CXCR7 Is Expressed in B Acute Lymphoblastic Leukemia (ALL) Cells and Mediates Their Transendothelial Migration.

The function of chemokine stromal cell-derived factor (SDF)-1 and its receptor CXCR4 in the pathobiology of hematological malignancies is known, but the role of another, newly identified receptor for SDF-1, RDC1/CXCR7, has not yet been defined. Thus we investigated whether CXCR7 plays a role in hematological malignancy. Using RT-PCR and flow cytometry analysis, we screened the expression of CXCR7 in 14 hematological cell lines and found that it is strongly expressed in all 5 B cell lines tested (Raji, NC- 37, NALM6, Ramos and REH) and weakly in two T cell lines( Jurkat and CEM) and 2 of 7 myeloid cell lines tested. Moreover, CXCR7 was also strongly expressed in all 9 patients diagnosed with B precursor ALL, very weakly in normal lymphocytes, and not at all in T precursor ALL samples, suggesting that CXCR7 is selectively expressed in B-cell ALL. Next we examined whether CXCR7 plays a role in the chemotaxis of these cells and found that its antagonist CCX733 (ChemoCentryx, Inc, Mountain View, CA) did not inhibit SDF-1-induced chemotaxis, indicating that CXCR7 does not play a significant role in the chemotactic activity of B-cell ALL. However, we found that CCX733 inhibited the transendothelial migration (TEM) of B cell lines and primary B precursor ALL cells in a dose-dependent manner, but the inactivated CXCR7 antagonist CCX226 did not. Moreover, transfection of the NALM6 with CXCR7 resulted in downregulation of CXCR4 and the TEM of such transfected cells was more inhibited by CCX733 than of wild-type cells. When we compared the roles of CXCR4 and CXCR7 in the TEM of these cells, we found that the CXCR7 antagonist totally inhibited the TEM of B cells at the low dose of 1mM, but the CXCR4 antagonist AMD3100 did not. On the other hand, the SDF- 1-induced TEM of T cell lines and primary T lymphoblasts was not significantly inhibited by CCX733, but was by AMD3100. CCX733 significantly prevented binding of NC-37 cell line to HUVEC (as shown by adhesion assay) which indicates that CXCR7 is likely required for the interaction of leukemic cells with endothelial cells and their extravasation. Moreover, because CXCR7 is known to be a receptor for the interferon-inducible T cell chemoattractant (I-TAC), we investigated whether the I-TAC/CXCR7 axis crosstalks with the SDF-1/CXCR7 axis in the trafficking of lymphoblastic cells. We found that

• I-TAC itself did not induce the TEM of NC-37 cells;

• pretreatment of NC-37 cells with I-TAC, but not with other chemokines (MCP-1 and MIG), inhibited TEM towards SDF-1;

• in contrast, pretreatment of MOLT-4 (T cells) with I-TAC did not block the TEM of MOLT-4 cells.

In conclusion, we demonstrated that CXCR7 is expressed on B lymphoblasts, and the SDF-1/CXCR7 axis plays a crucial role in the trafficking of these cells, indicating that CXCR7 could be a therapeutic target in patients diagnosed with B precursor ALL.