Functional and phenotypic heterogeneity of electrophoretically separated leukaemic B cells from patients with chronic lymphocytic leukaemia.

E-rosette negative largely leukaemic B cells from patients with chronic lymphocytic leukaemia (CLL) were separated by density gradient electrophoresis, on the basis of their surface charge. The separated cells were pooled in seven fractions, according to their relative position in the electrophoretic distribution, and analysed by functional tests and cell-surface phenotypes. The ability of electrophoretically separated B cells from patients with CLL to differentiate into plasma cells in the presence of T cells from normal donors and in the pokeweed mitogen induced differentiation system was investigated. Lymphocytes able to differentiate into plasma cells under these conditions were highly enriched in the low-mobility fractions V, VI and VII. These plasma cells were of leukaemic origin, because they expressed only the light chain present on the cell surface of the leukaemic B cells before stimulation. Lymphocytes with Fc (IgG) receptors were relatively enriched in the high and intermediate mobility fractions I-IV, but they were present in the remaining of the fractions in smaller proportions. Lymphocytes with Fc (IgM) receptors were present in all fractions, but only in very small proportions in the very high mobility fraction I. Cells with complement receptors I and II were present in all fractions. Analysis of the density of cell surface immunoglobulin expression using the fluorescence-activated cell sorter, revealed that fractions of high and intermediate electrophoretic mobility (I-V) contained cells with both low and intermediate density of surface immunoglobulin, whereas the low mobility fractions VI and VII contained predominantly cells with low density of surface immunoglobulin. These results revealed significant phenotypic and functional heterogeneity of B cells from patients with CLL, suggesting the presence of subpopulations of leukaemic B cells in different differentiation or maturation stages.