Abstract. Erythropoietin (EPO) is a hematopoietic hormone that protects against renal interstitial fibrosis in animal models; however, the mechanism underlying the anti-fibrotic activity of EPO has remained elusive. The present study aimed to elucidate this mechanism. Twenty-four male C57BL6 mice were randomly divided into four groups, each comprising six mice: i) control group (Sh); ii) unilateral ureteral obstruction (UUO) plus vehicle group (U+V); ⅲ) UUO plus 300 U/kg body weight recombinant human (rh)EPO (U+E1) and ⅳ) UUO plus 1,000 U/kg body weight rhEPO (U+E2). Seven days post-surgery, the mice were sacrificed for examination. UUO induced significant deposition of extracellular matrix, detected by picro-sirius red staining, which was decreased following rhEPO treatment. UUO also induced deposition of collagen I and fibronectin, rhEPO treatment was able to attenuate this effect at protein and mRNA levels. Compared with the control groups, UUO resulted in the accumulation of α-smooth muscle actin-positive cells in the interstitium, an effect which was ameliorated by rhEPO. Furthermore, rhEPO abrogated the UUO-induced increase in the number of bone marrow-derived myofibroblasts. Mechanistically, it was discovered that rhEPO decreased CXC chemokine ligand 16 (CXCL16) expression at protein level. However, treatment with rhEPO did not alter the protein expression of CC chemokine ligand 21 or CXCL12. These results suggested that rhEPO decreased fibrocyte accumulation via the suppression of renal CXCL16, which resulted in the attenuation of renal fibrosis. IntroductionChronic kidney disease induces irreversible kidney damage, and progressive renal fibrosis is frequently observed regardless of the underlying cause of the disease (1). The major effector cells that are associated with the development of progressive renal fibrosis are the interstitial myofibroblasts (2). Interstitial myofibroblasts have been proposed to originate from five potential sources, including circulating fibrocytes, pericytes, fibroblasts, the tubular epithelial-mesenchymal transition (EMT) and the endothelial-mesenchymal transition (3).Fibrocytes are bone marrow-derived mesenchymal progenitor cells, which express hematopoietic stem cell antigens, monocytic lineage markers and fibroblast products (4). Fibrocytes constitutively produce extracellular matrix (ECM) components, in addition to ECM-modifying enzymes, and are able to differentiate into myofibroblasts in vitro and in vivo under certain micro-environmental conditions (5). There is an increasing body of evidence suggesting that these cells contribute to the development of the novel myofibroblast population that is responsible for the production of ECM during renal fibrosis (6-11).Erythropoietin (EPO) is a hematopoietic hormone, the majority of which is produced by the adult kidneys. In addition to its erythropoietic effects, EPO exerts protective effects against acute ischemic and toxic renal injuries (12)(13)(14). EPO also protects against interstitial injuries, including interstitial ...