Influence of bone marrow stromal microenvironment on forodesine-induced responses in CLL primary cells.

Forodesine, a purine nucleoside phosphorylase inhibitor, displays in vitro activity in chronic lymphocytic leukemia (CLL) cells in presence of dGuo, which is the basis for an ongoing clinical trial in patients with fludarabine-refractory CLL. Initial clinical data indicate forodesine has significant activity on circulating CLL cells, but less activity in clearing CLL cells from tissues such as marrow. In tissue microenvironments, lymphocytes interact with accessory stromal cells that provide survival and drug-resistance signals, which may account for residual disease. Therefore, we investigated the impact of marrow stromal cells (MSCs) on forodesine-induced response in CLL lymphocytes. We demonstrate that spontaneous and forodesine-induced apoptosis of CLL cells was significantly inhibited by human and murine MSCs. Forodesine-promoted dGuo triphosphate (dGTP) accumulation and GTP and ATP depletion in CLL cells was inhibited by MSCs, providing a mechanism for resistance. Also, MSCs rescued CLL cells from forodesine-induced RNA- and protein-synthesis inhibition and stabilized and increased Mcl-1 transcript and protein levels. Conversely, MSC viability was not affected by forodesine and dGuo. Collectively, MSC-induced biochemical changes antagonized forodesine-induced CLL cell apoptosis. This provides a biochemical mechanism for MSC-derived resistance to forodesine and emphasizes the need to move toward combinations with agents that interfere with the microenvironment's protective role for improving current therapeutic efforts.

Diverse marrow stromal cells protect CLL cells from spontaneous and drug-induced apoptosis: development of a reliable and reproducible system to assess stromal cell adhesion-mediated drug resistance.

Marrow stromal cells (MSCs) provide important survival and drug resistance signals to chronic lymphocytic leukemia (CLL) cells, but current models to analyze CLL-MSC interactions are heterogeneous. Therefore, we tested different human and murine MSC lines and primary human MSCs for their ability to protect CLL cells from spontaneous and drug-induced apoptosis. Our results show that both human and murine MSCs are equally effective in protecting CLL cells from fludarabine-induced apoptosis. This protective effect was sustained over a wide range of CLL-MSC ratios (5:1 to 100:1), and the levels of protection were reproducible in 4 different laboratories. Human and murine MSCs also protected CLL cells from dexamethasone- and cyclophosphamide-induced apoptosis. This protection required cell-cell contact and was virtually absent when CLL cells were separated from the MSCs by micropore filters. Furthermore, MSCs maintained Mcl-1 and protected CLL cells from spontaneous and fludarabine-induced Mcl-1 and PARP cleavage. Collectively, these studies define common denominators for CLL cocultures with MSCs. They also provide a reliable, validated tool for future investigations into the mechanism of MSC-CLL cross talk and for drug testing in a more relevant fashion than the commonly used suspension cultures.

B-cell antigen receptor signaling enhances chronic lymphocytic leukemia cell migration and survival: specific targeting with a novel spleen tyrosine kinase inhibitor, R406.

Antigenic stimulation through the B-cell antigen receptor (BCR) is considered to promote the expansion of chronic lymphocytic leukemia (CLL) B cells. The spleen tyrosine kinase (Syk), a key component of BCR signaling, can be blocked by R406, a small-molecule Syk inhibitor, that displayed activity in CLL patients in a first clinical trial. In this study, we investigated the effects of BCR stimulation and R406 on CLL cell survival and migration. The prosurvival effects promoted by anti-IgM stimulation and nurselike cells were abrogated by R406. BCR triggering up-regulated adhesion molecules, and increased CLL cell migration toward the chemokines CXCL12 and CXCL13. BCR activation also enhanced CLL cell migration beneath marrow stromal cells. These responses were blocked by R406, which furthermore abrogated BCR-dependent secretion of T-cell chemokines (CCL3 and CCL4) by CLL cells. Finally, R406 inhibited constitutive and BCR-induced activation of Syk, extracellular signal-regulated kinases, and AKT, and blocked BCR-induced calcium mobilization. These findings suggest that BCR activation favors CLL cell homing, retention, and survival in tissue microenvironments. R406 effectively blocks these BCR-dependent responses in CLL cells, providing an explanation for the activity of R406 in patients with CLL.

High-level expression of the T-cell chemokines CCL3 and CCL4 by chronic lymphocytic leukemia B cells in nurselike cell cocultures and after BCR stimulation.

In lymphatic tissues, chronic lymphocytic leukemia (CLL) cells are interspersed with CD68(+) nurselike cells (NLCs), T cells, and other stromal cells that constitute the leukemia microenvironment. However, the mechanism regulating colocalization of CLL and these accessory cells are largely unknown. To dissect the molecular cross talk between CLL and NLCs, we profiled the gene expression of CD19-purified CLL cells before and after coculture with NLCs. NLC coculture induced high-level expression of B-cell maturation antigen and 2 chemoattractants (CCL3, CCL4) by CLL cells. CCL3/CCL4 induction in NLC cocultures correlated with ZAP-70 expression by CLL cells. High CCL3/CCL4 protein levels were found in CLL cocultures with NLCs, and CCL3/CCL4 induction was abrogated by R406, a Syk inhibitor, suggesting that NLCs induce these chemokines via B-cell receptor (BCR) activation. BCR triggering also caused robust CCL3/CCL4 protein secretion by CLL cells. High CCL3 and CCL4 plasma levels in CLL patients suggest that this pathway plays a role in vivo. These studies reveal a novel mechanism of cross talk between CLL cells and their microenvironment, namely, the secretion of 2 T-cell chemokines in response to NLC coculture and BCR stimulation. Through these chemokines, CLL cells can recruit accessory cells and thereby actively create a supportive microenvironment.