Macrophages in immunity to syphilis: suppressive effect of concurrent infection with Mycobacterium bovis BCG on the development of syphilitic lesions and growth of Treponema pallidum in tuberculin-positive rabbits.

Paired groups of male rabbits were challenged with Treponema pallidum and Mycobacterium bovis BCG. One group had been sensitized to BCG by inoculation 3 weeks before challenge. All animals were challenged intradermally at multiple sites with T. pallidum alone, BCG alone, and both organisms into the same sites. The resulting lesions were followed clinically and histologically. BCG lesions enlarged more rapidly in sensitized rabbits, but they were otherwise no different from those in the controls. T. pallidum lesions enlarged and regressed simultaneously in both groups, but in the BCG-sensitized animals they became twice as large as those in the unsensitized rabbits. Mixed BCG-T. pallidum lesions showed the greatest differences in the two groups of animals. Like the pure BCG lesions, they enlarged more rapidly in the sensitized rabbits but began to recede after 1 week. The corresponding lesions in the controls enlarged more slowly and reached their maximum size after 3 weeks when the receding lesions in the sensitized animals were much smaller. The most marked histological-histochemical difference between the two groups of animals was in the number and activation of macrophages. These cells were more numerous in the mixed lesions of BCG-sensitized animals than in similar lesions of the controls and more activated as determined by beta-galactosidase staining. Although sparsely distributed, activated macrophages were more numerous in the pur T. pallidum lesions of sensitized animals than in those of control animals. Silver-stained sections revealed fewer treponemes in mixed lesions of sensitized animals than in the mixed lesions of control animals. Quantitation of treponemes in pure T. pallidum versus mixed lesions was determined in two groups of rabbits challenged intratesticularly. The total number of treponemes per testis in the mixed lesions of BCG-sensitized rabbits was significantly less than the number in the mixed lesions of control animals, and also less than the number in pure T. pallidum lesions of both groups of animals.