Chlamydial vaccines--future trends

Towards a Chlamydia trachomatis vaccine: how close are we?

Chlamydia trachomatis is the leading cause of bacterial sexually transmitted infections and preventable blindness worldwide. The incidence of chlamydial sexually transmitted infections has increased rapidly and current antibiotic therapy has failed as an intervention strategy. The most accepted strategy for protection and/or control of chlamydial infections is a vaccine that induces both local neutralizing antibodies to prevent infections by the extracellular elementary bodies and a cell-mediated immune response to target the intracellular infection. This article will discuss the challenges in vaccine design for the prevention of chlamydial urogenital infection and/or disease, including selection of target antigens, discussion of effective delivery systems, immunization routes and adjuvants for induction of protective immunity at the targeted mucosal surface whilst minimizing severe inflammatory disease sequelae.

Identification and characterization of novel recombinant vaccine antigens for immunization against genital Chlamydia trachomatis

Chlamydia trachomatis infection is the most common sexually transmitted bacterial infection worldwide, with over 91 million cases estimated annually. An effective subunit vaccine against Chlamydia may require a multivalent subunit cocktail of antigens in a single formulation for broad coverage of a heterogeneous major histocompatibility complex population. Herein, we describe the identification of novel C. trachomatis antigens by CD4+ and CD8+ T-cell expression cloning, serological expression cloning, and an in silico analysis of the C. trachomatis genome. These antigens elicited human CD4+ T-cell responses, and a subset proved to be immunogenic and protective when administered as immunoprophylactic vaccines against C. trachomatis challenge. Candidate vaccines consisting of the prioritized C. trachomatis antigens adjuvanted in a GlaxoSmithKline proprietary AS01B adjuvant were prioritized based on induction of solid protection against challenge in C57BL/6 and BALB/c mice with C. trachomatis. Some of the vaccines prevented bacterial shedding and colonization of the upper genital tract to varying degrees by mechanisms that may include CD4+ T cells.

Delivery of Chlamydia vaccines

The plethora of ocular, genital and respiratory diseases of Chlamydia, including nongonococcal urethritis, cervicitis pelvic inflammatory disease, ectopic pregnancy, tubal factor infertility, conjunctivitis, blinding trachoma and interstitial pneumonia, and chronic diseases that may include atherosclerosis, multiple sclerosis, adult onset asthma and Alzheimer's disease, still pose a considerable public health challenge to many nations. Although antibiotics are effective against Chlamydia when effectively diagnosed, asymptomatic infections are rampart, making clinical presentation of complications often the first evidence of an infection. Consequently, the current medical opinion is that an effective prophylactic vaccine would constitute the best approach to protect the human population from the most severe consequences of these infections. Clinical and experimental studies have demonstration that Chlamydia immunity in animals and humans is mediated by T cells and a complementary antibody response, and the completion of the genome sequencing of several isolates of Chlamydia is broadening our knowledge of the immunogenic antigens with potential vaccine value. Thus, major advances have been made in defining the essential elements of a potentially effective subunit vaccine design and parameters for evaluation. However, the challenge to develop effective delivery systems and human compatible adjuvants that would boost the immune response to achieve long-lasting protective immunity remains an elusive objective in chlamydial vaccine research. In response to evolving molecular and cellular technologies and novel vaccinology approaches, considerable progress is being made in the construction of novel delivery systems, such as DNA and plasmid expression systems, viral vectors, living and nonliving bacterial delivery systems, the use of chemical adjuvants, lipoprotein constructs and the codelivery of vaccines and specific immuno-modulatory biological agonists targeting receptors for chemokines, Toll-like receptors, and costimulatory molecules. The application of these novel delivery strategies to Chlamydia vaccine design could culminate in timely achievement of an efficacious vaccine.

