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 x 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.