The regulation of haemopoietic stem cell and progenitor cell proliferation by humoral factors

Abstract

The mechanisms which regulate the growth fraction of the haemopoietic stem cell (CFU-S) and granulocyte macrophage progenitor cell (GM-CFC) have been investigated. In normal murine bone marrow (NMBM) a small proportion of the CFU-S are synthesising DNA (-10%). In contrast, in the bone marrow from mice regenerating after treatment with cytotoxic drugs and in developing haemopoietic tissues such as murine fetal liver a large proportion of the CFU-S (-40%) are synthesising DNA. Medium conditioned by normal murine and human bone marrow cells inhibited the proliferation of rapidly cycling CFU-S from regenerating bone marrow. This inhibitor was contained in a 50-100K daltons ultrafiltration fraction. In contra-distinction medium conditioned by human fetal liver cells stimulated the proliferation of CFU-S from NMBM. The stimulator was produced by adherent cells and was contained in a 30-50K daltons ultrafiltration fraction. An alternative assay for the humoral regulators of CFU-S proliferation was developed. Different numbers of haemopoietic cells were injected into lethally irradiated mice. Five days later they were injected with 2iCi of 125IUdR and sacrificed 2 hours later. There was a linear relationship between the log 125IUdR uptake into the spleen and femur and the log cell dose injected. Pre-treatment of haemopoietic cells with an S-phase specific cytotoxic drug resulted in a reduction in the 125IUdR incorporation into the spleen. This enabled the kinetic properties of a haemopoietic stem cell population to be assessed and the humoral 111 factors which modulate the growth fraction of these cells to be investigated. At early stages of gestation (11-14 weeks) in human fetal liver few GM-CFC are synthesising DNA, whereas later in gestation (>14 weeks) a large proportion of GM-CFC are in S-phase, Moore and Williams (1973b). Incubation of NMBM GM-CFC (approx 40% in DNA synthesis) with a supernatant from an early human fetal liver (11-14 weeks) reduced the proportion synthesising DNA to <5%. In contrast, the proportion of murine GM-CFC synthesising DNA was not affected by incubation with a supernatant from a late human fetal liver (>14 weeks). GM-CFC that had been switched out of cycle by incubation with a supernatant from an early gestation human fetal liver were switched back into cycle following incubation with a late human fetal liver supernatant. The inhibitor and stimulator of GM-CFC proliferation were both produced by non-adherent cells and were contained in >100K and 30-50K daltons ultrafiltration fractions repectively. It is likely that changes in the relative levels of a proliferation inhibitor and stimulator throughout gestation might control the proportion of GM-CFC in cycle.