Role of cell-mediated immunity in the resolution of secondary chlamydial genital infection in guinea pigs infected with the agent of guinea pig inclusion conjunctivitis

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.

Frequency of Chlamydia trachomatis-specific T cell interferon-γ and interleukin-17 responses in CD4-enriched peripheral blood mononuclear cells of sexually active adolescent females

An evaluation of CD4 T cell responses to candidate Chlamydia trachomatis vaccine antigens was conducted in an adolescent female cohort exposed through natural infection to explore antigen immunogenicity and correlation with protection from reinfection. The frequency of peripheral blood CD4 T cell IFN-γ and IL-17 responses to three candidate vaccine antigens, polymorphic membrane protein G (PmpG), F (PmpF), and major outer membrane protein (MOMP), were determined by ELISPOT; responses to chlamydial heat shock protein 60 (HSP60) and to elementary bodies (EB) were included for comparison. Responses of Infected (n=8), Seropositive/Uninfected (n=13), and Seronegative/Uninfected (n=18) participants were compared. The median CD4 IFN-γ response to EB was significantly increased in Infected (P=0.003) and Seropositive/Uninfected (P=0.002) versus Seronegative/Uninfected female subjects. Higher rates of positive IFN-γ responders to EB, PmpF, and MOMP were detected in Seropositive/Uninfected versus Seronegative/Uninfected participants (P=0.021). IL-17 responses were generally low. A positive IFN-γ response to any of the antigens tested was associated with a trend toward a reduced risk of reinfection, although not statistically significant. Among this adolescent cohort, chlamydial-specific CD4 IFN-γ but not IL-17 responses were detected in acutely and previously infected participants and a positive CD4 IFN-γ response was associated with a non-significant reduced risk of reinfection.

Animal models for studying female genital tract infection with Chlamydia trachomatis

Chlamydia trachomatis is a Gram-negative obligate intracellular bacterial pathogen. It is the leading cause of bacterial sexually transmitted disease in the world, with more than 100 million new cases of genital tract infections with C. trachomatis occurring each year. Animal models are indispensable for the study of C. trachomatis infections and the development and evaluation of candidate vaccines. In this paper, the most commonly used animal models to study female genital tract infections with C. trachomatis will be reviewed, namely, the mouse, guinea pig, and nonhuman primate models. Additionally, we will focus on the more recently developed pig model.

A novel guinea pig model of Chlamydia trachomatis genital tract infection

Genital Chlamydia trachomatis infections often result in pelvic inflammatory disease and sequelae including infertility and ectopic pregnancies. In addition to the already established murine models, the development of other animal models is necessary to study the safety and efficacy of prototype vaccine candidates. The intravaginal infection of guinea pigs with C. trachomatis has been tested in three independent studies. The first two studies investigated the effect of hormonal treatment of the animals prior to infection with serovars D and E. The results showed that estradiol treatment was required for sustained infection. The third study conducted an immunization-challenge experiment to explore the feasibility of measuring protection in this guinea pig model. C. trachomatis bacteria were sampled using vaginal swabs and measured by qPCR. Using immunohistochemistry the bacteria were detected in the oviducts 19 days post-infection, indicating that the estradiol treatment resulted in ascending infection. Furthermore, immunization of guinea pigs with live EB formulated with ISCOM matrix led to reduction of cervico-vaginal shedding and diminished the severity of pathology. In this study we have developed a new guinea pig model of C. trachomatis female genital tract infection for the purpose of evaluating potential vaccine candidates.

Pathogenesis of genital tract disease due to Chlamydia trachomatis

Although the pathologic consequences of C. trachomatis genital infection are well-established, the mechanism(s)that result in chlamydia-induced tissue damage are not fully understood. We reviewed in vitro, animal, and human data related to the pathogenesis of chlamydial disease to better understand how reproductive sequelae result from C. trachomatis infection. Abundant in vitro data suggest that the inflammatory response to chlamydiae is initiated and sustained by actively infected nonimmune host epithelial cells. The mouse model indicates a critical role for chlamydia activation of the innate immune receptor, Toll-like receptor 2, and subsequent inflammatory cell influx and activation, which contributes to the development of chronic genital tract tissue damage. Data from recent vaccine studies in the murine model and from human immunoepidemiologic studies support a role for chlamydia-specific CD4 Th1-interferon-g-producing cells in protection from infection and disease. However, limited evidence obtained using animal models of repeated infection indicates that, although the adaptive T cell response is a key mechanism involved in controlling or eliminating infection, it may have a double-edged nature and contribute to tissue damage. Important immunologic questions include whether anamnestic CD4 T cell responses drive disease rather than protect against disease and the role of specific immune cells and inflammatory mediators in the induction of tissue damage with primary and repeated infections. Continued study of the complex molecular and cellular interactions between chlamydiae and their host and large-scale prospective immunoepidemiologic and immunopathologic studies are needed to address gaps in our understanding of pathogenesis that thwart development of optimally effective control programs, including vaccine development.

