A predominant role for antibody in acquired immunity to chlamydial genital tract reinfection

A limited role for antibody in protective immunity induced by rCPAF and CpG vaccination against primary genital Chlamydia muridarum challenge

Mice deficient in B cells (micromT mice) were used to evaluate the role of antibody in enhanced chlamydial clearance and reduction of pathology afforded by vaccination with recombinant chlamydial protease-like activity factor (rCPAF). Enhanced, but comparable, chlamydial clearance was observed in micromT and wild-type (WT) mice after rCPAF+CpG vaccination. Chlamydia-induced pathology was present in mock-immunized animals, but at significantly greater levels in micromT than WT mice, whereas vaccinated micromT and WT mice exhibited similar reductions in pathology. Thus, antibodies may play a role in protection against chlamydial pathology after primary infection, but were largely dispensable in rCPAF+CpG-induced chlamydial clearance and reduction in pathology.

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.

Protection of wild-type and severe combined immunodeficiency mice against an intranasal challenge by passive immunization with monoclonal antibodies to the Chlamydia trachomatis mouse pneumonitis major outer membrane protein

Monoclonal antibodies (MAbs) to the Chlamydia trachomatis mouse pneumonitis (MoPn) major outer membrane protein (MOMP) were characterized for their ability to neutralize the infectivity of this organism in vitro and in vivo. One of the MAbs (MoPn-23) recognizes a nonlinear epitope in the MOMP, MAb MoPn-40 binds to a linear epitope in the variable domain 1 (VD1), and MAb MoPn-32 recognizes the chlamydial lipopolysaccharide. MAb MoPn-23 neutralized 50% of the infectivity of Chlamydia, as measured in vitro by using HAK (Fc gammaIII(-)) and HeLa-229 (Fc gammaIII(+)) cells at a concentration 100 times lower than MAb MoPn-40. MAb MoPn-32 had no neutralizing ability. In comparison to the control normal mouse immunoglobulin G, passive immunization of BALB/c mice with MAb MoPn-23 resulted in a highly significant protection against an intranasal (i.n.) challenge as determined by the change in body weight, the weight of the lungs, and the yield of Chlamydia inclusion-forming units (IFU) from the lungs. Passive immunization with MAb MoPn-40 resulted in a lower degree of protection, and MAb MoPn-32 afforded no protection. MAb MoPn-23 was also tested for its ability to protect wild-type (WT) and severe combined immunodeficient (SCID) C.B-17 mice against an i.n. challenge. Protection based on total body weight, lung weight, and yield of Chlamydia IFU was as effective in SCID as in WT C.B-17 mice. In conclusion, antibodies to MOMP can protect mice against a chlamydial infection in the presence or absence of T and B cells.

Molecular detection of Chlamydophila abortus in post-abortion sheep at oestrus and subsequent lambing

Enzootic abortion of ewes (EAE), resulting from infection with the bacterium Chlamydophila abortus (C. abortus), is a major cause of lamb loss in Europe. The purpose of this study was to assess the potential impact of the shedding of organisms in post-abortion ewes at oestrus and subsequent lambing on the epidemiology of EAE. Using a newly developed C. abortus specific real-time PCR assay, few chlamydial genomes could be detected in vaginal swabs taken from post-abortion ewes at oestrus. At subsequent parturition, all ewes lambed normally with no macroscopic or microbiological evidence of infection. Real-time PCR analysis of placental samples identified very few or no chlamydial genomes, which contrasted significantly with samples taken at the time of abortion, where an average of 2.7x10(7) chlamydial genomes per microgram of total tissue DNA was detected. Few genomes could also be detected from vaginal and cervical tissue samples and lymph nodes taken post-mortem. The results, although not discounting the possibility of a chronic low level persistent infection in post-abortion ewes, suggest that the low levels of chlamydial DNA detected during the periovulation period and at lambing do not significantly impact on the epidemiology of EAE. In terms of flock management, the products of abortion should be considered the major and principal source of infection for transmission to naïve ewes.

