AIMS: Matrix Metalloproteinase-9 (MMP-9) plays an important role in ventricular remodeling after acute myocardial infarction. The cardioprotectant adenosine may be involved in ventricular remodeling. We have shown that adenosine inhibits the secretion of MMP-9 by human neutrophils. This study investigated the effect of adenosine on MMP-9 production by human macrophages. METHODS: Cells used in this study were monocytes of healthy volunteers, a human monocyte cell line, and leukocytes from patients following myocardial infarction. Monocytes were differentiated to macrophages and treated with adenosine. RESULTS: Adenosine enhanced MMP-9 secretion by human macrophages in a time- and dose-dependent manner. Increasing the level of endogenous adenosine by inhibition of adenosine deaminase or adenosine transferase also increased MMP-9 secretion. Adenosine enhanced MMP-9 production when macrophages were activated by hypoxia or Toll-like receptor-4 ligands such as lipopolysaccharide, hyaluronan and heparan sulphate. The effect of adenosine was replicated by the A3 agonist IB-MECA and inhibited by silencing the A3 receptor. Adenosine improved monocyte capacity to migrate through a matrix of gelatin B, and this effect was blocked by inhibition of MMP-9 activity. The chemotactic capacity of macrophages was reduced by adenosine through a loss of expression of the monocyte chemotactic protein-1 receptor. Finally, MMP-9 expression was higher in blood cells from patients with acute myocardial infarction compared with healthy volunteers. CONCLUSION: Adenosine activates MMP-9 secretion by macrophages through its A3 receptor. The effect is in contrast to that observed in neutrophils, where adenosine inhibits MMP-9 secretion by the A2a receptor. These observations may have important implications for therapeutic strategies targeting adenosine receptors in the setting of myocardial infarction.