i) genital IgA most effectively inhibits Ct infection in vitro, (ii) human antibody-mediated inhibition of Ct infection is significantly influenced by the ompA genotype of the infecting strain, (iii) the genital antibody repertoire develops or matures differently compared to systemic antibody, and (iv) ompA genotype-specificity of inhibition of infection by whole serum can be overcome by high concentrations of Ct-specific IgG.

Chlamydia trachomatis Cross-Serovar Protection during Experimental Lung Reinfection in Mice

Chlamydia trachomatis causes most bacterial sexually transmitted diseases worldwide. Different major outer membrane proteins (MOMPs) define various serovars of this intracellular pathogen: In women, D to L3 can cause urethritis, cervicitis, salpingitis, and oophoritis, and, thus, infertility. Protective immunity might be serovar-specific since chlamydial infection does not appear to induce an effective acquired immunity and reinfections occur. A better understanding of induced cross-serovar protection is essential for the selection of suitable antigens in vaccine development. In our mouse lung infection screening model, we evaluated the urogenital serovars D, E, and L2 in this regard. Seven weeks after primary infection or mock-infection, respectively, mice were infected a second time with the identical or one of the other serovars. Body weight and clinical score were monitored for 7 days. Near the peak of the second lung infection, bacterial load, myeloperoxidase, IFN-γ, and TNF-α in lung homogenate, as well as chlamydia-specific IgG levels in blood were determined. Surprisingly, compared with mice that were infected then for the first time, almost independent of the serovar combination used, all acquired parameters of disease were similarly diminished. Our reinfection study suggests that efficient cross-serovar protection could be achieved by a vaccine combining chlamydial antigens that do not include nonconserved MOMP regions.

Chlamydia spreads to the large intestine lumen via multiple pathways

Chlamydia in the genital tract is known to spread via the blood circulation system to the large intestinal lumen to achieve long-lasting colonization. However, the precise pathways for genital Chlamydia to access to the large intestinal lumen remain unclear. The spleen was recently reported to be critical for the chlamydial spreading. In the current study, it was found that following intravaginal inoculation with Chlamydia, mice with or without splenectomy both produced infectious Chlamydia in the rectal swabs, indicating that spleen is not essential for genital Chlamydia to spread to the gastrointestinal tract. This conclusion was validated by the observation that intravenously inoculated Chlamydia was also detected in the rectal swabs of mice regardless of splenectomy. Careful comparison of the tissue distribution of live chlamydial organisms following intravenous inoculation revealed redundant pathways for Chlamydia to reach the large intestine lumen. The intravenously inoculated Chlamydia was predominantly recruited to the spleen within 12h and then detected in the stomach lumen by 24h, the intestinal lumen by 48h and rectal swabs by 72h. These observations suggest a potential spleen-to-stomach pathway for hematogenous Chlamydia to reach the large intestine lumen. This conclusion was supported by the observation made in mice under coprophagy-free condition. However, in the absence of spleen, hematogenous Chlamydia was predominantly recruited to the liver and then simultaneously detected in the intestinal tissue and lumen, suggesting a potential liver-to-intestine pathway for Chlamydia to reach the large intestine lumen. Thus, genital/hematogenous Chlamydia may reach the large intestinal lumen via multiple redundant pathways.

Immunopeptidomics for next-generation bacterial vaccine development

Antimicrobial resistance is an increasing global threat and alternative treatments substituting failing antibiotics are urgently needed. Vaccines are recognized as highly effective tools to mitigate antimicrobial resistance; however, the selection of bacterial antigens as vaccine candidates remains challenging. In recent years, advances in mass spectrometry-based proteomics have led to the development of so-called immunopeptidomics approaches that allow the untargeted discovery of bacterial epitopes that are presented on the surface of infected cells. Especially for intracellular bacterial pathogens, immunopeptidomics holds great promise to uncover antigens that can be encoded in viral vector- or nucleic acid-based vaccines. This review provides an overview of immunopeptidomics studies on intracellular bacterial pathogens and considers future directions and challenges in advancing towards next-generation vaccines.

Circulating immunity protects the female reproductive tract from Chlamydia infection

Anatomical positioning of memory lymphocytes within barrier tissues accelerates secondary immune responses and is thought to be essential for protection at mucosal surfaces. However, it remains unclear whether resident memory in the female reproductive tract (FRT) is required for Chlamydial immunity. Here, we describe efficient generation of tissue-resident memory CD4 T cells and memory lymphocyte clusters within the FRT after vaginal infection with Chlamydia Despite robust establishment of localized memory lymphocytes within the FRT, naïve mice surgically joined to immune mice, or mice with only circulating immunity following intranasal immunization, were fully capable of resisting Chlamydia infection via the vaginal route. Blocking the rapid mobilization of circulating memory CD4 T cells to the FRT inhibited this protective response. These data demonstrate that secondary protection in the FRT can occur in the complete absence of tissue-resident immune cells. The ability to confer robust protection to barrier tissues via circulating immune memory provides an unexpected opportunity for vaccine development against infections of the FRT.

Encapsulation of Recombinant MOMP in Extended-Releasing PLGA 85:15 Nanoparticles Confer Protective Immunity Against a Chlamydia muridarum Genital Challenge and Re-Challenge

Recently we reported the immune-potentiating capacity of a Chlamydia nanovaccine (PLGA-rMOMP) comprising rMOMP (recombinant major outer membrane protein) encapsulated in extended-releasing PLGA [poly (D, L-lactide-co-glycolide) (85:15)] nanoparticles. Here we hypothesized that PLGA-rMOMP would bolster immune-effector mechanisms to confer protective efficacy in mice against a Chlamydia muridarum genital challenge and re-challenge. Female BALB/c mice received three immunizations, either subcutaneously (SC) or intranasally (IN), before receiving an intravaginal challenge with C. muridarum on day 49 and a re-challenge on day 170. Both the SC and IN immunization routes protected mice against genital challenge with enhanced protection after a re-challenge, especially in the SC mice. The nanovaccine induced robust antigen-specific Th1 (IFN-γ, IL-2) and IL-17 cytokines plus CD4+ proliferating T-cells and memory (CD44high CD62Lhigh) and effector (CD44high CD62Llow) phenotypes in immunized mice. Parallel induction of antigen-specific systemic and mucosal Th1 (IgG2a, IgG2b), Th2 (IgG1), and IgA antibodies were also noted. Importantly, immunized mice produced highly functional Th1 avidity and serum antibodies that neutralized C. muridarum infectivity of McCoy fibroblasts in-vitro that correlated with their respective protection levels. The SC, rather than the IN immunization route, triggered higher cellular and humoral immune effectors that improved mice protection against genital C. muridarum. We report for the first time that the extended-releasing PLGA 85:15 encapsulated rMOMP nanovaccine confers protective immunity in mice against genital Chlamydia and advances the potential towards acquiring a nano-based Chlamydia vaccine.

