Humoral immune response to plasmid protein pgp3 in patients with Chlamydia trachomatis infection.

We identified, by two-dimensional electrophoretic analysis and microsequencing, a protein of Chlamydia trachomatis elementary bodies which corresponds to the polypeptide (pgp3) encoded by open reading frame 3 (ORF3). Amino acid analysis showed that the first residue (Gly) found in the native protein is the one encoded by the second ORF3 codon, implying a typical bacterial removal of the first Met residue. Relatively large amounts of recombinant pgp3 (r-pgp3) in a stable, water-soluble form were obtained by overexpressing ORF3 in Escherichia coli and purifying the product from periplasmic extracts under nondenaturing conditions. These r-pgp3 preparations allowed specific detection of anti-pgp3 antibodies by enzyme-linked immunosorbent assay. Analysis of a group of 170 sera from healthy blood donors and from patients who were seropositive or -negative for C. trachomatis and Chlamydia pneumoniae showed that an immune response to pgp3 occurs in the majority (ca. 81%) of patients with sexually transmitted diseases who are seropositive for C. trachomatis and generally correlates with the response to cell surface antigens. No reaction between r-pgp3 and C. pneumoniae-positive sera was detected.