Heterogeneity of B-cell leukemias demonstrated by the monoclonal antibody FMC7.

A new monoclonal antibody, FMC7, was studied in 68 patients with chronic B-cell leukemia. All 17 cases of prolymphocytic leukemia (B-PLL) and 8 of 9 of hairy-cell leukemia were positive. In contrast, FMC7 was negative in 32 of 38 chronic lymphocytic leukemias (B-CLL; p less than 0.001) and 4 cases of B-cell lymphoma. Four of the 6 positive B-CLL cases were in "prolymphocytoid" transformation; two of them had bright membrane Ig (SmIg) staining and may represent an intermediate form between B-CLL and B-PLL. Although there was a tendency for the intensity of the immunofluorescence reaction with FMC7 and SmIg to change in parallel, FMC7 did not correlate with any Ig class. In addition, almost all FMC7-negative B-CLL had weak expression of SmIg. FMC7 is different from other monoclonal antibodies raised against B-lineage cells in that it recognizes only some subsets, presumably those at a late stage of maturation. This property confers diagnostic potential to this reagent and may contribute to the better characterization of the B-cell neoplasias.