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Small vessel (microvascular) endothelial cells are in close contact with hematopoietic progenitor cells in the bone marrow and therefore may have an important role in hematopoiesis. Although other studies have shown that endothelial cells produce various colony-stimulating factors (CSFs), these studies examined large vessel endothelial cells, which are different in many respects from microvascular endothelial cells and which do not contact cells in the bone marrow. We show in this study that primary cultures of unstimulated rat fat capillary endothelial cells grown in serum-free medium produce a substantial amount of granulocyte-macrophage CSF (GM-CSF). The medium conditioned by these cells stimulated proliferation of two different lines of GM- CSF-responsive cells--PT-18 mast cells and FDC-P1 cells--and supported the growth of cells of the granulocyte and macrophage lines in cultures of rat bone marrow cells. The factor responsible for this activity had physical properties consistent with those of GM-CSF, namely, a similar apparent mol wt by gel filtration, resistance to repeated freeze-thaws, resistance to boiling for ten minutes but not for 30 minutes, and resistance to heating to 56 degrees C for one hour. The factor causing target cell stimulation was not B cell-stimulating factor-1 (BSF-1, or IL 4), since it failed to stimulate a BSF-1-responsive cell line HT2- JH, and target cells (PT-18) did not respond appreciably to recombinant BSF-1. Northern blot analysis of mRNA from rat fat capillary endothelial cells showed high levels of expression of GM-CSF, confirming that this factor is produced by microvascular endothelial cells. This is the first report of CSF production by unstimulated microvascular endothelial cells, demonstrating that these ubiquitous cells are capable of producing sizable amounts of at least one growth factor for hematopoietic progenitor cells. |
