Immunogenicity evaluation of a lipidic amino acid-based synthetic peptide vaccine for Chlamydia trachomatis

Lipidic amino acid-based synthetic peptides derived from the variable domains (VD) of Chlamydia trachomatis outer membrane protein 1 were evaluated as potential candidate sequences in a vaccine. A peptide sequence designated P2 from the VD IV of serovar B contained a B cell epitope capable of eliciting antibodies binding to serovar B elementary bodies (EB) and a T helper site capable of presentation by multiple H-2 alleles. Polymerization of the P2 into polylysine to form lipid core peptides (LCP) significantly enhanced immunogenicity compared with P2 monomer alone. The LCP system incorporates lipidic amino acids into the polylysine system and enhances lipophobicity and membrane binding effects of the peptide. A second peptide sequence derived from the VD I of serovar C was cosynthesized with P2 into lipidic polylysine LCP and was designated LCP-H1. Antibodies to this construct reacted at high titer with EB of the three major trachoma causing C. trachomatis serovars A, B, and C. LCP-H1 was immunogenic among four of five murine H-2 alleles. Pepscan analysis showed that the fine specificity of antibodies generated to LCP-H1 were directed to the predetermined neutralizing epitope sequences. An in vitro HAK cell neutralization assay showed that LCP-H1 elicited neutralizing antibodies to serovars A, B, and C, but these were of low titer. Because LCP-H1 antibodies bound to the peptide sequence with 10-100-fold higher titer than to EB, the low neutralization titers most likely result from conformational differences between the synthetic peptide and antigenic sites on the native organism. Modification of LCP-H1 to incorporate a predefined conformation may result in improved antigenic properties.