Myocardial infarction is associated with the rapid induction of mononuclear cell chemoattractants that promote monocyte infiltration into the injured area. Monocyte-to-macrophage differentiation and macrophage proliferation allow a long survival of monocytic cells, critical for effective healing of the infarct. In a canine infarction-reperfusion model, newly recruited myeloid leukocytes were markedly augmented during early reperfusion (5-72 h). By 7 days, the number of newly recruited myeloid cells was reduced, and the majority of the inflammatory cells remaining in the infarct were mature macrophages. Macrophage colony-stimulating factor (MCSF) is known to facilitate monocyte survival, monocyte-to-macrophage conversion, and macrophage proliferation. We demonstrated marked induction of MCSF mRNA in ischemic segments persisting for at least 5 days after reperfusion. MCSF expression was predominantly localized to mature macrophages infiltrating the infarcted myocardium; the expression of the MCSF receptor, c-Fms, a protein with tyrosine kinase activity, was found in these macrophages but was also observed in a subset of microvessels within the infarct. Many infarct macrophages expressed proliferating cell nuclear antigen, a marker of proliferative activity. In vitro MCSF induced monocyte chemoattractant protein-1 synthesis in canine venous endothelial cells. MCSF-induced endothelial monocyte chemoattractant protein-1 upregulation was inhibited by herbimycin A, a tyrosine kinase inhibitor, and by LY-294002, a phosphatidylinositol 3Ј-kinase inhibitor. We suggest that upregulation of MCSF in the infarcted myocardium may have an active role in healing not only through its effects on cells of monocyte/ macrophage lineage, but also by regulating endothelial cell chemokine expression.inflammation; reperfusion; chemokine; growth factors; macrophage colony-stimulating factor REPERFUSED MYOCARDIAL INFARCTION is associated with an intense inflammatory response (7,21,27) leading to healing and scar formation (16). Mononuclear cell chemoattractants are rapidly induced in the infarcted area and mediate monocyte and lymphocyte recruitment (4, 11, 23). Differentiation of monocytes into macrophages is important for their survival, allowing them to actively participate in the repair process through the production of cytokines and growth factors. We hypothesized that monocytes in reperfused infarctions undergo a maturation process, regulated by the local expression of macrophage colony-stimulating factor (MCSF) (8, 38), a hematopoietic growth factor that induces survival, proliferation, differentiation, and activation of mononuclear phagocytes (40,41). Our experiments demonstrated a marked induction of MCSF mRNA in the infarcted areas, associated with evidence of macrophage proliferation in the healing infarct. We present evidence suggesting direct effects of MCSF on endothelial cell phenotype and activity, possibly mediated through c-Fms, resulting in upregulation of monocyte chemoattractant protein-1 (MCP-1). We suggest that local...