The phenotype of a subpopulation(s) of human monocytes which has been shown to proliferate in vitro in response to macrophage colonystimulating factor (M-CSF or CSF-1) and granulocyte-macrophage CSF (GM-CSF) is as yet unknown. To identify this proliferating subpopulation(s) we demonstrated first that DNA synthesis was occurring under culture conditions suitable for flow cytometric evaluation. Flow cytometric analysis of surface antigen expression identified that after 5 days of culture the proliferating subpopulation of monocytes expressed CD14, CD13, CD33, CD11b, CD11c, CD87, HLA-DR, CD45RO, and did not express CD86, CD34, CD80, CD4, CD16, and CD56. In addition, these proliferating monocytes (representing approximately 5% of total monocytes) were shown to produce the proinflammatory cytokines interleukin-6 and tumor necrosis factor ␣ in response to lipopolysaccharide stimulation. Further characterization and subsequent isolation of this subpopulation of monocytes may provide new and important information necessary to understand inflammatory diseases such as rheumatoid arthritis, where local proliferation at the site of inflammation may be a key factor contributing to the chronicity of the disease. J. Leukoc. Biol. 66: 953-960; 1999.
There is evidence that a proportion of human monocytes can proliferate in vitro in response to colony-stimulating factor-1 (CSF-1, also known as M-CSF) and granulocyte-macrophage CSF (GM-CSF). To determine whether there are differences in DNA synthesis responses to these CSF, a large study using purified human peripheral blood monocytes from 45 donors was performed under optimized culture conditions. In contrast to the consistent response to CSF-1, approximately 20% of donors have monocytes that do not respond or have a minimal DNA synthesis response to GM-CSF stimulation. However, analysis demonstrated that no statistically significant differences exist in the levels of CSF-1 and GM-CSF-stimulated proliferation in monocytes. In addition, CSF-1 receptor (CSF-1R) blocking experiments indicated that a proportion of the GM-CSF-induced DNA synthesis is due to endogenous levels of CSF-1. As a further comparison of the actions of the two CSFs, CSF-1R and GM-CSFR levels were measured by flow cytometry, and it was shown that GM-CSFR levels decreased within 5 days of culture, independent of the conditions examined. In contrast, CSF-1R levels at day 5 approximated those measured in uncultured monocytes. Whether the proliferating subpopulation(s) express one or both CSF receptors at the beginning or at the end of culture is as yet unknown. The information obtained in this study will be useful for the design of strategies to enrich for the subpopulation in question based on CSF receptor expression.
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