Importance of species-specific antigens in the serodiagnosis of Chlamydia trachomatis reactive arthritis.

Objectives. To determine the most sensitive and specific method of anti-Chlamydia antibody measurement for the serodiagnosis of Chlamydia trachomatis reactive arthritis. Methods. Immunoblotting, enzyme-linked immunosorbent assays using six synthetic peptides or recombinant antigens and a microimmunofluorescence test were used to determine the presence of IgG, IgM and IgA in serum samples from 17 patients with C. trachomatis reactive arthritis. Twenty patients with other inflammatory arthropathies without evidence of urogenital C. trachomatis infection were used as controls. Results. The best association of sensitivity (76%) and specificity (85%) was obtained when IgG anduor IgA reactivity to two species-specific antigens was determined. These antigens were synthetic peptides, derived from species-specific epitopes in the variable domain IV of the major outer membrane protein (MOMP) (Labsystems, Finland) and recombinant polypeptide encoded by open reading frame 3 of the plasmid (pgp3). Conclusions. IgG anduor IgA anti-MOMP-derived peptides and anti-pgp3 could be useful for the diagnosis of probable C. trachomatis reactive arthritis. Reactive arthritis due to Chlamydia trachomatis is not always associated with obvious urogenital symptoms. Moreover, evidence of C. trachomatis infection by positive urethraluendocervical culture of bacteria or positive antigen or DNA detection can be missing at the time of the arthritis, and the diagnosis of the triggering infection relies on serology. However, both the sensitivity and specificity of serological tests need to be improved w1x. In two previous studies, we compared serum antiChlamydia antibody responses, obtained for patients with well defined urethraluendocervical infection, with those obtained for healthy blood donors. The best sensitivity (86%) with a specificity of 81% was obtained for immunoblot results, when the number of individuals with 010 IgG anduor 02 IgM responses to the different C. trachomatis antigens was considered w2x. Concerning antibody responses to peptides or recombinant antigens, the best sensitivity (79%) associated with the best specificity (82%) was obtained when IgG responses to both synthetic peptides, derived from species-specific epitopes in the variable domain IV of the major outer membrane protein (MOMP) (Labsystems, Finland) and recombinant polypeptide encoded by open reading frame 3 of the plasmid (pgp3) were considered w3x .M OMP is a surface-exposed, integral membrane protein of approximately 40 kDa, found in both the extracellular infectious elementary bodies and the non-infectious intracellular reticulate bodies. Pgp3 is a protein of approximately 27 kDa, predominantly found in chlamydial outer membrane complex preparation w4x. Therefore, we evaluated whether these new assays can also increase sensitivity and specificity of the serological diagnosis of C. trachomatis reactive arthritis. The specificity was evaluated from samples of patients with inflammatory arthropathies without evidence of C. trachomatis infection, half of the cases being other bacterial arthropathies. Such controls are necessary in view of unspecific reactions possibly due to inflammation anduor presence of cross-reacting antibodies, due to other bacterial infections.