IL-17/Th17 promotes type 1 T cell immunity against pulmonary intracellular bacterial infection through modulating dendritic cell function

Although their contribution to host defense against extracellular infections has been well defined, IL-17 and Th17 are generally thought to have limited impact on intracellular infections. In this study, we investigated the role and mechanisms of IL-17/Th17 in host defense against Chlamydia muridarum, an obligate intracellular bacterium, lung infection. Our data showed rapid increase in IL-17 production and expansion of Th17 cells following C. muridarum infection and significant detrimental impact of in vivo IL-17 neutralization by anti-IL-17 mAb on disease course, immune response, and dendritic cell (DC) function. Specifically, IL-17-neutralized mice exhibited significantly greater body weight loss, higher organism growth, and much more severe pathological changes in the lung compared with sham-treated control mice. Immunological analysis showed that IL-17 neutralization significantly reduced Chlamydia-specific Th1 responses, but increased Th2 responses. Interestingly, the DC isolated from IL-17-neutralized mice showed lower CD40 and MHC II expression and IL-12 production, but higher IL-10 production compared with those from sham-treated mice. In two DC-T cell coculture systems, DC isolated from IL-17-neutralized mice induced higher IL-4, but lower IFN-gamma production by Ag-specific T cells than those from sham-treated mice in cell priming and reaction settings. Adoptive transfer of DC isolated from IL-17-neutralized mice, unlike those from sham-treated mice, failed to protect the recipients against challenge infection. These findings provide in vivo evidence that IL-17/Th17 plays an important role in host defense against intracellular bacterial infection, and suggest that IL-17/Th17 can promote type 1 T cell immunity through modulating DC function.

Temporal delay of peak T-cell immunity determines Chlamydia pneumoniae pulmonary disease in mice

Severe chlamydial disease typically occurs after previous infections and results from a hypersensitivity response that is also required for chlamydial elimination. Here, we quantitatively dissected the immune and disease responses to repeated Chlamydia pneumoniae lung infection by multivariate modeling with four dichotomous effects: mouse strain (A/J or C57BL/6), dietary protein content (14% protein and 0.3% L-cysteine-0.9% L-arginine, or 24% protein and 0.5% L-cysteine-2.0% L-arginine), dietary antioxidant content (90 IU alpha-tocopherol/kg body weight versus 450 IU alpha-tocopherol/kg and 0.1% g L-ascorbate), and time course (3 or 10 days postinfection). Following intranasal C. pneumoniae challenge, C57BL/6 mice on a low-protein/low-antioxidant diet, but not C57BL/6 mice on other diets or A/J mice, exhibited profoundly suppressed early lung inflammatory and pan-T-cell (CD3delta(+)) and helper T-cell (CD45) responses on day 3 but later strongly exacerbated disease on day 10. Contrast analyses characterized severe C. pneumoniae disease as being a delayed-type hypersensitivity (DTH) response with increased lung macrophage and Th1 cell marker transcripts, increased Th1:Th2 ratios, and Th1 cytokine-driven inflammation. Results from functional analyses by DTH, enzyme-linked immunospot, and immunohistofluorescence assays were consistent with the results obtained by transcript analysis. Thus, chlamydial disease after secondary infection is a temporal dysregulation of the T-cell response characterized by a profoundly delayed T-helper cell response that results in a failure to eliminate the pathogen and provokes later pathological Th1 inflammation. This delayed T-cell response is under host genetic control and nutritional influence. The mechanism that temporally and quantitatively regulates the host T-cell population is the critical determinant in chlamydial pathogenesis.

Adoptive transfer of CD8alpha+ dendritic cells (DC) isolated from mice infected with Chlamydia muridarum are more potent in inducing protective immunity than CD8alpha- DC

Chlamydial infections are serious public health concerns worldwide. In this study, we examined the role of dendritic cell (DC) subsets in inducing protective immunity against chlamydial infection using an adoptive transfer approach. We found that CD11c+CD8alpha+ (double-positive, DP) DC, compared with CD11c+CD8alpha- (single-positive, SP) DC isolated from infected mice, are more potent inducers of protective immunity. Specifically, mice pretreated with DPDC from infected mice, upon infection with Chlamydia trachomatis mouse pneumonitis (MoPn), experienced significantly less severe body weight loss and in vivo chlamydial growth. Analysis of MoPn-driven cytokine production by immune cells revealed that mice that were treated with DPDC produced significantly higher levels of Th1 (TNF-alpha, IFN-gamma, and IL-12) but lower levels of Th2 (IL-4, IL-5, and IL-13)-related cytokines than the recipients of SPDC following infection challenge. Moreover, DPDC-treated mice displayed significantly higher levels of MoPn-specific IgG2a production and delayed-type hypersensitivity responses compared with SPDC-treated mice. Furthermore, DPDC isolated from infected mice produced higher amounts of IL-12 and IL-10 in vitro in comparison with SPDC. These data indicate that CD8alpha+ DC have a significantly higher capacity in inducing protective immunity compared with CD8alpha- DC, demonstrating the crucial role of DC1-like cells in eliciting protection against C. trachomatis infection.

