The major outer membrane protein of Chlamydia trachomatis: critical binding site and conformation determine the specificity of antibody binding to viable chlamydiae

Human antibody signatures towards the Chlamydia trachomatis major outer membrane protein after natural infection and vaccination

BACKGROUND

Chlamydia trachomatis (CT) Major Outer Membrane Protein (MOMP) holds a neutralising epitope in the Variable Domain 4 (VD4), and this region's immune dominance during infection is well known. This study aimed to assess the antibody response induced after infection and compare it for specificity and functionality to the response following vaccination with the vaccine CTH522, which contains VD4's from serovars D, E, F, and G.

METHODS

We assessed the antibody epitopes in MOMP by a high density peptide array. Furthermore, the role of the VD4 epitope in neutralisation was explored by competitive inhibition experiments with a fusion protein holding the neutralising VD4 linear epitope. This was done in two independent groups: 1) MOMP seropositive individuals infected with CT (n = 10, from case-control study) and 2) CTH522/CAF®01-vaccinated females (n = 14) from the CHLM-01 clinical trial.

FINDINGS

We identified the major antigenic regions in MOMP as VD4 and the conserved region just before VD3 in individuals infected with CT. The same regions, with the addition of VD1, were identified in vaccine recipients. Overall, the VD4 peptide responses were uniform in vaccinated individuals and led to inhibition of infection in vitro in all tested samples, whereas the VD4 responses were more heterogenous in individuals infected with CT, and only 2 out of 10 samples had VD4-mediated neutralising antibody responses.

INTERPRETATION

These data provide insights into the role of antibodies against MOMP VD4 induced after infection and vaccination, and show that their functionality differs. The induction of functional VD4-specific antibodies in vaccine recipients mimics previous results from animal models.

FUNDING

This work was supported by the European Commission through contract FP7-HEALTH-2011.1.4-4-280873 (ADITEC) and Fonden til Lægevidenskabens Fremme.

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.

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

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

Characterization of the humoral response induced by a peptide corresponding to variable domain IV of the major outer membrane protein of Chlamydia trachomatis serovar E

A 30-amino-acid peptide corresponding to variable domain IV (VD IV) of the major outer membrane protein of Chlamydia trachomatis serovar E was conjugated to keyhole limpet hemocyanin (KLH) and used to immunize mice. The resulting antisera (anti-KLH-VD IV sera) recognized all 15 serovars of C. trachomatis when assayed by indirect immunofluorescence and Western blotting. Probing of overlapping hexameric peptides representing VD IV with mouse anti-KLH-VD IV sera revealed that two main regions of the peptide were recognized by the antisera, the N terminus of the peptide, which contains B-complex-specific epitopes, and the middle region of the peptide, which contains a species-conserved domain. When used in an in vitro neutralization assay, these antisera were able to neutralize mainly serovars in the B complex. These data provide evidence that a linear peptide corresponding to VD IV can induce in vitro protection from C. trachomatis infectivity that is subspecies specific.

Functional and structural mapping of Chlamydia trachomatis species-specific major outer membrane protein epitopes by use of neutralizing monoclonal antibodies

Three monoclonal antibodies (MAbs), E4, L1-4, and L1-24, to the major outer membrane protein (MOMP) of Chlamydia trachomatis were identified that neutralized in vitro the infectivity of members of the B- and C-related complex as well as the mouse pneumonitis strain. MAbs L1-4, L1-24, and E4 gave a strong signal in an indirect immunofluorescence assay and/or Western immunoblot with all serovars of the lymphogranuloma venereum and trachoma biovars and a weak signal with the mouse biovar. In addition, C. psittaci and C. pneumoniae were also weakly recognized by MAbs L1-4 and L1-24. As determined by the technique of pneumoniae were also weakly recognized by MAbs L1-4 and L1-24. As determined by by the technique of overlapping peptides, all three MAbs showed reactivity to variable domain (VD) IV of MOMP. While all three MAbs had different recognition patterns, all strongly bound to the peptides TLNPTI and LNPTIA within the species-conserved region of VD IV. MAb E4 also recognized the peptide SATAIF in the subspecies region of VD IV. Peptides corresponding to VD IV of MOMP were synthesized and used in competitive inhibition experiments to determine the functional location of the epitope recognized by these three MAbs. Both the serological and neutralizing activities of MAb E4 were inhibited by the peptides ATAIFDTTTLNPTIAG and FDTTTLNPTIAG; however, none of the peptides made to the VD IV region blocked the neutralizing activity of MAbs L1-4 and L1-24. Therefore, the neutralizable domain of the epitope recognized by MAb E4 is contiguous and may be an important candidate for inclusion in a subunit vaccine.

Mapping antigenic sites on the major outer membrane protein of Chlamydia trachomatis with synthetic peptides

The antigenicity of the major outer membrane protein (MOMP) of Chlamydia trachomatis was comprehensively evaluated by using overlapping hexapeptide homologs of serovar B MOMP and polyclonal rabbit antisera in a peptide enzyme-linked immunosorbent assay. Of 367 hexapeptides, 152 showed reactivities with at least one antiserum. Seven hexapeptides located within variable domain (VD) IV (residues 288 to 316) were found to be most reactive in terms of their binding titer and frequency, suggesting that VD IV is the immunodominant region within the MOMP as detected by this assay. Peptide-reactive antibodies could also recognize corresponding epitopes on either viable or acetone-permeabilized organisms. The antigenic specificity and immunoaccessibility of epitopes located in VD IV were resolved by absorbing antisera with chlamydial elementary bodies. Six antigenic sites were found in this region and included a B-type-specific site (S1), four subserogroup-specific sites (S2 and S4 to 6), and one species-specific site (S3), each displaying varying degrees of surface exposures on elementary bodies from different C. trachomatis serovars.