Serum reactivity to Chlamydia trachomatis and C. pneumoniae antigens in patients with documented infection and in healthy children by microimmunofluorescence and immunoblotting techniques.

Chlamydia trachomatis and C. pneumoniae are both important human pathogens. Antigenic cross-reactivity between the two species may complicate serologic diagnosis of infection with one or the other agent. In this study we examined sera of persons with chlamydia infections and of healthy children by microimmunofluorescence (MIF) against C. trachomatis L2 antigen and against C. pneumoniae TW-183 antigen to explore the degree of cross-reactivity found by these two methods. Likewise, the cross-reactivity seen by immunoblot with sera of rabbits immunized against one of the antigens when tested on the other was examined. While among healthy children stratified by age, MIF seropositivity to C. pneumoniae TW-183 increased with age, no such trend was observed with respect to seropositivity to C. trachomatis L2 antigens, and 81% of children seropositive to TW-183 did not react on L2 antigen. Moreover, 27% of those positive on L2 antigen were negative on TW-183. Immunoblot analysis showed much greater antibody cross-reactivity than that detected by MIF. The immunoblot cross-reactivity was probably not attributable solely to double infection since sera of rabbits immunized to only one species of chlamydia reacted strongly with both chlamydiae in immunoblot analysis. The data presented need to be taken into account in the development of serologic tests based on a small number of antigens or on partially denatured antigens.