Retinal leukostasis, characterized as adhesion and entrapment of leukocytes in the retinal vasculature, is a key inflammatory process and an underlying cause of diabetic retinal complications, including, retinal vessel occlusion, nonperfusion, retinal endothelial cell death, and blood–retinal barrier leakage.
19–22 In the STZ-induced diabetic rat model, leukostasis has been observed as early as 3 days after diabetes induction and is spatially and temporally associated with capillary nonperfusion and blood–retinal barrier breakdown.
11,23 After topical administration of SAR 1118, we observed a significant decrease in the number of adherent leukocytes, visualized by FITC-concanavalin A binding in the retinal vasculature in comparison to that in diabetic nontreated retinas. Up to a 24.9% and 54.58% decrease in retinal leukostasis was observed in rats treated with 1% and 5% SAR 1118, respectively, establishing a dose-dependent decrease in retinal leukostasis. Furthermore, 5% SAR 1118 was able to reduce retinal leukostasis in diabetic retinas (47.5 ± 7.41) to levels comparable to that in the normal retinas (35.5 ± 8.18). The interaction of the endothelial cell surface adhesion molecule ICAM, with the leukocyte integrins, in particular with the β2 (CD18) subunit of the integrin molecule, expressed on various leukocytes, is important for the adhesion of leukocytes to endothelial cells.
9 Enhanced expression of both endothelial ICAM and leukocyte CD18 is observed in DR.
9,11,24 A recent study correlated the vitreous levels of ICAM to DME severity.
25 A previous study reported that mice lacking CD18 and ICAM have significantly fewer adherent leukocytes in retinal vasculature after diabetes induction with STZ.
7 Also, diabetic animals treated with an intraperitoneal injection of anti-ICAM-1 mAb demonstrated a decrease in leukostasis by 48.5% and blood–retinal barrier leakage by 85.6%.
11 A significant decrease in leukostasis by 62% was also observed in an in vivo model of experimentally induced diabetes, after an intravenous injection of anti-CD18 F(ab9)2 fragments.
9 In the present study, after topical administration of 5% SAR 1118 thrice daily, we observed a significant reduction in leukostasis by 54.58% (
P < 0.001), similar to levels of inhibition seen with the IP injection of anti-ICAM mAb. The present study is the first to describe a molecule designed to mimic the LFA-1 binding epitope of ICAM-1 within a small-molecule framework,
12 which can selectively reduce retinal leukostasis in diabetic retinas to a level comparable to that in normal retinas, after an ocular topical drop administration. MPO is a heme protein expressed in polymorphonuclear leukocytes (PMNs), including neutrophils, monocytes, and activated macrophages. Increases in leukostasis, leukocyte infiltration, and leukocyte-derived MPO have been implicated in the vascular dysfunction associated with diabetes and atherosclerosis.
26–29 An increase in tissue MPO activity is representative of an increase in tissue PMNs and has been used for measuring leukocyte infiltration and adhesion.
30 Previously, enhanced in vitro neutrophil adhesion to retinal endothelial cells was demonstrated by measuring MPO activity in response to elevated glucose and insulin.
31,32 In the present study, diabetic retinas demonstrated a greater than fourfold increase in retinal MPO activity compared with the normal nondiabetic retinas. Treatment with 1% and 5% SAR 1118 reduced the MPO activity by 10.8% and 28.3%, respectively. The reduction in MPO activity after SAR 1118, however, was not as statistically significant (
P < 0.05) as the leukostasis inhibition (
P < 0.001). This result may be attributable to the fact that LFA-1 antagonists are known to be more effective in inhibiting lymphocyte adhesion relative to PMNs (e.g., neutrophils),
33 and thus the higher residual MPO activity may reflect the preferential retinal infiltration of PMNs.
31,32