Cross‐linking of membrane IgM on B CLL cells: dissociation between intracellular free Ca2+ mobilization and cell proliferation

Abstract

It has been shown previously that chronic lymphocytic leukemia (CLL) B cells are frozen at different stages of activation with unique requirements for proliferation. Although most B CLL cells express surface IgM, anti‐γ antibodies are able to trigger only some of them to proliferate and/or respond to B cell growth factor (BCGF) or interleukin 2 (IL2), as normal B cellsIn this report we extend these observations using three different monoclonal antibodies (mAb) to human y chain (one mitogenic in soluble form for normal B cells, the two others mitogenic only when coupled on Sepharose 4B beads). Cells from only 3 out of 11 B CLL patients proliferated in the presence of either mitogenic anti‐μ. When the early events following surface Ig cross‐linking, such as calcium mobilization (by flow cytometry on indo‐1‐labeled cells), were studied all three mAb in soluble form were able to induce a similar increase in the intracellular free calcium concentration ([Ca²⁺I_i); analogous to [Ca²⁺], rise after the mitogenic F(ab′)₂ anti‐μ stimulation. This response was seen only in 8 out of the 12 CLL B cells studied. All B CLL cells, however, proliferate in response to a combination of phorbol ester 12,13‐dibutyrate (PBt2) and ionomycin. Therefore, three patterns of response to sIg cross‐linking by anti‐μ could be distinguished: cells from 4 out of 12 cases proliferate and mobilize Ca²⁺ upon anti‐y triggering (behaving like resting B lymphocytes); in 4 other cases anti‐μ lead to Ca²⁺ mobilization without cell proliferation; in the last 4 cases neither Ca²⁺ mobilization, IP3 generation (in the one case studied) nor cell proliferation are observed although these cells do proliferate directly in response to growth factors. Moreover, anti‐μ stimulation in this group leads to increased proliferation in response to BCGF and IL2 suggesting an anti‐Ig signaling independent of inositol phosphate metabolism. These results are interpreted in terms of differential anti‐μ signaling on B cells at different stages of activation.