An oligomer of the major outer membrane protein of Chlamydia psittaci is recognized by monoclonal antibodies which protect mice from abortion.

Monoclonal antibodies (MAbs) were generated against an ovine abortive strain of Chlamydia psittaci. A plaque reduction assay was used to select 19 neutralizing antibodies which appeared to be heterogeneous in isotype, specificity, and recognized proteins. Different neutralizing MAbs were tested for their protective abilities against abortion in a pregnant-mouse model. All of the protective MAbs selected had the same isotype, were serotype 1 specific, and recognized a protein of about 110 kDa by immunoblotting. The recognized epitopes were resistant to sodium dodecyl sulfate and reducing agents, but all of them were heat sensitive. The protein was able to form disulfide-linked polymers. Immunological cross-reaction studies with rabbit sera showed a link between the 110-kDa protein and the major outer membrane protein (MOMP). The 110-kDa protein was purified by immunoaffinity and shown to be dissociated after heating into MOMP by silver staining and immunoblotting. These results show homogeneity among protective MAbs directed to heat-sensitive epitopes located on an oligomer of the MOMP of C. psittaci.