Ultrastructural, immunologic, and functional studies on Sézary cells: a neoplastic variant of thymus-derived (T) lymphocytes.

The vast majority of human lymphoid neoplasms examined to date have been associated with a proliferation of bone marrow-dependent (B) lymphocytes. In an effort to delineate human tumors of T-cell (thymusdependent) lineage, use was made of the peripheral blood leukocytes of sixteen subjects with various forms of mycosis fungoides. The abnormal cells in the circulation of these patients are morphologically identical to those that infiltrate their nodes and skin. On electron microscopy, such neoplastic lymphocytes (Sézary cells) had "cerebriform" nuclei and an abundance of cytoplasmic fibrils not described heretofore. Sézary cells were nonadherent and nonphagocytic and usually responded to stimulation with phytohemagglutinin, refuting earlier suggestions that the cells represent monocytes or histiocytes. In contrast to chronic lymphocytic leukemia lymphocytes, the Sézary cells lacked surface immunoglobulin and receptors for complement. Ultrastructural analysis identified Sézary cells in the center of directly formed rosettes (E-rosettes) characterizing the behavior of T lymphocytes in this test. Though some Sézary cells lacked both T and B cell-surface properties, in general, these observations support the view that the Sézary cell is a neoplastic variant of a thymusderived lymphocyte.