Prior exposure to infection with Chlamydia pneumoniae can influence the T‐cell‐mediated response to Chlamydia trachomatis

Using T‐cell clones derived from patients with Chlamydia trachomatis (CT)‐induced reactive arthritis, we identified target antigens and mapped the peptide epitopes that were recognized. Several epitopes were conserved in homologous proteins of Chlamydia pneumoniae (CPN), and it was shown that these epitopes were generated following processing of the CPN proteins or CPN elementary bodies, i.e. the T‐cell clones were indeed CT and CPN cross‐reactive. Given that CPN infection is frequent, we wished to determine whether prior infection with CPN could have an effect on the response to subsequent infection with CT. First, we showed that the CPN antigen, OmcB, was recognized by polyclonal peripheral blood T cells from additional subjects with CT‐induced reactive arthritis; they were chosen to be HLA‐DR‐matched with the T‐cell clones used to map epitopes in OmcB. Responses to a peptide previously shown to be conserved in CT and CPN OmcB were also seen, but only in CPN‐seropositive individuals. These subjects also produced interferon‐γ (IFN‐γ) in response to CPN OmcB, and did not recognize a nonconserved epitope in OmcB. Secondly, OmcB‐responsive clones from CPN‐seropositive subjects were dominated by those recognizing the cross‐reactive epitope, despite the recent exposure of these subjects to CT. Lastly, healthy CPN‐seropositive subjects, without evidence of exposure to CT, showed greater responses, measured as IFN‐γ secretion, to CT proteins in vitro than those shown by seronegative subjects. This is consistent with the idea that prior CPN infection primes a Th1 T‐cell response to CT antigens. This finding is relevant to the pathogenesis of the sequelae of CT infection (trachoma, infertility and arthritis), which may be influenced by prior exposure to CPN, and to the choice of CT antigens as vaccine candidates.