TRANSFORMING GROWTH-FACTOR BETA(1) DIRECTLY AND REVERSIBLY INHIBITS THE INITIAL CELL DIVISIONS OF LONG-TERM REPOPULATING HEMATOPOIETIC STEM-CELLS

Hematopoiesis appears to be regulated, in part, by a balance between e xtracellular positive and negative growth signals, Transforming growth factor beta-1 (TGF-beta(1)) has been shown to be a negative regulator of primitive hematopoietic cells. This study examined the direct effe ct of TGF-beta(1) on the proliferation and differentiation of long-ter m repopulating hematopoietic stem cells (LTR-HSC) in vitro, We previou sly reported a cell fractionation approach that includes the selection of low Hoescht 33342/low Rhodamine 123 (low Ho/Rh) cell fractions tha t are highly enriched for long-term repopulating cells (LTR-HSC) and a lso clone to a very high efficiency in the presence of stem cell facto r (SCF) + interleukin-3 (IL-3) + IL-6: 90% to 100% of individually cul tured low Ho/Rh cells formed high proliferative potential clones, This high cloning efficiency of an LTR-HSC enriched cell population enable d proliferation inhibition studies to be more easily interpreted. In t his report, we show that the continuous presence of TGF-beta(1) direct ly inhibits the cell division of essentially all low Ho/Rh cells (in a dose-dependent manner) during their 0 to 5th cell division in vitro. Therefore, it follows that TGF-beta(1) must directly inhibit the proli feration of LTR-HSC contained within these low Ho/Rh cells, The time r equired for some low Ho/Rh cells to undergo their first cell division in vitro was also prolonged in the presence of TGF-beta(1) Furthermore , when low Ho/Rh cells were exposed to TGF-beta(1) for varying lengths of time before neutralization of the TGF-beta(1) by monoclonal antibo dy, the ability to form macroclones was markedly decreased after simil ar to 4 days of TGF-beta(1) exposure, In addition, 1 to 10 ng/mL of TG F-beta(1) resulted in a maintenance of high proliferative potential-co lony-forming cell (HPP-CFC) during 8 days of culture compared with los s of HPP-CFC in cultures with no added TGF-beta(1). In conclusion, thi s study shows that TGF-beta(1) directly inhibits the initial stages of proliferation of LTR-HSC and appears to slow the differentiation of d aughter cells of low Ho/Rh cells. (C) 1996 by The American Society of Hematology.