Recombinant Vibrio cholerae ghosts as a delivery vehicle for vaccinating against Chlamydia trachomatis

An efficacious vaccine is needed to control the morbidity and burden of rising healthcare costs associated with genital Chlamydia trachomatis infection. Despite considerable efforts, the development of reliable chlamydial vaccines using conventional strategies has proven to be elusive. The 40kDa major outer membrane protein (MOMP) of C. trachomatis is so far the most promising candidate for a subunit vaccine. The lack of satisfactory protective immunity with MOMP-based vaccine regimens to date would suggest that either MOMP alone is inadequate as a vaccine candidate or better delivery systems are needed to optimize the effect of MOMP. Recombinant Vibrio cholerae ghosts (rVCG) are attractive for use as non-living vaccines because they possess strong adjuvant properties and are excellent vehicles for delivery of antigens of vaccine relevance to mucosal sites. The suitability of the ghost technology for designing an anti-chlamydial vaccine was evaluated by constructing a rVCG vector-based candidate vaccine expressing MOMP (rVCG-MOMP) and assessing vaccine efficacy in a murine model of C. trachomatis genital infection. Intramuscular delivery of the rVCG-MOMP vaccine induced elevated local genital mucosal as well as systemic Th1 responses. In addition, immune T cells from immunized mice could transfer partial protection against a C. trachomatis genital challenge to nai;ve mice. These results suggest that rVCG expressing chlamydial proteins may constitute a suitable subunit vaccine for inducing an efficient mucosal T cell response that protects against C. trachomatis infection. Altogether, the potency and relatively low production cost of rVCG offer a significant technical advantage as a chlamydial vaccine.

Immunization with the Chlamydia trachomatis major outer membrane protein, using the outer surface protein A of Borrelia burgdorferi as an adjuvant, can induce protection against a chlamydial genital challenge

Two strains of mice C3H/HeN (H-2(k)) and BALB/c (H-2(d)) were immunized with the Chlamydia trachomatis mouse pneumonitis (MoPn) major outer membrane protein (MOMP) using the Borrelia burgdorferi outer surface protein A (OspA) as an adjuvant. As a control, groups of mice were inoculated with ovalbumin instead of MOMP. Female mice were immunized using three different routes: intramuscular (i.m.) plus subcutaneous (s.c.), intranasal (i.n.) and perivaginal and perisacral (p.vag.+p.sac.). Significant humoral and cell mediated immune responses developed particularly in mice inoculated by the i.m.+s.c. routes as determined by the levels of chlamydial specific antibody in the serum and genital secretions and a T-cell proliferative assay. Following immunization the animals were challenged in the genital tract with C. trachomatis MoPn and the course of the infection followed by vaginal cultures. Significant protection against infection was achieved in the C3H/HeN mice inoculated i.m.+s.c. with MOMP+OspA, as shown by the intensity and duration of vaginal cultures, and by the number of mice with positive cultures. On the other hand in BALB/c mice there was only a decrease in the number of animals with positive vaginal cultures. Six weeks after the challenge the mice were mated and the outcome of the pregnancy evaluated. In both the C3H/HeN and the BALB/c mice immunized i.m.+s.c. with MOMP+OspA there was significant protection against infertility as shown by the number of animals with bilateral fertility and number of embryos per uterine horn. In conclusion, immunization using C. trachomatis MOMP, and B. burgdorferi OspA as an adjuvant, can induce significant protection against a chlamydial genital challenge.

