Multiple myeloma (MM) is characterized by clonal proliferation of malignant plasma cells (PCs) in the bone marrow (BM) compartment. Interaction of plasma cells with the BM stromal cells (BMSCs) is critical for homing, growth and drug resistance acquisition of the malignant PCs. However, the functional significance of other cellular components of the MM milieu, which includes osteoclasts and immune effector cells, is less clear. Both MM-derived and stromal cell-produced factors, including cytokines and chemokines, are believed to participate in the cross-talk between the MM and stroma leading to disease progression.
We hypothesized an important role for CXCL12 (SDF-1) chemokine and its receptor CXCR4 in MM-stroma interactions and microenvironment formation. We now show that MM cell lines ARH77 and RPMI8226 and primary MM cells may produce high amounts of CXCL12 and co-express CXCR4 receptor. Co-culture of the MM cells with BMSCs significantly up-regulated both CXCR4 cell-surface expression and CXCL12 secretion by the MM cells. Enhanced CXCR4 signaling in the MM cells upon the interaction with BMSCs promoted the survival and proliferation of the cells in an autocrine way. Moreover, the paracrine effect of increased CXCL12 production on immune cell migration was tested. We found, that conditioned medium (CM) produced by MM cells cultured with BMSCs specifically attracted increased numbers of CXCR4-expressing PB CD14+ cells. Furthermore, CXCR4 inhibition, using neutralizing antibodies toward CXCR4, inhibited the MM-induced migration of CD14+ monocytes, suggesting the possible role of CXCR4/CXCL12 axis in monocyte recruitment to the site of the disease. We next examined the functional consequence of MM-macrophage interaction. We saw that PB-generated macrophages induced the proliferation of MM cells, even more effectively than BMSCs. Furthermore, co-culture with macrophages strongly increased the expression of various pro-inflammatory and pro-angiogenic factors by MM cells, including CCL2 (MCP-1), CCL4 (MIP1a), IL-1b, IL-8 and VEGF. Interestingly, expression of IL-10 by MM cells was also up-regulated following the interaction with macrophages, suggesting the possible reciprocal effect of MM-produced factors on macrophage phenotype polarization.
Taken together, our findings demonstrate that interaction of MM with BM stromal cells positively regulates the expression of CXCR4 and CXCL12 by MM cells, affecting both MM proliferation and CXCR4-dependent monocyte recruitment. The migrated monocytes may in turn interact with MM cells, support their growth and activate cytokine release, therefore producing favorable pro-inflammatory and pro-angiogenic environment and promoting disease progression. Overall, our data provide the basis for future targeting MM-BMSCs and MM-macrophage interactions with anti-CXCR4 agents as a therapeutic strategy to improve the outcome of patients with MM.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.