An Intrinsic Thymic Epithelial Abnormality Is Responsible for the Spontaneous Development of Predominantly Lymphocytic Thymomas in BUF/Mna Rats

The nature of tumorigenesis of predominantly lymphocytic thymoma was examined using an animal model. Rats of the inbred BUF/Mna strain were found spontaneously to develop predominantly lymphocytic thymomas, histologically indistinguishable from their human counterparts, at an incidence of virtually 100%. Thymic rudiments of BUF/Mna rats grafted 17 months previously under the renal capsule of young athymic ACI/NMs‐rnu/rnu rats also gave rise to similar lesions. The lymphocytes in the thymomas expressed T‐cell antigens (rat Lyt‐1 and Lyt‐2.3), as in the normal case, and ACI rat specific antigen. When BUF/Mna rats of thymoma age were irradiated with a lethal dose of 12 Gy and then received a single injection of bone marrow cells (8 × 10(7)) from BALB/c‐nu/nu mice, thymomas were re‐formed three weeks later (in 2 of 5 rats) with the replacement lymphocytes expressing mouse Thy‐1.2 antigen. These results indicate that an intrinsic thymic epithelial abnormality is responsible for the development of predominantly lymphocytic thymomas in BUF/Mna rats.