An in vitro model for immune control of chlamydial growth in polarized epithelial cells

Problems with Understanding Chlamydia trachomatis Immunology

The pay off for understanding Chlamydia. trachomatis (CT) immunology is the development of a vaccine. Two lines of research have contributed to our current understanding of CT immunology. The first is the Grayston model of type specific immunity and genus specific pathology which was elaborated by Caldwell and Morrison as the major outer membrane protein (MOMP) and heat shock protein 60 (HSP60) paradigm. The second is the murine model of C. muridarum (CM) infection which established the essential role of major histocompatibility complex class II and CD4 T cells in immunity. However neither approach has yielded a vaccine. I review these two lines of research and conclude with six problem areas in human CT immunology whose resolution may result in a vaccine.(122 words)

Discordance in the Epithelial Cell-Dendritic Cell Major Histocompatibility Complex Class II Immunoproteome: Implications for Chlamydia Vaccine Development

BACKGROUND

Chlamydia trachomatis and Chlamydia muridarum are intracellular bacterial pathogens of mucosal epithelial cells. CD4 T cells and major histocompatibility complex (MHC) class II molecules are essential for protective immunity against them. Antigens presented by dendritic cells (DCs) expand naive pathogen-specific T cells (inductive phase), whereas antigens presented by epithelial cells identify infected epithelial cells as targets during the effector phase. We previously showed that DCs infected by C trachomatis or C muridarum present epitopes from a limited spectrum of chlamydial proteins recognized by Chlamydia-specific CD4 T cells from immune mice.

METHODS

We hypothesized that Chlamydia-infected DCs and epithelial cells present overlapping sets of Chlamydia-MHC class II epitopes to link inductive and effector phases to generate protective immunity. We tested that hypothesis by infecting an oviductal epithelial cell line with C muridarum, followed by immunoaffinity isolation and sequencing of MHC class I- and II-bound peptides.

RESULTS

We identified 26 class I-bound and 4 class II-bound Chlamydia-derived peptides from infected epithelial cells. We were surprised to find that none of the epithelial cell class I- and class II-bound chlamydial peptides overlapped with peptides presented by DCs.

CONCLUSIONS

We suggest the discordance between the DC and epithelial cell immunoproteomes has implications for delayed clearance of Chlamydia and design of a Chlamydia vaccine.

Murine Endometrial Organoids to Model Chlamydia Infection

The obligate intracellular bacterium Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infections. Once internalized in host cells, C. trachomatis undergoes a biphasic developmental cycle within a membrane-bound compartment, known as the inclusion. Successful establishment of the intracellular niche relies on bacterial Type III effector proteins, such as Inc proteins. In vitro and in vivo systems have contributed to elucidating the intracellular lifestyle of C. trachomatis, but additional models combining the archetypal environment of infection with the advantages of in vitro systems are needed. Organoids are three-dimensional structures that recapitulate the microanatomy of an organ's epithelial layer, bridging the gap between in vitro and in vivo systems. Organoids are emerging as relevant model systems to study interactions between bacterial pathogens and their hosts. Here, we took advantage of recently developed murine endometrial organoids (EMOs) and present a C. trachomatis-murine EMO infection model system. Confocal microscopy of EMOs infected with fluorescent protein-expressing bacteria revealed that inclusions are formed within the cytosol of epithelial cells. Moreover, infection with a C. trachomatis strain that allows for the tracking of RB to EB transition indicated that the bacteria undergo a full developmental cycle, which was confirmed by harvesting infectious bacteria from infected EMOs. Finally, the inducible gene expression and cellular localization of a Chlamydia Inc protein within infected EMOs further demonstrated that this model is compatible with the study of Type III secreted effectors. Altogether, we describe a novel and relevant system for the study of Chlamydia-host interactions.

Perspective: my 37 year journey through Chlamydia research: Chlamydia antigen analysis using monoclonal antibodies and major histocompatibility complex molecules

Chlamydia antigen analysis enables understanding of disease pathogenesis, facilitates development of diagnostic immunoassays and is essential to the design of a subunit Chlamydia trachomatis vaccine. Using an autobiographical narrative, I review over three decades of antigen analysis research findings coming from my research laboratory and provide an outlook to the broader field of Chlamydia seroepidemiology and vaccinology. Based on the experiences of my scientific career I conclude with thoughts for young scientists newly entering the Chlamydia research field.

