Thymoma and
thymic carcinoma are the representative
tumors arising from the thymic epithelium.
Thymoma is well known for association with
autoimmune diseases including
myasthenia gravis, suggesting its
biological activity. Herein, recent progress in research of
thymoma is reviewed with reference to its immunological function.
Myasthenia gravis is frequently associated with WHO type B1 and B2
thymomas. These types of
thymomas hold a significant number of CD4(+)CD8(+) double-positive T cells, and at the same time, the neoplastic epithelial cells express
HLA-DR molecules at a slightly reduced level compared with the normal thymus. The impaired expression of
HLA-DR molecules in neoplastic epithelial cells of
thymomas possibly affects positive selection of CD4(+)CD8(-) single-positive T cells and may result in alteration of its repertoire. The function of
thymoma neoplastic cells as the cortical epithelium of the thymus and the morphological resemblance of
thymomas to the cortex suggest that
thymoma is of cortical epithelial origin; this might imply that
thymoma lacks the functional medulla where professional antigen-presenting cells are engaged in negative selection. These findings suggest that
thymoma generates autoreactive T cells causing autoimmunity. Further investigation on immunological function of
thymoma is supposed to elucidate the pathogenesis of
thymoma-related autoimmunity and the high affinity of
thymoma with
myasthenia gravis. In addition, studying the biology of
thymoma is also expected to contribute to further understanding of T-cell development and immunological tolerance in the human, because
thymoma can be considered an acquired thymus.