Characterization of mesenchymal progenitor cells isolated from human bone marrow by negative selection

Studies on the pathogenesis of osteoporosis and other metabolic bone diseases would be greatly facilitated by the development of approaches to assess changes in gene expression in osteoblast/osteoprogenitor populations in vivo without the potentially confounding effects of in vitro culture and expansion of the cells. While positive selection to identify a progenitor population in human marrow can be used to select for cells capable of osteoblast differentiation, each of the markers that have been used to identify marrow mesenchymal populations (alkaline phosphatase [AP], Stro-1, CD29, CD49a, CD73, CD90, CD105, CD166, CD44, CD146 and CD271) may be expressed on distinct subsets of marrow mesenchymal cells. Thus, positive selection with one or more of these markers could exclude a possibly relevant cell population that may undergo important changes in various clinical conditions. In the present report, we describe the isolation and characterization of human osteoprogenitor cells obtained by depletion of bone marrow cells of all hematopoietic lineage/hematopoietic stem cells and endothelial/endothelial precursor cells (lin-/CD34/CD31-). The yield of lin-/CD34/CD31- cells from ~10 mL of bone marrow (~80 million mononuclear cells) was ~80,000 cells (0.1% of mononuclear cells). While not selected on the basis of expression for the mesenchymal marker, Stro-1, 68% of these cells were Stro-1+. Using linear whole transcriptome amplification followed by quantitative polymerase chain reaction (QPCR) analysis, we also demonstrated that, compared to lin- cells (which are already depleted of hematopoietic cells), lin-/CD34/31- cells expressed markedly lower mRNA levels for the endothelial/hematopoietic markers, CD34, CD31, CD45, and CD133. Lin-/CD34/31- cells were also enriched for the expression of mesenchymal/osteoblastic markers, with a further increase in runx2, osterix, and AP mRNA expression following in vitro culture under osteogenic conditions. Importantly, lin-/CD34/31- cells contained virtually all of the mineralizing cells in human marrow: while these cells displayed robust calcium deposition in vitro, lin-/CD34/31+ cells demonstrated little or no mineralization when cultured under identical osteogenic conditions. Lin-/CD34/31- cells thus represent a human bone marrow population highly enriched for mesenchymal/osteoblast progenitor cells that can be analyzed without in vitro culture in various metabolic bone disorders, including osteoporosis and aging.