Expression of GITR ligand abrogates immunosuppressive function of ocular tissue and differentially modulates inflammatory cytokines and chemokines

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Abstract
The glucocorticoid‐induced TNF‐related receptor ligand (GITRL) was previously shown to be constitutively expressed at low levels in human eye, including retinal pigment epithelial (RPE) cells. By expressing enhanced yellow fluorescent protein‐tagged human GITRL in human RPE cells, we investigated the significance of expression of GITRL on human ocular tissue. Confocal immunofluorescence microscopy and flow cytometry confirmed the surface expression of GITRL on RPE cells. However, a soluble form of GITRL was also detected. Remarkably, expression of GITRL on the RPE cells abrogated RPE‐mediated immunosuppression of CD3+ T cells, implicated as a possible mechanism for ocular immune privilege. This abrogation of immunosuppression by GITRL‐RPE was dependent on GITR‐GITRL interaction and could not be mimicked by anti‐CD28 antibody. Analysis of cytokine profiles revealed high level of TGF‐beta during the immunosuppression by RPE cells while expression of GITRL abrogated the RPE cell‐induced TGF‐beta secretion. Expression of GITRL also stimulates secretion of an array of proinflammatory cytokines/chemokines from T cells. GITR‐GITRL interaction provides a unique proinflammatory costimulation that may signal through a different pathway than that of CD28‐B7 costimulation. This study implicated that GITRL could be a potential candidate for regulation of the ocular immune privilege and the balance between immune privilege and inflammation.