Haemopoiesis in mice genetically lacking granulocyte–macrophage colony stimulating factor during chronic infection with Mycobacterium avium

In order to test the role of granulocyte–macrophage colony stimulating factor (GM‐CSF) in haemopoiesis during chronic infection, mice with a targeted disruption of the gene for GM‐CSF were infected intraperitoneally with the facultative intracellular pathogen, Mycobacterium avium. The bacteria spread to lungs, liver and spleen and persisted for more than 10 weeks at levels between 105 and 106 CFU. Bacterial numbers did not differ significantly between infected GM‐CSF–/– and wild‐type mice, making this an excellent model in which to study the effects of GM‐CSF deficiency on haemopoietic cells without complications of interpretation relating to differences in bacterial load. Haemopoietic colony forming cells (CFC) in the bone marrow of GM‐CSF–/– mice before infection were not different from wild‐type. However, whereas CFC in wild‐type mice increased 1.5‐fold with infection, GM‐CSF–/– mice were unable to increase their CFC and numbers were significantly lower than in infected wild‐type mice. Cells attracted to the peritoneal cavity of the GM‐CSF–/– mice following i.p. injection of bacteria were notably lacking in the large, granular macrophages of activated appearance, which were a feature in wild‐type mice. Nitric oxide production by peritoneal cells from GM‐CSF–/– mice was deficient. Thus, GM‐CSF is not critical for haemopoiesis during chronic infection, but in its absence the mice are unable to increase their output of haemopoietic cells and there are deficiencies in macrophage activation.