Characterization of intrathymic and extrathymic T cell development in spontaneous thymoma Buffalo/Mna rats.

The developmental status of T cell lineages at prethymic, intrathymic and postthymic stages in spontaneous thymoma Buffalo/Mna (BUF/Mna) rats was characterized on the basis of surface phenotypes and some immune responses. The proportion of bone marrow cell populations was shown to be normal in thymoma rats by immunofluorescence flow cytometry (FACS). In spite of a cortex-predominant appearance of thymomas, the proportion of thymocyte populations was phenotypically normal as assessed by FACS. Double immunohistochemical stainings revealed that a substantial number of single-possible (CD4+CD8- or CD4-CD8+) thymocytes existed not only in the narrowed medullary areas but also in the enlarged cortical areas of the thymomas. In lymph nodes, the proportion of T cells increased with age, reaching 88% after 20 months of age. This increase was due mainly to an increase in the CD8+ population but not the CD4+ population, resulting in low CD4/CD8 ratios. An abnormal increase of Thy1+ immature T cells was also observed in the lymph nodes. However, these phenotypic changes in the T cell lineages in the thymoma rats were not so influential as to alter their immunological reactivities, such as the primary antibody response to a T-dependent antigen, the graft-versus-host reaction and the mixed lymphocyte reaction to allo-antigens. These results suggest the possible presence of some altered differentiation pathways for intrathymic and postthymic T cell development in BUF/Mna rats.