ACTIVATION OF HUMAN ORBITAL FIBROBLASTS THROUGH CD40 ENGAGEMENT RESULTS IN A DRAMATIC INDUCTION OF HYALURONAN SYNTHESIS AND PROSTAGLANDIN ENDOPEROXIDE H SYNTHASE-2 EXPRESSION - INSIGHTS INTO POTENTIAL PATHOGENIC MECHANISMS OF THYROID-ASSOCIATED OPHTHALMOPATHY

Human orbital fibroblasts play a putative role in the pathogenesis of thyroid-associated ophthalmopathy (TAO), We hypothesize that the hyalu ronan accumulation and inflammation in TAO derive from enhanced biosyn thetic activities of orbital fibroblasts, CD40, a member of the tumor necrosis factor-alpha receptor superfamily, is a critical signaling mo lecule expressed by B lymphocytes, Engagement of CD40 with CD154 or CD 40 ligand results in the activation of target genes. Orbital fibroblas ts also display CD40, Here we report that CD40 engagement leads to sub stantial increases in hyaluronan synthesis in orbital fibroblasts, The increase is approximately 8-fold above control values, is comparable to the induction elicited by IL-1 beta and could be attenuated with de xamethasone but not by SC 58125, a prostaglandin endoperoxide H syntha se-2 (PGHS-2)-selective inhibitor. PGHS-2 is also induced by CD40 enga gement in a time-dependent manner, and this is mediated through increa ses in levels of steady-state mRNA, The induction of PGHS-2 leads to a dramatically enhanced prostaglandin E-2 production that can be blocke d by SC 58125 and dexamethasone. CD40 ligand up-regulates the synthesi s of IL-1 alpha, and blocking this cytokine with exogenous IL-1 recept or antagonist (IL-1ra) or with IL-1 alpha neutralizing antibodies part ially attenuates the induction of PGHS-2. In contrast, CD40 ligand up- regulation of hyaluronan synthesis is unaffected by IL-1ra, CD40 cross -linking enhances mitogen-activated protein kinase activation, and int errupting this pathway attenuates the PGHS-2 induction. Thus the CD40/ CD40 ligand bridge represents a potentially important activational pat hway for orbital fibroblasts that may underlie the cross-talk between these cells and leukocytes. These findings may be relevant to the path ogenesis of TAO and provide insights into previously unrecognized, pot ential therapeutic targets.