Epithelial and Mesenchymal Cell Biology Pericytes and Perivascular Fibroblasts Are the Primary Source of Collagen-Producing Cells in Obstructive Fibrosis of the Kidney

Understanding the origin of scar-producing myofibroblasts is vital in discerning the mechanisms by which fibrosis develops in response to inflammatory injury. Using a transgenic reporter mouse model expressing enhanced green fluorescent protein (GFP) under the regulation of the collagen type I, 1 (coll1a1) promoter and enhancers, we examined the origins of coll1a1-producing cells in the kidney. Here we show that in normal kidney, both podocytes and pericytes generate coll1a1 transcripts as detected by enhanced GFP, and that in fibrotic kidney, coll1a1GFP expression accurately identifies myofibroblasts. To determine the contribution of circulating immune cells directly to scar production, wild-type mice, chimeric with bone marrow from coll-GFP mice, underwent ureteral obstruction to induce fibrosis. Histological examination of kidneys from these mice showed recruitment of small numbers of fibrocytes to the fibrotic kidney, but these fibrocytes made no significant contribution to interstitial fibrosis. Instead, using kinetic modeling and time course microscopy, we identified coll1a1-GFP-expressing pericytes as the major source of interstitial myofibroblasts in the fibrotic kidney. Our studies suggest that either vascular injury or vascular factors are the most likely triggers for pericyte migration and differentiation into myofibroblasts. Therefore, our results serve to refocus fibrosis research to injury of the vasculature rather than injury to the epithelium. (Am J Pathol 2008, 173:1617–1627; DOI: 10.2353/ajpath.2008.080433) The origin of scar-producing cell(s) in the kidney is of primary importance to understanding the mechanisms by which fibrosis develops in response to inflammatory injury. In the healthy kidney as in other organs, no clear endogenous precursor of the myofibroblast has been thought to exist. Derivation of myofibroblasts from bone marrow-derived CD34 circulating cells known as fibrocytes has been proposed in several studies, and conflicting data have been published suggesting that a separate population of bone marrow-derived myofibroblasts contribute to interstitial fibroblasts in the kidney. Injured epithelial cells undergoing epithelial to mesenchymal transition (EMT) have been proposed to contribute to kidney myofibroblasts as well, with some studies suggesting epithelial cells are the major source of fibroblasts in the kidney and elsewhere. We have recently reported using robust, reproducible genetic fate mapping techniques in vivo that epithelial cells make no significant contribution to myofibroblasts in mouse kidney fibrosis (Humphreys et al, manuscript submitted for publication). Our recent report suggested that fresh investigation of the source of myofibroblasts in the kidney was merited. Myofibroblasts are the cell type that generates and deposits collagen-I and collagen-III rich pathological extracellular matrix leading to irreversible fibrosis and causing organ dysfunction. Antibodies against the intermediate filament -SMA has been widely used as a marker for collagen producing myofibroblasts, and other less-widely accepted markers such as the transcription factor S100A4 have been reported to label myofibroblasts. However none of these markers is unique to myofibro-