Defining species-specific immunodominant B cell epitopes for molecular serology of Chlamydia spp.

ABSTRACT

Urgently needed species-specific ELISAs for detection of antibodies against Chlamydia (C.) spp. have been elusive due to high cross-reactivity of chlamydial antigens. To identify Chlamydia species-specific B cell epitopes for such assays, we ranked potential epitopes of immunodominant chlamydial proteins that are polymorphic among all Chlamydia species. High-scoring peptides were synthesized with N-terminal biotin followed by a serine-glycine-serine-glycine spacer, immobilized onto streptavidin-coated microtiter plates, and tested with mono-specific mouse hyperimmune sera against each Chlamydia species in chemiluminescent ELISAs. For each of nine Chlamydia species, 3-9 dominant polymorphic B cell epitope regions were identified on OmpA, CT618, PmpD, IncA, CT529, CT442, IncG, Omp2, TarP, and IncE proteins. Peptides corresponding to 16-40 amino acid-long species-specific sequences of these epitopes reacted highly and with absolute specificity with homologous, but not heterologous, Chlamydia mono-species-specific sera. Host-independent reactivity of such epitopes was confirmed by testing of 6 C. pecorum-specific peptides from 5 proteins with C. pecorum-reactive sera from cattle, the natural host of C. pecorum. The probability of cross-reactivity of peptide antigens from closely related chlamydial species or strains correlated with percent sequence identity, and declined to zero at less than 50% sequence identity. Thus, phylograms of B cell epitope regions predict the specificity of peptide antigens for rational use in genus-, species-, or serovar-specific molecular serology of Chlamydia spp. We anticipate that these peptide antigens will improve chlamydial serology by providing easily accessible assays to non-specialist laboratories. Our approach also lends itself to identification of relevant epitopes of other microbial pathogens.