Thymus and the carcinogenic induction of mouse leukemia.

The thymus may under certain conditions de termine susceptibility to the development of leu kemia in mice. The incidence of spontaneous leu kemia in Ak mice was remarkably reduced by thymectomy (2) ; extirpation of the thymus simi larly reduced the susceptibility of C57BL mice to the induction of leukemia by x-rays (4) and of DBA/2 mice to the leukemogenic activity of methylcholanthrene (10). Although thymic in volvement was observed infrequently in leukemia of C58 mice, thymectomy reduced the spontaneous incidence (9). For either the carcinogen-induced or spontaneous disease, susceptibility of thymectomized mice was restored by thymic grafting (9, 10). The thymus was either exclusively or more ex tensively involved than other organs in leukemias induced by estrogenic hormone (3) or the com bined action of estrogen and x-rays (7). When leu kemia was induced in DBA/2 mice by feeding methylcholanthrene (1), the thymus was usually heavily infiltrated. The thymus proved to be more sensitive than other lymphoid tissues to the leuke mogenic action of small doses of carcinogenic hy drocarbons which were injected directly into the tissue (11). Apparently, the thymus provides a favorable environment in mice of certain genetic constitu tion for the transformation of normal to leukemic lymphocytes. When the thymus of a susceptible parent strain was grafted into thymectomized hy brids between this and a resistant-to-leukemia strain, the FI hybrid leukemias were transplantable only to the F] hybrids and not to the high leukemia strain which served as the donor for grafted thy mus (8). This experiment indicates that the graft-