Immunopathogenesis of genital Chlamydia infection: insights from mouse models

Chlamydiae are pathogenic intracellular bacteria that cause a wide variety of diseases throughout the globe, affecting the eye, lung, coronary arteries and female genital tract. Rather than by direct cellular toxicity, Chlamydia infection generally causes pathology by inducing fibrosis and scarring that is largely mediated by host inflammation. While a robust immune response is required for clearance of the infection, certain elements of that immune response may also damage infected tissue, leading to, in the case of female genital infection, disease sequelae such as pelvic inflammatory disease, infertility and ectopic pregnancy. It has become increasingly clear that the components of the immune system that destroy bacteria and those that cause pathology only partially overlap. In the ongoing quest for a vaccine that prevents Chlamydia-induced disease, it is important to target mechanisms that can achieve protective immunity while preventing mechanisms that damage tissue. This review focuses on mouse models of genital Chlamydia infection and synthesizes recent studies to generate a comprehensive model for immunity in the murine female genital tract, clarifying the respective contributions of various branches of innate and adaptive immunity to both host protection and pathogenic genital scarring.

Chlamydia trachomatis: Cell biology, immunology and vaccination

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

T cell responses to Chlamydia

Chlamydia trachomatis is the most commonly reported sexually transmitted infection in the United States. The high prevalence of infection and lack of a vaccine indicate a critical knowledge gap surrounding the host's response to infection and how to effectively generate protective immunity. The immune response to C. trachomatis is complex, with cells of the adaptive immune system playing a crucial role in bacterial clearance. Here, we discuss the CD4+ and CD8+ T cell response to Chlamydia, the importance of antigen specificity and the role of memory T cells during the recall response. Ultimately, a deeper understanding of protective immune responses is necessary to develop a vaccine that prevents the inflammatory diseases associated with Chlamydia infection.

Innate IFN-γ Is Essential for Systemic Chlamydia muridarum Control in Mice, While CD4 T Cell-Dependent IFN-γ Production Is Highly Redundant in the Female Reproductive Tract

Protective immunity against the obligate intracellular bacterium Chlamydia has long been thought to rely on CD4 T cell-dependent gamma interferon (IFN-γ) production. Nevertheless, whether IFN-γ is produced by other cellular sources during Chlamydia infection and how CD4 T cell-dependent and -independent IFN-γ contribute differently to host resistance have not been carefully evaluated. In this study, we dissected the requirements of IFN-γ produced by innate immune cells and CD4 T cells for resolution of Chlamydia muridarum female reproductive tract (FRT) infection. After C. muridarum intravaginal infection, IFN-γ-deficient and T cell-deficient mice exhibited opposite phenotypes for survival and bacterial shedding at the FRT mucosa, demonstrating the distinct requirements for IFN-γ and CD4 T cells in host defense against Chlamydia In Rag1-deficient mice, IFN-γ produced by innate lymphocytes (ILCs) accounted for early bacterial control and prolonged survival in the absence of adaptive immunity. Although type I ILCs are potent IFN-γ producers, we found that mature NK cells and ILC1s were not the sole sources of innate IFN-γ in response to Chlamydia By conducting T cell adoptive transfer, we showed definitively that IFN-γ-deficient CD4 T cells were sufficient for effective bacterial killing in the FRT during the first 21 days of infection and reduced bacterial burden more than 1,000-fold, although mice receiving IFN-γ-deficient CD4 T cells failed to completely eradicate the bacteria from the FRT like their counterparts receiving wild-type (WT) CD4 T cells. Together, our results revealed that innate IFN-γ is essential for preventing systemic Chlamydia dissemination, whereas IFN-γ produced by CD4 T cells is largely redundant at the FRT mucosa.

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.

Analysis of complement deposition and processing on Chlamydia trachomatis

Chlamydia trachomatis (C. trachomatis) is the leading cause of sexually transmitted bacterial infections worldwide, with over 120 million annual cases. C. trachomatis infections are associated with severe reproductive complications in women such as extrauterine pregnancy and tubal infertility. The infections are often long lasting, associated with immunopathology, and fail to elicit protective immunity which makes recurrent infections common. The immunological mechanisms involved in C. trachomatis infections are only partially understood. Murine infection models suggest that the complement system plays a significant role in both protective immunity and immunopathology during primary Chlamydia infections. However, only limited structural and mechanistic evidence exists on complement-mediated immunity against C. trachomatis. To expand our current knowledge on this topic, we analyzed global complement deposition on C. trachomatis using comprehensive in-depth mass spectrometry-based proteomics. We show that factor B, properdin, and C4b bind to C. trachomatis demonstrating that C. trachomatis-induced complement activation proceeds through at least two activation pathways. Complement activation leads to cleavage and deposition of C3 and C5 activation products, causing initiation of the terminal complement pathway and deposition of C5b, C6, C7, C8, C9 on C. trachomatis. Interestingly, using immunoelectron microscopy, we show that C5b-9 deposition occurred sporadically and only in rare cases formed complete lytic terminal complexes, possibly caused by the presence of the negative regulators vitronectin and clusterin. Finally, cleavage analysis of C3 demonstrated that deposited C3b is degraded to the opsonins iC3b and C3dg and that this complement opsonization facilitates C. trachomatis binding to human B-cells.

