In vivo evaluation of leukocyte dynamics in the retinal and choroidal circulation

We have developed a new method to visualize leukocytes and evaluate their kinetics in the chorioretinal microcirculation of the living eyes. Nuclear staining dyes and a scanning laser ophthalmoscope were used to image leukocytes in the fundus. Acridine orange was used to visualize leukocytes in the retinal microcirculation. For imaging leukocytes in the choroid, indocyanine green was injected intravenously. Dynamics of leukocytes in the capillaries of the retina and choroid were quantitatively estimated in monkeys and rats. This method also allowed evaluation of leukocyte-endothelial interactions, such as rolling or firm adhesion, in vivo. Acridine orange leukocyte fluography was used to study leukocyte dynamics in the following experimentally induced microcirculatory disturbances of the retina: 1) interferon-associated retinopathy, 2) ischemia-reperfusion injury of the retina, and 3) experimental diabetes mellitus. 1) Interferon-associated retinopathy Systemic administration of interferon alpha enhanced leukocyte-endothelial interactions in the retina, which resulted in leukocyte rolling and entrapment in the retinal capillary beds. Leukocyte accumulation was also detected in the lung. The entrapment or accumulation of leukocytes in the microcirculation was inhibited by simultaneous administration of corticosteroids or other agents. These results suggested that leukocytes play a major role in the development of adverse effects of interferon, such as retinopathy or interstitial pneumonia. 2) Ischemia-reperfusion injury of the retina During reperfusion period after transient (60 min) retinal ischemia by optic nerve ligation, the rolling of leukocytes in the retinal veins was prominent and numerous leukocytes were trapped in the retinal capillaries. The number of rolling leukocytes was at a maximum 12 hours after reperfusion. Leukocyte entrapment peaked at 24 hours after reperfusion. By blocking adhesion molecules on the vascular endothelium, these leukocyte-endothelial interactions were effectively inhibited. Postischemic retinal atrophy was also inhibited by blocking adhesion molecules. These results suggested that leukocytes may be major players in the pathophysiology of ischemia reperfusion injury of the retina. 3) Experimental diabetes mellitus Leukocyte dynamics in the retina were studied in streptozotocin-induced diabetes and spontaneous diabetes (OLETF rats). In both diabetic models, leukocyte entrapment in the retinal capillaries was increased even in the early stages of diabetes. Fluorescein angiography revealed that trapped leukocytes disturbed the regional capillary blood flow in the downstream. Enhanced expression of adhesion molecules was observed in the capillary endothelium of the retina in the diabetic rats. Leukocyte entrapment in the retinal capillaries might cause microvascular occlusions and dysfunction, in turn causing diabetic retinopathy.