Populate Cellular Crescents in a Murine Model of Inflammatory Glomerulonephritis

Cellular crescents are a defining histologic finding in many forms of inflammatory glomerulonephritis. Despite numerous studies, the origin of glomerular crescents remains unresolved. A genetic cell lineage-mapping study with a novel transgenic mouse model was performed to investigate whether visceral glomerular epithelial cells, termed podocytes, are precursors of cells that populate cellular crescents. The podocytespecific 2.5P-Cre mouse line was crossed with the ROSA26 reporter line, resulting in irreversible constitutive expression of -galactosidase in doubly transgenic 2.5P-Cre/ROSA26 mice. In these mice, crescentic glomerulonephritis was induced with a previously described rabbit anti-glomerular basement membrane antiserum nephritis approach. Interestingly, -galactosidase-positive cells derived from podocytes adhered to the parietal basement membrane and populated glomerular crescents during the early phases of cellular crescent formation, accounting for at least one-fourth of the total cell mass. In cellular crescents, the proliferation marker Ki-67 was expressed in -galactosidase-positive and -galactosidase-negative cells, indicating that both cell types contributed to the formation of cellular crescents through proliferation in situ. Podocyte-specific antigens, including WT-1, synaptopodin, nephrin, and podocin, were not expressed by any cells in glomerular crescents, suggesting that podocytes underwent profound phenotypic changes in this nephritis model. Cellular glomerular crescents are a pathognomonic histologic finding in inflammatory glomerulonephritis that can have many different causes and pathogenic mechanisms (1). The histogenesis and origin of cellular crescents, which are cap-like multilayered accumulations of proliferating cells, have remained controversial. Although early ultrastructural studies suggested that crescents are formed by proliferating epithelial cells (2,3), subsequent studies with newly developed antibodies against leukocytes identified the presence of macrophages in cellular crescents (4). However, it soon became evident that different cell populations, particularly cells expressing epithelial antigens such as cytokeratin, may inhabit the crescents under different conditions (4–7). In fact, the composition of cellular crescents changes during the progression of disease after the inciting glomerular injury. Boucher et al. (8) demonstrated that epithelial cells predominated in crescents of patients during the early phases of disease. Later phases were characterized by rupture of the basement membrane of Bowman’s capsule and subsequent infiltration of cellular crescents, predominantly by macrophages. Therefore, the proportion of observed epithelial cells in cellular crescents varies among patients, depending on how far the glomerulonephritis has progressed in the affected glomeruli. The predominance of crescentic epithelial cells during the early phase of disease progression and of inflammatory cells during later phases of disease progression was supported by studies with various animal models of anti-glomerular basement membrane (GBM) glomerulonephritis. The animal models are characterized by synchronized progression of inflammation, allowing the study of crescent formation at defined time points. Similar to findings in human tissues, early cellular crescents in rodents were composed of epithelial cells. Rupture of the parietal basement membrane (PBM) was associated with fibrocellular infiltration of crescents, with a high incidence of macrophages/monocytes (9,10). It was widely assumed that crescentic epithelial cells originated entirely from parietal epithelial cells and that podocytes were not involved in the formation of cellular crescents. In a recent study with a murine anti-GBM model, a more detailed description of the earliest steps of cellular crescent formation was provided (11). In that study, crescent formation was preceded by the formation of podocyte bridges between the glomerular tuft and Bowman’s capsule. Podocytes extended processes that disrupted intercellular junctions between parietal epithelial cells and adhered to the PBM. This seemed to be the initiating event for cell proliferation on the capsular side and the formation of cellular crescents. Unlike bridging podocytes, Received May 7, 2003. Accepted October 2, 2003. Correspondence to Dr. Marcus J. Moeller, Institute for Anatomy and Cell Biology 1, University of Heidelberg, INF 307, 3.OG, D-69120 Heidelberg, Germany. Phone: 49-6221-548684; Fax: 49-6221-544951; E-mail: marcus.moeller@urz.uni-heidelberg.de