Bone Marrow–Derived Myofibroblasts Contribute to the Renal Interstitial Myofibroblast Population and Produce Procollagen I after Ischemia/Reperfusion in Rats Martine Broekema,* Martin C. Harmsen,* Marja J.A. van Luyn,* Jasper A. Koerts,* Arjen H. Petersen,* Theo G. van Kooten, † Harry van Goor, ‡ Gerjan Navis, § and Eliane R. Popa* *Department of Pathology & Laboratory Medicine, Medical Biology Section, and Departments of † Biomedical Engineering, ‡ Pathology, and § Nephrology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands Bone marrow– derived cells (BMDC) have been proposed to exert beneficial effects after renal ischemia/reperfusion injury (IRI) by engraftment in the tubular epithelium. However, BMDC can give rise to myofibroblasts and may contribute to fibrosis. BMDC contribution to the renal interstitial myofibroblast population in relation to fibrotic changes after IRI in rats was investigated. A model of unilateral renal IRI (45 min of ischemia) was used in F344 rats that were reconstituted with R26-human placental alkaline phosphatase transgenic BM to quantify BMDC contribution to the renal interstitial myofibro- blast population over time. After IRI, transient increases in collagen III transcription and interstitial protein deposition were observed, peaking on days 7 and 28, respectively. Interstitial infiltrates of BMDC and myofibroblasts reached a maximum on day 7 and gradually decreased afterward. Over time, an average of 32% of all interstitial -smooth muscle actin–positive myofibroblasts coexpressed R26-human placental alkaline phosphatase and, therefore, were derived from the BM. BMD myofibroblasts produced procollagen I protein and therefore were functional. The postischemic kidney environment was profibrotic, as demonstrated by increased transcription of TGF- and decreased transcription of bone morphogenic protein-7. TGF- protein was present predominantly in interstitial myofibroblasts but not in BMD myofibroblasts. In conclusion, functional BMD myofibroblasts infiltrate in the postischemic renal interstitium and are involved in extracellular matrix production. J Am Soc Nephrol 18: 165–175, 2007. doi: 10.1681/ASN.2005070730 R enal tubular ischemia/reperfusion injury (IRI), if not too severe, is, in principle, completely reversible (1), indicating that innate repair mechanisms are acti- vated upon damage. Regeneration and remodeling of the kidney results in recovery of renal function and morphology by tubular epithelial cell replacement (2,3). However, tubu- lointerstitial remodeling, as a result of an uncontrolled bal- ance between synthesis and degradation of extracellular ma- trix (ECM) proteins, also can result in tubulointerstitial fibrosis (4,5), which is an important risk factor for progres- sive renal function loss (6). Previously, we and others showed that after renal IRI, low numbers of bone marrow– derived cells (BMDC) engraft tu- buli and differentiate to tubular epithelium (7–10), possibly replacing damaged tubular epithelial cells. These data have been taken to support a therapeutic potential for BMD stem/ progenitor cells in renal tubular IRI. However, adverse ef- fects also have been reported in models of lung and liver injury, in which BMD stem/progenitor cells gave rise to myofibroblasts and contributed to fibrosis (11,12). Renal interstitial myofibroblasts are regarded as major producers of ECM proteins in fibrosis and therefore play a central role in its pathogenesis. Their origin remains the subject of discussion. Myofibroblasts may originate from injured tubular epithelial cells by epithelial-to-mesenchymal transition (13), differentiation of resident fibroblasts (14), or migration of perivascular smooth muscle cells (15). In mouse models of BM transplantation, it was shown that the BM is a source of myofibroblasts for many tissues, including the gut, lung, and kidney (10,16 –18). However, functionality of renal BMD myofibroblasts after IRI was not shown. Here, we investigated the contribution of BMDC to the renal interstitial myofibroblast population. Moreover, we studied the contribution of these cells to ECM production. To this end, we used a rat model of unilateral renal ischemia/ reperfusion in F344 rats that were reconstituted with R26- human placental alkaline phosphatase (hPAP) transgenic BM. Received July 15, 2005. Accepted October 11, 2006. Published online ahead of print. Publication date available at www.jasn.org. Address correspondence to: Martine Broekema, University Medical Center Gro- ningen, University of Groningen, Department of Pathology & Laboratory Medi- cine, Medical Biology Section, Hanzeplein 1, Room Z 2.09, 9713 GZ Groningen, The Netherlands. Phone: +31-50-3632417; Fax: +31-50-3619911; E-mail: m.broekema@med.umcg.nl Copyright © 2007 by the American Society of Nephrology ISSN: 1046-6673/-0165