Mucosal Vaccination with UV-Inactivated Chlamydia suis in Pre-Exposed Outbred Pigs Decreases Pathogen Load and Induces CD4 T-Cell Maturation into IFN-γ+ Effector Memory Cells

Chlamydia trachomatis (Ct) infections are the most frequent bacterial sexually transmitted disease, and they can lead to ectopic pregnancy and infertility. Despite these detrimental long-term sequelae, a vaccine is not available. Success in preclinical animal studies is essential for vaccines to move to human clinical trials. Pigs are the natural host to Chlamydia suis (Cs)-a chlamydia species closely related to Ct, and are susceptible to Ct, making them a valuable animal model for Ct vaccine development. Before making it onto market, Ct vaccine candidates must show efficacy in a high-risk human population. The high prevalence of human Ct infection combined with the fact that natural infection does not result in sterilizing immunity, results in people at risk likely having been pre-exposed, and thus having some level of underlying non-protective immunity. Like human Ct, Cs is highly prevalent in outbred pigs. Therefore, the goal of this study was to model a trial in pre-exposed humans, and to determine the immunogenicity and efficacy of intranasal Cs vaccination in pre-exposed outbred pigs. The vaccine candidates consisted of UV-inactivated Cs particles in the presence or absence of an adjuvant (TriAdj). In this study, both groups of vaccinated pigs had a lower Cs burden compared to the non-vaccinated group; especially the TriAdj group induced the differentiation of CD4⁺ cells into tissue-trafficking CCR7^- IFN-γ-producing effector memory T cells. These results indicate that Cs vaccination of pre-exposed pigs effectively boosts a non-protective immune response induced by natural infection; moreover, they suggest that a similar approach could be applied to human vaccine trials.

Antibody, but not B-cell-dependent antigen presentation, plays an essential role in preventing Chlamydia systemic dissemination in mice

The obligate intracellular bacterium Chlamydia trachomatis causes the most prevalent bacterial sexually transmitted infection worldwide. CD4 T cells play a central role in the protective immunity against Chlamydia female reproductive tract (FRT) infection, while B cells are thought to be dispensable for resolution of primary Chlamydia infection in mouse models. We recently reported an unexpected requirement of B cells in local Chlamydia-specific CD4 T-cell priming and bacterial containment within the FRT. Here, we sought to tackle the precise effector function of B cells during Chlamydia primary infection. Using mixed bone marrow chimeras that lack B-cell-dependent Ag presentation (MHCII^B - / - ) or devoid of circulating antibodies (AID^-/- × μS^-/- ), we show that Chlamydia-specific CD4 T-cell expansion does not rely on Ag presentation by B cells. Importantly, we demonstrate that antibody, but not B-cell-dependent Ag presentation, is required for preventing systemic bacterial dissemination following Chlamydia FRT infection.

Unexpected Role of CD8 T Cells in Accelerated Clearance of Salmonella enterica Serovar Typhimurium from H-2 Congenic mice

Salmonella infection can cause gastroenteritis in healthy individuals or a serious, systemic infection in immunocompromised patients and has a global impact. CD4 Th1 cells represent the main lymphocyte population that participates in bacterial clearance during both primary and secondary infections in mice of the H-2^b haplotype. Previous studies have used congenic mice to examine the function of major histocompatibility complex (MHC) molecules in elimination of this pathogen from the host. In this study, we further characterized the ability of H-2^b, H-2^k, and H-2^u molecules to influence adaptive immunity to Salmonella in MHC congenic mice. By depleting different cell populations during infection, we unexpectedly found that CD8 T cells, in addition to CD4 T cells, play a major role in accelerated clearance of bacteria from H-2^k congenic hosts. Our data suggest that CD8 T cells accelerate clearance in some MHC congenic mouse strains and could therefore represent an unexpected contributor to the protective efficacy of Salmonella vaccines outside the typical studies in C57BL/6 mice.

LFA-1 antagonist (BIRT377) similarly reverses peripheral neuropathic pain in male and female mice with underlying sex divergent peripheral immune proinflammatory phenotypes

Aim:

The majority of preclinical studies investigating aberrant glial-neuroimmune actions underlying neuropathic pain have focused on male rodent models. Recently, studies have shown peripheral immune cells play a more prominent role than glial cells in mediating pathological pain in females. Here, we compared the onset and duration of allodynia in males and females, and the anti-allodynic action of a potentially novel therapeutic drug (BIRT377) that not only antagonizes the action of lymphocyte function-associated antigen-1 (LFA-1) to reduce cell migration in the periphery, but may also directly alter the cellular inflammatory bias.

Methods:

Male and female mice were subjected to peripheral nerve injury chronic constriction injury (CCI) applying two methods, using either 4–0 or 5–0 chromic gut suture material, to examine potential sex differences in the onset, magnitude and duration of allodynia. Hindpaw sensitivity before and after CCI and application of intravenous BIRT377 was assessed. Peripheral and spinal tissues were analyzed for protein (multiplex electrochemiluminescence technology) and mRNA expression (quantitative real-time PCR). The phenotype of peripheral T cells was determined using flow cytometry.

Results:

Sex differences in proinflammatory CCL2 and IL-1β and the anti-inflammatory IL-10 were observed from a set of cytokines analyzed. A profound proinflammatory T cell (Th17) response in the periphery and spinal cord was also observed in neuropathic females. BIRT377 reversed pain, reduced IL-1β and TNF, and increased IL-10 and transforming growth factor (TGF)-β1, also an anti-inflammatory cytokine, in both sexes. However, female-derived T cell cytokines are transcriptionally regulated by BIRT377, as demonstrated by reducing proinflammatory IL-17A production with concurrent increases in IL-10, TGF-β1 and the anti-inflammatory regulatory T cell-related factor, FOXP3.

Conclusion:

This study supports that divergent peripheral immune and neuroimmune responses during neuropathy exists between males and females. Moreover, the modulatory actions of BIRT377 on T cells during neuropathy are predominantly specific to females. These data highlight the necessity of including both sexes for studying drug efficacy and mechanisms of action in preclinical studies and clinical trials.

