Aberrant Expression of BAFF System Molecules on B-Cell Precursor Leukemia Modulates Tumor Cell Survival: Novel Targets for Therapeutic Intervention.

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Identification of pathways that support the survival and proliferation of acute lymphoblastic leukemia (ALL) cells will provide novel targets for therapeutic intervention. BAFF and APRIL, with their receptors, BAFF-R, BCMA and TACI are members of the TNF family that play a critical role in the survival of mature B cells but not of normal B cell precursors. We sought to determine whether BAFF/APRIL and their receptors might be aberrantly expressed on B-cell precursor ALL. Surprisingly, tumor cells from 28 of 29 ALL patients expressed mRNA for at least one of these receptors. Using flow cytometric analysis, surface expression of BAFF-R, BCMA, and TACI protein was detected on 89%, 78% and 56% of B-lineage ALL, respectively. Binding assays using a BAFF fusion protein demonstrated that ALL cells expressed high-affinity receptors for BAFF. Stimulation of leukemia cells with BAFF fusion protein triggered the activation of the NFκB pathway with phosphorylation of NFκB2 p100 (Ser864), and its processing to p52. However, BAFF binding did not trigger phosphorylation of IKKα, suggesting that BAFF-mediated activation of NFκB on leukemia cells occurs in an IKK-independent manner. These observations demonstrate that B-cell precursor ALL express functional receptors for BAFF/APRIL ligands. To examine the biological significance of the BAFF system in precursor B-cell ALL, we performed survival and proliferation assays using a myc-tagged BAFF. Our results indicate that while BAFF promoted survival of B-ALL cells, it did not induce their proliferation. Moreover, BAFF binding to leukemia cells was accompanied by increased expression of Bcl-2. To determine the source of BAFF in the leukemic BM, expression of membrane-bound BAFF and APRIL was analyzed in both leukemia cells and cells from the BM microenvironment (BM endothelium, BM mesenchymal stem cells and BM stroma). Surprisingly, leukemia cells expressed BAFF in all patients tested, and APRIL was detected in 94% of the cases. BM microenvironmental cells were found to express membrane-bound BAFF but not APRIL. These findings demonstrate that leukemia B-cell precursors express functional receptors for BAFF system ligands. Importantly, we show that BAFF-mediated stimulation of leukemia cells triggers the activation of the NFκB pathway through an IKK-independent mechanism, and promotes the survival of leukemia cells. The differential expression of BAFF and APRIL on B-ALL cells and the BM microenviromental cells suggests that these ligands may act through both autocrine and juxtacrine mechanisms. Our studies indicate that the BAFF/APRIL ligand-receptor axis might provide new opportunities for therapeutic intervention in B-cell ALL.