Reactivation in vitro of immunocompetence in irradiated mouse spleen

Total-body X-irradiation has been shown to suppress -mmune responses to a large variety of antigenic stimulations (1), followed, in the case of sublethal doses, by gradual spontaneous recovery. It has been demonstrated that such a spontaneous recovery may not be manifest if thymectomy precedes irradiation (2--4). Experiments using thymus grafts (4-6) or thymuses placed within diffusion chambers (7) indicated that the thymus might influence this recovery by the liberation of humoral factors. Although the idea of humoral factor activity has been repeatedly stated by various groups for radiation recovery as well as for the normal development in the neonate (8-11), its mechanism of action is still obscure. Several suggestions can be made to explain the process of thymus activity; e.g. the thymus may activate cells residing in the lymphoid-depleted (or immature) organs, or it may activate stem cells to migrate and develop in these tissues. However, humoral activity of the thymus may reflect only a part of its role in the immune system (cf. references 12 and 13). For example, circulation of lymphoid cells between lymphoid organs (14) provides an additional factor to be considered in studies on the mechanism of the thymus effect and for identification of the tissue origin of immunocompetent cells in a regenerating system.