Protection of turkeys against Chlamydophila psittaci challenge by DNA and rMOMP vaccination and evaluation of the immunomodulating effect of 1α,25-dihydroxyvitamin D3

Plasmid DNA expressing the major outer membrane protein (MOMP) of an avian Chlamydophila psittaci serovar D strain and recombinant MOMP (rMOMP) with or without the immunomodulating adjuvant 1 alpha,25-dihydroxyvitamin D(3) have been tested for their ability to elicit an immune response and induce protection in turkeys against challenge with the same serovar. Three vaccination strategies were compared: priming and boosting with either pcDNA1::MOMP or rMOMP and priming with pcDNA1::MOMP followed by rMOMP boosting. Turkeys primed with pcDNA1::MOMP showed significant protection against Cp. psittaci challenge, turkeys primed with rMOMP did not. The steroid hormone 1 alpha,25-dihydroxyvitamin D(3) augmented serum and mucosal antibody titres. However, higher antibody titres were not related to better protection and even had a negative effect on especially bacterial excretion.

Tryptophan recycling is responsible for the interferon-gamma resistance of Chlamydia psittaci GPIC in indoleamine dioxygenase-expressing host cells

Comparative genomics indicates that vast differences in Chlamydia sp. host range and disease characteristics can be traced back to subtle variations in gene content within a region of the chromosome termed the plasticity zone. Genes required for tryptophan biosynthesis are located in the plasticity zone; however, the complement of genes encoded varies depending on the chlamydial species examined. Of the sequenced chlamydia genomes, Chlamydia psittaci GPIC contains the most complete tryptophan biosynthesis operon, encoding trpRDCFBA. Immediately downstream of the trp operon are genes encoding kynureninase and ribose phosphate pyrophosphokinase. Here, we show that, in GPIC, these genes are transcribed as a single transcript, the expression of which is regulated by tryptophan. Complementation analyses, using various mutant Escherichia coli isolates, indicate that the tryptophan biosynthesis, kynureninase and ribose phosphate pyrophosphokinase gene products are functional. Furthermore, growth of C. psittaci GPIC in HeLa cells, cultured in tryptophan-free medium, could be rescued by the addition of anthranilate, kynurenine or indole. In total, our results indicate that this complement of genes enables GPIC to recycle tryptophan and thus accounts for the interferon-gamma resistant phenotype displayed in indoleamine-2,3-dioxygenase-expressing host cells.

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.

Resolution of secondary Chlamydia trachomatis genital tract infection in immune mice with depletion of both CD4+ and CD8+ T cells

The essential role of T cells in the resolution of primary murine Chlamydia trachomatis genital tract infection is inarguable; however, much less is known about the mechanisms that confer resistance to reinfection. We previously established that CD4+ T cells and B cells contribute importantly to resistance to reinfection. In our current studies, we demonstrate that immune mice concurrently depleted of both CD4+ T cells and CD8+ T cells resisted reinfection as well as immunocompetent wild-type mice. The in vivo depletion of CD4+ and CD8+ T cells resulted in diminished chlamydia-specific delayed-type hypersensitivity responses, but antichlamydial antibody responses were unaffected. Our data indicate that immunity to chlamydial genital tract reinfection does not rely solely upon immune CD4+ or CD8+ T cells and further substantiate a predominant role for additional effector immune responses, such as B cells, in resistance to chlamydial genital tract reinfection.

Mucosal immunity to herpes simplex virus type 2 infection in the mouse vagina is impaired by in vivo depletion of T lymphocytes

Intravaginal (IVAG) inoculation of wild-type herpes simplex virus type 2 (HSV-2) in mice causes epithelial infection followed by lethal neurological illness, while IVAG inoculation of attenuated HSV-2 causes epithelial infection followed by development of protective immunity against subsequent IVAG challenge with wild-type virus. The role of T cells in this immunity was studied by in vivo depletion of these cells with monoclonal antibodies. Three groups of mice were used for each experiment: nonimmune/challenged mice, immune/challenged mice, and immune depleted mice [immune mice depleted of a T-cell subset(s) shortly before challenge with HSV-2]. Mice were assessed for epithelial infection 24 h after challenge, virus protein in the vaginal lumen 3 days after challenge, and neurological illness 8 to 14 days after challenge. Monoclonal antibodies to CD4, CD8, or Thy-1 markedly reduced T cells in blood, spleen, and vagina, but major histocompatibility complex class II antigens were still partially upregulated in the vaginal epithelium after virus challenge, indicating that virus-specific memory T-cell function was not entirely eliminated from the vagina. Nevertheless, immune mice depleted of CD4+ and CD8+ T cells, Thy-1+ T cells, or CD8+ T cells alone had greater viral infection in the vaginal epithelium than nondepleted immune mice, indicating that T cells contribute to immunity against vaginal HSV-2 infection. All immune depleted mice retained substantial immunity to epithelial infection and were immune to neurological illness, suggesting that other immune mechanisms such as virus-specific antibody may also contribute to immunity.