Antibodies from women urogenitally infected with C. trachomatis predominantly recognized the plasmid protein pgp3 in a conformation-dependent manner

Background

C. trachomatis organisms carry a cryptic plasmid that encodes 8 open reading frames designated as pORF1 to 8. It is not clear whether all 8 pORFs are expressed during C. trachomatis infection in humans and information on the functionality of the plasmid proteins is also very limited.

Results

When antibodies from women urogenitally infected with C. trachomatis were reacted with the plasmid proteins, all 8 pORFs were positively recognized by one or more human antibody samples with the recognition of pORF5 protein (known as pgp3) by most antibodies and with the highest titers. The antibody recognition of the pORFs was blocked by C. trachomatis-infected HeLa but not normal HeLa cell lysates. The pgp3 fusion protein-purified human IgG detected the endogenous pgp3 in the cytosol of C. trachomatis-infected cells with an intracellular distribution pattern similar to that of CPAF, a chlamydial genome-encoded protease factor. However, the human antibodies no longer recognized pgp3 but maintained recognition of CPAF when both antigens were linearized or heat-denatured. The pgp3 conformation is likely maintained by the C-terminal 75% amino acid sequence since further deletion blocked the binding by the human antibodies and two conformation-dependent mouse monoclonal antibodies.

Conclusion

The plasmid-encoded 8 proteins are both expressed and immunogenic with pgp3 as the most immunodominant antigen during chlamydial infection in humans. More importantly, the human anti-pgp3 antibodies are highly conformation-dependent. These observations have provided important information for further understanding the function of the plasmid-encoded proteins and exploring the utility of pgp3 in chlamydial diagnosis and vaccination.

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.

Chlamydia trachomatis infection: host immune responses and potential vaccines

Chlamydia trachomatis causes genital tract infections that affect men, women, and children on a global scale. This review focuses on innate and adaptive immune responses in the female reproductive tract (FRT) to genital tract infections with C. trachomatis. It covers C. trachomatis infections and highlights our current knowledge of genital tract infections, serovar distribution, infectious load, and clinical manifestations of these infections in women. The unique features of the immune system of the FRT will be discussed and will include a review of our current knowledge of innate and adaptive immunity to chlamydial infections at this mucosal site. The use of animal models to study the pathogenesis of, and immunity to, Chlamydia infection of the female genital tract will also be discussed and a review of recent immunization and challenge experiments in the murine model of chlamydial FRT infection will be presented.

Cloning, characterization and expression analysis of the gene for a putative lipopolysaccharide-induced TNF-alpha factor of the Pacific oyster, Crassostrea gigas

Lipopolysaccharide-induced TNF-alpha factor (LITAF) is an important transcription factor that mediates the expression of inflammatory cytokines, including TNF-alpha, in lipopolysaccharide (LPS)-induced processes. In the present study, the Pacific oyster Crassostrea gigas LITAF (Cg-LITAF) gene was cloned and characterized. The full-length Cg-LITAF cDNA consists of 906bp and encodes a polypeptide of 115 amino acids. The Cg-LITAF gene consists of three exons and two introns, with a length of approximately 1.8kb. The Cg-LITAF protein showed 34-45% amino acid sequence identity with other known LITAF sequences. Although the Cg-LITAF coding sequence (115 aa) is shorter than all previously reported LITAF genes, the LITAF domain which contains two CXXC motifs is well conserved. An in vivo expression study showed that Cg-LITAF mRNA was expressed predominantly in gills and moderately in digestive gland and labial palps of healthy oysters. The accumulation of Cg-LITAF mRNA in oyster haemocytes determined by real-time PCR showed the peak 12h after bacterial challenge. This expression pattern suggests that Cg-LITAF is a potent factor in the regulation of genes that are involved in innate defence mechanisms.

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.

