Myeloid cell kinetics in response to haemopoietic growth factors.

The morphologically recognizable cells of haemopoietic tissue comprise some 95% of the total and their kinetic performance is both flexible and adaptable to stress conditions. The effects of the common myeloid growth factors on these cells have been examined. Using the classically developed analyses of cell kinetics with tritiated thymidine labelling autoradiography, it is shown that continuously infused or repeated injections of G-CSF increases the proliferative activity of marrow haemopoietic tissue in both mouse and man, shortening the average cell cycle times by 35 and 65% respectively, amplifying neutrophil production so that levels in the peripheral blood rise 10-15-fold. This amplified production, amounting to 3-4 extra proliferation divisions, is mostly confined to the proliferatively active maturing neutrophil cell compartments. In addition, the neutrophil maturation time is also reduced, leading to a rapid and sustained release of postmitotic cells within 1-2 days, compared with the normal time of 4-6 days. Neither GM-CSF nor IL-3 generates any significant amplification of neutrophil production in mice but, in humans, GM-CSF stimulates cell proliferation in the bone marrow to a similar degree as doses G-CSF. Studies on the granulocyte-macrophage progenitor population show that the proliferation stimulus in response to GM-CSF is not confined to the maturing populations. In the maturing neutrophil precursor population, average cell cycle times are shortened by 60%, but in this case the overall maturation times are unaffected and their time of release into the circulation is normal. The response to GM-CSF, however, is not so straightforward as that to G-CSF. The peripheral half-life of the mature cells is considerably prolonged and this is consistent with some suggestion of functional impairment. In addition, a significant release of immature cells and eosinophils (also expanded in the bone marrow in response to GM-CSF) dilutes the neutrophilic response. Monocyte production is also stimulated by G- and GM-CSF and, though no direct measurement of proliferation has been made, stimulation at all stages of their proliferation, maturation and release are implied.