Discovery of Human-Specific Immunodominant <i>Chlamydia trachomatis</i> B Cell Epitopes

Abstract

<i>Chlamydia</i> species-specific serology is compromised by cross-reactivity of the gold standard microimmunofluorescence (MIF) or commercial enzyme-linked immunosorbent assays (ELISAs). This study was conducted to discover novel <i>C. trachomatis</i>-specific peptide antigens that were recognized only by the antibody response of the natural human host. We evaluated a library of 271 peptide antigens from immunodominant <i>C. trachomatis</i> proteins by reactivity with 125 <i>C. trachomatis</i> antibody-positive sera from women with PCR-confirmed <i>C. trachomatis</i> infection and 17 <i>C. trachomatis</i> antibody-negative sera from low-risk women never diagnosed with <i>C. trachomatis</i> infection. These <i>C. trachomatis</i> peptide antigens had been predicted <i>in silico</i> to contain B cell epitopes but had been nonreactive with mouse hyperimmune sera against <i>C. trachomatis</i> We discovered 38 novel human host-dependent antigens from 20 immunodominant <i>C. trachomatis</i> proteins (PmpD, IncE, IncG, CT529, CT618, CT442, TarP, CT143, CT813, CT795, CT223, PmpC, CT875, CT579, LcrE, IncA, CT226, CT694, Hsp60, and pGP3). Using these human sera, we also confirmed 10 <i>C. trachomatis</i> B cell epitopes from 6 immunodominant <i>C. trachomatis</i> proteins (OmpA, PmpD, IncE, IncG, CT529, and CT618) as host species-independent epitopes that had been previously identified by their reactivity with mouse hyperimmune sera against <i>C. trachomatis</i> ELISA reactivities against these peptides correlated strongly with the <i>C. trachomatis</i> microimmunofluorescence (MIF) text results (Pearson's correlation coefficient [<i>R</i>] = 0.80; <i>P</i> < 10^-6). These <i>C. trachomatis</i> peptide antigens do not cross-react with antibodies against other <i>Chlamydia</i> species and are therefore suitable for species-specific detection of antibodies against <i>C. trachomatis</i> This study identified an extended set of peptide antigens for simple <i>C. trachomatis</i>-specific ELISA serology.<b>IMPORTANCE</b> Current serological assays for species-specific detection of anti-<i>Chlamydia</i> species antibodies suffer from well-known shortcomings in specificity and ease of use. Due to the high prevalences of both anti-<i>C. trachomatis</i> and anti-<i>C. pneumoniae</i> antibodies in human populations, species-specific serology is unreliable. Therefore, novel specific and simple assays for chlamydial serology are urgently needed. Conventional antigens are problematic due to extensive cross-reactivity within <i>Chlamydia</i> spp. Using accurate B cell epitope prediction and a robust peptide ELISA methodology developed in our laboratory, we identified immunodominant <i>C. trachomatis</i> B cell epitopes by screening performed with sera from <i>C. trachomatis</i>-infected women. We discovered 38 novel human host-dependent antigens from 20 immunodominant <i>C. trachomatis</i> proteins, in addition to confirming 10 host-independent mouse serum peptide antigens that had been identified previously. This extended set of highly specific <i>C. trachomatis</i> peptide antigens can be used in simple ELISA or multiplexed microarray formats and will provide high specificity and sensitivity to human <i>C. trachomatis</i> serodiagnosis.