From Single Cells to Engineered and Explanted Tissues: New Perspectives in Bacterial Infection Biology

Cell culture techniques are essential for studying host-pathogen interactions. In addition to the broad range of single cell type-based two-dimensional cell culture models, an enormous amount of coculture systems, combining two or more different cell types, has been developed. These systems enable microscopic visualization and molecular analyses of bacterial adherence and internalization mechanisms and also provide a suitable setup for various biochemical, immunological, and pharmacological applications. The implementation of natural or synthetical scaffolds elevated the model complexity to the level of three-dimensional cell culture. Additionally, several transwell-based cell culture techniques are applied to study bacterial interaction with physiological tissue barriers. For keeping highly differentiated phenotype of eukaryotic cells in ex vivo culture conditions, different kinds of microgravity-simulating rotary-wall vessel systems are employed. Furthermore, the implementation of microfluidic pumps enables constant nutrient and gas exchange during cell cultivation and allows the investigation of long-term infection processes. The highest level of cell culture complexity is reached by engineered and explanted tissues which currently pave the way for a more comprehensive view on microbial pathogenicity mechanisms.

Chlamydia trachomatis infection results in a modest pro-inflammatory cytokine response and a decrease in T cell chemokine secretion in human polarized endocervical epithelial cells

The endocervical epithelium is a major reservoir for Chlamydia trachomatis in women, and genital infections are extended in their duration. Epithelial cells act as mucosal sentinels by secreting cytokines and chemokines in response to pathogen challenge and infection. We therefore determined the signature cytokine and chemokine response of primary-like endocervix-derived epithelial cells in response to a common genital serovar (D) of C. trachomatis. For these studies, we used a recently-established polarized, immortalized, endocervical epithelial cell model (polA2EN) that maintains, in vitro, the architectural and functional characteristics of endocervical epithelial cells in vivo including the production of pro-inflammatory cytokines. PolA2EN cells were susceptible to C. trachomatis infection, and chlamydiae in these cells underwent a normal developmental cycle as determined by a one-step growth curve. IL1α protein levels were increased in both apical and basolateral secretions of C. trachomatis infected polA2EN cells, but this response did not occur until 72h after infection. Furthermore, protein levels of the pro-inflammatory cytokines and chemokines IL6, TNFα and CXCL8 were not significantly different between C. trachomatis infected polA2EN cells and mock infected cells at any time during the chlamydial developmental cycle up to 120h post-infection. Intriguingly, C. trachomatis infection resulted in a significant decrease in the constitutive secretion of T cell chemokines IP10 and RANTES, and this required a productive C. trachomatis infection. Examination of anti-inflammatory cytokines revealed a high constitutive apical secretion of IL1ra from polA2EN cells that was not significantly modulated by C. trachomatis infection. IL-11 was induced by C. trachomatis, although only from the basolateral membrane. These results suggest that C. trachomatis can use evasion strategies to circumvent a robust pro-inflammatory cytokine and chemokine response. These evasion strategies, together with the inherent immune repertoire of endocervical epithelial cells, may aid chlamydiae in establishing, and possibly sustaining, an intracellular niche in microenvironments of the endocervix in vivo.

T lymphocyte immunity in host defence against Chlamydia trachomatis and its implication for vaccine development

Chlamydia trachomatis is an obligate intracellular bacterial pathogen that causes several significant human infectious diseases, including trachoma, urethritis, cervicitis and salpingitis, and is an important cofactor for transmission of human immunodeficiency virus. Until very recently, over three decades of research effort aimed at developing a C trachomatis vaccine had failed, due mainly to the lack of a precise understanding of the mechanisms for protective immunity. Although most studies concerning protective immunity to C trachomatis have focused on humoral immune responses, recent studies have clearly shown that T helper-1 (Th1)-like CD4 T cell-mediated immune responses play the dominant role in protective immunity. These studies suggest a paradigm for chlamydial immunity and pathology based on the concept of heterogeneity (Th1/Th2) in CD4 T cell immune responses. This concept for chlamydial immunity offers a rational template on which to base renewed efforts for development of a chlamydial vaccine that targets the induction of cell-mediated Th1 immune responses.