B-cell-deficient mice develop complete immune protection against genital tract infection with Chlamydia trachomatis

We evaluated the ability of mice made genetically deficient for B cells to resolve a primary infection and to develop protective immunity against vaginal challenge with a human isolate of Chlamydia trachomatis bacteria. The B-cell-deficient microMT mice cleared a primary ascending infection with similar or faster kinetics compared with wild-type mice. The presence of chlamydial inclusion bodies and the degree of inflammation in the upper genital tract was comparable and showed similar kinetics in microMT as in wild-type mice. Following resolution of the primary infection the mice were challenged by 100 ID50 of live bacteria and the level of protection and the extent of local inflammation was assessed. Strikingly, all microMT mice, as well as most of the wild-type mice, demonstrated complete immune protection with no bacterial shedding. While high titres of chlamydia-specific antibodies were stimulated locally and systemically in wild-type mice, no antibodies were detected in microMT mice. However, in both strains, immunohistochemical analysis of the upper genital tract demonstrated the presence of large numbers of CD4+ T cells and increased levels of interferon-gamma (IFN-gamma)-producing cells. The results unequivocally demonstrate that antibodies are not required for full protection to develop against ascending infection with a high dose of C. trachomatis in the female genital tract. Our study confirms the notion that cell-mediated immunity, in particular that owing to CD4+ T helper I (Th1)-type cells, is critical for host resistance against C. trachomatis in mice.

Genital tract infection with Chlamydia trachomatis fails to induce protective immunity in gamma interferon receptor-deficient mice despite a strong local immunoglobulin A response

CD4+ T cells have been found to play a critical role in immune protection against Chlamydia trachomatis infection. Since both humoral and cell-mediated antichlamydial immunity have been implicated in host protection, the crucial effector functions provided by the CD4+ T cells may rely on Th1 or Th2 functions or both. In the present study, we evaluated the development of natural immunity following vaginal infection with C. trachomatis serovar D in female gamma interferon receptor-deficient (IFN-gammaR-/-) mice with a disrupted Th1 effector system. We found that in comparison with wild-type mice, the IFN-gammaR-/- mice exhibited a severe ascending primary infection of prolonged duration which stimulated almost 10-fold-stronger specific local immunoglobulin A (IgA) and IgG responses in the genital tract. Following resolution of the primary infection and despite the augmented antibody responses to chlamydiae, the IFN-gammaR-/- mice were completely unprotected against reinfection, suggesting that local antibodies play a subordinate role in host protection against chlamydial infection. Immunohistochemical analysis of frozen sections of the genital tract revealed many CD4+ T cells in the IFN-gammaR-/- mice, with a dominance of interleukin 4-containing cells in mice following resolution of the secondary infection. However, in contrast to the findings with wild-type mice, the typical clusters of CD4+ T cells were not found in the IFN-gammaR-/- mice. Few and similarly distributed CD8+ T cells were observed in IFN-gammaR-/- and wild-type mice. Whereas chlamydia-infected macrophages from wild-type mice had no inclusion bodies (IB) and produced significant amounts of nitric oxide (NO) in the presence of IFN-gamma, macrophages from IFN-gammaR-/- mice contained many IB but no NO. These results indicate that CD4+ Th1 cells and IFN-gamma, rather than local antibodies, are critical elements in host immune protection stimulated by a natural ascending C. trachomatis infection in the female genital tract.

Immunity to reinfection and immunization of male guinea pigs against urethral infection with the agent of guinea pig inclusion conjunctivitis

BACKGROUND AND OBJECTIVES

There is little information available on immunity of males to chlamydial infection after recovering from a primary urethral infection or after immunization with chlamydial antigen. The guinea pig model of genital infection using the chlamydial agent of guinea pig inclusion conjunctivitis was utilized to evaluate the protective immune response in these circumstances.

GOAL

To determine whether immunity to reinfection develops after a primary urethral infection and whether immunity develops as a result of immunization with inactivated chlamydiae.

STUDY DESIGN

Groups of five male guinea pigs each in two separate experiments were infected in the urethra with chlamydiae and challenged with a fresh inoculum at either 30, 75, or 150 days after infection. The course of the challenge infection was then determined. Similarly, guinea pigs were immunized subcutaneously with ultraviolet-inactivated chlamydial elementary bodies and the course of urethral infection was determined when inoculated 2 weeks after immunization.

RESULTS

Male guinea pigs were highly resistant to reinfection after a primary urethral infection. Animals that were immunized with inactivated chlamydiae generally became infected upon challenge, but the intensity of the infection was markedly reduced.

CONCLUSIONS

Male guinea pigs possess protective mechanisms that make them more resistant to repeat chlamydial genital infection for a longer period of time than is seen in female guinea pigs. In addition, immunization of males with inactivated chlamydial antigen by a parenteral route is able to elicit a protective response to urethral infection with chlamydiae.

