Several isolates of Chlamydia pneumoniae were compared with each other and to Chlamydia trachomatis and Chlamydia psittaci by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblots. Protein profiles of the C. pneumoniae isolates appeared to be the same and were distinct from the other Chlamydia species. A 39.5-kilodalton (kDa) protein, similar in molecular weight to the major outer membrane proteins (MOMP) of C. trachomatis and C. psittaci, was found in the Sarkosyl-insoluble fraction, demonstrating its association with the outer membrane complex. In the outer membrane complex, the MOMP was shown to exist in disulfide-linked protein complexes. Electron microscopy of the Sarkosyl-extracted elementary bodies showed that the structural rigidity and pear-shaped morphology remained intact. Rabbit immune sera prepared against C. pneumoniae demonstrated immunoreactive proteins of 98-, 77-, 75-, 66-, 60-, 39.5-, 28-, and 17.5-kDa proteins. Cross-reactivity experiments revealed that most of the antigenic reactivities shared between C. psittaci and C. trachomatis extend to C. pneumoniae and that the 98-kDa protein recognition appeared to be C. pneumoniae specific. In contrast to the other Chlamydia spp., the recognition of the C. pneumoniae MOMP by homologous immune sera was weak and was cross-reactive with the MOMPs of the other Chlamydia species. These results suggest that the C. pneumoniae MOMP is less immunogenic and antigenically complex than are the MOMPs of C. trachomatis and C. psittaci.
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