PmpG303-311, a protective vaccine epitope that elicits persistent cellular immune responses in Chlamydia muridarum-immune mice

Urogenital Chlamydia serovars replicating in reproductive epithelium pose a unique challenge to host immunity and vaccine development. Previous studies have shown that CD4 T cells are necessary and sufficient to clear primary Chlamydia muridarum genital tract infections in the mouse model, making a protective CD4 T cell response a logical endpoint for vaccine development. Our previous proteomics studies identified 13 candidate Chlamydia proteins for subunit vaccines. Of those, PmpG-1 is the most promising vaccine candidate. To further that work, we derived a PmpG(303-311)-specific multifunctional Th1 T cell clone, designated PmpG1.1, from an immune C57BL/6 mouse and used it to investigate the presentation of the PmpG(303-311) epitope by infected epithelial cells. Epithelial presentation of the PmpG(303-311) epitope required bacterial replication, occurred 15 to 18 h postinfection, and was unaffected by gamma interferon (IFN-γ) pretreatment. Unlike epitopes recognized by other Chlamydia-specific CD4 T cell clones, the PmpG(303-311) epitope persisted on splenic antigen-presenting cells (APC) of mice that cleared primary genital tract infections. PmpG1.1 was activated by unmanipulated irradiated splenocytes from immune mice without addition of exogenous Chlamydia antigen, and remarkably, activation of PmpG1.1 by unmanipulated immune splenocytes was stronger 6 months postinfection than it was 3 weeks postinfection. Enhanced presentation of PmpG(303-311) epitope on splenic APC 6 months postinfection reflects some type of "consolidation" of a protective immune response. Understanding the antigen-presenting cell populations responsible for presenting PmpG(303-311) early (3 weeks) and late (6 months) postinfection will likely provide important insights into stable protective immunity against Chlamydia infections of the genital tract.

Plac8-dependent and inducible NO synthase-dependent mechanisms clear Chlamydia muridarum infections from the genital tract

Chlamydia trachomatis urogenital serovars replicate predominantly in genital tract epithelium. This tissue tropism poses a unique challenge for host defense and vaccine development. Studies utilizing the Chlamydia muridarum mouse model have shown that CD4 T cells are critical for clearing genital tract infections. In vitro studies have shown that CD4 T cells terminate infection by upregulating epithelial inducible NO synthase (iNOS) transcription and NO production. However, this mechanism is not critical, as iNOS-deficient mice clear infections normally. We recently showed that a subset of Chlamydia-specific CD4 T cell clones could terminate replication in epithelial cells using an iNOS-independent mechanism requiring T cell degranulation. We advance that work using microarrays to compare iNOS-dependent and iNOS-independent CD4 T cell clones. Plac8 was differentially expressed by clones having the iNOS-independent mechanism. Plac8-deficient mice had delayed clearance of infection, and Plac8-deficient mice treated with the iNOS inhibitor N-monomethyl-l-arginine were largely unable to resolve genital tract infections over 8 wk. These results demonstrate that there are two independent and redundant T cell mechanisms for clearing C. muridarum genital tract infections: one dependent on iNOS, and the other dependent on Plac8. Although T cell subsets are routinely defined by cytokine profiles, there may be important subdivisions by effector function, in this case CD4(Plac8).

Chlamydia-specific CD4 T cell clones control Chlamydia muridarum replication in epithelial cells by nitric oxide-dependent and -independent mechanisms

Chlamydia trachomatis serovars D-K are sexually transmitted intracellular bacterial pathogens that replicate in epithelial cells lining the human reproductive tract. It is clear from knockout mice and T cell depletion studies using Chlamydia muridarum that MHC class II and CD4 T cells are critical for clearing bacteria from the murine genital tract. It is not clear how CD4 T cells interact with infected epithelial cells to mediate bacterial clearance in vivo. Previous work using an epithelial tumor cell line showed that a Chlamydia-specific CD4 T cell clone was able to inhibit C. muridarum replication in vitro via induction of epithelial NO production. We have previously shown that Chlamydia-specific CD4 T cell clones can recognize and be activated by infected reproductive tract epithelial cells and block Chlamydia replication in them. We extend those observations by investigating the mechanism used by a panel of CD4 T cell clones to control Chlamydia replication in epithelial cells. We found that Chlamydia-specific CD4 T cell clones were cytolytic, but that cytolysis was not likely critical for controlling C. muridarum replication. For one, CD4 T cell clone-induced epithelial NO production was critical for controlling replication; however, the most potent CD4 T cell clones were dependent on T cell degranulation for replication control with only a minor additional contribution from NO production. We discuss our data as they relate to existing knockout mouse studies addressing mechanisms of T cell-mediated control of Chlamydia replication and their implications for intracellular epithelial pathogens in mouse models.

