Characterization of fluorescently labeled photoreceptors observed in carriers of choroideremia using multimodal adaptive optics imaging

Abstract

Purpose : To characterize patches of fluorescently labeled cone photoreceptors detected in female carriers of choroideremia (CHM) following intravenous injection of indocyanine green (ICG) dye using adaptive optics (AO) imaging (PMID:33796365).

Methods : Patches of fluorescently labeled photoreceptors (>50 cones) were identified using AO-ICG following multimodal AO imaging in 11 eyes from 6 carriers of CHM. Co-registered non-confocal split detection AO images were used to evaluate whether fluorescent patterns corresponded to cone photoreceptors, and outer retinal length (ORL) measurements were performed in AO-OCT volumes. For comparative purposes, histology of the outer retina from mice was obtained using custom-modified microscopes. Murine photoreceptors and retinal pigment epithelial (RPE) cells were assessed following both systemic injection of ICG as well as ex vivo incubation with ICG.

Results : In mice after systemic injection with ICG, RPE cells were labeled with ICG (PMID:27564519) but there was no observable ICG fluorescence in the photoreceptor layer. Photoreceptors were however readily labeled when detached from the RPE and incubated with ICG ex vivo, suggesting that the outer blood-retinal barrier function of the RPE prevents labeling of photoreceptors during normal physiologic conditions. In carriers, in addition to the ICG fluorescence pattern observed in the RPE mosaic, patches of ICG-labeled cone photoreceptors were also observed (23 distinct patches identified across 10 eyes). The excellent correspondence between fluorescently labeled photoreceptors visualized using AO-ICG and simultaneously acquired split detection images confirmed that these were indeed cones. There were no apparent differences in spacing, density, or ORL measurements of cones within these patches.

Conclusions : The discovery of ICG-labeled cone photoreceptors in patients, together with mouse histology data, suggests that there are focal disruptions to the outer blood-retinal barrier in carriers of CHM. These photoreceptors appear to be structurally normal and may be associated with interspersed enlarged RPE cells seen in CHM (IOVS 2021; 62:1900). These results demonstrate a possible novel application of AO-ICG as a functional assay for assessing the integrity of the outer blood-retinal barrier function of the RPE.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.