We tested the effects of Rituximab (anti-CD20) and IDEC-152 (anti-CD23) on apoptosis of B-cell malignancies, using established non-Hodgkin’s B-Cell lymphoma cell lines and freshly isolated Chronic Lymphocytic Leukemia (CLL) B-cells. We used monolayers of stably transfected CHO-cells expressing FcRγIII-A to present antibody to B-cells and promote crosslinking. Established B-cell lymphomas (n = 3) were cultured in the presence of FcRγIIIA-expressing CHO monolayer with or without MAbs and apoptosis was measured by annexin V/propidium iodide staining at various times thereafter. Both antibodies induced time-dependent apoptosis of B-cell lymphoma cell lines. After 48 hrs of treatment with either Rituximab or IDEC-152, the majority of the malignant B-cells were apoptotic (remaining viable cells = 28.7% ± 0.2137% for Rituximab and 30.87% ± 0.7332% for IDEC-152). Rituximab and IDEC-152 also induced marked increases in caspase activity in B-cell lymphoma cell lines, with fold-increases above baseline control cells of 25 ± 0.9031 and 24 ± 0.3839, respectively. In contrast, neither Rituximab nor IDEC-152 induced striking effects on primary CLL B-cells (n = 6). We therefore tested the combination of Rituximab or IDEC-152 with other agents that target anti-apoptotic proteins, exploring whether more efficient induction of apoptosis can be achieved. We cultured lymphoma cell lines and primary CLL specimens with chemical antagonists of XIAP (

Schimmer, et al.
Cancer Cell
), an anti-apoptotic protein that inhibits effector caspases. When used at concentrations where XIAP antagonists alone were non-apoptotic (approximately 2.5 μM), a significant increase in apoptosis was achieved in cultures of lymphoma and CLL cells treated with either Rituximab or IDEC-152. These findings suggest that Rituximab or IDEC-152 may more efficiently induce apoptosis of malignant B-cells when combined with an apoptosis-sensitizing agent. (Supported by CA-81534; CA-78040; and an unrestricted grant from Genentech, Inc.).

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