Th1 cytokine responses fail to effectively control Chlamydia lung infection in ICOS ligand knockout mice

ICOS ligand (ICOSL) plays an important role in controlling specific aspects of T cell activation, differentiation, and function. Th1-type immune responses have been shown to be critical in host defense against chlamydial infections. To assess the role of ICOSL-ICOS interaction in host defense against chlamydial infection, we compared the immune responses and pathological reactions in ICOSL gene knockout (KO) and wild-type (WT) mice following Chlamydia muridarum lung infection. The results showed that ICOSL KO mice exhibited greater body weight loss, higher pathogen burden, and more severe histopathology in their lung than did WT mice. Cytokine analysis revealed that ICOSL KO mice produced lower levels of Th2 (IL-4 and IL-5) and anti-inflammatory (TGF-beta1 and IL-10) cytokines, but higher Th1-related (IFN-gamma and IL-12p40/IL-23) and proinflammatory (IL-6 and TNF-alpha) cytokines. ICOSL KO mice also showed reduced Chlamydia-specific Ab levels in their sera and lung homogenates. In addition, ICOSL KO mice demonstrated significantly lower ICOS expression in T cells and lower Th17 responses than did WT mice. Finally, we showed that ICOS-ICOSL interaction and cell-cell contact are essential for CD4(+) T cells to inhibit chlamydial growth in the cultured lung fibroblasts. The data suggest that ICOSL plays a significant role in immunoregulation and protective immunity against Chlamydia infections and that the Th1 skew in cytokine responses per se is not sufficient for effective control of Chlamydia infections.

Chlamydia muridarum-specific CD4 T-cell clones recognize infected reproductive tract epithelial cells in an interferon-dependent fashion

During natural infections Chlamydia trachomatis urogenital serovars replicate predominantly in the epithelial cells lining the reproductive tract. This tissue tropism poses a unique challenge to host cellar immunity and future vaccine development. In the experimental mouse model, CD4 T cells are necessary and sufficient to clear Chlamydia muridarum genital tract infections. This implies that resolution of genital tract infection depends on CD4 T-cell interactions with infected epithelial cells. However, no laboratory has shown that Chlamydia-specific CD4 T cells can recognize Chlamydia antigens presented by major histocompatibility complex class II (MHC-I) molecules on epithelial cells. In this report we show that MHC-II-restricted Chlamydia-specific CD4 T-cell clones recognize infected upper reproductive tract epithelial cells as early as 12 h postinfection. The timing of recognition and degree of T-cell activation are dependent on the interferon (IFN) milieu. Beta IFN (IFN-beta) and IFN-gamma have different effects on T-cell activation, with IFN-beta blunting IFN-gamma-induced upregulation of epithelial cell surface MHC-II and T-cell activation. Individual CD4 T-cell clones differed in their degrees of dependence on IFN-gamma-regulated MHC-II for controlling Chlamydia replication in epithelial cells in vitro. We discuss our data as they relate to published studies with IFN knockout mice, proposing a straightforward interpretation of the existing literature based on CD4 T-cell interactions with the infected reproductive tract epithelium.

The chlamydial inclusion preferentially intercepts basolaterally directed sphingomyelin-containing exocytic vacuoles

