Upregulation of CXCR4 Is Essential for the Migration and Survival of Quiescent CML Progenitor Cells in the Bone Marrow Microenvironment after Imatinib Treatment.

Chronic myeloid leukemia (CML) is driven by constitutively activated Bcr-Abl tyrosine kinase, which causes the defective adhesion of CML cells to bone marrow (BM) stroma. The overexpression of p210^Bcr-Abl was reported to down-regulate CXCR4 expression, resulting in cell migration defects in CML. We proposed that the tyrosine kinase inhibitors imatinib or INNO-406 may restore CXCR4 expression and cause the migration of CML cells to BM microenvironment niches which results in acquisition of stroma-mediated chemoresistance of CML progenitor cells. In KBM5 and K562 cells, imatinib or INNO-406 increased CXCR4 expression and migration. Imatinib induced a G_0/1 cell cycle block in CML cells, which was further enhanced in a bone marrow-derived stromal cell (MSC) co-culture system. Quiescent KBM5 cells migrating to stromal cells were resistant to Imatinib-induced cell death. These anti-apoptotic effects were abrogated by inhibition of CXCR4 (AMD3465) or Integrin-linked kinase (QLT0267). BM CD34⁺ cells from newly diagnosed (n=15) or blast crisis CML patients (n=5) expressed markedly lower CXCR4 levels compared with normal CD34⁺ BM progenitor cells (n=12) (p<0.05), and was associated with deficient migration of CD34⁺ CML cells. A longitudinal analysis of peripheral blood samples from CML patients in blast crisis treated with imatinib (n=5) revealed induction of CXCR4 in peripheral blood CD34⁺ progenitor cells within the first few days after initiation of treatment, which coincided with decreased WBC and absolute blast counts. After prolonged (3–10 day) exposure to imatinib, however, CXCR4 levels of CD34⁺ PB CML cells decreased, frequently below the starting levels. CXCR4 immunostaining on blasts/immature cells in BM biopsy sections from patients demonstrated that 5 of 7 patient samples were CXCR4-negative prior to imatinib therapy; 4 of these 5 became CXCR4 positive after imatinib treatment. While patients in blast-phase had short-lived hematologic responses to imatinib, Ph⁺ clone remained predominant in the bone marrow, and all patients ultimately relapsed with recurrence of blasts in the BM. Altogether, these findings suggest that the up-regulation of CXCR4 by imatinib promotes migration of CML cells to BM stroma, causing G_0/1 cell-cycle arrest and enhancing the survival of quiescent CML progenitor cells within bone marrow microenvironment. This provides the rationale for interfering with the protective effects of BM stroma cells by inhibiting CXCR4 and/or integrin signaling, which could be of benefit in eradicating residual quiescent CML cells.