Leu-M1 antigen expression in advanced (tumor) stage mycosis fungoides

The authors describe two patients with clinically and histopathologically documented advanced (tumor) stage mycosis fungoides. In each case the large, pleomorphic neoplastic cells lacked the monoclonal antibody-defined cell surface antigens commonly associated with immature and mature T-cells, i.e., T11, Leu-1, T3, T4, T6, T8, and T10, but expressed various T-cell-associated activation antigens, such as HLA-DR, Tac, and T21. Leu-M1, a monocyte-associated antigen, was not expressed by the small, cerebriform neoplastic cells in the plaque stage lesions of either patient. However, Leu-M1 was expressed by most of the large, pleomorphic neoplastic cells present in the nodular lesions of both patients. The pattern of Leu-M1 antigen expression was identical to that previously reported in the Reed-Sternberg cells of Hodgkin's disease. Identification of these two patients suggests using caution in the interpretation of the results of immunophenotypic analysis of cutaneous lymphoid neoplasms and that Leu-M1 should not be used as a diagnostic indicator of Hodgkin's disease or a histiocytic-derived neoplasm. These studies also suggest that Leu-M1 may be preferentially expressed on a subpopulation of activated, rapidly proliferating, and/or dedifferentiated neoplastic T-cells that proliferate in the advanced (tumor) stages of mycosis fungoides.