Purpose: Dendritic cells (DC) play a crucial role in ocular surface defence. DC can be visualised in vivo by confocal microscopy but have not yet been fully characterised in humans. This study investigated the diurnal variation, topographical distribution, and repeatability of DC density and morphology measurement. Methods: In vivo confocal microscopy was conducted on 20 healthy participants (mean age 32.7±6.4 years, 50% F) at baseline and repeated after 30 minutes, 2, 6, and 24 hours. Images were captured at the corneal centre, inferior whorl, corneal periphery, limbus, and bulbar conjunctiva. DC density was counted manually, and morphology of DC was assessed for the largest cell body size, presence of dendrites, presence of long dendrites, and presence of thick dendrites. Mixed model analysis, non-parametric analyses, Bland & Altman plots, the Coefficient of Repeatability (CoR), and kappa were used. Results: There were no significant changes in DC density (p≥0.74) or morphology (p>0.07) at any location over the 24-hour period. Highest DC density was observed at the corneal limbus followed by the peripheral cornea (p<0.001), with lowest density at the corneal centre, inferior whorl, and bulbar conjunctiva. Most DC at the corneal periphery, limbus, and bulbar conjunctiva had larger cell bodies compared to the corneal centre (p≤0.01), and presence of long dendrites was observed mostly at non-central locations. DC with thick dendrites were mostly observed at the limbus. Day-to-day CoR for DC density ranged from ±28.1 cells/mm2 at the corneal centre to ±56.4 cells/mm2 at the limbus. Day-to-day agreement of DC morphology determined by kappa ranged from 0.5 to 0.95 for cell body size, 0.60 to 0.95 for presence of dendrites, and 0.55 to 0.80 for presence of long dendrites, at various locations. Conclusions: No diurnal changes are apparent in corneal or conjunctival DC. Substantial topographical differences exist in DC density and morphology. In vivo confocal microscopy provides good repeatability of DC density and acceptable agreement of DC morphology. Keywords: cornea, conjunctiva, dendritic cell, diurnal variation, antigen capture capacity, migratory capacity.

The authors have declared no competing interest.

This study did not receive any funding.

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The study was approved by the Human Research Ethics Committee of University of New South Wales Sydney.

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