LOW-LEVEL CD20 EXPRESSION ON T-CELL MALIGNANCIES

Although initially thought to be a B lineage restricted antigen, low ' 'density'' or antibody binding capacity (ABC) CD20 has recently been d etected on subset(s) of normal T lymphocytes (Hultin et al.: Cytometry 14:196-204, 1993). We report low ABC CD20 expression in three (two ch ildren, one adult) cases of T-acute lymphoblastic leukemia (T-ALL). CD 20 and other pertinent antigens were detected using a direct dual colo r method with a Becton Dickinson FACScan(R) flow cytometer and Simulse t(R) software. Only one cell population based on light scatter was not ed in each case that immunophenotypically represented almost a pure po pulation of malignant cells expressing T lymphocyte antigens (for exam ple, CD7 98%, 92%, and 100%, respectively). A total of 95%, 87%, and 7 9% of the cells from the three cases expressed CD20 with an unusual lo w ABC compared to the customary ''bright'' CD20 expression on normal B lymphocytes. Other B lymphocyte associated antigens, such as CD19, CD 22, Dr, and immunoglobulin light chains, were negative. Eleven other T lymphocytic malignancies from 1991 to 1993 were CD20 negative, includ ing three other case of T-ALL (one adult and two children). One unusua l case of intestinal small lymphocytic non-Hodgkin's lymphoma with a n atural killer/T lymphocytic immunophenotype not described in this repo rt appeared to be CD20(''dim''+). Low ABC CD20 expression by T lymphoc ytic malignancies may provide a more unique immunophenotypic ''fingerp rint'' to help support the diagnosis of T cell neoplasia vs. normal/re active T cells (for example, low ABC CD20 cells represent only 2.4 +/- 1.5% of normal peripheral blood lymphocytes). This characteristic mig ht also facilitate monitoring patients for residual or recurrent disea se. The prognostic significance of CD20 co-expression is uncertain and must await further studies. (C) 1994 Wiley-Liss, Inc.