Detection of Chlamydia trachomatis antigen by radioimmunoassay

A sensitive indirect radioimmunoassay (RIA) was developed for detection of chlamydial antigens in infected, irradiated McCoy cell culture. Polystyrene beads were used as the solid phase, guinea pig antichlamydial immunoglobulins were used as the captive antibodies, rabbit antichlamydial immunoglobulins were used as the secondary antibodies, and 125I- labelled sheep antirabbit immunoglobulin was used as the indicator antibodies. The immunizations were done intracutaneously with purified genital and lymphogranuloma venereum Chlamydia trachomatis strains grown in the yolk sacs of embryonated eggs. The bound radioactivity was a function of the amount of chlamydial antigen and the method demonstrated the antigen approximately 20 hours post infection. Also noninfectious chlamydial antigen was detectable by RIA. The sensitivity of the assay was about 10 ng/ml for purified antigen and less than 10 inclusions in the cell culture. Each chlamydial serotype could be detected. The RIA was found to be more sensitive than iodine staining and as rapid and sensitive as immunofluorescence method to demonstrate chlamydial infection in cell culture.