Comparison of anaplastic large cell Ki-1 lymphomas and microvillous lymphomas in their immunologic and ultrastructural features

Anaplastic large cell Ki-1 malignant lymphomas (MLs) resemble microvillous lymphoma in having a pleomorphic infiltrate with a prominent sinus growth pattern. Ultrastructural features of anaplastic large cell Ki-1 MLs and their immunologic relationship to the microvillous MLs have not been thoroughly evaluated. We have studied 23 anaplastic large cell Ki-1 MLs immunologically as well as 14 cases ultrastructurally, and compared them with 7 cases of microvillous MLs. Anaplastic large cell Ki-1 MLs were predominantly T-cell in type (13 cases) with three cases marking as B;in seven cases the immunophenotype was not clearly defined. Six microvillous MLs expressed monotypic cytoplasmic or surface immunoglobin and the remaining case had a probable B-cell phenotype (LN-1⁺, UCHL1^-). All microvillous MLs were Ki-1/Ber-H2 (CD30) negative. Epithelial membrane antigen (EMA) marked most anaplastic large cell Ki-1 MLs, except those of B-cell type, whereas all microvillous MLs were EMA negative. By electron microscopy, both lymphomas had features of transformed lymphocytes although anaplastic large cell Ki-1 MLs generally had more nuclear irregularity and variability from cell to cell. Numerous cytoplasmic processes were present in three anaplastic large cell MLs and in all microvillous MLs. The ultrastructural features of the cytoplasmic projections were not sufficiently distinctive to differentiate these two lymphomas. It is apparent that at least two forms of MLs may have a sinus growth pattern and that these MLs cannot be differentiated by morphology alone. Full characterization requires a battery of immunological markers and ultrastructural studies; even then there is overlap of these MLs. The majority of microvillous MLs, are Ki-1^-, EMA^-, and have a B-cell phenotype, but a small population (21% in this study) of Ki-1⁺ MLs have numerous cytoplasmic processes. The biological and clinical significance of cytoplasmic projections in these lymphomas are unknown.