Using SD-OCT to Study the Effect of VEGF on Fluid Transport across the RPE In Vivo

Abstract

Purpose: : Macular edema is a common symptom in several eye conditions and a major reason for the loss of visual acuity. Anti-VEGF therapy is thought to be effective in treating edema by targeting the leakiness of retinal vessels. However, it is the retinal pigment epithelium (RPE) that maintains the outer retina in a dehydrated state. We have recently shown that VEGF can regulate the barrier function of the RPE in vitro. Utilizing spectral domain optical coherence tomography (OCT), we have developed an in vivo model system to study the effects of VEGF on RPE barrier function and fluid transport.

Methods: : Rabbits were anaesthetized and intraocular pressure (IOP) was maintained at 10, 15 or 20mmHg. Subretinal blebs (1-5μL) containing PBS, BSA (100ng/μL), VEGF (100ng/μL) or PLGF (100ng/μL) were created. Bleb reabsorption was followed for 90 minutes with infrared fundus images and high resolution OCT scans across the entire bleb-filled area. In selected experiments, 5 μL of ZM323881 (10μM), a relatively selective VEGF-R2 receptor inhibitor, was injected into the vitreous one hour prior to the creation of VEGF-filled blebs. Fluorescein angiograms (100mg/kg, intravenous injection) were used to visualize RPE barrier leakage.

Results: : In blebs containing PBS and at an IOP of 10mmHg, fluid transport across the RPE was 3.6±0.7μL/mm²min. Raising the IOP to 15 and 20 mmHg increased transport rates to 8.9±0.8 and 13.7±1.3μL/mm²min, respectively. Blebs containing BSA exhibited a fluid transport rate of 3.2±0.8μL/mm²min. VEGF containing blebs did not show any significant reabsorption during the time they were monitored. Pretreatment with ZM323881 inhibited the VEGF-induced effect on fluid transport across the RPE. PLGF did not cause any statistically significant alteration in the rate of reabsorption compared to BSA. VEGF-filled blebs showed significant early-phase fluorescein leakage while PBS-filled blebs did not show any early phase leakage.

Conclusions: : In these experiments, OCT technology was successfully applied to increase our understanding of basic physiological properties of the RPE barrier. The result that VEGF induces a significant reduction in the reabsorption is consistent with a localized breakdown of the RPE barrier function. These data establish the RPE as an important factor in the pathophysiology of edema, and VEGF as a major cytokine that can modulate RPE function in vivo.

Keywords: pump/barrier function • vascular endothelial growth factor • retinal pigment epithelium