DIAGNOSTIC MICROBIOLOGY Detection of Chlamydia trachomatis -speci®c antibodies in human sera by recombinant major outer-membrane protein polyantigens PER MYGIND, GUNNA CHRISTIANSEN, KENNETH PERSSON  and SVEND BIRKELUND Department of Medical Microbiology and Immunology, University of Aarhus, DK-8000 Aarhus, Denmark and  Department of Virology, University Hospital, Malmo È, Sweden This study was performed to generate and evaluate recombinant antigens for use in a species-speci®c Chlamydia trachomatis immunoassay. In a molecular genetic approach, fragments of the C. trachomatis major outer-membrane protein MOMP) were produced as fusion proteins to create three different constructs encompassing the variable domains I, II and IV of selected C. trachomatis serovars. The recombinant MOMP polyantigens were af®nity-puri®ed and used in an enzyme-linked immunosorbent assay. Antibody detection was evaluated with 103 patient sera and the results were compared with titres obtained in the micro-immuno¯uorescence test. The results showed that the generated MOMP polyantigens detected the presence of C. trachomatis-speci®c human antibodies with little cross-reaction to C. pneumoniae-speci®c antibodies. When compared to the micro-immuno¯uorescence assay the MOMP polyantigen detected the presence of anti-C. trachomatis IgG antibodies with a sensitivity of 80% and a speci®city of 91%. Introduction The eubacterial genus Chlamydia comprises a group of obligately intracellular pathogens that proliferate through a unique biphasic developmental cycle. The organisms alternate between an infective elementary body, EB) and a replicating form reticulate body, RB). The genus consists of four species ± C. trachomatis, C. pneumoniae, C. pecorum and C. psittaci. C. tracho- matis and C. pneumoniae cause a wide variety of infections in man, wherease C. psittaci and C. pecorum are animal pathogens of particular interest in veterinary medicine. Occasionally, C. psittaci causes severe infections of the respiratory tract in man known as psittacosis or ornithosis. C. trachomatis is divided into serovars, all of which are human pathogens except the mouse pneumonitis serovar MoPn). The trachoma biovars serovars A± C) cause conjunctivitis, leading to trachoma, the primary cause of preventable blindness in third world countries. Serovars D±K are all pathogens of the urogenital tract. They are the most common cause of sexually transmitted genital infections, eliciting local acute epithelial infections, which can lead to pelvic in¯ammatory disease. Chronic infections by C. tracho- matis serovars D±K occasionally cause tubal obstruc- tion or ectopic pregnancy. The serovars L1±L3 of C. trachomatis infect both macrophages and epithelial cells and are spread systemically through the lymphatic tissue, causing the invasive disease known as lympho- granuloma venereum [1]. C. pneumoniae causes pneumonia, bronchitis and pharyngitis [2]. Recently, this human pathogen has been associated with chronic disease such as asthma, chronic bronchitis, acute myocardial infarction and coronary artery disease [3±5]. Studies show that anti- C. pneumoniae antibodies are widespread in the adult population 50%) [6]. The aim of this study was to develop a serological assay able to measure speci®c antibodies to C. trachomatis genital serovars with little cross-reaction to C. pneumoniae-speci®c antibodies. In cases of upper genital tract infection, it is important to use serology tests that can distinguish antibodies to C. trachomatis from those associated with non-genital chlamydiae. This is the main problem of Chlamydia serology today. A large retrospective study showed that antibodies to C. pneumoniae account for up to half of all chlamydia IgG-positive patients attending genitourinary medicine clinics [7]. Similar results were obtained when Received 16 Aug. 1999; revised version received 20 Oct. 1999; accepted 21 Oct. 1999. Corresponding author: Dr P. Mygind e-mail: perm@ biobase.dk). J. Med. Microbiol. Ð Vol. 49 2000), 457±465 # 2000 The Pathological Society of Great Britain and Ireland ISSN 0022-2615