Clearance of Chlamydia trachomatis from the murine genital mucosa does not require perforin-mediated cytolysis or Fas-mediated apoptosis

A Class II-Restricted CD8γ13 T-Cell Clone Protects During Chlamydia muridarum Genital Tract Infection

BACKGROUND

The T-cell response to chlamydia genital tract infections in humans and mice is unusual because the majority of antigen-specific CD8 T cells are not class I restricted (referred to here as "unrestricted" or "atypical"). We previously reported that a subset of unrestricted murine chlamydia-specific CD8 T cells had a cytokine polarization pattern that included interferon (IFN)-γ and interleukin (IL)-13.

METHODS

In this study, we investigated the transcriptome of CD8γ13 T cells, comparing them to Tc1 clones using microarray analysis. That study revealed that CD8γ13 polarization included IL-5 in addition to IFN-γ and IL-13. Adoptive transfer studies were performed with Tc1 clones and a CD8γ13 T-cell clone to determine whether either influenced bacterial clearance or immunopathology during Chlamydia muridarum genital tract infections.

RESULTS

To our surprise, an adoptively transferred CD8γ13 T-cell clone was remarkably proficient at preventing chlamydia immunopathology, whereas the multifunctional Tc1 clone did not enhance clearance or significantly alter immunopathology. Mapping studies with major histocompatibility complex (MHC) class I- and class II-deficient splenocytes showed our previously published chlamydia-specific CD8 T-cell clones are MHC class II restricted.

CONCLUSIONS

The MHC class II-restricted CD8 T cells may play an important role in protection from intracellular pathogens that limit class I antigen presentation or diminish CD4 T-cell numbers or impair their function.

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.

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.

An Ancient Molecular Arms Race: Chlamydia vs. Membrane Attack Complex/Perforin (MACPF) Domain Proteins

Dynamic interactions that govern the balance between host and pathogen determine the outcome of infection and are shaped by evolutionary pressures. Eukaryotic hosts have evolved elaborate and formidable defense mechanisms that provide the basis for innate and adaptive immunity. Proteins containing a membrane attack complex/Perforin (MACPF) domain represent an important class of immune effectors. These pore-forming proteins induce cell killing by targeting microbial or host membranes. Intracellular bacteria can be shielded from MACPF-mediated killing, and Chlamydia spp. represent a successful paradigm of obligate intracellular parasitism. Ancestors of present-day Chlamydia likely originated at evolutionary times that correlated with or preceded many host defense pathways. We discuss the current knowledge regarding how chlamydiae interact with the MACPF proteins Complement C9, Perforin-1, and Perforin-2. Current evidence indicates a degree of resistance by Chlamydia to MACPF effector mechanisms. In fact, chlamydiae have acquired and adapted their own MACPF-domain protein to facilitate infection.

CD8+ T cells define an unexpected role in live-attenuated vaccine protective immunity against Chlamydia trachomatis infection in macaques

Trachoma, caused by the obligate intracellular organism Chlamydia trachomatis, is the world's leading cause of preventable blindness for which a vaccine is needed. We have previously shown that a plasmid-deficient live-attenuated trachoma vaccine delivered ocularly to macaques elicited either solid or partial protective immunity against a virulent ocular challenge. Solidly protected macaques shared the same MHC class II alleles implicating CD4(+) T cells in superior protective immunity. Understandably, we sought to define T cell immune correlates in these animals to potentially improve vaccine efficacy. In this study, following a 2-y resting period, these macaques were boosted i.m. with the live-attenuated trachoma vaccine and their peripheral T cell anamnestic responses studied. Both solidly and partially protected macaques exhibited a CD4(+) and CD8(+) T cell anamnestic response following booster immunization. CD8(+) but not CD4(+) T cells from solidly protected macaques proliferated against soluble chlamydial Ag. We observed a more rapid T cell inflammatory cytokine response in tears of solidly protected animals following ocular rechallenge. Most notably, depletion of CD8(+) T cells in solidly protected macaques completely abrogated protective immunity. Collectively, our findings support the conclusion that CD8(+) T cells play an important but unexpected role in live-attenuated trachoma vaccine-mediated protective immunity.

Perforin is detrimental to controlling [corrected] C. muridarum replication in vitro, but not in vivo

CD4 T cells are critical for clearing experimental Chlamydia muridarum genital tract infections. Two independent in vitro CD4 T cell mechanisms have been identified for terminating Chlamydia replication in epithelial cells. One mechanism, requiring IFN-γ and T cell-epithelial cell contact, terminates infection by triggering epithelial production of nitric oxide to chlamydiacidal levels; the second is dependent on T cell degranulation. We recently demonstrated that there are two independent in vivo clearance mechanisms singly sufficient for clearing genital tract infections within six weeks; one dependent on iNOS, the other on Plac8. Redundant genital tract clearance mechanisms bring into question negative results in single-gene knockout mice. Two groups have shown that perforin-knockout mice were not compromised in their ability to clear C. muridarum genital tract infections. Because cell lysis would be detrimental to epithelial nitric oxide production we hypothesized that perforin was not critical for iNOS-dependent clearance, but posited that perforin could play a role in Plac8-dependent clearance. We tested whether the Plac8-dependent clearance was perforin-dependent by pharmacologically inhibiting iNOS in perforin-knockout mice. In vitro we found that perforin was detrimental to iNOS-dependent CD4 T cell termination of Chlamydia replication in epithelial cells. In vivo, unexpectedly, clearance in perforin knockout mice was delayed to the end of week 7 regardless of iNOS status. The discordant in vitro/in vivo results suggest that the perforin's contribution to bacterial clearance in vivo is not though enhancing CD4 T cell termination of Chlamydia replication in epithelial cells, but likely via a mechanism independent of T cell-epithelial cell interactions.

Vaginal chlamydial clearance following primary or secondary infection in mice occurs independently of TNF-α

The role of TNF-α in chlamydial clearance is uncertain. Antibody-mediated depletion of TNF-α in mice and guinea pigs has been shown not to significantly affect chlamydial clearance, whereas production of TNF-α in addition to IFN-γ from T cells has been shown to correlate with enhanced clearance. The aim of our study is to evaluate the mechanistic role of TNF-α in clearance of primary and secondary chlamydial infection from the genital tract (GT) using C57BL/6 TNF-α deficient (TNF-α^−/−) and wild type (WT) mice. Chlamydial shedding from the lower GT was evaluated following primary and secondary intravaginal challenge. Also, antibody and antigen specific cytokine responses were analyzed from the infected GT and spleens, and oviduct pathology determined to analyze the role of TNF-α in upper GT pathological sequelae. MHC II^−/− mice, known to display muted adaptive immune responses and failure to resolve genital chlamydial infections, were used as a negative control. Following both primary and secondary genital chlamydial infection, TNF-α^−/− mice exhibited elevated granzyme B production, but similar IFN-γ and antibody responses. Importantly, absence of TNF-α did not significantly alter the resolution of infection. However, TNF-α^−/− mice displayed significantly reduced upper genital tract (UGT) pathology compared to WT mice. This study demonstrates mechanistically that optimal chlamydial clearance following primary and secondary chlamydial genital infection can occur in the complete absence of TNF-α, and considered with the reduction of upper GT pathology in TNF-α^−/− mice, suggests that targeted induction of anti-chlamydial TNF-α responses by vaccination may be unnecessary, and moreover could be potentially pathogenic.

