Table 1.
Sensitivity and Specificity of Markers Used to Identify Cardiac Fibroblasts
| Marker | Sensitivity | Specificity |
|---|---|---|
| Vimentin | Labels all fibroblasts 180, 181. | Also expressed by other cells of mesenchymal origin (endothelial cells [182], vascular smooth muscle cells [183], etc.). |
| α-SMA | Expressed by activated myofibroblasts in fibrotic hearts 22, 41, 138. Not expressed by quiescent fibroblasts (137). | Also expressed by vascular mural cells. |
| Col1α1 | Synthesis of structural collagens is a hallmark of fibroblasts in normal and remodeling hearts 42, 141. | Although synthesis of structural collagens by cells other than fibroblasts has been reported, expression of Col1α1 in cardiac endothelial cells, immune cells, vascular smooth muscle cells, and pericytes is negligible when compared to fibroblasts (141). Because of labeling of the surrounding matrix, antibodies to collagens may be suboptimal for fibroblast identification. Col1α1-GFP reporter mice represent a robust tool for identification of fibroblasts in many organs, including the heart (42). |
| Periostin | Expressed by fibroblasts in neonatal hearts but not by fibroblasts in normal adult hearts (184). Highly expressed in activated cardiac fibroblasts after injury 185, 186. | May also be expressed by subsets of vascular smooth muscle cells (187). |
| Fibronectin ED-A | Highly expressed by activated myofibroblasts (188). | Deposited in the matrix (189). May also colocalize with macrophages, endothelial cells, and other cell types 190, 191. |
| PDGFRα | Highly expressed in cardiac fibroblasts in normal (41) and pressure-overloaded myocardium (141). | Although vascular smooth muscle cells have been reported to express PDGFRα, especially under conditions of stress (192), PDGFRα-GFP reporter lines seem to predominantly identify cells with fibroblast-like characteristics (193). |
| DDR2 | High expression in cardiac fibroblasts in normal adult hearts (194), colocalizing with vimentin and col1α1 (195). May also be expressed in various subpopulations of infarct fibroblasts and myofibroblasts (196). | DDR2 expression has been reported in activated endothelial cells (197) and in stretched vascular smooth muscle cells (198). It is unclear whether this affects the specificity of DDR2 for fibroblasts in injured and remodeling hearts. |
| Antigen recognized by MEFSK4 | The MEFSK4 antibody labels through flow cytometry almost all PDGFRα+, Col1α1+ cardiac fibroblasts (14). No antibodies are available for immunohistochemistry. | MEFSK4 has been reported to label a small subpopulation of pericytes (14). |
| Cluster of differentiation 90 (Thy1) | Identifies a subpopulation of fibroblasts in the normal and remodeling myocardium 14, 141, 199, 200. | Also expressed by immune cells, lymphatic endothelial cells, and pericytes (201). |
| Sca1 | Identifies a subpopulation (∼60%) of PDGFRα+, Col1α1+ fibroblasts in the murine heart (14). | Lacks specificity. In Sca1-GFP reporter mice, Sca1 expression colocalized with endothelial and pericyte markers (202). |
| Tcf21 | Labels the majority of fibroblast-like cells in normal myocardium (19). In infarcted and pressure-overloaded hearts, accumulation of Tcf21+ fibroblast-like cells is noted; however, according to a single report, Tcf21 may not label activated α-SMA+ myofibroblasts (203). | Relatively specific for fibroblast populations. Not expressed by immune cells (CD45+) (203), endothelial cells, and vascular smooth muscle cells (19). |
| FSP1 | No expression in fibroblasts in the normal adult myocardium. In the infarcted and pressure-overloaded heart, there is a marked expansion of FSP1+ cells. The majority of these cells cannot be identified as α-SMA+ myofibroblasts 141, 184. | Lacks specificity. The majority of FSP1+ cells in injured and remodeling hearts are endothelial cells, macrophages, and vascular smooth muscle cells 184, 204. |
| FAP | Not expressed in normal cardiac fibroblasts (174). Labels many activated fibroblasts in infarcted rat hearts and in human myocardial samples from patients with post-infarction heart failure (205). | Specific for activated fibroblasts. However, in human failing hearts, FAP expression has been reported in small populations of inflammatory cells and endothelial cells (205). |
Col1α1 = collagen 1α1; DDR2 = discoidin domain receptor 2; FAP = fibroblast-activation protein; FSP1 = fibroblast-specific protein 1; GFP = green fluorescent protein; PDGFRα = platelet-derived growth factor α; Sca1 = stem cell antigen–1; SMA = smooth muscle actin; Tcf21 = transcription factor 21.