Immunohistochemical detection of E2F-1 in non-Hodgkin's lymphomas: a survey of 124 cases.

E2F-1 is a transcription factor that mediates cell cycle progression from the G1 to S phase and is normally regulated by a group of proteins, including cyclin D1. Although deregulation of E2F-1 is implicated in neoplastic transformation, in situ examination of this protein has not been performed to date. Using an immunohistochemical technique applied to routinely fixed, paraffin-embedded tissue sections, we evaluated E2F-1 expression in reactive lymphoid tissues and in 124 cases of non-Hodgkin's lymphoma (NHL) of various types. In reactive lymphoid tissues, E2F-1 was expressed predominantly by large noncleaved cells in germinal centers and by a small subset of cortical thymocytes. Mantle zones and splenic marginal zones were negative. Among the NHLs, four types had a relatively high percentage (> 20%) of E2F-1-positive cells: mantle cell lymphoma (19 of 19), lymphoblastic lymphoma (5 of 5), small noncleaved cell lymphoma (4 of 6), and hepatosplenic gammadelta T-cell lymphoma (3 of 3). The consistent detection of many E2F-1-positive cells in mantle cell lymphoma is in contrast to other small B-cell NHLs (n = 29), which had relatively few (< 10%) E2F-1-positive cells. This finding suggests that immunohistochemical staining for E2F-1 as a supplement to the existing markers, such as cyclin D1, might be useful in the differential diagnosis of NHLs composed of small B cells.