Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of mature B lymphocytes in the peripheral blood (PB), lymph nodes (LN) and bone marrow (BM). Increasing evidence suggests that CLL cells depend on survival and proliferation signals provided by stroma cells in LN and BM. The chemokine receptor CXCR4 (CD184) and its ligand stromal cell-derived factor-1 (SDF-1) play an important role in trafficking of lymphocytes and may guide CLL cells to stroma cell niches. ZAP70 expression has prognostic value in CLL but the functional consequences of ZAP70 expression remain incompletely defined. Given that ZAP70 has been implicated in CXCR4 signaling its expression could enhance migration to SDF-1 and thereby promote interactions with stroma cells. As measured by flow cytometry, CXCR4 expression on leukemic cells obtained from different anatomic sites differed; cells from the PB (n=24, median 71% above isotype control) expressed CXCR4 more strongly than cells from BM (n=21, median 39%) and from LN (n=9, median 24%). Expression of CD69, an activation marker, followed a reverse pattern with cells from LN and BM typically showing higher expression than cells from PB, albeit with not detectable difference in expression in several patients. In vitro CLL cells from PB migrated in a dose dependent manner to SDF-1, and cells that had migrated down-modulated CXCR4 expression (89% before migration - 54% after migration). After exposure to SDF-1 CXCR4 expression decreased rapidly and remained virtually absent for at least 24 hours. Several mechanisms apparently decrease CXCR4 expression after contact with SDF-1, including internalization (given rapid re-expression of CXCR4 when SDF-1 is washed off after short exposure), protein degradation or inhibition of translation (evidenced by a decrease in total CXCR4 protein on Western blots), and mRNA degradation or transcriptional inhibition (decrease in mRNA levels more than 6 hours from SDF-1 exposure). In vitro migration of ZAP70(+) CLL cells toward SDF-1 through a 5μm membrane (Migration Index [MI] of 12.0, n=5) was significantly increased compared to ZAP70(−) CLL cells (MI of 2.9, n=4, p<0.05). To exclude effects of contaminating cells we repeated these assays with purified CLL cells (negative selection) with similar results. To model the complex interactions of CLL cells with stroma, we cultured PB derived leukemic cells with or without murine marrow stroma cells (S17). CXCR4 expression on CD19+ cells decreased from 90% without S17 to 50% when cultured on S17 cells, consistent with the known SDF-1 secretion by the murine stroma cell line. Conversely, CD69 expression increased from 58% without S17 to 71% with S17 cells. In addition, culturing of CLL cells on an S17 stroma cell layer extended their survival by several weeks when compared to cultures without S17 cells. Our data is consistent with a model in which CLL cells migrate along an SDF-1 gradient to stroma cell niches in BM and LN where they are activated. ZAP70 expression is associated with more effective migration in an SDF-1 gradient and thereby may facilitate access to growth and survival signals which then could contribute to the more progressive nature of ZAP70(+) CLL. The interaction between leukemic cells and stroma may represent a novel target for therapy of patients with CLL.
Disclosure: No relevant conflicts of interest to declare.