Conditioned medium from primary porcine endothelial cells alone promotes the growth of primitive human haematopoietic progenitor cells with a high replating potential: Evidence for a novel early haematopoietic activity

The authors have recently shown that direct contact with primary porcine microvascular endothelial cell monolayers (PMVECs) in combination with haematopoietic growth factors enhances the expansion of primitive human haematopoietic CD34⁺ bone marrow progenitor cells. It is now demonstrated that serum-free conditioned medium (PMVEC CM, concentrated 70 x for proteins > 30 kDa) from untreated PMVECs contains haematopoietic growth factor activity that enhances the in vitro proliferation, haematopoietic cell production, and colony cell formation of primitive human haematopoietic progenitor cells. In combination with exogeneously added human growth factors such as interleukin 3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF) and EPO, PMVEC CM enhances the proliferation and colony growth of human haematopoietic CD34⁺ cells. In contrast, PMVEC CM has no significant synergistic activity on either stem cell factor (SCF) or flt3-ligand-induced CD34⁺ cell proliferation, cell production or colony formation. Blocking mAbs against the c-kit receptor have no effect on PMVEC CM-induced CD34⁺ cell proliferation at titres that completely suppress SCF-induced proliferation. Moreover, it is shown that this haematopoietic growth factor supports the proliferation and colony formation of murine, non-human primate, and porcine marrow progenitor cells without any apparent species-specific restrictions in its activity. These finding suggest that PMVEC CM contains a novel early haematopoietic activity.