Granulocyte colony stimulating factor expands hematopoietic stem cells within the central but not endosteal bone marrow region.

Granulocyte colony stimulating factor (G-CSF) is clinically well established for the mobilization of hematopoietic stem cells (HSC). Extensive data on the underlying mechanism of G-CSF induced mobilization is available; however, little is known regarding the functional effect of G-CSF on HSC within the bone marrow (BM). In this study we analyzed the proportion and number of murine HSC in the endosteal and central bone marrow regions after 4 days of G-CSF administration. We demonstrate that the number of HSC, defined as CD150(+)CD48(-)LSK cells (LSKSLAM cells), increased within the central BM region in response to G-CSF, but not within the endosteal BM region. In addition the level of CD150 and CD48 expression also increased on cells isolated from both regions. We further showed that G-CSF mobilized proportionally fewer LSKSLAM compared to LSK cells, mobilized LSKSLAM had colony forming potential and the presence of these cells can be used as a measure for mobilization efficiency. Together we provide evidence that HSC in the BM respond differently to G-CSF and this is dependent on their location. These findings will be valuable in developing new agents which specifically mobilize HSC from the endosteal BM region, which we have previously demonstrated to have significantly greater hematopoietic potential compared to their phenotypically identical counterparts located in other regions of the BM.

Phenotypically identical hemopoietic stem cells isolated from different regions of bone marrow have different biologic potential.

Hemopoietic stem cells (HSCs) reside within a specified area of the bone marrow (BM) cavity called a "niche" that modulates HSC quiescence, proliferation, differentiation, and migration. Our previous studies have identified the endosteal BM region as the site for the HSC niche and demonstrated that hemopoietic stem and progenitor populations (HSPCs, LSK) isolated from different BM regions exhibit significantly different hemopoietic potential. In this study, we have analyzed subpopulations of LSK cells isolated from different regions of the BM and showed that CD150(+)CD48(-)LSK HSCs within the endosteal BM region have superior proliferative capacity and homing efficiency compared with CD150(+)CD48(-)LSK HSCs isolated from the central BM. Furthermore, we show, for the first time, that a subset of CD150(+)CD48(+)LSK progenitor cells, previously defined as B-lymphoid primed hemopoietic cells, are capable of multilineage reconstitution, however, only when isolated from the endosteal region. In addition, we provide evidence for an unrecognized role of CD48 in HSC homing. Together, our data provide strong evidence that highly purified HSCs show functional differences depending on their origin within the BM and that the most primitive HSCs reside within the endosteal BM region.