Contemporary approaches to designing and evaluating vaccines against Chlamydia

The clinically relevant pathologic consequences of primary ocular, genital, or respiratory human infection by members of the genus Chlamydia are conjunctivitis, cervicitis, urethritis and sinusitis. The major complications and sometimes debilitating evolutionary outcomes of these infections include: trichiasis and cicatrizing trachoma, endometritis or pelvic inflammatory disease and involuntary tubal factor infertility and bronchopulmonary pneumonia. These diseases, in addition to other chlamydia-associated chronic syndromes (e.g., artherosclerosis, multiple sclerosis and Alzheimer's disease), pose serious public healthcare and huge budgetary concerns. The current medical opinion is that an efficacious prophylactic vaccine is a sine qua non--to control the morbidity of chiamydial infection in the human population. The research goal for an efficacious human chlamydial vaccine has faced key challenges to define the elements of protective immunity to facilitate vaccine evaluation, the judicious selection of appropriate vaccine candidates that possess stable antigenic and immunologic properties and the development of effective delivery vehicles and adjuvants to boost immune effectors to achieve long-term protective immunity. Progress in the functional immunobiology of Chlamydia has established the essential immunologic paradigms for vaccine selection and evaluation, including the obligatory requirement for a vaccine to induce T-helper Type 1 immune response that controls chlamydiae. Recent advances in chlamydial genomics and proteomics should enhance the identification of likely chlamydial gene products that fulfill the antigenic requirements of putative vaccine candidates. Major inroads are however needed in the construction and development of novel and effective delivery systems, such as vectors and adjuvants. This review summarizes the status of contemporary chlamydial vaccine research and promising trends fueling the growing optimism for an efficacious vaccine. The unified approach to vaccines for the genus Chlamydia is validated by the several conserved genes and common immunogenic proteins among member species and the similarity of immune effectors controlling Chlamydia species in animals and humans.

Immunization with the Chlamydia trachomatis mouse pneumonitis major outer membrane protein by use of CpG oligodeoxynucleotides as an adjuvant induces a protective immune response against an intranasal chlamydial challenge

Recently, we have shown that a vaccine consisting of a purified preparation of the Chlamydia trachomatis mouse pneumonitis (MoPn) major outer membrane protein (MOMP) and Freund's adjuvant can protect mice against a genital challenge. Here, we wanted to determine if CpG motifs could be used as an immune modulator to the MOMP to induce protection in mice against an intranasal (i.n.) challenge. One-week-old BALB/c mice were immunized intramuscularly and subcutaneously either once or three times at 2-week intervals with MOMP and CpG suspended in aluminum hydroxide (alum). Negative controls received ovalbumin, CpG, and alum. Positive controls were immunized i.n. with C. trachomatis MoPn elementary bodies (EB). Six weeks after the last immunization, mice were challenged i.n. with 10(4) inclusion-forming units (IFU) of the C. trachomatis MoPn serovar. Mice that received MOMP, CpG, and alum had a strong immune response, as shown by a high titer of serum antibodies to Chlamydia and significant lymphoproliferation of T-cells following stimulation with C. trachomatis EB. After the i.n. challenge mice immunized with MOMP, CpG, and alum showed significantly less body weight loss than the corresponding control mice immunized with ovalbumin, CpG, and alum. Ten days after the challenge the animals were euthanized, their lungs were weighed, and the numbers of IFU in the lungs were determined. The average weight of the lungs of the mice immunized with MOMP, CpG, and alum was significantly less than average weight of the lungs of the mice immunized with ovalbumin, CpG, and alum. Also, the average number of IFU recovered per mouse immunized with MOMP, CpG, and alum was significantly less than the average number of IFU per mouse detected in the mice inoculated with ovalbumin, CpG, and alum. In conclusion, our data show that CpG sequences can be used as an effective adjuvant with the C. trachomatis MoPn MOMP to elicit a protective immune response in mice against a chlamydial respiratory challenge.

Prediction of the membrane-spanning beta-strands of the major outer membrane protein of Chlamydia

There is preliminary experimental evidence indicating that the major outer-membrane protein (MOMP) of Chlamydia is a porin. We tested this hypothesis for the MOMP of the mouse pneumonitis serovar of Chlamydia trachomatis using two secondary structure prediction methods. First, an algorithm that calculates the mean hydrophobicity of one side of putative beta-strands predicted the positions of 16 transmembrane segments, a structure common to known porins. Second, outer loops typical of porins were assigned using an artificial neural network trained to predict the topology of bacterial outer-membrane proteins with a predominance of beta-strands. A topology model based on these results locates the four variable domains (VDs) of the MOMP on the outer loops and the five constant domains on beta-strands and the periplasmic turns. This model is consistent with genetic analysis and immunological and biochemical data that indicate the VDs are surface exposed. Furthermore, it shows significant homology with the consensus porin model of the program FORESST, which contrasts a proposed secondary structure against a data set of 349 proteins of known structure. Analysis of the MOMP of other chlamydial species corroborated our predicted model.

