Bone Marrow Stroma (BMS) Prevents Apoptosis of Lymphoma Cells by Upregulation of BMS-Derived B-Cell Activating Factor (BAFF) and Activation of NF-kB.

Stromal cells are an essential component of the bone marrow microenvironment that regulate or supports tumor survival. The bone marrow stroma has long been recognized as a "sanctuary site" for lymphoma cells during traditional chemotherapy. In this study, we demonstrated that adhesion of the B-cell lymphoma cell lines SUDH-4 and 10 to bone marrow stroma inhibited mitoxantrone-induced apoptosis. This adhesion-dependent inhibition of mitoxantrone-induced apoptosis correlated with decreased activation of caspases- 8 and 9, and cleavage of caspase 3 and PARP. EMSA analysis demonstrated significantly increased NF-kB binding activity in cells adhered to stroma cells compared to lymphoma cells in suspension. This DNA binding activity could be attributed to cell adhesion-mediated proteolysis of p100 NF-kappaB2 resulted in generation of active p52, which translocates to the nucleus in complex with p65 and RelB. Furthermore, co-culture with stromal cells also induced expression of the NF-kB-regulated anti-apoptotic molecules, XIAP, cIAP₁ and cIAP_2. In addition, because of the knowledge that BAFF shown to be critical for maintenance of normal B cell homeostasis as well as the survival of malignant B cells, we characterized the functional significance of BAFF in BMS-mediated drug resistance and survival. BAFF was detected on BMS cell line HS-5 and BMS cells derived from lymphoma patients by flow cytometry. Concentrations of BAFF were 3- to 25-fold higher in bone marrow aspirate than in peripheral blood. Lymphoma cell adhesion to HS-5 cells significantly increased BAFF secretion evidenced by both ELISA and immunoblotting. Addition of BAFF counteracted mitoxantrone-induced apoptosis, and elicited a reduction in spontaneous apoptosis in primary lymphomas via activation of NF-kB activation. Finally neutralization of BAFF by TACI enhanced lymphoma cell response to chemotherapy and overcame cell-adhesion mediated drug resistance, suggesting that lymphoma cells utilize BAFF as survival factor. These data indicate that stromal cells prevent apoptosis of lymphoma cells by upregulation of BAFF and induction of NF-kB-regulated anti-apoptotic proteins. Bone marrow-derived BAFF plays a key role in lymphoma cell survival and drug resistance in bone marrow microenvironment.