Recently in flow rounds Maryalice presented the immunphenotype of a CLL patient that had no CD20 expression. This was true for at least two clones and three different flurochromes. I have never seen a case of CLL without some level of CD20 expression. It may be decreased by cryopreservation on occasion but often the results from immunophenotyping a whole blood sample are not available for comparison. Carleton Stewart has quite a collection of CLL variants but I don't recall seeing CD20 negative CLL. However, when speaking with him about this problem at the San Diego Mtg, he wanted to review his data to see which reagents were negative. CD5 negative CLL can be problematic but CD20 expression is present and may be increased. A recent letter (CD20-Negative Relaspse in B-CellLymphoma Aftrer Treatment With Rituximab by T Kinoshita, H Nagai, T Murate and H Saito in the J Clin Oncol 16 (12):3916, 1998) suggests that previous in vivo exposure to anti CD20 resulted in its further down regulation in some patients and may be a mechanism of resistance in such patients. I am curious to know if others have seen this phenomena. Maryalice Stetler-Stevenson Director Flow Cytometry Unit Laboratory of Pathology, NCI, NIH Gerald E. Marti Flow and Image Cytometry Section Laboratory of Medical and Molecular Genetics Division of Cell and Gene Therapies CBER OTRR FDA
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