ROLE OF THE THYMUS IN RESTORATION OF

Abstract

A study was made of the thymus''s role in the restoration of immune competence and the regeneration of the lymphoid system in X-irradiated mice. Thymectomy of adult mice followed by total body irradiation at sublethal doses postponed or abolished the spontaneous reactivation of the homo-graft reaction against genetically foreign tumors. Retransplantation of intact thymuses can reactivate the immune response. The depletion of the lymphoid system in spleen and lymph nodes of thymectomized and irradiated mice was studied. Following depletion of the lymphatic follicles, erythroblastic noduli proliferate in the spleen. In thymectom-ized animals, their appearance precedes that of irradiated controls. In spleens of the irradiated animals, erythropoiesis predominated for about 4 weeks after which regression of erythropoiesis took place with a concomitant regeneration of typical lymphoid follicles. In thymectom-ized animals, erythropoiesis persisted for a much longer time (2 months and more), and following its eventual regression the regeneration of the lymphoid elements was either suppressed or abolished. Re-grafting of thymuses to thymectomized animals was followed by a lymphoid regeneration in the spleen. Tentative interpretation of the results of immune reaction following thymus grafts of isologous and homologous origins seem to support the concept that the thymus induces the differentiation to immune competent cells of lymphoblasts originated in the spleen or bone marrow, rather than being itself a tissue origin for immune competent cells.