The major CD8 T cell effector memory subset in the normal and Chlamydia trachomatis-infected human endocervix is low in perforin

BACKGROUND

The local tissue microenvironment plays an important role in the induction, homing, maintenance and development of effector functions of T cells. Thus, site-specific differences in phenotypes of mucosal and systemic T cell populations have been observed. Chlamydia trachomatis most commonly infects the endocervix in women, yet little is known about Chlamydia-specific effector T cell immunity at this unique mucosal site. Our previous flow-cytometry-based study of cervical-cytobrush retrieved cells indicated that CD8 T cells are significantly increased in the C. trachomatis-infected human endocervix. The cytolytic function of CD8 T cells is important in the protective immunity against many intracellular pathogens, and requires the cytolytic granule perforin to facilitate the entry of other molecules that mediate the lysis of target cells. Determination of perforin expression of the CD8 T cell population in the endocervix would therefore provide insights on the granule-mediated cytolytic potential of these cells at this site.

RESULTS

Our histological data revealed that C. trachomatis-infected tissues have significantly higher numbers of CD3 and CD8 T cells compared to non-infected tissues (p<0.01), and that the majority of CD8+ cells do not express perforin in situ. A subsequent flow cytometric analysis of paired blood and endocervix-derived cells (n=16) revealed that while all the CD8 T cell subsets: naïve, effector memory (TEM), central memory (TCM) and terminally differentiated effector memory (TEMRA) can be found in the blood, the endocervix is populated mainly by the TEM CD8 T cell subset. Our data also showed that perforin expression in the TEM population is significantly lower in the endocervix than in the blood of C. trachomatis positive women (n=15; p<0.0001), as well as in C. trachomatis-negative individuals (n=6; p<0.05). Interestingly, our in vitro co-culture study suggests that the exposure of HeLa 229 cervical epithelial cells to IFN gamma could potentially induce a decrease in perforin content in CD8 TEM cells in the same microenvironment.

CONCLUSIONS

The low perforin content of CD8 TEM cells in the endocervix, the local site of C. trachomatis infection in women, may reflect the unique immunological environment that balances immune protection against sexually transmitted infections and immune- tolerance to support conception.

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).

Enhanced upper genital tract pathologies by blocking Tim-3 and PD-L1 signaling pathways in mice intravaginally infected with Chlamydia muridarum

Background

Although Tim-3 & PD-L1 signaling pathways play important roles in negatively regulating immune responses, their roles in chlamydial infection have not been evaluated.

Methods

Neutralization antibodies targeting Tim-3 and PD-L1 were used to treat mice. Following an intravaginal infection with C. muridarum organisms, mice with or without the dual antibody treatment were compared for live chlamydial organism shedding from the lower genital tract and inflammatory pathology in the upper genital tract.

Results

Mice treated with anti-Tim-3 and anti-PD-L1 antibodies displayed a time course of live organism shedding similar to that of mice treated with equivalent amounts of isotype-matched IgG molecules. The combined antibody blocking failed to alter either the lower genital tract cytokine or systemic humoral and cellular adaptive responses to C. muridarum infection. However, the antibody blocking significantly enhanced C. muridarum-induced pathologies in the upper genital tract, including more significant hydrosalpinx and inflammatory infiltration in uterine horn and oviduct tissues.

Conclusions

The Tim-3 and PD-L1-mediated signaling can significantly reduce pathologies in the upper genital tract without suppressing immunity against chlamydial infection, suggesting that Tim-3 and PD-L1-mediated negative regulation may be manipulated to attenuate tubal pathologies in women persistently infected with C. trachomatis organisms.

Protective immunity against mouse upper genital tract pathology correlates with high IFNγ but low IL-17 T cell and anti-secretion protein antibody responses induced by replicating chlamydial organisms in the airway

To search for optimal immunization conditions for inducing protective immunity against upper genital tract pathologies caused by chlamydial intravaginal infection, we compared protection efficacy in mice immunized intranasally or intramuscularly with live or inactivated Chlamydia muridarum organisms. Mice immunized intranasally with live organisms developed strong protection against both vaginal shedding of infectious organisms and upper genital tract pathologies. The protection correlated with a robust antigen-specific T cell response with high IFNγ but low IL-17. Although a significant level of IL-5 was also detected, these mice maintained an overall Th1-dorminant immunity following immunization and challenge infection. On the contrary, mice immunized intranasally with inactivated organisms or intramuscularly with live or inactivated organisms produced high levels of IL-17 and still developed significant upper genital tract pathologies. High titers of antibodies against chlamydial secretion antigens were detected only in mice immunized intranasally with live organisms but not mice in other groups, suggesting that the intranasally inoculated live organisms were able to undergo replication and immune responses to the chlamydial secretion proteins may contribute to protective immunity. These observations have provided important information on how to develop subunit vaccines for inducing protective immunity against urogenital infection with Chlamydia trachomatis organisms.

Tumor necrosis factor alpha production from CD8+ T cells mediates oviduct pathological sequelae following primary genital Chlamydia muridarum infection

The immunopathogenesis of Chlamydia trachomatis-induced oviduct pathological sequelae is not well understood. Mice genetically deficient in perforin (perforin(-/-) mice) or tumor necrosis factor alpha (TNF-α) production (TNF-α(-/-) mice) displayed comparable vaginal chlamydial clearance rates but significantly reduced oviduct pathology (hydrosalpinx) compared to that of wild-type mice. Since both perforin and TNF-α are effector mechanisms of CD8(+) T cells, we evaluated the role of CD8(+) T cells during genital Chlamydia muridarum infection and oviduct sequelae. Following vaginal chlamydial challenge, (i) mice deficient in TAP I (and therefore the major histocompatibility complex [MHC] I pathway and CD8(+) T cells), (ii) wild-type mice depleted of CD8(+) T cells, and (iii) mice genetically deficient in CD8 (CD8(-/-) mice) all displayed similar levels of vaginal chlamydial clearance but significantly reduced hydrosalpinx, compared to those of wild-type C57BL/6 mice, suggesting a role for CD8(+) T cells in chlamydial pathogenesis. Repletion of CD8(-/-) mice with wild-type or perforin(-/-), but not TNF-α(-/-), CD8(+) T cells at the time of challenge restored hydrosalpinx to levels observed in wild-type C57BL/6 mice, suggesting that TNF-α production from CD8(+) T cells is important for pathogenesis. Additionally, repletion of TNF-α(-/-) mice with TNF-α(+/+) CD8(+) T cells significantly enhanced the incidence of hydrosalpinx and oviduct dilatation compared to those of TNF-α(-/-) mice but not to the levels found in wild-type mice, suggesting that TNF-α production from CD8(+) T cells and non-CD8(+) cells cooperates to induce optimal oviduct pathology following genital chlamydial infection. These results provide compelling new evidence supporting the contribution of CD8(+) T cells and TNF-α production to Chlamydia-induced reproductive tract sequelae.