Evaluation of Chlamydia psittaci subfraction and subunit preparations for their protective capacities

A mouse toxicity potency test was used to evaluate the protective capacity of Chlamydia psittaci (Baker strain) subfraction and subunit preparations. The elementary body (EB), Chlamydial outer membrane complex (COMC), major outer membrane protein (MOMP), lipopolysaccharide (LPS), and MOMP mixed together with LPS were utilized as antigens in vaccine preparation. The EBs and COMCs were prepared by centrifugation and detergent extraction as previously described for Chlamydia trachomatis. Isolation and purification of MOMP and LPS were achieved by subjecting infected tissue culture fluids to polyacrylamide gel electrophoresis and subsequent elution from the gel. Chromatographic purification of the MOMP from EBs resulted in a preparation which also contained minor quantities of LPS. After vaccination and intraperitoneal challenge with C. psittaci (Cello strain), the MOMP presented in EBs, COMCs or in a purified form, was capable of protecting mice from death, whereas LPS did not demonstrate this capability. Further comparison of the protective capacity of these antigens indicated that the electrophoretically purified MOMP was the least protective when suboptimal levels of antigens were administered to mice. However, the level of protection afforded by electrophoretically purified MOMP could be increased to that observed with the chromatographically purified MOMP by the addition of electrophoretically purified LPS to the vaccine. These results suggest that the MOMP is the major protective antigen of C. psittaci and that the LPS component, although not protective on its own, may play a role in the potentiation of a protective immune response.

Immune Protection Against Chlamydia trachomatis in Females

Despite significant advances in our understanding of the biology and antigenic structure of Chlamydia trachomatis, and the epidemiology and clinical spectrum of chlamydial disease, the magnitude of morbidity from human chlamydial infections remains an important public health concern. Control of chlamydial disease will likely depend on a multidisciplinary approach, including the development of immunoprophylactic or immunotherapeutic strategies. Reasonable progress has been made in understanding specific immune mechanisms that contribute to host immunity in experimental models of chlamydial infection. However, studies of human immunity have not been so successful. This is particularly evident in that studies to address the development and role of mucosal immune responses to urogenital chlamydial infections have not been forthcoming. The following review is a brief summary of our current knowledge of protective immunity to chlamydial urogenital infections of females. It is not meant to be exhaustive, but instead to touch upon aspects of protective immunity that have been described in both human and experimental animal models of chlamydial infection.

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.

A peptide of Chlamydia trachomatis shown to be a primary T-cell epitope in vitro induces cell-mediated immunity in vivo

Chlamydiae are a major cause of infertility and preventable blindness and there is currently no effective vaccine in humans or rodents against these organisms. We have previously shown that a peptide of 12 amino acids (termed TINKP) from a conserved region of the major outer membrane protein (MOMP) of Chlamydia trachomatis (C. trachomatis) is a primary T-cell epitope in humans. Here we showed that when dendritic cells (DC) from C3H or BALB/c mice were pulsed in vitro with the peptide they stimulated proliferation of syngeneic T cells in vitro indicating that the peptide is also a primary T-cell epitope in mice. Since the skin is a rich source of DC, we immunized mice from each strain with an intradermal injection of the peptide. Humoral and cell-mediated immunity to peptide, MOMP or whole elementary bodies (EB) of C. trachomatis (F/NI1/GU) were assessed. No antibody response to TINKP was observed. However, immunized mice showed recall responses to all three chlamydial antigens. T-cell-mediated immunity in the absence of antibody was induced by a single injection of the peptide intradermally. C. trachomatis isolated from the human genital tract causes salpingitis in mice. Preliminary studies in susceptible C3H mice indicated that intradermal injection of peptide conferred some protection against the development of salpingitis. Thus, a primary T-cell epitope identified by in vitro stimulation using DC can also initiate cell-mediated immunity in vivo and this approach may be useful in the development of vaccines.

Experimental Chlamydia pneumoniae infection in mice: effect of reinfection and passive immunization

NIH/S mice were infected intranasally with Chlamydia pneumoniae isolate Kajaani 6 and rechallenged after either 28 or 70 days. A partial resistance to reinfection, indicated by a reduced recovery of live organisms, was noted at both time points of rechallenge: positive isolations from lung homogenates and/or bronchoalveolar lavage fluids were observed in fewer mice and the yields of isolated chlamydiae remained smaller, as compared to primary infection. However, a previous infection did not confer any protection against inflammatory changes. A strong peribronchial and perivascular inflammation with infiltrating lymphocytes and plasma cells was noted in the lungs of primary infected, as well as reinfected, mice. The effect of passive immunization was also studied. When mice were given convalescent or hyperimmune sera intraperitoneally before inoculation, lower C. pneumoniae isolation yields were detected. As in the rechallenge experiment, marked inflammation could still be seen in the lungs, now with polymorphonuclear leukocyte infiltration. The results suggest that immunological reactions play a role in the pathogenesis of C. pneumoniae infection. Antibodies may be important in reducing the amount of infective elementary bodies, but complete clearing of C. pneumoniae could not be achieved in these experiments, even less a protection against inflammatory lung changes.