Dendritic cells pulsed with a recombinant chlamydial major outer membrane protein antigen elicit a CD4(+) type 2 rather than type 1 immune response that is not protective

Chlamydia trachomatis is an obligate intracellular bacterium that infects the oculogenital mucosae. C. trachomatis infection of the eye causes trachoma, the leading cause of preventable blindness. Infections of the genital mucosae are a leading cause of sexually transmitted diseases. A vaccine to prevent chlamydial infection is needed but has proven difficult to produce by using conventional vaccination approaches. Potent immunity to vaginal rechallenge in a murine model of chlamydial genital infection has been achieved only by infection or by immunization with dendritic cells (DC) pulsed ex vivo with whole inactivated organisms. Immunity generated by infection or ex vivo antigen-pulsed DC correlates with a chlamydia-specific interleukin 12 (IL-12)-dependent CD4(+) Th1 immune response. Because of the potent antichlamydial immunizing properties of DC, we hypothesized that DC could be a powerful vehicle for the delivery of individual chlamydial antigens that are thought to be targets for more conventional vaccine approaches. Here, we investigated the recombinant chlamydial major outer membrane protein (rMOMP) as a target antigen. The results demonstrate that DC pulsed with rMOMP secrete IL-12 and stimulate infection-sensitized CD4(+) T cells to proliferate and secrete gamma interferon. These immunological properties implied that rMOMP-pulsed DC would be potent inducers of MOMP-specific CD4(+) Th1 immunity in vivo; however, we observed the opposite result. DC pulsed ex vivo with rMOMP and adoptively transferred to naive mice generated a Th2 rather than a Th1 anti-MOMP immune response, and immunized mice were not protected following infectious challenge. We conclude from these studies that the immunological properties of ex vivo pulsed DC are not necessarily predictive of the immune response generated in vivo following adoptive transfer. These findings suggest that the nature of the antigen used to pulse DC ex vivo influences the Th1-Th2 balance of the immune response in vivo.

Immunization with the Chlamydia trachomatis mouse pneumonitis major outer membrane protein can elicit a protective immune response against a genital challenge

Infertility, ectopic pregnancy, and chronic abdominal pain are frequent complications of genital infections with Chlamydia trachomatis. In an attempt to produce a vaccine to protect against this pathogen we purified and refolded the C. trachomatis mouse pneumonitis (MoPn) major outer membrane protein (MOMP). This preparation, mixed with Freund's adjuvant using vortexing or sonication, was used to immunize BALB/c mice that were subsequently challenged in the upper genital tract. Vaginal cultures were taken on a weekly basis, and mice were mated 6 weeks after the challenge. Gels of the vortexed MOMP showed a predominant band with a molecular size of 62 kDa and weaker bands at 42 and 132 kDa, while the sonicated MOMP had a single band with a molecular size of 42 kDa. Following immunization with these two preparations, strong humoral and cell-mediated immune responses were detected in the mice inoculated with the vortexed MOMP. On the other hand, mice immunized with the sonicated MOMP had a strong humoral immune response but a relatively weak cell-mediated immune response, as determined by a T-cell lymphoproliferative assay and level of cytokine production by splenocytes. Vaginal cultures showed that the mice immunized with the vortexed MOMP were significantly protected, as determined by a decrease in the number of animals with positive cultures, the length of time the mice shed viable organisms, and the number of inclusion-forming units recovered per mouse. Animals immunized with the sonicated MOMP, on the other hand, showed a weaker level of protection based on the same three parameters. After mating, the number of fertile animals and number of embryos per mouse were significantly higher for the mice immunized with vortexed MOMP, but not for the mice immunized with sonicated MOMP, compared to those of the control groups. In conclusion, immunization with a purified and refolded preparation of the C. trachomatis MoPn MOMP confers a significant level of protection in mice against a genital challenge.

