Development of nonthymic lymphomas in thymectomized NFS mice exposed to split-dose X-irradiation.

Split-dose X-irradiation efficiently induced Thy-1-positive thymic lymphomas (80%) in intact NFS mice within 12 months after irradiation. A high incidence (67%) of nonthymic lymphomas and leukemias was observed in the thymectomized NFS mice. Development of nonthymic lymphomas and leukemias in these mice started about 2 months later than that of thymic lymphomas and increased significantly 10 months onward after the last irradiation, when the development of thymic lymphomas in intact mice had already come to an end. These nonthymic lymphomas and leukemias involved predominantly the spleen and the mesenteric lymph nodes. Twelve out of 18 lymphomas and leukemias were examined immunocytologically. All of these tumors except one were diagnosed as lymphomas including one plasmacytoma. One case was diagnosed as myelomonocytic leukemia because the leukemic cells were highly positive for nonspecific esterase and negative for chrolacetate esterase. All lymphomas tested were negative for thy-1.2, and five of them expressed surface immunoglobulins. From these results, nonthymic lymphomas developed in thymectomized and X-irradiated NFS mice were classified as B-cell lymphomas probably including non-T/non-B cell lymphomas. Present findings demonstrated that a low incidence of nonthymic lymphomas in intact NFS mice exposed to split-dose X-irradiation should be ascribed to a longer latency since most of the mice died of thymic lymphomas prior to the development of overt nonthymic lymphomas.