Role for CD8+ T cells in antichlamydial immunity defined by Chlamydia-specific T-lymphocyte clones

The role of CD8+ T cells in antichlamydial immunity was investigated in a murine model of chlamydial genital infection by using T-cell clones generated against the Chlamydia trachomatis agent of mouse pneumonitis (MoPn). Two CD8+ T-cell clones tested (2.1F and 2.14-9) were chlamydia antigen specific and MHC restricted and reacted against MoPn as well as the Chlamydia psittaci agent of guinea pig inclusion conjunctivitis and C. trachomatis serovar E, suggesting the recognition of a genus-specific antigen. Upon adoptive transfer into persistently MoPn-infected nu/nu mice, 55.6% of the recipients of clone 2.1F (15 of 27) resolved the infection but recipients of clone 2.14-9 did not. The ability to resolve the MoPn infection correlated with the capacity of clone 2.1F to elaborate a combination of gamma interferon and tumor necrosis factor alpha. The results suggested that in addition to CD4+ T cells, CD8+ T cells may also contribute to antichlamydial T-cell immunity in vivo.

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

A polarized epithelial culture system and chlamydia-specific T-cell lines and clones were employed to investigate the ability and mechanisms by which T cells control the growth of chlamydiae in epithelial cells. Monolayers of polarized mouse epithelial cells were infected with the Chlamydia trachomatis agent of mouse pneumonitis (MoPn) and then exposed to antigen-stimulated MoPn-specific T-cell lines and clones. The results revealed that in vivo-protective MoPn-specific T-cell lines and clone 2.14-0 were capable of inhibiting the growth of MoPn in polarized epithelial cells. In contrast, the nonprotective MoPn-specific T-cell clone 2.14-3, naive splenic T cells, and a control T-cell clone could not inhibit the growth of MoPn in epithelial cells. Transmission electron microscopic analysis of infected epithelial cells which were exposed to clone 2.14-0 confirmed the absence of an established infection, as deduced from the virtual absence of inclusions in the cells. Antigen-specific activation of clone 2.14-0 was required for the MoPn-inhibitory function, since the absence of antigenic stimulation or stimulation with a heterologous chlamydial agent did not result in MoPn growth inhibition. Activation of clone 2.14-0 resulted in acquisition of the capacity to inhibit growth of both homologous (MoPn) and heterologous chlamydial agents. Close interaction between epithelial cells and clone 2.14-0 was required for the MoPn-inhibitory action, because separation of the cell types by a filter with a pore size of 0.45, 3.0, or even 8.0 microns abrogated MoPn inhibition. Protective T cells may act at close range in the epithelium to control chlamydial growth, possibly involving short-range-acting cytokines. The ability of antigen-stimulated T-cell lines and clones to inhibit chlamydial growth in polarized epithelial cultures could be a useful method for identifying protective T-cell clones and antigenic peptide fragments containing protective epitopes.

Vaccines for bacterial sexually transmitted infections: a realistic goal?

Bacterial infections of the genital tract (gonorrhea, chlamydia, chancroid, syphilis) are common and cause significant morbidity. Their importance is heightened by recent appreciation of their roles in facilitation of transmission of the human immunodeficiency virus (HIV). Each is capable of causing repeated infections, suggesting lack of permanent broadly effective immunity. An effective vaccine has yet to be developed for any of these diseases. Rapid progress in understanding the molecular basis for pathogenesis of infection, including mechanisms for escape from otherwise effective immune surveillance and mechanisms for causing injury to host cells, has stimulated renewed efforts to make vaccines for some of these infections. Progress has been greatest for Neisseria gonorrhoeae and Chlamydia trachomatis. Present emphasis is on the major or principal outer membrane proteins of N. gonorrhoeae and C. trachomatis, based on evidence for neutralizing antibodies directed against surface-exposed variable domains of each of these proteins. Other surface-exposed proteins, including the iron-repressible transferrin receptor in gonococci and certain heat-shock proteins in chlamydia, also may be targets for vaccines. Although much remains to be learned, cautious optimism is warranted.