Susceptibility of mice to vaginal infection with Chlamydia trachomatis mouse pneumonitis is dependent on the age of the animal

Mice from three strains, BALB/c (H-2(d)), C3H (H-2(k)), and C57BL/6 (H-2(b)), ranging from 5 to 14 weeks of age, were inoculated intravaginally with different doses of the Chlamydia trachomatis mouse pneumonitis serovar. Vaginal swabs taken at weekly intervals showed that the percentage of animals with positive cultures and the number of inclusion-forming units recovered per mouse were higher in the younger animals. Furthermore, vaginal shedding lasted longer in the young mice than in the older mice. In addition, following mating higher rates of infertility and a decrease in the number of embryos were observed in the infected young mice. In conclusion, susceptibility to a chlamydial vaginal infection is dependent on the age of the mice, with the older animals being more resistant.

Expression of genes encoding Th1 cell-activating cytokines and lymphoid homing chemokines by chlamydia-pulsed dendritic cells correlates with protective immunizing efficacy

We studied the expression of cytokines, chemokines, and chemokine receptors by the RNase protection assay in chlamydia-pulsed dendritic cells to better understand their potent anti-chlamydial immunizing properties. We found that chlamydia-pulsed dendritic cells express a complex profile of inflammatory and immunomodulatory molecules. These include CCR-7, interleukin-12, and interferon-induced protein 10, molecules that might influence the homing of pulsed dendritic cells to the site of chlamydial infection and the induction of a local protective CD4(+) Th1 cellular immunity.

Mutagenesis and functional reconstitution of chlamydial major outer membrane proteins: VS4 domains are not required for pore formation but modify channel function

Chlamidial organisms are obligate intracellular pathogens containing highly antigenic porin-like major outer membrane proteins (MOMPs). MOMP epitopes are of substantial medical interest, and they cluster within four relatively short variable (VS) domains. If MOMPs adopt a beta-barrel fold, like bacterial porins, the VS domains may form extramembranous loops and the conserved regions of the protein may correspond to predicted membrane-located beta-strands. However, molecular studies on native MOMPs have been hampered by the need to culture chlamydiae in eukaryotic host cells and purification and reconstitution remain problematic. In addition, the organisms are difficult to manipulate genetically, and it has also been difficult to functionally reconstitute recombinant MOMPs. To help overcome these problems and improve our understanding of MOMP structure and function, we cloned and expressed C. trachomatis and C. psittaci MOMPs and functionally reconstituted them at the single-channel level. We measured significant functional differences between the two proteins, and by removing and exchanging VS4, we tested the hypothesis that the largest variable domain forms an extramembranous loop that contributes to these differences. Proteins in which VS4 was deleted continued to form functional ion channels, consistent with the idea that the domain forms an extramembranous protein loop and incompatible with models in which it contributes to predicted membrane-located beta-strands. Additionally, the properties of the chimeric proteins strongly suggested that the VS4 domain interacts closely with other regions of the protein to form the channel entrance or vestibule. Our approach can be used to probe structure-function relationships in chlamydial MOMPs and may have implications for the generation of effective antichlamydial vaccines.

Induction of protective immunity against Chlamydia trachomatis genital infection by a vaccine based on major outer membrane protein-lipophilic immune response-stimulating complexes

The significance of delivery systems in modern vaccine design strategies is underscored by the fact that a promising vaccine formulation may fail in vivo due to an inappropriate delivery method. We evaluated the immunogenicity and efficacy of a candidate vaccine comprising the major outer membrane protein (MOMP) of Chlamydia trachomatis delivered with the lipophilic immune response-stimulating complexes (ISCOMs) as a vehicle with adjuvant properties, in a murine model of chlamydial genital infection. Immunocompetent BALB/c mice were immunized intranasally (IN) or intramuscularly (IM) with MOMP, MOMP-ISCOMs, and live or heat-inactivated C. trachomatis serovar D. The level of local genital mucosal Th1 response was measured by assaying for antigen-specific Th1 cell induction and recruitment into the genital mucosa at different times after immunization. Immunization with MOMP-ISCOMs by the IM route induced the greatest and fastest local genital mucosal Th1 response, first detectable 2 weeks after exposure. Among the other routes and regimens tested, only IN immunization with MOMP-ISCOMs induced detectable and statistically significant levels of local genital mucosal Th1 response during the 8-week test period (P < 0.001). In addition, when T cells from immunized mice were adoptively transferred into syngeneic naive animals and challenged intravaginally with Chlamydia, recipients of IM immunization of MOMP-ISCOMs cleared their infection within 1 week and were resistant to reinfection. Animals that received IN immunization of MOMP-ISCOMs were partially protected, shedding fewer chlamydiae than did control mice. Altogether, the results suggested that IM delivery of MOMP-ISCOMs may be a suitable vaccine regimen potentially capable of inducing protective mucosal immunity against C. trachomatis infection.