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.

Chlamydia vaccine candidates and tools for chlamydial antigen discovery

The failure of the inactivated Chlamydia-based vaccine trials in the 1960s has led researchers studying Chlamydia to take cautious and rational approaches to develop safe and effective chlamydial vaccines. Subsequent research efforts focused on three areas. The first is the analysis of the immunobiology of chlamydial infection in animal models, with supporting clinical studies, to identify the immune correlates of both protective immunity and pathological responses. Second, recent radical improvements in genomics, proteomics and associated technologies have assisted in the implementation of creative approaches to search for suitable vaccine candidates. Third, progress in the analysis of host response and adjuvanticity regulating both innate and adaptive immunity at the mucosal site of infection has led to progress in the design of optimal delivery and adjuvant systems for enhancing protective immunity. Considerable progress has been made in the first two areas but research efforts to better define the factors that regulate immunity at mucosal sites of infection and to develop strategies to boost protective immunity via immunomodulation, effective delivery systems and potent adjuvants, have remained elusive. In this article, we will summarize progress in these areas with a focus on chlamydial vaccine antigen discovery, and discuss future directions towards the development of a safe and effective chlamydial vaccine.

Induction of protective immunity by vaccination against Chlamydia trachomatis using the major outer membrane protein adjuvanted with CpG oligodeoxynucleotide coupled to the nontoxic B subunit of cholera toxin

The present study was undertaken to test the efficacy of immunization with the native major outer membrane protein (nMOMP) of Chlamydia trachomatis mouse pneumonitis (MoPn) serovar in combination with a novel immunostimulatory adjuvant consisting of CpG oligodeoxynucleotide (ODN) linked to the nontoxic B subunit of cholera toxin (CTB-CpG) to elicit a protective immune response to C. trachomatis. High levels of Chlamydia-specific IgG antibodies were detected in the sera from BALB/c mice immunized intramuscularly and subcutaneously (i.m.+s.c.) with the nMOMP/CTB-CpG vaccine or with nMOMP adjuvanted with a mixture of CT and CpG ODN (CT+CpG). Further, these immunization schemes gave rise to significant T-cell-mediated Chlamydia-specific immune responses. No Chlamydia-specific humoral or cell-mediated immune responses were detected in the control mice vaccinated with ovalbumin together with either CTB-CpG or CT+CpG. Following an intranasal challenge with C. trachomatis the groups of mice immunized with nMOMP plus CTB-CpG, CT+CpG or live C. trachomatis were found to be protected based on their change in body weight and lung weight as well as number of inclusion forming unit recovered from the lungs, as compared with control groups immunized with ovalbumin plus either adjuvants. Interestingly, IFN-gamma-producing CD4(+), but not CD8(+), T-cells showed a significant correlation with the outcomes of the challenge. In conclusion, nMOMP in combination with the novel adjuvant CTB-CpG elicited a significant antigen-specific antibody and cell-mediated immune responses as well as protection against a pulmonary challenge with C. trachomatis.

Human guanylate binding proteins potentiate the anti-chlamydia effects of interferon-gamma

Chlamydiae are obligate intracellular pathogens that are sensitive to pro-inflammatory cytokine interferon-gamma. IFN-gamma-inducible murine p47 GTPases have been demonstrated to function in resistance to chlamydia infection in vivo and in vitro. Because the human genome does not encode IFN-gamma-inducible homologues of these proteins, the significance of the p47 GTPase findings to chlamydia pathogenesis in humans is unclear. Here we report a pair of IFN-gamma-inducible proteins, the human guanylate binding proteins (hGBPs) 1 and 2 that potentiate the anti-chlamydial properties of IFN-gamma. hGBP1 and 2 localize to the inclusion membrane, and their anti-chlamydial functions required the GTPase domain. Alone, hGBP1 or 2 have mild, but statistically significant and reproducible negative effects on the growth of Chlamydia trachomatis, whilst having potent anti-chlamydial activity in conjunction with treatment with a sub-inhibitory concentration of IFN-gamma. Thus, hGBPs appear to potentiate the anti-chlamydial effects of IFN-gamma. Indeed, depletion of hGBP1 and 2 in cells treated with IFN-gamma led to an increase in inclusion size, indicative of better growth. Interestingly, chlamydia species/strains harboring the full-length version of the putative cytotoxin gene, which has been suggested to confer resistance to IFN-gamma was not affected by hGBP overexpression. These findings identify the guanylate binding proteins as potentiators of IFN-gamma inhibition of C. trachomatis growth, and may be the targets of the chlamydial cytotoxin.

Involvement of CD252 (CD134L) and IL-2 in the expression of cytotoxic proteins in bacterial- or viral-activated human T cells

Regulation of cytotoxic effector molecule expression in human CTLs after viral or bacterial activation is poorly understood. By using human autologous dendritic cells (DCs) to prime T lymphocytes, we found perforin only highly up-regulated in virus- (HSV-1, vaccinia virus) but not in intracellular bacteria- (Listeria innocua, Listeria monocytogenes, Mycobacterium tuberculosis, Chlamydophila pneumoniae) activated CTLs. In contrast, larger quantities of IFN-gamma and TNF-alpha were produced in Listeria-stimulated cultures. Granzyme B and granulysin were similarly up-regulated by all tested viruses and intracellular bacteria. DCs infected with HSV-1 showed enhanced surface expression of the costimulatory molecule CD252 (CD134L) compared with Listeria-infected DC and induced enhanced secretion of IL-2. Adding blocking CD134 or neutralizing IL-2 Abs during T cell activation reduced the HSV-dependent up-regulation of perforin. These data indicate a distinct CTL effector function in response to intracellular pathogens triggered via differing endogenous IL-2 production upon costimulation through CD252.

A MyD88-dependent early IL-17 production protects mice against airway infection with the obligate intracellular pathogen Chlamydia muridarum

We found that IL-17, a signature cytokine of Th17, was produced early in the innate immunity phase after an intranasal infection with the obligate intracellular pathogen Chlamydia muridarum. The airway IL-17, which peaked at 48 h after infection, was dependent on live chlamydial organism replication and MyD88-mediated signaling pathways. Treatment with antibiotics or knockout of the MyD88 gene, but not Toll/IL receptor domain-containing adapter-inducing IFN-beta, can block the early IL-17 production. Treatment of mice with an anti-IL-17-neutralizing mAb enhanced growth of chlamydial organisms in the lung, dissemination to other organs, and decreased mouse survival, whereas treatment with an isotype-matched control IgG had no effect. Although IL-17 did not directly affect chlamydial growth in cell culture, it enhanced the production of other inflammatory cytokines and chemokines by Chlamydia-infected cells and promoted neutrophil infiltration in mouse airways during chlamydial infection, which may contribute to the antichlamydial effect of IL-17. These observations suggest that an early IL-17 response as an innate immunity component plays an important role in initiating host defense against infection with intracellular bacterial pathogens in the airway.

