Effects of filgrastim on granulopoietic cells of mice pretreated with methotrexate.

We have evaluated the effect of filgrastim on proliferation and differentiation activity of granulopoietic cells in mice pretreated with methotrexate. Filgrastim was injected daily, from day 8 to 28 after cytotoxic agent administration. The granulopoiesis changes were measured by assessment of GM-CFU cells content, marrow and spleen granuloid cells pool as well as circulating neutrophils. In MTX pretreated mice, bone marrow GM-CFU oscillating values were higher than normal values, but these changes were not followed by high proliferative activity in granuloid precursor cell compartment. After MTX treatment, filgrastim administration was unable to stimulate marrow granulopoiesis as observed in normal mice. In the spleen, MTX led to dramatic changes in the proliferative activity of GM-CFU cells, but did not result in spleen granuloid cell changes. However, filgrastim treatment induced a spleen granuloid amplification, similar to the changes observed in circulating neutrophils values. We suggest that these findings can be explained by inhibition of differentiation of marrow GM-CFU cells into the more mature granulopoietic cells and/or by an inhibited proliferative activity of marrow granuloid cells. They can be also explained in terms of an unfavorable marrow microenvironment for granulopoiesis, contrary to a supportive spleen microenvironment.