T-cell development in human thymoma

Human thymoma is derived from thymic epithelial cells and often associated with a large number of conical thymocytes. Since thymic epithelial cells play key roles in T-cell development in the normal thymus, we hypothesized that the neoplastic epithelial cells of thymoma may support T-cell differentiation. We attempted to reconstitute the T-cell development in vitro by using neoplastic epithelial cells isolated from thymoma. CD34, a stem cell marker, was expressed on a proportion of CD4^-CD8^- cells in thymoma. These CD34⁺CD4^-CD8^- cells also expressed both IL-7R α-chain and common γ-chain. Purified CD4^-CD8^- cells from thymomas were cultured with the neoplastic epithelial cells, and their differentiation into CD4⁺CD8⁺ cells via CD4 single positive intermediates was observed within 9 days' co-culture in the presence of recombinant IL-7. The CD34⁺CD4^-CD8^- cells purified from a normal thymus also differentiated to CD4⁺CD8⁺ cells in an allogeneic co-culture with the neoplastic epithelial cells of thymoma. In addition, a pleural dissemination from thymoma contained a large amount of cortical thymocytes. These results suggest that the neoplastic epithelial cells retain the function of thymic epithelium and can support T-cell development in thymomas.