Conjunctival chlamydial 16S ribosomal RNA expression in trachoma: is chlamydial metabolic activity required for disease to develop?

BACKGROUND: Nucleic acid amplification testing frequently detects conjunctival Chlamydia trachomatis infection in subjects without clinical signs of trachoma. It is unclear whether such subjects are actually infected. We measured chlamydial 16S ribosomal RNA (rRNA) expression, a marker of chlamydial metabolic activity, in comparison with the quantitation of a chlamydial DNA target in subjects exposed to trachoma.
METHODS: Subjects from 2 Gambian villages where trachoma was endemic were examined. Conjunctival samples were tested for the presence of C. trachomatis DNA using a quantitative real-time polymerase chain reaction (PCR) assay for the omp1 gene and for the expression of C. trachomatis 16S rRNA using a 1-step, real-time reverse-transcriptase PCR assay.
RESULTS: A total of 248 people were examined. The prevalence of clinically active trachoma was 16.9%. C. trachomatis was detected in 19.8% and 6.8% of subjects by the omp1 and 16S rRNA assays, respectively. For subjects with positive results for both tests, the number of copies of 16S rRNA was approximately 100-fold greater than the number of copies of the omp1 gene. In samples from subjects in whom the omp1 gene was detected in the absence of 16S rRNA, typically only a few copies of omp1 were present. The expression of 16S rRNA was strongly associated with the presence of clinical signs of active trachoma.
CONCLUSIONS: The use of 16S rRNA expression for the detection of chlamydial metabolic activity appears to usefully discriminate established infections from the inoculation of the conjunctiva with dead or subviable organisms, which probably occurs frequently in settings in which trachoma is endemic. The data support conclusions from primate challenge studies that live Chlamydiae species or antigens derived from them are needed to provoke the clinical signs of disease.