Fibrocytes are monocyte progenitor cells that have been implicated in normal and pathological tissue remodeling. Among the prominent chemokine receptors expressed by these cells is CXCR4 which, with its cognate ligand CXCL-12, directs fibrocytes to sites of fibrosis. Fibrocytes have been implicated in the pathogenesis of thyroid-associated ophthalmopathy, the ocular manifestation of Graves’ disease (GD), by virtue of their unique accumulation as CD34(+) orbital fibroblasts (OFs). Fibrocytes also express high levels of functional thyrotropin receptor (TSHR). Here we determined CXCL-12 and CXCR4 expression in fibrocytes and GD-OF and whether that pathway interacts with TSHR. CXCL-12 is highly expressed in GD-OF while CXCR4 levels are dramatically higher in fibrocytes. Levels of these proteins are differentially regulated by TSH in a cell-type specific manner. Further, CXCL-12 enhances the induction by TSH of IL-6 in fibrocytes but attenuates this induction in GD-OF. In contrast, in pure CD34(+) OF, the interplay between TSH and CXCL-12 reverts to that observed in fibrocytes. Our results indicate that CXCL-12 enhances TSH actions in fibrocytes but inhibits them in GD-OF, a dichotomy imposed by factors emanating from CD34(-) OF. They also suggest a potentially important modulatory role for CD34(-) OF in determining the factors that modulate pathological TSHR signaling in the TAO orbit.
Intersection of chemokine and thyrotropin receptor pathways in human fibrocytes: Emergence of CXCL-12/CXCR4 crosstalk potentially relevant to thyroid-associated ophthalmopathy.