Immunogenic and protective ability of the two developmental forms of Chlamydiae in a mouse model of infertility

To compare the ability of elementary bodies (EB) and reticulate bodies (RB) of the Chlamydia trachomatis mouse pneumonitis (MoPn) biovar to induce a protective immune response, two groups of BALB/c mice were inoculated and boosted twice, with UV-inactivated EB or RB in Freund's adjuvant. Two weeks after the last immunization mice were challenged with C. trachomatis in the ovarian bursa. Vaginal cultures collected for 6 weeks after the intrabursal challenge showed that mice inoculated with EB were significantly protected, while mice inoculated with RB were not. Six weeks after the genital challenge mice were mated. Mice immunized with EB showed significant protection as demonstrated by the number of animals which were fertile and the number of embryos present in the uterine horns. In contrast, no significant protection against infertility was observed in the mice immunized with RB.

Vaccination of mice with DNA plasmids coding for the Chlamydia trachomatis major outer membrane protein elicits an immune response but fails to protect against a genital challenge

A DNA plasmid encoding the gene of the major outer membrane protein (MOMP) of the Chlamydia trachomatis mouse pneumonitis (MoPn) serovar and three plasmids containing the variable domains (VD) of the MOMP were constructed. Female mice were inoculated with the plasmids and 60 days later were challenged in the genital tract with C. trachomatis. Six weeks after challenge female mice were caged with male mice and the course of the mating followed. Mice immunized with the MOMP plasmids mounted weak humoral and cell mediated immune responses. However, following the genital challenge no significant differences in vaginal shedding were observed between the groups immunized with the MOMP and control plasmids. In addition, the fertility rates were similar in the experimental and negative control groups. In conclusion, vaccination with DNA plasmids encoding the MOMP elicited a modest immune response but did not protect against infection or disease.

STD vaccines--an overview

OBJECTIVES

To describe the role and current status of vaccine research against sexually transmitted diseases (STDs).

METHODS

The available literature was reviewed with particular emphasis on bacterial STDs.

RESULTS

Strategic approaches to possible implementation of STD vaccine programmes were analysed. The status of vaccines against bacterial STDs (syphilis, chancroid, gonorrhoea, and chlamydia) is described in detail.

CONCLUSIONS

The development of safe and effective STD vaccines offers a potent tool for the control of STDs, including direct and indirect prevention of HIV infection. Future priorities should be in the development of vaccines against gonorrhoea, chlamydia, and syphilis. When such vaccines become available, caution should be exercised to ensure that they do not interfere with the effectiveness of other prevention programmes.

Monoclonal immunoglobulin A antibody to the major outer membrane protein of the Chlamydia trachomatis mouse pneumonitis biovar protects mice against a chlamydial genital challenge

