Immunophenotypic and genotypic characterization of diffuse mixed non-Hodgkin's lymphomas.

Diffuse, mixed small and large cell lymphomas (DML) are a heterogeneous group of non-Hodgkin's lymphomas. There are only a few published immunophenotypic and/or genotypic studies of DML, and they include a small number of cases. It is unclear whether these neoplasms are monoclonal, oligoclonal, or perhaps of dual lineage. Using monoclonal antibodies (UCHL1, MB2, MT1, LN1, LN2, and L26) that are effective in paraffin-embedded, B5-fixed tissue, 13 cases of DML were studied. This method allowed for improved correlation between cell morphology and immunophenotype compared with frozen section immunohistology. In addition, Southern blotting/DNA hybridization was used to identify directly the lineage of the neoplastic cell population. In each case a population of large B lymphocytes was demonstrated by immunohistology. In six of the cases, a single class of immunoglobulin light chains was detected by frozen section immunohistology, cytospin immunocytology, or both supporting the hypothesis that the B lymphocytes are monoclonal. In almost all the cases (12 of 13), the small cell population consisted predominantly of T lymphocytes. Immunoglobulin gene rearrangements were detected in seven cases, but no T cell receptor gene rearrangements were detected. It was concluded that DML are monoclonal lymphomas of B cell lineage with a non-neoplastic T cell component.