Evidence for neoplastic cell differentiation in mediastinal T lymphoblastic lymphoma

Immunostaining of frozen sections from a mediastinal T lymphoblastic lymphoma (T-LL) revealed the existence of two neoplastic cell populations characterized by different degrees of maturation. Several large nodules of 3A1+/T11+/T9+ T6-/T4-/T8-/T3- lymphoid cells, resembling normal early thymocytes, were surrounded by 3A1+/T11+/T9+/T6+/T4+/T8+/T3- cells resembling normal cortical thymocytes. The junctional area between early and cortical lymphocytes was occupied by numerous Leu-M3+/PAM-1+/DR+ reticular macrophages which were also characterized by J5 reactivity. Cytokeratin+/keratin+ epithelial cells were absent. Immunostaining of paraffin sections and of cytocentrifuge smears obtained from tumour cell suspensions revealed that a consistent percentage (8%) of neoplastic lymphoblasts were S-100+. Our findings are consistent with a cortical T-LL presenting areas of dedifferentiated cells or, alternatively, with an early T-LL whose cells were able to differentiate into cortical thymocytes, perhaps through the interaction with a specialized subset of J5+ macrophages.