In order to analyze the protective role that IgA may play in a chlamydial infection two IgA monoclonal antibodies (mAb), MoPn 4-2 and MoPn 13-2, were raised against the major outer membrane protein (MOMP) of the Chlamydia trachomatis mouse pneumonitis (MoPn) biovar. mAb MoPn 4-2 was found to be serovar specific while mAb MoPn 13-2 was species specific. mAb MoPn 4-2 recognized a surface exposed conformational epitope as shown by its ability to bind to native EBs and nonreduced MOMP while failing to bind to heat and trypsin treated EBs, to reduced MOMP and to synthetic MOMP peptides. In contrast, mAb MoPn 13-2 recognized a nonconformational epitope since it was able to bind treated EBs, to reduced MOMP and to the synthetic peptide MTTWNPTISGSGI located in variable domain 4 of the MOMP. Both mAbs agglutinated intact EBs and had in vitro neutralizing activity. However, mAb MoPn 4-2 had a 20-fold higher in vitro neutralizing ability when compared to mAb MoPn 13-2 (50% neutralization at 5 micrograms ml-1 vs 100 micrograms ml-1). In an in vitro in vivo infectivity assay, mAb MoPn 4-2 protected mice against infertility when C. trachomatis MoPn elementary bodies were preincubated with the mAb before inoculation. In addition, passive transfer of mAb MoPn 4-2 resulted in significant protection as measured by a decrease in the number of mice infected, and in the intensity and duration of vaginal shedding. These results support previous findings suggesting that IgA antibodies can play a role in protection against a chlamydial infection, and further encourage work to develop vaccination strategies that elicit mucosal immunity.

Intranasal immunization induces long-term protection in mice against a Chlamydia trachomatis genital challenge

In an attempt to confer long-term protective immunity, BALB/c female mice were immunized intranasally with 10(4) inclusion-forming units (IFU) of the Chlamydia trachomatis mouse pneumonitis biovar (MoPn). Animals were subsequently challenged in the ovarian bursa with 10(5) C. trachomatis MoPn IFU at 60, 120, or 180 days post-intranasal immunization. Two control groups were included in the study. One control was sham immunized and mock challenged, and another group was sham immunized and challenged with 10(5) C. trachomatis MoPn IFU. Vaginal cultures were collected at regular intervals following the intrabursal challenge. In comparison with the sham-immunized mice, the animals that were intranasally immunized with C. trachomatis had significant protection, as shown by a reduction in the number of animals that had positive vaginal cultures and by a decrease in the intensity and length of the shedding. Furthermore, histopathological characterization of the genital tract following challenge, in the three groups of mice, showed a minimal inflammatory infiltrate in the C. trachomatis-immunized animals, when compared with the sham-immunized control group. Subsequently, the three groups of female mice that were challenged at 60, 120 and 180 days postimmunization were mated at 6 weeks following the challenge. Overall, in the mice intranasally immunized with C. trachomatis the fertility rates and the number of embryos were similar to those in the sham-immunized and mock-challenged group. In contrast, there was a significant increase in infertility in the groups of mice that were sham immunized and C. trachomatis challenged. In conclusion, intranasal immunization with C. trachomatis induces long-term protection against a genital challenge as shown by a decrease in the infection and infertility rates when compared with sham-immunized animals. Thus, this model may help to characterize the parameters of the immune response that are important in maintaining long-term protection and may aid in identifying the antigenic determinants involved in eliciting protection.

Chlamydial envelope components and pathogen-host cell interactions

Few bacterial pathogens are as widespread in nature or as capable of eliciting such a diversity of disease syndromes as are the chlamydiae. As obligate intracellular organisms, they pose a special research challenge in defining the molecular components and mechanisms for productive growth within host cells and the overall progress of infection throughout host tissue. Although a comprehensive view of chlamydial envelope composition and respective functions in pathogenesis is far from complete, ongoing investigations continue to expose new and intriguing avenues for exploration.

A new computer model for estimating the impact of vaccination protocols and its application to the study of Chlamydia trachomatis genital infections

We describe a new computer model for studying the impact of vaccination protocols on the prevalence of disease. This state transition model uses two parameters, the forward rate and the cure rate, that are derived from epidemiological data to compute the percentage of individuals infected in the population. The cure rate is the percentage of individuals that are cured within one time step (which in this study is one year) and the forward rate is used to calculate the number of new infections due to transmission. The forward rate and the cure rate are incorporated into an update function that has the property that if the vaccination efficacy is 0%, or no vaccination is applied, then the percentage of individuals infected stays constant. We present computer simulations of this model designed to assess the influence of two variables on the prevalence of Chlamydia trachomatis infection in a study population. More specifically, we determined the effect of vaccines with efficacies ranging from 50% to 100% and we analysed the impact on the population for vaccines efficacious for periods of 10, 20 and 40 years. The results of the computer simulation show that even the least efficacious vaccination programme rapidly decreases the prevalence of C. trachomatis infection in the population. On the other hand, a vaccine that is efficacious for a period of only 1 year had minimal impact on the prevalence of the disease in the total population.

