[en] Mesenchymal stem cells (MSC) have the ability to support and maintain hematopoiesis in vitro. However, mechanisms implicated in this support are not fully characterized. In the present study, the role of stromal-derived factor-1 (SDF-1)/CXCR4 axis in the interactions between MSC and hematopoietic stem/progenitor cells (HSPC) was studied. Human bone marrow MSC were plated as feeder layers in Dexter-type long-term cultures (LTC) with human cord blood CD34(+) HSPC. Cultures were supplemented weekly with neutralizing antibodies against CXCR4 or SDF-1 for 5 wk. LTC-initiating cell (IC) activity was strongly dependent on the SDF-1/CXCR4 axis, as both antibodies significantly decreased secondary colony-forming cell production. To assess the effect of SDF-1/CXCR4 axis on progenitor cell proliferation, LTC-IC killing assays were carried out: in LTC of CD34(+) cells in contact with MSC, treatment with anti-CXCR4 antibody significantly reduced the number of cycling progenitors. These results indicate that the SDF-1/CXCR4 axis promotes HSPC proliferation in contact with MSC. Interestingly, when HSPC were separated from MSC by a semipermeable membrane, LTC-IC activity became CXCR4 independent. Multiplex analysis of MSC-conditioned medium revealed that in addition to SDF-1, MSC produced stimulatory and inhibitory factors, such as interleukin (IL)-6, IL-11, granulocyte macrophage-colony stimulating factor as well as monocyte-chemoattractant protein-1. Altogether, human MSC support hematopoiesis in Dexter-type cultures through the activation of the SDF-1/CXCR4 axis. Our data further suggest that SDF-1 stimulates retention of HSPC in MSC niches which expose them to stimulatory and inhibitory factors in a paracrine manner.