Myofibroblasts in murine cutaneous fibrosis originate from adiponectin-positive intradermal progenitors.

Fibrosis of the skin and internal organs is the distinguishing pathologic hallmark of systemic sclerosis (SSc; scleroderma) and is responsible for its major complications (1,2). Skin fibrosis is caused by the presence of myofibroblasts, which are regarded as the primary fibrogenic effector cells in SSc driving excessive accumulation of fibrillar collagens and other extracellular matrix proteins in the dermis (3). Although multiple sources of myofibroblast precursors have been proposed, the origin of myofibroblasts within fibrotic lesions remains uncertain (4). In response to transforming growth factor β (TGFβ), Wnt ligands, biomechanical signals, and other profibrotic cues, fibroblasts can acquire a myofibroblast phenotype with prominent expression of α-smooth muscle actin (α-SMA) stress fibers (3). Myofibroblasts might also originate from bone marrow–derived progenitor cells or may arise via epithelial–mesenchymal transition (EMT) and endothelial–mesenchymal transition (endoMT) (3,5). The significance of these cell fate decisions, and their contribution to fibrosis, remain the subject of considerable debate. A less well-appreciated pathologic hallmark of SSc is loss of intradermal adipose tissue (6). The lesional skin of patients with SSc and animals with experimentally induced fibrosis shows attenuation of intradermal adipose tissue subjacent to reticular dermis, with shrunken and misshapen adipocytes of variable diameter surrounded by fibrillar collagen (see Supplementary Figure 1, available on the Arthritis & Rheumatology web site at http://onlinelibrary.wiley.com/doi/10.1002/art.38990/abstract). The loss of intradermal adipose tissue accounts for the characteristic skin tethering in SSc. A similar phenomenon is noted in animal models of cutaneous fibrosis induced by bleomycin (7), HOCl (8), or angiotensin II (9), overexpression of constitutively active TGFβ receptor type I (TGFβRI) (10), or in sclerodermatous graft-versus-host disease (11). Mice with heritable cutaneous fibrosis, including Fra-2–transgenic (12), fibrillin-1–mutant (13), tsk1 (14), and tsk2 mice (15), display comparable atrophy of intradermal white adipose tissue. Moreover, we have shown that dermal fibrosis in transgenic mice with adipocyte-specific expression of Wnt-10b is accompanied by complete disappearance of intradermal adipose tissue (16). Adipocytes exert a regulatory influence on connective tissue metabolism through both cell-autonomous and paracrine mechanisms (17). However, despite the prominence of intradermal adipose tissue loss in patients with SSc and in experimental murine models, the characteristics and functional sequelae of this phenomenon are unknown. In the current study, we sought to investigate the process of intradermal adipose loss and to determine the origin of dermal myofibroblasts in cutaneous fibrosis. Our results show that intradermal adipose tissue loss preceded the onset of dermal fibrosis and was accompanied by a decline in the expression of peroxisome proliferator–activated receptor γ (PPARγ), the adipogenic master regulator, followed by the expression of myofibroblast-associated genes. Cell fate mapping studies in mice revealed that dermal fibrosis was associated with the recruitment of adipocyte-derived mesenchymal cells into lesional dermis. These cells accounted for a great majority of the accumulating myofibroblasts. Ex vivo treatment of subcutaneous adipocytes with TGFβ resulted in loss of morphologic and biochemical adipocytic characteristics and acquisition of myofibroblast features in the absence of significant proliferation. This process was accompanied by large-scale transcriptional reprograming, with decreasing expression of many adipogenic genes and a reciprocal increase in the expression of pivotal fibrogenic genes. Taken together, these observations indicate that the majority of myofibroblasts populating fibrotic skin originate from intradermal adipocytic progenitors through a process termed adipocyte–myofibroblast transition (AMT). Intradermal adipose tissue loss might therefore represent a primary event in the pathogenesis of cutaneous fibrosis.