Protection against infertility in a BALB/c mouse salpingitis model by intranasal immunization with the mouse pneumonitis biovar of Chlamydia trachomatis

Female BALB/c mice were immunized intranasally with the mouse pneumonitis biovar of Chlamydia trachomatis and subsequently challenged in the ovarian bursa (C. trachomatis immunized, C. trachomatis challenged). Two groups of mice served as controls. One group was sham immunized intranasally with mock-infected HeLa 229 cell extracts and was challenged in the ovarian bursa with C. trachomatis MoPn (sham immunized, C. trachomatis challenged). The second control group was sham immunized and not challenged (sham immunized, nonchallenged). Before challenge, the C. trachomatis-immunized, C. trachomatis-challenged animals mounted a significant humoral response as shown by high immunoglobulin G (IgG), IgM, and IgA levels and high levels of neutralizing antibodies in serum and moderate IgG and IgA titers in vaginal secretions. Reactivity by Western blot (immunoblot) to the lipopolysaccharide, 30-, 40- (major outer membrane protein), and 60-kDa cysteine-rich proteins and 75- and 100-kDa chlamydial components could be demonstrated. However, reactivity to the 60-kDa heat shock protein was only observed 22 days after challenge. In addition, this group of animals mounted a significant immune response to chlamydial antigens, as shown by a lymphocyte proliferation assay, compared with the sham-immunized nonchallenged mice. After intrabursal challenge, there was no C. trachomatis shedding from the vagina in the C. trachomatis-immunized, C. trachomatis-challenged animals, while 63% of the sham-immunized, C. trachomatis-challenged mice had a positive C. trachomatis culture. In addition, histological sections from the genital tract showed, at 2 weeks postchallenge, a marked acute inflammatory reaction in the sham-immunized, C. trachomatis-challenged animals while in the C. trachomatis-immunized, C. trachomatis-challenged mice there was minimal inflammatory reaction. When the animals were mated, only 12% of the mice from the sham-immunized, C. trachomatis-challenged mice were fertile. In contrast, 94 and 80% of the sham-immunized, nonchallenged and C. trachomatis-immunized, C. trachomatis-challenged mice, respectively, were fertile.

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.

A poliovirus hybrid expressing a neutralization epitope from the major outer membrane protein of Chlamydia trachomatis is highly immunogenic

Trachoma and sexually transmitted diseases caused by Chlamydia trachomatis are major health problems worldwide. Epitopes on the major outer membrane protein (MOMP) of C. trachomatis have been identified as important targets for the development of vaccines. In order to examine the immunogenicity of a recombinant vector expressing a chlamydial epitope, a poliovirus hybrid was constructed in which part of neutralization antigenic site I of poliovirus type 1 Mahoney (PV1-M) was replaced by a sequence from variable domain I of the MOMP of C. trachomatis serovar A. The chlamydial sequence included the neutralization epitope VAGLEK. This hybrid was viable, grew very well compared with PV1-M, and expressed both poliovirus and chlamydial antigenic determinants. When inoculated into rabbits, this hybrid was highly immunogenic, inducing a strong response against both PV1-M and C. trachomatis serovar A. Antichlamydia titers were 10- to 100-fold higher than the titers induced by equimolar amounts of either purified MOMP or a synthetic peptide expressing the VAGLEK epitope. Furthermore, rabbit antisera raised against this hybrid neutralized chlamydial infectivity both in vitro, for hamster kidney cells, and passively in vivo, for conjunctival epithelia of cynomolgus monkeys. Because poliovirus infection induces a strong mucosal immune response in primates and humans, these results indicate that poliovirus-chlamydia hybrids could become powerful tools for the study of mucosal immunity to chlamydial infection and for the development of recombinant chlamydial vaccines.