T-cell hyperplasia of lymphoid tissues of rabbits infected with Treponema pallidum: evidence for a vigorous immune response.

Specific identifications by immunofluorescence of infecting organisms and lymphoid cells in lymphoid organs and testes of rabbits were compared with the light microscopic appearances of these cells and organs on days 10, 11, 13, and 20 after intratesticular inoculation with Treponema pallidum (Nichols strain). Large numbers of T. pallidum were observed in the interstitial tissues of the testes on days 10 and 11. These numbers had declined markedly by day 13, and by day 20 only rare organisms (estimated as fewer than one to three per cross section) were seen. Organisms were also easily identified in much smaller numbers in the lymph nodes and spleen on days 13 and 20. Disappearance of organisms from the testes were associated with infiltration of large numbers of T cells. Marked follicular and diffuse cortical hyperplasia of the lymph nodes as well as follicular and periarterial hyperplasia of the spleen were observed. Specific immunofluorescence revealed large numbers of T cells in the diffuse cortex of the lymph nodes and the periarteriolar zones of the spleen. There was also a periportal infiltration of T cells in the liver. It is concluded that rabbits infected intratesticularly with T. pallidum mount an intense immune response that effectively eliminates most of the infecting organisms. However, despite this response, surviving T. pallidum may be identified not only at the original site of infection, but also disseminated in lymphoid organs. Normal mechanisms for controlling immune responses apparently shut down the specific response at a time when infecting organisms have not been completely eradicated from the host's tissues.