Regeneration of thymus grafts. II. Effects on immunological capacity.

Mice thymectomized at birth were grafted at 1 week of age with thymus tissue under the kidney capsule. The implants were excised after a period of 1, 2 or 3 weeks and the response of the mice to sheep erythrocytes and to allogeneic skin grafts was tested. Thymectomized mice that had not received thymus implants had twenty times less antibody-plaque-forming cells per million spleen cells than sham-thymectomized controls and failed to reject foreign skin. Some evidence of restoration of immune capacities was obtained in mice bearing for 1 to 2 weeks either normal thymus implants or thymus tissue irradiated with 500 R. By contrast, thymus tissue irradiated with 2000 R failed to influence neonatally thymectomized mice with respect to their immunological capacities. Most thymectomized mice bearing thymus implants, whether normal, irradiated with 500 or 2000 R, had blood lymphocyte levels within the normal range. Cytological analysis of the lymph nodes and spleen of mice bearing normal thymus implants revealed only very few thymus-derived cells but the proportion of these cells was significantly increased after specific immunization. No thymus-derived cells were however detected in the lymphoid tissues of mice bearing irradiated thymus implants, not even in those that were capable of responding to antigenic stimuli. It is concluded that the thymus plays an essential role in inducing the differentiation of immunologically competent cells from non-competent precursors and that this function is dependent on the integrity of the thymus epithelial-reticular cells.