Effect of Interferon-γ on the Expression of Transforming Growth Factor-β by Human Corneal Fibroblasts: Role in Corneal Immunoseclusion

Human corneal fibroblasts (HCF) inhibit T cell alloresponse in mixed leukocyte response-human corneal fibroblast coculture. The inhibition is contact independent, insensitive to indomethacin, and is enhanced by pretreatment of HCF with interferon-γ (IFN-γ). To investigate cytokine-dependent mechanisms of inhibition of T cell alloresponse by HCF, the capacity of cultured HCF to produce transforming growth factor-β (TGF-β) and the modulatory role of IFN-γ on their TGF-β production were investigated by radioreceptor binding inhibition assay (RRA) and the standard mink cell bioassay (BIA). The net total TGF-β concentration of 4 day culture supernatants from IFN-γ-treated HCF, measured by RRA, was 11.5 ng/ml. The net total bioactive TGF-β concentrations of 4 day culture supernatants from HCF, before and after treatment with IFN-γ, measured by BIA, were 2.0 and 4.8 ng/ml, respectively. These findings indicate that HCF produce TGF-β and increase their TGF-β output under the influence of the proinflammatory cytokine IFN-γ. Media-borne TGF-β binding proteins appeared to be primarily responsible for the discrepancy between the TGF-β values measured by RRA and BIA. Active exclusion of TGF-β binding proteins from intraocular fluids may have an important role in the maintenance of TGF-β-dependent ocular immune privilege. Corneal fibroblasts may utilize TGF-β--dependent mechanisms to maintain the immunosecluded environment of the cornea and to preserve the homeostasis of corneal optical competency. Interferon-γ may enhance corneal immunoseclusion by upregulating the TGF-β output of the corneal fibroblasts.