Lipopolysaccharide-induced upregulation of tumor necrosis factor-alpha (TNF-alpha) and TNF receptors in rat sciatic nerve

The proinflammatory and lipopolysaccharide (LPS)-inducible cytokine tumor necrosis factor alpha (TNF-alpha) has been shown to enhance primary sensory nociceptive signaling. However, the precise cellular sites of TNF-alpha and TNF receptors synthesis are still a matter of controversy. Therefore, we focused our study on TNF-alpha, TNFR1, and TNFR2 protein synthesis and expression patterns in sciatic nerve of controls and rats under systemic challenge with LPS. The enzyme-linked immunosorbent (ELISA) assay showed that the protein level of TNF-alpha reached peak at 6 h. Double immunofluorescence revealed that LPS-induced expression of TNF-alpha exclusively located in a subpopulation of Schwann cells, endothelial cells, and macrophages, which increased at late time point in the rat sciatic nerve. Positive staining of TNF receptors were also found in Schwann cells and a few endothelial cells. These observations have demonstrated the production of this proinflammatory cytokine by peripheral nerve glia especially Schwann cells. Synthesized TNF-alpha might directly act on peripheral nerve glia via TNF receptors, but the inherent mechanisms remain unknown. Further studies are needed to confirm the pathogenic role of tumor necrosis factor in the early stage of inflammation.

Immune-mediated control of Chlamydia infection

Infection with the bacterium Chlamydia trachomatis can lead to a variety of diseases, including ectopic pregnancy, infertility and blindness. Exposure of the host to C. trachomatis stimulates multiple innate and adaptive immune effectors that can contribute towards controlling bacterial replication. However, these effectors are often insufficient to resolve the infection and prevent re-infection, and the continued presence of C. trachomatis within the host may induce immune effectors to chronically produce inflammatory cytokines. This may eventually lead to the tissue pathologies associated with the infection. Reducing the incidence and sequelae of infection will ultimately require the development of a C. trachomatis vaccine that can stimulate sterilizing immunity while avoiding immune-mediated pathology.

Differential CD28 and inducible costimulatory molecule signaling requirements for protective CD4+ T-cell-mediated immunity against genital tract Chlamydia trachomatis infection

Th1 cells and gamma interferon (IFN-gamma) production play critical roles in protective immunity against genital tract infections by Chlamydia trachomatis. Here we show that inducible costimulatory molecule (ICOS)(-/-) mice develop greatly augmented host resistance against chlamydial infection. Protection following a primary infection was characterized by strong Th1 immunity with enhanced CD4(+) T-cell-mediated IFN-gamma production in the genital tract and high expression of T-bet in the draining para-aortic lymph node. This Th1 dominance was associated with low expression of interleukin 10 (IL-10) mRNA in the uteruses of protected ICOS(-/-) mice. By contrast, CD28(-/-) mice were severely impaired in their adaptive immune response, demonstrating a lack of CD4(+) T cells and IFN-gamma in the genital tract, with a substantial delay in bacterial elimination compared to that seen in wild-type (WT) mice. Upon reinfection, WT mice exhibited a transient local infection with evidence of regulatory T-cell (Treg)/Foxp3 mRNA and a more balanced Th1 and Th2 response in the genital tract than ICOS(-/-) mice, whereas 90% of the latter mice developed sterile immunity, poor expression of local Treg/Foxp3 mRNA, and macroscopic signs of enhanced local immunopathology. Therefore, different requirements for CD28 signaling and ICOS signaling clearly apply to host protection against a genital tract infection by C. trachomatis. Whereas, CD28 signaling is critical, ICOS appears to be dispensable and can have a dampening effect on Th1 development by driving Th2 immunity and anti-inflammation through IL-10 production and promotion of the Foxp3(+) Treg populations in the genital tract. Both the CD28-deficient and the ICOS-deficient mice demonstrated poor specific antibody production, supporting the fact that antibodies are not needed for protection against genital tract chlamydial infections.

