Chlamydia pneumoniaeMajor Outer Membrane Protein Is a Surface-Exposed Antigen That Elicits Antibodies Primarily Directed against Conformation-Dependent Determinants

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Abstract
The major outer membrane protein (MOMP) ofChlamydia trachomatisserovariants is known to be an immunodominant surface antigen. Moreover, it is known that theC. trachomatisMOMP elicits antibodies that recognize both linear and conformational antigenic determinants. In contrast, it has been reported that the MOMP ofChlamydia pneumoniaeis not surface exposed and is immunorecessive. We hypothesized that the discrepancies betweenC. trachomatisandC. pneumoniaeMOMP exposure on intact chlamydiae and immunogenic properties might be because the focus of the host's immune response is directed to conformational epitopes of theC. pneumoniaeMOMP. We therefore conducted studies aimed at defining the surface exposure of MOMP and the conformational dominance of MOMP antibodies. We present here a description ofC. pneumoniaespecies-specific monoclonal antibody (MAb), GZD1E8, which recognizes a conformational epitope on the surface ofC. pneumoniae. This MAb is potent in the neutralization ofC. pneumoniaeinfectivity in vitro. Another previously describedC. pneumoniaespecies-specific monoclonal antibody, RR-402, displayed very similar characteristics. However, the antigenic determinant recognized by RR-402 has yet to be identified. We show by immunoprecipitation ofC. pneumoniaewith GZD1E8 and RR-402 MAbs and by mass spectrometry analysis of immunoprecipitated proteins that both antibodies GZD1E8 and RR-402 recognize the MOMP ofC. pneumoniaeand that this protein is localized on the surface of the organism. We also show that human sera fromC. pneumoniae-positive donors consistently recognize the MOMP by immunoprecipitation, indicating that the MOMP ofC. pneumoniaeis an immunogenic protein. These findings have potential implications for bothC. pneumoniaevaccine and diagnostic assay development.