Endothelial cells suppress granulocyte clusters and stimulate large granulocyte colonies in culture.

Endothelial cells have been shown to produce granulopoietic colony stimulating activity (CSA) under the regulatory control of a humoral factor, MRA produced by blood monocytes. An endothelial cell-derived granulopoietic inhibitory factor has also been described. To further define these apparently paradoxical observations, human bone marrow mononuclear cells were co-cultured with umbilical cord derived endothelial cells in a plasma clot system in vitro. To enhance the sensitivity of the assay for growth effects attributable to the endothelial cells (or their products) alone, an exogenous source of CSA (e.g. a peripheral blood leukocyte feeder layer) was not used. On day ten of culture, less than or equal to 1% endothelial cells markedly stimulated the growth of early granulocyte progenitors (large diaminofluorine positive (DAF+) GM-CFUc) (p less than .01) and a linear dose response relationship was confirmed (p less than .001). Late granulocyte progenitors (DAF+ clusters) were coincidently suppressed by less than or equal to 2% endothelial cells (p less than .01). No effect of endothelial cells on intermediate progenitors (small GM-CFUc) was demonstrated at any concentration. Similar effects were observed with the addition of 5% to 30% endothelial conditioned medium (ECM) (p less than .01). When cohort cultures were evaluated serially, suppression of clusters was observed by day four and stimulation of large GM-CFUcs by day six. These varied effects on different stages of granulocyte differentiation suggest that endothelial cell derived CSA(S) may be of biological relevance in the regulation of granulopoiesis.