Antigenic and immunogenic properties of recombinants from Salmonella typhimurium and Salmonella minnesota rough mutants expressing in their lipopolysaccharide a genus-specific chlamydial epitope.

Rough mutants from Salmonella typhimurium and Salmonella minnesota were transformed with a plasmid containing a 6.5-kilobase insert of DNA from Chlamydia trachomatis assumed to encode a glycosyltransferase. Transformation resulted in the expression of a genus-specific chlamydial epitope on the lipopolysaccharide (LPS) of the recombinant strains. Proteinase K-digested whole-cell lysates of the recombinants and of controls were subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by silver staining or Western blot analysis. Two LPS populations were detected in the recombinants, the parent LPS and a faster-migrating component. The latter stained with monoclonal antibody against the genus-specific chlamydial epitope and was not seen in the controls. LPS was extracted and purified from recombinants of S. minnesota R595 and R4 and characterized by the passive hemolysis and passive hemolysis inhibition assays and by hydrolysis kinetics. Different antigenic determinants could be distinguished from each other by the passive hemolysis inhibition test with monospecific antigen-antibody reactions. Rabbits were immunized with heat-killed recombinant bacteria to study the immunogenic properties of the recombinants. In all animals, antibodies were raised against the parent core specificity and against the chlamydia-specific epitope. The data show that the recombinant bacteria are useful as immunogens to prepare polyclonal antisera against chlamydiae and that LPS isolated from them exhibits the same antigenic determinants as chlamydial LPS and may thus be used as a substitute for chlamydial LPS in serological assays.