Chlamydia pneumoniae major outer membrane protein is a surface-exposed antigen that elicits antibodies primarily directed against conformation-dependent determinants.

The major outer membrane protein (MOMP) of Chlamydia trachomatis serovariants is known to be an immunodominant surface antigen. Moreover, it is known that the C. trachomatis MOMP elicits antibodies that recognize both linear and conformational antigenic determinants. In contrast, it has been reported that the MOMP of Chlamydia pneumoniae is not surface exposed and is immunorecessive. We hypothesized that the discrepancies between C. trachomatis and C. pneumoniae MOMP exposure on intact chlamydiae and immunogenic properties might be because the focus of the host's immune response is directed to conformational epitopes of the C. pneumoniae MOMP. We therefore conducted studies aimed at defining the surface exposure of MOMP and the conformational dominance of MOMP antibodies. We present here a description of C. pneumoniae species-specific monoclonal antibody (MAb), GZD1E8, which recognizes a conformational epitope on the surface of C. pneumoniae. This MAb is potent in the neutralization of C. pneumoniae infectivity in vitro. Another previously described C. pneumoniae species-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 of C. pneumoniae with GZD1E8 and RR-402 MAbs and by mass spectrometry analysis of immunoprecipitated proteins that both antibodies GZD1E8 and RR-402 recognize the MOMP of C. pneumoniae and that this protein is localized on the surface of the organism. We also show that human sera from C. pneumoniae-positive donors consistently recognize the MOMP by immunoprecipitation, indicating that the MOMP of C. pneumoniae is an immunogenic protein. These findings have potential implications for both C. pneumoniae vaccine and diagnostic assay development.