Novel Chlamydia muridarum T cell antigens induce protective immunity against lung and genital tract infection in murine models

Using a combination of affinity chromatography and tandem mass spectrometry, we recently identified 8 MHC class II (I-A(b)) -bound Chlamydia peptides eluted from dendritic cells (DCs) infected with Chlamydia muridarum. In this study we cloned and purified the source proteins that contained each of these peptides and determined that three of the eight peptide/protein Ags were immunodominant (PmpG-1, RplF, and PmpE/F-2) as identified by IFN-gamma ELISPOT assay using splenocytes from C57BL/6 mice recovered from C. muridarum infection. To evaluate whether the three immunodominant Chlamydia protein Ags were also able to protect mice against Chlamydia infection in vivo, we adoptively transferred LPS-matured DCs transfected ex vivo with the cationic liposome DOTAP (N-[1-(2,3-dioleoyloxy)propyl]-N,N,N-trimethylammonium methyl-sulfate) and individual PmpG-1(25-500aa), RplF, or PmpE/F-2 (25-575 aa) proteins. The results showed that the transfected Chlamydia proteins were efficiently delivered intracellularly into DCs. Mice vaccinated with DCs transfected with individual Chlamydia protein PmpG-1(25-500), RplF, or PmpE/F-2(25-575) exhibited significant resistance to challenge infection as indicated by reduction in the median Chlamydia inclusion forming units in both the lung and genital tract models. The major outer membrane protein was used as a reference Ag but conferred significant protection only in the genital tract model. Overall, vaccination with DCs transfected with PmpG-1(25-500) exhibited the greatest degree of protective immunity among the four Chlamydia Ags tested. This study demonstrates that T cell peptide Ags identified by immunoproteomics can be successfully exploited as T cell protein-based subunit vaccines and that PmpG-1(25-500) protein may be a suitable vaccine candidate for further evaluation.

A chlamydial type III-secreted effector protein (Tarp) is predominantly recognized by antibodies from humans infected with Chlamydia trachomatis and induces protective immunity against upper genital tract pathologies in mice

Chlamydia trachomatis genome is predicted to encode a type III secretion system consisting of more than 40 open reading frames (ORFs). To test whether these ORFs are expressed and immunogenic during chlamydial infection in humans, we expressed 55 chlamydial ORFs covering all putative type III secretion components plus control molecules as fusion proteins and measured the reactivity of these fusion proteins with antibodies from patients infected with C. trachomatis in the urogenital tract (24 antisera) or in the ocular tissue (8 antisera). Forty-five of the 55 proteins were recognized by at least 1 of the 32 human antisera, suggesting that these proteins are both expressed and immunogenic during chlamydial infection in humans. Tarp, a putative type III secretion effector protein, was identified as a novel immunodominant antigen due to its reactivity with the human antisera at high frequency and titer. The expression and immunogenicity of Tarp were confirmed in cell culture and mouse systems. Tarp was mainly associated with the infectious form of chlamydial organisms and became undetectable between 13 and 24 h during the infection cycle in cell culture. Mice intravaginally infected with C. muridarum developed Tarp-specific humoral and cellular immune responses. More importantly, immunization of mice with Tarp induced Th1-dominant immunity that significantly reduced the shedding of live organisms from the lower genital tract and attenuated inflammatory pathologies in the fallopian tube tissues. These observations have demonstrated that Tarp, an immunodominant antigen identified by human antisera, can induce protective immunity against chlamydial infection and pathology in mice.

Role of CD8(+)T cells in the host response to Chlamydia

Chlamydia infections constitute a major public health problem. Although multiple arms of the immune system participate in the control of Chlamydia in infected hosts, T lymphocytes are essential. This review focuses on the roles that CD8(+)T cells may play in immunoprotection and immunopathology following recognition of Chlamydia-infected cells.

Antigen-specific CD4+ T cells produce sufficient IFN-gamma to mediate robust protective immunity against genital Chlamydia muridarum infection

Chlamydia has been shown to evade host-specific IFN-gamma-mediated bacterial killing; however, IFN-gamma-deficient mice exhibit suboptimal late phase vaginal Chlamydia muridarum clearance, greater dissemination, and oviduct pathology. These findings introduce constraints in understanding results from murine chlamydial vaccination studies in context of potential implications to humans. In this study, we used mice deficient in either IFN-gamma or the IFN-gamma receptor for intranasal vaccination with a defined secreted chlamydial Ag, chlamydial protease-like activity factor (CPAF), plus CpG and examined the role of IFN-gamma derived from adoptively transferred Ag-specific CD4+ T cells in protective immunity against genital C. muridarum infection. We found that early Ag-specific IFN-gamma induction and CD4+ T cell infiltration correlates with the onset of genital chlamydial clearance. Adoptively transferred IFN-gamma competent CPAF-specific CD4+ T cells failed to enhance the resolution of genital chlamydial infection within recipient IFN-gamma receptor-deficient mice. Conversely, IFN-gamma production from adoptively transferred CPAF-specific CD4+ T cells was sufficient in IFN-gamma-deficient mice to induce early resolution of infection and reduction of subsequent pathology. These results provide the first direct evidence that enhanced anti-C. muridarum protective immunity induced by Ag-specific CD4+ T cells is dependent upon IFN-gamma signaling and that such cells produce sufficient IFN-gamma to mediate the protective effects. Additionally, MHC class II pathway was sufficient for induction of robust protective anti-C. muridarum immunity. Thus, targeting soluble candidate Ags via MHC class II to CD4+ T cells may be a viable vaccine strategy to induce optimal IFN-gamma production for effective protective immunity against human genital chlamydial infection.

The chlamydial plasmid-encoded protein pgp3 is secreted into the cytosol of Chlamydia-infected cells

The chlamydial cryptic plasmid encodes eight putative open reading frames (ORFs), designated pORF1 to -8. Antibodies raised against these ORF proteins were used to localize the endogenous proteins during chlamydial infection. We found that the pORF5 protein (also known as pgp3) was detected mainly in the cytosol of Chlamydia-infected cells, while the remaining seven proteins were found inside the chlamydial inclusions only. The pgp3 distribution pattern in the host cell cytosol is similar to but not overlapping with that of chlamydial protease/proteasome-like activity factor (CPAF), a chlamydial genome-encoded protein known to be secreted from chlamydial inclusions into the host cell cytosol. The anti-pgp3 labeling was removed by preabsorption with pgp3 but not CPAF fusion proteins and vice versa, demonstrating that pgp3 is a unique secretion protein. This conclusion is further supported by the observation that pgp3 was highly enriched in cytosolic fractions and had a minimal presence in the inclusion-containing nuclear fractions prepared from Chlamydia-infected cells. The pgp3 protein was detected as early as 12 h after infection and was secreted by all chlamydial species that carry the cryptic plasmid, suggesting that there is a selection pressure for maintaining pgp3 secretion during chlamydial infection. Although expression of pgp3 in the host cell cytosol via a transgene did not alter the susceptibility of the transfected cells to the subsequent chlamydial infection, purified pgp3 protein stimulated macrophages to release inflammatory cytokines, suggesting that pgp3 may contribute to chlamydial pathogenesis.