Mimicry of a neutralizing epitope of the major outer membrane protein of Chlamydia trachomatis by anti-idiotypic antibodies

The major outer membrane protein (MOMP) is a primary target antigen for the development of chlamydial vaccine. This protein is composed of four variable domains (I to IV) flanked by constant regions. Some of the variable domains contain antigenic determinants that elicit a neutralizing antibody response. Murine monoclonal antibodies (MAbs) against three nonoverlapping epitopes of MOMP were developed. One of these, called DP10, bound to all serovars, as shown by immunoblot analysis, and neutralized chlamydial infectivity for hamster kidney (HaK) cells in a complement-independent in vitro assay. Furthermore, analysis of the fine specificity of this MAb showed that it recognized a synthetic peptide contained within variable domain IV of the MOMP. Anti-idiotypic antibodies (aId) directed against this anti-MOMP MAb were produced in rabbits. These aId specifically bound to the relevant idiotype (DP10) and inhibited the binding of anti-MOMP MAb (DP10) to MOMP preparations in a dose-dependent fashion. The specificity of our aId for the binding site of anti-MOMP MAb is further suggested by the binding inhibition of affinity-purified aId to DP10 by the synthetic peptide defined by the idiotype. In addition, these aId also reacted with anti-MOMP antisera from rats and mice, suggesting an idiotypic cross-reactivity between these species. Finally, immunization of naive mice with aId induced an antibody response directed against the peptide defined by our anti-MOMP MAb and with neutralizing activity. Taken together, these data suggest that aId mimic a neutralization site on MOMP and could serve as a surrogate antigen to induce protective immunity against Chlamydia trachomatis.

Lack of allelic polymorphism for the major outer membrane protein gene of the agent of guinea pig inclusion conjunctivitis (Chlamydia psittaci)

The major outer membrane protein gene (omp1) was sequenced for each of six Chlamydia psittaci (guinea pig inclusion conjunctivitis [GPIC]) strains isolated from guinea pigs. Five of the isolates were obtained in the United States during the 1960s and 1970s, including the prototype strain isolated by Murray in 1962. The other isolate was obtained from a guinea pig in England. The nucleotide sequence of the omp1 gene for each strain was identical. The lack of omp1 allelic polymorphism among GPIC isolates suggests that, unlike C. trachomatis, the GPIC agent lacks antigenic variation in the major outer membrane protein.

Cell-mediated immunity to Chlamydia pneumoniae measured as lymphocyte blast transformation in vitro

The purpose of the present study was to analyze Chlamydia pneumoniae-induced, antigen-specific, cell-mediated immunity. Peripheral blood mononuclear cells of four persons infected with C. pneumoniae Kajaani 6 and 17 healthy volunteers were stimulated with antigen composed of whole elementary bodies of C. pneumoniae Kajaani 6 (CP-Ag). Definitive antigen-specific lymphoproliferation (LP) responses were developed after recent infection. The LP responses of healthy people to CP-Ag varied considerably. There was no clear correlation between LP responses to CP-Ag and those to an antigen prepared from Chlamydia trachomatis serotype L2 (r > or = 0.50, P < 0.1). A larger study is required to demonstrate whether the LP responses to CP-Ag can be used for the diagnosis of C. pneumoniae infection.

Effect of gamma interferon on resolution of murine chlamydial genital infection

Mice infected in the genital tract with the Chlamydia trachomatis agent of mouse pneumonitis were treated with monoclonal rat anti-gamma interferon (anti-IFN-gamma) antibody to determine whether IFN-gamma participated in the resolution of the infection. In two experiments, anti-IFN-gamma antibody treatment resulted in significantly prolonged infections. In support of these data, passive administration of recombinant IFN-gamma to chronically infected nu/nu mice was able to bring about resolution of the infection in some animals.

Susceptibility to reinfection after a primary chlamydial genital infection is associated with a decrease of antigen-specific T cells in the genital tract

We tested the hypothesis that the intensity of specific antichlamydial T cell-mediated immunity in the genital tract of female guinea pigs infected intravaginally with the chlamydial agent of guinea pig inclusion conjunctivitis would determine the resistance or susceptibility to reinfection after a primary chlamydial infection. T cell-enriched lymphocytes were isolated by collagenase treatment of genital tract tissues from either infected or control uninfected female guinea pigs at various times after infection. The nylon wool-enriched T lymphocytes were evaluated for expression of antigen-specific T cell-mediated immunity in vitro by using a blast transformation assay. Both uninfected and infected genital tracts contained T cells, as evidenced by reactivity to concanavalin A, although a greater number of T lymphocytes was detected in the genital tracts of infected animals compared with that in controls. Significant antigen-specific T-cell activity could be detected in the genital tract tissue by 7 days after a primary genital tract infection with the chlamydial agent of guinea pig inclusion conjunctivitis. When antigen-specific activity was assessed at different times after infection, the intensity of the response of genital tract-associated T lymphocytes was directly proportional to the degree of resistance of the animals to genital challenge. Thus, susceptibility of animals to reinfection by chlamydiae appears to be associated with the intensity of the local T cell-mediated immune responses in the genital tract of infected animals.