IL-10 producing B cells rescue mouse fetuses from inflammation-driven fetal death and are able to modulate T cell immune responses

Understanding the mechanisms leading to fetal death following maternal subclinical infections is crucial to develop new therapeutic strategies. Here we addressed the relevance of IL-10 secreting B cells (B10) in the maintenance of the immune balance during gestation. µMT females lacking mature B cells presented normal pregnancies, although their fetuses were smaller and their Treg pool did not expand as in B cell sufficient controls. Pregnant µMT females were more susceptible to LPS despite having less Treg; their fetuses died at doses compatible with pregnancy in WT animals. Adoptive transfer of IL-10 negative B effector cells or B cells from IL-10 deficient mice did not modify this outcome. The transfer of B10 cells or application of recombinant murine IL-10 reduced the fetal loss, associated with a normalization of Treg numbers and cytokine modulation at the feto-maternal interface. B cell-derived IL-10 suppressed the production of IL-17A and IL-6 by T cells and promoted the conversion of naïve cells into Treg. B10 cells are required to restore the immune balance at the feto-maternal interface when perturbed by inflammatory signals. Our data position B cells in a central role in the maintenance of the balance between immunity and tolerance during pregnancy.

Diversity in the T cell response to Chlamydia-sum are better than one

Chlamydia trachomatis is responsible for an increasing number of sexually transmitted infections in the United States and is a common cause of serious pathology in the female reproductive tract (FRT). Given the impact and incidence of these infections, the production of an effective Chlamydia vaccine is a public health priority. Mouse models of Chlamydia infection have been utilized to develop a detailed and mechanistic understanding of protective immunity in the FRT. These studies reveal that MHC class-II restricted Chlamydia-specific CD4 T cells are critical for primary bacterial clearance and provide effective protection against secondary infection in the FRT. Despite the clear importance of IFN- γ produced by CD4 Th1 cells, there are also suggestions of wider functional heterogeneity in the CD4 T cell response to Chlamydia infection. Understanding the role of this diversity in the CD4 T helper cell response in the FRT should allow a more nuanced view of CD4 T cell biology in the context of Chlamydia infection and may be critical for vaccine development. Here, we summarize our current understanding of CD4 T helper subsets in the clearance of Chlamydia and discuss some areas where knowledge needs to be further extended by additional experimentation.

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.

Caveolin-mediated endocytosis of the Chlamydia M278 outer membrane peptide encapsulated in poly(lactic acid)-Poly(ethylene glycol) nanoparticles by mouse primary dendritic cells enhances specific immune effectors mediated by MHC class II and CD4+ T cells

We previously developed a Chlamydia trachomatis nanovaccine (PPM) by encapsulating a chlamydial M278 peptide within poly(lactic acid)-poly(ethylene glycol) biodegradable nanoparticles that immunopotentiated Chlamydia-specific immune effector responses in mice. Herein, we investigated the mechanistic interactions of PPM with mouse bone marrow-derived dendritic cells (DCs) for its uptake, trafficking, and T cell activation. Our results reveal that PPM triggered enhanced expression of effector cytokines and chemokines, surface activation markers (Cd1d2, Fcgr1), pathogen-sensing receptors (TLR2, Nod1), co-stimulatory (CD40, CD80, CD86) and MHC class I and II molecules. Co-culturing of PPM-primed DCs with T cells from C. muridarum vaccinated mice yielded an increase in Chlamydia-specific immune effector responses including CD3+ lymphoproliferation, CD3+CD4+ IFN-γ-secreting cells along with CD3+CD4+ memory (CD44high and CD62Lhigh) and effector (CD44high and CD62Llow) phenotypes. Intracellular trafficking analyses revealed an intense expression and colocalization of PPM predominantly in endosomes. PPM also upregulated the transcriptional and protein expression of the endocytic mediator, caveolin-1 in DCs. More importantly, the specific inhibition of caveolin-1 led to decreased expression of PPM-induced cytokines and co-stimulatory molecules. Our investigation shows that PPM provided enhancement of uptake, probably by exploiting the caveolin-mediated endocytosis pathway, endosomal processing, and MHC II presentation to immunopotentiate Chlamydia-specific immune effector responses mediated by CD4+ T cells.

B Cell Presentation of Chlamydia Antigen Selects Out Protective CD4γ13 T Cells: Implications for Genital Tract Tissue-Resident Memory Lymphocyte Clusters

Surveillance and defense of the enormous mucosal interface with the nonsterile world are critical to protecting the host from a wide range of pathogens. Chlamydia trachomatis is an intracellular bacterial pathogen that replicates almost exclusively in the epithelium of the reproductive tract. The fallopian tubes and vagina are poorly suited to surveillance and defense, with limited immune infrastructure positioned near the epithelium. However, a dynamic process during clearing primary infections leaves behind new lymphoid clusters immediately beneath the epithelium. These memory lymphocyte clusters (MLCs) harboring tissue-resident memory (Trm) T cells are presumed to play an important role in protection from subsequent infections. Histologically, human Chlamydia MLCs have prominent B cell populations. We investigated the status of genital tract B cells during C. muridarum infections and the nature of T cells recovered from immune mice using immune B cells as antigen-presenting cells (APCs). These studies revealed a genital tract plasma B cell population and a novel genital tract CD4 T cell subset producing both gamma interferon (IFN-γ) and interleukin-13 (IL-13). A panel of CD4 T cell clones and microarray analysis showed that the molecular fingerprint of CD4γ13 T cells includes a Trm-like transcriptome. Adoptive transfer of a Chlamydia-specific CD4γ13 T cell clone completely prevented oviduct immunopathology without accelerating bacterial clearance. Existence of a CD4γ13 T cell subset provides a plausible explanation for the observation that human peripheral blood mononuclear cell (PBMC) Chlamydia-specific IFN-γ and IL-13 responses predict resistance to reinfection.

