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In the classification of acute leukemia, the presence of myeloperoxidase (MPO) within the leukemic blasts indicates myeloid leukemia. Previous studies compared enzyme cytochemistry (EC) or flow cytometry (FC) with immunocytochemistry, in detecting MPO. Our study is the first direct comparison of EC with 3-color FC in a large group of acute leukemias. We studied 26 cases of acute myeloid leukemia (AML) and 4 cases of B-precursor acute lymphoblastic leukemia (B-ALL). Classification was according to the French-American-British criteria. The cells were analyzed for MPO expression by 3-color FC after cell permeabilization followed by staining with anti-MPO antibody. For FC, the blasts were defined by a combination of light scatter characteristics and dim CD45 expression. Concordance between EC and FC was seen in 27 of 30 cases (23/26 AML and all B-ALL), including all AML cases of M1, M2, M3, and M4 subtypes. In 1 of 4 AML-MO and 2 of 5 AML-M5a cases, FC demonstrated the presence of MPO in 8%, 86%, and 94% blasts; EC detected none. Three-color FC may be more sensitive than routine EC in demonstrating the presence of MPO in AML cases and may offer the advantage of multiparametric analysis.

CD20 is a 35-kDa protein that is expressed early in B-cell ontogeny and is lost during terminal B-cell differentiation into plasma cells. It is thought to be B-cell-specific. However, the CD20 antigen, detected by the monoclonal antibody L26, has been reported in some cases of T-cell lymphoma. This report describes a case of a malignant lymphoma coexpressing T-cell-lineage antigens and CD20 and characterization of a CD20+ T-cell population in the peripheral blood of healthy donors. The tumor cells were pleomorphic medium-sized cells that expressed a range of T-cell-specific antigens, including CD2, CD3, CD4, CD5, CD6, CD7, and beta F1. In addition, the tumor cells expressed CD20 on frozen (B1) and paraffin sections (L-26). Stains for other pan-B cell antigens, including CD19 and CD22, and immunoglobulin light and heavy chains were negative. To determine whether this unusual coexpression of T-cell-lineage antigens and CD20 represented aberrant antigen expression or neoplastic transformation of an unusual normal T-cell subset, the authors examined specimens of peripheral blood lymphocytes from healthy donors for evidence of a CD20+ T-cell population by using three-color immunofluorescence analysis by flow cytometry. Two distinct populations of CD20+ cells were observed in peripheral blood. One expressed bright CD20 (6.6% to 23.7%, mean 14.47% of peripheral blood lymphocytes) and other B-cell associated antigens, whereas the other expressed dim CD20 (.94% to 11.90%, mean 3.50% of peripheral blood lymphocytes) and coexpressed CD3. Approximately two thirds (52.8% to 82.3%, mean 64.1%) of the dim CD20 cells were CD8+ and one third (19.2% to 74.1%, mean 37.5) CD4+. These cells also expressed CD5 and the alpha-beta chain of the T-cell receptor and lacked CD19 and CD22. These results indicate that CD20 is expressed on some normal peripheral blood T cells. CD20 expression by T-cell lymphomas may represent neoplastic transformation of a normal subset of CD20+ T cells rather than aberrant antigen expression by neoplastic cells. The nature of the CD20 antigen on T cells and the function of the normal population remain to be determined.

The histologic distinctions between normal choroid plexus and choroid plexus papilloma and between choroid plexus papilloma and choroid plexus carcinoma are sometimes difficult. The authors performed the silver nucleolar organizer region (AgNOR) technique, immunohistochemistry for proliferating cell nuclear antigen (PCNA), and DNA ploidy analysis by flow cytometry on 9 samples of normal choroid plexus, 8 papillomas, and 13 carcinomas to evaluate whether these techniques can aid in these differential diagnoses. Significant differences were found in the mean AgNOR count between normal choroid plexus (1.35 +/- 0.11) and choroid plexus papillomas (2.42 +/- 0.81) (P < 0.001), but not between choroid plexus papillomas and carcinomas. In the normal choroid plexus, AgNORs were smooth and round; in the papillomas and carcinomas, however, they varied in size and shape. Compound AgNORs were commonly present in the tumors but were essentially absent in controls. Antibody to PCNA did not stain normal choroid plexus cells (except for focal staining in one sample of normal choroid plexus adjacent to a carcinoma) but stained many papilloma and carcinoma cells. DNA ploidy analysis demonstrated aneuploidy in some papillomas and carcinomas but could not be used for the distinction of normal choroid plexus from papillomas. These results suggested that the AgNOR technique and PCNA immunohistochemistry could be used to distinguish normal choroid plexus from choroid plexus papilloma in small, diagnostically difficult biopsy specimens.