Resolution of chlamydial genital infection with antigen-specific T-lymphocyte lines

To determine cell-mediated immune mechanisms involved in the resolution of chlamydial genital infection of mice, we utilized an established murine model in which it has been demonstrated that resolution of infection occurs independently of the antibody response. Splenic T lymphocytes were obtained from mice that had previously been immunized with viable elementary bodies of the mouse pneumonitis agent (MoPn), a Chlamydia trachomatis biovar. Antigen-reactive T lymphocytes were maintained and expanded in vitro by frequent restimulation with UV light-inactivated MoPn in the presence of antigen-presenting cells and recombinant interleukin-2 (rIL-2). Flow cytometry indicated that this cell line was at least 92% positive for the pan-specific T-cell marker Thy1.2. Stimulation of the cells in the presence of syngeneic antigen-presenting cells plus MoPn antigen and in the absence of exogenous IL-2 induced the cells to produce IL-2 activity in culture supernatants. Following adoptive transfer, this T-lymphocyte line was effective in inducing resolution of an ongoing MoPn genital infection in congenitally athymic nude mice which otherwise maintain chronic unresolved infections. The line was less efficient in resolving the infection after longer periods in culture. An additional T-lymphocyte line was derived from the spleens of athymic mice that had received the first line and had resolved the infection. These T cells were also capable of inducing resolution of the infection. Lastly, this cell line was treated with specific antibody and complement to delete either CD4+ or CD8+ T lymphocytes in an attempt to enrich for T-cell subpopulations prior to transfer into infected athymic mice. The anti-CD4-treated line was essentially depleted of CD4 cells, while the anti-CD8-treated line was only partially enriched for CD4 cells, with a large proportion of CD8 cells still present. Nude mice that received either of the treated T-cell lines or the parental cell line were capable of resolving the infection, although the line with increased numbers of CD4 cells was more efficient than either the parental line or the CD8 line.

Isolation of Chlamydia pneumoniae from the lungs of patients infected with the human immunodeficiency virus

Chlamydia pneumoniae is being recognized as a common cause of respiratory tract infections. Bronchoalveolar lavage specimens from human immunodeficiency virus-infected patients were examined by culture for this pathogen. Of 50 specimens examined, 5 (10%) were positive for C. pneumoniae. Four of these (80%) were also positive for other pathogens frequently implicated as causes of respiratory disease in this patient population. C. pneumoniae may frequently inhabit the respiratory tracts of human immunodeficiency virus-infected individuals.

Immunization against chlamydial genital infection in guinea pigs with UV-inactivated and viable chlamydiae administered by different routes

Female guinea pigs were immunized with viable or UV light-inactivated chlamydiae (agent of guinea pig inclusion conjunctivitis), belonging to the species Chlamydia psittaci, by intravenous, subcutaneous, oral, or ocular routes. All animals were then inoculated vaginally with viable chlamydiae to determine the extent of protection against challenge infection induced by the various regimens. The course of genital infection was significantly reduced in intensity in all groups of animals except the unimmunized controls and those animals immunized orally with inactivated antigen. Guinea pigs immunized with viable antigen were more likely to develop resistance to challenge infection and, in general, had a significantly greater degree of protection than animals immunized with inactivated antigen. No one route seemed superior in producing a protective response. Animals in all groups demonstrating protection developed serum and secretion immunoglobulin G antibody responses to chlamydiae. Lymphocyte proliferative reactions to chlamydial antigen were variable among groups. Immunoblot analysis of serum and secretions indicated a wide range of antibody specificities, but most protected animals produced antibodies to the major outer membrane protein, lipopolysaccharide, and the 61-kilodalton protein. No definitive associations could be made between the increased ability of immunization with viable organisms to produce resistance to challenge infection and a particular immune parameter. These data indicate that viable chlamydiae given by various routes are able to induce a strong immune response which can provide resistance against reinfection in some cases or at least reduce the degree of infection to a greater degree than inactivated antigen. However, complete resistance to genital tract infection may be difficult to obtain and alternate immunizations strategies may have to be developed.

Humoral immune response to chlamydial genital infection of mice with the agent of mouse pneumonitis

The objective of this study was to characterize the humoral immune response to chlamydial genital infection of mice with the mouse pneumonitis agent (MoPn). With an enzyme-linked immunoabsorbent assay, immunoglobulin G antibodies to MoPn were first detected in plasma by day 14. Peak plasma antibody concentrations were reached by day 49, and this response did not decline significantly throughout the 300-day monitoring period. Immunoglobulin A against MoPn could first be detected in pooled vaginal washes by day 21 after infection and had reached peak concentrations by day 28, but anti-MoPn immunoglobulin G was not consistently present in secretions. The antibody response in secretions had declined slightly by day 300. Immunoblot analysis revealed that the early phase of the plasma antibody response to MoPn as a result of genital infection was against lipopolysaccharide, the major outer membrane protein, and a 62-kilodalton (kDa) protein. In secretions, early-phase immunoglobulin A antibodies were directed to the major outer membrane protein and lipopolysaccharide. Late reactions to 15-, 22-, and 83-kDa proteins in plasma were noted. Late reactions to the 62-kDa protein in secretions were also noted. The cause of these late responses remains unexplained. When mice were challenged intravaginally with MoPn at 50-day intervals after the primary infection, it was found that mice inoculated on day 100 or after were susceptible to reinfection. Susceptibility could not be related to a decline in the antibody concentration in plasma or secretions or in the antibody response to specific components of MoPn as measured by immunoblot analysis.