Differential signaling pathways are initiated in macrophages during infection depending on the intracellular fate of Chlamydia spp

Chlamydia muridarum and Chlamydia caviae have equivalent growth rates in mouse epithelial cells but only C. muridarum replicates inside mouse macrophages, while C. caviae does not. Macrophages infected with C. muridarum or C. caviae were used to address the hypothesis that the early signaling pathways initiated during infection depend on the fate of chlamydiae in the host cell. Transmission electron microscopy of C. muridarum-infected macrophages showed intact chlamydial elementary bodies and reticulate bodies 2 h postinfection in compact vacuoles. Conversely, in macrophages infected with C. caviae, chlamydiae were observed in large phagocytic vacuoles. Furthermore, C. caviae infections failed to develop into inclusions or produce viable bacteria. Expression of proinflammatory cytokines TNFα, IL-1β and MMP13 was similar in C. caviae- or C. muridarum-infected macrophages at 3 h postinfection, indicating that chlamydial survival is not required for initiation of these responses. IL-1β secretion, dependent on inflammasome activation, occurred in C. caviae-infected macrophages despite no chlamydial growth. Conversely, IFNβ mRNA was observed only in C. muridarum- but not in C. caviae-infected macrophages. These data demonstrate that differential signaling events are initiated during a productive versus nonproductive chlamydial infection in a macrophage.

Structural and Immunological Characterization of Novel Recombinant MOMP-Based Chlamydial Antigens

Chlamydia is the most common cause of bacterial sexually transmitted infections worldwide. While infections resolve with antibiotic treatment, this is often neglected in women due to frequent asymptomatic infections, leading to disease progression and severe sequelae (pelvic inflammatory disease, ectopic pregnancy, infertility). Development of a vaccine against Chlamydia is crucial. Whole organism-based vaccines have short-lived activity, serovar/subgroup-specific immunity and can cause adverse reactions in vaccinated subjects. The Chlamydia major outer membrane protein (MOMP) is a prime candidate for a subunit vaccine. MOMP contains four regions of sequence variability (variable domains, VDs) with B-cell and T-cell epitopes that elicit protective immunity. However, barriers for developing a MOMP-based vaccine include solubility, yield and refolding. We have engineered novel recombinant antigens in which the VDs are expressed into a carrier protein structurally similar to MOMP and suitable for recombinant expression at a high yield in a correctly folded and detergent-free form. Using a carrier such as the PorB porin from the human commensal organism N. lactamica, we show that PorB/VD chimeric proteins are immunogenic, antigenic and cross-reactive with MOMP. VDs are unique for each serovar but if combined in a single vaccine, a broad coverage against the major Chlamydia serovars can be ensured.

The Genital Tract Virulence Factor pGP3 Is Essential for Chlamydia muridarum Colonization in the Gastrointestinal Tract

The cryptic plasmid is essential for Chlamydia muridarum dissemination from the genital tract to the gastrointestinal (GI) tract. Following intravaginal inoculation, a C. muridarum strain deficient in plasmid-encoded pGP3 or pGP4 but not pGP5, pGP7, or pGP8 failed to spread to the mouse gastrointestinal tract, although mice infected with these strains developed productive genital tract infections. pGP3- or pGP4-deficient strains also failed to colonize the gastrointestinal tract when delivered intragastrically. pGP4 regulates pGP3, while pGP3 does not affect pGP4 expression, indicating that pGP3 is critical for C. muridarum colonization of the gastrointestinal tract. Mutants deficient in GlgA, a chromosome-encoded protein regulated by pGP4, also consistently colonized the mouse gastrointestinal tract. Interestingly, C. muridarum colonization of the gastrointestinal tract positively correlated with pathogenicity in the upper genital tract. pGP3-deficient C. muridarum strains did not induce hydrosalpinx or spread to the GI tract even when delivered to the oviduct by intrabursal inoculation. Thus, the current study not only has revealed that pGP3 is a novel chlamydial colonization factor in the gastrointestinal tract but also has laid a foundation for investigating the significance of gastrointestinal Chlamydia.

A Chlamydia-Specific TCR-Transgenic Mouse Demonstrates Th1 Polyfunctionality with Enhanced Effector Function

Chlamydia is responsible for millions of new infections annually, and current efforts focus on understanding cellular immunity for targeted vaccine development. The Chlamydia-specific CD4 T cell response is characterized by the production of IFN-γ, and polyfunctional Th1 responses are associated with enhanced protection. A major limitation in studying these responses is the paucity of tools available for detection, quantification, and characterization of polyfunctional Ag-specific T cells. We addressed this problem by developing a TCR-transgenic (Tg) mouse with CD4 T cells that respond to a common Ag in Chlamydia muridarum and Chlamydia trachomatis Using an adoptive-transfer approach, we show that naive Tg CD4 T cells become activated, proliferate, migrate to the infected tissue, and acquire a polyfunctional Th1 phenotype in infected mice. Polyfunctional Tg Th1 effectors demonstrated enhanced IFN-γ production compared with polyclonal cells, protected immune-deficient mice against lethality, mediated bacterial clearance, and orchestrated an anamnestic response. Adoptive transfer of Chlamydia-specific CD4 TCR-Tg T cells with polyfunctional capacity offers a powerful approach for analysis of protective effector and memory responses against chlamydial infection and demonstrates that an effective monoclonal CD4 T cell response may successfully guide subunit vaccination strategies.

Dual Immunization with SseB/Flagellin Provides Enhanced Protection against Salmonella Infection Mediated by Circulating Memory Cells

The development of a subunit Salmonella vaccine has been hindered by the absence of detailed information about antigenic targets of protective Salmonella-specific T and B cells. Recent studies have identified SseB as a modestly protective Ag in susceptible C57BL/6 mice, but the mechanism of protective immunity remains undefined. In this article, we report that simply combining Salmonella SseB with flagellin substantially enhances protective immunity, allowing immunized C57BL/6 mice to survive for up to 30 d following challenge with virulent bacteria. Surprisingly, the enhancing effect of flagellin did not require flagellin Ag targeting during secondary responses or recognition of flagellin by TLR5. Although coimmunization with flagellin did not affect SseB-specific Ab responses, it modestly boosted CD4 responses. In addition, protective immunity was effectively transferred in circulation to parabionts of immunized mice, demonstrating that tissue-resident memory is not required for vaccine-induced protection. Finally, protective immunity required host expression of IFN-γR but was independent of induced NO synthase expression. Taken together, these data indicate that Salmonella flagellin has unique adjuvant properties that improve SseB-mediated protective immunity provided by circulating memory.

