The CXCR4 chemokine receptor in acute and chronic leukaemia: A marrow homing receptor and potential therapeutic target

Chemokine (C-X-C motif) receptor 4 (CXCR4) is essential for homing and maintenance of haematopoietic stem cells in distinct stromal cell niches within the marrow. Chemotactic responsiveness of haematopoietic stem cells is restricted to the ligand for CXCR4, stromal cell-derived factor-1 (SDF-1/CXCL12), which is constitutively secreted by marrow stromal cells. Myeloid and lymphoid leukaemia cells also express CXCR4 that induces leukaemia cell chemotaxis and migration beneath marrow stromal cells. CXCR4 expression levels have a major prognostic impact in acute myeloid leukaemia. There is growing in vitro and in vivo evidence that CXCR4 expression by leukaemia cells allows for homing and their retention within the marrow. As such, leukaemia cells appear to utilise CXCR4 to access niches that are normally restricted to progenitor cells, and thereby reside in a microenvironment that favours their growth and survival. CXCR4- and integrin-mediated contact between leukaemia cells and stromal cells protects leukaemia cells from spontaneous and chemotherapy-induced cell death and therefore may represent a mechanism to explain minimal residual disease and subsequent relapses commonly seen in the treatment of these diseases. This review summarises our current knowledge regarding the importance of CXCR4 in acute and chronic leukaemia, discusses the importance of CXCR4 detection by flow cytometry in the diagnostic workup of leukaemia patients, and introduces the potential role of CXCR4-targeting compounds for the treatment of leukaemia patients.