Chlamydiae replicate intracellularly within a unique vacuole termed the inclusion. The inclusion circumvents classical endosomal/lysosomal pathways but actively intercepts a subset of Golgi-derived exocytic vesicles containing sphingomyelin (SM) and cholesterol. To further examine this interaction, we developed a polarized epithelial cell model to study vectoral trafficking of lipids and proteins to the inclusion. We examined seven epithelial cell lines for their ability to form single monolayers of polarized cells and support chlamydial development. Of these cell lines, polarized colonic mucosal C2BBe1 cells were readily infected with Chlamydia trachomatis and remained polarized throughout infection. Trafficking of (6-((N-(7-nitrobenz-2-oxa-1, 3-diazol-4-yl) amino)hexanoyl)sphingosine) (NBD-C(6)-ceramide) and its metabolic derivatives, NBD-glucosylceramide (GlcCer) and NBD-SM, was analyzed. SM was retained within L2-infected cells relative to mock-infected cells, correlating with a disruption of basolateral SM trafficking. There was no net retention of GlcCer within L2-infected cells and purification of C. trachomatis elementary bodies from polarized C2BBe1 cells confirmed that bacteria retained only SM. The chlamydial inclusion thus appears to preferentially intercept basolaterally-directed SM-containing exocytic vesicles, suggesting a divergence in SM and GlcCer trafficking. The observed changes in lipid trafficking were a chlamydia-specific effect because Coxiella burnetii-infected cells revealed no changes in GlcCer or SM polarized trafficking.

Differences in Chlamydia trachomatis serovar E growth rate in polarized endometrial and endocervical epithelial cells grown in three-dimensional culture

In vitro studies of obligate intracellular chlamydia biology and pathogenesis are highly dependent on the use of experimental models and growth conditions that mimic the mucosal architecture and environment these pathogens encounter during natural infections. In this study, the growth of Chlamydia trachomatis genital serovar E was monitored in mouse fibroblast McCoy cells and compared to more relevant host human epithelial endometrium-derived HEC-1B and cervix-derived HeLa cells, seeded and polarized on collagen-coated microcarrier beads, using a three-dimensional culture system. Microscopy analysis of these cell lines prior to infection revealed morphological differences reminiscent of their in vivo architecture. Upon infection, early chlamydial inclusion distribution was uniform in McCoy cells but patchy in both epithelial cell lines. Although no difference in chlamydial attachment to or entry into the two genital epithelial cell lines was noted, active bacterial genome replication and transcription, as well as initial transformation of elementary bodies to reticulate bodies, were detected earlier in HEC-1B than in HeLa cells, suggesting a faster growth, which led to higher progeny counts and titers in HEC-1B cells upon completion of the developmental cycle. Chlamydial development in the less relevant McCoy cells was very similar to that in HeLa cells, although higher progeny counts were obtained. In conclusion, this three-dimensional bead culture system represents an improved model for harvesting large quantities of infectious chlamydia progeny from their more natural polarized epithelial host cells.

Immunopathogenesis of chlamydia trachomatis infections in women

OBJECTIVE

To develop a model of pathogenesis by which Chlamydia trachomatis progresses from acute to chronic infection, and finally serious disease (salpingitis, tubal occlusion).

DESIGN

Review of current literature located through web-based Medline searches using key words: Chlamydia trachomatis, immunology, cytokines, heat shock protein, infertility.

RESULT(S)

Cell-mediated immune mechanisms appear to be critical in determining whether acute infection is resolved or progresses into chronicity with pathological outcome. What determines the particular immune pathway depends on a range of determinants-HLA subtype and human genetics, cytokine profile, infectious load, route of infection, and endocrinology. A clearer picture of the natural history of chlamydial pathology may assist in providing better predictors of those women who may go on to develop significant sequelae after infection.

CONCLUSION(S)

Predicting those who may develop serious disease, including infertility, may contribute to improved management of such persons during earlier stages of infection and assist in prevention.

A mathematical model for the investigation of the Th1 immune response to Chlamydia trachomatis

Chlamydia are bacterial pathogens of humans and animals causing the important human diseases trachoma, sexually transmitted chlamydial disease and pneumonia. Of the human chlamydial diseases, sexually transmitted disease caused by Chlamydia trachomatis is a major public health concern. Chlamydia trachomatis replicates intracellularly and is characterised by a complex developmental cycle. Chlamydia is susceptible to humoral and cell-mediated immunity. Here we investigate the Th1 cell-mediated immune response against Chlamydia-infected cells as the response changes over the chlamydial developmental cycle. We suggest a form for the immune response over one developmental cycle by modelling the change in the number of intracellular chlamydial particles and assume peptides are presented in proportion to the number of replicating forms of chlamydial particles. We predict, perhaps non-intuitively, that persistent Chlamydia should be induced and forced not to return to the lytic cycle. We also suggest that extending the length of the time of the lytic cycle will effectively decrease the required efficacy of the Th1 response to eliminate the pathogen. We produce plots of active disease progression, control and clearance for varying levels of Th1 effectiveness.