Live-attenuated influenza viruses as delivery vectors for Chlamydia vaccines

Effective delivery systems are needed to design efficacious vaccines against the obligate intracellular bacterial pathogen, Chlamydia trachomatis. Potentially effective delivery vehicles should promote the induction of adequate levels of mucosal T-cell and antibody responses that mediate long-term protective immunity. Antigen targeting to the nasal-associated lymphoid tissue (NALT) is effective for inducing high levels of specific immune effectors in the genital mucosa, and therefore suitable for vaccine delivery against genital chlamydial infection. We tested the hypothesis that live attenuated influenza A viruses are effective viral vectors for intranasal delivery of subunit vaccines against genital chlamydial infection. Recombinant influenza A/PR8/34 (H1N1) viruses were generated by insertion of immunodominant T-cell epitopes from chlamydial major outer membrane protein into the stalk region of the neuraminidase gene. Intranasal immunization of mice with viral recombinants resulted in a strong T helper 1 (Th1) response against intact chlamydial elementary bodies. Also, immunized mice enjoyed a significant state of protective immunity (P > 0.002) by shedding less chlamydiae and rapidly clearing the infection. Furthermore, a high frequency of Chlamydia-specific Th1 was measured in the genital mucosal and systemic draining lymphoid tissues within 24 hr after challenge of vaccinated mice. Moreover, multiple epitope delivery provided a vaccine advantage over single recombinants. Besides, long-term protective immunity correlated with the preservation of a robustly high frequency of specific Th1 cells and elevated immunoglobulin G2a in genital secretions. Because live attenuated influenza virus vaccines are safe and acceptable for human use, they may provide a new and reliable approach to deliver efficacious vaccines against sexually transmitted diseases.

Expression library immunization confers partial protection against Chlamydia muridarum genital infection

Protective sequences of Chlamydia muridarum were identified as potential vaccine candidates by screening a genomic DNA expression library and assessing the immune responses of mice immunized with individual library clones following vaginal challenge with live Chlamydia. Groups of female BALB/c mice were immunized intra-abdominally by gene gun delivery of DNA three times at three-weekly intervals with individual library clones expressing chlamydial protein fragments and humoral and cell-mediated immune responses were evaluated. Chlamydia-specific cytokines including tumour necrosis factor-alpha (TNF-alpha) interleukin-10 (IL-10), interleukin-4 (IL-4), interleukin-12 (IL-12) and interferon-gamma (IFN-gamma) were detected in mice immunized either with selected DNA clones in spleen cells (0.2-135.2 pg/mL) or lymph nodes (0.15-84.9 pg/mL). The most protective antigen identified was TC0512, a putative outer membrane protein (OMP). Immunization of mice with this clone elicited T-helper type-1 (Th-1) and T-helper type-2 (Th-2) cytokines as well as and IgG1 and IgG2a in sera of these animals. Ten days after the last immunization, animals were challenged intra-vaginally with 5 x 10(4) inclusion-forming units (IFUs) of C. muridarum. At 9 days following challenge TC0512 showed a 73% reduction in the number of recoverable Chlamydia compared with vector only immunized controls. Six additional clones were identified that also conferred varying degrees of protection against live chlamydial challenge. Significant protection against the initial stages of infection was shown by two DNA clones (encoding hypothetical proteins) and five clones showed enhanced clearance of chlamydial infection following DNA immunization and live chlamydial challenge. These results demonstrate that the C. muridarum genome can be screened for individual vaccine candidates by genetic immunization and that the screen produces novel and partially protective vaccine candidates.

The protective efficacy of chlamydial protease-like activity factor vaccination is dependent upon CD4+ T cells

We have previously determined the protective efficacy of intranasal vaccination with chlamydial protease-like activity factor (CPAF) against genital chlamydial infection. Since T-helper 1 (Th1) responses are important for anti-chlamydial immunity, we examined the contribution of CD4(+) T cells in CPAF mediated immunity against intravaginal (i.vag.) Chlamydia muridarum infection in C57BL/6 mice. CPAF+IL-12 vaccination induced antigen-specific CD4(+) T cells that secreted elevated levels of IFN-gamma, and generated strong humoral responses. The protective effects of CPAF vaccination against genital chlamydial challenge were abrogated by anti-CD4 neutralizing antibody treatment. Moreover, anti-chlamydial immunity could be adoptively transferred to naïve recipients using CPAF-specific CD4(+) T cells. Therefore, CPAF mediated anti-chlamydial immunity is highly dependent upon antigen-specific CD4(+) T cells.

