INFLAMMATORY CYTOKINES INDUCE INTERCELLULAR ADHESION MOLECULE-1 (ICAM–1) mRNA SYNTHESIS AND PROTEIN SECRETION BY HUMAN RETINAL PIGMENT EPITHELIAL CELL CULTURES

Retinal inflammatory diseases in man are associated with an upregulation in the expression of intercellular adhesion molecule-1 (ICAM–1) in cells within the retina and with an increase in soluble ICAM–1 within the vitreous. These studies suggest that this protein may contribute to immunopathological processes within the eye. The effects of inflammatory mediators on the regulation of the expression and secretion of ICAM–1 by human retinal pigment epithe-lial cell cultures (HRPE) were investigated in order to identify the possible source of soluble ICAM–1 and the conditions which enhance its production. Immunofluorescence studies on TNF–α and/or IFN–γ treated HRPE cells demonstrated cellular expression of ICAM–1 which was predominantly localized to intercellular junctions. Moreover, treatment of HRPE for 24 h with tumour necrosis factor alpha (TNF–/) (10 ng/ml), interferon gamma (IFN–γ) (500 u/ml), interleukin 1 alpha (IL–1α) (10 ng/ml) and IL–1β (10 ng/ml) results in the secretion of ICAM–1, ranging from 9 to 13 ng per 10 6 cells. IFN–γ acts synergistically with (TNF–α) and IL–1 in the secretion of ICAM–1 by HRPE. Only 1.75 ng of soluble ICAM–1 was detected in untreated HRPE cells. In contrast, lipopolysaccharide (LPS), IL–6, IFN–α or TGF–β did not exhibit any influence on ICAM–1 secretion by these cells. Northern blot analysis reveals an increased expression of ICAM–1 mRNA in HRPE stimulated with IFN–g, TNF–α or IL–1 for 24 h. In untreated cells, ICAM–1 mRNA is not detectable. There is a progressive increase in ICAM–1 mRNA levels in cytokine-treated HRPE, that reaches steady state by 12 h. Furthermore, a close correlation is noted between ICAM–1 mRNA levels and the secretion of ICAM–1 protein, suggesting regulation at the level of gene transcription. ICAM–1 secretion by RPE might actively participate in the immune reactions in the retina, by recruiting and activating lymphocytes, and contribute to the immunopathological processes in inflammatory diseases.