Distinct function of Th1 and Th2 type delayed type hypersensitivity: protective and pathological reactions to chlamydial infection

The role of delayed-type hypersensitivity (DTH) to chlamydial infection has been shown to be a double-edged sword to the host. Reported animal and human studies have, on the one hand, shown that DTH is associated with protective immunity against chlamydial infection and, on the other hand, shown links to immunopathology. Using a murine lung infection model, we recently demonstrated that there might be two different functional types of DTH induced by chlamydial infection based on its association with cytokine patterns. Th1 type DTH is associated with protection while Th2 type DTH is associated with immunopathology. The Th2 type DTH demonstrated in IFNgamma gene knockout (KO) mice is characterized by eosinophil infiltration in addition to mononuclear cell infiltration that exists in Th1 DTH, observed in wild-type C57BL/6 mice and IL-10 KO mice. In addition, the inflammatory cells in IFNgamma KO mice fail to target the cellular sites of chlamydial inclusions in infected tissues and fail to clear the infection. The functional differences in Th1 and Th2 type DTH responses may account for the dual role DTH plays in chlamydial protective immunity and immunopathology.

The potential for vaccine development against chlamydial infection and disease

Chlamydia trachomatis and Chlamydia pneumoniae appear to share a common immunobiology with about 80% of their protein coding genes being orthologs. Progress in DNA vaccine development for C. trachomatis suggests that such a subunit approach may prove useful for C. pneumoniae. The recent finding that it is possible to select for chlamydiae with targeted mutations in key metabolic genes together with the new knowledge of the chlamydia genome also suggests that it may be possible to develop live attenuated strains of chlamydiae for use as vaccine.

The intercellular adhesion molecule type-1 is required for rapid activation of T helper type 1 lymphocytes that control early acute phase of genital chlamydial infection in mice

Recent studies in animal models of genital chlamydial disease revealed that early recruitment of dendritic cells and specific T helper type-1 (Th1) cells into the genital mucosae is crucial for reducing the severity of the acute phase of a cervico-vaginal infection and arresting ascending disease. These immune effectors are therefore important for preventing major complications of genital chlamydial infection. Other in vitro studies showed that intercellular adhesion molecule-1 (ICAM-1) plays a role in the antichlamydial action of specific CD4+ and CD8+ T cells. In the present study, we investigated the clinicopathological consequences of ICAM-1 deficiency during chlamydial genital infection in ICAM-1 knockout (ICAM-1KO) mice, and analysed the cellular and molecular immunological bases for any observed pathology or complication. Following a primary genital infection of female ICAM-l-/- and ICAM-1+/+ mice, the intensity of the disease during the first 3 weeks (as assessed by shedding of chlamydiae in the genital tract) was significantly greater in ICAM-1KO mice than in ICAM-1+/+ mice (P < 0.0001), although both ICAM-l-/- and ICAM-1+/+ mice subsequently cleared the primary infection. There was greater ascending disease during the initial stage of the infection, and a higher incidence of tubal disease (hydrosalpinx formation) after multiple infections in ICAM-l-/- mice. Analysis of the cellular and molecular bases for the increased acute and ascending disease in ICAM-l-/- mice revealed that the high affinity of ICAM-1 for leucocyte function antigen type-1 is a property that promotes rapid activation of specific Th1 cells, as well as their early recruitment into the genital mucosa. Moreover, ICAM-1 was more important for naive T-cell activation than primed Th1 cells, although its absence delayed or suppressed immune T-cell activation by at least 50%. Taken together, these results indicated that ICAM-1 is crucial for rapid T-cell activation, early recruitment and control of genitally acquired Chlamydia trachomatis.

Vaccines against Chlamydia: approaches and progress

Infections of the eye and genital tract with the bacterium Chlamydia trachomatis are a major cause of morbidity worldwide and are costly to treat. Development of a vaccine capable of protecting against infection or severe disease presents special challenges but would be the most effective long-term option for control of chlamydial disease. Progress has been made in understanding protective and pathological immune mechanisms in these infections, and a number of potential vaccine candidates have been developed.

