by the eye in severe TAO. Clearly, the details surrounding disordered hyaluronan accumulation might prove instructive in the development of strategies for halting the progression of this disease.

ORBITAL FIBROBLASTS DISPLAY CELL-SURFACE CD40 AND RESPOND TO CD154

CD40, a member of the TNF-a receptor family, is highly expressed by orbital fibroblasts (4). Preliminary evidence suggests that the levels of CD40 expression may be higher in orbital fibroblasts than those found in several other types of cells. This expression is potentially important because CD40 is a critical antigenic molecule on B-cells and represents a major conduit through which B-lymphocytes are activated by T-cells. When CD40 is ligated by its natural ligand, CD154, expression of several orbital fibroblast genes is induced. Levels of PGHS-2, mPGES, IL-1a, IL-6, and IL-8 are increased (4,18). The demonstration of CD40 on orbital fibroblasts suggests that T-cells might directly influence the activity of these cells and thus may represent a direct route through which they could be activated. Interferon g increases the levels of CD40 on fibroblasts and enhances the magnitude of fibroblast response to CD154 (18). Thus, a predominant Th1 immune response could condition the orbital fibroblast and facilitate the impact T-cells and mast cells might have on fibroblastic activity.

Widespread expression of CD40 on numerous cell types suggests that this receptor and its ligand(s) may be involved with the regulation of many metabolic events. Cells other than T-lymphocytes have now been shown capable of expressing CD154 (platelets and mast cells are examples) and thus the CD40=CD154 bridge may represent a particularly important conduit through which heterogeneous cells might cross-talk. With regard to Graves’ disease, this pathway may represent an important therapeutic target, the interruption of which might allow attenuation of several aspects of the immune response.

FIBROBLASTS FROM SEVERAL ANATOMIC

REGIONS CAN EXPRESS THE THYROTROPIN

RECEPTOR

Cloning and characterization of the thyrotropin (TSH) receptor more than 10 years ago has been accompanied by substantial inquiry concerning its expression in extrathyroidal tissues. The earliest studies examined the presence of TSHR mRNA in tissues of the orbit using RT-PCR-based assays. Feliciello et al. (24) found the transcript in normal orbital tissues and those from patients with TAO. This important study was soon followed by several reports of orbital fibroblasts expressing TSHR mRNA (25–27). These studies used more reliable and somewhat quantitative methods of detecting low abundance mRNA, such as northern blot analysis, nuclease protection, and in situ hybridization. In addition, TSHR protein was found expressed by cultured orbital fibroblasts (28). Subsequently, extraorbital tissues and fibroblasts from some of those depots were found to also express TSHR (29,30). Among the first observations concerning TSHR expression occurring outside the thyroid gland were suggestions that the ectopic receptor might represent a truncated protein (31). The investigators believed it was inactive in that addition of TSH to the cultures of eye muscle-derived fibroblasts failed to enhance cAMP generation or to upregulate glycosaminoglycan production. Later studies have verified that a full-length receptor is expressed in extrathyroidal tissues. For instance, fat from infants expresses high levels of TSHR while lower levels were demonstrated in adipose tissue from adults (32). More recently, TSHR protein and mRNA have been found in fibroblasts=preadipocytes from omental and abdominal wall fat of adults (30). Moreover, the receptor is competent to signal through the p70s6k pathway (30). At issue is whether TSHR is expressed widely by cells of the fibroblast lineage or only in the orbit. If the latter proved to be the case, it would be possible to implicate anatomically restricted TSHR expression as the basis for regional distribution of disease manifestations. Two reports have appeared recently suggesting that the transition from orbital

fibroblasts incubated in a differentiation-provoking medium, into mature adipocytes resulted in enhanced TSHR expression (33,34). The claim was based on the increase in cAMP production elicited by TSH under those culture conditions. An important possibility, apparently not adequately controlled for in either study, relates to the theoretical contribution of cAMP enhancing components found in the medium. Thus, although these findings are consistent with earlier reports supporting the concept that adipogenic differentiation is accompanied by enhanced levels of TSHR, it remains a possibility that cell differentiation did not influence cAMP levels in those particular studies. In any event, it would appear that TSHR expression is not limited to orbital fibroblasts but rather may be widespread among the fibroblasts found in many connective=adipose tissue depots. The role of the TSHR in the pathogenesis of extra-thyroidal Graves’ disease remains uncertain.

CONCLUSIONS

Orbital fibroblasts may play an important role in the pathogenesis of TAO. Unfortunately, most of the evidence supporting this concept is circumstantial and based on extensive studies conducted in cell culture. To date, these fibroblasts have been shown to exhibit phenotypic attributes that differ considerably from those of cells derived from other anatomic sites. Whether the differences found in vitro will ultimately prove relevant to human disease remains to be determined. Confirmation of their active participation in the disease will await the development of robust animal models for TAO.

ACKNOWLEDGEMENTS

The expert assistance of Ms Connie Madrigal is gratefully acknowledged. This work has been supported in part by National Institutes of Health grants DKOL3121, EY08976 and EY11708.

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