Peripheral blood hematopoietic progenitor/stem cells proliferate to form colonies in liquid culture but require contact with vascular endothelial cells and gm‐csf

To test the hypothesis whether peripheral blood hematopoietic progenitor/stem cells (PBSCs) interact with vascular endothelial cells during events leading to extramedullary hematopoiesis, we cocultured T‐cell depleted, peripheral blood mononuclear cells obtained from cytokine treated primates in liquid culture containing a monolayer of porcine aortic endothelial cells (PAECs) for 7 days. Recombinant human granulocyte‐macrophage colony‐stimulating factor (GM‐CSF) added to cocultures of PBSC‐PAEC stimulated colony formation, while only a few clusters were observed in cultures without GM‐CSF. In contrast, colony formation was not stimulated when either interleukin 1 (IL‐1) or IL‐3 were added to the cultures. Colony and cluster formation in response to GM‐CSF was dose dependent; 20 ±5 colonies/5,000 cells were formed at 3 U/ml, and optimal colony formation of 42 ±11/5,000 cells occurred at 100 U/ml. Colonies formed in the presence of GM‐CSF were large, and most contained >200 cells. Morphological and phenotypical characterization of cells from isolated colonies suggested that the majority of cells were predominantly immature myeloid elements. However, there was also a low but consistent frequency of megakaryocytic lineage cells. Thus, PBSCs interact with non‐bone marrow‐ derived vascular endothelial cells and proliferate, but only in the presence of GM‐CSF, suggesting that PBSC interaction with vascular endothelial cells in vivo could lead to extramedullary hematopoiesis.