Inhibition of intracellular multiplication of human strains of Chlamydia trachomatis by nitric oxide

It was previously shown that murine T cell clones could inhibit the intracellular growth of the mouse strain of Chlamydia trachomatis by cytokine-mediated induction of the inducible nitric oxide synthase (iNOS) system in epithelial cells, an effect enhanced by direct epithelial-T cell interaction via specific adhesion molecules. These findings and other recent reports showing that human mucosal epithelial cells secrete nitric oxide (NO) via iNOS expression would suggest that mucosal epithelial-derived NO may be involved in mucosal defense against Chlamydia and other pathogens that infect epithelial cells. As an initial approach to investigating whether NO contributes to chlamydial control in humans, the present studies evaluated the susceptibility of human isolates of C. trachomatis to NO delivered by chemical donors or via induction of the epithelial iNOS system by a cytokine-secreting T cell clone. It was found that a chlamydial-specific, cytokine-secreting, murine T lymphocyte clone (clone 2.14-0) could inhibit the intraepithelial growth of human strains of Chlamydia trachomatis (serovar E and H, and Lymphogranuloma venerum type L2) via the iNOS pathway when the clone was co-cultured with chlamydial-infected epithelial cells. Furthermore, treatment of infected epithelial cells with 50 microM of the NO donor, S-nitroso-L-glutathione, resulted in significant inhibition (approximately 70%) of chlamydial multiplication, while the NO scavenger, myoglobin plus ascorbate, could reverse the effect, demonstrating that NO could directly inhibit human strains of Chlamydia. The results are consistent with the hypothesis that the IFN-gamma-inducible iNOS pathway can contribute to chlamydial control in humans.

Molecular mechanism of T-cell control of Chlamydia in mice: role of nitric oxide in vivo

T-cell-mediated immunity is crucial for the control of Chlamydia in mice. Recent evidence from studies in an in vitro model of the mucosal epithelium, the polarized epithelial-lymphocyte co-culture (PELC) system, indicated that protective murine T cells mediated intracellular inhibition of the Chlamydia trachomatis agent of mouse pneumonitis (MoPn) at least partly by activating the interferon-gamma (IFN-gamma)-inducible nitric oxide synthase (iNOS) pathway. To investigate whether nitric oxide played a role in controlling chlamydial infection in vivo, the protective capacity of a chlamydial-specific T-cell clone (clone 2.14-0) was analysed in mice in the presence of a specific inhibitor of iNOS. The results revealed that the ability of this clone to clear Chlamydia in vivo is in part mediated by induction of nitric oxide (NO) production. The L-arginine analogue and iNOS inhibitor, NG-monomethyl-L-arginine monoacetate (MLA), increased the chlamydial burden in infected mice and inhibited the ability of clone 2.14-0 to clear genital MoPn infection in vivo. The results are consistent with the working hypothesis that the IFN-gamma-inducible iNOS pathway is involved in the control of Chlamydia by T lymphocytes in mice.

The molecular mechanism of T-cell control of Chlamydia in mice: role of nitric oxide

T-cell mediated immunity (CMI) is crucial for protection against genital chlamydial infection in mice. To define the underlying molecular mechanism for this protection, several T-cell clones generated against the Chlamydia trachomatis agent of mouse pneumonitis (MoPn) were analysed in an in vitro model of the mucosal epithelium, the polarized epithelial-lymphocyte co-culture (PELC) system, for immunobiological functions that correlated with chlamydial inhibition. The six clones analysed were classified as protective or non-protective on the basis of their ability to cure genital chlamydial infection in syngeneic mice. The results revealed a direct relationship between the ability of a clone to protect in vivo and to inhibit the multiplication of MoPn in vitro. Also, the protective ability of a clone correlated with its capacity to elaborate relatively high levels of interferon-gamma (IFN-gamma) and to induce nitric oxide (NO) production. Moreover, neutralizing anti-IFN-gamma antibodies used alone at 50 micrograms/ml or in combination with anti-tumour necrosis-factor (TNF-alpha), and the L-arginine analogue and NO synthase inhibitor, NG-monomethyl-L-arginine monoacetate (MLA), could significantly suppress the ability of protective clones to inhibit MoPn in epithelial cells. The results suggested that the IFN-gamma-inducible NO synthease pathway is important for chlamydial control in mice. Furthermore, IFN-gamma could stimulate infected murine epithelial cells (line TM3) to secrete NO, resulting in inhibition of MoPn growth. However, the degree of MoPn inhibition obtained with IFN-gamma alone was less than that observed when T cells were co-cultured with infected epithelial cells. T-cell-derived NO could partly explain the enhanced chlamydial inhibition when T cells were co-cultured with infected epithelial cells. These results are consistent with the hypothesis that, besides T-cell-derived IFN-gamma, other factors associated with lymphoepithelial interactions are likely to contribute an important role in chlamydial control by T cells in mice.

