Attachment of Mycoplasma pneumoniae to host cell by means of a specialized terminus initiates infection. Monoclonal antibodies to a surface protein (Pl) inhibit this process, and react with a region of the tip covered with peplomer-like particles. Since antibodies against the Pl protein are generated by natural and experimental infection and by immunization, the substance may be an important determinant of protective immunity.

In previous studies with hyperimmune rabbit sera and monoclonal antibodies against the P1 protein of Mycoplasma pneumoniae, we obtained evidence of a shared antigenic determinant with a single protein of Mycoplasma genitalium. Because of biologic and morphologic similarities between these two human Mycoplasma species, attempts were made to characterize this cross-reacting protein of M. genitalium (designated MgPa). The protein was surface exposed and had an estimated molecular size of 140 kilodaltons. Electron microscopy with monoclonal antibodies produced against either MgPa or P1 demonstrated that MgPa is located over the surface of the terminal structure of M. genitalium which is covered by a nap layer. These immunologic and morphologic findings suggest that the MgPa protein of M. genitalium could be the counterpart of the P1 protein of M. pneumoniae. Mycoplasma genitalium is a new mycoplasma isolated from the urethral specimens of human patients with nongonococcal urethritis (23, 30, 31). Although technical problems have impeded large-scale surveys for the prevalence of the organism, seroepidemiologic data and experimental infections suggest its pathogenicity. Antibodies specific to M. genitalium, but not to Chlamydia trachomatis and Mycoplasma hominis, have been demonstrated in 40% of 31 women with acute pelvic inflammatory disease (29). When inoculated intraurethrally, this organism induced urethritis in young male chimpanzees (28). The organism could be reisolated from the experimentally infected animals for at least 13 weeks, and the chimpanzees developed a marked antibody response. This evidence implicates M. genitalium in the etiology of human genitourinary tract disease. Ultrastructural studies indicated that M. genitalium possesses a differentiated termiinal structure covered by a peplomer-like nap (30). Furthermore, adherence of M. genitalium to Vero cell membranes appeared to be mediated by the nap area (31). This evidence suggests that M. genitalium may possess an attachment mechanism similar to that described earlier for Mycoplasma pneumoniae (2, 9, 12, 13). This possibility is supported by previous findings that M. genitalium shares extensive serologic cross-reactivity with M. pneumoniae (3, 11, 21) and that a monoclonal antibody specific to the P1 protein of M. pneumoniae was reactive with a single protein of M. genitalium by immunoblotting (7). In the present study, further characterization of this cross-reacting protein by immunological and ultrastructural means is described.

No abstract