Enhanced primary resistance to Treponema pallidum infection and increased susceptibility to toxoplasmosis in T-cell-depleted guinea pigs.

Strain 2 guinea pigs made T-cell deficient by thymectomy and irradiation and protected with syngeneic bone-marrow cells (TXB guinea pigs) have a surprisingly high level of resistance to cutaneous syphilis and to the dissemination of treponemes to the draining lymph node. Compared with normal euthymic controls infected with Treponema pallidum Nichols, syphilitic TXB guinea pigs developed fewer and less severe skin lesions and their lymph nodes contained lower numbers of treponemes. Associated with this evidence for enhanced innate resistance was the ability of the TXB host to produce, during each test interval of a primary infection, more antitreponemal antibodies than that of their euthymic counterparts. Similar levels of partial protection against cutaneous and disseminated syphilitic infection and elevated antibody levels occurred in challenged normal guinea pigs passively immunized with lymphocytes from T. pallidum-infected TXB donors. In contrast, the capacity of the TXB host to be protected against a lethal infection with the unrelated intracellular protozoan parasite Toxoplasma gondii was greatly impaired unless it received an intravenous infusion of normal syngeneic thymocytes. These seemingly paradoxical results are explained primarily in terms of a residual T-helper-cell population in the TXB guinea pig which is large and competent enough to generate antisyphilis, but not anti-Toxoplasma, immunity.