CCR7 Deficiency Allows Accelerated Clearance of Chlamydia from the Female Reproductive Tract

Immune mechanisms responsible for pathogen clearance from the female reproductive tract (FRT) are incompletely defined; in particular, the contribution of lymphocyte trafficking to this process is unclear. CCR7-deficient mice have profoundly altered lymphocyte recirculation and display ectopic formation of lymphocyte aggregates within mucosal nonlymphoid tissues, including the FRT. In this study, we investigated how altered lymphocyte distribution in CCR7-deficient mice would affect host responses to Chlamydia muridarum within the reproductive tract. As expected, CCR7-deficient mice exhibited reduced lymphocyte trafficking to lymph nodes and a corresponding increase in T cell populations within the FRT. After intravaginal infection with Chlamydia, CCR7-deficient mice displayed markedly reduced Ag-specific CD4 T cell responses within the local draining iliac lymph nodes, yet robust Th1 and Th17 responses were prominent in the FRT. In addition, Chlamydia-specific Ab responses were dysregulated in CCR7-deficient mice, displaying an unexpected increase in the systemic IgA responses. Importantly, prominent mucosal immune responses in CCR7-deficient mice increased the efficiency of bacteria clearance from the FRT while reducing tissue-associated inflammation and pathology. Thus, increased numbers of lymphocytes within the FRT result in pathogen clearance with reduced immune-mediated pathology.

T cell expression of IL-18R and DR3 is essential for non-cognate stimulation of Th1 cells and optimal clearance of intracellular bacteria

Th1 cells can be activated by TCR-independent stimuli, but the importance of this pathway in vivo and the precise mechanisms involved require further investigation. Here, we used a simple model of non-cognate Th1 cell stimulation in Salmonella-infected mice to examine these issues. CD4 Th1 cell expression of both IL-18R and DR3 was required for optimal IFN-γ induction in response to non-cognate stimulation, while IL-15R expression was dispensable. Interestingly, effector Th1 cells generated by immunization rather than live infection had lower non-cognate activity despite comparable IL-18R and DR3 expression. Mice lacking T cell intrinsic expression of MyD88, an important adapter molecule in non-cognate T cell stimulation, exhibited higher bacterial burdens upon infection with Salmonella, Chlamydia or Brucella, suggesting that non-cognate Th1 stimulation is a critical element of efficient bacterial clearance. Thus, IL-18R and DR3 are critical players in non-cognate stimulation of Th1 cells and this response plays an important role in protection against intracellular bacteria.

The cryptic plasmid is more important for Chlamydia muridarum to colonize the mouse gastrointestinal tract than to infect the genital tract

Chlamydia has been detected in the gastrointestinal tracts of both animals and humans. However, the mechanism by which Chlamydia colonizes the gut remains unclear. Chlamydia muridarum is known to spread from the genital to the gastrointestinal tracts hematogenously. The C. muridarum plasmid is a key pathogenic determinant in the mouse upper genital tract although plasmid-deficient C. muridarum is still able to colonize the upper genital tract. We now report that plasmid-deficient C. muridarum exhibits significantly delayed/reduced spreading from the mouse genital to the gastrointestinal tracts. C. muridarum with or without plasmid maintained similar levels in the mouse circulatory system following intravenous inoculation but the hematogenous plasmid-deficient C. muridarum was significantly less efficient in colonizing the gastrointestinal tract. Consistently, plasmid-deficient C. muridarum failed to restore normal colonization in the gastrointestinal tract even after intragastric inoculation at a high dose. Thus, we have demonstrated a plasmid-dependent colonization of C. muridarum in the gastrointestinal tract, supporting the concept that C. muridarum may have acquired the plasmid for adaptation to the mouse gastrointestinal tract during oral-fecal transmission. Since the plasmid is more important for C. muridarum to colonize the gastrointestinal tract than to infect the genital tract, the current study has laid a foundation for further defining the host pathways targeted by the plasmid-encoded or -regulated chlamydial effectors.

Update on Chlamydia trachomatis Vaccinology

Attempts to produce a vaccine to protect against Chlamydia trachomatis-induced trachoma were initiated more than 100 years ago and continued for several decades. Using whole organisms, protective responses were obtained. However, upon exposure to C. trachomatis, disease exacerbation developed in some immunized individuals, precluding the implementation of the vaccine. Evidence of the role of C. trachomatis as a sexually transmitted pathogen started to emerge in the 1960s, and it soon became evident that it can cause acute infections and long-term sequelae in women, men, and newborns. The main focus of this minireview is to summarize recent findings and discuss formulations, including antigens, adjuvants, routes, and delivery systems for immunization, primarily explored in the female mouse model, with the goal of implementing a vaccine against C. trachomatis genital infections.

A Protective Vaccine against Chlamydia Genital Infection Using Vault Nanoparticles without an Added Adjuvant

Chlamydia trachomatis genital infection is the most common sexually transmitted bacterial disease, causing a significant burden to females due to reproductive dysfunction. Intensive screening and antibiotic treatment are unable to completely prevent female reproductive dysfunction, thus, efforts have become focused on developing a vaccine. A major impediment is identifying a safe and effective adjuvant which induces cluster of differentiation 4 (CD4) cells with attributes capable of halting genital infection and inflammation. Previously, we described a natural nanocapsule called the vault which was engineered to contain major outer membrane protein (MOMP) and was an effective vaccine which significantly reduced early infection and favored development of a cellular immune response in a mouse model. In the current study, we used another chlamydial antigen, a polymorphic membrane protein G-1 (PmpG) peptide, to track antigen-specific cells and evaluate, in depth, the vault vaccine for its protective capacity in the absence of an added adjuvant. We found PmpG-vault immunized mice significantly reduced the genital bacterial burden and histopathologic parameters of inflammation following a C. muridarum challenge. Immunization boosted antigen-specific CD4 cells with a multiple cytokine secretion pattern and reduced the number of inflammatory cells in the genital tract making the vault vaccine platform safe and effective for chlamydial genital infection. We conclude that vaccination with a Chlamydia-vault vaccine boosts antigen-specific immunities that are effective at eradicating infection and preventing reproductive tract inflammation.

