Potential role for bone marrow-derived fibrocytes in the orbital fibroblast heterogeneity associated with thyroid-associated ophthalmopathy

Abstract

Fibroblast heterogeneity has been recognized for decades, but the basis for multiple phenotypes among these cells has been investigated only recently. More than 15 years ago, Bucalla and his colleagues described for the first time a population of fibroblast-like cells among circulating mononuclear blood cells. Subsequently these mesenchymal cells, termed fibrocytes, have been characterized and found to participate in normal and pathological tissue remodelling. In this review, I have attempted to present the evidence generated thus far suggesting that fibrocytes are participants in autoimmune diseases where tissues are injured and undergo remodelling. Aspects of their phenotype suggest that they are well suited to help orchestrate immune responses through mononuclear cell recruitment and their ability to produce inflammatory mediators and extracellular matrix molecules. These attributes also raise the possibility that they might be useful targets against which therapeutic agents might be aimed.

Fig. 3

Thyroid-stimulating hormone receptor (TSHR) displayed on fibrocytes generated from peripheral blood mononuclear cells (PBMCs) can function to initiate cytokine production. Cultured cells, in this case from a patient with Graves' disease, were treated with bovine thyroid-stimulating hormone (bTSH) (5 mU/ml) or interleukin (IL)-1β (10 ng/ml) for 48 h. The medium was subjected to enzyme-linked immunosorbent assays specific for (a) IL-6 or (b) tumour necrosis factor (TNF)-α. Data are expressed as the mean ± standard error of the mean of three replicate culture wells from a representative experiment (*P < 0·001). (Reprinted with permission; Douglas, RS et al. Increased generation of fibrocytes in thyroid-associated ophthalmopathy, Copyright 2010, The Endocrine Society.)

Fig. 2

Fibrocytes cultivated from peripheral blood mononuclear cells (PBMCs) express high levels of thyroid-stimulating hormone receptor (TSHR) regardless of whether they derive from (a) patients with Graves' disease or (b) healthy donors. (c) These levels are comparable to those found on primary human thyroid epithelial cells. (d) In contrast, undifferentiated orbital fibroblasts fail to express TSHR. (e) Fibrocytes differentiated into adipocytes accumulate intracellular lipid droplets staining with Oil Red O. (f) TSHR levels on fibrocytes remain elevated following differentiation. In (a), fibrocytes were preincubated with bovine thyroid-stimulating hormone (bTSH) (5 mU/ml) prior to staining with anti-TSHR antibodies. (Reprinted with permission; Douglas, RS et al. Increased generation of fibrocytes in thyroid-associated ophthalmopathy, Copyright 2010, The Endocrine Society.)

Fig. 1

CD34⁺LSP-1⁺ TSHR⁺ fibrocytes can be identified in the orbital tissue of patients with TAO but are absent in tissues from healthy donors. (a) CD34 expression (arrows, green FITC) in TAO-derived tissue (inset, negative control staining). (b) Absent CD34 expression in healthy tissue (inset, positive staining control). (c) LSP-1 expression in TAO-derived tissue [red, arrows, nuclei counterstained with DAPI (blue)] (inset negative control). (d) Absence of LSP-1 expression in healthy tissue (inset negative control). (e) CD31 expression in disease-derived tissue is limited to vascular endothelium (red, arrows). (f) H and E stained consecutive thin-sections of the same orbital tissue (40×). (g) Fibrocytes present in orbital tissue from patients with TAO co-express CD34 and TSHR. Nuclei were counterstained with DAPI (blue). Thin sections were then subjected to confocal microscopy. Inset contains a negative staining control. (Reprinted with permission; Douglas, RS et al. Increased generation of fibrocytes in thyroid-associated ophthalmopathy, Copyright 2010, The Endocrine Society.)

Fig. 4

Theoretical model of orbital fibroblast derivation and the molecular events occurring in thyroid-associated ophthalmopathy (TAO). HAS, hyaluronan synthase; TSHR, thyroid stimulating hormone receptor; IGF-1R, insulin-like growth factor-1 receptor; Col1, collagen 1; TNF-α, tumour necrosis factor-α.