Epithelial and Mesenchymal Cell Biology Leukocytes Induce Epithelial to Mesenchymal Transition after Unilateral Ureteral Obstruction in Neonatal Mice

Urinary tract obstruction during renal development leads to tubular apoptosis, tubular atrophy, and interstitial fibrosis. Epithelial to mesenchymal transition (EMT) has been proposed as a key mechanism of myofibroblast accumulation in renal fibrosis. We studied the interplay of leukocyte infiltration, tubular apoptosis, and EMT in renal fibrosis induced by unilateral ureteral obstruction (UUO) in neonatal mice. We show that leukocytes mediate tubular apoptosis and EMT in the developing kidney with obstructive nephropathy. Blocking leukocyte recruitment by using the chemokine receptor-1 antagonist BX471 protected against tubular apoptosis and interstitial fibrosis, as evidenced by reduced monocyte influx, a decrease in EMT, and attenuated collagen deposition. EMT was rapidly induced within 24 hours after UUO along with up-regulation of the transcription factors Snail1 and Snail2/Slug, preceding the induction of -smooth muscle actin and vimentin. In the presence of BX471, the expression of chemokines, as well as of Snail1 and Snail2/Slug, in the obstructed kidney was completely attenuated. This was associated with reduced macrophage and T-cell infiltration, tubular apoptosis, and interstitial fibrosis in the developing kidney. Our findings provide evidence that leukocytes induce EMT and renal fibrosis after UUO and suggest that chemokine receptor-1 antagonism may prove beneficial in obstructive nephropathy. (Am J Pathol 2007, 171:861–871; DOI: 10.2353/ajpath.2007.061199) Congenital obstructive nephropathy is a frequent cause of kidney failure in infants and children. Chronic unilateral ureteral obstruction (UUO) leads to interstitial inflammation, tubular apoptosis, and interstitial fibrosis. Central to these events is the influx of macrophages and lymphocytes into the tubulointerstitium. Macrophages release proinflammatory cytokines, cytotoxic substances, and induce apoptosis in tubular cells. Furthermore, macrophages are critical in promoting extracellular matrix production and fibroblast proliferation. Recently, it has been shown that fibroblasts at sites of inflammation originate either from the bone marrow or from an epithelial to mesenchymal transition (EMT) of tubular cells at sites of injury. This process of EMT is characterized by the loss of epithelial adhesion, polarity, and epithelial cell-specific markers such as E-cadherin. In addition, cells undergoing EMT gain the expression of fibroblast (vimentin) and smooth muscle [ -smooth muscle actin ( -SMA)] markers. These changes lead to an intermediate mesenchymal phenotype, termed the myofibroblast, which is motile and invasive and produces extracellular matrix. Molecular markers for EMT include loss of E-cadherin, increased expression of vimentin and SMA, nuclear localization of activated -catenin, and increased production of the transcription factors Snail1 and Snail2/Slug. Snail1 acts as a key regulator of EMT by suppressing Ecadherin transcription, increasing matrix metalloproteinases, and modulating tight junction protein expression. Snail2/Slug inhibits E-cadherin production, thereby facilitating detachment of epithelial cells, thus enabling them to migrate. Both Snail1 and Snail2 are crucial in the initiation and progression of EMT. Growth factors including transforming growth factor1 (TGF1), fibroblast growth factor-2, and epidermal growth factor have been found to induce EMT. TGF1 stimulates extracellular matrix