Mice transgenic for the hemopoietic growth factor, granulocyte-macrophage colony-stimulating factor (GM-CSF), exhibit a sustained elevation of GM-CSF levels and a 50-100-fold elevation of peritoneal macrophage cell numbers. The excess cell numbers were found to be generated in pre-adult life, with numbers remaining relatively constant thereafter. In the pre-adult period, no abnormalities were noted in the number or composition of blood, bone marrow, or spleen cells, the type or number of GM progenitor cells in the marrow or spleen, or the rate of appearance of newly formed monocytes in the peripheral blood. Peritoneal macrophages in pre-adult transgenic mice exhibited elevated mitotic activity and, after tritiated thymidine labeling, a more rapid accumulation of labeled progeny. The increase in peritoneal macrophage cell numbers appears, therefore, to be based on a GM-CSF-induced increase in local proliferative activity by peritoneal macrophages. This increased activity declined at the age of 8-10 wk, in parallel with a change in the morphology of the transgenic macrophages and an increase in binucleate and multinucleate macrophages arising by cell fusion. This change in macrophage phenotype was restricted to the transgenic mice and may therefore be a consequence of continued overstimulation by GM-CSF.

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