Caspase-1 contributes to Chlamydia trachomatis-induced upper urogenital tract inflammatory pathologies without affecting the course of infection

Chlamydia trachomatis infection induces inflammatory pathologies in the upper genital tract, potentially leading to ectopic pregnancy and infertility in the affected women. Caspase-1 is required for processing and release of the inflammatory cytokines interleukin-1beta (IL-1beta), IL-18, and possibly IL-33. In the present study, we evaluated the role of caspase-1 in chlamydial infection and pathogenesis. Although chlamydial infection induced caspase-1 activation and processing of IL-1beta, mice competent and mice deficient in caspase-1 experienced similar courses of chlamydial infection in their urogenital tracts, suggesting that Chlamydia-activated caspase-1 did not play a significant role in resolution of chlamydial infection. However, when genital tract tissue pathologies were examined, the caspase-1-deficient mice displayed much reduced inflammatory damage. The reduction in inflammation was most obvious in the fallopian tube tissue. These observations demonstrated that although caspase-1 is not required for controlling chlamydial infection, caspase-1-mediated responses can exacerbate the Chlamydia-induced inflammatory pathologies in the upper genital tract, suggesting that the host caspase-1 may be targeted for selectively attenuating chlamydial pathogenicity without affecting the host defense against chlamydial infection.

Chlamydia trachomatisInfection Alters the Development of Memory CD8+T Cells

The obligate intracellular bacterium Chlamydia trachomatis is the most common cause of bacterial sexually transmitted disease in the United States and the leading cause of preventable blindness worldwide. Prior exposure to C. trachomatis has been shown to provide incomplete protection against subsequent infection. One possible explanation for the limited immunity afforded by prior C. trachomatis infection is poor activation of Chlamydia-specific memory CD8+ T cells. In this study, we examined the development of CD8+ memory T cell responses specific for the Chlamydia Ag CrpA. The percentage of CrpA63-71-specific T cells expressing an effector memory T cell phenotype (IL-7R+ CD62low) was dramatically diminished in mice immunized with C. trachomatis, compared with mice immunized with vaccinia virus expressing the CrpA protein. These alterations in memory T cell development were correlated with a significant reduction in the capacity of convalescent mice to mount an enhanced recall response to Chlamydia Ags, compared with the primary response. CrpA-specific memory T cells primed during VacCrpA infection also failed to respond to a challenge with Chlamydia. We therefore investigated whether C. trachomatis infection might have a global inhibitory effect on CD8+ T cell activation by coinfecting mice with C. trachomatis and Listeria monocytogenes and we found that the activation of Listeria-specific naive and memory CD8+ T cells was reduced in the presence of C. trachomatis. Together, these results suggest that Chlamydia is able to alter the development of CD8+ T cell responses during both primary and secondary infection, perhaps accounting for the incomplete protection provided by prior Chlamydia infection.

The hypothetical protein CT813 is localized in the Chlamydia trachomatis inclusion membrane and is immunogenic in women urogenitally infected with C. trachomatis

Using antibodies raised with chlamydial fusion proteins, we have localized a protein encoded by hypothetical open reading frame CT813 in the inclusion membrane of Chlamydia trachomatis. The detection of the C. trachomatis inclusion membrane by an anti-CT813 antibody was blocked by the CT813 protein but not unrelated fusion proteins. The CT813 protein was detected as early as 12 h after chlamydial infection and was present in the inclusion membrane during the entire growth cycle. All tested serovars from C. trachomatis but not other chlamydial species expressed the CT813 protein. Exogenously expressed CT813 protein in HeLa cells displayed a cytoskeleton-like structure similar to but not overlapping with host cell intermediate filaments, suggesting that the CT813 protein is able to either polymerize or associate with host cell cytoskeletal structures. Finally, women with C. trachomatis urogenital infection developed high titers of antibodies to the CT813 protein, demonstrating that the CT813 protein is not only expressed but also immunogenic during chlamydial infection in humans. In all, the CT813 protein is an inclusion membrane protein unique to C. trachomatis species and has the potential to interact with host cells and induce host immune responses during natural infection. Thus, the CT813 protein may represent an important candidate for understanding C. trachomatis pathogenesis and developing intervention and prevention strategies for controlling C. trachomatis infection.

Immunology of Chlamydia infection: implications for a Chlamydia trachomatis vaccine

Sexually transmitted Chlamydia trachomatis infections are a serious public-health problem. With more than 90 million new cases occurring annually, C. trachomatis is the most common cause of bacterial sexually transmitted disease worldwide. Recent progress in elucidating the immunobiology of Chlamydia muridarum infection of mice has helped to guide the interpretation of immunological findings in studies of human C. trachomatis infection and has led to the development of a common model of immunity. In this review, we describe our current understanding of the immune response to infection with Chlamydia spp. and how this information is improving the prospects for development of a vaccine against infection with C. trachomatis.

Perforin and lymphohistiocytic proliferative disorders

Perforin is critical for cytotoxicity mediated by granules present in natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). Perforin-deficient mice have impaired cytotoxicity by NK cells and CTLs, resulting in failure to control infections with certain viruses or bacteria. Infection of perforin-deficient mice with lymphocytic choriomeningitis virus results in haemophagocytic lymphohistiocytosis and elevated levels of pro-inflammatory cytokines. Mutations throughout the perforin gene have been identified in patients with familial haemophagocytic lymphohistiocytosis (FHL) type 2. These patients present with fever, hepatosplenomegaly, pancytopenia, have marked elevations of T-helper type 1 and type 2 cytokines, and have impaired NK cell and CTL cytotoxicity. A number of infectious pathogens have been implicated as triggering the onset of disease. Identification of mutations in perforin as the cause of FHL should allow prenatal diagnosis of the disorder. While stem cell transplantation is curative, gene therapy might be effective in the future.

Cytokine and fibrogenic gene expression in the conjunctivas of subjects from a Gambian community where trachoma is endemic

The role of immunity in blinding trachoma is unclear. Conjunctival gene expression was measured in a population where trachoma is endemic. Proinflammatory (tumor necrosis factor alpha and interleukin-1beta [IL-1beta]), anti-inflammatory (IL-10), and fibrogenic (matrix metalloprotease 9) gene expression was increased in active trachoma. Markers indicative of T-cell response (gamma interferon, IL-4, IL-12p40, and perforin) were increased when chlamydial infection was present.

