Lymphocytic Leukemia Cells through the Bidirectional Crosstalk with Chronic Stromal Endothelial Cells Establish a

Chronic lymphocytic leukemia (CLL) is a clonal B cell disorder of unknown origin. Accessory signals from the microenvironment are critical for the survival, expansion, and progression of malignant B cells. We found that the CLL stroma included microvascular endothelial cells (MVECs) expressing BAFF and APRIL, two TNF family members related to the T cell-associated B cell-stimulating molecule CD40L. Constitutive release of soluble BAFF and APRIL increased upon engagement of CD40 on MVECs by CD40L aberrantly expressed on CLL cells. In addition to enhancing MVEC expression of CD40, leukemic CD40L induced cleavases that elicited intracellular processing of pro-BAFF and pro-APRIL proteins in MVECs. The resulting soluble BAFF and APRIL proteins delivered survival, activation, Ig gene remodeling, and differentiation signals by stimulating CLL cells through TACI, BAFF-R, and BCMA receptors. BAFF and APRIL further amplified CLL cell survival by upregulating the expression of leukemic CD40L. Inhibition of TACI, BCMA, and BAFF-R expression on CLL cells; abrogation of CD40 expression in MVECs; or suppression of BAFF and APRIL cleavases in MVECs reduced the survival and diversification of malignant B cells. These data indicate that BAFF, APRIL, and CD40L form a CLL-enhancing bidirectional signaling network linking neoplastic B cells with the microvascular stroma. C hronic lymphocytic leukemia (CLL) is an incurable disease of unknown etiology characterized by progressive accumulation of clonal B lymphocytes with mature morphology and phenotype (1). The clinical course of CLL is variable and correlates with the mutational status of Ig H chain variable (V H) genes and the percentage of leukemic cells expressing CD38 and z-chain–associated protein 70 (ZAP-70) (2– 4). The presence of unmutated V H genes and expression of CD38 and ZAP-70 are biological variables associated with worse prognosis. B cell intrinsic factors such as abnormal expression of antiapoptotic NF-kB and Bcl-2 proteins and accumulation of heterogeneous genomic lesions regulate the survival, expansion, and clonal evolution of CLL cells (5, 6). B cell extrinsic factors such as antigenic stimulation may also be important, as leukemic B cells from unrelated groups of CLL patients express restricted Ig VDJ genes encoding stereotyped Ag-binding V H regions (7–9). Accessory signals from the microenvironment further contribute to the pathogenesis of CLL (10, 11). Indeed, stromal cells such as nurselike cells, mesenchymal cells, follicular dendritic cells (DCs), and macrophages recruit CLL cells by secreting CXCL12 and CXCL13 chemokines and enhance CLL cell survival by releasing BAFF or BLyS and a proliferation-inducing ligand (APRIL) …