B-Cell Receptor Signaling Enhances Migration of B-Cell Chronic Lymphocytic Leukemia Cells in Response to the Chemokine Stromal Cell-Derived Factor-1 (SDF-1/CXCL12).

There is growing evidence that the microenvironment confers survival signals to Chronic Lymphocytic Leukemia (CLL) B-cells that may result in disease progression and resistance to therapy. In the marrow or secondary lymphoid tissues, CLL cells are in close contact with non-tumoral accessory cells, such as mesenchymal stromal cells or nurselike cells. We previously characterized SDF-1 (CXCL12) as a central mediator for CLL cell migration and interaction with the protective microenvironment. Constitutive secretion of CXCL12 attracts CLL cells to stroma or NLC through its cognate receptor, CXCR4. These accessory cells protect CLL cells from spontaneous or drug-induced apoptosis, which is contact-dependent and partially mediated by CXCL12. B-cell receptor (BCR) signaling has been considered another important regulator of CLL cell survival. Typically, CLL cell that lack somatic mutations in the immunoglobulin (Ig) variable region (V) genes and display high levels of the tyrosine kinase ZAP-70 strongly responds to anti-IgM stimulation. Because both, CXCL12 stimulation and BCR signaling may represent important mechanism for maintenance of CLL cell within the microenvironment, we examined whether anti-IgM stimulation affects CXCL12 responses in correlation with the ZAP-70 status.

BCR signaling was modulated either by crosslinking the BCR with IgM or by blocking the tyrosine kinase Syk. Effective BCR cross-linking with anti-IgM antibodies was demonstrated by phosphorylation of Syk and p44/42 MAP kinase. In ZAP-70 positive cells, BCR crosslinking resulted in a robust activation of Syk, p44/42 MAP kinases, and protein kinase B (Akt). ZAP-70 negative CLL cells displayed a weaker activation of p44/42 upon IgM crosslinking. Pretreatment of CLL cells with anti-IgM resulted in an enhanced calcium mobilization upon CXCL12 stimulation. This was not due to changes in surface expression of CXCR4. Accordingly, Syk inhibition by piceatannol resulted in a loss of calcium response upon CXCL12 stimulation. Furthermore, anti-IgM stimulation significantly increased CLL cell chemotaxis towards CXCL12 1.4 ± 1.2fold (n=9, p=0.027), and Syk inhibition by piceatannol decreased chemotaxis to 0.6 ± 0.2fold of controls (n=8). In these experiments, we could not detect differences between ZAP-70 positive or negative cells. However, there was a strong difference regarding the spontaneous, CXCL12-dependent migration of CLL cells beneath marrow stromal cells (pseudoemperipolesis). BCR crosslinking significantly increased pseudoemperipolesis of ZAP-70 expressing CLL cells 13.4 ± 21.0fold (n=7, p=0.043), whereas there was no significant increase in pseudoemperipolesis of ZAP-70 negative cells (1.4 ± 0.2fold increase, n=8). Syk inhibition by piceatannol significantly decreased the pseudoemperipolesis of ZAP-70 positive as well as ZAP-70 negative CLL cells to 0.4 ± 0.07 of controls (n=5, p=0.043). Interestingly, spontaneous migration of CLL cells beneath follicular dendritic cells (HK cells) was also significantly enhanced by anti-IgM stimulation, in particular in ZAP-70 positive cases. In summary, BCR signaling enhances calcium mobilization, CLL cell migration to CXCL12, and pseudoemperipolesis beneath marrow stroma or follicular dendritic cells. These data suggest that BCR stimulation co-operates with CXCL12 for localization and/or maintenance of CLL cells within distinct tissue microenvironments.