Human CD8+T Cells Recognize the 60-kDa Cysteine-Rich Outer Membrane Protein fromChlamydia trachomatis

The intracellular bacterial pathogen Chlamydia is sequestered from the host cell cytoplasm by remaining within an inclusion body during its replication cycle. Nevertheless, CD8(+) T cells recognizing Chlamydia Ags in the context of MHC class I molecules are primed during infection. We have recently described derivation of Chlamydia-specific human CD8(+) T cells by using infected dendritic cells as a surrogate system to reflect Chlamydia-specific CD8(+) T cell responses in vivo. These CD8(+) T cell clones recognize chlamydial Ags processed via the conventional class Ia processing pathway, as assessed by treatment of infected APC with lactacystin and brefeldin A, suggesting that the Ags are translocated from the chlamydial inclusion into the host cell cytosol. In this study, outer membrane protein 2 (OmcB) was identified as the Ag recognized by one of these Chlamydia-specific human CD8(+) T cells, and we defined the HLA*A0101-restricted epitope from this Ag. CD8(+) T cell responses to this epitope were present at high frequencies in the peripheral blood of both of two HLA*A0101 donors tested. In vitro chlamydial growth was completely inhibited by the OmcB-specific CD8(+) T cell clone independently of lytic mechanisms. OmcB is a 60-kDa protein that has been postulated to be associated with the Chlamydia outer membrane complex. The subcellular localization of OmcB to the cytosol of infected cells, as determined by conventional MHC class I Ag processing and presentation, suggests the possibility of an additional, cytosolic-associated function for this protein.

Cell death and inflammation during infection with the obligate intracellular pathogen, Chlamydia

Infections by Chlamydia are followed by a strong inflammatory response, which is necessary to eliminate the infection, but at the same time is responsible for the pathology of infection. Resistance of infected cells against apoptosis induced by external ligands, together with the effects of IFNgamma secreted during infection, would be expected to contribute to persistence of infection. Secretion of TNFalpha plays an important role during clearance of the chlamydiae, but also triggers apoptosis of uninfected cells in infected tissues. Apoptosis of infected host-cells towards the end of the infection cycle is thought to participate in the release of chlamydiae from infected cells and propagation of the infection. Dysregulation of the apoptotic program during infection leads to a less efficient infection, but paradoxically, results in a higher inflammatory response and more severe pathology.

Transcutaneous immunization with combined cholera toxin and CpG adjuvant protects against Chlamydia muridarum genital tract infection

Chlamydia trachomatis is a pathogen of the genital tract and ocular epithelium. Infection is established by the binding of the metabolically inert elementary body (EB) to epithelial cells. These are taken up by endocytosis into a membrane-bound vesicle termed an inclusion. The inclusion avoids fusion with host lysosomes, and the EBs differentiate into the metabolically active reticulate body (RB), which replicates by binary fission within the protected environment of the inclusion. During the extracellular EB stage of the C. trachomatis life cycle, antibody present in genital tract or ocular secretions can inhibit infection both in vivo and in tissue culture. The RB, residing within the intracellular inclusion, is not accessible to antibody, and resolution of infection at this stage requires a cell-mediated immune response mediated by gamma interferon-secreting Th1 cells. Thus, an ideal vaccine to protect against C. trachomatis genital tract infection should induce both antibody (immunoglobulin A [IgA] and IgG) responses in mucosal secretions to prevent infection by chlamydial EB and a strong Th1 response to limit ascending infection to the uterus and fallopian tubes. In the present study we show that transcutaneous immunization with major outer membrane protein (MOMP) in combination with both cholera toxin and CpG oligodeoxynucleotides elicits MOMP-specific IgG and IgA in vaginal and uterine lavage fluid, MOMP-specific IgG in serum, and gamma interferon-secreting T cells in reproductive tract-draining caudal and lumbar lymph nodes. This immunization protocol resulted in enhanced clearance of C. muridarum (C. trachomatis, mouse pneumonitis strain) following intravaginal challenge of BALB/c mice.

Chlamydia-infected cells continue to undergo mitosis and resist induction of apoptosis

Both anti- and proapoptotic activities have been reported to occur during chlamydial infection. To reconcile the apparent controversy, we compared host cell apoptotic responses to infection with 17 different chlamydial serovars and strains. None of the serovars caused any biologically significant apoptosis in the infected host cells. Host cells in chlamydia-infected cultures can continue to undergo DNA synthesis and mitosis. Chlamydia-infected cells are resistant to apoptosis induction, although the extent of the antiapoptotic ability varied between serovars. These observations have demonstrated that an anti- but not proapoptotic activity is the prevailing event in chlamydia-infected cultures.

Role of proapoptotic BAX in propagation of Chlamydia muridarum (the mouse pneumonitis strain of Chlamydia trachomatis) and the host inflammatory response

The BCL-2 family member BAX plays a critical role in regulating apoptosis. Surprisingly, bax-deficient mice display limited phenotypic abnormalities. Here we investigate the effect of BAX on infection by the sexually transmitted pathogen, Chlamydia muridarum (the mouse pneumonitis strain of Chlamydia trachomatis). Bax(-/-) cells are relatively resistant to Chlamydia-induced apoptosis, and fewer bacteria are recovered after two infection cycles from Bax(-/-) cells than from wild-type cells. These results suggest that BAX-dependent apoptosis may be used to initiate a new round of infection, most likely by releasing Chlamydia-containing apoptotic bodies from infected cells that could be internalized by neighboring uninfected cells. Nonetheless, infected Bax(-/-) cells die through necrosis, which is normally associated with inflammation, more often than infected wild-type cells. These studies were confirmed in mice infected intravaginally with C. muridarum; since the infection disappears more quickly from Bax(-/-) mice than from wild-type mice, secretion of proinflammatory cytokines is increased in Bax(-/-) mice, and large granulomas are present in the genital tract of Bax(-/-) mice. Taken together, these data suggest that chlamydia-induced apoptosis via BAX contributes to bacterial propagation and decreases inflammation. Bax deficiency results in lower infection and an increased inflammatory cytokine response associated with more severe pathology.