Collateral Damage: Detrimental Effect of Antibiotics on the Development of Protective Immune Memory

Antibiotic intervention is an effective treatment strategy for many bacterial infections and liberates bacterial antigens and stimulatory products that can induce an inflammatory response. Despite the opportunity for bacterial killing to enhance the development of adaptive immunity, patients treated successfully with antibiotics can suffer from reinfection. Studies in mouse models of Salmonella and Chlamydia infection also demonstrate that early antibiotic intervention reduces host protective immunity to subsequent infection. This heightened susceptibility to reinfection correlates with poor development of Th1 and antibody responses in antibiotic-treated mice but can be overcome by delayed antibiotic intervention, thus suggesting a requirement for sustained T cell stimulation for protection. Although the contribution of memory T cell subsets is imperfectly understood in both of these infection models, a protective role for noncirculating memory cells is suggested by recent studies. Together, these data propose a model where antibiotic treatment specifically interrupts tissue-resident memory T cell formation. Greater understanding of the mechanistic basis of this phenomenon might suggest therapeutic interventions to restore a protective memory response in antibiotic-treated patients, thus reducing the incidence of reinfection.

Sensing the enemy, containing the threat: cell-autonomous immunity to Chlamydia trachomatis

The bacterium Chlamydia trachomatis is the etiological agent of the most common sexually transmitted infection in North America and Europe. Medical complications resulting from genital C. trachomatis infections arise predominantly in women where the initial infections often remain asymptomatic and thus unrecognized. Untreated asymptomatic infections in women can ascend into the upper genital tract and establish persistence, ultimately resulting in extensive scarring of the reproductive organs, pelvic inflammatory disease, infertility and ectopic pregnancies. Previously resolved C. trachomatis infections fail to provide protective immune memory, and no effective vaccine against C. trachomatis is currently available. Critical determinants of the pathogenesis and immunogenicity of genital C. trachomatis infections are cell-autonomous immune responses. Cell-autonomous immunity describes the ability of an individual host cell to launch intrinsic immune circuits that execute the detection, containment and elimination of cell-invading pathogens. As an obligate intracellular pathogen C. trachomatis is constantly under attack by cell-intrinsic host defenses. Accordingly, C. trachomatis evolved to subvert and co-opt cell-autonomous immune pathways. This review will provide a critical summary of our current understanding of cell-autonomous immunity to C. trachomatis and its role in shaping host resistance, inflammation and adaptive immunity to genital C. trachomatis infections.

IFNγ is Required for Optimal Antibody-Mediated Immunity against Genital Chlamydia Infection

Defining the mechanisms of immunity conferred by the combination of antibody and CD4⁺ T cells is fundamental to designing an efficacious chlamydial vaccine. Using the Chlamydia muridarum genital infection model of mice, which replicates many features of human C. trachomatis infection and avoids the characteristic low virulence of C. trachomatis in the mouse, we previously demonstrated a significant role for antibody in immunity to chlamydial infection. We found that antibody alone was not protective. Instead, protection appeared to be conferred through an undefined antibody-cell interaction. Using gene knockout mice and in vivo cellular depletion methods, our data suggest that antibody-mediated protection is dependent on the activation of an effector cell population in genital tract tissues by CD4⁺ T cells. Furthermore, the CD4⁺ T cell-secreted cytokine gamma interferon (IFN-γ) was found to be a key component of the protective antibody response. The protective function of IFN-γ was not related to the immunoglobulin class or to the magnitude of the Chlamydia-specific antibody response or to recruitment of an effector cell population to genital tract tissue. Rather, IFN-γ appears to be necessary for activation of the effector cell population that functions in antibody-mediated chlamydial immunity. Our results confirm the central role of antibody in immunity to chlamydia reinfection and demonstrate a key function for IFN-γ in antibody-mediated protection.

Intravenous Inoculation with Chlamydia muridarum Leads to a Long-Lasting Infection Restricted to the Gastrointestinal Tract

Chlamydia has been detected in the gastrointestinal tracts of both animals and humans. However, it remains unclear whether the chlamydial organisms can be introduced into the gastrointestinal tract via pathways independent of the oral and anal routes. We have recently shown that Chlamydia muridarum spreads from the genital tract to the gastrointestinal tract potentially via the circulatory system. To test whether hematogenous C. muridarum can spread to and establish a long-lasting colonization in the mouse gastrointestinal tract, we inoculated mice intravenously with a luciferase-expressing C. muridarum strain and monitored its distribution. After tail vein inoculation, most luciferase-generated bioluminescence signals were detected in the mouse abdominal area throughout the experiment. The ex vivo imaging revealed that the abdominal signals came from the gastrointestinal tract tissues. Simultaneous monitoring of chlamydial organisms in individual organs or tissues revealed an initial stage of systemic spreading followed by a long-lasting infection in the gastrointestinal tract. A retro-orbital vein inoculation of the C. muridarum organisms at a lower dose in a different mouse strain also led to colonization of the gastrointestinal tract. We have demonstrated that intravenous C. muridarum inoculation can result in colonization of the gastrointestinal tract, suggesting that the chlamydial organisms may use the sexual behavior-independent circulation pathway to infect the gastrointestinal tract.

Characterisation of CD4 T cells in healthy and diseased koalas (Phascolarctos cinereus) using cell-type-specific monoclonal antibodies

The koala (Phascolarctos cinereus) is an arboreal herbivorous marsupial that is an Australian icon. Koalas in many parts of Australia are under multiple threats including habitat destruction, dog attacks, vehicular accidents, and infectious diseases such as Chlamydia spp. and the koala retrovirus (KoRV), which may contribute to the incidence of lymphoma and leukaemia in this species. Due to a lack of koala-specific immune reagents and assays there is currently no way to adequately analyse the immune response in healthy, diseased or vaccinated animals. This paper reports the production and characterisation of the first anti-koala CD4 monoclonal antibody (mAb). The koala CD4 gene was identified and used to develop recombinant proteins for mAb production. Fluorochrome-conjugated anti-CD4 mAb was used to measure the levels of CD4(+) lymphocytes collected from koala spleens (41.1%, range 20-45.1%) lymph nodes (36.3%, range 19-55.9%) and peripheral blood (23.8%, range 17.3-35%) by flow cytometry. Biotin-conjugated anti-CD4 mAb was used for western blot to determine an approximate size of 52 kDa for the koala CD4 molecule and used in immunohistochemistry to identify CD4(+) cells in the paracortical region and germinal centres of spleen and lymph nodes. Using the anti-CD4 mab we showed that CD4 cells from vaccinated, but not control, koalas proliferated following in vitro stimulation with UV-inactivated Chlamydia pecorum and recombinant chlamydial antigens. Since CD4(+) T cells have been shown to play a pivotal role in clearing chlamydial infection in both human and mouse infections, using this novel antibody will help determine the role CD4(+) T cells play in protection against chlamydial infection in koalas and also enhance our knowledge of how KoRV affects the koala immune system.

