FIBRONECTIN INCREASES BOTH NONADHERENT CELLS AND CFU-GM WHILE COLLAGEN INCREASES ADHERENT CELLS IN HUMAN NORMAL LONG-TERM BONE-MARROW CULTURES

Normal haematopoietic proliferation and differentiation occur within t he human bone marrow microenvironment which is comprised of stromal ce lls including fibroblasts, adipocytes, macrophages and endothelial cel ls as well as the extracellular matrix made of collagen, fibronectin, laminin, vitronectin, thrombospondin and haemonectin. All haematopoiet ic progenitor cells including primitive LTC-IC, multilineage CFU-mix, myeloid CFU-GM and erythroid BFU-E adhere to the heparin-binding domai ns of the extracellular matrix component fibronectin. Human long-term bone marrow cultures (LTHBMC) represent the best available approximati on for the in vivo marrow microenvironment in which the proliferation and differentiation of haematopoietic progenitor cells depend on the p resence of marrow stromal cells and their attendant matrices. Since ex tracellular matrix components have been shown to promote myelopoiesis in long-term murine bone marrow cultures, we have examined the effect of two main components of the extracellular matrix: fibronectin and co llagen type I on myelopoiesis in LTHBMC in an effort to increase the m yeloid progenitor cell production. The present study revealed differen t modulatory effects for these two components. Collagen significantly increased the adherent fraction of LTHBMC (p < 0.05) but always result ed in a decreased myeloid progenitor cell (CFU-GM) production througho ut the whole 8 weeks of culture. On the other hand, fibronectin signif icantly increased the number of both non-adherent cells, CFU-GMs (p < 0.01) and to a lesser extent the number of adherent cells as well as m aintaining the LTHBMC up to 14 weeks. Fibronectin has been previously shown to stimulate the development of CFU-GMs in short-term semisolid cultures and to play an active role in haematopoietic progenitor cell- microenvironment interactions. Therefore, the presence of fibronectin in LTHBMC could increase both the productivity and longevity of myelop oiesis in the system. The integration of fibronectin in the ex vivo ex pansion systems currently undergoing development would ensure a sustai ned effective cumulative production of the myeloid progenitor cells (C FU-GMs), and consequently could accelerate the rate of haematological recovery in transplanted patients.