Chlamydia vaccines: strategies and status

The ultimate goal of current chlamydial vaccine efforts is to utilise either conventional or modern vaccinology approaches to produce a suitable immunisation regimen capable of inducing a sterilising, long-lived heterotypic protective immunity at mucosal sites of infection to curb the severe morbidity and worldwide prevalence of chlamydial infections. This lofty goal poses tremendous challenges that include the need to clearly define the relevant effectors mediating immunity, the antigens responsible for inducing these effectors, the anti-chlamydial action(s) of effectors, and establishment of the most effective method of vaccine delivery. Tackling these challenges is further compounded by the biological complexity of chlamydia, the existence of multiple serovariants, the capacity to induce both protective and deleterious immune effectors, and the occurrence of asymptomatic and persistent infections. Thus, novel molecular, immunological and genetic approaches are urgently needed to extend the frontiers of current knowledge, and develop new paradigms to guide the production of an effective vaccine regimen. Progress made in the last 15 years has culminated in various paradigm shifts in the approaches to designing chlamydial vaccines. The dawn of the current immunological paradigm for antichlamydial vaccine design has its antecedence in the recognition that chlamydial immunity is mediated primarily by a T helper type1 (Th1) response, requiring the induction and recruitment of specific T cells into the mucosal microenvironment. Additionally, the ancillary role of humoral immune response in complementing the Th1-driven protective immunity, through ensuring adequate memory and optimal Th1 response during a reinfection, has been recognised. With continued progress in chlamydial genomics and proteomics, select chlamydial proteins, including structural, membrane and secretory proteins, are being targeted as potential subunit vaccine candidates. However, the development of an effective adjuvant, delivery vehicle or system for a potential subunit vaccine is still an elusive objective in these efforts. Promising delivery vehicles include DNA and virus vectors, bacterial ghosts and dendritic cells. Finally, a vaccine still represents the best approach to protect the greatest number of people against the ocular, pulmonary and genital diseases caused by chlamydial infections. Therefore, considering the urgency and the enormity of these challenges, a partially protective vaccine preventing certain severe sequelae would constitute an acceptable short-term goal to control Chlamydia. However, more research efforts and support are needed to achieve the worthy goal of protecting a significant number of the world's population from the devastating consequences of chlamydial invasion of the human mucosal epithelia.

The role of IFN-gamma in the outcome of chlamydial infection

Chlamydia are intracellular bacteria which infect many vertebrates, including humans. They cause a myriad of severe diseases, ranging from asymptomatic infection to pneumonia, blindness or infertility. IFN-gamma plays an important role in defense against acute infection and in the establishment of persistence. Chlamydia have evolved mechanisms to escape IFN-gamma functions. IFN-gamma-mediated effector mechanisms may involve effects on the metabolism of tryptophan or iron, on the inducible NO synthase (iNOS), on the secretion of chemokines and adhesion molecules or on the regulation of T-cell activities. IFN-gamma is secreted by the innate and the adaptive arms of the immune system. Within the former, Chlamydia-infected macrophages express IFN-gamma that in turn mediates resistance to infection. IFN-alpha/beta are pivotal for both IFN-gamma- and iNOS-mediated resistance to chlamydial infection in macrophages.

Persistent Chlamydia trachomatis infections resist apoptotic stimuli

Microbial modulation of apoptosis has added a new dimension of understanding to the dynamic interaction between the human host and its microbial invaders. Persistent infection can be a by-product of inhibition of apoptosis and may significantly impact the pathogenesis of diseases caused by organisms such as Chlamydia trachomatis. We compared apoptotic responses among HeLa 229 cells acutely and persistently infected and mock infected with serovar A/HAR-13. Persistence was induced by gamma interferon at 0.2 and 2.0 ng/ml. Cells were treated with etoposide or staurosporine at 24-h intervals and assayed for apoptosis by cell count, DNA ladder formation, and cytochrome c translocation. From the 24- to 120-h time points, infected cultures were 87 and 90% viable for etoposide and staurosporine treatment, respectively, and produced no DNA ladder, and cytochrome c remained in the mitochondria. In contrast, mock-infected cells were 22 and 37% viable for etoposide (P = 0.0001) and staurosporine (P = 0.01), respectively, and displayed characteristic DNA ladders, and cytochrome c was translocated. We found that resistance to apoptotic stimuli was identical in acute and persistent infections. Since cytochrome c was not translocated from the mitochondrion, caspase-9 activity was likely not involved. The expression of chlamydial hsp60, a known stimulator of inflammation in vivo, was measured in both active and persistent infections by Western blot, with increased production in the latter with or without staurosporine treatment. Chlamydial disregulation of apoptosis and the ensuing persistence of organisms offer an alternative pathogenic mechanism for chlamydial scarring observed in trachoma and infertility populations via sustained inflammation induced by immunoreactive molecules such as hsp60.

Modulation of P2Z/P2X(7) receptor activity in macrophages infected with Chlamydia psittaci

Given the role that extracellular ATP (ATP(o))-mediated apoptosis may play in inflammatory responses and in controlling mycobacterial growth in macrophages, we investigated whether ATP(o) has any effect on the viability of chlamydiae in macrophages and, conversely, whether the infection has any effect on susceptibility to ATP(o)-induced killing via P2Z/P2X(7) purinergic receptors. Apoptosis of J774 macrophages could be selectively triggered by ATP(o), because other purine/pyrimidine nucleotides were ineffective, and it was inhibited by oxidized ATP, which irreversibly inhibits P2Z/P2X(7) purinergic receptors. Incubation with ATP(o) but not other extracellular nucleotides inhibits the growth of intracellular chlamydiae, consistent with previous observations on ATP(o) effects on growth of intracellular mycobacteria. However, chlamydial infection for 1 day also inhibits ATP(o)-mediated apoptosis, which may be a mechanism to partially protect infected cells against the immune response. Infection by Chlamydia appears to protect cells by decreasing the ability of ATP(o) to permeabilize macrophages to small molecules and by abrogating a sustained Ca(2+) influx previously associated with ATP(o)-induced apoptosis.

Immunity to murine Chlamydia trachomatis genital tract reinfection involves B cells and CD4(+) T cells but not CD8(+) T cells

CD4(+) T-helper type 1 (Th1) responses are essential for the resolution of a primary Chlamydia trachomatis genital tract infection; however, elements of the immune response that function in resistance to reinfection are poorly understood. Defining the mechanisms of immune resistance to reinfection is important because the elements of protective adaptive immunity are distinguished by immunological memory and high-affinity antigen recognition, both of which are crucial to the development of efficacious vaccines. Using in vivo antibody depletion of CD4(+) and CD8(+) T cells prior to secondary intravaginal challenge, we identified lymphocyte populations that functioned in resistance to secondary chlamydial infection of the genital tract. Depletion of either CD4(+) or CD8(+) T cells in immune wild-type C57BL/6 mice had a limited effect on resistance to reinfection. However, depletion of CD4(+) T cells, but not CD8(+) T cells, in immune B-cell-deficient mice profoundly altered the course of secondary infection. CD4-depleted B-cell-deficient mice were unable to resolve a secondary infection, shed high levels of infectious chlamydiae, and did not resolve the infection until 3 to 4 weeks following the discontinuation of anti-CD4 treatment. These findings substantiated a predominant role for CD4(+) T cells in host resistance to chlamydial reinfection of the female genital tract and demonstrated that CD8(+) T cells are unnecessary for adaptive immune resistance. More importantly, however, this study establishes a previously unrecognized but very significant role for B cells in resistance to chlamydial reinfection and suggests that B cells and CD4(+) T cells may function synergistically in providing immunity in this model of chlamydial infection. Whether CD4(+) T cells and B cells function independently or dependently is unknown, but definition of those mechanisms is fundamental to understanding optimum protective immunity and to the development of highly efficacious immunotherapies against chlamydial urogenital infections.

