Fifty-three cases of chronic lymphocytic leukemia (CLL) were studied for the presence of the B cell IgM Fc receptor (Fc microR) using an aggregated IgM reagent. Restricted surface immunoglobulin, using conventional immunofluorescent techniques and FACS analysis, was detected in 43 cases (81%). The cells in the remaining ten cases (19%) expressed negligible surface immunoglobulin (slg-) and did not form E rosettes (E-), but this “null” subset clearly expressed the B cell Fc microR. The coincident membrane expression of the B1 antigen and the la- like antigen, as well as serial studies showing surface membrane light chain acquisition (in one patient), provided additional evidence for the B cell origin of this slg-E- subset. This subgroup of CLL appears to correspond phenotypically to a normal counterpart at a stage of B cell differentiation between the pre-B cell and the slgM+ early B cell. The B cell Fc microR appears to be a consistent and potentially useful marker for sl gE (“null”) CLL.

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