Bone marrow accessory cells regulate human bone precursor cell development

Objective. Much remains to be learned about the intimate relationship betwe en bone marrow and its surrounding tissue: the bone. We hypothesized that b one marrow accessory cell populations might regulate the development of hum an bone precursor cells. Materials and Methods. We used immunologic phenotyping, and isolation metho ds to fractionate subpopulations of nonadherent, low-density (NALD) human b one marrow cells. These cells were examined for their ability to support th e serum-free survival, proliferation, and expression of bone proteins by hi ghly purified populations of human bone precursor cells. Quantitative asses sment of the accessory cell populations as well as human bone precursor cel ls phenotype was performed using multiparameter flow cytometry. Bone protei n expression was evaluated by immunocytochemistry, Western analysis, and en zymatic analysis (for alkaline phosphatase activity). Results. Human bone marrow contains a cell population that stimulates the d evelopment of purified bone precursor cells. Feeder-layer studies demonstra te that these osteopoietic accessory cells (OACs) do not require cell-cell interaction to promote bone precursor cell development but, rather, produce soluble molecules responsible for their effects. Flow cytometric analyses reveal that bone marrow derived B cells, T cells, macrophages, natural kill er cells, and endothelial cells do not produce this stimulatory factor. The (growth) factor cannot be replaced by addition of exogenous cytokines, The isolation of human transforming growth factor beta receptor type II (TGF-b eta RII)-positive cells increases OAC-specific activity in bone cell ex viv o expansion cultures. Moreover, isolation of OAC bone marrow cells characte rized by high TGF-beta RII expression, relatively low cellular complexity, and small size yields a population that is highly enriched for OACs. Conclusion. We conclude that human bone marrow contains a population of OAC s that are an obligate requirement for the early phases of bone cell develo pment ex vivo. (C) 2000 International Society for Experimental Hematology. Published by Elsevier Science Inc.