Evaluation of Chlamydia psittaci subfraction and subunit preparations for their protective capacities

A mouse toxicity potency test was used to evaluate the protective capacity of Chlamydia psittaci (Baker strain) subfraction and subunit preparations. The elementary body (EB), Chlamydial outer membrane complex (COMC), major outer membrane protein (MOMP), lipopolysaccharide (LPS), and MOMP mixed together with LPS were utilized as antigens in vaccine preparation. The EBs and COMCs were prepared by centrifugation and detergent extraction as previously described for Chlamydia trachomatis. Isolation and purification of MOMP and LPS were achieved by subjecting infected tissue culture fluids to polyacrylamide gel electrophoresis and subsequent elution from the gel. Chromatographic purification of the MOMP from EBs resulted in a preparation which also contained minor quantities of LPS. After vaccination and intraperitoneal challenge with C. psittaci (Cello strain), the MOMP presented in EBs, COMCs or in a purified form, was capable of protecting mice from death, whereas LPS did not demonstrate this capability. Further comparison of the protective capacity of these antigens indicated that the electrophoretically purified MOMP was the least protective when suboptimal levels of antigens were administered to mice. However, the level of protection afforded by electrophoretically purified MOMP could be increased to that observed with the chromatographically purified MOMP by the addition of electrophoretically purified LPS to the vaccine. These results suggest that the MOMP is the major protective antigen of C. psittaci and that the LPS component, although not protective on its own, may play a role in the potentiation of a protective immune response.