The effect of inflammatory agents on the blood-retinal barrier

The blood-retinal barrier (BRB), like that of the blood-brain barrier (BBB), forms a selective interface between the blood and the neural parenchyma. During inflammatory diseases of the retina there is a large scale increase in leucocyte infiltration and breakdown of the BRB. It is not entirely clear, however, what the causative factors are in BRB disruption, but recently cytokines have been implicated. An understanding of the role of cytokines within retinal inflammatory diseases, such as posterior uveitis where BRB breakdown leads to macular oedema and impaired vision, may lead to improved therapies and disease control. Cytokines have been detected within the eyes of patients with uveitis. Also, the injection of cytokines to experimental animals has been shown to cause leucocyte recruitment and an increase m vascular permeability. However, it is not known whether cytokines have a direct effect on increasing vascular permeability, if they induce the release of vasoactive mediators, or if they exert their effects through leucocyte recruitment. It was therefore hypothesised that the injection of a proinflammatory cytokine to the vitreous of the Lewis rat will induce a leucocytic infiltration to the retina and breakdown of the BRB. Inhibitor studies may then identify the mechanism by which BRB dysfunction is occurring. The effect of the proinflammatory cytokines interleukin-1β (IL-1β), tumour necrosis factor-α (TNF-α) and interleukin-6 (IL-6) upon the Lewis rat BRB were investigated. Following an intravitreal injection of cytokine the structural integrity of the retina was evaluated using both light and electron microscopy. The permeability of the BRB was assessed by either introducing the small molecular weight tracer [14C]-mannitol, or the large molecular weight tracer horseradish peroxidase, into the circulation. Following administration of IL-1β a biphasic opening of the BRB was found, as determined by increased [14C]-mannitol extravasation. The initial breakdown occurred at 4 hr post injection (PI), corresponding to the appearance of leucocytes within the retinal vessels and adhering to the endothelium. The BRB permeability then decreased although this did not correspond to a reduction in the number of infiltrating leucocytes. A second, larger, and more prolonged increase in barrier permeability was detected at 24 to 48 hr PI which coincided with peak infiltration of leucocytes. Inhibitor and leucocyte depletion studies indicated that the IL-1β-induced breakdown of the BRB was mediated by histamine and cyclooxygenase metabolites (prostaglandins and thromboxanes), and associated with leucocyte infiltration to the retina. Following the intravitreal injection of TNF-α a monophasic and reversible opening of the BRB occurred but without the accompanying large scale cellular infiltrate. Immunohistochemical studies revealed the upregulation of major histocompatibility complex (MHC) class II molecules within the retina of TNF-α injected eyes. It was also demonstrated that there was an opening of the BRB of the non-injected contralateral eye which may be due to a neuronal reflex arc mechanism. Administration of IL-6 caused a minimal inflammatory cell infiltrate, but did not increase the permeability of the BRB at the dose examined. The intravitreal injection of the cytokines examined in this study exhibited different effects on the retina and BRB of the Lewis rat. IL-1β caused a biphasic breakdown of the BRB that appeared to be the consequence of classic inflammatory mediators and dependent on leucocyte infiltration, whereas TNF-α caused a monophasic increase in BRB permeability which appeared to be independent of leucocytic involvement. IL-6 had no effect on barrier integrity. Therefore, it appears that, in vivo, proinflammatory cytokines differ in their ability to induce leucocyte recruitment and cause increased vascular permeability. The mechanism by which TNF-α caused BRB breakdown is unknown, but the IL-1β induced biphasic breakdown of the BRB is dependent on the presence of leucocytes recruited to the retina and the release of vasoactive mediators such as histamine.