Characterization of circulating osteoblast lineage cells in humans

We recently identified circulating osteoblastic cells using antibodies to osteocalcin (OCN) or alkaline phosphatase (AP). We now provide a more detailed characterization of these cells. Specifically, we demonstrate that 46% of OCN positive (OCN^pos) cells express AP, and 37% also express the hematopoietic/endothelial marker CD34. Using two different anti-OCN antibodies and forward/side light scatter characteristics by flow cytometry, we find that OCN^pos cells consist of two distinct populations: one population exhibits low forward/side scatter, consistent with a small cell phenotype with low granularity, and a second population has higher forward/side scatter (larger and more granular cell). The smaller, low granularity population also co-expresses CD34, whereas the larger, more granular cells are CD34 negative. Using samples from 26 male subjects aged 28 to 68 years, we demonstrate that the concentration of circulating OCN^pos cells increases as a function of age (R = 0.59, P = 0.002). By contrast, CD34^pos cells tend to decrease with age (R = - 0.31, P = 0.18); as a consequence, the ratio of OCN^pos:CD34^pos cells also increase significantly with age (R = 0.54, P = 0.022). These findings suggest significant overlap between circulating cells expressing OCN and those expressing the hematopoietic/endothelial marker CD34. Further studies are needed to define the precise role of circulating OCN^pos cells not only in bone remodeling but rather also potentially in the response to vascular injury.