The immunobiology and immunopathology of chlamydial infections

Chlamydiae are obligate intracellular bacterial pathogens of eukaryotic cells responsible for a wide variety of important human and animal infections. In humans, chlamydial infections are generally localised to superficial epithelial or mucosal surfaces, are frequently asymptomatic and may persist for long periods of time if untreated, inducing little protective immunity. Nevertheless, neutralising antibodies of limited efficacy are produced against the main chlamydial outer envelope protein, while gamma interferon (IFN gamma) is chlamydiastatic and paradoxically may play a role both in chlamydial persistence and in protective immunity. Delayed hypersensitivity responses to chlamydiae caused by repeated or persistent infection are thought to be important in the development of the severe scarring sequelae characteristic of cicatricial trachoma and of chronic salpingitis. Chlamydial heat shock proteins bearing close homology with their human equivalents may be major targets for immunopathological responses and their expression is upregulated in IFN gamma induced persistent infection. C. pneumoniae, a common cause of acute respiratory infection in humans, may persist in coronary arteries and is strongly implicated as a risk factor in atherosclerosis and in acute myocardial infarction. This paper reviews the immunology and immunopathology of chlamydial infections in the context of the unique biology of this fascinating but challenging group of organisms.

CD4+ T cells play a significant role in adoptive immunity to Chlamydia trachomatis infection of the mouse genital tract

The ability of CD4+ and CD8+ T cells to adoptively immunize mice against Chlamydia trachomatis infection of the mouse genital tract was studied. Adoptive transfer experiments were performed with splenic CD4+ or CD8+ T cells obtained from mice following resolution of a primary genital tract infection and after a secondary chlamydial challenge. The results show that donor CD4+ T cells, but not CD8+ T cells, obtained from mice following resolution of a primary infection or after secondary challenge were effective in transferring significant antichlamydial immunity to the genital tracts of naive animals. The lymphokine profiles in the culture supernatants of proliferating Chlamydia-specific CD4+ T cells obtained from mice following resolution of a primary infection and after secondary challenge were assayed by an enzyme-linked immunoadsorbent assay. Protective CD4+ T cells restimulated in vitro secreted interleukin 2, gamma interferon, and interleukin 6, lymphokine profiles characteristic of both Th1- and Th2-like responses. Resting CD4+ T cells obtained from mice 4 months following resolution of a primary infection were also capable of conferring significant levels of adoptive protective immunity to naive mice. These findings support an important role for CD4+ T cells in acquired immunity to chlamydial infection of the genital tract and indicate that protective CD4+ immune responses in this model are relatively long lived.

Effect of gamma-irradiation on the effector function of T lymphocytes in microbial control

A chlamydial-specific T cell clone, capable of inhibiting the growth of infectious Chlamydia in vivo and in vitro, was employed to investigate the effect of gamma-irradiation on the ability of effector T cells to control infections. Clone 2.14-0 (CD4+), specific for the Chlamydia trachomatis biovar agent of mouse pneumonitis (MoPn), was irradiated with varying doses (0, 2.5, 5.0, 10.0, 20.0 and 40.0 Gy) and its biological functions and ability to inhibit the intraepithelial growth of MoPn were assessed. The results revealed that although gamma-irradiation drastically reduced the proliferative response of the clone to antigen, it did not affect the phenotype and the effector function of inhibiting chlamydial growth. The preservation of anti-chlamydial function after gamma-irradiation correlated with the retention of IFN-gamma and TNF-alpha secretion in response to antigenic stimulation. We conclude that the biologic functions of T cells requiring proliferation and differentiation are more likely to be adversely affected by gamma-irradiation on the short-term, but the effector functions, possibly associated with cytokines and cytolysis, may be preserved among persisting effector T cells in an irradiated individual.