Basic Science Articles Isolation and Characterization of Nontubular Sca-1  Lin  Multipotent Stem/Progenitor Cells from Adult Mouse Kidney Benjamin Dekel,* Lior Zangi,* Elias Shezen,* Shlomit Reich-Zeliger,* Smadar Eventov-Friedman,* Helena Katchman,* Jasmin Jacob-Hirsch, † Ninette Amariglio, † Gideon Rechavi, † Raanan Margalit,* and Yair Reisner* *Department of Immunology, Weizmann Institute of Science, Rehovot, and † Department of Pediatric Hemato-Oncology and Functional Genomics, Sheba Medical Center, Tel-Hashomer, Israel Tissue engineering and cell therapy approaches aim to take advantage of the repopulating ability and plasticity of multipotent stem cells to regenerate lost or diseased tissue. Recently, stage-specific embryonic kidney progenitor tissue was used to regenerate nephrons. Through fluorescence-activated cell sorting, microarray analysis, in vitro differentiation assays, mixed lymphocyte reaction, and a model of ischemic kidney injury, this study sought to identify and characterize multipotent organ stem/progenitor cells in the adult kidney. Herein is reported the existence of nontubular cells that express stem cell antigen-1 (Sca-1). This population of small cells includes a CD45-negative fraction that lacks hematopoietic stem cell and lineage markers and resides in the renal interstitial space. In addition, these cells are enriched for 1-integrin, are cytokeratin negative, and show minimal expression of surface markers that typically are found on bone marrow– derived mesenchymal stem cells. Global gene profiling reveals enrichment for many genes downstream of developmental signaling molecules and self-renewal pathways, such as TGF-/bone morphogenic protein, Wnt, or fibroblast growth factor, as well as for those that are involved in specification of mesodermal lineages (myocyte enhancer factor 2A, YY1-associated factor 2, and filamin-). In vitro, they are plastic adherent and slowly proliferating and result in inhibition of alloreactive CD8  T cells, indicative of an immune- privileged behavior. Furthermore, clonal-derived lines can be differentiated into myogenic, osteogenic, adipogenic, and neural lineages. Finally, when injected directly into the renal parenchyma, shortly after ischemic/reperfusion injury, renal Sca- 1  Lin  cells, derived from ROSA26 reporter mice, adopt a tubular phenotype and potentially could contribute to kidney repair. These data define a unique phenotype for adult kidney– derived cells, which have potential as stem cells and may contribute to the regeneration of injured kidneys. J Am Soc Nephrol 17: 3300 –3314, 2006. doi: 10.1681/ASN.2005020195 R egenerative medicine is focused on the development of cells, tissues, and organs for the purpose of restoring function through transplantation (1). In this regard, the use of stem cells offers new and powerful strategies for future tissue development and engineering. Stem cells are defined by two major criteria: Pluripotentiality and self-renewal capacity. Perhaps the most characterized stem cell is one that resides in the adult bone marrow (BM), that is, the hematopoietic stem cell (HSC), which gives rise to all blood cell types (2). In addition, mesenchymal stem cells (MSC) are multipotent cells that can be isolated from adult BM and several other tissues and induced in vitro and in vivo to differentiate into a variety of mesenchymal tissues, including bone, cartilage, tendon, fat, BM stroma, and muscle (3). Recently, it was suggested that BM- derived stem cells can cross boundaries and give rise to a broader array of differentiated cell types, that is, turning blood into liver, brain, pancreas, skin, intestine, and eventually kid- ney (4 –11). However, this phenomenon remains controversial for several reasons. First, several studies cast doubt on the biologic significance and even the existence of such transdiffer- entiation (12,13). Second, some other investigations that do show a donor cell phenotype in parenchymal cells after trans- plantation of BM-derived stem cells suggest that it occurs through cell fusion and not by transdifferentiation and gener- ation of cells de novo (14 –16). Therefore, the term “regenera- tion” is used erroneously in this context and should be replaced by “reparative,” if indeed that is shown to be the result of stem cell fusion. Third, whether by transdifferentiation or cell fusion, the efficiency of these processes under basal conditions and even when inflicting tissue injury is low, underscoring the functional capacity of adult BM-derived stem cells. For in- stance, in the kidney, several studies have shown that BM- derived cells adopt the phenotype of proximal tubular cells during acute tubular injury (10,11), as well as that of glomerular Received February 21, 2005. Accepted August 30, 2006. Published online ahead of print. Publication date available at www.jasn.org. B.D. and L.Z. contributed equally to this work. Address correspondence to: Dr. Yair Reisner, Weizmann Institute of Science, Department of Immunology, Rehovot, Israel. Phone: +972-8-9344023; Fax: +972- 8-9344145; E-mail: yair.reisner@weizmann.ac.il Copyright © 2006 by the American Society of Nephrology ISSN: 1046-6673/1712-3300