LPS-induced TNF-alpha factor (LITAF)-deficient mice express reduced LPS-induced cytokine: Evidence for LITAF-dependent LPS signaling pathways

Previously we identified a transcription factor, LPS-Induced TNF-alpha Factor (LITAF), mediating inflammatory cytokine expression in LPS-induced processes. To characterize the role of LITAF in vivo, we generated a macrophage-specific LITAF-deficient mouse (macLITAF(-/-)). Our data demonstrate that in macrophages (i) several cytokines (such as TNF-alpha, IL-6, sTNF-RII, and CXCL16) are induced at lower levels in macLITAF(-/-) compared with LITAF(+/+) control macrophages; (ii) macLITAF(-/-) mice are more resistant to LPS-induced lethality. To further identify LITAF signaling pathways, we tested mouse TLR-2(-/-), -4(-/-), and -9(-/-) and WT peritoneal macrophages exposed to LPS. Using these cells, we now show that LITAF expression can be induced after challenge either with LPS from Porphyromonas gingivalis via agonism at TLR-2, or with LPS from Escherichia coli via agonism at TLR-4, both requiring functional MyD88. We also show that, in response to LPS, the MyD88-dependent LITAF pathway differs from the NF-kappaB pathway. Furthermore, using a kinase array, p38alpha was found to mediate LITAF phosphorylation and the inhibition of p38alpha with a p38-specific inhibitor (SB203580) blocked LITAF nuclear translocation and reduced LPS-induced TNF-alpha protein levels. Finally, macLITAF(-/-) macrophages rescued by LITAF cDNA transfection restored levels of TNF-alpha similar to those observed in WT cells. We conclude that LITAF is an important mediator of the LPS-induced inflammatory response that can be distinguished from NF-kappaB pathway and that p38alpha is the specific kinase involved in the pathway linking LPS/MyD88/LITAF to TNF.

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.

Intranasal vaccination with a defined attenuated Francisella novicida strain induces gamma interferon-dependent antibody-mediated protection against tularemia

Francisella tularensis is an intracellular gram-negative bacterium that is the causative agent of tularemia and a potential bioweapon. We have characterized the efficacy of a defined F. novicida mutant (DeltaiglC) as a live attenuated vaccine against subsequent intranasal challenge with the wild-type organism. Animals primed with the F. novicida DeltaiglC (KKF24) mutant induced robust splenic gamma interferon (IFN-gamma) and interleukin-12 (IL-12) recall responses with negligible IL-4 production as well as the production of antigen-specific serum immunoglobulin G1 (IgG1) and IgG2a antibodies. BALB/c mice vaccinated intranasally (i.n.) with KKF24 and subsequently challenged with wild-type F. novicida (100 and 1,000 50% lethal doses) were highly protected (83% and 50% survival, respectively) from the lethal challenges. The protection conferred by KKF24 vaccination was shown to be highly dependent on endogenous IFN-gamma production and also was mediated by antibodies that could be adoptively transferred to naive B-cell-deficient mice by inoculation of immune sera. Collectively, the results demonstrate that i.n. vaccination with KKF24 induces a vigorous Th1-type cytokine and antibody response that is protective against subsequent i.n. challenge with the wild-type strain. This is the first report of a defined live attenuated strain providing protection against the inhalation of F. novicida.

Chlamydia trachomatis polymorphic membrane protein D is a species-common pan-neutralizing antigen

Infections caused by the obligate intracellular pathogen Chlamydia trachomatis have a marked impact on human health. C. trachomatis serovariants are the leading cause of bacterial sexually transmitted disease and infectious preventable blindness. Despite decades of effort, there is no practical vaccine against C. trachomatis diseases. Here we report that all C. trachomatis reference serotypes responsible for sexually transmitted disease and blinding trachoma synthesize a highly conserved surface-exposed antigen termed polymorphic membrane protein D (PmpD). We show that Ab specific to PmpD are neutralizing in vitro. We also present evidence that Ab against serovariable-neutralizing targets, such as the major outer membrane protein, block PmpD neutralization. This finding suggests that a decoy-like immune evasion strategy may be active in vivo whereby immunodominant type-specific surface antigens block the neutralizing ability of species-common PmpD Ab. Collectively, these results show that PmpD is a previously uncharacterized C. trachomatis species-common pan-neutralizing target. Moreover, a vaccine protocol using recombinant PmpD to elicit neutralizing Ab in the absence of immunodominant type-specific Ab might be highly efficacious and surpass the level of protection achieved through natural immunity.