CXCL12+ stromal cells as bone marrow niche for CD34+ hematopoietic cells and their association with disease progression in myelodysplastic syndromes.

The bone marrow microenvironment, known as 'hematopoietic stem cell niche,' is essential for the survival and maintenance of hematopoietic stem cells. Myelodysplastic syndromes (MDS) are a group of clonal hematopoietic stem cell diseases, which eventually result in leukemic transformation (acute myelogenous leukemia with myelodysplasia-related changes, AML-MRC). However, the precise components and functions of the MDS niche remain unclear. Recently, CXCL12-abundant reticular cells were shown to act as a hematopoietic stem cell niche in the murine bone marrow. Using immunohistochemistry, we show here that CXCL12(+) cells were located in the cellular marrow or perivascular area, and were in contact with CD34(+) hematopoietic cells in control and MDS/AML-MRC bone marrow. MDS bone marrow exhibited higher CXCL12(+) cell density than control or AML, not otherwise specified (AML-NOS) bone marrow. Moreover, AML-MRC bone marrow also exhibited higher CXCL12(+) cell density than control bone marrow. CXCL12(+) cell density correlated positively with bone marrow blast ratio in MDS cases. CXCL12 mRNA level was also higher in MDS bone marrow than in control or AML-NOS bone marrow. In vitro coculture analysis revealed that overexpression of CXCL12 in stromal cells upregulated BCL-2 expression of leukemia cell lines. Triple immunostaining revealed that the CD34(+) hematopoietic cells of MDS bone marrow in contact with CXCL12(+) cells were BCL-2-positive and TUNEL-negative. In the bone marrow of MDS cases, CXCL12-high group showed significantly higher Bcl-2(+)/CD34(+) cell ratio and lower apoptotic cell ratio than CXCL12-low group. Moreover, CXCL12-high refractory cytopenia with multilineage dysplasia (RCMD) cases had a greater tendency to progress to refractory anemia with excess blasts (RAEBs) or AML-MRC than CXCL12-low RCMD cases. These results suggest that CXCL12(+) cells constitute the niche for CD34(+) hematopoietic cells, and may be associated with the survival/antiapoptosis of CD34(+) hematopoietic cells and disease progression in MDS. Thus, CXCL12(+) cells may represent a novel MDS therapeutic target.