Administration of antithymocyte serum to syphilitic rabbits inhibits development of resistance to reinfection with Treponema pallidum.

Three groups of female rabbits were inoculated i.v. with T. pallidum. One group of the animals was treated with horse anti-rabbit thymocyte serum (ATS) one day before and during the next two weeks after infection. Two other groups, one treated with normal horse serum (NHS) in the same way as the previous one and the other untreated, served as controls. After various time intervals, all the animals were tested for the content of T and B lymphocytes in the peripheral blood, humoral (VDRL and TPI tests), and cell-mediated (SDH test) responses. Two days after testing, animals of each group were challenged i.d. with T. pallidum, and observed for appearance of syphilitic lesions. The treatment of syphilitic rabbits with ATS caused a significant reduction in the number of T lymphocytes and a distinct inhibition of development of cell-mediated immunity associated with the lack of resistance to reinfection with T. pallidum till the 16th week of the infection; the level of B lymphocytes and the development of serological responsiveness was not affected by ATS treatment. The results are interpreted as indicating the protective role of cell-mediated immunity in syphilis.