Identification and enrichment of human osteoprogenitor cells by using differentiation stage-specific monoclonal antibodies

A major problem in developmental bone biology is the inability to clearly identify early progenitor cells of the osteogenic and related lineages. Identification of these cells is important for the study of their normal development and for determination of potential changes in skeletal diseases. The objective of the present study was to obtain specific markers for early progenitor cells. Monoclonal antibodies were raised against human marrow stromal fibroblastic cell cultures, known to be rich in progenitors for the stromal lineages. Antibodies were selected initially by their reactivity with these marrow cultures and their immunohistochemical localization in human fetal tissues, in progenitor cell regions adjacent to osteoblastic cells. Antibody HOP-26 was strongly reactive with cells in marrow stromal colonies at early stages of differentiation, before the induction of alkaline phosphatase activity, and decreased dramatically after the cells reached confluence. In sections of human fetal limb, binding of HOP-26 was restricted to cells in close proximity to the developing bone, in periosteum, and between the developing bone trabeculae. In adult trabecular bone tissue, HOP-26 was reactive with occasional cells present within the marrow spaces with osteoblasts, adipocytes, and fibrous tissue unreactive. No antibody binding was detected in sections of skin, muscle, appendix, brain, tonsil, or liposarcoma, or cultured SaOS II, MG63, or skin cells. In primary cell suspensions, HOP-26 was unreactive with blood cells but strongly reactive with 0.59 ± 0.27% of nucleated marrow cells. The antigen associated with these cells was detectable both intracellularly and on the cell surface, and by using immunopanning, HOP-26 selected the marrow stromal fibroblastic colony-forming units (CFU-F). HOP-26 provides the means to identify osteogenic progenitor cells directly and with high specificity. The present studies demonstrate the value of this antibody in providing enriched populations of progenitor cells for experimental studies of osteogenic differentiation and in histopathology.