Adhesion Molecules Associated with Female Genital Tract Infection

Efforts to develop vaccines that can elicit mucosal immune responses in the female genital tract against sexually transmitted infections have been hampered by an inability to measure immune responses in these tissues. The differential expression of adhesion molecules is known to confer site-dependent homing of circulating effector T cells to mucosal tissues. Specific homing molecules have been defined that can be measured in blood as surrogate markers of local immunity (e.g. α4β7 for gut). Here we analyzed the expression pattern of adhesion molecules by circulating effector T cells following mucosal infection of the female genital tract in mice and during a symptomatic episode of vaginosis in women. While CCR2, CCR5, CXCR6 and CD11c were preferentially expressed in a mouse model of Chlamydia infection, only CCR5 and CD11c were clearly expressed by effector T cells during bacterial vaginosis in women. Other homing molecules previously suggested as required for homing to the genital mucosa such as α4β1 and α4β7 were also differentially expressed in these patients. However, CD11c expression, an integrin chain rarely analyzed in the context of T cell immunity, was the most consistently elevated in all activated effector CD8⁺ T cell subsets analyzed. This molecule was also induced after systemic infection in mice, suggesting that CD11c is not exclusive of genital tract infection. Still, its increase in response to genital tract disorders may represent a novel surrogate marker of mucosal immunity in women, and warrants further exploration for diagnostic and therapeutic purposes.

Antibody and Cytokine Responses of Koalas (Phascolarctos cinereus) Vaccinated with Recombinant Chlamydial Major Outer Membrane Protein (MOMP) with Two Different Adjuvants

Developing a vaccine against Chlamydia is key to combating widespread mortalities and morbidities associated with this infection in koalas (Phascolarctos cinereus). In previous studies, we have shown that two or three doses of a Recombinant Major Outer Membrane Protein (rMOMP) antigen-based vaccine, combined with immune stimulating complex (ISC) adjuvant, results in strong cellular and humoral immune responses in koalas. We have also separately evaluated a single dose vaccine, utilising a tri-adjuvant formula that comprises polyphosphazine based poly I: C and host defense peptides, with the same antigen. This formulation also produced strong cellular and humoral immune responses in captive koalas. In this current study, we directly compared the host immune responses of two sub-groups of wild Chlamydia negative koalas in one population vaccinated with the rMOMP protein antigen and adjuvanted with either the ISC or tri-adjuvant formula. Overall, both adjuvants produced strong Chlamydia-specific cellular (IFN-γ and IL-17A) responses in circulating PBMCs as well as MOMP-specific and functional, in vitro neutralising antibodies. While the immune responses were similar, there were adjuvant-specific immune differences between the two adjuvants, particularly in relation to the specificity of the MOMP epitope antibody responses.

Expression of CD11c Is Associated with Unconventional Activated T Cell Subsets with High Migratory Potential

CD11c is an α integrin classically employed to define myeloid dendritic cells. Although there is little information about CD11c expression on human T cells, mouse models have shown an association of CD11c expression with functionally relevant T cell subsets. In the context of genital tract infection, we have previously observed increased expression of CD11c in circulating T cells from mice and women. Microarray analyses of activated effector T cells expressing CD11c derived from naïve mice demonstrated enrichment for natural killer (NK) associated genes. Here we find that murine CD11c+ T cells analyzed by flow cytometry display markers associated with non-conventional T cell subsets, including γδ T cells and invariant natural killer T (iNKT) cells. However, in women, only γδ T cells and CD8+ T cells were enriched within the CD11c fraction of blood and cervical tissue. These CD11c+ cells were highly activated and had greater interferon (IFN)-γ secretory capacity than CD11c- T cells. Furthermore, circulating CD11c+ T cells were associated with the expression of multiple adhesion molecules in women, suggesting that these cells have high tissue homing potential. These data suggest that CD11c expression distinguishes a population of circulating T cells during bacterial infection with innate capacity and mucosal homing potential.

Subunit vaccines for the prevention of mucosal infection with Chlamydia trachomatis

Chlamydia trachomatis is the most common preventable cause of tubal infertility in women. In high-income countries, despite public health control efforts, C. trachomatis case rates continue to rise. Most medium and low-income countries lack any Chlamydia control program; therefore, a vaccine is essential for the control of Chlamydia infections. A rationally designed Chlamydia vaccine requires understanding of the immunological correlates of protective immunity, pathological responses to this mucosal pathogen, identification of optimal vaccine antigens and selection of suitable adjuvant delivery systems that engender protective immunity. Fortunately, Chlamydia vaccinology is facilitated by genomic knowledge and by murine models that reproduce many of the features of human C. trachomatis infection. This article reviews recent progress in these areas with a focus on subunit vaccine development.

Immunopathogenesis of Chlamydial Infections

Chlamydial infections lead to a number of clinically relevant diseases and induce significant morbidity in human populations. It is generally understood that certain components of the host immune response to infection also mediate such disease pathologies. A clear understanding of pathogenic mechanisms will enable us to devise better preventive and/or intervention strategies to mitigate the morbidity caused by these infections. Over the years, numerous studies have been conducted to explore the immunopathogenic mechanisms of Chlamydia-induced diseases of the eye, reproductive tract, respiratory tract, and cardiovascular systems. In this article, we provide an overview of the diseases caused by Chlamydia, animal models used to study disease pathology, and a historical context to the efforts to understand chlamydial pathogenesis. Furthermore, we discuss recent findings regarding pathogenesis, with an emphasis on the role of the adaptive immune response in the development of chlamydial disease sequelae. Finally, we summarize the key insights obtained from studies of chlamydial pathogenesis and avenues that remain to be explored in order to inform the next steps of vaccine development against chlamydial infections.