Further characterization of prolymphocytic leukemia cells as a tumor of activated B cells.

Peripheral blood mononuclear cells from 24 patients with prolymphocytic leukemia (PLL) were isolated using a Ficoll-Hypaque gradient and stained by indirect immunofluorescence using a wide panel of monoclonal antibodies against B cell restricted and associated antigens, including HLA DR (Ia), CD19, CD21 (C3dR) surface membrane immunoglobulin (Slg), CD10 (CALLA), C3b, B5, CD25 (TAC), PCA1, T9, and T10. The cells were also tested for the FMC7, defined previously on PLL cells and the RAB1, a newly described hairy cell leukemia antigen. Thirteen out of the 24 samples expressed with variable intensity all the above antigens. While Ia, CD19, CD20, FMC7, and RAB1 were strongly or moderately expressed in all, the complement receptors (CD21 and C3b) were only weakly expressed in 12 cases; and the activation antigens B5, TAC, T9, T10, and PCA1 were found with variable intensity in two-thirds of the cases. In 50% of the cases tested, the CD5 antigen (usually strongly expressed on B CLL cells) was weakly to moderately expressed. These findings (absence or weak expression of complement receptors with variable expression of activation antigens) suggest that the PLL cells are activated B cells. When stimulated in vitro by anti-mu and TPA, (phorbol ester) tumor cells showed a decrease in CD21 and Slg and a stronger expression of CD25, T9, T10, and PCA1, with evidence of Ig secretion in four out of the seven cases studied. This confirms that the PLL cells arrested at an advanced stage of differentiation progressed narrowly to more differentiated cells. In view of our findings, we believe that the term prolymphocytic leukemia is inaccurate to define the stage of cell differentiation, and we suggest calling the disease preplasmacytic leukemia.