Correlation of immunity in experimental syphilis with serum-mediated aggregation of Treponema pallidum rare outer membrane proteins.

We have previously shown by freeze-fracture electron microscopy that serum from infection-immune syphilitic rabbits aggregates the low-density membrane-spanning Treponema pallidum rare outer membrane proteins (TROMPs). The purpose of this study was to determine if a relationship could be demonstrated between acquired immunity in experimental rabbit syphilis, serum complement-dependent treponemicidal antibody, and antibody directed against TROMPs as measured by the aggregation of TROMP particles. Three groups of T. pallidum-infected rabbits were treated curatively with penicillin at 9 days, 30 days, and 6 months postinfection to generate various degrees of immunity to challenge reinfection. Sera from rabbits completely susceptible to localized and disseminated reinfection possessed a low titer of treponemicidal antibody (/=50% of a treponemal suspension) and showed a correspondingly low level of TROMP aggregation (16.5% of the total number of outer membrane particles counted) similar to normal serum controls (13. 4%); the number of particles within these aggregates never exceeded three. Sera from partially immune rabbits, which were susceptible to local reinfection but had no evidence of dissemination, showed an increase in the titer of treponemicidal antibody (1:16) compared to the completely susceptible group (</=1:1). Although no significant increase was observed in the total number of TROMP particles aggregated (18.9%) compared to the number in controls (13.4%), approximately 15% of these aggregates did exhibit a significant increase in the number of particles per aggregate (4 to 5 particles) compared to controls (</=3 particles), indicating a measurable increase in anti-TROMP antibody. Finally, sera from rabbits completely immune to both local and disseminated reinfection possessed both high titers of treponemicidal antibody (1:128) and significant aggregation of TROMP (88.6%); approximately 50% of these aggregates contained four to six particles. The results indicate that complete immunity in experimental rabbit syphilis correlates with antibody that kills T. pallidum and aggregates TROMPs, suggesting that TROMPs are molecules which contribute to the development of acquired immunity.