Chlamydia trachomatis infection: incidence, health costs and prospects for vaccine development

Chlamydia trachomatis infection is now the most common sexually transmitted disease worldwide. World Health Organisation figures estimated that 89 million new cases of genital Chlamydia infections occurred in 1995, highlighting the worldwide prevalence of infections and the economic burden on healthcare delivery. A number of methods have been developed for detection of chlamydial infection, which vary in sensitivity and specificity. No single method has yet gained general acceptance and in many countries Chlamydia infections are not reported, suggesting that the above figures may be an underestimate of the problem. As yet there is no consensus as to what constitutes a protective immune response against genital Chlamydia infection. Studies in animal models have shown that cell-mediated immunity, both Th1-driven macrophage activation and cytotoxic T cell responses, as well as antibody can mediate protection at different stages of the chlamydial life cycle. A successful vaccine would probably need to elicit both a systemic cell-mediated immune response to limit/resolve established infections and a mucosal IgA response to reduce bacterial shedding and the resulting spread of infection to partners of infected individuals. The immune response to Chlamydia, either through natural infection or following immunisation, also has the potential to enhance inflammation and to act as a driving force for constant mutation in the variable regions of the major outer membrane protein. As a result a constant prevalence of infection is maintained even in an immune population through the emergence of new allelic variants. Immune responses against antigens such as the 60 kDa heat shock protein can exacerbate inflammation through molecular mimicry and must not be elicited as a result of vaccination. Thus there are many challenges for the development of a successful vaccine which must elicit immunity against multiple serovars while at the same time minimising damaging pro-inflammatory immune responses.

CD8+ T cell effector mechanisms in resistance to infection

Based on T cell subset depletion studies and the analysis of gene knockout mice, it is evident that CD8(+) T cells contribute to resistance against intracellular infections with certain viral, protozoan, and bacterial pathogens. Although they are known primarily for their capacity to kill infected cells, CD8(+) T cells elaborate a variety of effector mechanisms with the potential to defend against infection. Microbes use multiple strategies to cause infection, and the nature of the pathogenhost interaction may determine which CD8(+) T cell effector mechanisms are required for immunity. In this review, we summarize our current understanding of the effector functions used by CD8(+) T cells in resistance to pathogens. Analyses of mice deficient in perforin and/or Fas demonstrate that cytolysis is critical for immunity against some, but not all, infections and also reveal the contribution of cytolysis to the pathogenesis of disease. The role of CD8(+) T cell-derived cytokines in resistance to infection has been analyzed by systemic treatment with neutralizing antibodies and cytokine gene knockout mice. These studies are complicated by the fact that few, if any, cytokines are uniquely produced by CD8(+) T cells. Thus, the requirement for CD8(+) T cell- derived cytokines in resistance against most pathogens remains to be defined. Finally, recent studies of human CD8(+) T cells reveal the potential for novel effector mechanisms in resistance to infection.

Chlamydia and associated arthritis

An inflammatory arthritis is known to follow urogenital infection with the intracellular bacterium Chlamydia trachomatis in some individuals, and recent research results have elucidated important aspects of the characteristics of this Chlamydia-associated joint disease. Although the several extra-articular features of Chlamydia-induced arthritis have been defined clinically, their detailed causes remain largely unexplained. Current data indicate that the clinical characteristics of joint disease associated with C. trachomatis infection and those associated with postenteric arthritis are not easily distinguishable, although the response of each to antibiotic therapy does differ. The biologic characteristics of Chlamydia and enteric organisms in the joint show profound differences, and these are probably responsible for the variable responses to drug treatment. Molecular analyses of synovial C. trachomatis have demonstrated that long-term infection of the joint occurs primarily in synovial tissue and that the organism exhibits highly unusual biologic properties in its synovial context. These unusual molecular, biochemical, and other characteristics provide explanations for the frequent culture negativity of joint materials for C. trachomatis and for several other aspects of the arthritogenic process. Much remains to be learned concerning the behavior of this organism in the joint and concerning its interaction with its synovial host cells.

In situ analysis of the evolution of the primary immune response in murine Chlamydia trachomatis genital tract infection

Adaptive immune responses contribute to the resolution of Chlamydia trachomatis genital tract infection and protect against reinfection, but our understanding of the mechanisms of those protective responses is incomplete. In this study, we analyzed by in situ immunohistochemistry the progression of the inflammatory and cytokine responses in the genital tracts of mice vaginally infected with C. trachomatis strain mouse pneumonitis. The cellular inflammatory response was characterized by an initial elevation in myeloid cells in the vagina (day 3) and uterine horns (day 7), followed by a marked rise in the number of T cells, predominantly CD4(+) cells. CD8(+) T cells and CD45R(+) B cells were also detected but were much less numerous. Perivascular clusters of CD4(+) T cells, which resembled clusters of T cells seen in delayed-type hypersensitivity responses, were evident by 2 weeks postinfection. Following the resolution of infection, few CD8(+) T cells and CD45R(+) B cells remained, whereas numerous CD4(+) T cells and perivascular clusters of CD4(+) T cells persisted in genital tract tissues. Interleukin-12 (IL-12)- and tumor necrosis factor alpha (TNF-alpha)-producing cells were observed in vaginal tissue by day 3 of infection and in uterine tissues by day 7. Cells producing IL-4 or IL-10 were absent from vaginal tissues at day 3 of infection but were present in uterine tissues by day 7 and were consistently more numerous than IL-12- and TNF-alpha-producing cells. Thus, the evolution of the local inflammatory response was characterized by the accumulation of CD4(+) T cells into perivascular clusters and the presence of cells secreting both Th1- and Th2-type cytokines. The persistence of CD4(+)-T-cell clusters long after infection had resolved (day 70) may provide for a readily mobilizable T-cell response by which previously infected animals can quickly respond to and control a secondary infectious challenge.

Degradation of transcription factor RFX5 during the inhibition of both constitutive and interferon gamma-inducible major histocompatibility complex class I expression in chlamydia-infected cells

We have previously shown that the obligate intracellular pathogen chlamydia can suppress interferon (IFN)-gamma-inducible major histocompatibility complex (MHC) class II expression in infected cells by degrading upstream stimulation factor (USF)-1. We now report that chlamydia can also inhibit both constitutive and IFN-gamma-inducible MHC class I expression in the infected cells. The inhibition of MHC class I molecule expression correlates well with degradation of RFX5, an essential downstream transcription factor required for both the constitutive and IFN-gamma-inducible MHC class I expression. We further demonstrate that a lactacystin-sensitive proteasome-like activity identified in chlamydia-infected cell cytosolic fraction can degrade both USF-1 and RFX5. This proteasome-like activity is dependent on chlamydial but not host protein synthesis. Host preexisting proteasomes may not be required for the unique proteasome-like activity. These observations suggest that chlamydia-secreted factors may directly participate in the proteasome-like activity. Efforts to identify the chlamydial factors are underway. These findings provide novel information on the molecular mechanisms of chlamydial evasion of host immune recognition.